WO2018224167A1

WO2018224167A1 - Lithium battery and use of triphenylphosphine oxide as an electrolyte additive therein

Info

Publication number: WO2018224167A1
Application number: PCT/EP2017/064147
Authority: WO
Inventors: Juhyon Lee; Thomas Koester; Kolja BELTROP; Sven Klein; Xin Qi; Tobias PLACKE; Martin Winter; Liang TAO; Jian Yan; Lin Lu; Chengdu Liang
Original assignee: Bayerische Motoren Werke Aktiengesellschaft; Contemporary Amperex Technology Co., Limited
Priority date: 2017-06-09
Filing date: 2017-06-09
Publication date: 2018-12-13

Abstract

The present invention relates to a lithium battery comprising an anode comprising an active anode material, a cathode comprising an active cathode material comprising lithium nickel manganese cobalt oxide (NMC), and an electrolyte separating anode and cathode, wherein the electrolyte comprises a solvent or solvent mixture and lithium hexafluorophosphate, wherein the electrolyte further comprises triphenylphosphine oxide. Moreover, the present invention further relates to the use of triphenylphosphine oxide as additive in the lithium battery for enhancing one characteristic selected from the group consisting of reversible capacity, Coulombic efficiency, cyclic stability and combinations thereof.

Description

Lithium battery and use of triphenylphosphine oxide as an electrolyte additive therein

The present invention relates to a lithium battery and use of triphenylphosphine oxide as an electrolyte additive therein.

Conceptually, there should be no net chemical changes during the operation of the battery in the electrolyte, and all

Faraday processes should be performed within the electrodes. Therefore, the electrolyte can be regarded as an inert component in the battery, and therefore must be stable both against cathode and anode surfaces. This electrochemical stability of the electrolyte, which is usually realized in a kinetic (passivation) and not a thermodynamic manner in actual devices, is of particular importance for rechargeable battery systems, even though these are difficult to fulfil because of the strong oxidizing and reducing nature of the cathode and anode.

A basic prerequisite for the components used in the

electrolyte for lithium-ion batteries, especially solvents, is therefore that they are anhydrous or more precisely aprotic; that is, the solvent must not contain active protons which can react with lithium. In addition, the solvent should be in a liquid state in the service temperature range.

A disadvantage of conventional electrolytes based on lithium hexafluorophosphate in carbonates for lithium-ion batteries is in particular the low oxidative stability of 4.5 V against Li / Li⁺. The electrolyte is stable only up to this voltage, whereas outside this range the oxidative decomposition of the electrolyte and associated dissolution of the cathode

material occur.

Lithium-nickel-manganese-cobalt oxides, also referred to as "NMC", is one preferred cathode active material for lithium- ion batteries with a high energy density or high power density. However, also in this case decomposition of the electrolyte and the dissolution of the cathode material occurs at 4.4 V. The result is a low cycle stability and therefore battery life.

In 2016, W. Qiu, J. Xia, L. Chen, J.R. Dahn, Journal of Power Sources, 318 (2016) 228-234 reported on the effectiveness of methyl phenyl carbonate and diphenyl carbonate as electrolyte additives for high voltage NMC-811. The addition of 1 wt . % diphenyl carbonate to the standard electrolyte [1M LiPFg in ethylene carbonate : ethyl methyl carbonate (EC:EMC) 3:7] greatly improves the Coulombic efficiency, reduces the self- discharge rate during storage and improves the capacity retention.

The work of J. Li, L.E. Downie, L. Ma, W. Qiu, J.R. Dahn, Journal of the Electrochemical Society, 162 (2015) A1401- A1408 focused on the failure mechanism of NMC-811/graphite full cells with and without the use of different additives including vinylene carbonate (VC) , prop-l-ene-1 , 3-sultone (PES), tris (trimethylsilyl) phophite (TTMSPi) and methylene methanedisulfonate (MMDS) . They were able to show a

beneficial effect by using control electrolyte [1M LiPF6 in 3:7 v:v ethylene carbonate (EC) : ethylmethyl carbonate (EMC)] with 2 wt . % VC in terms of an increased capacity retention and lower voltage drop. The object of the present invention is to provide a lithium battery with improved stability.

