WO2014067891A1

WO2014067891A1 - Lithium-ion secondary battery comprising lithium cobalt phosphate as cathode active material

Info

Publication number: WO2014067891A1
Application number: PCT/EP2013/072485
Authority: WO
Inventors: Elena Markevich; Ronit SHARABI; Katia FRIDMAN; Gregory Salitra; Doron Aurbach; Ran Elazari
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2012-10-30
Filing date: 2013-10-28
Publication date: 2014-05-08
Anticipated expiration: 2015-04-30

Abstract

Lithium ion secondary battery comprising LiCoPO₄ A lithium ion secondary battery comprising: (i) a cathode comprising a cathode active material selected from LiCoPO₄; (ii) an anode comprising an anode active material that can reversibly occlude and release lithium ions; and (iii) a non-aqueous electrolyte comprising at least one lithium salt, at least one non-aqueous organic solvent selected from fluorinated carbonates and at least one optionally fluorinated boroxine.

Description

LITHIUM-ION SECONDARY BATTERY COMPRISING LITHIUM COBALT PHOSPHATE AS

CATHODE|ACTIVE MATERIAL

Description 1 . FIELD OF THE INVENTION

The present invention relates to a lithium ion secondary battery with improved capacity retention property, faradaic efficiency and durability. 2. BACKGROUND

High voltage Li-ion batteries attracted high attention as potential power sources for electric vehicles due to the high energy density of all the commercialized rechargeable batteries. Lithium cobalt phosphate (UC0PO4) with an olivine structure possesses high operating voltage (red-ox potential of 4.8 V vs. Li/Li⁺), flat voltage profile, and a high theoretical capacity of about 170 mAh/g (Phadhi et al., J. Electrochem. Soc, 1997, 144, 1 188). However, L1C0PO4 has shown a fast fading of discharge capacity upon charge-discharge cycling (Wolfenstine et al., J. Power Sources, 2005, 144, 226; Bramnik et al., J. Solid State Electrochem., 2004, 8, 558; Jin et al., J Solid State Electrochem., 2008, 12, 105-1 1 1 ; Li et al., Electrochem. Comm., 2009, 1 1 , 95-98; Wang et al., J. Power Sources, 2010, 195, 6884-6887; Tan et al., J. Alloys Compd., 2010, 502, 407-410).

Present standard Li-ion batteries have an operating voltage of about 3.3 to 3.8 V. The difference of the operating voltage has great influence on the stability of the electrolytes used. The electro- lytes commonly used in standard Li-ion batteries are usually not suited for high voltage Li-ion batteries since they are not as stable at higher operating voltage as at lower operating voltage.

Use of fluorinated ethylene carbonate (FEC) as a component in electrolyte solutions has been reported. As particularly reported, the addition of FEC improves discharge capacity retention and coulombic efficiency of Si|Li half-cell (Choi et al., J. Power Sources, 2006, 161 , 1254-1259 and Nakai et al., J. Electrochem. Soc, 201 1 , 158, A798-A801 ), and of graphite/Li cells (McMillan et al., J. Power Sources, 1999, 81 -82, 20-26). As further reported, the addition of FEC improves (i) cycling efficiency for lithium metal deposition and dissolution (Mogi et al., J. Electrochem. Soc, 2002, 149, A1578-A1583) and (ii) capacity retention of LiMn₂04/graphite Li-ion cells at elevated temperature (Ryou et al., Electrochemica Acta, 2010, 55, 2073-2077).

US 201 1/0143216 discloses lithium secondary batteries which include a positive electrode containing a lithium transition-metal oxyanion compound of the formula LiFeP04 as a positive electrode active material, a negative electrode and a non-aqueous electrolyte solution, which con- tains vinylene carbonate, such as fluoroethylene carbonate, and a solvent and/or a solute that decomposes at a potential more positive than that of vinylene carbonate. As disclosed in this publication, different transition-metal oxyanion compound in which some of the Fe is replaced by other transition metals such as Co, Ni, Mn or mixtures thereof may be used; however, such cathodes are not exemplified. US 201 1/0223490 discloses nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte and a separator, wherein the positive electrode active material is composed of a mixture of a lithium-cobalt composite oxide containing at least both zirconium and magnesium, and a lithium-manganese-nickel composite oxide containing at least both manganese and nickel; and the nonaqueous electrolyte includes fluoro- ethylene carbonate and dimethyl carbonate and further includes an additive such as 2-propynyl 2-(methane sulfonyloxy)propionate (PMP) or 2-propynyl methane sulfonate (MSP).

An object of the present invention was to provide an electrolyte for high voltage Li-ion second- ary batteries comprising LiCoP0₄ as cathode active material which allows long operation of the batteries and high voltage Li-ion secondary batteries comprising LiCoP0₄ as cathode active material with prolonged cycle stability and life time.

3. SUMMARY OF INVENTION

In one aspect, the present invention thus provides a lithium ion secondary battery comprising:

(i) a cathode comprising a cathode active material selected from LiCoP0₄;

(ii) an anode comprising an anode active material that can reversibly occlude and release lithium ions; and

(iii) a non-aqueous electrolyte comprising at least one lithium salt and at least one nonaqueous organic solvent selected from fluorinated carbonates.

In another aspect, the present invention relates to the use of electrolyte (iii) in lithium ion secondary batteries comprising a cathode active material selected from LiCoP0₄. In a further as- pect the present invention relates to a non-aqueous electrolyte comprising at least one lithium salt, at least one non-aqueous organic solvent selected from fluorinated carbonates and at least one optionally fluorinated boroxine of formula (I) as defined below and to the use of said electrolyte in lithium ion secondary batteries comprising a cathode active material selected from LiCoP0₄.

4. BRIEF DESCRIPTION OF THE FIGURES

Figs. 1 A-1 B show SEM image (1 A) and XRD pattern (1 B) of carbon-coated LiCoP0₄ olivine powder. The LiCoP0₄ powder consists of rods having a diameter of 50-200 nm and a length of about 1 μηη. The material has an orthorhombic, olivine-like structure as described in X-ray Powder Diffraction Data Files 00-032-0552.

Figs. 2A-2B show curves of discharge capacity (2A) and irreversible capacity (2B) vs. cycle number obtained upon galvanostatic cycling (C/8 h rates) of LiCoP0₄ electrodes (30°C). 1 M LiPFe/EC-DMC 1 :1 solution taken in amount of 15 μΙ/mg (open triangulars) or 5 μΙ/mg (full triangulars) of active electrode mass; 1 M LiPFe/FEC-DMC 1 :4 solution taken in amount of 15 μΙ/mg (open circles) or 5 μΙ/mg (full circles) of active electrode mass; 0.5 wt.-% of TMB in 1 M UPF6/FEC-DMC 1 :4 solution taken in amount of 5 μΙ/mg of active electrode mass (open squares).

Fig. 3 shows the discharge capacity vs. cycle number obtained upon galvanostatic cycling (C/2 h rate) of LiCoP0₄ electrodes (30°C) with the electrolyte composition comprising 1 wt.-% of TMB in 1 M LiPFe/FEC-DMC 1 :4 taken in amount of 5 μί/mg of active electrode mass.

