DE102011084009A1 - Lithium-ion cells with improved properties - Google Patents

Abstract

Disclosed is a lithium-ion cell having at least one negative and at least one positive electrode, which are interconnected via an ion-conducting electrolyte, wherein the electrolyte comprises a solvent and a conductive salt component dissolved therein and wherein the solvent in addition to ethylene carbonate at least two carbonates from the group with diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and propylene carbonate and optionally additionally at least one co-solvent from the group comprising butylene carbonate, dipropyl carbonate, methyl propyl carbonate and ethyl propyl carbonate.

Description

The present invention relates to a lithium-ion cell having at least one negative and at least one positive electrode, which are connected to one another via an ion-conducting electrolyte, wherein the electrolyte comprises a solvent and a conductive salt component dissolved therein.

The term "battery" originally meant several galvanic cells connected in series. Today, however, individual galvanic cells are often referred to as a battery. During the discharge of a battery, an energy-supplying chemical reaction takes place, which is composed of two electrically coupled but spatially separated partial reactions. A partial reaction taking place at a comparatively lower redox potential takes place at the negative electrode, one at a comparatively higher redox potential at the positive electrode. During the discharge, electrons are released at the negative electrode by an oxidation process, resulting in an electron current via an external load to the positive electrode, from which a corresponding amount of electrons is taken up. So at the positive electrode, a reduction process takes place. At the same time, an ion current corresponding to the electrode reaction occurs within the cell. This ion current is ensured by an ionically conductive electrolyte. In secondary cells and batteries, this discharging reaction is reversible, so there is the possibility of reversing the conversion of chemical energy into electrical discharge. If the terms anode and cathode are used in this context, the electrodes are usually named according to their discharge function. The negative electrode is in such cells so the anode, the positive electrode, the cathode.

Among the secondary cells and batteries comparatively high energy densities of lithium-ion batteries are achieved. As a rule, these batteries have so-called composite electrodes which, in addition to electrochemically active components, also comprise electrochemically inactive components. As electrochemically active components (often referred to as active materials) for lithium-ion batteries are basically all materials in question, which can absorb lithium ions and release again. The state of the art in this regard for the negative electrode in particular carbon-based particles such as graphitic carbon or for the intercalation of lithium capable non-graphitic carbon materials. Furthermore, it is also possible to use metallic and semi-metallic materials which can be alloyed with lithium. For example, the elements tin, antimony and silicon are able to form intermetallic phases with lithium. For the positive electrode, the active materials industrially used at this time include mainly lithium cobalt oxide (LiCoO ₂ ), LiMn ₂ O ₄ spinel (LiMn ₂ O ₄ ), lithium iron phosphate (LiFePO ₄ ), and derivatives such as LiNi _1/3 Mn _1/3 Co _1/3 O ₂ or LiMnPO ₄ . All electrochemically active materials are usually contained in the form of particles in the electrodes.

As electrochemically inactive components, electrode binders and current conductors should be mentioned in the first place. Via current conductors, the transport of the electrons from and to the electrodes takes place. Electrode binders ensure the mechanical stability of the electrodes as well as the mutual contacting of the particles of the electrochemically active material and their connection to the current conductors. To improve the electrical connection of the electrochemically active particles to the current collector conductivity-improving additives contribute, which are also subsumed herein under the collective term "electrochemically inactive components". All electrochemically inactive components should be electrochemically stable at least in the potential range of the respective electrode and have a chemically inert character compared to common electrolyte solutions. Common electrolyte solutions are e.g. Solutions of lithium salts such as lithium hexafluorophosphate in organic solvents such as ethers or esters of carbonic acid.