This object is achieved according to the invention in a first aspect by a lithium battery according to claim 1, in a second aspect by the use of triphenylphosphine oxide in a lithium battery as defined in the first aspect according to claim 10. Preferred embodiments are shown in the dependent claims. The following definitions apply, if applicable, to all aspects of the invention.

Lithium battery According to the present invention, the terms "lithium battery", "lithium ion battery", "rechargeable lithium ion battery" and "lithium ion secondary battery" are used

synonymously. The terms also include the terms "lithium-ion accumulator" and "lithium-ion cell" as well as all lithium or alloy batteries. Thus, the term "lithium battery" is used as a generic term for the aforementioned terms used in the prior art. It means both rechargeable batteries (secondary

batteries) as well as non-rechargeable batteries (primary batteries) . In particular, a "battery" for the purposes of the present invention also comprises a single or only

"electrochemical cell". Preferably, two or more such

electrochemical cells are connected together in a "battery", either in series (i.e., successively) or in parallel. Electrodes

The electrochemical cell according to the invention has at least two electrodes, i. e. a positive (cathode) and a negative electrode (anode) .

Both electrodes each have at least one active material. This is capable of absorbing or emitting lithium ions and at the same time absorbing or emitting electrons.

The term "positive electrode" means the electrode which, when the battery is connected to a load, for example to an

electric motor, is capable of receiving electrons. It is the cathode in this nomenclature.

The term "negative electrode" means the electrode which is capable of emitting electrons during operation. It represents the anode in this nomenclature. The electrodes comprise inorganic material or inorganic compounds or substances which can be used for or in or on an electrode or as an electrode. These compounds or substances can, under the working conditions of the lithium-ion battery, accept (insert) and also release lithium ions due to their chemical nature. In the present specification, such material is referred to as "active cathode material" or "active anode material" or generally "active material". For use in an electrochemical cell or battery, this active material is preferably applied to a support or carrier, preferably to a metallic support, preferably aluminum for the cathode or copper for the anode. This support is also referred to as a "collector" or collector film. Cathode (positive electrode)

According to the present invention, the active material for the positive electrode or active cathode material comprises or preferably consists of nickel manganese cobalt oxide (NCM) . NCM has the general formula (LiNi_xCo_yMn₁-_x-_y0₂) with each of x and y not including zero and x + y being smaller than 1. By changing the content of each transition metal, for example, LiNi_xCo_yMni-_x__y02 selected from the group

consisting of Li i_1/3Coi/3Mn_1/302 (NCM-111), LiNio.5Coo.2Mno.3O2 (NCM-532), LiNio.eCoo.2Mno.2O2 (NCM-622), LiNio.7Coo.15Mno.15O2,

LiNio.8Coo.1Mno.1O2 (NCM-811), LiNi₀.85Coo.o75Mno.o750₂ and mixtures thereof can be used. Preferred are Ni-rich NMCs due to their higher specific capacity of 180-190 mAh g^_1 at the upper cut^¬ off potential of 4.3 V vs. Li/Li⁺, with NCM-622 and NCM-811 being more preferred and NCM-811 being in particular

preferred .

The active material may also contain mixtures of the above active cathode material with a second or more of, for example, one of the following active cathode materials.

More specifically, as the second active material for the positive electrode or active cathode material all materials known from the related art can be used. These include, for example, LiCo0₂, NCA, high-energy NCM or HE-NCM, lithium-iron phosphate, Li-Manganese spinel (LiMn₂0₄) , Li-Manganese nickel oxide (LMNO) or lithium-rich transition metal oxides of the type (Li₂Mn03) _x (L1MO2) i-_x · Preferably, a material selected from a group consisting of a lithium-transition metal oxide

(hereinafter also referred to as "lithium metal oxide"), layered oxides, spinels, olivine compounds, silicate

compounds, and mixtures thereof is used as such a second active cathode material. Such active cathode materials are described, for example, in Bo Xu et al . "Recent progress in cathode materials research for advanced lithium ion

batteries", Materials Science and Engineering R 73 (2012) 51- 65. Another preferred cathode material is HE-NCM. Layered oxides and HE-NCM are also described in the patents US Pat. Nos. 6,677,082 B2, 6,680,143 B2 and US Pat. No. 7,205,072 B2 of Argonne National Laboratory.

