FR2494502A1 - Rechargeable lithium battery electrolyte - contains di:ethyl ether, co-solvent and lithium salt - Google Patents

Abstract

The cell comprises an Li-contg. electrode and an electrolyte of diethyl ether, a cosolvent and an Li salt. The diethyl ether is present as greater than or equal to 70 (pref. 80, esp. greater than or is 90)vol.%, and the cosolvent is greater than or equal to 5 vol.%, the amts. selected so that the increase in conductivity, compared with pure diethyl ether, is proportionally greater than or equal to any decrease in cycling efficiency, compared with pure diethyl ether. The cosolvent is pref. present in an amt. that does not decrease cycling efficiency compared with pure diethyl ether and esp. increases it, and the Li salt is pref. present in an amt. preventing sepn. of the electrolyte into layers. Pref. electrolytes provide cycling efficiencies over 97% with large numbers of 100% efficient cycles, e.g. over 200. They exhibit low Li reactivity and are readily available in pure form.

Description

Non-aqueous electrolytes for electrochemical cells using electrodes containing lithium.

 The present invention relates to nonaqueous electrolytes for electrochemical cells using electrodes containing lithium.

 In a secondary element using a lithium anode, it is desirable to use an electrolyte system making it possible to obtain good charge-discharge performance, good conductivity and a reasonable price. The number of times a lithium cell can be recharged and the efficiency of each recharge measures the charge-discharge performance. The charge-discharge performance of a lithium electrode is essentially linked to the resistance capacity of the solvent of the electrolyte to reduction by lithium which is a powerful reducing agent. When a reduction occurs, reaction products form on the surface of the lithium electrode which prevent regular subsequent coating during the recharging cycle, therefore dendritic growth and therefore poor lithium morphology. A poor lithium morphology can also result from a poor penetration power of the electrolyte or specific adsorption effects of the electrolyte. Poor lithium morphology can cause: 1) electrical isolation of part of the lithium coating preventing it from being stripped off during discharge, 2) short circuits between the electrodes due to dendritic growth and 3) a speed high chemical reduction due to the increase in the electrode surface.

Other important factors are that the solvent can dissolve the amount of salt necessary to obtain an appropriate conductivity as well as the conductivity characteristics of the particular salt and solvent chosen.

 Many primary and secondary elements of the prior art use a mixture of two or more solvents in an electrolyte system. Mixed solvents are described in US Pat. Nos. 3,778,310, 3,468,716r 3,567,515.3578500.3 877,983,4060674 and 3,185,590 and in GB Patent Nos. 1,343-853.

Although in several of the aforementioned patents ethyl ether (or diethyl ether; hereinafter ODE) is mentioned as a possible component of the solvent and is even used as sole solvent in an example of the patent
US na 3,185,590, the very low conductivity of ethyl ether poses problems. Although to improve the conductivity we can mix ethyl ether with a more conductive cosolvent, in general, in a secondary element, we can expect a corresponding decrease in charge-discharge performance because more conductive co-solvents react more with lithium.

The aforementioned patents mention numerous solvents and combinations of solvents which can be used and in particular: unsaturated heterocycles and saturated ethers such as ethyl ether and tetrahydrofuran (US Pat. Nos. 3,778,310 and GB Nos. 1,343,853); pentacyclic esters, aliphatic ethers such as ethyl ether and tetrahydrofuran, cyclic ketones and aliphatic nitriles (patent
US No. 3,468,716); sulfur dioxide, trialkyl borates, boric acid esters, tetraalkyl sillcwtes, nitroalkanes, lactams, acetals, ortho-esters, mono-ethers such as ethyl ether, ethers cyclics such as tetrahydrofuran, dialkyl sulfates and alkyl sulfonates (US Pat. Nos. 3,567,515 and 3,578,500); tetrahydrofuran, dimethyl carbonate, propylene carbonate, dimethoxy-1,2 ethane, dimethylformamide, trimethyl carbonate, ethyl N, N-dimethylcarbamate, dimethyl ether of diethylene glycol, cyclic ethers such as than 1,3-dioxolane, 4-methyl-1,3-dioxolane, ethylene oxide, propylene oxide, butylene oxide, dioxane, and tetrahydrofuran and aliphatic ethers such as 1,2-dimethoxy ethane, dimethyl ether of diethylene glycol and diethyl ether of diethylene glycol (US Patent No. 3,877,983); ethers, esters, sulfones, sulfites, nitrites and nitrates (US Patent No. 4,060,674); and ethers such as ethyl ether, amines, amides, sulfoxides and nitriles (US Patent No. 3,185,590).