Fig. 4. shows XPS spectra of LiCoP0₄ electrodes cycled for an equal period of time in EC- based electrolyte solution (panel A); FEC-based electrolyte solution (panel B); and said FEC- based electrolyte with the addition of 0.5 wt.-% of TMB (panel C).

Fig. 5 shows the charge and discharge capacity vs. cycle number obtained upon galvanostatic cycling (C/8 h rate) at 30 °C of LiCoPC /Li cells comprising the electrolyte composition containing 1 wt.-% TMB in 1 M UPF6/EC-DMC 1 :1 (full and open triangles, comparative) and of LiCoPC /Li cells comprising the electrolyte composition containing 1wt.-% of TMB in 1 M

UPF6/FEC-DMC 1 :1 (full and open circles, inventive).

Fig. 6 shows the discharge capacity vs. cycle number obtained upon galvanostatic cycling (C/8 rate) of LiCoPC cathode against Si-anodes at 30°C. Electrolyte solution compositions were 1 M LiPFe/FEC-DMC 1 :4 without TMB (open circles) and with the addition of 1 wt.-% TMB (full circles).

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a lithium ion secondary battery comprising:

(i) a cathode comprising a cathode active material selected from LiCoP0₄;

Further object of the present invention is the use of electrolyte (iii) in lithium ion secondary batteries comprising a cathode active material selected from LiCoP0₄. Another object of the pre- sent invention is a non-aqueous electrolyte comprising at least one lithium salt, at least one non-aqueous organic solvent selected from fluorinated carbonates and at least one optionally fluorinated boroxine of formula (I) as defined below and its use in lithium ion secondary batteries comprising a cathode active material selected from LiCoP0₄. The inventive secondary lithium ion batteries comprising LiCoP0₄ as cathode active material and fluorinated carbonate-based electrolyte solutions show a significantly better capacity retention and higher coulomb efficiency, compared with those of secondary lithium ion batteries comprising only the non-fluorinated carbonate-based electrolyte solution as shown in the experiments. The stability of the LiCoP0₄ cathode in a lithium ion secondary battery in the delithiated state is significantly improved by the addition of a fluorinated carbonate to the electrolyte solution, due to the fact that the fluorinated carbonate forms a surface protective film on the

LiCoP0₄ cathode surface. A further improvement of the capacity retention and coulomb efficiency of the inventive lithium ion secondary batteries is achieved by addition of an optionally fluorinated boroxine of formula (I) as defined below. The use of the inventive electrolyte comprising at least one lithium salt, at least one non-aqueous organic solvent selected from fluorinated carbonates and at least one optionally fluorinated boroxine of formula (I) in lithium ion batteries comprising a cathode active material selected from LiCoP0₄ leads to better capacity retention of such batteries.

As further found and shown in Example 1 , the capacity retention and coulomb efficiency of the LiCoP0₄/Li cells were further improved by increasing the active cathode mass : electrolyte vol- ume ratio, i.e. decreasing the volume of the electrolyte solution in the cells. In contrast, a similar increase of the active cathode mass : electrolyte volume ratio of the non-fluorinated carbonate- based electrolyte solution resulted in a modest improvement only in the capacity retention of the cells, along with a decrease of the irreversible capacity of those cells. In the following the invention is described in detail.

In the context of the present invention the term "lithium ion battery" means a rechargeable electrochemical cell wherein during discharge lithium ions move from the negative electrode (anode) to the positive electrode (cathode) and during charge the lithium ions move from the positive electrode to the negative electrode, i.e. the charge transfer is performed by lithium ions. Usually lithium ion batteries comprise a cathode containing as cathode active material a lithium ion- containing transition metal compound, for example transition metal oxide compounds with layer structure like UC0O2, LiNi02, and LiMn02, or transition metal phosphates having olivine structure like LiFeP0₄ and LiMnP0₄, or lithium-manganese spinels which are known to the person skilled in the art in lithium ion battery technology.

The term "cathode active material" denotes the electrochemically active material in the cathode, e.g. the transition metal oxide intercalating/deintercalating the lithium ions during

charge/discharge of the battery. Depending on the state of the battery, i.e. charged or dis- charged, the cathode active material contains more or less lithium ions. The term "anode active material" denotes the electrochemically active material in the anode, e.g. carbon intercalating/deintercalating the lithium ions during charge/discharge of the battery.

According to the present invention the cathode active material is selected from LiCoP0₄. The cathode may also be referred to as LiCoP0₄ cathode. The LiCoP0₄ may be doped with Fe, Mn, Ni, V, Mg, Al, Zr, Nb, Tl, Ti, K, Na, Ca, Si, Sn, Ge, Ga, B, As, Cr, Sr, or rare earth elements, i.e., a lanthanide, scandium and yttrium. LiCoP0₄ with olivine structure is particularly suited according the present invention due to its high operating voltage (red-ox potential of 4.8 V vs. Li/Li⁺), flat voltage profile and a high theoretical capacity of about 170mAh/g. According to one embod- iment of the present invention the LiCoP0₄ does not contain further cations and anions in significant amounts. Many elements are ubiquitous. For example, sodium, potassium and chloride are detectable in certain very small proportions in virtually all inorganic materials. In the context of the present invention, proportions of less than 0.5 % by weight of cations or anions are disregarded, i.e. amounts of cations or anions below 0.5 % by weight are regarded as nonsignificant. Any LiCoPC comprising less than 0.5 % by weight of sodium is thus considered to be sodium-free in the context of the present invention. Correspondingly, any LiCoPC compris- ing less than 0.5% by weight of sulfate ions is considered to be sulfate-free in the context of the present invention.

The cathode may further comprise electrically conductive materials like electrically conductive carbon and may further comprise usual components like binders. Compounds suited as electri- cally conductive materials and binders are known to the person skilled in the art. For example, the cathode may comprise carbon in a conductive polymorph, for example selected from graphite, carbon black, carbon nanotubes, graphene or mixtures of at least two of the aforementioned substances. In addition, the cathode may comprise one or more binders, for example one or more organic polymers like polyethylene, polyacrylonitrile, polybutadiene, polypropylene, poly- styrene, polyacrylates, polyvinyl alcohol, polyisoprene and copolymers of at least two comono- mers selected from ethylene, propylene, styrene, (meth)acrylonitrile and 1 ,3-butadiene, especially styrene-butadiene copolymers, and halogenated (co)polymers like polyvinlyidene chloride, polyvinly chloride, polyvinyl fluoride, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, copolymers of tetrafluoroethylene and hexafluoropropylene, copolymers of tetrafluoroethylene and vinylidene fluoride and polyacrylnitrile. Preferably the cathode comprises a LiCoPC /C composite material. The preparation of a suited cathode comprising a LiCoPC /C composite material is described in Markevich et al., Electrochem. Comm., 2012, 15, 22-25.

Furthermore, the cathode may comprise a current collector which may be a metal wire, a metal grid, a metal web, a metal sheet, a metal foil or a metal plate. A suited metal foil is aluminum foil.

