WO2025159209A1 - Secondary battery component, secondary battery, and method for manufacturing secondary battery component - Google Patents

Abstract

To provide a secondary battery component that can provide excellent cycle characteristics when used in a secondary battery. A secondary battery component comprising a metal complex represented by formula (1): Q_(n-m) - M - L_m (1) wherein M is an n-valent metal, m is an integer of 0 or more and (n-1) or less, Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato, and L is a ligand selected from the group consisting of phenolato and naphtolato.

Description

SECONDARY BATTERY COMPONENT, SECONDARY BATTERY, AND METHOD FOR MANUFACTURING SECONDARY BATTERY COMPONENT

The present disclosure relates to a secondary battery component, a secondary battery, and a method for manufacturing a secondary battery component.

Background

Lithium (Li) metal can deliver the highest theoretical specific capacity (3860 mAh/g) and lowest standard redox potential (-3.04 V vs. standard hydrogen electrodes) among all types of anodes (negative electrodes) for rechargeable Li batteries. However, Li metal anodes still suffer from poor nucleation homogeneity and uncontrollable growth of Li dendrites during repetitive deposition/stripping processes. These issues are usually accompanied by low Coulombic efficiency (CE) as well as the limited lifespan of lithium metal batteries (LMBs), caused by pulverization loss of Li anode and sudden dendrite-induced internal short circuits. These drawbacks impede the widespread use of LMBs in practical applications.

These impediments are mostly related to the unstable interface between anodes and electrolytes caused by the high reductive activity and colossal volume change of Li metal anodes. Routine methods for tackling these interfacial issues can be generally divided into two groups. According to a first group, routine methods involve constructing artificial interfaces on the Li anodes to replace the unstable electrolyte-derived solid electrolyte interphase(SEI). For example, an artificial interface comprising inorganic salts such as LiF has been proposed (NPLs 1 and 2).

According to a second group, decomposition of the electrolyte is controlled to achieve a lithium fluoride (LiF)-rich and stable SEI spontaneously, by regulating the electron loss/gain and dissociation chemistry on the anode interface. Consequently, electrolyte or interfacial engineering is adopted to induce the generation of stable SEI layers. For example, the strategy of electrolyte engineering to ameliorate the generation of SEI interphase has been proposed (NPLs 3 to 10).

NPL 1: Yin, Y.-C. et al. Metal chloride perovskite thin film based interfacial layer for shielding lithium metal from liquid electrolyte. Nature Communications 11, 1761, doi:10.1038/s41467-020-15643-9 (2020).
NPL 2: Cheng, X.-B. et al. Implantable Solid Electrolyte Interphase in Lithium-Metal Batteries. Chem 2, 258-270,
doi:https://doi.org/10.1016/j.chempr.2017.01.003 (2017).
NPL 3: Ren, X. et al. Localized High-Concentration Sulfone Electrolytes for High-Efficiency Lithium-Metal Batteries. Chem 4, 1877-1892, doi:https://doi.org/10.1016/j.chempr.2018.05.002 (2018).
NPL 4: Langdon, J. & Manthiram, A. Crossover Effects in Lithium-metal Batteries with a Localized High Concentration Electrolyte and High-nickel Cathodes. Advanced Materials n/a, 2205188,
doi:https://doi.org/10.1002/adma.202205188 (2022).
NPL 5: Basile, A., Bhatt, A. I. & O’Mullane, A. P. Stabilizing lithium metal using ionic liquids for long-lived batteries. Nature Communications 7, ncomms11794, doi:10.1038/ncomms11794 (2016).
NPL 6: Sun, H. et al. High-Safety and High-Energy-Density Lithium Metal Batteries in a Novel Ionic-Liquid Electrolyte. Advanced Materials 32, 2001741, doi:https://doi.org/10.1002/adma.202001741 (2020).
NPL 7: Yu, Z. et al. Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries. Nature Energy 5, 526-533, doi:10.1038/s41560-020-0634-5 (2020).
NPL 8: Fan, X. et al. All-temperature batteries enabled by fluorinated electrolytes with non-polar solvents. Nature Energy 4, 882-890, doi:10.1038/s41560-019-0474-3 (2019).
NPL 9: Zheng, J. et al. Electrolyte additive enabled fast charging and stable cycling lithium metal batteries. Nature Energy 2, 17012,
doi:10.1038/nenergy.2017.12 (2017).
NPL 10: Biswal, P. et al. The early-stage growth and reversibility of Li electrodeposition in Br-rich electrolytes. Proceedings of the National Academy of Sciences 118, e2012071118, doi:10.1073/pnas.2012071118 (2021).

Summary

However, the protective layer proposed by NPLs 1 and 2 may inevitably crack and is not regenerated during cycling, thereby triggering severe Li-electrolyte reactions at the exposed surface consuming the active Li and electrolytes.

The SEI formed by using the electrolytes proposed by NPLs 3 to 10 can reduce the consumption of Li but nevertheless constantly consumes the electrolyte. For example, the so-formed SEI layer is weak against mechanical accommodation of Li irregular growth, and it undergoes continuous formation and breakdown upon cycling to further increase the consumption of electrolytes. Therefore, an excess amount of electrolyte (the electrolyte-to-capacity ratio > ~40 μL/mAh) and Li (negative-to-positive capacity ratio > 2) are needed to guarantee the cycling stability of cells, which lowers the energy density of the battery.

To satisfy the energy density required for practical batteries, the ratio of the negative-to-positive capacity and electrolyte-to-capacity are required to be around 1 and below 10 μL/mAh, respectively. These requirements call for not only a stable SEI layer without unnecessary electrolyte loss, but also the highly reversible Li stripping/deposition processes to avoid the loss of active Li. Unfortunately, due to the competitive multi-order reaction kinetics of different solvents and salts, simultaneously regulating the constituents of SEI and Li stripping/deposition behaviors is still challenging, especially under such realistic operation conditions. An approach to rationally design a stable interface on a Li anode to induce the generation of SEI and stabilize the Li stripping/deposition process is important for realizing practical high-energy-density LMBs.

The present disclosure aims to provide a secondary battery component that can provide excellent cycle characteristics when used in a secondary battery.

The present inventors invented Li metal anode which has a Li-rich molecular interfacial layer using a specific metal complex including 8-hydroxyquinolinolato-lithium (Liq). The present inventors found that 8-hdroxyquinolinolato-lithium (Liq), which is known as a material for an electron injection layer, may form a coating with a well-defined thickness by deposition (e.g., thermal evaporation or transfer printing). When the coating is provided on an anode metal, the resulting dense interface can effectively prevent the oxidation reaction of corrosive species (e.g., O₂, CO₂, H₂O, N₂, etc.) in ambient conditions, which is confirmed by X-ray diffraction experiments. Benefiting from the Li-rich dense stacking structure and abundant lithiophilic pyridinic nitrogen of Liq molecules, the Liq-Li anodes are protected from the parasitic reactions with the electrolyte, demonstrating even Li nucleation and a stable deposition/stripping process. Furthermore, combined measurements of X-ray photoelectron spectra (XPS) and cryogenic transmission microscopy (cryo-TEM) reveal that the Liq interface layer induces the precise generation of stable fluorinated SEI. Consequently, the growth of Li dendrites during repetitive deposition/stripping processes is markedly suppressed, while realizing improved efficiency. This approach is available not only in lithium-metal batteries, but also in secondary batteries using other metals as the anode.

Specifically, the present disclosure is as follows:
[1] A secondary battery component comprising a metal complex represented by formula (1):
Q_(n-m) - M - L_m (1)
wherein
M is an n-valent metal,
m is an integer of 0 or more and (n-1) or less,
Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato, and
L is a ligand selected from the group consisting of phenolato and naphtolato.
[2] The secondary battery component according to [1], the component comprises a layer that includes the metal complex represented by formula (1).
[3] The secondary battery component according to [1] or [2], wherein Q is unsubstituted or substituted 8-hydroxyquinolinolato.
[4] The secondary battery component according to any one of [1] to [3], wherein m is 0.
[5] The secondary battery component according to any one of [1] to [4], wherein M is a monovalent metal.
[6] The secondary battery component according to [5], wherein M is Li, Na, K or Ag.
[7] The secondary battery component according to any one of [1] to [4], wherein M is a divalent metal.
[8] The secondary battery component according to [7], wherein M is Mg, Ca, Zn or Cu.
[9] The secondary battery component according to any one of [1] to [4], wherein M is a trivalent metal.
[10] The secondary battery component according to [9], wherein M is Al.
[11] The secondary battery component according to any one of [1] to [6], wherein the complex is represented by formula (2), (3) or (4).
wherein
R²¹, R²², R²³, R²⁴, R²⁵, and R²⁶ are each independently selected from the group consisting of a hydrogen atom, a halogen atom, and a hydroxyl group,
R³¹, R³², R³³, R³⁴, R³⁵, R³⁶, and R³⁷ are each independently selected from the group consisting of a hydrogen atom, a halogen atom, and a hydroxyl group,
R⁴¹, R⁴², R⁴³, R⁴⁴, R⁴⁵, R⁴⁶, R⁴⁷, R⁴⁸, R⁴⁹, and R⁵⁰ are each independently selected from the group consisting of a hydrogen atom, a halogen atom, and a hydroxyl group.
[12] The secondary battery component according to any one of [1] to [11], wherein the secondary battery component is a separator or an anode.
[13] A secondary battery comprising a secondary battery component according to any one of [1] to [12].
[14] The secondary battery according to [13], wherein the secondary battery is a lithium metal battery, a lithium-oxygen battery, a lithium-sulfur battery, a sodium metal battery, a sodium-oxygen battery, a sodium-sulfur battery, a potassium metal battery, a zinc metal battery, a magnesium metal battery, a calcium metal battery, an aluminum metal battery, an aluminum-sulfur battery, or a lithium ion secondary battery.
[15] A method for manufacturing a secondary battery component, comprising applying a metal complex represented by formula (1):
Q_(n-m) - M - L_m (1)
wherein
M is an n-valent metal,
m is an integer of 0 or more and (n-1) or less,
Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato, and
L is a ligand selected from the group consisting of phenolato and naphtolato,
to a substrate to obtain a secondary battery component according to any one of [1] to [12].
[16] The method for manufacturing a secondary battery component according to [15], comprising depositing the metal complex represented by formula (1) on the substrate.
[17] The method for manufacturing a secondary battery component according to [15] or [16], wherein the compound of formula (1) is deposited on the substrate by thermal evaporation under reduced pressure.
[18] The method for manufacturing a secondary battery component according to [15] or [16], wherein the compound of formula (1) is deposited on the substrate by transfer printing.

