JP2018032552A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery excellent in Li precipitation resistance in a short pulse large current cycle and a long pulse small current cycle. A lithium ion secondary battery includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode includes a negative electrode active material. The negative electrode active material contains soft carbon and hard carbon. The hard carbon has a mass ratio of 10% by mass or more and 30% by mass or less with respect to the total of the soft carbon and the hard carbon. Soft carbon has a (002) plane spacing of 0.350 nm or more and 0.360 nm or less. [Selection diagram] Figure 1

Description

  The present disclosure relates to a lithium ion secondary battery.

  Japanese Patent Laying-Open No. 2003-051309 (Patent Document 1) discloses a carbon material having a (002) plane spacing of 0.370 nm or more and 0.400 nm or less as a negative electrode active material of a lithium ion secondary battery.

Japanese Patent Laid-Open No. 2003-051309

According to Patent Document 1, since the movement of lithium ions (Li ⁺ ) is fast inside a carbon material having a (002) spacing (hereinafter also referred to as “d ₀₀₂ ”) of 0.370 nm or more, input / output It is said that the characteristics will be improved.

  However, a usage pattern in which a large current is repeatedly charged and discharged with a short pulse width (hereinafter also referred to as “short pulse high current cycle”) and a usage pattern in which a small current is repeatedly charged and discharged with a long pulse width (hereinafter referred to as “long”). There is room for improvement in the input characteristics (also referred to as “pulse small current cycle”), that is, lithium (Li) precipitation resistance.

  Hereinafter, the technical configuration and effects of the present disclosure will be described. However, the action mechanism of the present disclosure includes estimation, and the scope of the invention of the present disclosure should not be limited by the correctness.

  The lithium ion secondary battery of the present disclosure includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode includes a negative electrode active material. The negative electrode active material contains soft carbon and hard carbon. Hard carbon has a mass ratio of 10% by mass to 30% by mass with respect to the total of soft carbon and hard carbon. Soft carbon has a (002) plane interval of 0.350 nm or more and 0.360 nm or less.

In a short pulse large current cycle and a long pulse small current cycle, the negative electrode may not accept lithium ions (Li ⁺ ), and Li may precipitate on the surface of the negative electrode. In the short pulse large current cycle, it is considered that the negative electrode potential falls below 0 V (vs. Li / Li ⁺ ) and Li is precipitated mainly due to overvoltage due to large current charging. On the other hand, in the long pulse small current cycle, it is considered that Li precipitates because the Li ⁺ storage site is insufficient in the vicinity of the surface of the negative electrode.

Hard carbon has a higher charging potential than soft carbon. Therefore, the generation of overvoltage can be suppressed by mixing hard carbon with soft carbon. Furthermore, according to the new knowledge of the present disclosure, the insertion resistance of Li ⁺ is remarkably low in soft carbon having a (002) plane spacing (d ₀₀₂ ) of 0.350 nm or more and 0.360 nm or less. Thereby, the lithium ion secondary battery of this indication is excellent in Li precipitation tolerance in a short pulse high current cycle.

Hard carbon is less crystalline than soft carbon and has a complex crystal structure. Due to its complicated structure, hard carbon has a plurality of sites capable of storing Li ^{+ in} addition to the graphite-like interlayer. By mixing hard carbon with soft carbon, the shortage of the Li ⁺ storage site during the long pulse small current cycle can be compensated. Thereby, the lithium ion secondary battery of this indication is excellent also in Li precipitation tolerance in a long pulse small current cycle.

  However, hard carbon shall have the mass ratio of 10 mass% or more and 30 mass% or less with respect to the sum total of soft carbon and hard carbon. When the hard carbon is less than 10% by mass, the Li precipitation resistance in a long pulse small current cycle is not sufficient. When hard carbon exceeds 30 mass%, Li precipitation tolerance in a short pulse high current cycle will fall. This is thought to be due to the lack of soft carbon that is resistant to large current charging.

  As mentioned above, the lithium ion secondary battery of this indication is excellent in Li precipitation tolerance in a short pulse large current cycle and a long pulse small current cycle.

FIG. 1 is a graph showing the relationship between the capacity retention rate and the mass ratio of hard carbon in a short pulse large current cycle and a long pulse small current cycle. FIG. 2 is a graph showing the relationship between the capacity retention ratio in a short pulse high current cycle and the d _{002 of} soft carbon.

  Hereinafter, an embodiment of the present disclosure (hereinafter referred to as “the present embodiment”) will be described. However, the following description does not limit the scope of the present disclosure.

