EP2572399A1

EP2572399A1 - Additive for electrolytes in rechargeable lithium ion batteries

Info

Publication number: EP2572399A1
Application number: EP11719774A
Authority: EP
Inventors: Guenter Eichinger
Original assignee: Li Tec Battery GmbH
Current assignee: Li Tec Battery GmbH
Priority date: 2010-05-19
Filing date: 2011-05-16
Publication date: 2013-03-27
Also published as: WO2011144317A1; US20140017547A1; DE102010020992A1; CN103004004A

Abstract

The invention relates to a method for reducing the loss of electrical capacitance of a rechargeable lithium ion battery when charging and discharging, comprising: (i) introducing an esterified aliphatic dicarboxylic acid into the electrolyte contained in the battery, said electrolyte comprising an organic solvent and a conducting salt.

Description

Additive for electrolytes in rechargeable lithium ion batteries

The invention relates to a method for reducing the electrical capacity loss of a rechargeable lithium ion battery in cyclic charging and discharging operation, wherein organic esters are added to the electrolyte of the battery. Further objects of the invention relate to the electrolyte of the battery as well as the battery containing the electrolyte.

Secondary batteries, in particular lithium ion secondary batteries, are used as energy stores for mobile information devices because of their high energy density and high capacity. In addition, such batteries are also used for tools, electrically powered automobiles and for hybrid cars.

At such batteries high demands are made in terms of electrical capacity and energy density. They should remain stable, especially in the charge and discharge cycle, i. to suffer the least possible loss of electrical capacity.

It is known that secondary batteries such as rechargeable lithium ion cells are subject to a decrease in the electric capacity or an increase in the internal resistance with the charge / discharge cycles. According to a conceivable theory, covering layers are made responsible for this decrease, which can form from the electrolyte contained in the battery by deposition on the electrodes (SEI (solid electrolyte interface)). Through such cover layers, the internal resistance of the battery increases, whereby the capacity decreases.

DE 10 2006 025 471 A1 proposes to counteract this layer formation by adding silicon compounds to the electrolyte. It is also proposed to additionally use acyclic monocarboxylic ester compounds such as methyl formate, ethyl acetate, ethyl acetate, propyl ester, sec-butyl ester, butyl ester, methyl propionate and ethyl ester for stabilization.

DE 10 2006 055 770 A1 proposes to increase the stability by an electrolyte containing lithium bis (oxalato) borate dissolved in a suitable solvent. It is an object of the present invention to reduce the capacity loss of a rechargeable lithium ion battery in cyclic charging and discharging operation, not resorting to the compounds mentioned above in the prior art. The object is achieved in that one or more esterified aliphatic dicarboxylic acids is added to the electrolyte of the battery in the loading and unloading operation to reduce the capacity loss of a rechargeable lithium-ion battery. The subject matter of the present invention is thus a method for reducing the electrical capacity loss of a rechargeable lithium ion battery during charging and discharging operation comprising step (i):

(i) introducing an esterified aliphatic dicarboxylic acid into a nonaqueous electrolyte contained in the battery and comprising at least an organic solvent and a conductive salt. Further objects of the invention are also an electrolyte for a rechargeable lithium-ion battery, which contains at least one esterified aliphatic dicarboxylic acid, and a rechargeable lithium-ion battery containing the electrolyte according to the invention.

The term "lithium ion battery" includes terms such as "lithium ion secondary battery", "lithium battery", "lithium ion secondary battery", and "lithium ion cell". This means that the term "lithium-ion battery" is used as a generic term for the terms used in the prior art.

According to the invention, the introduction of the aliphatic dicarboxylic acid ester into the electrolyte reduces the electrical capacity loss occurring in the charge and discharge cycle.

The term "bringing in" is synonymous with terms such as "admit" or "mix" or "pour".

The electrical capacity loss is particularly pronounced during the first charge / discharge cycles, since it comes here to the formation of the mentioned outer layers on the electrodes.

In one embodiment, the method according to the invention is characterized in that the capacity loss after the first loading and unloading and the second loading and unloading is reduced.

The capacity loss of the battery, which is irreversible, can then be determined by determining the capacity of the battery after the first charging and after the first discharging or the second charging and the second discharging. Suitable methods are known to the person skilled in the art. In one embodiment of the method, the capacity loss may be expressed as a percentage capacity loss (Q - Q ₂ ) * 100% / Qi, where Q is the capacity after the first charge and discharge and Q _{2 is} the capacity after the second charge. For each additional cycle, the corresponding capacity loss can be determined analogously.

In one embodiment, the esterified dicarboxylic acid is selected so that when the esterified dicarboxylic acid is added to the electrolyte and the battery is started up, the capacity loss is lower than the capacity loss of the battery if its electrolyte does not contain esterified dicarboxylic acid and the battery is put into operation.

Preferably, the esterified dicarboxylic acid is selected such that the irreversible capacity loss is at most 90% of the irreversible capacity loss of the battery when put into service in the absence of the esterified dicarboxylic acid, preferably at most 90%, more preferably at most 85%.

Already nominally minor improvements in said capacity behavior, for example by 1 to 5%, are considered to be significant improvements.