Advantageous Effect

The present disclosure provides a secondary battery component that can provide excellent cycle characteristics when used in a secondary battery.

In the accompanying drawings:
FIG. 1: a, Schematic illustration of the relative electron energies at the interface between the Li metal anode and electrolyte to form a thermodynamically stable SEI layer. Eg is the window of the electrolyte for thermodynamic stability.
b, Ultraviolet photoemission spectra of the pristine Li metal anode including the secondary electron cutoff (left) and valance features (right).
FIG. 2: a, Schematic illustration of evaporated 8-hydroxyquinolinolato-lithium (Liq) on the surface of a lithium foil, which inhibits H₂O, O₂, and CO₂ in air to react with Li foils.
b, Photo of 10 nm Liq coated Li foil (thickness: ~0.17 mm) in air.
c,d, (c) Schematic illustration of the formation process of SEI on the pristine Li anode, and (d) the Liq-Li anode, respectively.
FIG. 3: a-f, Simulated crystal and molecule structure of (a) LiOH, (b) Li₂O, (c) Li₂CO₃, (d) LiF, (e) Li₂S, and (f) Liq and the corresponding structure after losing a Li atom.
FIG. 4: a, Size distribution of deposited Li on the pristine Cu, LiF-Cu, and Liq-Cu.
b-d, Schematic illustration of nucleation and growth of deposited Li on Cu surface: (b) pristine Cu, (c) LiF-Cu, and (d) Liq-Cu.
FIG. 5: a, Schematic illustration of thermal evaporation of Liq on Li/Cu foils and polymer separators.
b-g, Optical photos of pristine (b) Cu and (c) Li, 10 nm LiF on (d) Cu and (e) Li, 10 nm Liq on (f) Cu and (g) Li in the Ar glovebox.
FIG. 6: a, b, Ultraviolet photoemission spectra of Li metal anode with 5 nm Liq including (a) the secondary electron cutoff and (b) valance features as a function of Ar-ion sputtering time.
c, Interfacial electronic distribution of Liq molecule and Li metal anode.
d, Schematic illustration of the electronic structure at the interface between Liq molecular layer and Li metal anode.
FIG. 7: In situ XRD results of oxidation process of (a) pristine Li foil, (b) 10 nm LiF-Li foil, and (c) 10 nm Liq-Li foil in ambient (room temperature = ~25 °C and humidity = ~30%). The thickness of Li foil is 0.17 mm.
FIG. 8: a, 2D grazing-incidence wide-angle X-ray scattering pattern of 10 nm Liq molecule interlayer on Cu film.
b, The corresponding intensity versus azimuth distribution of plane (Qz = ~0.5).
FIG.9: a, Galvanostatic Li electrodeposition voltage profiles for a range of Liq thicknesses at the fixed current density of 0.05 mA/cm².
b, Li nucleation overpotentials measured at the fixed current density of 0.05 mA/cm² for Liq thickness ranging from 0 to ~50 nm.
FIG.10: a-c, Cryo-transmission electron microscopy images of Li nucleation on Cu grids (a) pristine one, (b) deposited with LiF interphase, and (c) deposited with Liq interphase. The deposition thickness of LiF and Liq molecular interlayer is ~10 nm. Scale bar, 1 μm.
d, Li nucleation overpotentials measured at the fixed current density of 0.05 mA/cm² for the pristine Cu, LiF-Cu, and Liq-Cu. The thickness of LiF and Liq on Cu current collector is around 10 nm.
e, The anodic current density in cyclic voltammograms of Li|Cu under different scan cycles at the scan rate of 2 mV/s.
f, CE as a function of cycle number for pristine Cu/Li, LiF-Cu/Li, and Liq-Cu/Li asymmetric cells with 20 μL LiPF₆ electrolyte (EC/DMC/DEC: 1:1:1 vol%) at the fixed current density of 0.25 mA/cm².
g, CE as a function of cycle number for Liq-Cu/Li asymmetric cell with 10 μL LiPF₆ electrolyte (EC/DMC/DEC: 1:1:1 vol%) at the fixed current density of 0.5 mA/cm².
FIG.11: a, b, Cryo-TEM images of Li nucleation on the surface of Liq-Cu.
FIG.12: a, b, Cryo-TEM images of (a) Li nucleation on the surface of LiF-Cu and (b) corresponding fast Fourier transform (FFT).
c,d, High-resolution cryo-TEM images of (c) Li (002) and (d) Li₂O (111).
FIG.13: Initial cyclic voltammograms of the pristine Li|Cu cell, LiF-Li|LiF-Cu cell, Liq-Li|Liq-Cu cell at the scan rate of 2 mV/s with the scan window of -0.3-2.0 V.
FIG.14: a-c, Cyclic voltammetry curves of asymmetric cells of pristine (a) Li|Cu, (b) LiF-Li|LiF-Cu, (c) Liq-Li|Liq-Cu at a fixed scanning rate of 2 mV/s.
FIG.15: a, Voltage profiles of Liq-Li|Liq-Cu asymmetric cell under a fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm² at 100th, 200th, 300th, 400th, and 500th.
b, Voltage profiles of Liq-Li|Liq-Cu asymmetric cell under the different fixed current densities of 0.25, 0.5, 1, and 2 mA/cm² with the corresponding capacity of 0.5, 1, 2, and 4 mAh/cm².
c-e, Surface morphologies of (c) pristine Cu, (d) 10 nm LiF-Cu, and (e) 10 nm Liq-Cu after depositing Li at a fixed current density of 0.25 mAh/cm² and areal capacity of 0.5 mAh/cm².
FIG.16: a-c, Surface morphologies of (a) pristine Cu, (b) 10 nm LiF-Cu, and (c) 10 nm Liq-Cu after depositing Li at a fixed current density of 0.2 mA/cm² for an areal capacity of 4 mAh/cm².
d-f, Surface morphologies of (d) pristine Cu, (e) 10 nm LiF-Cu, and (f) 10 nm Liq-Cu after depositing Li at a fixed current density of 0.2 mAh/cm² for an areal capacity of 6 mAh/cm².
FIG.17: a-d, Cross-section morphologies of LiF-Cu after depositing an areal capacity of (a) 4 mAh/cm² and (b) 8 mAh/cm², Liq-Cu after depositing an areal capacity of (c) 4 mAh/cm² and (d) 8 mAh/cm² at a fixed current density of 0.2 mA/cm².
e, Comparison of Li deposition thickness on LiF-Cu and Liq-Cu at the areal capacities of 4 mAh/cm² and 8 mAh/cm².
FIG.18: a-d, X-ray photoelectron spectra (XPS) results of the Li SEI formed on pristine Cu, 10 nm LiF-Cu, and 10 nm Liq-Cu for the (a) C 1s, (b) Li 1s, (c) F 1s, and (d) O 1s regions.
e-j, Cryo-TEM images of the SEI layers formed on the (e-h) pristine Cu grid and (i-k) 10 nm Liq-Cu grid.
FIG.19: The composition ratio of F 1s in SEI layer of Li deposits on pristine Cu, Cu grid with LiF interlayer, and Cu grid with Liq interlayer.
FIG.20: a-c, Schematic illustration of deposited Li and corresponding SEI layers on (a) pristine Cu grid, (b) 10 nm LiF-Cu grid, and (c) 10 nm Liq-Cu grid.
d-f, Corresponding Cryo-TEM images of deposited Li on (d) pristine Cu grid, (e) 10 nm LiF-Cu grid, (f) 10 nm Liq-Cu grid at a fixed current density of 0.05 mA/cm² for 20 min.
FIG.21: a, b, Cryo-TEM image of (a) SEI layer of deposited Li on LiF-Cu grid and (b) corresponding fast Fourier transformation image. High-resolution cryo-TEM images of (c) Li₂CO₃ (200) and Li₂O (111), (d) Li (110) and LiF (220).
FIG.22: a, Galvanostatic Li plating/stripping voltage profiles for the pristine Li|Li, LiF Li|Li, and Liq Li|Li symmetric cells at a fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm². The thickness of Li anode: 0.6 mm.
b-e, Surface morphology of the pristine Li (b) before cycling, and (c) pristine Li after cycling 1000 h, (d) LiF-Li after cycling 1000 h, (e) Liq-Li after cycling 2000 h at a fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm².
f, Galvanostatic Li plating/stripping voltage profiles for the Li|Li symmetric cells with 10 nm Liq Celgard (registered trademark) separator at a fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm². The thickness of Li anode: 0.17 mm.
g, Nyquist plots of the initial pristine Li|Li, LiF Li|Li, and Liq Li|Li symmetric cells.
h, Galvanostatic Li plating/stripping voltage profiles for the Li|Li symmetric cells with 10 nm Liq at different current densities and capacities. The thickness of Li anode: 0.17 mm
FIG.23: a-f, Galvanostatic Li plating/stripping voltage profiles of 10 nm Liq Li|Li symmetric cell under a fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm² at (a) 0-20 h, (b) 400-420 h, (c) 800-820 h, (d) 1200-1220 h, (e) 1600-1620 h, and (f) 1980-2000 h.
FIG.24: a, Galvanostatic Li plating/stripping voltage profiles of 10 nm Liq 60 μm Li|Li symmetric cell under the different current densities of 0.5 mA/cm², 1.0 mA/cm², and 2.0 mA/cm², corresponding to the areal capacity of 1.0 mAh/cm², 2.0 mAh/cm², and 4.0 mAh/cm² continuously after cycling for 2000 h at the fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm².
b-d, Voltages profiles under the fixed current density of 0.5 mA/cm² and capacity of 1.0 mAh/cm² at (b) 2000-2020 h, (c) 2200-2220h, (d) 2380-2400 h.
e-g, Voltages profiles under the fixed current density of 1.0 mA/cm² and capacity of 2.0 mAh/cm² at (e) 2400-2420 h, (f) 2600-2620h, (g) 2780-2800 h.
h-j, Voltages profiles under the fixed current density of 2.0 mA/cm² and capacity of 4.0 mAh/cm² at (h) 2820-2840 h, (i) 3000-3020h, (j) 3180-3200 h.
FIG.25: a-f, Galvanostatic Li plating/stripping voltage profiles of Li|Li symmetric cell with 10 nm @ Celgard (registered trademark)separator under the fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm² at (a) 0-20 h, (b) 450-470h, (c) 900-920 h, (d) 1350-1370 h, (e) 1800-1820 h, and (f) 2500-2520 h.
FIG.26: a, b, Nyquist plots of the pristine Li|Li, LiF-Li|Li, and Liq-Li|Li symmetric cells after cycling for (a) 20 cycles and (b) 50 cycles with magnified one inset.
FIG.27: a-c, Voltage profiles of 10 nm Liq-Li|Li symmetric cell under the fixed current density of 1 mA/cm² and capacity of 2.0 mAh/cm² at (a) 0-10 h, (b) 100-110 h, and (c) 270-280 h.
d-f, Voltage profiles of 10 nm Liq-Li|Li symmetric cell under the fixed current density of 2 mA/cm² and capacity of 4 mAh/cm² at (d) 280-290 h, (e) 480-490 h, and (f) 670-680 h.
g-i, Voltage profiles of 10 nm Liq Li|Li symmetric cell under the fixed current density of 3 mA/cm² and capacity of 6 mAh/cm² at (g) 690-700 h, (h) 900-910 h, and (i) 1070-1080 h. j-l, Voltage profiles of 10 nm Liq Li|Li symmetric cell under the fixed current density of 5 mA/cm² and capacity of 10 mAh/cm² at (j) 1090-1100 h, (k) 1300-1310 h, and (l) 1470-1480 h.
FIG.28: a, Cycling profiles of a discharge capacity for the pristine Li|LFP and Liq-Li|LFP full cells with excess Li, N/P ratio of ~5.0, and N/P ratio of ~1.9 at charging/discharging rate of 2 C.
b, Cycling profiles of the discharge gravimetric capacity (left y axis) and areal capacity (right y axis) of 10 nm Liq on Li for LFP cell at a current density of 0.5 C with N/P ratio of ~1.0 and lean electrolyte condition of 10 μL/mAh.
c, d Cycling profiles of the discharge capacity (left axis) and areal capacity (right axis) of (c) Liq-Li|LFP and (d) Liq-Li|NCM-811 pouch cells at the charging/discharging rates of 0.2 C/0.33 C with the electrolyte ratio of 3.0 g/Ah.
FIG.29: a, Cycling profiles of a discharge capacity for the pristine Li|LFP, 10 nm LiF-Li|LFP and 10 nm Liq-Li|LFP full cells with excess Li at charging/discharging rate of 2 C.
b-d, Corresponding discharge curves of (b) pristine Li|LFP, (c) 10 nm LiF-Li|LFP, and (d) 10 nm Liq-Li|LFP for initial and after 270 cycles.
FIG.30: a, Cycling profiles of a discharge capacity for Liq-Li|LFP full cells with N/P ratio of ~5.0 at charging/discharging rate of 2 C.
b, c, (b) Galvanostatic charge/discharge curves of 10 nm Liq-Li|LFP full cell under a fixed current density of 2 C with a N/P ratio of ~5.0 and electrolyte ratio of 75 μL/mAh at 1st, 50th, 100th, 150th, and 220th cycles and (c) corresponding coulombic efficiency during cycling.
FIG.31: a, Cycling profiles of a discharge capacity for Liq-Li|LFP full cells with N/P ratio of ~1.9 at charging/discharging rate of 2 C.
b, c, (b) Galvanostatic charge/discharge curves of 10 nm Liq-Li|LFP full cell under a fixed current density of 2 C with a N/P ratio of ~1.9 and electrolyte ratio of 75 μL/mAh at 1st, 50th, 100th, 150th, and 220th cycles and (c) corresponding coulombic efficiency during cycling.
FIG.32: a, Galvanostatic charge/discharge curves of 10 nm Liq-Li|LFP full cell under a fixed current density of 0.5 C with an N/P ratio of ~1.0 and electrolyte ratio of 10 μL/mAh at 1st, 50th, 100th, 150th, and 220th cycles. b, corresponding coulombic efficiency during cycling.
FIG.33: a, Cycling profiles of the discharge gravimetric capacity (left y axis) and areal capacity (right y axis) for deposited Li on 10 nm Liq-Cu foil|LFP full cell under a fixed current density of 0.5 C with an N/P ratio of ~1.0 and electrolyte ratio of 10 μL/mAh.
b, Corresponding galvanostatic charge/discharge voltage profiles under a fixed current density of 0.5 C at 1st, 20th, 50th, 75th, and 100th cycles.
c, Corresponding Coulombic efficiency for 100 cycles.
FIG.34: a, Cycling profiles of a discharge capacity for Liq-Li|NCA full cells with N/P ratio of ~1.0 at charging/discharging rate of 0.2 C.
b, c, (b) Galvanostatic charge/discharge curves of 10 nm Liq-Li|NCA full cell under a fixed current density of 0.2 C with an N/P ratio of ~1.0 and electrolyte ratio of 10 μL/mAh at 1st, 25th, 50th, 75th, and 100th cycles and (c) corresponding coulombic efficiency during cycling.
FIG.35: a, Photos of the pouch cells of Liq-Li|NCM-811 with a capacity of ~0.7 Ah and Liq-Li|LFP with a capacity of ~0.45 Ah. The electrolyte ratio of the pouch cells is ~3.0 g/Ah.
b, Corresponding initial Coulombic efficiency of the pouch cells.
FIG.36: a, b, Galvanostatic charge/discharge voltage profiles of (a) Liq-Li|LFP pouch cell and (b) the Liq-Li|NCM-811 pouch cell with the fixed charging rate of 0.2 C and discharging rate of 0.33 C at 1st, 20th, 50th, 75th, and 100th cycles.
FIG.37:a, Energy level alignments between the Li deposited with Liq molecules showing their highest occupied system orbitals (HOSO), lowest unoccupied system orbitals (LUSO), and vacuum level, and reported different complexes in the electrolyte showing their simulated reduction potential relative to the vacuum level.
b, Energy diagram and electronic structure of bare Li anode with inevitably oxidized surface and electrolyte. c, Energy diagram and electronic structure of Liq-Li anode and electrolyte. E_{vac (s)}: the vacuum energy level at the surface. Φ: work function. E_{reduction potential}: the reduction potential relative to the vacuum energy level.
d,e, Schematic illustration of (d) reaction between the Li anode with an oxidized surface and electrolyte for SEI generation, and (e) reaction between Li anode coated with Liq molecular layer and electrolyte for SEI generation.