According to one embodiment of the present invention the cathode has a thickness of from 25 to 200 μηη, preferably of from 30 to 100 μηη, based on the whole thickness of the cathode with- out the thickness of the current collector.

According to the present invention electrolyte (iii) functions as a medium that transfers lithium ions participated in the electrochemical reaction taking place in the battery. The lithium salt(s) present in the electrolyte are usually solvated in the non-aqueous organic solvent. Electrolyte (iii) is also referred to as electrolyte solution according to the present invention. Organic solvents used in lithium ion secondary batteries, in general, and in the secondary battery of the present invention and in the inventive electrolyte in particular, have a high dielectric constants and a low viscosity, and therefore increase ionic dissociation by promoting ionic conductance. Electrolyte (iii) comprises at least one fluorinated carbonate. The at least one fluorinated carbonate is usually selected from cyclic and linear carbonates which are partially or fully fluorinated and mixtures thereof. Examples of linear fluorinated carbonates are partially or fully fluori- nated dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate and ethylmethyl carbonate. Examples of cyclic fluorinated carbonates are partially or fully fluorinated ethylene carbonate and propylene carbonate. Partially fluorinated means, that only a part of the substitutable hydrogen atoms of the carbonate is substituted by F, totally fluorinated means, that all substitutable hydrogen atoms of the carbonate are substituted by F. Depending on the respective molecule the partially or fully fluorinated dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, and ethylmethyl carbonate, ethylene carbonate, or propylene carbonate may be substituted with 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, or 14 fluorine atoms. Preferably the fluorinated carbonates are selected from mono-, di-, tri-, tetra-, penta-, or hexa- fluorinated dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate, and mixtures thereof.

Concrete examples of fluorinated dimethyl carbonates are fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, trifluoromethyl methyl carbonate, bis(fluoromethyl) carbonate, bis(difluoro)methyl carbonate, and bis(trifluoro)methyl carbonate. Concrete examples of fluorinated ethylmethyl carbonates are 2-fluoroethylmethyl carbonate, ethylfluoromethyl carbonate, 2,2-difluoroethylmethyl carbonate, 2-fluoroethylfluoromethyl carbonate, ethyldifluoromethyl carbonate, 2,2,2-trifluoroethylmethyl carbonate, 2,2- difluoroethylfluoromethyl carbonate, 2-fluoroethyldifluoromethyl carbonate, and ethyltrifluorome- thyl carbonate.

Concrete examples of fluorinated diethyl carbonates are ethyl-(2-fluoroethyl) carbonate, ethyl- (2,2-difluoroethyl) carbonate, bis(2-fluoroethyl) carbonate, ethyl-(2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl-2'-fluoroethyl carbonate, bis(2,2-difluoroethyl) carbonate, 2,2,2-trifluoroethyl-2'- fluoroethyl carbonate, 2,2,2-trifluoroethyl-2',2'-difluoroethyl carbonate, and bis(2,2,2- trifluoroethyl) carbonate.

Concrete examples of fluorinated ethylene carbonates are monofluorinated ethylene carbonate, 4,4-difluoroethylene carbonate, and 4,5-difluoroethylene carbonate. Concrete examples of fluorinated propylene carbonates are monofluorinated propylene carbonate, 5,5-difluoropropylene carbonate, and 4,5-difluoropropylene carbonate.

Preferably the cyclic fluorinated carbonate is monofluorinated ethylene carbonate. In certain embodiments, the non-aqueous organic solvent composing the electrolyte solution in the lithium ion secondary battery of the present invention comprises a mixture of a linear fluorinated carbonate and a cyclic fluorinated carbonate. According to a preferred embodiment the electrolyte (iii) further comprises at least one nonaqueous organic solvent selected from non-fluorinated carbonates. The non-fluorinated carbonates may be selected from cyclic and linear carbonates and mixtures thereof. Examples of linear carbonates are dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate and ethylmethyl carbonate. Examples of cyclic carbonates are ethylene carbonate, propylene carbonate, butylene carbonate and pentylene carbonate. Preferably the at least one non-fluorinated carbonate is selected from dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate and mixtures thereof. The weight ratio between the fluorinated carbonates and the non-fluorinated carbonates present in said electrolyte (iii) as described above may be in the range of 1 :200 to 1 :1 , preferably 1 :100 to 1 :2, more preferably 1 :50 to 1 :2, most preferably 1 :25 to 1 :3, in particular in the range of from more than 1 :9 up to 1 :3 by weight, respectively. In order to obtain an electrolyte solution having desired dielectric constant and viscosity, a mixture of at least two carbonates is usually used, wherein one of said carbonates has a high dielectric constant and viscosity, and another one of said at least one solvent has a low dielectric constant and viscosity. In particular cases, such mixtures consist of a cyclic carbonate(s) and a linear carbonate(s), wherein the ratio between the cyclic and linear carbonates in the mixture is determined so as to obtain a desired dielectric constant and viscosity. Preferred cyclic carbonate^) : linear carbonate(s) ratios are in a range of 1 :1 to 1 :9, e.g., 1 :1 , 1 ;2, 1 :3, 1 :4, 1 :5, 1 :6, 1 :7, 1 :8 and 1 :9, respectively, by volume. Alternatively, in order to obtain an electrolyte solution having desired dielectric constant and viscosity, a sole fluorinated cyclic carbonate such as a fluorinated propylene carbonate may be used.

In certain embodiments, electrolyte (iii) in the lithium ion secondary battery of the present invention comprises (a) a mixture of a linear fluorinated carbonate and a cyclic fluorinated carbonate. In other embodiments, said non-aqueous organic solvent comprises (b) a mixture of a linear fluorinated carbonate and a cyclic non-fluorinated carbonate, and in further embodiments, said non-aqueous organic solvent comprises (c) a mixture of a linear non-fluorinated carbonate and a cyclic fluorinated carbonate.

If the electrolyte (iii) in the lithium ion secondary battery of the present invention comprises a mixture of a linear non-fluorinated carbonate and a cyclic fluorinated carbonate, the ratio be- tween the cyclic fluorinated carbonate and the linear non-fluorinated carbonate in the electrolyte (iii) may be in a range of 1 :200 to 1 :1 by weight, preferably 1 :100 to 1 :2 by weight, more preferably 1 :50 to 1 :2 by weight, most preferably 1 :25 to 1 :3 by weight, even more preferred in the range of from more than 1 :9 up to 1 :3, respectively. In particular such embodiments, said cyclic fluorinated carbonate is fluoroethylene carbonate or fluoropropylene carbonate, preferably fluo- roethylene carbonate. In a preferred embodiment of the invention the electrolyte (iii) of the inventive lithium ion secondary battery comprises at least one optionally fluorinated boroxine of formula (I)

r-.3 ^ „1

ET

^'B'

R²

(I) wherein R¹, R², and R³ are independently from each other are selected from (Ci-C6)alkyl, (Ci- C6)alkoxy, (C5-C7)aryl, and (C5-C7)aryloxy, and wherein alkyl, alkyloxy, aryl and aryloxy may be independently from each other substituted by one or more substituents selected from F, (Ci- C6)alkyl, (Ci-Ce)alkoxy, (C5-C7)aryl, and (C5-C7) aryloxy and each alkyl, alkoxy, aryl, and aryloxy may be substituted by one or more F. The term "(Ci-C6)alkyl" as used herein typically means a straight or branched hydrocarbon radical having 1 -6 carbon atoms and includes, e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl, sec- butyl, isobutyl, tert-butyl, n-pentyl, iso-pentyl, 2,2-dimethylpropyl, n-hexyl, iso-hexyl and the like. Preferred are (Ci-Cs)alkyl groups, more preferably methyl and ethyl. The term "(Ci-C6)alkoxy" as used herein refers to a group of the general formula -0-(Ci- C6)alkyl. Preferred are -0-(Ci-C3)alkyl groups, more preferred are methoxy or ethoxy.