Of particular importance in this context is that even at the first charge / discharge cycle of secondary lithium-ion cells (the so-called formation) to form a cover layer on the surface of the electrochemically active materials in the anode (see D. Aurbach, H. Teller, M. Koltypin, E. Levi, Journal of Power Sources 2003, 119-121, 2 ). This top layer is referred to as "solid electrolyte interphase" (SEI) and usually consists mainly of electrolyte decomposition products and a certain amount of lithium, which is accordingly no longer available for further charge / discharge reactions. Ideally, the SEI is only permeable to the extremely small lithium ions, but is also an obstacle to these during charging and discharging processes. The thicker the SEI is, the greater the impedance of the affected cell. The thickness of the SEI should therefore not increase as much as possible after the formation. Another important feature of the SEI is that it prevents further direct contact of the electrolyte solution with the electrochemically active components in the anode, thereby protecting it from further decomposition.

This is desirable insofar as the reaction of electrolytic solution with the electrochemical active components usually associated with gas evolution. Since the housing of lithium-ion cells usually have little tolerance to the associated pressures, any gassing is basically highly undesirable. For this reason, the electrolytes of lithium-ion cells often contain quite high levels of the solvent ethylene carbonate, which contributes very positively to the safety properties of lithium-ion cells.

For example, in the EP 0 683 537 B1 Described lithium ion cells having a negative electrode comprising a carbon material having a degree of crystallinity of more than 80%, wherein the electrolyte is a mixture of two organic solvents (ethylene carbonate and dimethyl carbonate) and a lithium salt is used. In addition, the electrolyte contains as an additive vinylene carbonate or a Vinylencarbonatderivat, which is added to selectively form an SEI in the formation.

An electrolyte mixture of diethyl carbonate and ethylene carbonate as the solvent, lithium hexafluorophosphate as the conductive salt and vinylene carbonate as an additive is known from DE 10 2004 014 629 A1 known.

However, ethylene carbonate - probably because of its high viscosity - has a negative effect on the performance of lithium-ion cells, especially at low temperatures.

It is an object of the present invention to provide lithium-ion cells with an optimized electrolyte that are superior to conventional cells in terms of performance, especially at lower temperatures, without sacrificing safety. In addition, a better stability of the cells at higher temperatures, in particular at 60 ° C, should be ensured.

This object is achieved by the lithium-ion cell with the features of claim 1. Preferred embodiments of the cell according to the invention are specified in the dependent claims 2 to 11. The wording of all claims is hereby incorporated by reference into the content of this specification.

Lithium-ion cells according to the invention, like the generic lithium-ion cells mentioned in the introduction, have at least one negative and at least one positive electrode which are connected to one another via an ion-conducting electrolyte, wherein the electrolyte comprises a solvent and a conductive salt component dissolved therein. In particular, lithium-ion cells according to the invention are distinguished by the fact that the solvent comprises, in addition to ethylene carbonate, at least one, preferably at least two, carbonates from the group comprising diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and propylene carbonate (PC).

EC, DEC, DMC, EMC and PC belong to the organic carbonates (acyclic and cyclic carbonic acid esters) and are highly volatile, polar solvents, which are characterized among other things by their low toxicity.

The solvent particularly preferably comprises at least three of the stated carbonates, in particular the components EC, DEC and EMC.

• • zwischen 5 und 80 %, bevorzugt zwischen 10 und 60 %, EC
• • zwischen 5 und 80 %, bevorzugt zwischen 10 und 60 %, DEC
• • zwischen 5 und 80 %, bevorzugt zwischen 10 und 60 %, DMC
• • zwischen 5 und 90 %, bevorzugt zwischen 10 und 60 %, EMC
• • zwischen 0 und 70 %, bevorzugt zwischen 5 und 40 %, PC

Preferably, the solvent contains the components EC and DEC and / or DMC and / or EMC and / or PC in the following proportions:

• • between 5 and 80%, preferably between 10 and 60%, EC
• • between 5 and 80%, preferably between 10 and 60%, DEC
• • between 5 and 80%, preferably between 10 and 60%, DMC
• • between 5 and 90%, preferably between 10 and 60%, EMC
• • between 0 and 70%, preferably between 5 and 40%, PC

The percentages are in each case percent by weight, again based on the total weight of all components of the electrolyte which are liquid at room temperature and normal pressure.