Examples of olivine compounds are lithium phosphates of the sum formula L1XPO₄ with X = Mn, Fe, Co or Ni, or combinations thereof .

Examples of lithium metal oxide, spinel compounds and layered oxides are lithium manganate, preferably LiMn204, lithium cobaltate, preferably LiCo0₂, lithium nickelate, preferably LiNi0₂, or mixtures of two or more of these oxides or mixed oxides thereof.

In order to increase the electrical conductivity, further compounds may be present in the active material, preferably carbon-containing compounds, or carbon, preferably in the form of conductive carbon black or graphite. The carbon can also be introduced in the form of carbon nanotubes. Such additives are preferably applied in an amount of from 0.1 to 10% by weight, preferably from 1 to 8% by weight, based on the mass of the positive electrode applied to the support.

Anode (negative electrode) The active material for the negative electrode or active anode material can be any of the materials known from the related art. Thus, according to the present invention there is no limitation with regard to the negative electrode. In particular, it is also possible to use mixtures of different active anode materials.

The active anode material may be selected from the group consisting of lithium metal oxides, such as lithium titanium oxide, metal oxides (e.g. Fe₂<03, ZnO, ZnFe₂0₄) , carbonaceous materials such as graphite (synthetic graphite, natural graphite) graphene, mesocarbon , doped carbon, hard carbon, soft carbon, fullerenes, mixtures of silicon and carbon, silicon, lithium alloys, metallic lithium and mixtures thereof. Niobium pentoxide, tin alloys, titanium dioxide, tin dioxide, silicon or oxides of silicon can also be used as the electrode material for the negative electrode. The active anode material may also be a material alloyable with lithium. This may be a lithium alloy or a non-lithiated or partially lithiated precursor to this, resulting in a lithium alloy formation. Preferred lithium-alloyable

materials are lithium alloys selected from the group

consisting of silicon-based, tin-based and antimony-based alloys. Such alloys are described, for example, in the review article W.-J. Zhang, Journal of Power Sources 196 (2011) 13- 24. Electrode binders

The materials used for the positive or for the negative electrode, such as the active materials, are held together by one or more binders which hold these materials on the

electrode or on the current collector.

The binder (s) may be selected from the group consisting of polyvinylidene fluoride (PVdF) , polyvinylidene fluoride-hexa- fluoro-propylene co-polymer (PVdF-HFP) polyethylene oxide (PEO) , polytetrafluoroethylene, polyacrylate, styrene- butadiene Rubber, and carboxymethylcellulose (CMC) , and mixtures and copolymers thereof. Styrene-butadiene rubber and optionally carboxymethylcellulose and / or the other binders such as PVdF are preferably present in an amount of 0.5-8% by weight based on the total amount of the active material used in the positive or negative electrode. Separator

The lithium battery according to the invention preferably has a material which separates the positive electrode and the negative electrode from each other. This material is

permeable to lithium ions, ie it emits lithium ions, but is a non-conductor for electrons. Such materials used in lithium ion batteries are also referred to as separators.

In a preferred embodiment within the meaning of the present invention, polymers are used as separators. In one

embodiment, the polymers are selected from the group

consisting of: cellulose, polyester, preferably polyethylene terephthalate ; polyolefin, preferably polyethylene,

polypropylene; polyacrylonitrile ; polyvinylidene fluoride; polyvinylidene hexafluoropropylene ; polyetherimide ;

polyimide, polyether; polyether ketone or mixtures thereof. The separator has porosity so that it is permeable to lithium ions. In a preferred embodiment within the meaning of the present invention, the separator consists of at least one polymer. Electrolyte

The term "electrolyte" preferably means a liquid in which a lithium conducting salt is dissolved, preferably the liquid is a solvent for the conducting salt, and the Li conductive salt is preferably present as an electrolyte solution.

According to the present invention LiPF6 is used as lithium conductive salt. It is possible to use a second or more conductive salt, such as LiBF₄.