The term "(C5-C7)aryl" as used herein denotes a 5- to 7-membered aromatic cycle. A preferred (C5-C7)aryl is phenyl. The term "(C5-C7) aryloxy" as used herein means a group of the general formula -0-(C5-C7)aryl like phenoxy.

Examples of (Ci-Ce)alkyl substituted by one or more F are CH2F, CHF2, CF3, CH2CH2F, CH2CHF2, CH2CF3, CHFCH2F, CHFCHF2, CHFCFs, CF2CH2F, CF2CHF2, CF2CF3, and so on. Examples of (Ci-Ce)alkyloxy substituted by one or more F are OCH2CH2F, OCH2CHF2,

OCH2CF3, OCH2CH2CH2F, OCH2CH2CHF2, OCH2CH2CF3, OCH2CHFCH2F, OCH2CHFCHF2, OCH2CHFCF3, OCH2CF2CH2F, OCH2CF2CHF2, and OCH2CF2CF3. Examples of (C₅-C₇)aryl substituted by one or more F are 2-, 3- and 4-mono-F-phenyl, 2,3-di-F-phenyl, 2,4-di-F-phenyl and 2,4,6-tri-F-phenyl. Examples of (C5-C7) aryloxy substituted by one or more F are 2-, 3- and 4-mono-F-phenoxy, 2,3-di-F-phenoxy, 2,4-di-F-phenoxy and 2,4,6-tri-F-phenoxy.

The term "optionally fluorinated boroxine of formula (I)" as used herein means that the boroxine of formula (I) may be substituted by one or more F. If the boroxine is substituted by one or more F, the boroxine may be partially or fully substituted by F, i.e. partially or fully fluorinated. For example, the boroxine may be substituted with 1 to 39 F, in particular with 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14 or 15 F. Non-limiting examples of optionally fluorinated boroxines of formula (I) are optionally fluorinated tri(Ci-C6)alkyl-boroxines, optionally fluorinated tri(Ci-C6)alkoxy-boroxines, optionally fluorinated tri(C5-C7)aryl-boroxines, and optionally fluorinated tri(C5-C7)aryloxy-boroxines. Non-limiting examples of non-fluorinated tri(Ci-C6)alkyl-boroxine according to the invention include, without being limited to, trimethylboroxine (TMB), triethylboroxine, tripropylboroxine, triisopropylboroxine, tributylboroxine, triisobutylboroxine, tripentylboroxine, triisopentylboroxine, trihexylboroxine, and triisohexylboroxine; and non-limiting examples of non-fluorinated tri(Ci- C6)alkoxy-boroxine according to the invention include trimethoxyboroxine, triethyxoboroxine, tripropyloxyboroxine, triisopropyloxy boroxine, tributoxyboroxine, triisobutoxyboroxine, tripen- tyloxyboroxine, triisopentyloxyboroxine, trihexyloxyboroxine or triisohexyloxyboroxine. Non- limiting examples of fluorinated tri(Ci-C6)alkyl-boroxines are the fluorinated derivatives of the non-fluorinated tri(Ci-C6)alkyl-boroxines mentioned above which are substituted by 1 , 2, 3, 4, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, or 15 F like tri(monofluoromethyl)boroxine,

tri(difluoromethyl)boroxine, tri(trifluoromethyl)boroxine, tri(monofluoroethyl)boroxine, tri(difluoroethyl)boroxine, tri(trifluoroethyl)boroxine, tri(tetrafluoroethyl)boroxine,

tri(pentafluoroethyl)boroxine, tri(monofluoropropyl)boroxine, tri(monofluoroisopropyl)boroxine, tri(monofluorobutyl)boroxine, tri(monofluoroisbutyl)boroxine, tri(monofluoropentyl)boroxine, tri(monofluoroisopentyl)boroxine, tri(monofluorohexyl)boroxine, and

tri(monofluoroisohexyl)boroxine. Non-limiting examples of fluorinated (Ci-C6)alkoxy-boroxines are the fluorinated derivatives of the above mentioned non-fluorinated (Ci-C6)alkoxy-boroxines which are substituted by 1 , 2, 3, 4, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, or 15 F like

tri(monofluoroethyloxy)boroxine, tri(difluoroethyloxy)boroxine, tri(trifluoroethyloxy)boroxine, tri(monofluoropropyloxy)boroxine, tri(difluoropropyloxy)boroxine, tri(trifluoropropyloxy)boroxine, tri(tetrafluoropropyloxy)boroxine, tri(pentafluoropropyloxy)boroxine,

tri(monofluoroisopropyloxy)boroxine, tri(monofluorobutyloxy)boroxine,

tri(monofluoroisbutyloxy)boroxine, tri(monofluoropentyloxy)boroxine,

tri(monofluoroisopentyloxy)boroxine, tri(monofluorohexyloxy)boroxine, and

tri(monofluoroisohexyloxy)boroxine.

Non-limiting examples of non-fluorinated tri(C5-C7)aryl-boroxines and of tri(C5-C7)aryloxy- boroxines are triphenylboroxine and triphenoxyboroxine, respectively. Non-limiting examples of fluorinated tri(C5-C7)aryl-boroxines are tri(2-F-phenyl)boroxine, tri(3-F-phenyl)boroxine, tri(4-F- phenyl)boroxine, tri(2,3-di-F-phenyl)boroxine, tri(2,4-di-F-phenyl)boroxine, and tri(2,4,6-tri-F- phenyl)boroxine, and non-limiting examples of tri(C5-C7)aryloxy-boroxines are, tri(2-F- phenoxy)boroxine, tri(3-F-phenoxy)boroxine, tri(4-F-phenoxy)boroxine, tri(2,3-di-F- phenoxy)boroxine, tri(2,4-di-F-phenoxy)boroxine, and tri(2,4,6-tri-F-phenoxy)boroxine.

The concentration of the optionally fluorinated boroxine of formula (I) in electrolyte (iii) com- prised in the inventive lithium ion secondary battery is usually 0.1 % to 5% by weight, based on the total wheight of the electrolyte solution, preferably 0.1 % to 2% by weight and most preferred 0.25% to 2% by weight. In particular such embodiments, electrolyte (iii) further comprises trimethylboroxine.