Optionally, the electrolyte of a lithium ion cell according to the invention may additionally comprise at least one co-solvent selected from the group consisting of butylene carbonate (BC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC). The proportion of the co-solvent is optionally less than 10%, based on the total weight of all liquid at room temperature and normal pressure components of the electrolyte.

In a preferred embodiment of the lithium-ion cell according to the invention, the solvent has the three components EC, DEC and EMC in each case in the same proportions, ie in a ratio of 1: 1: 1.

It has surprisingly been found that mixtures which comprise, in addition to EC, at least one, preferably at least two, components from the group comprising DEC, DMC, EMC and PC are particularly suitable as electrolyte constituents of lithium-ion cells, in particular if they are present in the stated proportions available. In particular by the simultaneous addition of at least two of said carbonates to EC, the cycle behavior of a secondary lithium-ion cell according to the invention in a temperature range between minus 20 ° C and 60 ° C can be significantly improved. Especially at an elevated temperature of 60 ° C, significant improvements were observed.

The conducting salt component dissolved in the solvent is particularly preferably a mixture of at least two lithium salts, in particular a mixture which contains, in addition to a fluorinated lithium phosphate (lithium fluorophosphate), at least one further lithium salt, preferably a lithium borate and / or a lithium alkylsulfonylimide.

The lithium fluorophosphate is particularly preferably lithium hexafluorophosphate (LiPF ₆ ), the at least one further lithium salt is preferably selected from the group comprising lithium bis (oxalato) borate (LiBOB), lithium difluoro (oxalato) borate (LiFOB), lithium (fluorosulfonyl) (nonafluorobutanesulfonyl ) imide (LiFNFSI), lithium (trifluoromethanesulfonyl) imide (LiTFSI) and mixtures of said lithium salts.

By using the mixture of the at least two lithium salts, the cycle stability and also the current-carrying capacity of the lithium-ion cells according to the invention could in some cases be drastically increased. This is especially true for the combination of lithium hexafluorophosphate with LiBOB. It is believed that these positive effects are due to the formation of a particularly stable SEI on the surface of the active materials.

For the functionality of the electrolyte in a temperature range between -20 ° C and 60 ° C, it has proved to be particularly advantageous to the solvent, the lithium fluorophosphate in a molar concentration between 0.1 and 4.0 M, preferably between 0.5 and 2 , 0 M, in particular between 0.8 and 1.4 M add.

For the at least one further lithium salt, a molar concentration between 0.01 and 1.0 M, preferably between 0.05 and 0.5 M, in particular between 0.1 and 0.3 M, has proven to be particularly advantageous.

The weight ratio of the lithium fluorophosphate to the at least one further conductive salt in the conductive salt component should in preferred embodiments be in a range between 20: 1 and 4: 1, in particular about 10 to 1, in order to achieve the above effects.

In particularly preferred embodiments, the electrolyte of a lithium-ion cell according to the invention comprises at least one additive, in particular from the group with vinylene carbonate (VC), cyclohexylbenzene (CHB), biphenyl (BP), diphenyl ether (DPE), toluene (TOL), xylene (XYL), 1,3-propane sultone (PS), propensultone (PRS), butane sultone (BS), propargyl methane sulfonate (PMS), thiophene (TP) and succinic anhydride (BSA). It has proved to be advantageous for the electrolyte to contain the at least one additive in a proportion of between 0.1 and 10% by weight, based on the total weight of all liquid constituents of the electrolyte which are liquid at room temperature and normal pressure.