The two aspects of the present invention will be described in more detail below.

In a first aspect, the present invention relates to a lithium battery comprising an anode comprising an active anode material, a cathode comprising an active cathode material comprising lithium nickel manganese cobalt oxide

LiNi_xMn_yCo_z02 (NMC) , wherein 0<x<l, 0<y<l, 0<z<l, and x+y+z=l, a separator separating anode and cathode, and an electrolyte, wherein the electrolyte comprises a solvent or solvent mixture and lithium hexafluorophosphate, wherein the

electrolyte further comprises triphenylphosphine oxide.

Surprisingly, it has been found that the lithium battery according to the present invention comprising NCM as active cathode material and triphenylphosphine oxide as electrolyte additive, compared to the electrolyte without additive exhibits higher cycle stability and service life. In

addition, dissolving of the cathode material is suppressed. Finally, a lower self-discharge occurs.

Without being bound to a theory, it is believed that the addition of the additive triphenylphosphine oxide to an electrolyte containing LiPF6 causes in situ formation of a cathode passivation layer on the NCM active cathode material which is particularly effective at charge terminating

potentials of more than 4.4 V against Li / Li and kinetically inhibits the release of metals from the active cathode matrix and the oxidative decomposition of the electrolyte.

The additive triphenylphosphine oxide is usable in a wide temperature range, is relatively non-toxic, and readily available .

The additive triphenylphosphine oxide is therefore

advantageously suitable as an additive for LiPF₆-containing electrolytes for commercial lithium-ion batteries based on NCM active cathode materials.

Preferably, the electrolyte according to the invention comprises the additive triphenylphosphine oxide, dissolved in an organic solvent. The electrolyte is, for example,

obtainable by introducing and dissolving lithium

hexafluorophosphate and the additive triphenylphosphine oxide into a solvent or a solvent mixture.

In preferred embodiments, from 0.01 to 10% by weight, preferably from 0.1 to 5% by weight, preferably from 0.2 to 1% by weight, in particular from 0.25 to 0.75% by weight of the additive triphenylphosphine oxide, in terms of the amount of electrolyte used comprising lithium hexafluorophosphate in a solvent or solvent mixture. In preferred embodiments, the concentration of lithium hexafluorophosphate in the electrolyte is in the range from> 0.1 M to < 2 M, preferably in the range from > 0.5 M to <1.5 M, particularly preferably in the range from> 0.7 M to < 1.2 M. In a particularly preferred embodiment, the concentration of lithium hexafluorophosphate in the electrolyte is 1 M. In preferred embodiments, the electrolyte comprises an organic solvent, an ionic liquid and / or a polymer matrix. Preferably, the electrolyte comprises lithium

hexafluorophosphate, triphenylphosphine oxide, and an organic solvent. It has been found that the additive

triphenylphosphine oxide has good solubility in organic solvents, especially in cyclic and / or linear carbonates. This advantageously allows the use of triphenylphosphine oxide in LiPF₆-containing liquid electrolytes.

In preferred embodiments, the organic solvent is selected from the group consisting of ethylene carbonate (EC) , propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate (EMC) , acetonitrile, glutaronitrile, adiponitrile, pimelonitrile, gamma-butyrolactone, gamma- valerolactone, dimethoxyethane, dioxalane, methyl acetate, ethyl methane sulfonate, dimethyl methyl phosphonate and / or mixture thereof. Suitable organic solvents are, in

particular, selected from the group consisting of cyclic carbonates such as ethylene carbonate and propylene carbonate and linear carbonates such as diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate and mixtures thereof.

Preferably the organic solvent is selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and mixtures thereof. A preferred solvent is ethylene carbonate. Ethylene carbonate is also referred to as 1 , 3-dioxolan-2-one according to the IUPAC nomenclature. Ethylene carbonate is commercially available. Ethylene carbonate has a high boiling point and a high flame point. It is also advantageous that ethylene carbonate allows a high conductivity due to a good salt dissociation .