<Lithium ion secondary battery>
The lithium ion secondary battery of this embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode, the negative electrode, and the nonaqueous electrolyte are accommodated in a predetermined housing. The positive electrode and the negative electrode constitute an electrode group. The electrode group can be configured, for example, by winding a positive electrode and a negative electrode in a spiral shape. Alternatively, the electrode group may be configured by alternately stacking positive electrodes and negative electrodes.

  The housing is made of, for example, aluminum alloy or stainless steel. The outer shape of the housing may be rectangular (flat rectangular parallelepiped) or cylindrical. The housing may be an aluminum laminate film bag or the like as long as it has a predetermined sealing property.

<Negative electrode>
The negative electrode is typically a strip-shaped sheet. The negative electrode includes a negative electrode active material. The negative electrode typically includes a current collector and a negative electrode mixture layer. The negative electrode has a thickness of about 10 to 150 μm, for example. The current collector may be, for example, a copper (Cu) foil. The negative electrode mixture layer is applied to the surface of the current collector, for example. The negative electrode mixture layer includes a negative electrode active material and a binder. For example, the negative electrode mixture layer includes 95 to 99.5% by mass of a negative electrode active material and 0.5 to 5% by mass of a binder. The binder may be, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyacrylic acid (PAA), and the like.

《Soft carbon and hard carbon》
The negative electrode active material includes soft carbon and hard carbon.
“Soft carbon” (also referred to as graphitizable carbon) is a kind of non-graphitic carbon (amorphous carbon). Soft carbon is graphitized relatively easily by heat treatment at 2000 ° C. or higher. Soft carbon is produced, for example, by heat treating petroleum coke, pitch coke, or the like. The heat treatment temperature is, for example, about 500 to 2000 ° C.

Soft carbon has an average particle diameter of about 0.5 to 10 μm, for example. The average particle size in the present specification indicates a particle size of 50% cumulative from the fine particle side in the volume-based particle size distribution measured by the laser diffraction scattering method. Soft carbon has a BET specific surface area of about 1 to 10 m ² / g, for example. The BET specific surface area of this specification shows the specific surface area measured by BET method.

“Hard carbon” (also referred to as non-graphitizable carbon) is a kind of non-graphitic carbon. Hard carbon hardly develops a graphite structure even by heat treatment at 2500 ° C. or higher. Hard carbon is less reactive with Li ⁺ than soft carbon. However, hard carbon has abundant Li ⁺ storage sites. Hard carbon is generated by carbonizing a resin or the like. Examples of the hard carbon raw material include phenol resin, polyacrylonitrile, polyimide, and the like. Carbonization temperature is about 1000-1500 degreeC, for example. Hard carbon has an average particle diameter of about 0.5 to 10 μm, for example. Hard carbon, for example, has a BET specific surface area of about 1 to 10 m ² / g.

  Hard carbon has a mass ratio of 10% by mass to 30% by mass with respect to the total of soft carbon and hard carbon. When the hard carbon is less than 10% by mass, the Li precipitation resistance in a long pulse small current cycle is not sufficient. When hard carbon exceeds 30 mass%, Li precipitation tolerance in a short pulse high current cycle will fall.

  From the viewpoint of the balance between the Li precipitation resistance in the long pulse small current cycle and the Li precipitation resistance in the short pulse high current cycle, the hard carbon preferably has a mass ratio of 10% by mass to 20% by mass. As long as the negative electrode active material contains soft carbon and hard carbon, the negative electrode active material is allowed to contain graphite as long as the effects expected in the present embodiment are not impaired.

<< (002) spacing>
The soft carbon of this embodiment has a (002) plane spacing (d ₀₀₂ ) of 0.350 nm or more and 0.360 nm or less. At d ₀₀₂ in this range, the insertion resistance of Li ⁺ is remarkably low. It is considered that when d ₀₀₂ is 0.350 nm or more, the change amount (expansion amount) of the lattice volume accompanying the insertion of Li ⁺ becomes small. However, if d ₀₀₂ exceeds 0.360 nm, the electron density on the soft carbon surface decreases, and it is considered that the Li ⁺ insertion reaction does not proceed easily.

“D ₀₀₂ ” in the present specification is measured by a powder X-ray diffraction method. The (002) diffraction line of the carbon material typically appears around 2θ = 25 to 28 °. “D ₀₀₂ ” is the Bragg equation:
d ₀₀₂ = λ / (2 sin θ)
Is required. In the formula, “λ” indicates the wavelength of X-rays. “Θ” indicates the Bragg angle. The Bragg angle is ½ of the diffraction angle (2θ).