DETAILED DESCRIPTION

<Secondary battery component>

The present disclosure relates to a secondary battery component comprising a metal complex represented by formula:
Q_(n-m) - M - L_m (1)
wherein
M is an n-valent metal,
m is an integer of 0 or more and (n-1) or less,
Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato,
L is a ligand selected from the group consisting of phenolato and naphtolato.

In one embodiment, the present disclosure relates to a secondary battery component comprising a layer that includes a metal complex represented by formula:
Q_(n-m) - M - L_m (1)
wherein
M is an n-valent metal,
m is an integer of 0 or more and (n-1) or less,
Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato,
L is a ligand selected from the group consisting of phenolato and naphtolato.

The secondary battery component may be a component that is incorporated into a secondary battery as it is, or a component used in the manufacture of a secondary battery.

(Metal complex)
A metal complex represented by the above formula (1) (hereinafter, also referred to as a “metal complex”) is explained below.

The variable "n" in the definition of M is an integer equal to or greater than 1, preferably an integer of 1 to 5, more preferably an integer of 1 to 3, particularly 1.

When n is 1, M is a monovalent metal including Li, Na, K, and Ag.
When n is 2, M is a divalent metal including Mg, Ca, Zn, and Cu.
When n is 3, M is a trivalent metal including Al.
M is preferably Li in terms of better interfacial compatibility and higher ionic conductivity

The variable "m" in formula (1) is an integer of 0 or more and (n-1) or less, preferably 0, that is, it is preferred that no group L is present in formula (1) in terms of chemical stability and compatibility.
When n is 1, m is 0, that is no group L is present in formula (1).
When n is 2, m is 0 or 1, preferably 0.
When n is 3, m is 0, 1, or 2, preferably 0.

Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato, preferably unsubstituted or substituted 8-hydroxyquinolinolato.

Examples of the substituent include a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom), alkyl groups (e.g., alkyl group having 1 to 4 carbon atoms), hydroxyl, preferably a fluorine atom. When substituent(s) are present, one or two substituents are preferred. The position of the substituent is preferably, meta position.
As Q, 8-hydroxyquinolinolato, 2-(2-pyridyl)phenolato, or 2-(2',2''-bipyridin-6'-yl)phenolato, unsubstituted or substituted at meta position with a fluorine atom, are preferred.

L is a ligand selected from the group consisting of phenolato and naphtolato.

Metal complexes represented by formula (1) wherein M is Li include the following:
wherein
R²¹, R²², R²³, R²⁴, R²⁵, and R²⁶ are each independently selected from the group consisting of a hydrogen atom, a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom) and a hydroxyl group, preferably, a hydrogen atom or a halogen atom, and more preferably, a hydrogen atom or a fluorine atom,
R³¹, R³², R³³, R³⁴, R³⁵, R³⁶, and R³⁷ are each independently selected from the group consisting of a hydrogen atom, a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom) and a hydroxyl group, preferably, a hydrogen atom or a halogen atom, and more preferably, a hydrogen atom or a fluorine atom.
R⁴¹, R⁴², R⁴³, R⁴⁴, R⁴⁵, R⁴⁶, R⁴⁷, R⁴⁸, R⁴⁹ and R⁵⁰ are each independently selected from the group consisting of a hydrogen atom, a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom) and a hydroxyl group, preferably, a hydrogen atom or a halogen atom, and more preferably, a hydrogen atom or a fluorine atom.

The Lithium complex represented by formula (2) is preferred in terms of, in some embodiments, chemical stability, commercial availability and inexpensive price.