In further preferred embodiments of the present invention electrolyte (iii) comprises at least one compound of general formula (II)

R⁴ is cyclohexyl or (hetero)aryl, which may be substituted by one or more substituent selected independently from each other from F, CI, Br, I, and (C1-C6) alkyl, wherein (C1-C6) alkyl may be substituted by one or more substituent selected independently from each other from F, CI, Br and I; and

R⁵, R⁶, R⁷, R⁸, and R⁹ may be same or different and are independently from each other selected from H, F, CI, Br, I, (Ci-Ce) alkyl, wherein (Ci-Ce) alkyl may be substituted by one or more substituent selected independently from each other from F, CI, Br and I.

The term "(Ci-Ce)" is defined as above. "(Hetero)aryl" means (C5-C7) aryl or (C5-C7) heteroaryl. "Heteroaryl" means aryl wherein 1 to 3 C atoms are replaced independently by N, S or O. Aryl may be phenyl, heteroaryl may be furanyl or pyridyl. Preferred compounds of formula (I) are biphenyl and cyclohexylphenyl.

If a compound of general formula (II) is present in electrolyte (iii), its concentration is usually 0.01 to 5 wt.-%, based on the total weight of the electrolyte, preferably 0.1 to 2 wt.-% and most preferred 0.1 to 0.5 wt.-%.

According to particular preferred embodiments electrolyte (iii) contains at least one optionally fluorinated boroxine of formula (I) and at least one compound of general formula (II). The con- centration of the at least one optionally fluorinated boroxine of formula (I) is usually 0.1 % to 5% by weight, based on the total weight of the electrolyte solution, preferably 0.1 % to 2% by weight and most preferred 0.25% to 2% by weight and the concentration of the at least one compound of general formula (II) is usually 0.01 to 5 wt.-%, based on the total weight of the electrolyte, preferably 0.1 to 2 wt-% and most preferred 0.1 to 0.5 wt.-%.

Electrolyte (iii) comprises at least one lithium salt. The lithium salt functions as a source of lithium ions, enabling basic operations of the lithium ion secondary battery and promoting movement of lithium ions between the cathode and anode. Suited lithium salts are for example of LiPF₆, LiBF₄, LiSbFe, LiAsF₆, UCIO4, UCF3SO3 or UC4F9SO3 and mixtures thereof. Preferably the lithium salt in electrolyte (iii) is LiPF6. In other embodiments, said lithium salt is a mixture of LiPFewith one or more of L1BF4, LiSbF6, LiAsF6, UCIO4, UCF3SO3 or UC4F9SO3 or mixtures thereof. According to the present invention, it is desirable to use a lithium salt having a low lat- tice energy, high dissociation degree, excellent ion conductivity, thermal stability and oxidation resistance. In addition, the concentration of the lithium salt should preferably be in the range of 0.1 to 2.0 M. When a lithium salt in a concentration less than 0.1 M is used, the conductivity of the electrolyte solution is decreased and the performance thereof may thus be degraded. On the other hand, when a lithium salt in a concentration higher than 2.0 M is used, the viscosity of the electrolyte solution is increased and the movement of the lithium ions within said solution may thus be reduced.

In certain embodiments, the lithium ion secondary battery of the present invention, in any of the configurations defined above, comprises an electrolyte comprising a solution of LiPF6 in a non- aqueous organic solvent comprising dimethyl carbonate and mono-fluorinated ethylene carbonate.

In certain particular such embodiments, said electrolyte solution further comprises TMB, preferably wherein the amount of the TMB in said solution is 0.5% to 2% by weight.

In other particular such embodiments, the ratio between the mono-fluorinated ethylene carbonate and the dimethyl carbonate in said organic solvent is 1 :4, by weight, respectively. More particular such embodiments are those wherein the ratio between the mono-fluorinated ethylene carbonate and the dimethyl carbonate in said organic solvent is 1 :4, by weight, and the amount of TMB in the electrolyte solution is about 0.5% to 1 % by weight.

As shown in the examples the inventive lithium ion secondary batteries and inventive electrolyte (iii) show improved properties without the presence of vinylene carbonate. According to further embodiments of the present invention electrolyte (iii) does not contain vinylene carbonate. This means in particular no vinylene carbonate is added to electrolyte (iii). Preferably, the concentration of vinylene carbonate in electrolyte (iii) of the inventive batteries is below the detection limit of vinylene carbonate.

The anode comprised within the lithium ion secondary battery of the present invention compris- es an anode active material that can reversibly occlude and release lithium ions. Particular anode active materials that can be used include, without being limited to, carbonaceous material that can reversibly occlude and release lithium ions. Carbonaceous materials suited are crystalline carbon such as a graphite material, more particularly, natural graphite, graphitized cokes, graphitized MCMB, and graphitized MPCF; amorphous carbon such as coke, mesocarbon mi- crobeads (MCMB) fired below 1500°C, and mesophase pitch-based carbon fiber (MPCF); hard carbon and carbonic anode active material (thermally decomposed carbon, coke, graphite) such as a carbon composite, combusted organic polymer, and carbon fiber. Further anode active materials are lithium metal, lithium alloys and materials containing an element capable of forming an alloy with lithium. Non-limiting examples of materials containing an element capable of forming an alloy with lithium include a metal, a semimetal, or an alloy thereof. It should be understood that the term "alloy" as used herein refers to both alloys of two or more metals as well as alloys of one or more metals together with one or more semimetals. If an alloy has metallic properties as a whole, the alloy may contain a nonmetal element. In the texture of the alloy, a solid solution, a eutectic (eutectic mixture), an intermetallic compound or two or more thereof coexist. Examples of such metal or semimetal elements include, without being limited to, titanium (Ti), tin (Sn), lead (Pb), aluminum, indium (In), zinc (Zn), antimony (Sb), bis- muth (Bi), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), hafnium (Hf), zirconium (Zr) yttrium (Y), and silicon (Si). Metal and semimetal elements of Group 4 or 14 in the long-form periodic table of the elements are preferable, and especially preferable are titanium, silicon and tin, in particular silicon.

Examples of tin alloys include ones having, as a second constituent element other than tin, one or more elements selected from the group consisting of silicon, magnesium (Mg), nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium (Ti), germanium, bismuth, antimony and chromium (Cr). Examples of silicon alloys include ones having, as a second constituent element other than silicon, one or more elements selected from the group consisting of tin, magnesium, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony and chromium.

