DE102006055770A1 - Electrolyte with lithium bis(oxalato)borate, dissolved in solvent from carbonate or ester and at least an additive, useful e.g. in electro-chemical cells, preferably in lithium cells, lithium ion cells and lithium ion polymer cells - Google Patents

Abstract

Electrolyte with lithium bis(oxalato)borate, dissolved in a solvent from at least a carbonate or ester and at least an additive.

Description

The The present invention relates to electrolytes for use in electrochemical Cells, especially lithium cells, lithium ion cells and lithium ion polymer cells but also supercapacitors. It was found that some additives in electrolytes with lithium bis (oxalato) borate as conductive salt compared to the solution without Additively bring about significant improvements in the cyclability of lithium-ion cells, while other additives cause significant deterioration.

was standing the technique and problem

For the electrochemical Behavior of a lithium cell, lithium ion cells of lithium ion polymer cells the choice of the electrolyte is crucial. you distinguishes liquid Electrolytes, polymer electrolytes and gel electrolytes.

A liquid electrolyte consists of at least one conducting salt dissolved in one or more liquid solvents. Since the liquid electrolyte can not separate the electrodes, the use of a separator is absolutely necessary. For this purpose, a porous film of polyolefins, such as Polyproplyen or polyethylene is usually used. Suitable conductive salts are inorganic lithium salts such as, for example, lithium hexafluorophosphate, lithium terafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, but also organic lithium salts such as lithium tetraalkylborates, lithium tetraarylborates, lithium trifluoromethylsulfonate. The publication EP 0 698 301 shows spiroborates such as lithium bis (bezodiolato) borate and lithium (salicylato) borate. As solvents, for example, linear and cyclic ethers or organic carbonates are used. A polymer electrolyte is used when the conductive salt is dissolved in one or more polymers. For this example, polyethers such as poly (ethylene oxide), vinyl esters and phosphazines are used. A gel electrolyte is used if the polymer also contains one or more solvents from the abovementioned group in addition to the conductive salt. With a suitable choice of the components of the gel electrolyte forms a free-standing film, which not only allows the ion transport, but also effectively separates the electrodes, the use of a separator is therefore no longer necessary.

• – Hydrolyse des LiPF₆ zu PF₅, und HF sowie ihren Folgeprodukten [1], [2], [3] durch Wasserspuren,
• – Reaktion der Hydrolyseprodukte mit eingesetzten Komponenten [1], [2] und den Stromableitern (Cu, Al),
• – geringe thermische Stabilität [3],
• – hohe Kosten des Leitsalzes,
• – Toxizität der Hydrolyseprodukte [3],
• – LiPF₆ und LiBF₄ können nicht mit Manganspinell [3] eingesetzt werden, der ökologisch unbedenklicher und wesentlich wirtschaftlicher wäre
• – hohe Kosten der LiCoO₂- oder LiNiO₂-Kathodenmaterialien, geringe Umweltverträglichkeit der LiCoO₂- oder LiNiO₂-Elektroden und des Leitsalzes.

On an industrial scale, electrolytes containing lithium hexafluorophosphate (LiPF ₆ ) as conducting salt are usually used. This is thermally unstable because it dissociates at elevated temperature to form phosphorus pentafluoride and lithium fluoride. Furthermore, security problems result from the highly exothermic reaction of lithium hexafluorophosphate with the anode. The prior art electrolytes based on fluorine-containing lithium salts have a number of disadvantages

• Hydrolysis of LiPF ₆ to PF ₅ , and HF and their derivatives [1], [2], [3] by traces of water,
• Reaction of the hydrolysis products with used components [1], [2] and the current conductors (Cu, Al),
• Low thermal stability [3],
• High cost of conductive salt,
• - toxicity of hydrolysis products [3],
• - LiPF ₆ and LiBF ₄ can not be used with manganese spinel [3], which would be ecologically safer and much more economical
• High cost of the LiCoO ₂ or LiNiO ₂ cathode materials, low environmental compatibility of the LiCoO ₂ or LiNiO ₂ electrodes and the conducting salt.