In a preferred embodiment, the organic solvent comprises a mixture of ethylene carbonate and at least one further organic solvent, preferably gamma-butyrolactone . Preference is also given to binary mixtures of carbonates, in particular of ethylene carbonate, with a further carbonate, for example diethyl carbonate, dimethyl carbonate and / or ethyl methyl carbonate, in particular ethyl methyl carbonate.

The ratio of ethylene carbonate and the at least one further organic solvent, preferably ethylmethyl carbonate, is preferably in the range from >1 : 99 to <99: 1, preferably in the range from >1 : 9 to <9: 1 : 7 to ≤1: 1. If not stated differently, the ratio indicated relates to the weight parts of the solvents. A high conductivity in a temperature range from -25 °C. to +60 °C was advantageously achieved in a solvent mixture of ethylene carbonate and ethyl methyl carbonate in the ratio 1 : 1.

Preference is also given to ternary mixtures comprising at least one carbonate as solvent. Particular preference is given to mixtures of ethylene carbonate with a further solvent, for example ethyl methyl carbonate, and a compound which is suitable for forming a so-called solid electrolyte interphase (SEI), a solid electrolyte interface. The

electrolyte can therefore also comprise additives, in

particular film-forming electrolyte additives. In preferred embodiments, the electrolyte comprises a compound selected from the group consisting of chloroethylene carbonate, fluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, ethylene sulfite, ethylene sulfate, propane sulfonates, sulfites, preferably dimethyl sulfite and propylene sulfite, sulfates, butyrolactones , phenylethylene carbonate, vinyl acetate and trifluoropropylene carbonate. Among the compounds based on carbonate, chlorine-substituted or fluorine-substituted carbonates are preferred, in

particular fluoroethylene carbonate (FEC) . The additives can improve the battery performance, for example the capacity or the cycle life. In particular, fluoroethylene carbonate can lead to improved long-term stability of a cell.

Preferably the electrolyte contains at least one further additive, in particular a compound selected from the group consisting of chloroethylene carbonate, fluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, ethylene sulfite, ethylene sulfate, propane sulfonates, sulfites, preferably dimethyl sulfite and propylene sulfite, sulfates, butyrolactones optionally substituted by F, CI or Br, phenylethylene carbonate, vinyl acetate,

trifluoropropylene carbonate and mixtures thereof, preferably fluoroethylene carbonate, in the range from >0.1% by weight to <10% by weight, preferably in the range from >1% by weight to <5%, more preferably in the range from >2% by weight to <3% by weight, based on the total weight of the electrolyte.

The organic solvent preferably comprises a mixture of ethylene carbonate and at least one further organic solvent, preferably selected from the group consisting of linear carbonates, in particular ethyl methyl carbonate, and

fluoroethylene carbonate. Thus, fluoroethylene carbonate can form a protective layer on a graphite cathode and reduce excess potentials of the electrode. Ionic liquids have also proved to be very

promising solvents because they combine a high thermal as well as electrochemical stability with a high ionic

conductivity. In particular, this is advantageous for use with lithium-2-methoxy-l , 2 , 2-tetrafluoro-ethanesulfonate . Preferred ionic liquids include a cation selected from the group consisting of 1 , 2-dimethyl-3-propylimidazolium (DMPI +) , 1,2-diethyl 3 , 5-dimethylimidazolium (DEDMI +) , N-alkyl-N- methylpiperidinium (PIPIR +) , N-alkyl-N-methylmorpholinium (MORPIR +) and mixtures thereof and an anion selected from the group consisting of trimethyl-n-hexylammonium (TMHA +) and N-alkylpyrrolidinium comprising bis

(trifluoromethanesulfonyl) imide (TFSI), bis

(pentafluoroethanesulfonyl ) imide (BETI), bis

( fluorosulfonyl ) imide (FSI), 2 , 2 , 2-trifluoro-N-

(trifluoromethanesulfonyl) acetamide (TSAC) Tetrafluoroborate (BF4-) , pentafluoroethane trifluoroborate (C₂F₅BF₃-) ,

hexafluorophosphate (PF₆-) , tris (pentafluoroethane)

trifluorophosphate ( (C₂F₅) 3PF3-) , and mixtures thereof.