A general X-ray diffractometer (XRD) is used for the measurement. CuKα rays are used for X-rays. The measurement is performed a plurality of times (for example, three times or more) for one sample. The average value of a plurality of times is adopted as d ₀₀₂ of the sample.

The measurement conditions of XRD are as follows, for example.
Target: Cu
Tube voltage: 40 kV
Tube current: 40 mA
Monochromator: Graphite Divergence Slit (DS): 0.5 °
Scattering Slit (SS): Opening Receiving Slit (RS): 0.5 mm
Scanning range: 20-40 °
Scanning speed: 0.1 sec / step

From the viewpoint of Li precipitation resistance in a short pulse high current cycle, d ₀₀₂ of soft carbon is preferable as it approaches 0.355 nm. Hard carbon, for example, may have a 0.400nm following d ₀₀₂ more than 0.380 nm.

《Positive electrode》
The positive electrode is typically a strip-shaped sheet. The positive electrode includes a positive electrode active material. The positive electrode typically includes a current collector and a positive electrode mixture layer. The positive electrode has a thickness of about 10 to 150 μm, for example. The current collector may be, for example, an aluminum (Al) foil. The positive electrode mixture layer is applied to the surface of the current collector, for example. The positive electrode mixture layer includes a positive electrode active material, a conductive material, a binder, and the like. For example, the positive electrode mixture layer includes 80 to 98% by mass of a positive electrode active material, 1 to 10% by mass of a conductive material, and 1 to 10% by mass of a binder.

Examples of the positive electrode active material include lithium-containing metal oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ; lithium-containing phosphorus such as LiFePO ₄ An acid salt or the like may be used. The conductive material may be, for example, carbon black such as acetylene black or furnace black; vapor grown carbon fiber (VGCF) or the like. For example, the binder may be polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like.

《Nonaqueous electrolyte》
The non-aqueous electrolyte is typically a liquid electrolyte (electrolytic solution). However, the nonaqueous electrolyte may be a gel electrolyte or a solid electrolyte.

  The liquid electrolyte includes an aprotic solvent and a supporting electrolyte salt. The aprotic solvent may be, for example, a mixture of a cyclic carbonate and a chain carbonate. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like. Examples of the chain carbonate include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The mixing ratio of the cyclic carbonate and the chain carbonate is about “cyclic carbonate: chain carbonate = 1: 9 to 5: 5” in volume ratio.

The supporting electrolyte salt is dissolved in an aprotic solvent. Examples of the supporting electrolyte salt include LiPF ₆ , LiBF ₄ , Li [(FSO ₂ ) ₂ N], and the like. The supporting electrolyte salt has a concentration of about 0.5 to 2.0 mol / l, for example.

<< Separator >>
The lithium ion secondary battery of this embodiment may include a separator. The separator is a porous membrane. The separator is disposed between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode and holds the nonaqueous electrolyte in the internal void.

  The separator may be a porous film made of, for example, polyethylene (PE) or polypropylene (PP). The separator may have a multilayer structure. For example, the separator may be configured by laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order. For example, the separator has a thickness of about 5 to 30 μm.

  Examples will be described below. However, the following examples do not limit the scope of the invention of the present disclosure.

<Production of lithium ion secondary battery>
Lithium ion secondary batteries (Examples 1 to 7 and Comparative Examples 1 to 5) having the following configurations were manufactured. Hereinafter, the lithium ion secondary battery may be abbreviated as “battery”.

<Battery configuration>
Cathode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Negative electrode active material: soft carbon (BET specific surface area: about 5 m ² / g), hard carbon (BET specific surface area: about 5 m ² / g)
Separator: Porous membrane having a three-layer structure of PP / PE / PP Nonaqueous electrolyte: LiPF ₆ (1 mol / l), EC: DMC: EMC (3: 4: 3 vol)
Case: Cylindrical case Positive / negative capacity ratio (negative electrode capacity / positive electrode capacity): 1.2
Rated capacity: 500mAh

The mass ratio of hard carbon to the total of soft carbon and hard carbon, and d ₀₀₂ of soft carbon are shown in Table 1 below. d ₀₀₂ was measured by the method described above.

<Evaluation>
The battery was evaluated as follows. In the following, “C” is used as the unit of the current rate. “1C” indicates a current rate at which 0% SOC reaches 100% SOC after charging for one hour. “SOC (State Of Charge)” indicates the ratio of the current charge capacity to the full charge capacity of the battery.