The carbonate additive VC can significantly improve the performance of a lithium-ion cell. Due to the low dissociation energies of the carbonate-carbon bond in the VC, it competes with electrolytic solvents in the SEI formation, it reacts preferentially with electrochemical active materials. As a consequence, the chemical composition and probably also the morphology of the SEI is likely to change. This has a particularly positive effect on the safety properties of the lithium-ion cell according to the invention, since the decomposition of the electrolyte can be at least partially prevented.

Preferably, the VC is added in amounts of at most between 0.1 and 2 wt .-%, based on the total mass of the electrolyte, since it can otherwise lead to increased gassing of the cell. In addition, at least one further stabilizing additive may also be added to the electrolyte. PS, PRS, PMS and / or BS are preferably used according to the invention for this purpose.

CHB, XYL, TOL, DPE and BP are preferably added to the electrolyte of a lithium-ion cell according to the invention as an overcharge additive, in particular in a proportion of 1 to 5% by weight, based on the total mass of the electrolyte. If a cell according to the invention is overloaded, these additives prevent or slow down a decomposition reaction and heat generation.

A lithium-ion cell according to the invention is, in particular, a cell in which the at least one negative and the at least one positive electrode are formed as flat layers and are constituents of an electrode-separator composite which is present as a coil.

Preferably, the lithium-ion cell according to the invention is housed in a button cell housing, particularly preferably in a housing, as in the DE 10 2009 008 859 A1 is described. The content of this document is hereby incorporated by reference into the content of this specification.

Further features and advantages of the present invention will become apparent from the embodiments in conjunction with the subclaims. The individual features can be implemented individually or in combination with one another in one embodiment of the invention. The embodiments are merely illustrative and for a better understanding of the invention and are in no way limiting.

Examples

Comparative studies were carried out in which lithium-ion cells according to the invention were compared with reference cells. The cells compared were identical, they differed only in the electrolyte composition. On the positive electrode side, they had lithium cobalt oxide as the active material, while the negative electrode side had graphite. The electrolyte used in each case had the following composition:

reference electrolyte

• The electrolyte contained 1 M LiPF ₆ as conductive salt component
• As solvent the electrolyte contained a mixture of EC and DEC in the weight ratio 4: 5 (EC: DEC)
• As additives, the electrolyte contained 2% by weight BS, 2% by weight CHB and 0.5% by weight VC

Electrolyte 1 (according to the invention)

• The conductive salt component contained 0.9 M LiPF ₆ and 0.1 M LiBOB
• As solvent, the electrolyte contained a mixture of EC, DEC and EMC in the weight ratio 1: 1: 1 (EC: DEC: DEC)
• - As co-solvent, the electrolyte contained 5 wt .-% PC
• - As additives, the electrolyte contained 3 wt .-% PS, 0.5 wt .-% VC and 0.1 wt .-% TP

Electrolyte 2 (according to the invention)

• The conductive salt component contained 0.9 M LiPF ₆ and 0.1 M LiBOB
• As solvent, the electrolyte contained a mixture of EC, DEC and EMC in the weight ratio 1: 1: 1 (EC: DEC: DEC)
• - As co-solvent, the electrolyte contained 5 wt .-% PC
• - As additives, the electrolyte contained 3 wt .-% PS, 1.5 wt .-% CHB, 0.5 wt .-% VC, 2 wt .-% TP and 0.5 wt .-% BP

Cells with the reference electrolyte and the electrolytes 1 and 2 were cycled at 60 ° C in the voltage range between 3.0 and 4.2 volts. The charge / discharge rate was 1C / 1C in cccv mode (cv Limit C / 50). The observed drop in discharge capacity is in 1 shown. It can be clearly seen that the cells with the electrolyte 1 and 2 have a significantly higher cycle stability compared to the cells with the reference electrolyte.