Since the capacity loss is generally proportional to the increase of the internal resistance of the battery, the capacity loss can also be expressed indirectly by determining the increase of the internal resistance. It is also possible to express the capacity loss via the voltage applied to the battery in the charging and discharging operation or the removable current. The term "method of reducing electrical capacity loss" is synonymous with the terms "method of reducing the increase in electrical capacity loss" Internal Resistance "or" Voltage Reduction Reduction Procedure "or" Power Loss Reduction Procedure ".

Instead of the capacity loss can thus be determined in one embodiment, the voltage loss of the battery.

In one embodiment of the method, the capacitance loss is expressed as the voltage loss (L ^ -U ₂ ) * 100% / U ₁ , where U-, the voltage after the first charge and discharge and U _{2 is} the voltage after the second charge. The voltage loss can be determined accordingly for each further cycle.

In one embodiment of the method, the esterified aliphatic dicarboxylic acid is selected such that the voltage loss is expressed as (Ui-U ₂ ) * 100% / Ui, where Ui is the voltage after the first charge and discharge and U _{2 is} the voltage after the second charge is less than 10%.

In another embodiment of the method, the esterified aliphatic dicarboxylic acid is selected such that the stress loss expressed as (L ^ -U ₂ ) * 100% / where L is the voltage after the first charge and discharge and U _{2 is} the voltage after the second charge is less than 5%.

In a further embodiment of the method, the esterified aliphatic dicarboxylic acid is selected such that the voltage loss is expressed as (Ui-U ₂ ) * 100% / L, where Ui is the voltage after the first charge and discharge and U ₂ the voltage after the second Loading is less than 1%. The electrical capacity loss can be counteracted in particular by adding esterified aliphatic dicarboxylic acids of the formula RrOOC- (CH 2) _x -COO-R ₂ to the electrolyte of a lithium-ion battery, where x is a is an even number between 0 and 12 and Ri and R _{2 are} independently an unbranched or branched alkyl radical having 1 to 8 carbon atoms.

The esters used for the invention are either commercially available and / or can be prepared by conventional methods known to those skilled in the art, for example by esterification of the dicarboxylic acids with the corresponding alcohols.

It is possible to use symmetrical esters, ie esters which have the same alcohol component. In this embodiment, Ri and R _{2 are} identical.

The use of unsymmetrical esters is also possible. Such esters have different esterification components, ie Ri and R ₂ are different.

It is possible to use individual esters as well as mixtures of two or more different esters. In one embodiment, the esters are esters of dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, suberonic acid, sebacic acid.

In one embodiment, esters of adipic acid are used as dicarboxylic esters.

Adipic acid esters known from the prior art can be used, ie dimethyl, diethyl, dipropyl, dibutyl, dipentyl, dihexyl adipate. While these compounds are used in the art as plasticizers, they are used for the present invention for reducing the electrical capacity loss of a rechargeable lithium ion battery in charging and discharging operation. For example, DE 699 04 932 T2 discloses plasticizers for the production of separators or electrodes used in electrochemical cells. These plasticizers are used to form porous polymer structures. Dimethyl, diethyl, dipropyl, dibutyl, dioctyl adipate are disclosed as suitable plasticizers. Dimethyl succinate, dimethyl suberate and sebacate are also suitable. The plasticizers are removed prior to activation of the electrochemical cell, for example by extraction.

DE 699 1 1 751 T2 discloses a rechargeable battery structure in the form of a laminate. The structure is prepared in the presence of an adipic acid ester having as alcohol component an alcohol of up to six carbon atoms. For the preparation of dimethyl, diethyl, dipropyl, dibutyl, dipentyl, Dihexyladipat be used, which are used in the manufacturing process of the structure as a plasticizer. The document discloses that the dimethyl adipate (DMA) used in the examples can be removed from the battery structure as well as in the electrolyte of the battery in an amount of 5 to 20% by weight. As shown in Examples I and II, the initial loss of the battery structure containing DMA after the first cycle is larger than that in the structure without DMA. Thus, DE 751 as a whole teaches to remove the plasticizers (esters) prior to activation of the cell.

In one embodiment of the present invention adipic acid esters are used in which the alcohol component contains an unbranched alkyl radical having 1 to 6 carbon atoms. In this embodiment, x = 4 and Ri and R ₂ are identical, and are methyl, ethyl, propyl, butyl, pentyl, or hexyl.

In another embodiment, x = 4 and Ri and R ₂ are identical, and are ethyl, propyl, butyl, pentyl, or hexyl. In a further embodiment, adipic acid esters are used in which the alcohol component contains a branched alkyl radical having 3 to 6 carbon atoms. A further embodiment is characterized in that x = 4 and R ₁ and R _{2 are} an unbranched or branched alkyl radical having 2 to 6 carbon atoms. Particularly effective adipic acid esters are also diethyl adipate and / or dibutyl adipate.

In this embodiment, the process according to the invention is characterized in that the esterified aliphatic dicarboxylic acid is diethyl adipate and / or dibutyl adipate.

In another embodiment, x = 0 and the esterified aliphatic dicarboxylic acid is selected from dimethyloxalate, diethyl oxalate, dipropyloxalate, dibutyl oxalate, diphenyl oxalate, dihexyl oxalate.