 The Applicant has found that, surprisingly, when degermed volumes of certain cosolvents are mixed with ethyl ether to increase the conductivity, the charge-discharge performance instead of being reduced is increased compared to that which 'is obtained with pure ethyl ether. Certain high volumes of cosolvent only produce a slight decrease in charge-discharge performance proportionally less than the gain in conductivity. The invention relates to an electrolyte which is relatively inexpensive and which has good conductivity, good charge-discharge performance, low reactivity with lithium and which can easily be stored in pure form.

 The invention relates more particularly to an electrolyte comprising ethyl ether, a cosolvent and a lithium salt such as Lias6. According to preferred embodiments, the ethyl ether constitutes at least 70% of the volume of the solvent and the cosolvent constitutes at least 5% of the volume of the solvent.

The invention will be better understood on reading the following description made with reference to the appended drawings in which
Figure 1 is a schematic representation of a rechargeable battery that can use the electrolyte of the invention
Figure 2 is a graph of the relationship between the average yield per cycle and the percentage of ethyl ether in four binary electrolytes
Figure 3 is a graph of the relationship between the average yield per cycle and the concentration of
LiAsF6 in the electrolyte composition LiAsF6 / 90 ODE 10 THF, (THF being the abbreviation of tetrahydrofuralLne)
Figure 4 is a graph of the relationship between the conductivity and the percentage of ethyl ether in the binary electrolyte LiAsF6 1.5 M / ODE :: TBF
Figure 5 is a graph of the relationship between the average yield per cycle and the proportions of the solvent components in c # inq saturated ternary electrolytes; and
FIG. 6 is a comparative graph of the average yield per cycle of several binary electrolytes.

 Embodiments of the invention will now be described.

 FIG. 1 is a schematic representation of an example of a rechargeable electrochemical element in which the electrolyte of the invention can be used. The illustrated battery consists of a sealed case 24 in which are placed several flat negative electrodes 26 consisting essentially of practically pure lithium deposited on nickel substrates, several flat positive electrodes 28 consisting of titanium disulfide deposited on deployed nickel and several separators 30 in the form of flat porous sheets made of a microporous polypropylene film (such as that marketed under the Celgard brand) or glass fibers.

The positive and negative electrodes are alternated and a separator is placed between each pair of positive and negative electrodes. The positive electrodes of several elements are electrically coupled in parallel to a positive terminal 32 by suitable electrical conductors (not shown) according to conventional battery manufacturing technology and the negative electrodes of several elements are similarly connected to a battery terminal negative 34. The electrolyte of the invention surrounds the electrodes and the separators and fills the interstices of the separators;
The advantages of the invention appear better on examining the results of the tests for determining the charge-discharge performance and the conductivity.

The tests are designed to reproduce the charging and discharging characteristics of the lithium secondary electrode of a battery used in practice. A key experiment to evaluate the stability of the electrolyte consists in carrying out coating-skinning cycles of delithium with a substrate of lithium and not of nickel in a semi-cell. To do this, a known quantity of lithium is deposited on a nickel electrode, then successively a lower charge of lithium is removed and deposited. The amount of excess lithium partly determines the number of 100% cycles that are obtained. For example, in a typical lithium on lithium type experiment, 4.5 C / cm 2 of lithium are deposited on a nickel electrode; 1.1 C / cm2 are then removed to leave 3.4 C / cm2 of lithium.