Silicon as anode active material that can reversibly occlude and release lithium ions may be used in different forms, e.g. in the form of nanowires, nanotubes, nanoparticles, films, nanopo- rous silicon or silicon nanotubes. The silicon may be deposited on a current collector. The current collector may be a metal wire, a metal grid, a metal web, a metal sheet, a metal foil or a metal plate. Preferred the current collector is a metal foil, e.g. a copper foil. Thin films of silicon may be deposited on metal foils by any technique known to the person skilled in the art, e.g. by sputtering techniques. It is preferred that the surface densitiy of the silicon in the thin films is in the region ranging from 0.2 mg/cm² to 2 mg/cm². The thickness of the silicon thin films is preferably 0.5 μηη up to 50 μηη, more preferred 1 μηη up to 20 μηη and most preferred 1 μηη up to 10 μηη. One method for preparing anodes having a thin film of silicon is explicitly described in the examples. According to one embodiment of the present invention the anode comprises a thin film of silicon. It is also possible to use a silicon/carbon composite as anode active material according to the present invention.

Preferably the anode active material present in the inventive lithium ion secondary battery is selected from carbonaceous material that can reversibly occlude and release lithium ions, particularly preferred the carbonaceous material that can reversibly occlude and release lithium ions is selected from crystalline carbon, hard carbon and amorphous carbon. In another preferred embodiment the anode active material present in the inventive lithium ion secondary battery is selected from silicon that can reversibly occlude and release lithium ions. The anode and cathode may be made by preparing an electrode slurry composition by dispersing the electrode active material, a binder, a conductive material and a thickener, if desired, in a solvent and coating the slurry composition onto a current collector. The current collector may be a metal wire, a metal grid, a metal web, a metal sheet, a metal foil or a metal plate. Preferred the current collector is a metal foil, e.g. a copper foil or aluminum foil.

As shown herein and stated above, by reducing the volume of the electrolyte, in particular by reducing the FEC-based electrolyte solution from 15 μΙ/mg of LiCoP0₄ (referred also as electrode active mass), to 5 μΙ/mg of electrode active mass, the capacity retention and coulomb effi- ciency of the LiCoP0₄/Li cells were further improved. In certain embodiments, the volume of electrolyte (iii) in the lithium ion secondary battery of the present invention, in any one of the configurations defined above, is 10 μΙ or less per 1 mg of the cathode active material, preferably about 5 μΙ or less per 1 mg of the cathode active material and in particular about 1 μΙ or less per 1 mg of the cathode active material. The term "about" as used herein means within an accepta- ble error range for a particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, "about" can mean a range of up to 20%, preferably up to 10%, more preferably up to 5%, and still more preferably up to 1 % of a given value. Where particular values are described in the application and claims, unless otherwise stated the term "about" meaning within an acceptable error range for the particular value should be assumed.

The lithium ion secondary battery of the present invention usually includes a separator that prevents a short between the cathode and anode. Such a separator may be made of a polymer membrane such as a polyolefin, polypropylene or polyethylene membrane, a multi-membrane thereof, a micro-porous film, or a woven or non-woven fabric.

The lithium secondary battery of the present invention may be configured in various types, such as a cylindrical, pouch or rectangular shaped battery. A unit battery having a structure of cathode/separator/anode, a bi-cell having a structure of cathode/separator/anode/separator/cathode, or a battery stack including a plurality of unit batteries may be formed using above-described lithium secondary battery including the electrolyte, cathode, anode and separator. As demonstrated herein, the lithium ion secondary battery of the present invention has significantly improved properties, more particularly, improved capacity retention property and faradaic efficiency.

The capacity retention of a lithium ion secondary battery is the fraction of the full capacity avail- able from a battery under specified conditions of discharge after it has been cycled for a particular number of cycles. The faradaic/coulomb efficiency of a lithium ion secondary battery is the ratio of discharge capacity: charge capacity, expressed in percents and calculated according to the formula measured as [(discharge capacity/charge capacity)* 100]. The term "improving the capacity retention property and faradaic efficiency of a lithium ion secondary battery" as used herein thus means that during the cycling the battery retains more portion of its discharge (faradaic) capacity and, at the same time, less charge is expended in parasitic reactions.

In another aspect the present invention refers to a non-aqueous electrolyte comprising at least one lithium salt, at least one fluorinated carbonate and comprising at least one optionally fluori- nated boroxine of formula (I). The components of the inventive electrolyte are the same as described above for electrolyte (iii) and may be used in the amounts as given for electrolyte (iii). The inventive electrolyte may contain further additives as described above for electrolyte (iii) and may have any one of the features as described above for the electrolyte (iii) comprised in the inventive Li ion secondary battery provided that the electrolyte contains at least one optionally fluorinated boroxine of formula (I), at least one lithium salt, and at least one fluorinated carbonate. This means the inventive electrolyte may for example further comprise at least one non- aqueous organic solvent selected from non-fluorinated carbonates as described above and/or may further comprise at least one compound of formula (II) as described above.

The inventive electrolyte is especially useful as electrolyte for lithium ion batteries comprising L1C0PO4 as cathode active material. Lithium ion secondary batteries comprising L1C0PO4 as cathode active material and comprising the inventive electrolyte show improved capacity retention properties and faradaic efficiencies in comparison with such lithium ion secondary batteries comprising an electrolyte containing only a partially fluorinated carbonate or only boroxine. Therefore, the use of said inventive electrolyte comprising at least one lithium salt, at least one fluorinated carbonate and comprising at least one optionally fluorinated boroxine of formula (I) as electrolyte for lithium ion batteries comprising UC0PO4 as cathode active material is an object of the present invention, too. The use of the inventive electrolyte in lithium ion batteries comprising UC0PO4 as cathode active material wherein the anode active material is selected from group consisting of carbonaceous material that can reversibly occlude and release lithium ions and from silicon that can reversibly occlude and release lithium ions is especially preferred.

In another aspect, the present invention thus relates to a method of using the inventive electrolyte and improving the capacity retention property and faradaic efficiency of a lithium ion secondary battery, said method comprising placing the inventive electrolyte comprising at least one lithium salt, at least one fluorinated carbonate and comprising at least one optionally fluorinated boroxine of formula (I), into a lithium ion secondary battery comprising a cathode comprising a cathode active material selected from L1C0PO4 and an anode comprising an anode active material that can reversibly occlude and release lithium ions. The inventive electrolyte placed into the lithium ion secondary battery may comprise further additives and features as described above. In this respect the present invention relates to a method of improving the capacity retention property and faradaic efficiency of a lithium ion secondary battery, said method comprising placing the electrolyte as described above into a lithium ion secondary battery comprising a cathode comprising a cathode active material selected from L1C0PO4; and an anode comprising an an- ode active material that can reversibly occlude and release lithium ions. The term "placing the electrolyte into a lithium ion secondary battery" is intended to include all techniques to place, insert or apply the electrolyte in or to the lithium secondary battery known to the person skilled in the art like injecting the electrolyte into the assembled battery or immersing one or more components of the cell like anode, cathode or separator in the electrolyte solution before assembling of the battery.