Another series of cyclization was carried out at 20 ° C. Here, too, cycling was performed in the voltage range between 3.0 and 4.2 volts, but the discharge rate was increased from cycle to cycle (0.1C, 0.2C, 0.5C, 1C, 2C, 3C, 5C, 8C and 10C). The observed drop in discharge capacity is in 2 shown. It can be clearly seen that the cells with the electrolytes 1 and 2 are clearly superior to the cells with the reference electrolyte at higher discharge rates.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0683537 B1 [0007]
• DE 102004014629 A1 [0008]
• DE 102009008859 A1 [0031]

Cited non-patent literature

• D. Aurbach, H. Teller, M. Koltypin, E. Levi, Journal of Power Sources 2003, 119-121, 2 [0005]

Claims (11)

Lithium-ion cell having at least one negative and at least one positive electrode, which are interconnected via an ion-conducting electrolyte, wherein the electrolyte comprises a solvent and a conductive salt component dissolved therein, characterized in that the solvent in addition to ethylene carbonate at least two carbonates from the group with diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and propylene carbonate, and optionally additionally at least one co-solvent from the group comprising butylene carbonate, dipropyl carbonate, methyl propyl carbonate and ethyl propyl carbonate. Lithium-ion cell according to claim 1, characterized in that the solvent Between 5 and 80%, preferably between 10 and 60%, of ethylene carbonate as well Between 5 and 80%, preferably between 10 and 60%, diethyl carbonate, and / or Between 5 and 80%, preferably between 10 and 60%, of dimethyl carbonate, and / or Between 5 and 90%, preferably between 10 and 60%, ethyl methyl carbonate, and / or Between 0 and 70%, preferably between 5 and 40%, of propylene carbonate, wherein the percentages are in each case percentages by weight, based on the total weight of the liquid constituents of the electrolyte. Lithium-ion cell according to claim 1, characterized in that the conductive salt component is a mixture of at least two lithium salts, in particular a mixture which contains, in addition to a lithium fluorophosphate, in particular lithium hexafluorophosphate, at least one further lithium salt, preferably a lithium borate and or a Lithiumalkylsulfonylimid. Lithium-ion cell according to claim 2, characterized in that the at least one further lithium salt is selected from the group with lithium bis (oxalato) borate (LiBOB), lithium difluoro (oxalato) borate (LiFOB), lithium (fluorosulfonyl) (nonafluorobutansulfonyl) imide (LiFNFSI), lithium (trifluoromethanesulfonyl) imide (LiTFSI) and mixtures of said lithium salts. Lithium-ion cell according to one of claims 2 or 3, characterized in that the lithium fluorophosphate in a molar concentration between 0.1 and 4.0 M, preferably between 0.5 and 2.0 M, in particular between 0.8 and 1.4 M, contained in the solvent. Lithium-ion cell according to one of claims 2 to 4, characterized in that the at least one further lithium salt in a molar concentration between 0.01 and 1.0 M, preferably between 0.05 and 0.5 M, in particular between 0 , 1 and 0.3 M, is contained in the solvent. Lithium-ion cell according to one of claims 2 to 5, characterized in that the weight ratio of the lithium fluorophosphate to the at least one further conductive salt in the conductive salt component is between 20: 1 and 4: 1, in particular about 10: 1. Lithium-ion cell according to one of the preceding claims, characterized in that the electrolyte contains at least one additive, in particular at least one additive from the group with vinylene carbonate, cyclohexylbenzene, biphenyl, diphenyl ether, toluene, xylene, 1,3-propanesultone, propensultone, Butane sultone, propargyl methanesulfonate, thiophene and succinic anhydride. Lithium-ion cell according to claim 7, characterized in that the electrolyte contains the at least one additive in a proportion of between 0.1 and 10 wt .-%, based on the total weight of all liquid constituents of the electrolyte. Lithium-ion cell according to one of the preceding claims, characterized in that the at least one negative and the at least one positive electrode are formed as flat layers and are components of an electrode-separator composite, which is present as a coil. Lithium-ion cell according to one of the preceding claims, characterized in that it is housed in a button cell housing.

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