In another embodiment, x = 1 and the esterified aliphatic dicarboxylic acid is selected from dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, dipentyl malonate, dihexyl malonate. In another embodiment, x = 2 and the esterified aliphatic dicarboxylic acid is selected from dimethyl succinate, diethyl succinate, dipropyl succinate, dibutyl succinate, dipentyl succinate, dihexyl succinate.

In another embodiment, x = 3 and the esterified aliphatic dicarboxylic acid is selected from dimethyl glutarate, diethyl glutarate, dipropyl glutarate, dibutyl glutarate, dipentyl glutarate, dihexyl glutarate.

In another embodiment, x = 5 and the esterified aliphatic dicarboxylic acid is selected from dimethyl pimelate, diethyl pimelate, dipropyl pimelate, dibutyl pimelate, dipentyl pimelate, dihexyl pimelate. In another embodiment, x = 6 and the esterified aliphatic dicarboxylic acid is selected from dimethyl suberate, diethyl suberate, dipropyl suberate, dibutyl suberate, dipentyl suberate, dihexyl suberate. In another embodiment, x = 7 and the esterified aliphatic dicarboxylic acid is selected from dimethyl azelate, diethyl azelate, dipropyl azelate, dibutyl azelate, dipentyl azelate, dihexyl azelate.

In another embodiment, x = 8 and the esterified aliphatic dicarboxylic acid is selected from dimethyl sebacate, diethyl sebacate, dipropyl sebacate, dibutyl sebacate, dipentyl sebacate, or dihexyisebacate.

The esterified aliphatic dicarboxylic acid can be introduced in relatively large amounts in the electrolyte. It is generally effective even when incorporated in amounts up to 20% by weight, based on the total weight of organic solvent and esterified aliphatic dicarboxylic acid.

In one embodiment, the esterified aliphatic dicarboxylic acid is incorporated in the electrolyte in an amount of from 0.1% to 20% by weight, based on the total weight of organic solvent and esterified aliphatic dicarboxylic acid.

Preferably, the esterified aliphatic dicarboxylic acid is incorporated in an amount of 0.5 to 5% by weight, more preferably 1 to 4% by weight.

The introduction of the esterified aliphatic dicarboxylic acid can be carried out by adding in the form of pouring into the electrolyte. It has been found that a very good activity can be achieved even at relatively low levels, ie in amounts of less than 5% by weight or even up to 4% by weight. In another embodiment, the esterified aliphatic dicarboxylic acid is incorporated in the electrolyte in an amount of 0.5 to 5 weight percent, based on the total weight of organic solvent and esterified aliphatic dicarboxylic acid.

In another embodiment, the esterified aliphatic dicarboxylic acid is incorporated in the electrolyte in an amount of 1 to 4% by weight, based on the total weight of organic solvent and esterified aliphatic dicarboxylic acid.

It has been found that with diethyl adipate and dibutyl adipate potent compounds are available, with which the capacity loss can be very advantageously lowered. These compounds have already proven to be very effective in a concentration range between 1 and 4 wt .-%.

The electrolyte used in the lithium-ion battery is nonaqueous. It comprises at least one organic solvent and one conducting salt. Preferably, the electrolyte for lithium-ion batteries comprises an organic solvent and a conductive salt dissolved therein, preferably based on lithium.

Preferred lithium salts have inert anions and are non-toxic. Suitable lithium salts are preferably lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis (trifluoromethylsulfonyl imide), lithium trifluoromethanesulfonate, lithium tris (trifluoromethylsulfonyl) methide, lithium tetrafluoroborate, lithium perchlorate, lithium tetrachloroaluminate, lithium chloride, lithium (bisoxalato) borate, and mixtures thereof.

In one embodiment, the lithium salt is selected from LiPF _6, LiBF _4, LiCI0 _4, LiAsF _6, LiCF ₃ S0 _3, LiN (CF ₃ S0 ₂₎ _2, LiC (CF ₃ S0 ₂₎ _3, LiS0 ₃ C _x F _{2x + 1} , LiN (S0 ₂ C _x F ₂ x ₊ i) 2 or LiC (S0 ₂ C _x F _{2x +} i) ₃ with 0 <x <8, Li [(C ₂ O ₄ ) ₂ B], and mixtures of two or more of these salts. Preferably, the electrolyte is present as an electrolyte solution. Suitable solvents are preferably inert. Suitable solvents include, for example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, methylpropycarbonate, butylmethyl carbonate, ethylpropyl carbonate, dipropyl carbonate, cyclopentanones, sulfolanes, dimethylsulfoxide, 3-methyl-1,3-oxazolidine-2-one, γ-butyrolactone, 1, 2-diethoxymethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, nitromethane, 1,3-propanesultone, and mixtures of two or more of these solvents.

In one embodiment, the conductive salt is LiPF ₆ .

In another embodiment, the organic solvent is selected from ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, dipropyl carbonate, or a mixture of two or more thereof.

The electrolyte may include other adjuvants commonly used in electrolytes for lithium ion batteries. For example, these are free-radical scavengers such as biphenyl, flame retardant additives such as organic phosphoric acid esters or hexamethylphosphoramide, or acid scavengers such as amines. So-called Überladeadditive such as cyclohexylbenzene may also be included in the electrolyte.