The invention will now be illustrated by the following non-limiting examples. 6. EXAMPLES Experimental

Carbon-coated LiCoPC powder was prepared by hydrothermal synthesis as described in Markevich et al., Electrochem. Comm., 2012, 15, 22-25, and had an orthorhombic, olivine-like structure as described in X-ray Powder Diffraction Data Files 00-032-0552 (X-ray Powder Diffraction Data Files 00-032-0552, American Society for Testing Materials). The carbon content in the powder was determined by an Eager, Inc. Model 200 analyzer and comprised 1 .53 % by weight. The surface area of a sample of the powder prepared was calculated using the Brunau- er-Emmett-Teller (BET) equation from the adsorption isotherm of N2 gas at 77 K using an Auto- sorb-1 -MP apparatus (Quantachrome Corporation), and was equal to 1 1.7 m²/g. High-resolution scanning electron microscopy (HRSEM) images and energy-dispersive X-ray spectroscopy (EDS) data were obtained using a FEI xHR- SEM Magellan 400L microscope, equipped with the Oxford Industries INCAx-sight energy dispersive spectrometer. X-ray diffraction (XRD) patterns were recorded with a BRUKER-AXS, D8-Advance diffractometer using Cu Και radiation. Fig. 1 shows SEM image and XRD pattern of the carbon-coated LiCoPC olivine powder prepared, consisting of rods having a diameter of 50 to 200 nm and a length of about 1 μηη.

The cathode sheets were fabricated by spreading a slurry (a suspension of LiCoPC powder and carbon black in a PVdF/N-methylpyrrolidon-solution) on an aluminum foil current collector with a doctor blade device. Typically, the electrodes contained 2-3 mg of active mass.

The electrolyte solutions were 1 M of LiPF6 in either ethylenecarbonate (EC)+dimethylcarbonate (DMC) 1 :1 (EC-based) or fluoroethylenecarbonate (FEC)+DMC 1 :4 (FEC-based) mixtures (both Li-battery grade from Merck, KGaA). Trimethyl boroxine (TMB, 99%) was purchased from Al- drich.

Specific electrolyte solutions were (a) 1 M of LiPF6 in EC+DMC (1 :1 ) (EC-based) solution taken (b) 1 M of LiPF₆ in FEC+DMC (1 :4) (FEC-based); (c) 0.5 wt.-% of TMB in 1 M LiPF₆ in FEC- DMC (1 :4) solution; (d) 1 wt.-% of TMB in 1 M LiPF₆ in FEC-DMC (1 :1 ); and (e) 1 wt.-% of TMB in 1 M LiPF₆ in EC-DMC (1 :4) solution. Silicon thin film electrodes were prepared by DC magnetron sputtering (Angstrom Sciences Inc., USA) of n-type silicon (99.999%, Kurt J. Lesker, USA), at a pressure of about 5x10-3 Torr of argon (99.9995%) onto the roughened copper foil (Oxygenfree, SE-Cu58, Schlenk Metallfolien GmbH & Co. KG) as described in R. Elazari, et al.; Electrochem. Comm. 2012, 14 , 21 -24. The surface density of the obtained a-Si film was 0.39 mg/cm² (~1 .8 μηη thick). Before the use of the film Si electrodes as anodes in full cells, they were galvanostatically pre-passivated and partially pre-lithiated in two electrode coin type cells containing Li counter electrodes. Two-electrode cells comprising silicon film electrodes, PE separator (Setela Tonen, Japan), an electrolyte solution, and Li counter electrodes were assembled in a glove box filled with pure argon and sealed in 2032 coin-cells (NRC, Canada). After that five galvanostatic cycles of Si electrodes were performed with the voltage cut-off limits of 10mV and 1 .2V and current density of 120 mA/g in the first cycle and 600 mA g in four subsequent cycles. Finally, the Si electrodes were discharged galvanostatically down to 50 mV vs. Li/Li+, withdrawn from Si/Li cells in the glove box and used for the preparation of the complete cells. A Raman spectrum of the Si anode was measured using a Raman microscope spectrometer (Labram, HR-800/Jobin Yvon Horiba) with a 632.8 nm line of a He-Ne laser with a power attenuated to O.l mWat the samples' surface. The result is shown in Fig. 1 b revealing the absence of the peak at 520 cm-¹, which is characteristic of crystalline silicon. Thus, it may be concluded that the Si film is totally amorphous. Two-electrode cells comprising LiCoPC electrodes, polyethylene (PE) separator (Setela Tonen, Japan), an electrolyte solution and Li foil negative electrodes or preliminary passivated and partially lithiated silicon film negative electrodes were assembled in a glove box filled with pure argon and sealed in 2032 coin-cells (NRC, Canada). Galvanostatic cycling of Si/Li cells (pre- passivation procedure), LiCoPC /Li and LiCoPC /Si cells was carried out at 30°C using an Arbin model BT2000 battery tester (Arbin Instruments, USA).

X-ray photoelectron spectroscopy (XPS) measurements of the cycled and thoroughly washed with DMC electrodes were performed using the AXIS-HS system (Kratos Analytical, Inc., England) using monochromatic Al Ka radiation. All binding energies (BE) were corrected with re- spect to the binding energy value of the C 1 s at 285 eV.

Example 1 : The cycling of LiCoPC /Li cells in EC-based vs. FEC-based electrolyte solution

Cycling results (C/8 h rates) of LiCoPC /Li cells in two electrolyte solutions (EC- and FEC- based) taken in two different amounts are shown in Fig. 2 (capacity [mAh/g] vs. cycle number). For the cells cycled in the FEC-based electrolyte solution (electrolyte solution (a) at 15 μΙ/g: open circles and electrolyte solution (a) at 5 μΙ/g: full circles), significantly better capacity retention and higher coulomb efficiency, compared to those of the cells cycled with EC-based solution (electrolyte solution (b) at 15 μΙ/g: open triangulars and electrolyte solution (b) at 5 μΙ/g: full triangulars), were observed. Both the capacity retention and coulomb efficiency were improved when the volume of the electrolyte solution in the cells cycled with the FEC-based electrolyte solution was decreased from 15 μΙ/mg of LiCoPC (open circles) to 5 μΙ/mg of LiCoPC ifull cir- cles). Similar decrease in the electrolyte volume related to the electrodes mass performed for the EC-based electrolyte solution (full triangulars vs. open triangulars) resulted only in a modest improvement in the capacity retention along with a decrease of the irreversible capacity of the cells.

Further improvement of the cycling behavior of LiCoPC /Li cells was achieved by the addition of TMB (0.5 weight %) to the FEC-based electrolyte solution (electrolyte solution (c) at 5 μΙ/g: open squares in Fig. 2 (capacity [mAh/g] vs. cycle number)). The discharge capacity vs. cycle number obtained upon galvanostatic cycling (C/2 h rate) of LiCoPC /Li cells (30°C) with the electrolyte solution composition comprising 1 wt.-% of TMB in 1 M LiPFe/FEC-DMC 1 :4 taken in amount of 5 μΙ_/ΓΤ^ of active electrode mass (electrolyte solution (d)) is shown in Fig. 3 (capacity [mAh/g] vs. cycle number).

Example 2: Characterization of LiCoPC /C cathodes cycled for an equal period of time in EC- and FEC-based electrolyte solutions

XPS spectra of LiCoPC /C cathodes cycled for an equal period of time in EC- and FEC-based electrolyte solutions, as well as in the FEC-based electrolyte with the addition of 0.5 wt.-% of TMB are shown in Fig. 4, and the surface chemical compositions of these electrodes are com- pared in Table 1.