Metal complexes represented by formula (1) wherein M is a metal other than Li include the following:
wherein
R⁵¹, R⁵², R⁵³, R⁵⁴, R⁵⁵, and R⁵⁶ are each independently selected from the group consisting of a hydrogen atom, a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom) and a hydroxyl group, preferably, a hydrogen atom or a fluorine atom.

wherein
M² is Mg, Zn, or Cu
R⁶¹, R⁶², R⁶³, R⁶⁴, R⁶⁵, R⁶⁶, R^61’, R^62’, R^63’, R^64’, R^65’, and R^66’are each independently selected from the group consisting of a hydrogen atom, a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom) and a hydroxyl group, preferably, a hydrogen atom or a fluorine atom.

wherein
R⁷¹, R⁷², R⁷³, R⁷⁴, R⁷⁵, R⁷⁶, R^71’, R^72’, R^73’, R^74’, R^75’, R^76’, R^71”, R^72”, R^73”, R^74”, R^75”, and R^76” are each independently selected from the group consisting of a hydrogen atom, a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom) and a hydroxyl group, preferably, a hydrogen atom or a halogen atom, more preferably, a hydrogen atom or fluorine atom.

(Metal complex layer)
The present secondary battery component comprises a layer that includes the metal complex (hereinafter referred to as a “metal complex layer”).

The metal complex included in the metal complex layer may be alone or two or more, preferably alone.

The metal complex layer may be formed on a substrate that is included in the secondary battery component by deposition.

For example, the metal complex layer may be formed on the substrate via thermal evaporation under reduced pressure. For example, the metal complex layer may be formed by placing the substrate in a reduced-pressure vessel, evaporating the complex in powder form, and depositing it on the substrate in the vessel.

The pressure in the vessel may be 10^-2 or less. The pressure is, preferably 10^-5 Torr or less, more preferably 10^-6 Torr or less. The lower limit of the pressure is not particularly limited and may be 10^-11 Torr or more, for example, 10^-10 Torr or more. The temperature is not limited as long as it is above the temperature at which the metal evaporates, and depends on the pressure. For example, the temperature may be -50°C or higher.

The evaporation rate may be 0.1 to 100 angstrom/s. The evaporation rate is, preferably 0.2 angstrom/s or more, more preferably 0.5 angstrom/s or more, and preferably 50 angstrom/s or less, more preferably 20 angstrom/s or less.

Another condition of thermal evaporation is the temperature of the substrate. The temperature of the substrate for the metal complex layer may be less than or equal to 100 °C. The temperature is, preferably 0 °C or more, more preferably 25 °C or more, and preferably 80 °C or less, more preferably 50 °C or less.

Alternatively, the metal complex layer may be formed on the substrate by transfer printing. An example of transfer printing is as follows: a second substrate which is different from the substrate that is included in the secondary battery component is prepared and a layer including the metal complex is formed on the second substrate via thermal evaporation. The second base material with the metal complex layer is then pressed onto the substrate, with the metal complex layer of the second base material in contact with the substrate, to obtain a laminate. The metal complex layer of the second substrate is in contact with the substrate. Then, the second substrate is peeled off from the laminate to obtain the substrate where the metal complex layer is provided on its surface.

The condition of transfer printing is not particularly limited. In the case where the metal complex layer is a Li complex layer such as Liq molecule layer, transfer printing is advantageous in terms of efficiency, especially in case the substrate uses Li metal (e.g., Li anode). Owing to the electron-withdrawing properties of its molecule structure, the Liq molecule exhibits a stronger electrostatic interaction with Li metal (e.g., Li anode), as compared with the interaction with polyolefins such as polypropylene (PP) or polyethylene commonly used for separator by physisorption. Specifically, the process is as follows: a polyolefin substrate (e.g. a PP separator) with a Liq molecular layer (e.g., a 20 nm Liq molecule layer) by thermal evaporation is pressed on the surface of Li metal substrate (e.g., Li anode) and peeled to transfer the Liq molecular layer to the Li metal substrate. The conditions for thermal evaporation mentioned above may be applied to the conditions of the thermal evaporation in the transfer printing. The pressure at which the molecular layer is pressed on the substrate is not particularly limited, and a pressure of 0.1 mPa or higher and 100 mPa or lower may be employed.

A clean Li metal substrate (e.g., Li anode) without carbon contaminants may be provided, for example, by continuous Ar⁺ sputtering in an ultra-high vacuum condition. According to the process, the transfer of the Liq molecular layer on the separator to the Li anode can be easily realized in the stacking or compression process during the assembly of Li metal batteries.

The above thermal evaporation and transfer printing methods are suitable for the roll-to-roll process, and are thus the preferred industrial methods.

Methods of forming the metal complex layer are not limited to the methods mentioned above, and include PVD (Physical Vapor Deposition) and coating the metal complex solution. The metal complex solution may be prepared by derivatizing the metal complex to improve its solubility in the solvent.

The thickness of the metal complex layer may be less than or equal to 500 nm. The thickness is, preferably 5 nm or more, more preferably 20 nm or more, and preferably 300 nm or less, more preferably 100 nm or less.

(Anode)
The secondary battery component may be an anode or a separator. When the secondary battery component is an anode, the anode material may be used as the substrate, and the metal complex layer can be provided on the surface of the anode material. The anode material may be a metal foil containing a metal which is the anode active substance of the secondary battery. The anode active substance is a substance that acts as anode parts or replenishment materials for anode part when used in a secondary battery.

Examples of the metal foil include a foil made of lithium, sodium, potassium, magnesium, calcium, zinc, aluminum, or an alloy containing at least one of these metals. For batteries where the anode active substance is lithium, lithium foil is preferred.

The metal species of the metal complex is preferably contained in the metal foil, and the metal species of the metal complex and the metal species of the metal foil are preferably the same. When the metal species of the metal complex and the metal species of the metal foil are different, it is preferred that the metal species of the metal complex can form an alloy with the metal species of the metal foil.

The thickness of the metal foil may be 10 to 500 μm. The thickness is, preferably 20 μm or more, more preferably 50 μm or more, and preferably 400 μm or less, more preferably 200 μm or less.

The metal foil may have a current collector. Examples of the current collector include copper foil, carbonaceous paper, and related fabrics.

In a secondary battery using a metal anode, the anode active substance, which is metal, can be deposited on the current collector as anode by electrochemical method or thermal evaporation. The substrate of the anode may be a current collector, and the metal complex layer can be provided on the surface of the anode material or current collector.

The anode material is not limited to a metal foil, and may be a compound capable of storing and discharging ions. Anode active materials used in lithium-ion secondary batteries may be used. Carbon or silicon materials may be mentioned as examples. Carbon materials include graphite (natural graphite, artificial graphite, etc.), hardly graphitizable carbon, easily graphitizable carbon, nanotubes, etc. Silicon materials include SiO, Si, etc.

Using the above anode as a substrate, a prelithiated anode may be obtained by depositing a lithium complex layer (e.g., Liq layer) on the substrate. This method is advantageous because the anode may be prelithiated by depositing lithium complex on the surface of the anode rather than inserting lithium into the anode. Further, the lithium complex is stable, and is unlikely to cause problems of lithium oxidation.

The method for preparing a prelithiated anode using Liq is not limited to the abovementioned methods, and any known method may be used. For example, a prelithiated anode may be obtained by applying lithium complex to the substrate. In preparing a prelithiated anode, Liq may be applied to the anode material, or Liq may be incorporated into the anode. The anode active material and Liq may be mixed to form a prelithiated anode.

(Separator)
When the secondary battery component is a separator, the separator material may be used as the substrate, and the metal complex layer can be provided on the surface of the separator material. The separator material may be a porous membrane, a woven fabric, or a woven fabric made of a polymer including polyolefin, polypropylene, polyimide, polyvinylidene difluoride, poly(vinylidene fluoride-co-hexafluoropropylene), zein, gelatin, and cellulose, or made of glass fibers.
The thickness of the separator material may be 10 to 100 μm. The thickness is, preferably 12 μm or more, more preferably 20 μm or more, and preferably 80 μm or less, more preferably 40 μm or less.
The separator with deposited metal complex layer can be provided on the surface of anode part directly as the component in the secondary battery. Additionally, the deposited metal complex layer can be covered on the surface of anode by transfer printing of separators.

<Secondary battery>
The present disclosure also relates to a secondary battery comprising a secondary battery component according to the present disclosure. The secondary battery may be a secondary battery fabricated by using a secondary battery component according to the present disclosure. The secondary battery generally has a positive electrode, a negative electrode and an electrolyte. The anode of the present disclosure may be used for the negative electrode of a secondary battery.

An active substance used for the positive electrode may be selected depending on the type of battery. The positive electrode may have a cathode current collector. The electrolyte may be selected depending on the type of battery.

A separator may be placed between the positive and negative electrodes to prevent a short circuit. The separator may be the separator of the present disclosure.

Examples of the secondary battery include a lithium metal battery, a lithium-oxygen battery, a lithium-sulfur battery, a sodium metal battery, a sodium-oxygen battery, a sodium-sulfur battery, a potassium metal battery, a zinc metal battery, a magnesium metal battery, a calcium metal battery, an aluminum metal battery, and an aluminum-sulfur battery.

The following are explanations taking Li metal battery as an example.
A lithium metal battery is a battery with a lithium metal anode, in which lithium metal is deposited on the negative electrode when charging, and lithium ions are eluted from the negative electrode when discharging.

A lithium metal battery has a positive electrode, a negative electrode having a lithium metal layer, and an electrolyte. The anode of the present disclosure may be used for the negative electrode or replenishment material of negative electrode.

An active substance used for the positive electrode includes lithium composite oxide. Lithium composite oxide is not particularly limited, and the examples include, LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, LiV₂O₅, Li₄Ti₅O₁₂, LiNi_xCo_yMn_zM_aO₂ (wherein x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), LiV₂O₅, olivine-type LiMPO₄ (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Cr), LiNi_xCo_yAl_zO₂ (wherein 0.9 < x + y + z < 1.1).

The positive electrode may have a current collector. Examples of the current collector include aluminum foil, carbonaceous paper or related fabrics.

The electrolyte may be a liquid electrolyte or a solid electrolyte.

The liquid electrolyte may contain a nonaqueous organic solvent and a lithium salt. The nonaqueous organic solvent is not particularly limited, and examples include carbonate (e.g. dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), Methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC); ether (e.g. dimethyl ether, 1,2-dimethoxyethane, dibutyl ether, polyethylene glycol dimethyl ether, tetrahydrofuran); ester (e.g. methyl acetate, ethyl acetate, n-propyl acetate, γ-butyrolactone); ketone (e.g. hexanone); alcohol.