Adjuvants which can influence the formation of the "solid electrolyte interface" layer (SEI) on the electrodes, preferably carbon-containing electrodes, can also be used in the electrolyte.

It is known from the prior art that due to the reactivity of vinylene carbonate when stored lithium ion batteries at elevated Temperature can come to gas formation. It is also known that this gassing can be prevented by adding a stabilizing additive. Such an additive is, for example, propane sultone (PS). However, it is also known that such sultones have or may have carcinogenic properties. Surprisingly, it has now been found that the esterified aliphatic dicarboxylic acids used according to the invention can advantageously be used in combination with vinylene carbonate, so that the use of propanesultone can be dispensed with. Accordingly, an embodiment is advantageous in which the electrolyte contains at least one of the esterified aliphatic dicarboxylic acids and vinylene carbonate.

Another embodiment is also characterized in that the electrolyte contains at least one of the esterified aliphatic dicarboxylic acids and vinylene carbonate but no propanesultone.

A further subject of the invention is a nonaqueous electrolyte for a rechargeable lithium ion battery, characterized in that said electrolyte comprises:

at least one organic solvent;

at least one conductive salt;

at least one esterified aliphatic dicarboxylic acid, preferably an esterified aliphatic dicarboxylic acid of the formula Ri-OOC- (CH 2) _x -COO-Px ₂ , where x is an even number between 0 and 12 and Ri and R _{2 are} independently an unbranched or branched alkyl radical 1 to 8 carbon atoms;

wherein the esterified aliphatic dicarboxylic acid in an amount of 0.5 to 4 wt.% Is contained in the electrolyte, preferably from 1 to 2.5 wt .-% based on the total weight of organic solvent and esterified aliphatic dicarboxylic acid. In this case, the electrolyte has the said composition, in particular during the charging and / or discharging operation of the lithium-ion battery. A further subject of the invention is also an electrolyte for a rechargeable lithium ion battery, characterized in that said electrolyte comprises:

at least one organic solvent;

at least one conductive salt;

at least one esterified aliphatic dicarboxylic acid, preferably an esterified aliphatic dicarboxylic acid of the formula of the formula R OOC- (CH 2) _x -COO-R ₂ , wherein x is an even number between 0 and 12 and Ri and R _{2 are} independently an unbranched or branched alkyl radical with 1 to 8 carbon atoms; wherein dimethyl adipate is excluded.

In one embodiment of the electrolyte, diethyl adipate, dipropyl adipate, dibutyl adipate, dipentyl adipate, dihexyl adipate as added versed dicarboxylic acid are excluded in addition to dimethyl adipate.

In one embodiment, the nonaqueous electrolyte of the invention for a rechargeable lithium ion battery is characterized in that said electrolyte comprises: at least one organic solvent;

at least one conductive salt; at least one esterified aliphatic dicarboxylic acid of the formula R OOC- (CH 2) _x -COO-R ₂ , where x = 4 and R 1 and R _{2 are} identical, and are ethyl or butyl. Another object of the invention is also a lithium-ion battery, which comprises a positive electrode, a negative electrode, a separator and the electrolyte according to the invention. The present invention also relates to the use of any of the above methods or electrolytes for reducing said capacity loss.

The term "positive electrode" means the electrode that is capable of accepting electrons when the battery is connected to a consumer, such as an electric motor. Then the positive electrode is the cathode. The term "negative electrode" means the electrode that is able to emit electrons when in use. Then the negative electrode is the anode. The anode of the battery of the invention may be made of a variety of materials suitable for use with a lithium ion electrolyte battery. For example, the negative electrode may contain lithium metal or lithium in the form of an alloy, either in the form of a foil, a grid, or in the form of particles held together by a suitable binder. The use of lithium metal oxides such as lithium titanium oxide is also possible. In principle, all materials which are capable of forming lithium intercalation compounds can be used. Suitable materials for the negative electrode then include, for example, graphite, synthetic graphite, carbon black, mesocarbon, doped carbon, fullerenes, niobium pentoxide, tin alloys, tin dioxide, silicon, titanium dioxide, and mixtures of these substances.

The cathode of the battery according to the invention preferably comprises a compound having the formula LiMP0 ₄ , wherein M is at least one transition metal cation of the first row of the Periodic Table of the Elements, this transition metal cation preferably from the group consisting of Mn, Fe, Ni and Ti or a combination of these elements is chosen, and preferably has an olivine structure, preferably parent olivine, wherein Fe is particularly preferred.

For the lithium ion battery according to the invention, a lithium iron phosphate with olivine structure of the empirical formula LiFePO ₄ can be used. However, it is also possible to use a lithium iron phosphate containing an element M selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, B and Nb. Furthermore, it is also possible that the lithium iron phosphate contains carbon to increase the conductivity.

In a further embodiment, the lithium iron phosphate with olivine structure used for the production of the positive electrode has the empirical formula LixFei _-y M _y PO ₄ , wherein M represents at least one element selected from the group consisting of Mn, Cr, Co, Cu, Ni, V , Mo, Ti, Zn, Al, Ga, B and Nb, with 0.05 <x <1, 2 and 0 <y <0.8.