Preferred N-alkyl-N-methylpyrrolidinium (PYRIR +) cations are selected from the group consisting of N-butyl-N- methylpyrrolidinium (PYR14 +) , N-methyl-N-propylpyrrolidinium (PYR13 +) and mixtures thereof.

Preferred ionic liquids are selected from the group

consisting of N-butyl-N-methylpyrrolidinium bis

(trifluoromethanesulfonyl) imide (PYR14TFSI), N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PYR13TFSI), and mixtures thereof.

Further suitable electrolyte materials are polymer

electrolytes, where the polymer electrolyte can be present as gel polymer electrolyte or solid polymer electrolyte. Solid polymer electrolytes exhibit good properties with regard to the requirements for future accumulator generations. They allow for a solvent-free construction, which is easy to manufacture and manifold in shape. In addition, the energy density can be increased since the three-layer structure made of electrolyte separator electrolyte is omitted so that only a thin polymer film is required between the electrodes. Solid electrolytes are generally chemically and electrochemically stable to electrode materials and do not escape from the cell. Gel polymer electrolytes usually comprise an aprotic solvent and a polymer matrix.

Preferred polymers for solid polymer electrolytes and gel polymer electrolytes are selected from the group consisting of homo- or copolymers of polyethylene oxide (PEO) ,

polypropylene oxide (PPO) , polyvinylidene fluoride (PVdF) , polyvinylidenefluoridehexafluoropropylene (PVdF-HFP) , polyacrylonitrile (PAN) , polymethylmethacrylate (PMMA ) , Polyethylmethacrylate (PEMA) , polyvinyl acetate (PVAc) , polyvinyl chloride (PVC) , polyphophazenes , polysiloxanes , polyvinyl alcohol (PVA) , homo- and (block) copolymers comprising functional side chains selected from the group consisting of ethylene oxide, propylene oxide,

acrylonitrile, siloxanes and mixtures thereof.

According to the present invention, the active material for the positive electrode or active cathode material comprises or preferably consists of lithium nickel manganese cobalt oxide LiNixMnyCoz02 (NMC) , wherein 0<x<l, 0<y<l, 0<z<l, and x+y+z=l. Alternatively, the general formula (LiNi_xCo_yMni-_x-_y02 ) with each of x and y not including zero and x + y being smaller than 1 can be used. LiNi_xCo_yMni-_x-_y02 materials

selected from the group consisting of Li ii/3Coi/3Mni/302 (NMC- 111), LiNio.5Coo.2Mno.3O2 (NMC-532), LiNio.6Coo.2Mno.2O2 (NMC-622), LiNio.7Coo.15Mno.15O2, LiNio.8Coo.1Mno.1O2 (NMC-811), Li io.85Coo.o75 n₀.o7502 and mixtures thereof are preferred. More preferred are Ni-rich NMCs with 0.5≤x<l due to their higher specific capacity of 180-190 mAh g^_1 at the upper cut-off potential of 4.3 V vs. Li/Li⁺, with NMC-622 and NMC-811 being still more preferred and NMC-811 being in particular

preferred .

In addition, a disproportionation and dissolution of

manganese, as well as other transition metals, from the active cathode material can be further kinetically inhibited in the NCM-cathode active materials by the addition of lithium 2-pentafluoroethoxy-1 , 1 , 2-tetrafluoroethane sulfonate to the electrolyte containing LiPF₆. In preferred embodiments, the anode comprises an active anode material selected from a group consisting of carbon,

graphite, mixtures of silicon and carbon / graphite, silicon, lithium, lithium metal oxide, lithium-alloyable materials, and mixtures thereof. Graphite is particularly preferred.

In a second aspect of the invention, the present invention is directed to the use of triphenylphosphine oxide as additive in a lithium battery as defined in the first aspect of the present invention for enhancing one characteristic selected from the group consisting of reversible capacity, Coulomb efficiency, cyclic stability and combinations thereof.

The lithium-ion battery according to the invention is

particularly suitable for this purpose because of its high- voltage stability.

Examples and figures which serve to illustrate the present invention are given below. The Figures show:

FIG. 1 shows the results of LiPF6-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) without additive in LP50 (EC/MC (1:1)) with respect to the charge capacity, discharge capacity and efficiency.