Table 1 : Results of XPS quantitative analysis of overall surface composition of LiCoPC /C electrodes cycled in three electrolyte solutions

In all cases, the volume of the electrolyte solution was 5 μΙ/mg of electrode active mass. As clearly shown, the better was the performance of the cell, the higher content of fluorine atoms was found on the surface of the cathode. To illustrate, the atomic ratio F:0 equal to 0.35 in the case of the EC-based electrolyte grows up to 0.6 for the FEC-based solution and reaches 0.7 for the TMB-containing electrolyte. As the content of fluorine on the surface increases the content of cobalt reduces, indicating the formation of more perfect or thick surface film. The lowest content of carbon on the surface of the electrode cycled in the EC-based solution may be explained by an exfoliation of the carbon coating film due to the dissolution of the underlying layer of the active LiCoPC material as was shown in Markevich et al., Electrochem. Comm., 2012, 15, 22-25.

Example 3: The cycling of LiCoPC /Li cells in EC-based and FEC-based electrolyte with TMB (comparative and inventive examples)

Cycling results of LiCoPC /Li cells in two electrolyte solutions are shown in Fig. 5 (capacity [mAh/g] vs. cycle number). The galvanostatic cycling was performed at C/8 h rate at 30 °C. Electrolyte solution (e) (1 wt.-% TMB in 1 M LiPF₆/EC-DMC 1 :1 ) is shown with full (charge) and open (discharge) triangles (comparative example). Electrolyte solution (d) (1 wt.-% TMB in 1 M LiPFe/FEC-DMC 1 :1 ) is shown with full (charge) and open (discharge) circles (inventive example). The addition of TMB to an EC-based electrolyte solution not containing FEC yields a lower discharge capacity of the LiCoPC /Li cell than the addition of TMB to the FEC-based electrolyte solution.

Example 4: The cycling of LiCoPC /Si cells in FEC-based electrolyte with and without TMB (inventive examples)

Cycling results of LiCoPC /Si cells in two electrolyte solutions are shown in Fig. 6 (capacity [mAh/g] vs. cycle number). The galvanostatic cycling was performed at C/8 h rate at 30 °C. Electrolyte solution compositions were 1 M LiPFe/FEC-DMC 1 :4 without TMB (open circles) and with the addition of 1 % TMB (full circles). As can be seen the addition of TMB to an FEC-based electrolyte solution leads to a higher capacity of the cell.

Claims

A lithium ion secondary battery comprising:

(i) a cathode comprising a cathode active material selected from LiCoPC ;

(iii) a non-aqueous electrolyte comprising at least one lithium salt and at least one nonaqueous organic solvent selected from fluorinated carbonates and at least one optionally fluorinated boroxine of formula (I)

(I) wherein R¹, R², and R³ are independently from each other are selected from (Ci- C6)alkyl, (Ci-C6)alkoxy, (C5-C7)aryl, and (C5-C7)aryloxy, and

wherein alkyl, alkyloxy, aryl and aryloxy may be independently from each other substituted by one or more substituents selected from F, (Ci-Ce)alkyl, (Ci-Ce)alkoxy, (C5-C7)aryl, and (C5-C7)aryloxy and

each alkyl, alkoxy, aryl, and aryloxy may be substituted by one or more F.

The battery of claim 1 , wherein said anode active material is a carbonaceous material.

The battery of claim 2, wherein said carbonaceous material is selected from crystalline carbon, hard carbon and amorphous carbon.

4. The battery of claim 1 , wherein said anode active material is silicon.

The battery of any of claims 1 to 4, wherein the concentration of the optionally fluorinated boroxine of formula (I) in said electrolyte (iii) is 0.1 % to 5% by weight, based on the total weight of the electrolyte, preferably 0.1 to 2 wt.-% and most preferred 0.25 to 2 wt.-%.

6. The battery of any one of claims 1 to 5, wherein the electrolyte (iii) comprises at least one non-aqueous organic solvent selected from non-fluorinated carbonates, and the weight ratio between the fluorinated carbonates and the non-fluorinated carbonates present in said electrolyte (iii) is in the range of 1 :200 to 1 :1 , preferably 1 :100 to 1 :2, more preferably 1 :50 to 1 :2, most preferably 1 :25 to 1 :3, in particular in the range of from more than 1 :9 up to 1 : 3 by weight, respectively. The battery of any one of claims 1 to 6, wherein the electrolyte (iii) further comprises at least one compound of general formula (II)

R⁴ is cyclohexyl or aryl, which may be substituted by one or more substituent selected independently from each other from F, CI, Br, I, and (C-i-Ce) alkyi, wherein (C-i-Ce) alkyi may be substituted by one or more substituent selected independently from each other from F, CI, Br and I; and

R⁵, R⁶, R⁷, R⁸, and R⁹ may be same or different and are independently from each other selected from H, F, CI, Br, I, (C-i-Ce) alkyi, wherein (C-i-Ce) alkyi may be substituted by one or more substituent selected independently from each other from F, CI, Br and I.

The battery of claim 7, wherein the concentration of compound of formula (II) in the electrolyte (iii) is 0.01 to 5 wt.-%, based on the total weight of the electrolyte, preferably 0.1 to 2 wt.-% and most preferred 0.1 to 0.5 wt.-%. 9. The battery of any one of claims 1 to 8 wherein the electrolyte (iii) comprises at least one optionally fluorinated -boroxine of formula (I) and at least one compound of general formu-

10. The battery of any one of claim 1 to 9, wherein the lithium salt in the electrolyte (iii) is LiPF₆, or a mixture of LiPF₆with LiBF₄, LiSbF₆, LiAsF₆, LiCI0₄, LiCF₃S0₃ , LiC₄F₉S0₃ or mixtures thereof.

1 1 . The battery of any one of claims 1 to 10, wherein the electrolyte (iii) does not contain vi- nylene carbonate.

12. A non-aqueous electrolyte for lithium ion secondary batteries as defined in any one of claims 1 and 5 to 1 1 comprising at least one lithium salt and at least one non-aqueous organic solvent selected from fluorinated carbonates and at least one optionally fluorinated boroxine of formula (I).

wherein R¹ , R², and R³ are independently from each other are selected from (Ci-C6)alkyl, (Ci-Ce)alkoxy, (C5-C7)aryl, and (C5-C7)aryloxy, and

wherein alkyl, alkyloxy, aryl and aryloxy may be independently from each other substituted by one or more substituents selected from F, (Ci-Ce)alkyl, (Ci-Ce)alkoxy, (C5-C7)aryl, and (C5-C7) aryloxy and

each alkyl, alkoxy, aryl, and aryloxy may be substituted by one or more F.

13. Use of the electrolyte of claim 12 as electrolyte in lithium ion secondary batteries comprising a cathode active material selected from UC0PO4 and an anode active material selected from carbonaceous material that can reversibly occlude and release lithium ions or from silicon that can reversibly occlude and release lithium ions.