In one embodiment, x = 1 and y = 0.

The positive electrode preferably contains the lithium iron phosphate in the form of nanoparticles. The nanoparticles can take any shape, that is, they can be coarse-spherical or elongated.

In one embodiment, the lithium iron phosphate has a particle size measured as a Dg ₅ value of less than 15 pm. Preferably, the particle size is smaller than 10 pm.

In a further embodiment, the lithium iron phosphate has a particle size measured as D ₉₅ value between 0.005 pm to 10 pm. In a further embodiment, the lithium iron phosphate has a particle size measured as D ₉₅ value of less than 10 pm, wherein the D ₅₀ value is 4 pm ± 2 pm and the D ₁₀ value is less than 1, 5 pm. The values given can be determined by measurement using static laser light scattering (laser diffraction, laser diffractometry). The methods are known from the prior art.

According to a preferred embodiment, the cathode may also be a lithium manganate, preferably spinel-type LiMn ₂ O, a lithium cobaltate, preferably LiCoO ₂ , or a lithium nickelate, preferably LiNiO ₂ , or a mixture of two or three of these oxides, or a lithium mixed oxide comprising nickel, manganese and cobalt (NMC).

In a preferred embodiment, the cathode comprises at least one active material of a lithium-nickel-manganese-cobalt mixed oxide (NMC), which is not present in a spinel structure, in a mixture with a lithium manganese oxide (LMO) in spinel structure.

It is preferred that the active material comprises at least 30 mol%, preferably at least 50 mol% NMC and at least 10 mol%, preferably at least 30 mol% LMO, in each case based on the total moles of the active material of the cathodic electrode (ie not based on the cathodic Total electrode, which in addition to the active material may also comprise conductivity additives, binders, stabilizers, etc.).

It is preferred that NMC and LMO together account for at least 60 mole% of the active material, more preferably at least 70 mole%, more preferably at least 80 mole%, even more preferably at least 90 mole%, each based on the total moles of active material of the cathodic electrode (ie not based on the cathodic electrode as a whole, which in addition to the active material can still comprise conductivity additives, binders, stabilizers, etc.). In principle, there are no restrictions with respect to the composition of the lithium-nickel-manganese-cobalt mixed oxide, except that this oxide in addition to lithium at least 5 mol%, preferably in each case at least 15 mol%, more preferably in each case at least 30 mol% of nickel, manganese and cobalt must contain, in each case based on the total number of moles of transition metals in lithium-nickel-manganese-cobalt mixed oxide. The lithium-nickel-manganese-cobalt mixed oxide can be doped with any other metals, in particular transition metals, as long as it is ensured that the abovementioned molar minimum amounts of Ni, Mn and Co are present.

In this case, a lithium-nickel-manganese-cobalt mixed oxide of the following stoichiometry is particularly preferred: wherein the proportion of Li, Co, Mn, Ni and O can each vary by +/- 5%. In the positive electrode, the lithium iron phosphate or lithium oxide (s) used and the materials used for the negative electrode (a) are generally held together by a binder holding these materials on the electrode. For example, polymeric binders can be used. As the binder, polyvinylidene fluoride, polyethylene oxide, polyethylene, polypropylene, polytetrafluoroethylene, polyacrylate, ethylene (propylene-diene monomer) copolymer (EPDM), and mixtures and copolymers thereof may preferably be used.

The separator used for the battery must be permeable to lithium ions to ensure ion transport for lithium ions between the positive and negative electrodes. On the other hand, the separator must be insulating for electrons.

The separators known from the prior art can be used. In one embodiment, microporous films or membranes can be used. In one embodiment, the films or membranes comprise polyolefins. Suitable polyolefins are preferably polyethylene, polypropylene, or laminates of polyethylene and polypropylene.

In a further embodiment, separators are used which comprise nonwoven polymer fibers.

In one embodiment, the separator of the battery according to the invention comprises a nonwoven made of nonwoven polymer fibers, also known as "non-woven fabrics", which are not electrically conductive. The term "fleece" is used synonymously with terms such as "knitted fabric" or "felt". Instead of the term "unwoven" the term "not woven" is used.

Preferably, the nonwoven is flexible and has a thickness of less than 30 pm. Methods for producing such nonwovens are known in the art.

Preferably, the polymer fibers are selected from the group of polymers consisting of polyacrylonitrile, polyolefin, polyester, polyimide, polyether imide, polysulfone, polyamide, polyether.

Suitable polyolefins are, for example, polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride. Preferred polyesters are preferably polyethylene terephthalates.

In a preferred embodiment, the separator comprises a nonwoven, which is coated on one or both sides with an inorganic material. The term "coating" also includes that the ionic conductive inorganic material may be located not only on one side or both sides of the web, but also within the web. The material used for the coating is preferably at least one compound from the group of Oxides, phosphates, sulfates, titanates, silicates, aluminosilicates at least one of zirconium, aluminum or lithium.

The ion-conductive inorganic material is preferably ion-conducting in a temperature range of -40 ° C to 200 ° C, i. ion-conducting for the lithium ions.

In a preferred embodiment, the ion-conducting material comprises or consists of zirconium oxide.