To avoid these problems, Barthel et al. introduced the class of lithium borates [4] to [7]. This group has shown that boron can be tetrahedrally linked to a variety of bidentate ligands containing -OH, -CO (OH), or -SO ₃ H and combinations thereof.

One Representative of these salts, the borate with oxalic acid, containing twice two CO (OH) groups can be made simple and inexpensive; so is lithium bis [oxalato (2 -)] borate (1 -) (LiBOB), almost at the same time proposed by Wietelmann [3] and Angell [8] as conductive salt for lithium-ion batteries Service. Although Wietelmann states conductivities of LiBOB in propylene carbonate / 1,2-dimethoxyethane in [3], a proposal for however, he does not make a usable electrolyte.

• – höhere Sicherheit aufgrund großer thermischer Stabilität [9],
• – enthält kein Fluor, daher höhere Sicherheit [1], [10] und Umweltverträglichkeit,
• – geringe Korrosion an Aluminiumstromableitern [11],
• – sehr gute SEI-Bildung am Anodenmaterial [12],
• – Eignung für den Einsatz bei hohen Temperaturen (>40°C) [9], [10],
• – schwach koordinierend [13], daher relativ hohe Leitfähigkeit in Standardlösungsmitteln [11],
• – geringe Hydrolyseneigung [1],
• – geringe Molmasse, daher hohe Energie- und Leistungsdichte [8],
• – geringe Herstellungskosten,
• – Kompatibilität mit Manganspinell als Kathodenmaterial [9].

This salt is distinguished from the previously used conductive salts by a number of properties, which makes it very suitable for technical use:

• - higher safety due to high thermal stability [9],
• - contains no fluorine, therefore higher safety [1], [10] and environmental compatibility,
• - low corrosion of aluminum conductors [11],
• Very good SEI formation on the anode material [12],
• - Suitability for use at high temperatures (> 40 ° C) [9], [10],
• - weakly coordinating [13], therefore relatively high conductivity in standard solvents [11],
• Low hydrolysis tendency [1],
• Low molecular weight, therefore high energy and power density [8],
• - low production costs,
• - Compatibility with manganese spinel as cathode material [9].

The combination of these positive properties [13] makes this salt a very good substitute for the previously used LiPF _6. However, since the compositions of the previous electrolyte solutions have been designed for salts such as LiPF ₆ , the electrolyte compositions for lithium bis (oxalato) borate must be used optimize it again so that it can be used successfully in lithium-ion batteries [14].

One important parameter of the performance of a lithium-ion battery describes is their life. Regarding this is in contrast to a primary battery not the self-loading rate is crucial. Much more puts on a secondary battery the cycle stability the decisive criterion for the durability. This is the number of cycles, with which the battery can be charged and discharged until the capacity on a part of their nominal capacity, mostly 80% has fallen off [15]. At higher Temperatures and big ones Discharge rates are increasing a decomposition of the electrolyte or electrode materials on, so that usually a significant reduction in life expectancy a battery yields [16].

has both the salt and the solvent or the solvent mixture a sufficient electrochemical stability, so the cycle stability of the battery decisively through the so-called "solid electrolyte interface ", short SEI [17], influenced [18]. The boundary layer that is between training the anode and the electrolyte plays a very important one Role in lithium metal, lithium ion and other alkaline and alkaline earth batteries [18]. For primary batteries through this boundary layer security, self-discharge, high current capability, the low temperature behavior and the Faraday efficiency of the battery certainly. For secondary batteries is she in addition for the Cycle yield, lifetime and irreversible capacity loss, responsible for the first cycle. The aim is therefore to this layer, which forms on the anode, for the corresponding Scope of application. As the temperature increases Effects for the decrease in cycle stability and increase in internal resistance are responsible, it makes sense to optimize the cycle stability at higher temperatures perform [19].

methods to improve the SEI and thus to optimize the batteries can be divided into two categories. For one thing, there are methods where the anode material is pre-treated outside the battery before use becomes. One of these methods is, for example, the reductive electrode surface Pretreatment conditions, thereby greatly reducing the irreversible capacity is reached [19]. By targeted oxidation of the electrode surface can be also achieve an improvement in cycle stability [18], [20] and [21]. By adding metallic nanoparticles of Ni [22] or Ag [23], [24] the anode becomes irreversible in terms of impedance capacity and their stability improved. The other methods are in-situ methods in which the SEI passes through Adding additives to the electrolyte solution is optimized. Because the Specified composition and pretreatment of the electrode materials was, only this approach could the optimization is chosen become.