The lithium salt is not particularly limited, and examples include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, lithium bis(fluorosulfonyl)imide (LiFSI).

The electrolyte may be impregnated in a porous separator located between the negative electrode and the positive electrode. The separator is not particularly limited, and examples include glass fiber, polyester, polyolefin (e.g. polyethylene, polypropylene), polytetrafluoroethylene (PTFE). The separator may be a non-woven fabric or a woven fabric. The separator of the present disclosure may be used.

The lithium metal battery may be produced by any known method. The shape of the battery is also not particularly limited, and examples include a cylindrical battery, rectangular battery, pouch-type battery, and coin-type battery.

The secondary battery may be a lithium ion secondary battery. For example, a lithium complex layer, including a Liq layer, may be deposited on an anode containing carbon or silicon materials to obtain a prelithiated anode.
The method for preparing a prelithiated anode using Liq is not limited to the abovementioned methods, and any known method may be used.
The components of a lithium ion secondary battery other than the anode (cathode, electrolyte, separator, etc.) are not limited and may be any known components.

A lithium complex layer may be deposited on the separator. By placing the lithium complex layer and the anode to face each other, a prelithiated anode may be obtained. The components of a lithium ion secondary battery other than the separator (anode, cathode, electrolyte, etc.) are not limited and may be any known components.

<Design rationale of the metal complex interface on metal anodes>
The design rationale of the metal complex interface on metal anodes is explained below using a Li metal anode with a Liq molecular layer as an example.

During the generation of SEI in the initial charging/discharging cycles, the excess electrons from the Li metal anodes tunnel through the interface to reach the electrolyte, resulting in the corrosion of the Li metal surface and degradation of the electrolyte.

FIG. 1 a is a schematic illustration of the relative electron energies at the interface between the anode and electrolyte of a thermodynamically stable Li metal battery. The Femi level of the Li metal anode is higher than the lowest unoccupied molecular orbital (LUMO) of the electrolyte. Using ultraviolet photoelectron spectroscopy (UPS) (FIG. 1 b), it was found that the inevitable surface oxidation or organic contamination during the fabrication or long-term storage process further leads to the decrease in the work function (Wf) of Li metal anodes. The lower Wf of Li metal anodes indicates that more electrons from Li metal anodes can easily react with the electrolyte, resulting in severe consumption of the anodes and electrolytes to achieve a relative equilibrium.

Optimizing the surface electron injection and transport at the interface between the Li metal anode and the electrolyte is considered to be an important factor that impacts the SEI evolution and stabilization of the Li metal anodes.

8-quinolinolate lithium (Liq), which is usually used as the electron-injection layer in organic/hybrid light emitting diode devices, may be precisely deposited on the surface of the Li metal anodes/copper (Cu) foils via thermal evaporation in a vacuum system. FIG. 2 a is a schematic illustration of evaporated 8-hydroxyquinolinolato-lithium (Liq) on the surface of a lithium foil, which inhibits H₂O, O₂, and CO₂ in air to react with Li foils.

As explained in the examples below, the Liq molecular layer on the Li metal anode can decrease the work function (Wf) of the Li metal anode via interfacial dipole. The excess electrons of the Li metal anode can be easily transported to the interface, which participates in the decomposition of electrolyte and generation of SEI.

As explained in the examples below, the continuous Li foil with a Liq molecular layer in air further exhibits a metallic surface without obvious oxidation. Additionally, the Li-ion conduction behavior of the interfacial layer is also another key point for SEI generation and further Li deposition/stripping process. As shown in FIG. 2 a, the Liq molecule exhibits a lithiophilic chemical structure, which consists of a lithium atom bonded to a hydroxyquinolate ligand.

The potential Li⁺ conduction properties were evaluated, and the calculated dissociation energy for the Liq molecule to lose a Li atom is lower than that of the prevalent inorganic species (Li₂O, Li₂CO₃, LiF, LiOH, and Li₂S) observed in SEIs of Li metal anodes (FIG. 3 and Table 1). The lower dissociation energy of the Liq molecule indicates that the Li-ion in the Liq molecule can easily migrate from the molecule skeleton, which is beneficial to regulating the Li⁺ distribution on the surface of Li metal anode.

Table 1: Calculated dissociation energies of a single Li atom from the Li-containing crystal and molecules.
Note: dis-Li means the dissociation of Li from the molecules or crystal structure.

In combination with the previous findings and key results, the SEI evolution on the Li metal anode is portrayed in FIG. 2 c, d. The dissociated Li⁺ are coordinated with anion and solvent molecules, which creates numerous solvated Li⁺ clusters in the liquid electrolyte. The shell of solvated clusters, as the potential sources for the generation of SEIs on the Li anode, carry the solvated Li⁺ to the surface of the Li metal anode. For the pristine Li anode, spontaneous electron transfer between solvent/anion molecules in clusters and active Li anode occurs, resulting in the subsequent decomposition of the electrolyte and consumption of the Li anode to form the SEI region (FIG. 2 c). The organic/inorganic decomposition precipitations from the electrolyte and the Li metal anode are distributed heterogeneously in the SEI layer, forming a mosaic structure with both crystalline and amorphous microphases.

Comparatively, the Liq molecular interlayer achieves effective electron transfer by the interfacial dipoles at the interface between the Li anode and electrolyte for Li⁺ reduction/oxidation. The formation of the resulting SEI layer is regulated by avoiding the byproducts of the reaction between the solvent and the Li anode and subsequent surface corrosion of the Li anode (FIG. 2 d). The generated SEI on the Liq-Li anode exhibits a multilayer nanostructure that possesses a uniform inorganic-rich (e.g., Li₂O, Li₂CO₃, and LiF) outer layer and an amorphous inner layer. Beyond that, owing to the Li-rich and electron-conductive properties of the Liq molecule, the Liq molecular interlayer provides a uniform Li⁺ concentration distribution and facilitates electron distribution on the surface of the Li anode. These features effectively reduce the interfacial polarization of Li⁺ concentration and partial current density, leading to a stable Li deposition/stripping process and stabilization of the SEI layer.

<Mechanism of Liq or similar molecular layer on protection of Li anode>
Possible mechanism for Liq-mediated SEI formation may be mentioned as follows (FIG.37). Upon the injection of external electrons and physical contact between Liq-Li and electrolyte, spontaneous electron transfer occurs between the induced interface states of Liq-Li and Ereduction levels (i.e., the reduction potential relative to the vacuum level) of more species in the electrolyte, leading to the instant formation of the initial SEI layer compared to using bare Li (FIG. 37 b and c). This is because the Liq molecular layer effectively narrows the energy gap between Li and electrolyte components at the Li/electrolyte interface through interfacial dipole, thereby enabling a more facile and rapid SEI formation with largely reduced electrolyte and Li consumption compared to the formation process using bare Li. Additionally, the reductive reactions of solvents and salts largely occur on the stacked dipolar Liq molecular layer by interfacial electron transfer instead of on the Li metal surface, which helps reduce Li consumption during the SEI formation process to improve Coulombic efficiency.

<Size distribution of Li deposit and growth mechanism>
The size distribution of Li deposit and growth mechanism on different Cu surfaces based on the results as shown in the examples below are depicted in FIG. 4. As compared with that of Li deposit on pristine Cu surfaces and surfaces with a LiF molecular layer, the average diameter of Li deposit on the Cu grid with Liq molecular is smaller with a relatively concentrated distribution. Under the finite nucleation site on the Cu surface, Li deposits nucleate randomly on the surface of Cu and grow freely in the vertical direction (FIG. 4 b). For Li nucleation on the Cu surface with a LiF layer (FIG. 4 c), the initial Li nucleation and deposition prefer to occur between LiF particles rather than underneath them, due to the poor electrical conductivity and strong chemical bonding with the underlying Cu surface. Once the nucleation occurs, further Li deposition is prone to occur on the existing nucleation sites to perpendicularly form the dendritic microstructure rather than forming additional Li nuclei. Such low nucleation density leads to a high initial nucleation barrier and subsequent nonuniform deposition.

For Li nucleation on the Cu surface with the Liq molecular layer (FIG. 4 d), the Liq molecule consists of a Li-rich chemical structure with a lithiophilic pyridinic nitrogen, which provides numerous Li nucleation sites and abundant Li⁺ replenishment. Furthermore, benefiting from the excellent electron transport ability of Liq molecule, the molecular interlayer supplies sufficient electrons for Li nucleation and deposition. As the vertical Li growth barrier at the interface exceeds the surface energy of lateral growth, the Li deposits turn to initially grow laterally to fill the intergranular voids. In this way, the uniform nucleation process with high density and initial lateral deposition of Li are simultaneously achieved by introduction of Liq molecular interlayer.

As mentioned above, a design of the Li-rich molecular interfacial layer on Li metal anodes using 8-quinolinolate lithium (Liq) with a well-defined thickness as the SEI precursor is provided.
Benefiting from the Li-rich dense stacking structure and abundant lithiophilic pyridinic nitrogen of Liq molecules, the Liq-Li anodes are protected from parasitic reactions with ambient air and electrolyte.
Furthermore, the Liq interface layer ameliorates the interfacial electron transport and induces the precise generation of structurally stable fluorinated SEI towards uniform Li⁺ nucleation and diffusion as well as growth. Apart from the efficient and stable Li plating/stripping process, the utilization of Liq molecular interlayer enables stable cycling of LMBs under lean electrolyte, limited Li excess, and high-capacity conditions.
These prospective results allow the simple and practical creation of diverse ranges and types of Liq molecular interlayers on commercially available Li/Cu foils or separators, which enables construction of reliable LMBs with a contemporary selection of cathodes.
This approach is available not only in lithium-metal batteries, but also in secondary batteries using other metals as the anode.

Examples

More specific descriptions of the present disclosure based on certain examples are provided in the following paragraphs.