《Short pulse high current cycle test》
The battery capacity before cycling was measured. The battery SOC was adjusted to 70% in a 25 ° C. environment. The battery was placed in a thermostat set to −10 ° C. One cycle of the following pulse charge and pulse discharge was defined as one cycle, and 1000 cycles were executed.
Pulse charging: 40C x 5sec
Pulse discharge: 40C x 5sec

  The battery capacity after cycling was measured. The capacity retention rate was calculated by dividing the battery capacity after the cycle by the battery capacity before the cycle. The results are shown in Table 1 below. The values shown in each column of “Short Pulse High Current Cycle” in Table 1 below are relative values when the capacity retention rate of Comparative Example 2 is 100. The capacity decreased in this test is considered to be an irreversible capacity due to Li precipitation. It is shown that the higher the capacity retention rate in this test, the better the Li precipitation resistance in a short pulse high current cycle.

《Long pulse small current cycle test》
The battery capacity before cycling was measured. The battery SOC was adjusted to 70% in a 25 ° C. environment. The battery was placed in a thermostat set at 25 ° C. One cycle of the following pulse charge and pulse discharge was defined as one cycle, and 1000 cycles were executed.
Pulse charging: 10C x 100sec
Pulse discharge: 10C x 100sec

  The battery capacity after cycling was measured. The capacity retention rate was calculated by dividing the battery capacity after the cycle by the battery capacity before the cycle. The results are shown in Table 1 below. The values shown in each column of “Long pulse small current cycle” in Table 1 below are relative values when the capacity retention rate of Comparative Example 2 is 100. The capacity decreased in this test is considered to be an irreversible capacity due to Li precipitation. It is shown that the higher the capacity retention rate in this test, the better the Li precipitation resistance in a long pulse small current cycle.

<Result>
As shown in Table 1 above, examples in which the hard carbon is 10% by mass or more and 30% by mass or less and the d _{002 of the} soft carbon is 0.350 nm or more and 0.360 nm or less are used in a short pulse high current cycle. It is excellent in both Li precipitation resistance and Li precipitation resistance in a long pulse small current cycle. In contrast, in the comparative example that does not satisfy the same condition, at least one of the Li precipitation resistance in the short pulse high current cycle and the Li precipitation resistance in the long pulse small current cycle are not sufficient.

FIG. 1 is a graph showing the relationship between the capacity retention rate and the mass ratio of hard carbon in a short pulse large current cycle and a long pulse small current cycle. FIG. 1 shows the results of Examples 3 to 5 and Comparative Examples 2 to 4 in Table 1 above. In Examples 3 to 5 and Comparative Examples 2 to 4, d _{002 of the} soft carbon is 0.355 nm.

As shown in FIG. 1, when the hard carbon content is 10% by mass or more, the capacity retention rate of the long pulse small current cycle is greatly improved. It is thought that the shortage of the Li ⁺ storage site is compensated by the mixing of hard carbon. Furthermore, when the hard carbon is 10% by mass or more, the capacity retention rate of the short pulse high current cycle is also improved. It is considered that the generation of overvoltage is suppressed by mixing hard carbon.

  However, when the hard carbon exceeds 30% by mass, the capacity retention rate of the short pulse large current cycle is lowered. This is thought to be due to the lack of soft carbon that is resistant to large current charging.

FIG. 2 is a graph showing the relationship between the capacity retention ratio in a short pulse high current cycle and the d _{002 of} soft carbon. FIG. 2 shows the results of Examples 1 to 7 and Comparative Examples 1 to 5 in Table 1 above. As shown in FIG. 2, d ₀₀₂ is at 0.360nm following range of 0.350 nm, the capacity retention rate short pulse high-current cycle is higher projects. In this range, it is considered that the insertion resistance of Li ⁺ into soft carbon is remarkably low.

  The embodiments and examples disclosed herein are illustrative in all aspects and should not be construed as being restrictive. The scope of the present disclosure is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

Claims (1)

Positive electrode,
With negative electrode and non-aqueous electrolyte,
The negative electrode includes a negative electrode active material,
The negative electrode active material contains soft carbon and hard carbon,
The hard carbon has a mass ratio of 10% by mass to 30% by mass with respect to the total of the soft carbon and the hard carbon.
The soft carbon has a (002) plane spacing of 0.350 nm or more and 0.360 nm or less,
Lithium ion secondary battery.

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