Additives that are added to the electrolyte can in turn be divided into three categories. One strategy is to use substances that serve as precursors for an SEI. These compounds have a significantly greater reaction rate than the other components of the battery, whereby the SEI is largely determined by these additives and not by the composition of the electrolyte. Examples of these additives are listed in Table 1. The additives shown in this table correspond to typical battery solvents whose reactivity has been increased by derivatization with functional groups or by exchange with homologous atoms in order to increase their reaction rate at the electrode surface. Dimethyl dicarbonate decomposes into methanol and CO ₂ , both of which contribute to the formation of the SEI as LiOMe and Li ₂ CO ₃ [25].

Tabelle 1 Additive für Lithium- und Lithium-Ionen-Batterien, die als SEI Vorläufersubstanzen dienen

Table 1 Additives for lithium and lithium-ion batteries, which serve as SEI precursors

One Another approach of a battery additive is the addition of metal salts [33] where the metal ion is not in the electrode materials can intercalate and form a film that is permeable to lithium ions. The mode of action of this layer corresponds to the method of Honbo [23], except that here the SEI is generated in situ and not is previously evaporated in a high vacuum.

tetraethylene with its polyglycol ether structure has a similar structure to an SEI on, which results from polymerization of carbonates. Therefore, will this substance as building block for a appropriate SEI [27]. The ethyl esters, toluene and nitromethane also serve to train an SEI [16].

Tabelle 2 Additive für Lithium- und Lithium-Ionen-Batterien

Table 2 Additives for lithium and lithium-ion batteries

The additives mentioned are used in the prior art in lithium and lithium-ion batteries in which LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ and LiPF ₃ (CF ₂ CF ₃ ) are used as conductive salts. In DE 199 10 968 Lithiumspiroborates are provided as an additive, but the electrolyte is not based on LiBOB as conductive salt, which is a clear difference from the present invention. In DE 100 42 149 Isocyanates are added to improve the hydrolytic stability of fluoride-containing electrolytes.

By the listed Although additives can extend the life of the battery and mitigate adverse effects such as RF development, but not the fundamental ones Resolve disadvantages caused by the use of fluorine-containing electrolytes consist. This requires an electrolyte with LiBOB as conductive salt. To In the prior art, the additives mentioned are not with lithium (bis) oxalatoborat used as conductive salt. about the use of additives in batteries with chelatoborates, in particular Lithium bis (oxalato) borate, as conducting salt has not yet been reported. This is an essential novelty of this invention see.

Solution and achieved advantages

In order to overcome the drawbacks described above, it has been the object of the present invention, by means of the additives suitable for this electrolyte, to provide an improved electrolyte with LiBOB as conductive salt, which does not have the disadvantages of the prior art. As has already been shown, although the use of additives in electrolytes with conductive salts such as LiPF ₆ etc. known from the literature. What is new, however, is the use of selected additives in an electrolyte with LiBOB as conductive salt. In DE 101 11 410 [1] also describes a corresponding electrolyte, which differs significantly on closer inspection. In claim 1 of DE 101 11 410 [1] aliphatic esters and ethers are claimed at levels of over 35%, that is as the main constituent of the electrolyte, while the esters used as an additive in the present invention are limited to below 10%. The compound classes with 5-40% content differ from the additives mentioned in the disclosure of the invention. It should be noted that claim 1 of [1] does not refer to "compounds which contain at least one carboxylic acid ester group and one ether group", ie compounds with ester group or ether group are not claimed here alone It should be noted that the tetra- or pentaethylene glycol dimethyl ether having a degree of polymerization of 4 or 5 does not fall under the customary definition of the polyethylene glycol ethers but is an oligoethylene glycol ether.

The claims of DE 102 09 429 are with those of DE 101 11 410 largely identical, only the composition in claim 5 varies slightly. However, this is not relevant to the argument why reference is made here to the above statements.