<Experimental methods>

(Materials)
8-Quinolinolato lithium (Liq, >99.5%), polyvinylidene difluoride (PVDF) and lithium fluoride (LiF, >99.9%) were purchased from Sigma-Aldrich, USA. Lithium foils (Li, >99.9%) with the thickness around 0.17 mm were fabricated by the HONJO Chemical, Co. The lithium chips (Li, >99.9%) with the thickness around 0.6 mm were fabricated by the Xiamen TOB New Energy Technology Co. The copper foils with the thickness around 9 μm were purchased from the MTI Co.. Lithium hexafluorophosphate (LiPF₆), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were all purchased from Sigma-Aldrich. Commercial Celgard (registered trademark) microporous monolayer membranes were purchased from Celgard LLC, USA. Lithium iron phosphate (LiFePO₄) and LiNi_0.815Co_0.15Al_0.035O₂ (NCA) were supplied by BTR New Material Group Co.,ltd. LiNi0_.8Co_0.1Mn_0.1O₂ (NCM) was supplied by Changsha Research Institute of Mining & Metallurgy Co., Ltd. All chemicals are analytical grade and used without further purification.

(Evaporation of LiF/Liq on lithium anodes/copper foils)
The LiF/Liq powders were added into quartz crucibles and transferred into the vacuum evaporation system through the glovebox. The vacuum of evaporation system was below 6×10^-6 Torr during thermal evaporation. The LiF/Liq molecules were deposited on the surface of Li metal anodes/copper foils with different deposition thicknesses from 10 to 50 nm via the masks (active area: 1 × 1 cm²). The evaporation rate was controlled around 0.3 angstrom/s. After that, the samples were directly transferred into the Ar glovebox without contamination of laboratory ambient conditions (~40 % relative humidity).

(Evaporation of Liq on polymetric separators)
The Liq powder was added into a quartz crucible and transferred into the vacuum evaporation system through the glovebox. The vacuum of evaporation system was below 6×10^-6 torr during thermal evaporation. For assembling coil cells, the Liq molecules were deposited on the surface of separators via the masks (active area: 1.8 × 1.8 cm²). For assembling pouch cells, the Liq molecules were deposited on the surface of separators without the mask. The evaporation rate was controlled around 0.3 angstrom/s. After that, the samples were directly transferred into the Ar glovebox without contamination of laboratory ambient conditions.

(Characterizations)
The surface morphologies of lithium metal anode and deposited lithium on copper foils were examined by scanning electron microscope (FEI Helios G3 UC) with a transfer holder that was protected by Ar gas during transfer. The Titan G2 Transmission Electron Microscope (TEM, Thermo Fisher Scientific) with a Gatan-613 cooling holder was used to investigate the crystal structure of Li metal and solid electrolyte interface. The TEM has a post-specimen spherical-aberration corrector (CEOS Gmbh), for correction of the TEM images. 400 mesh Cu grids were coated with specific thickness of LiF/Liq molecules and then used as a carrier for Li deposition. The Li was deposited with the areal capacity of 0.1 mAh/cm² at the current density of 0.25 mA/cm². After dissembled from coin cells, the samples were washed using dioxolane (DOL) and dried in the Ar glovebox. Then the samples were loaded on the cooling holder in the Ar glovebox, which was subsequently sealed in the Ar-filled holder container. The holder was quickly inserted into the TEM and added into the liquid nitrogen to freeze at the low temperature of -180 °C. All cryo-TEM images were taken at cryogenic temperature of -180 °C with an operation voltage of 300 kV. X-ray photoelectron (XPS) (Al-Kα = 1486.6 eV) and ultraviolet photoelectron spectra (UPS) (He-Iα = 21.22 eV) were collected by a photoelectron spectrometer (XPS-AXIS Ultra HAS, Kratos). The binding energy scales for XPS and UPS were calibrated by measuring the Femi edge (EF = 0 eV) and Au 4f_7/2 (84.0 eV) on a clean Au surface. The energy resolutions for UPS and XPS were 0.14 and 0.7 eV, respectively. UV and X-ray induced damages were examined by acquiring five consecutive spectra and further comparison. The final spectra were obtained by averaging the five individual curves if no visible changes were observed. An Ar-filled transfer vessel was employed to transfer the specimens into the XPS system to avoid the air contamination. X-ray diffraction results were recorded by a Bruker D8 Discover diffractometer (Bruker AXS, Cu X-ray source). The Li metal samples were assembled in Ar-filled sealed container for further XRD measurements. Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted at a Xeuss 3.0 SAXS/WAXS laboratory beamline at Vacuum Interconnected Nanotech Workstation (Nano-X) in China with Kα X-ray of Cu source (operated at 50 kV, 0.06 mA, 1.542 angstrom). GIWAXS patterns were recorded by a two-dimensional X-ray detector (Eiger2 R 1M, Dectris). The incident angle was set to 0.18°.

(Electrochemical characterization)
The charge/discharge plots were performed using galvanostatic charge/discharge stations (Neware BTS-CT-3008-TC 5.X.). All coin cells (CR 2025 and CR 2032 type) were assembled and disassembled in an Ar glovebox (H₂O and O₂ < 0.1 ppm). 1 M lithium hexafluorophosphate (LiPF₆)/EC:DMC:DEC (1:1:1 vol %) was used as electrolyte.
The cyclic voltammetry and electrochemical impedance spectroscopy (EIS) tests were performed on an electrochemical workstation (Metrohm Autolab). EIS curves were obtained with the frequency range from 100 kHz to 0.01 Hz. For Li|Cu half cells, Li capacity of 0.5 or 1 mAh/cm² were plated/stripped onto the current collectors at the current density of 0.25 and 0.5 mA/cm² at room temperature with a cutoff voltage (1.5 V).
To investigate the deposition density, Li capacities of 4, 6, and 8 mAh/cm² were adopted at a current density of 0.25 mA/cm². The symmetric Li|Li cells were performed with the areal capacity of 0.5, 2, 4, 6, 10 mAh/cm² at the current densities of 0.25, 1, 2, 3, and 5 mA/cm² at room temperature. The Li|NCA cells were activated for the initial three cycles at 0.1 C to stabilize the solid electrolyte interface.
The working electrodes were a mixture of LFP/NCA/NCM microparticles, poly (vinylidene difluoride) and acetylene black in the weight ratio of 90:5:5. The average active mass of LFP, NCM, and NCA were around 18, 18, and 25 mg/cm², respectively.
The full cells were assembled with controlled capacity ratio of the negative electrode to the positive electrode (N/P ratio). The electrolyte in each cell was controlled as 40, 20, or 10 μL (for the cell with a low electrolyte weight to cathode capacity (E/C) ratio). The working potential windows of the full cell for Li|LFP, Li|NCA, and Li|NCM were 2.7-4.0 V, 2.8-4.3 V, and 2.8-4.2 V, respectively. All evaluation of full cells were carried at room temperature.
The pouch cells were assembled by pairing 4 layers LFP/NCM cathodes and corresponding Li metal anodes with the dimensions of 4.7 × 7.7 cm². The pouch cells were cycled at charging rate of 0.2 C and discharging rate of 0.33 C.

(Theoretical stimulation)
The DFT calculations were carried out using the Vienna Ab-initio Simulation Package (VASP) with the frozen-core all-electron projector-augment-wave (PAW) method. The Perdew-Burke-Ernzerhof (PBE) of generalized gradient approximation (GGA) was adopted to describe the exchange and correlation potential. The cutoff energy for the plane-wave basis set was set to 450 eV. The 3-layer 2×2 LiF (200), LiF (111), Li₂CO₃ (002), 3 × 3 Li₂O (111), and 2 × 3 LiOH (101) supercells were used to investigate the dissociation of Li atoms. A vacuum region of 15 angstrom was added above the supercell model to minimize the interactions between neighboring systems. The C₉H₆LiNO and L₂S molecules were placed in a 30 angstrom × 30 angstrom × 30 angstrom vacuum box. The Gamma k-point mesh6 was used, and geometry optimizations were performed until the forces on each ion were reduced below 0.01 eV/ angstrom.

The dissociation energies of Li atom were calculated by the following formula (1):
Edis = E(dissociated) - E(pure)……………………………….(1)
where E(dissociated) and E(pure) are the total energies of dissociated and pure systems, respectively.

<Example 1: Liq molecular layer on lithium anodes/copper foils>
Liq or LiF was deposited on the surface of the Li metal anodes/copper foils via thermal evaporation by the method explained above.
FIG. 2 b is a photo of 10 nm Liq coated Li foil (thickness: ~0.17 mm) in air. FIG. 5 b-g are optical photos of pristine (b) Cu and (c) Li, 10 nm LiF on (d) Cu and (e) Li, 10 nm Liq on (f) Cu and (g) Li in the Ar glovebox.

Ultraviolet photoelectron spectra (UPS) of Li metal anode with 5 nm Liq was collected by the method explained above. FIG. 6 a, b, illustrate the ultraviolet photoemission spectra including (a) the secondary electron cutoff and (b) valance features as a function of Ar-ion sputtering time.

According to the photoemission secondary electron cutoff energy of UPS results, Li metal anode with 5 nm Liq molecular layer exhibits Wf of ~1.52 eV (FIG. 6 a). The obtained Wf is similar to the simulated Wf of Li metal that adsorbs different polar gas molecules but much lower than that of clean Li metal (~2.9 eV). The extracted highest occupied molecular orbital (HOMO) of Liq molecule is ~2.13 eV (FIG. 6 b). With the increase in sputtering time from 5 to 10 min, the HOMO energy of Liq molecule decreases from ~2.13 eV to ~2.05 eV. The corresponding Wf of Li metal anode increases from ~1.52 eV to ~1.72 eV. With further sputtering, the occupied states of UPS spectra exhibit the Femi level of Li metal without the HOMO features of Liq molecules. The Wf of Li metal anode increases from ~1.72 eV to ~2.22 eV, which is still lower than the clean Li metal surface due to remaining oxidized states of Li. Combined with the charge density distribution at the interface between Liq molecule and Li metal (FIG. 6 c), these results imply that the Liq molecular layer on the Li metal anode can decrease the Wf of Li metal anode via interfacial dipole as illustrated in FIG.6 d.

Owing to the high chemical reactivity, the Li metal anodes are prone to air contamination (e.g., O₂, CO₂, H₂O, etc.), leading to capacity loss and even safety issues during practical storage and application. In situ X-ray diffraction analyses of Li metal anodes (FIG. 7) were conducted in ambient air (room temperature, humidity ~30%) to further investigate the effectiveness of the molecular layer in protecting the Li metal from air contamination. (FIG. 7 illustrate the in situ XRD results of oxidation process of (a) pristine Li foil, (b) 10 nm LiF-Li foil, and (c) 10 nm Liq-Li foil in ambient (room temperature = ~25 °C and humidity = ~30%). The thickness of Li foil is 0.17 mm.