FIG. 2 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) in LP50 (EC/EMC (1:1)) on the charge capacity, discharge

capacity and efficiency.

FIG. 3 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) in LP50 (EC/EMC (1:1)) in comparison to those without additive on the discharge capacity and coulombic efficiency. FIG. 4 shows the results of LiPF6-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) without additive in LP57 (EC/EMC (3:7)) with respect to the charge capacity, discharge capacity and efficiency. FIG. 5 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) in LP57 (EC/EMC (3:7)) on the capacity, and efficiency. FIG. 6 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) in LP50 (EC/EMC (1:1)) in comparison to those without additive on the discharge capacity and coulombic efficiency.

Example 1

Preparation of electrolyte solutions

The electrolyte mixtures were mixed in a glove box with a ¾0 and O2 content below 0.5 ppm. All indicated mixing ratios are based on the mass ratio (% by weight) .

An electrolyte containing 1 M LiPF₆ in EC: EMC (1: 1; in the following also referred to as "LP50") was prepared by

charging 50% by weight of ethylene carbonate (EC) and 50% by weight of ethyl methyl carbonate (EMC) and dissolving in this solvent mixture the required amount of L1PF₆ , resulting in a concentration of 1 M LiPF6. This electrolyte served as comparative electrolyte. For the preparation of the additive electrolytes according to the invention, triphenylphosphine oxide (TPPO) was added to this electrolyte mixture. The proportion of the additive (A) indicated in % by weight refers to the electrolyte (E) with additive (A) , not to the entire electrolyte mixture including additive, that is, W (A) = m (A) / (m (E) + m (A)).

Electrochemical investigations:

The experiments were carried out in a 3-electrode arrangement in modified Swagelok® tees (tube connectors, Stainless steel body) with a nickel manganese cobalt oxide (NMC-811) - Electrode (12 mm diameter) as working electrode and graphite as anode (12 mm diameter) . The measurements at constant current were carried out at 20 C ± 2 ° C. The cyclization of the NMC-811 half-cells was carried out in the potential range from 2.8 V to 4.3 V against Li / Li +.

The following test plan was used:

After two forming cycles with a charge and discharge rate (C and D rate) of 0.1C, the cyclization behavior at a charging and discharging rate of 0.5C was tested over 100 cycles.

Results

Fig. 1 shows the results of LiPF6-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) without additive in LP50 (EC/MC (1:1)) with respect to the charge capacity, discharge capacity and efficiency. The capacity at cycle 100 was 120 mAh/g. The capacity retention after 100 cyclGS WcLS 85%. The Coulombic efficiency after the first cycle was 64.3%.

FIG. 2 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) in LP50 (EC/MC (1:1)) on the charge capacity, discharge capacity and efficiency. The capacity at cycle 100 was 173 mAh/g. This translates into a capacity gain compared to LP50 without additive of 44%. The capacity retention after 100 cycles was 93%. The Coulombic efficiency after the first cycle was

85.8%.

FIG. 3 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) in LP50 (EC/MC (1:1)) in comparison to those without additive on the discharge capacity and coulombic efficiency. As is apparent from the diagram, the results show that the

electrical cells comprising 0.5 wt% TPPO are always superior compared to the ones without TPPO regarding Coulombic

efficiency and discharge capacity over the whole range of 100 cycles .

Example 2

The same general testing conditions as in Example 1 were used except that 1M LiPF₆ in EC / EMC (3:7; in the following referred to as "LP57") was used as electrolyte. Results

Fig. 4 shows the results of LiPF6-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) without additive in LP57 (EC/MC (3:7)) with respect to the charge capacity, discharge capacity and efficiency. The capacity at cycle 60 was 142 mAh/g. The capacity retention after 60 cyclGS WcLS 88%. The Coulombic efficiency after the first cycle was 73.1%. FIG. 5 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li⁺) in LP57 (EC/MC (3:7)) on the charge capacity, discharge capacity and efficiency. The capacity at cycle 60 was 177 mAh/g. This translates in a capacity gain compared to LP57 without additive (at cycle 60; please see Fig. 3) of 25%. The

capacity retention after 60 cycles was 95%. The Coulombic efficiency after the first cycle was 86.2%. FIG. 6 shows the influence of triphenylphosphine oxide (TPPO) as an additive in LiPF₆-containing electrolytes in lithium half cells with NCM (2.8 V to 4.3 V against Li / Li +) in LP50 (EC/MC (1:1)) in comparison to those without additive on the discharge capacity and coulombic efficiency. As is apparent from the diagram, the results show that the

effieciency and discharge capacity over the whole range of 100 cycles.