The Object of the present invention is achieved by an electrolyte according to the appended claim 1 solved. Respective advantageous developments are defined in the subclaims.

there it is the advantage of the present invention that the electrolyte is chemically, thermally and electrochemically stable and high Conductivity over one huge Temperature range has. Furthermore, this electrolyte is inexpensive and sure. Due to the additives added, the electrochemical Cells in which the additive-enhanced electrolyte is used becomes, a strongly extended Lifetime the LiBOB electrolyte without additives, the state of the art match, up.

Tabelle 3 Verbesserung der Elektrolyteigenschaften durch Additivzusatz

Table 3 Improvement of the electrolyte properties through additive addition

table 3 gives an overview of the Improvement of service life through the addition of additives. For the tests were on purpose very adverse conditions with high currents of 1C, the considerable depth of discharge of 3.0 V and a relatively high Temperature of 60 ° C selected.

Because of these conditions, the life expectancy given in the table was set at a residual capacity of 50% of the initial value, with five-fold increases for suitable additives. In addition, in the table, the internal resistances of the cell after 50 and 500 cycles, R ₅₀ and R ₅₀₀ , were respectively given with respect to the end of the charging (first value) and discharging (2nd value). These values, which are also important for the cell properties, correlate well with the lifespan defined above. In 1 the influence of tetraethylene glycol dimethyl ether as an additive on the capacity of a test cell can be clearly recognized.

As in Table 4 leads the addition of additives does not necessarily lead to an improvement. Much more to lead the additives also known from the literature nitromethane, α-bromo-γ-butyrolactone, 1,3-benzodioxolane, Dimethyl dicarbonate and vinyl tri-thiocarbonate deteriorate the cyclizability, which is why they use this electrolyte in this Concentration are not suitable. The choice of suitable additives So it's not trivial. Striking are here in particular the high internal resistance R50 and R500 for Vinyl trithiocarbonate, the worst additive.

Tabelle 4 Verschlechterung der Elektrolyteigenschaften durch Additivzusatz

Table 4 Deterioration of the electrolyte properties by additive addition

Corresponding In the foregoing, the present invention provides an electrolyte with lithium bis (oxalato) borate solved in a solvent from at least one carbonate and at least one of the in Table 3 or structurally related additive.

Prefers it is the solvent a mixture of a first substituted or unsubstituted (Di) alkyl or alkylene carbonate, a second substituted or unsubstituted (bi) alkyl or alkylene carbonate and at least another solvent from the group of substituted or unsubstituted (di) alkyl carbonates or from the group of substituted or unsubstituted alkyl acetates. Dialkyl carbonate means a dialkyl carbonate or an alkyl carbonate. Specific examples are organic carbonates such as ethylene carbonate, Propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate. By using such a mixture can the conductivity while maintaining an elevated stability be further increased.

In this connection can Saturated substituents or mono- or polyunsaturated, aliphatic or aromatic, straight-chain or branched hydrocarbons optionally containing functional groups such as ether, ester, hydroxy, Carbonyl ~ phosphonyl, sulfonyl, halogen or nitrogen containing groups may include.

Furthermore can In addition, to the aforementioned solvents various other compounds in a solvent according to the present invention Be included in the invention, such. the aforementioned linear and cyclic ethers such as tetrahydrofuran, 2-methyl-tetrahydrofuran and dimethoxyethane.

description of the embodiments

Example 1:

The electrolyte consists of a mixture of 66% by weight of ethylene carbonate and 33% by weight of propylene carbonate, in which 0.5 mol / kg LiBOB are dissolved. 4% by weight of tetraethylene glycol dimethyl ether are added as an additive to the electrolyte. A test cell was cycled with currents of 1C and a discharge depth of 3.0V at a temperature of 60 ° C. The measured data are in 2 played. Applying a drop in capacitance to 50% of the initial value as a life, this increases by the additive addition of 56 to 264 cycles. This corresponds to almost five times the life, so a marked improvement. After 500 cycles, the residual capacity with additive is 38% compared to 20% without additive. The addition of the additive thus causes a nearly twice as large capacity.