For pristine Li and another one coated with a 10 nm LiF molecular layer (LiF-Li), diffraction peaks indexed to LiOH and Li₂O emerge gradually as a function of exposure time to air. As compared with the counterparts, the Li metal anode with a 10 nm Liq molecular layer exhibits a stable chemical structure in air for 5 h.

<Example 2: Nucleation and deposition of Li on Liq-Cu >

(2D grazing incidence wide-angle X-ray scattering)
The 2D grazing incidence wide-angle X-ray scattering (GIWAXS) measurement was used to investigate the orientation of Liq molecules. FIG. 8 a illustrates 2D grazing-incidence wide-angle X-ray scattering pattern of 10 nm Liq molecule interlayer on Cu film. Qz denotes the out-of-plane direction and Qxy the in-plane direction. The scattering vector Qz relates to the diffraction angle 2θ by following formula (2):
Qz = 2π/λ(sin ω + sin (2θ - ω))………………………(2)
The isotropic orientation of the molecules shows a ring in the 2D GIWAXS pattern, while spots indicate the strong orientation of molecules. The 10 nm Liq molecular interlayer on polycrystal Cu substrate (FIG. 8 a) exhibits a ring at Qz = 0.5/nm, which is assigned into the stacking of Liq molecules with ~1.26 nm. Furthermore, the intensity of corresponding intensity versus azimuth distribution of plane (Qz = ~0.5) in FIG. 8 b demonstrates the obvious out-of-plane orientation of Liq molecules.

(Li nucleation overpotentials)
To further identify the Li nucleation and deposition on the Liq molecular interlayer, the galvanostatic nucleation overpotential of Li metal from Li|Cu cells were conducted. The Li nucleation overpotentials (μnuc) are calculated according to the following equation (1):
μnuc = μmtc - μtip……………………………….………(1)
where μmtc and μtip are the mass-transfer controlled potential and tip potential, respectively.
The following literature is available in this regard.
Kim, M. S. et al. “Langmuir-Blodgett artificial solid-electrolyte interphases for practical lithium metal batteries”. Nature Energy 3, 889-898, doi:10.1038/s41560-018-0237-6 (2018).

With the increasing of Liq molecular thickness, the μtip gradually increases owing to the emerging favorable nucleation sites. According to the galvanostatic voltage profiles and statistics (FIG. 9 and Table 2), the μnuc value of the Liq molecular layer based on thickness from 0 to ~50 nm is 44.6, 34.2, 33.1, 27.5, 31.2, and 27.2 mV, respectively (FIG.10 d), indicating a trend of decreasing in energy barrier for Li nucleation on Cu surface. As compared with LiF-Cu (~37.2 mV), the Liq molecular layer still exhibits a lower barrier (~34.2 mV) for nucleation with the same thickness of 10 nm. Utilization of an ultrathin molecular layer is highly applicable for thin Li metal anodes, which are required to construct reliable LMBs.

Table 2: Tip potential and mass-transfer controlled potential of Li nucleation on pristine Cu, LiF-Cu, and Liq-Cu with different thickness of Liq molecular layer.

(Cryo-TEM measurement)
Furthermore, cryo-TEM was employed to comparatively investigate the Li nucleation and deposition process on the pristine Cu grids, Cu grids with the 10 nm LiF and Liq molecular layers, respectively. The stability and morphology of deposited Li are mainly controlled by kinetic regime, due to the lower activation barrier for diffusion at room temperature and temporal freedom before reaching the favorable face-centered cubic hollow sites on the Cu surface. The Li nucleation on the surface of pristine Cu exhibits a vertical and whisker-like morphology with the uneven distribution of nucleation sites (FIG.10 a). Under the same deposition condition, the Li nucleation on the surface of Cu with LiF layers shows a relatively dense and uniform nucleation micromorphology with few whisker-like deposits (FIG.10 b). The deposits during the Li nucleation on the surface of Cu with Liq molecular layer displays a dense and flat micromorphology (FIG.10 c). The deposits on Cu with Liq and LiF layer are also composed of numerous Li nanocrystals with corresponding lattice space of ~0.18 nm, which is assigned into (002) plane of metallic Li (FIG.11 and FIG.12).

(Cyclic Li stripping/plating process)
The regulation and stabilization of molecular interlayer on SEI generation and nucleation/deposition are further investigated by cyclic Li stripping/plating process. The initial CV curves of Li stripping/plating process on the Cu surface with LiF and Liq molecular layer exhibits higher peak current densities with good symmetry than that of pristine Cu (FIG.13),
corresponding to rapid Li⁺ transport and reversible nucleation/deposition kinetics.

With the continuous Li stripping/plating process, the peak current densities of stripping and plating process gradually increase due to the generation of SEI layer and corresponding Li⁺ conduction pathways. For pristine Cu, even after 20 cycles, the corresponding stripping/plating process still fails to reach up to a thermodynamic equilibrium, possibly attributed to the unstable stripping/plating process itself or repeated rupture/regeneration of SEI layer. Comparatively, the cyclic Li stripping process on the Cu surface with Liq molecular layer reaches the stable and highest peak current density in a few cycles (FIG.10 e), indicating that the Liq interlayer induces the generation of stable SEI layer rapidly and enhances the electrochemical kinetics of Li stripping/plating process. Additionally, the overpotentials of stripping/plating process on the Cu surface with Liq molecular interlayer is stable, which is always lower than the overpotential on pristine Cu and Cu with LiF interlayer (FIG.14).

(CE measurements in Li|Cu cells)
Furthermore, the CE measurements in Li|Cu cells were conducted to further investigate the cyclic stability of Li stripping/plating process on different Cu surface (FIG.10 f). Specifically, the cells equipped with LiF and Liq molecular layer exhibit the stable CEs of ~99.85% and ~99.99%, respectively over 500 cycles under a current density of 0.25 mA/cm² and areal capacity of 0.5 mAh/cm². By contrast, the cell with pristine Cu shows an inferior lifetime, in which CE fades to 77.36% after 60 cycles. Besides, the cell with Liq interlayer is always stable with the overpotential of only ~13 mV even after 500 cycles (FIG.15 a). As the increasing of current density and corresponding Li stripping/plating capacity, the cell with Liq interlayer still displays good stability and reversibility (FIG.15 b). Account of the inner polarization and inevitable consumption of Li as well as electrolyte, the cell with Liq interlayer was conducted under a lean electrolyte (~10 μL) and controlled Li anode, for better prediction of molecular interlayers on the stabilization of Li stripping/deposition in practical condition. The cell with Liq interlayer still shows a stable CE of ~99.99% at a current density of 0.5 mA/cm² and an areal capacity of 1 mAh/cm² (FIG.10 g).

The major determinants of CEs are highly correlated to the topography of Li deposits and SEI features. With the deposition capacity of 0.5 mAh/cm² at a fixed current density of 0.25 mA/cm², the Li deposits on the Cu surface with Liq interlayer (FIG.15 e) exhibit a more packed micromorphology than that of pristine Cu and Cu surface with LiF interlayer (FIG.15 c, d). Towards coupling with a high-loading cathode (> 4 mAh/cm²), the areal capacity of deposited Li on Cu needs to be comparable for practical high-energy LMBs 34. The high areal capacity of Li (~4 mAh/cm² and ~8 mAh/cm²) was deposited on different Cu surface at a current density of 0.2 mA/cm². Under the deposition capacity of ~4 mAh/cm² (FIG.16 a-c), the Li deposits on the Cu surface with Liq interlayer display a dense and flat morphology. Similarly, for the Cu surface with LiF interlayer, the deposits also show a relatively compact morphology with obvious grain boundaries. In contrast, the deposits on pristine Cu exhibit a mossy-like structure with low taping density. With the increasing deposition capacity to 8 mAh/cm² (FIG.16 d-f), the deposition on Cu surface Liq interlayer is still more compact than that of pristine Cu and Cu with LiF interlayer.

(Cross-section morphologies)
Besides, the deposition thickness can be estimated from the cross-section images of Li deposits on LiF and Liq interlayer according to the thickness of Cu foil (FIG.17 a-d). Specifically, under the deposition capacity of 4 mAh/cm², the deposition thickness of Liq and LiF interlayer are around 18.1 μm and 16.3 μm, corresponding to the 90.5% and 81.5% of the thickness of theoretical bulk Li metal (0.534 g/cm³) in FIG.17 e. Similarly, with increasing deposition capacity to 6 mAh/cm², the deposition thickness of Liq and LiF interlayer are around 35.0 μm and 25.8 μm, corresponding to the 87.5% and 64.5% of the thickness of theoretical bulk Li metal. These results show that dense Li deposition can be achieved by regulation of the Liq molecular interlayer.

<Example 3: Nanostructure and constituents of the formed SEI>
The influence of different molecular interlayer on the generation of SEI layers was investigated by the combination of XPS and cryo-TEM on the Li deposits. In the C 1s spectra (FIG.18 a), the peaks assigned to the C-C, C-O, (CH₂CH₂O)_n, (CH₂CH₂OCH₂O)_n, and C-F_x species originate from the decomposition of the electrolyte and surface reaction with molecular interlayer. In this work, all the fluorinated species come from the decomposition of Li salts in the electrolyte and further subsequent reactions. The SEI layer induced by Liq interlayer exhibits emerging fluorinated carbon species as compared with that of the pristine one and LiF interlayer. Specifically, the F 1s spectra of different samples are deconvoluted into four peaks of LiF and CFx species (x = 1-), respectively (FIG.18 c and FIG.19). For SEI of Li deposits on pristine Cu, the relative ratio of LiF and fluorinated carbon species are ~10.3% and ~16.2%. With the introduction of LiF interlayer, the corresponding proportion of LiF in the SEI layer is increasing (~15.2%). Comparatively, the fluorinated species in the Liq-generated SEI layer are mainly composed of fluorinated carbonaceous species (~23.2%) with a small amount of LiF (~3.9%).

These results indicate that the Liq molecular interlayer can efficiently reduce the initial side reactions during the decomposition of Li salt in electrolyte, which simultaneously contributes to a fluorinated SEI by the generation of fluorinated carbonaceous species.