Furthermore, a separator may be used, which consists of an at least partially permeable carrier, which is not or only poorly electron-conducting. This support is coated on at least one side with an inorganic material. As an at least partially permeable carrier an organic material is used, which is designed as non-woven web. The organic material is in the form of polymer fibers, preferably polymer fibers of polyethylene terephthalate (PET). The nonwoven fabric is coated with an inorganic ion-conducting material which is preferably ion-conducting in a temperature range of -40 ° C to 200 ° C. The inorganic ion-conducting material preferably comprises at least one compound from the group of oxides, phosphates, sulfates, titanates, silicates, aluminosilicates with at least one of the elements zirconium, aluminum, lithium, particularly preferably zirconium oxide. The inorganic ion-conducting material preferably has particles with a largest diameter of less than 100 nm.

Such a separator is marketed in Germany, for example, under the trade name "Separion ^®" by the company Evonik AG. Methods for producing such separators are known from the prior art, for example from EP 1 017 476 B1, WO 2004/021477 and WO 2004/021499. In principle, too large pores and holes in separators used in secondary batteries can lead to an internal short circuit. The battery can then discharge itself very quickly in a dangerous reaction. In this case, such large electrical currents can occur that a closed battery cell can even explode in the worst case. For this reason, the separator can contribute significantly to the safety or lack of security of a lithium high performance or lithium high energy battery. Polymer separators generally stop any current transport through the electrolyte above a certain temperature (the so-called "shut-down temperature", which is about 120 ° C). This happens because at this temperature, the pore structure of the separator collapses and all pores are closed. The fact that no ions can be transported, the dangerous reaction that can lead to an explosion, comes to a standstill. However, if the cell continues to be heated due to external circumstances, the so-called "break-down temperature" is exceeded at approx. 150 to 180 ° C. From this temperature, the separator melts and contracts. In many places in the battery cell, there is now a direct contact between the two electrodes and thus to a large internal short circuit. This leads to an uncontrolled reaction, which can end with an explosion of the cell, or the resulting pressure must be reduced by a pressure relief valve (a rupture disk) often under fire phenomena.

In the separator used in the battery according to the invention comprising a nonwoven made of non-woven polymer fibers and the inorganic coating, it can only come to shutdown (shutdown), when melted by the high temperature, the polymer structure of the carrier material and penetrates into the pores of the inorganic material and this thereby closing. On the other hand, there is no break-down (breakdown) in the separator, since the inorganic particles ensure that a complete melting of the sepa- rators can not enter. This ensures that there are no operating states in which a large-area short-circuit can occur.

By the type of nonwoven used, which has a particularly suitable combination of thickness and porosity, separators can be produced that can meet the requirements for separators in high-performance batteries, especially lithium high-performance batteries. The simultaneous use of particle particles of exactly matched oxide particles for the production of the porous (ceramic) coating results in a particularly high porosity of the finished separator, the pores still being sufficiently small to prevent undesired "lithium oxide" growth. Whiskers "through the separator to prevent.

Due to the high porosity of the separator, however, care must be taken to ensure that no or as little dead space as possible is created in the pores.

The separators which can be used for the battery according to the invention also have the advantage that the anions of the conducting salt partly adhere to the inorganic surfaces of the separator material, which leads to an improvement in the dissociation and thus to a better ion conductivity in the high-current range.

The separator which can be used for the battery according to the invention, comprising a flexible nonwoven with a porous inorganic coating located on and in this nonwoven, the material of the nonwoven being selected from non-woven, non-electrically conductive polymer fibers, is also distinguished by the fact that the nonwoven a thickness of less than 30 microns, a porosity of more than 50%, preferably from 50 to 97% and a pore radius distribution has, wherein at least 50% of the pores have a pore radius of 75 to 150 pm. The separator particularly preferably comprises a nonwoven which has a thickness of 5 to 30 μm, preferably a thickness of 10 to 20 μm. Also particularly important is a homogeneous distribution of pore radii in the web as indicated above. An even more homogeneous pore radius distribution in the nonwoven, in combination with optimally matched oxide particles of a certain size, leads to an optimized porosity of the separator.

The thickness of the substrate has a great influence on the properties of the separator, since on the one hand the flexibility but also the sheet resistance of the electrolyte-impregnated separator depends on the thickness of the substrate. Due to the small thickness, a particularly low electrical resistance of the separator is achieved in the application with an electrolyte. The separator itself has a very high electrical resistance, since it itself must have insulating properties. In addition, thinner separators allow increased packing density in a battery pack so that one can store a larger amount of energy in the same volume.

Preferably, the web has a porosity of 60 to 90%, more preferably from 70 to 90%. The porosity is defined as the volume of the web (100%) minus the volume of the fibers of the web, ie the proportion of the volume of the web that is not filled by material. The volume of the fleece can be calculated from the dimensions of the fleece. The volume of the fibers results from the measured weight of the fleece considered and the density of the polymer fibers. The large porosity of the substrate also allows a higher porosity of the separator, which is why a higher uptake of electrolytes with the separator can be achieved.