Claims

Claims (We claim)

1. A lithium battery comprising:

-an anode comprising an active anode material

-a cathode comprising an active cathode material comprising lithium nickel manganese cobalt oxide Li i_xMnyCo_z02

(NMC) , wherein 0<x<l, 0<y<l, 0<z<l, and x+y+z=l

- a separator separating anode and cathode, and an

electrolyte wherein the electrolyte comprises a solvent or solvent mixture and lithium hexafluorophosphate,

characterized in that

the electrolyte further comprises triphenylphosphine oxide.

2. The lithium battery according to claim 1, wherein the active cathode material is selected from the group consisting of NMC with 0.5<x<l, preferably LiNi₀ _ ₆Mn₀ _ 2^Co0.2°2

(NMC622)and LiNi₀ _ ₈Mn₀ _ _]_Coo .1O2 (NMC811), in particular

LiNiQ _ ₈Mn₀. iCo₀. i0₂ (NMC811) . 3. The lithium battery according to claim 1 or 2, wherein, in terms of the total amount of electrolyte comprising lithium hexafluorophosphate in a solvent or solvent mixture, 0.01 to 10 wt.-%, preferably 0.1 to 5 wt.-%, more preferably 0.2 to 1 wt.-%, in particular 0.25 to 0.75 wt.-% triphenylphosphine oxide.

4. The lithium battery according to one of the preceding claims, wherein the concentration of lithium

hexafluorophosphate is in the range of 0.1 M to 2 M,

preferably 0.5 M to 1.5 M, more preferably 0.7 M to 1.2 M. 5. The lithium battery according to one of the preceding claims, wherein the solvent or solvent mixture is selected from an organic solvent or solvent mixture, an ionic liquid and/or a polymer matrix.

6. The lithium battery according to one of the preceding claims, wherein the organic solvent or solvent mixture is selected from the group consisting of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, acetonitrile, glutaronitrile, adiponitrile, pimelonitrile, gamma-butyrolactone, gamma- valerolactone, dimethoxyethane, 1 , 3-dioxalane, methylacetate and/or mixtures thereof, preferably selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and/or mixtures thereof.

7. The lithium battery according to one of the preceding claims, wherein the organic solvent mixture comprises, preferably consists of, a mixture from ethylene carbonate and at least one further solvent, preferably ethyl methyl

carbonate, prefably in a ratio in terms of weight parts of > 1 :99 to < 99: 1, more preferably from > 1 :9 to < 9: 1 , in particular from ≥ 3 : 7 to ≤ 1 : 1.

8. The lithium battery according to one of the preceding claims, wherein the electrolyte further comprises an additive selected from the group consisting of chlorethylene

carbonate, fluorethylene carbonate, vinylene carbonate, vinylethylene carbonate, ethylene sulfite, ethylene sulfate, propane sulfonate, sulfite, preferably dimethylsulfite and propylene sulfite, sulfate, butyrolactone optionally

substituted with F, CI or Br, phenylethylene carbonate, vinylacetate and trifluoropropylene carbonate.

9. The lithium battery according to one of the preceding claims, wherein the active anode material is selected from the group consisting of carbon, graphite, mixtures of silicon and carbon/graphite, silicon, tin, lithium-metal oxide, materials that form alloys with lithium, composites und mixtures thereof, preferably carbon, graphite, mixtures of silicon and carbon/graphite and composites und mixtures thereof .

10. Use of triphenylphosphine oxide as additive in a lithium battery as defined in one of claims 1 to 9 for enhancing one characteristic selected from the group consisting of

reversible capacity, Coulomb efficiency, cyclic stability and combinations thereof.