The nanostructures of the SEI on Li deposits were further characterized by cryo-TEM analysis. For the pristine Cu surface, the Li deposits appear as a porous structure with several voids (FIG.20 a, d). he Li deposits on the Cu surface with LiF interlayer show a randomly distributed columnar micromorphology without obvious voids (FIG.20 b, e). n comparison, the Li deposits on the Cu surface with Liq interlayer display a dense and flat surface (FIG.20 c, f).

Furthermore, the specific nanocrystalline structure of SEI layer of Li deposit on different Cu surface was investigated by high-resolution TEM analysis. The SEI on deposited Li of pristine Cu exhibits a porous and mosaic structure that consists of an amorphous phase and embedded Li, Li₂O, and LiF nanocrystals (FIG.18 e-h). Additionally, the SEI on deposited Li of Cu with LiF interlayer also displays a mosaic structure with embedded nanocrystals (e.g. Li, Li₂O, Li₂CO₃, and LiF) without obvious voids (FIG.21). Notably, the SEI of Li deposits on the Cu surface with Liq interlayer shows a continuous shell that consists of LiF nanocrystals on the surface of amorphous phase (FIG.18 i-k). The XPS results and cryo-TEM observation synergistically confirm the composition of the fluorinated SEI and the distribution of LiF nanocrystals in the Liq-induced SEI.

<Example 4: Stability of Li stripping/deposition in symmetric cells >

(Reversibility of long-term Li plating/stripping evaluation 1)
The reversibility of long-term Li plating/stripping was evaluated by symmetric Li|Li cells (Li thickness: ~60 μm) with different molecular interlayers using the galvanostatic cycling test at 0.25 mA/cm² (FIG.22 a). The symmetric cell with Liq interlayer exhibits good plating/stripping cyclability for 2000 hours (FIG.23). The corresponding overpotential is only ~12 mV, which is much lower than that of a symmetric cell with LiF interlayer (~40 mV). Comparatively, the symmetric cell with pristine Li anodes exhibits a substantially large overpotential with inferior reversibility and cyclability. After cycling for 2000 hours, the symmetric cell with Liq interlayer continuously cycles at 0.5 mA/cm², 1.0 mA/cm², and 2.0 mA/cm² for 400 hours, respectively (FIG.24 a), which still shows steady cyclability and reversibility (FIG.24 b-j).

Additionally, the nanomorphology of Li anodes after cycling in symmetric cells is utilized to further investigate the advantages of Liq interlayer. After long-term cycling, the Li anode with Liq interlayer still maintains a smooth and dense surface without obvious dendrite growth as compared with pristine Li before cycling (FIG.22 b, e). In contrast, the surface of pristine Li anode and Li anode with LiF interlayer simultaneously show the loose and porous structure (FIG.22 c, d), attributed to the dendrite growth and generation of inactive Li.

(Reversibility of long-term Li plating/stripping evaluation 2)
To further demonstrate the practical accessibility of Liq molecular interlayer, the symmetric Li|Li cell equipped with Liq modified separator was conducted using galvanostatic cycling at 0.25 mA/cm² (FIG.22 f). The resulting symmetric cell also shows a stable plating/stripping cyclability at a fixed current density of 0.25 mA/cm² and capacity of 0.5 mAh/cm² for 2800 hours (FIG.25 a-f). Besides, the initial interfacial impedance of symmetric cell with pristine Li anode, Li anode with LiF interlayer, and Li anode with Liq interlayer are ~162 Ω, ~80 Ω, and ~98 Ω, respectively, at an uncycled condition (FIG.22 g). With the generation and stabilization of the SEI layer, the interfacial impedances in symmetric cells with LiF and Liq interlayers gradually decrease during the cycling. After cycling for 50 cycles, the interfacial impedance of symmetric cells with LiF and Liq interlayer decreases to ~41 Ω and ~53 Ω, which is much smaller than that of the symmetric cell with pristine Li (~818 Ω) (FIG.26 a, b).

Furthermore, even when the current density continuously increases from 1 mA/cm² to 5 mA/cm², corresponding to the areal capacity from 2 mAh/cm² to 10 mAh/cm², the symmetric cell with Liq interlayer still exhibits low overpotentials and steady cyclability for over 1400 hours (FIG.22 h and FIG.27).

<Example 5: Cycling stability of LMBs under the controlled N/P ratio and lean electrolyte conditions>
To further demonstrate the advantage and stability of the molecular interlayer, the coin-cell-based LMBs with the LFP cathode and different Li anodes are cycled at a fixed current density of 2 C (FIG.28 a and FIG.29). After over 250 cycles, the cell with Liq interlayer on Li anode shows a capacity retention of ~98.7%, which is much higher than that of cells with pristine Li (~39.6%) and LiF interlayer on Li (~80.5%). For practical LMBs, the capacity ratio of the negative electrode to the positive electrode (N/P ratio) and electrolyte content needs to be controlled to further improve the energy density of LMBs. With the N/P ratio decreasing from ~5.0 to ~1.9, the capacity retention of Liq-Li|LFP cells under excess electrolyte conditions are ~99.6% and ~98.2% for 200 cycles at 2 C, respectively (FIG.30 and FIG.31). To further control the N/P ratio (~1.0) and the electrolyte content (10 μL/mAh), the Liq-Li|LFP cell exhibits a capacity retention of ~96.3% at 0.5 C for over 135 cycles (FIG.28 b and FIG.32). Even with deposited Li on Liq-Cu as the anode, the Liq-Li|LFP cell still shows a capacity retention of ~83.8% under an N/P ratio of ~1 and lean electrolyte (10 μL/mAh) as well (FIG.33).

Besides, after adopting another typically unstable cathode, the Liq-Li|NCA cell under the same well-controlled condition exhibits a capacity retention of ~67.3% at 0.2 C for 100 cycles (FIG.34). The potential practical application of Liq interlayer on Li anode is further demonstrated in the pouch cells by pairing with LFP and NCM-811 cathodes. The capacities of Liq-Li|LFP cells and Liq-Li|NCM-811 cells are ~0.45 Ah and ~0.7 Ah, respectively, with fixed electrolyte ratios of ~3.0g/Ah (FIG.35 a). The average initial CE of Liq-Li|LFP cells and Liq-Li|NCM-811 cells are ~89% and ~91% (FIG.35 b). The Liq-Li|LFP cell and Li|NCM-811 cell exhibit the champion capacity retention of ~94.3% for over 100 cycles and ~85% for 230 cycles at a charging/discharging rate of 0.33 C/0.2 C (FIG.28 c, d). The corresponding voltage profiles show stable trends that slightly move horizontally towards the left or remain almost constant (FIG.36), indicating the stable electrochemical performance of the Liq-Li anode in the pouch-format LMBs.

According to the present disclosure, a secondary battery component that can provide excellent cycle characteristics when used in a secondary battery is provided.

Claims (18)

1. A secondary battery component comprising a metal complex represented by formula (1):
   Q_(n-m) - M - L_m (1)
   wherein
   M is an n-valent metal,
   m is an integer of 0 or more and (n-1) or less,
   Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato, and
   L is a ligand selected from the group consisting of phenolato and naphtolato.
   
2. The secondary battery component according to claim 1, the component comprises a layer that includes the metal complex represented by formula (1).
   
3. The secondary battery component according to claim 1 or 2, wherein Q is unsubstituted or substituted 8-hydroxyquinolinolato.
   
4. The secondary battery component according to any one of claims 1 to 3, wherein m is 0.
   
5. The secondary battery component according to any one of claims 1 to 4, wherein M is a monovalent metal.
   
6. The secondary battery component according to claim 5, wherein M is Li, Na, K or Ag.
   
7. The secondary battery component according to any one of claims 1 to 4, wherein M is a divalent metal.
   
8. The secondary battery component according to claim 7, wherein M is Mg, Ca, Zn or Cu.
   
9. The secondary battery component according to any one of claims 1 to 4, wherein M is a trivalent metal.
   
10. The secondary battery component according to claim 9, wherein M is Al.
    
11. The secondary battery component according to any one of claims 1 to 6, wherein the complex is represented by formula (2), (3) or (4).
    wherein
    R²¹, R²², R²³, R²⁴, R²⁵, and R²⁶ are each independently selected from the group consisting of a hydrogen atom, a halogen atom, and a hydroxyl group,
    R³¹, R³², R³³, R³⁴, R³⁵, R³⁶, and R³⁷ are each independently selected from the group consisting of a hydrogen atom, a halogen atom, and a hydroxyl group,
    R⁴¹, R⁴², R⁴³, R⁴⁴, R⁴⁵, R⁴⁶, R⁴⁷, R⁴⁸, R⁴⁹, and R⁵⁰ are each independently selected from the group consisting of a hydrogen atom, a halogen atom, and a hydroxyl group.
    
12. The secondary battery component according to any one of claims 1 to 11, wherein the secondary battery component is a separator or an anode.
    
13. A secondary battery comprising the secondary battery component according to any one of claims 1 to 12.
    
14. The secondary battery according to claim 13, wherein the secondary battery is a lithium metal battery, a lithium-oxygen battery, a lithium-sulfur battery, a sodium metal battery, a sodium-oxygen battery, a sodium-sulfur battery, a potassium metal battery, a zinc metal battery, a magnesium metal battery, a calcium metal battery, an aluminum metal battery, an aluminum-sulfur battery, or a lithium ion secondary battery.
    
15. A method for manufacturing a secondary battery component, comprising applying metal complex represented by formula (1):
    Q_(n-m) - M - L_m (1)
    wherein
    M is an n-valent metal,
    m is an integer of 0 or more and (n-1) or less,
    Q is a ligand selected from the group consisting of unsubstituted or substituted 8-hydroxyquinolinolato, unsubstituted or substituted 2-(2-pyridyl)phenolato, and unsubstituted or substituted 2-(2',2''-bipyridin-6'-yl)phenolato, and
    L is a ligand selected from the group consisting of phenolato and naphtolato,
    to a substrate to obtain a secondary battery component according to any one of claims 1 to 12.
    
16. The method for manufacturing a secondary battery component according to claim 15, comprising depositing the metal complex represented by formula (1) on the substrate.
    
17. The method for manufacturing a secondary battery component according to Claim 15 or 16, wherein the compound of formula (1) is deposited on the substrate by thermal evaporation under reduced pressure.
    
18. The method for manufacturing a secondary battery component according to Claim 15 or 16, wherein the compound of formula (1) is deposited on the substrate by transfer printing.