In order to obtain a separator having insulating properties, it has as polymer fibers for the non-woven fabric preferably non-electrically conductive fibers of polymers as defined above, which are preferably selected from polyacrylonitrile (PAN), polyesters such. B. Polyethylene terephthalate (PET) and / or polyolefin (PO), such as. As polypropylene (PP) or polyethylene (PE), or mixtures of such polyolefins.

The polymer fibers of the nonwovens preferably have a diameter of from 0.1 to 10 μm, more preferably from 1 to 4 μm.

Particularly preferred flexible nonwovens have a basis weight of less than 20 g / m ² , preferably from 5 to 10 g / m ² . The separator has a porous, electrically insulating, ceramic coating on and in the fleece. The porous inorganic coating on and in the nonwoven preferably has oxide particles of the elements Li, Al, Si and / or Zr with an average particle size of 0.5 to 7 μm, preferably 1 to 5 μm and very particularly preferably 1 , 5 to 3 pm up. The separator particularly preferably has a porous inorganic coating on and in the nonwoven, the aluminum oxide particles having an average particle size of from 0.5 to 7 μm, preferably from 1 to 5 μm and very particularly preferably from 1.5 to 3 pm, which are bonded to an oxide of the elements Zr or Si. In order to achieve the highest possible porosity, preferably more than 50% by weight and more preferably more than 80% by weight of all particles are within the abovementioned limits of average particle size. As already described above, the maximum particle size is preferably 1/3 to 1/5 and particularly preferably less than or equal to 1/10 of the thickness of the nonwoven used. The separator preferably has a porosity of from 30 to 80%, preferably from 40 to 75% and particularly preferably from 45 to 70%. The porosity refers to the achievable, ie open pores. The porosity can be determined by the known method of mercury porosimetry or can be calculated from the volume and density of the feedstock used, if it is assumed that only open pores are present. The separators used for the battery according to the invention are also distinguished by the fact that they can have a tensile strength of at least 1 N / cm, preferably of at least 3 N / cm and very particularly preferably of 3 to 10 N / cm. The separators can preferably be bent without damage to any radius down to 100 mm, preferably down to 50 mm and most preferably down to 1 mm. The high tensile strength and the good bendability of the separator have the advantage that changes in the geometries of the electrodes occurring during the charging and discharging of a battery can be through the separator without being damaged. The flexibility also has the advantage that commercially standardized winding cells can be produced with this separator. In these cells, the electrode / separator layers are spirally wound together in a standardized size and contacted. The combination of the negative electrode and the positive electrode, in particular the separator and the electrolyte containing the esterified dicarboxylic acid, preferably the esterified dicarboxylic acid of the formula RrOOC- (CH 2) _x -COO-R ₂ , where x is an even number between 0 and 12 and R ₁ and R _{2 are} independently an unbranched or branched alkyl group having 1 to 8 carbon atoms, resulting in a lithium ion battery which, in addition to reducing the irreversible capacity loss in charge / discharge operation, advantageously also reduces the increase of the AC internal resistance, a decrease in the increase of the DC internal resistance after storage of the battery and a reduction in the increase of the irreversible capacity loss after storage of the battery has.

The preparation of the lithium-ion battery according to the invention can be carried out in principle by methods known in the art.

For example, for the production of the positive electrode, the active material used, for example, the lithium iron phosphate, as a powder on the Electrode deposited and compacted into a thin film, optionally using a binder. The other electrode may be laminated on the first electrode, the separator being laminated in the form of a foil beforehand on the negative or the positive electrode. It is also possible to simultaneously process the positive electrode, the separator and the negative electrode under mutual lamination. The composite of electrodes and separator is then surrounded by a housing. Electrolyte can be filled as described in the prior art. This combination of improved properties is extremely beneficial to the use of the inventive lithium-ion battery in the charge / discharge cycle as a driving force for mobile information devices, tools, electric powered automobiles, and hybrid-powered automobiles. Examples

A lithium ion battery with Separion® as separator contained as the electrolyte a mixture of ethylene carbonate and propylene carbonate in a ratio of 1: 1 and was 1.15 molar of LiPF ₆ . The electrolyte contained 1, 5 wt% vinylene carbonate and 2 wt .-% biphenyl based on the total weight of the electrolyte. The battery was charged with high current pulses at a starting temperature of 75 ° C. and then discharged (charging: 150 A, 5 pulses (628 W), discharging: 225 A, 1 pulse (790 W), total duration of test: 1 h). The voltage Ui at the beginning of the test cycle and the voltage U ₂ at the end of the first test cycle were measured, wherein 2% by weight of the compounds mentioned in the table were added to the electrolyte. The reference electrolyte contained no added ester. Furthermore, the propionic acid methyl ester known from the prior art was also tested for comparison: First test cycle u ₂ [V] (U, - U ₂ ) * 100 / U,

[%]

Reference (no ester) 3.33 2.84 14.7

Propionic acid methyl ester 3.49 3.2 8.3

Diethyl adipate 3.52 3.44 2.27

Dibutyl adipate 3.69 2.62 0.18

Diethyl sebacate 3.71 3.65 0.16

Thereafter, a second test cycle was performed, where Ui was the voltage at the beginning of the second test cycle and U _{2 was} the voltage at the end of the second test cycle:

The results confirm the good cycle stability, i. the low voltage loss compared to the reference and the comparative tester despite the relatively high test temperature.