The figure shows the Coulomb charging efficiency that is close to 100% (filled triangles, left-hand scale), the falling capacity of the cell (right scale, filled circles) and the converted charge quantities Q ⁺ and Q ^-, (right scale, filled rectangles).

Example 2:

The electrolyte consists of a mixture of 66% by weight of ethylene carbonate and 33% by weight of propylene carbonate, in which 0.5 mol / kg LiBOB are dissolved. 4% by weight of toluene are added as an additive to the electrolyte. A test cell was cycled with currents of 1C and a discharge depth of 3.0V at a temperature of 60 ° C. The measured data are in 3 played. Set a drop in capacity to 50% off value as a service life, this increases by the additive addition of 56 to 141 cycles. This corresponds to an increase in life by a factor of 2.5, so a significant improvement. After 500 cycles, the residual capacity with additive is 29% compared to 20% without additive. The addition of the additive thus causes a 50% higher capacity. Symbols and scales are like in 2 to understand.

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Claims (12)

Electrolyte with lithium bis (oxalato) borate (LiBOB), solved in a solvent from at least one carbonate or ester and addition of at least one Additive. An electrolyte according to claim 1, wherein the solvent is a mixture of a first substituted or unsubstituted (di) alkyl or alkylene carbonate, a second substituted or unsubstituted (di) alkyl or alkylene carbonate and at least one further solvent from the group of substituted or unsubstituted (di ) alkylcarbonates or from the group of substituted or unsubstituted esters and adding at least one additive. An electrolyte according to the preceding claims, wherein the solvent a mixture of propylene carbonate, ethylene carbonate and at least selected a connection from the group dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, γ-butyrolactone, Ethyl butyrate, ethyl propionate and ethyl acetate. This electrolyte is at least one additive from the group methyl chloroformate, methyl haloformate, Ethylene sulfite, vinylene carbonate, silver salts such as silver hexafluorophosphate, Oligoethylene oxides, such as tetraethylene glycol dimethyl ether, Toluene, formic acid methyl ester, ethyl formate, Ameisensäurepropylester, formate, Ameisensäurepentylester, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, pentyl acetate, Propionic acid methyl ester, ethyl propionate, propionate, propionate, pentyl propionate, methyl butyrate, butyrate, Buttersäurepropylester, Buttersäurebutlyester, pentyl butyrate, Pentansäuremethylester, pentanoate, Pentansäurepropylester, Pentansäurebutylester, Pentansäurepentylester, Acetonitrile, 1,2-dimethoxyethane, dimethyl sulfoxide, 1,3-dioxolane, Sulfolane, tetrahydrofuran, 1,4-dioxane, Pentaethylene glycol dimethyl ether, tetraethylene monoethyl ether, substituted or unsubstituted cyclohexylbenzene. The amount of added additive or added additives is 0.01 to 10% by mass of the electrolyte. Electrolyte according to one of the preceding claims, characterized characterized in that the concentration of lithium bis (oxalato) borate at least 0.08 moles per kilogram of solvent or solvent mixture. Electrolyte according to claim 3 or 4, characterized that the solvent at least propylene carbonate, ethylene carbonate and ethyl acetate. Electrolyte according to claim 3 or 4, characterized that the solvent at least propylene carbonate, ethylene carbonate and dimethyl carbonate includes. Electrolyte according to claim 3 or 4, characterized that the solvent at least propylene carbonate, ethylene carbonate and dimethyl carbonate and ethyl acetate. Electrolyte according to claim 5 or 7, characterized the proportion of ethyl acetate in the solvent is at least 5% by mass is. Electrolyte according to Claim 7, characterized that the sum of the proportions of ethyl acetate and dimethyl carbonate on the solvent at least 6 percent by mass. Electrolyte according to Claim 7, characterized that the sum of the proportions of ethyl acetate and dimethyl carbonate on the solvent at least 6 and at most 95 percent by mass. Electrolyte according to one of Claims 3 to 10, characterized the proportion of ethylene carbonate is at least 3 percent by mass. Electrolyte according to claim 11, characterized in that the proportion of ethylene carbonate in the solvent is at least 3 and at the most 70 percent by mass.