Claims

claims

A method of reducing the electrical capacity loss of a rechargeable lithium ion battery in charge and discharge operation comprising:

(i) introducing an esterified aliphatic dicarboxylic acid into a nonaqueous electrolyte contained in the battery and comprising at least an organic solvent and a conductive salt.

Process according to Claim 1, characterized in that the esterified aliphatic dicarboxylic acid has the formula RrOOC- (CH 2) _x -COO-R ₂ , where x is an even number between 0 and 12 and R 1 and R ₂ independently of one another are an unbranched or branched alkyl radical with 1 to 8 carbon atoms.

A method according to claim 2, characterized in that Ri and R _{2 are} identical.

A method according to claim 3, characterized in that x = 4 and Ri and R _{2 are} ethyl, propyl, butyl, pentyl, or hexyl.

Method according to one of the preceding claims, characterized in that the esterified aliphatic dicarboxylic acid is diethyl adipate or dibutyl adipate.

Method according to one of claims 1 to 3, characterized in that

x = 5 and the esterified aliphatic dicarboxylic acid is selected from dimethyl pimelate, diethyl pimelate, dipropyl pimelate, dibutyl pimelate, dipentyl pimelate, dihexyl pimelate; or x = 6 and the esterified aliphatic dicarboxylic acid is selected from dimethylsuberate, diethylsuberate, dipropylsuberate, dibutylsuberate, dipentylsuberate, dihexylsuberate; or

x = 7 and the esterified aliphatic dicarboxylic acid is selected from dimethyl azelate, diethyl azelate, dipropyl azelate, dibutyl azelate, dipentyl azelate, dihexyl azelate; or

x = 8 and the esterified aliphatic dicarboxylic acid is selected from dimethyl sebacate, diethyl sebacate, dipropyl sebacate, dibutyl sebacate, dipentyl sebacate, or dihexyl sebacate.

Method according to one of the preceding claims, characterized in that the esterified aliphatic dicarboxylic acid is introduced in an amount of 0.1% by weight to 20% by weight into the electrolyte, based on the total weight of organic solvent and esterified aliphatic dicarboxylic acid, or that the esterified aliphatic dicarboxylic acid is introduced in an amount of 0.5 to 5 wt .-% in the electrolyte, or that the esterified aliphatic dicarboxylic acid in an amount of 1 to 4 wt .-% is introduced into the electrolyte.

Process according to one of the preceding claims, characterized in that the conductive salt is selected from lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis (trifluoromethylsulfonyl imide), lithium trifluoromethanesulfonate, lithium tris (trifluoromethylsulfonyl) methide, lithium tetrafluoroborate, lithium perchlorate, lithium tetrachloroaluminate, lithium chloride, Lithium (bisoxalato) borate, and mixtures thereof.

Method according to one of the preceding claims, characterized in that the organic solvent is selected from ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethylpropyl carbonate, dipropyl carbonate, or a mixture of two or more thereof.

A nonaqueous electrolyte for a rechargeable lithium ion battery, characterized in that said electrolyte comprises:

at least one organic solvent;

at least one conductive salt;

at least one esterified aliphatic dicarboxylic acid, preferably an esterified aliphatic dicarboxylic acid of the formula RrOOC- (CH 2) _x -COO-R ₂ , where x is an even number between 0 and 12 and Ri and R _{2 are} independently an unbranched or branched alkyl radical having 1 to 8 carbon atoms; wherein the esterified aliphatic dicarboxylic acid in an amount of 0.5 to 4 wt .-% is contained in the electrolyte, preferably from 1 to 2.5 wt .-% based on the total weight of organic

Solvent and esterified aliphatic dicarboxylic acid.

Non-aqueous electrolyte for a rechargeable lithium ion battery, characterized in that said electrolyte comprises:

at least one organic solvent; at least one conductive salt;

at least one esterified aliphatic dicarboxylic acid of the formula Ri- OOC- (CH 2) _x -COO-R ₂ , where x = 4 and Ri and R _{2 are} identical and are ethyl or butyl.

at least one organic solvent; at least one conductive salt; at least one esterified aliphatic dicarboxylic acid, preferably an esterified aliphatic dicarboxylic acid of the formula R ₁ OOC- (CH 2) _x -COO-R ₂ , where x is an even number between 0 and 12 and R ₁ and R ₂ are each independently an unbranched or branched alkyl radical 1 to 8 carbon atoms;

wherein dimethyl adipate is excluded as esterified dicarboxylic acid.

Electrolyte for a rechargeable lithium ion battery according to claim 12, characterized in that diethyl adipate, dipropyl adipate, dibutyl adipate, dipentyl adipate, dihexyl adipate are excluded as additives to the electrolyte.

Method according to one of claims 1 to 9 or electrolyte according to one of claims 10 to 13, characterized in that the electrolyte comprises at least one of the esterified aliphatic dicarboxylic acids and vinylene carbonate.

A lithium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that the electrolyte is an electrolyte according to any one of claims 10 to 14.

Use of a method according to any one of claims 1 to 9 or an electrolyte according to any one of claims 10 to 14 for reducing the electrical capacity loss of a rechargeable lithium ion battery in charging and discharging operation.