In the following coating and skinning cycles, 1.1 C / cm2 is used. If each cycle had a yield of 100%, the cyclic functioning of the cell would continue indefinitely with a reserve of lithium corresponding to 3.4 C / cm2, (the yield being
Stripped Q / coated Q). Of course, each stripping cycle has a yield of less than 100% so that each stripping starts the reserve of lithium while maintaining a cycle apparently at 100% until the nickel substrate is reached. At this time the excess lithium is used up and the average yield per cycle E can be calculated.

where Q5 is the stripped lithium charge, Qex is the excess amount of lithium and n is the number of cycles at 100%. First of all, the value of E rises rapidly when n increases; beyond 100 cycles at 100%, a large increase in n results in only a small increase in E.

 All the values of E in the present description have been calculated from experiments in which the current density was fixed at 5 mA / cm 2. If current densities below this value are used, even higher values of E can be expected, according to studies carried out with electrolytes consisting of tetrahydrofuran and 2-methyl tetrahydrofuran / LiAsF6. As some of the figures also show, the values of E for a given solvent mixture also vary with the salt concentration.

 FIG. 2 is a graph of the relationship between the average yield per # cycle (E% on the ordinate) and the -concentration in ethyl ether (in percentages by volume, on the abscissa) of four binary electrolytes in which the cosolvent is a saturated ether . In the four mixtures, the salt is LiÂsF6 (at different concentrations). The graph shows that the volume ratio ODE: co-solvent for which the maximum value of the average yield per cycle is obtained depends on the saturated co-solvent used. The preferred ratios (for the LiAsF6 concentrations indicated) are: 90 ODE: 10 dioxolane (2.5 M); 95 ODE: 5 TBF (1.5 M); 90 ODE: 10 TKF (2.5 M) and 80 ODE: 20 dimethoxyethane (DME) (2.0 M).

 Figure 2 shows that as expected, when the proportion of ethyl ether drops below about 70%, the average yield per cycle decreases rapidly. The unexpected phenomenon illustrated by the curves is that the use of small amounts of any of the saturated cosolvents does not reduce the average yield per cycle but causes a higher yield than that obtained with pure ethyl ether.

Figure 2 shows that any proportion of THF, DME or dioxolane Up to about 20% leads to an electrolyte whose yield is higher than that of ethyl ether at 100%. The open chain structure of ethyl ether seems to be the cause of its low reactivity towards lithium and therefore of the high value of its average yield per cycle. The mechanism by which the addition of THF, DME or dioxolane, which are all compounds with relatively reactive structures, increases the average yield per cycle, is not known.

 An important advantage of mixtures with a high content of ethyl ether is their ability to dissolve large amounts of salt, so that these mixtures are useful in various situations.

Figure 3 where the average yield per cycle (%, on the ordinate) is represented as a function of the concentration of LiAsF6 (moles / l on the abscissa), shows that for the mixture 90 ODE: 10 TKF, the maximum yield taler is obtained medium per cycle with a concentration of
LiAsF6 of 2.5 M. This mixture, as shown in Figure 6 where the average yield per cycle (%, on the ordinate) is represented as a function of the number of cycles (on the abscissa), also makes it possible to obtain more than 200 cycles 100%.

 Figure 4 shows that the conductivity of the electrolyte (Ksp x 10 2 (Ohm.cm on the ordinate) staccrott when the THF content increases (the abscissa scale represents the concentration of ethyl ether in percentages by volume). Therefore any proportion of TKF up to approximately 20 makes it possible to obtain both an average yield per higher cycle (see FIG. 2) and a higher conductivity compared to 100% ethyl ether.

However in certain cases, a proportion of THF greater than 20% may be desirable if the conductivity is very high and if the user can accept losing on the average yield per cycle in order to obtain a higher conductivity. An important advantage of the invention is that in certain ranges of proportions, even when a decrease in yield is observed (that is to say an increase in the difference with respect to the yield of 100%) it is so surprising proportionally less than the gain in conductivity. the same considerations apply to the choice of the desired proportion of DME, dioxolane or any other co-solvent used in the invention.

 Figure 5 shows the relationship between the average yield per cycle S (, on the ordinate) and the number of cycles on the abscissa for various ODE: DME: THF mixtures. The maximum yield is obtained with 2.25 M LiAsF6 in a mixture 86 ODE: 5 DME: 9 THF; however, this mixture has a lower yield than that of the best binary mixture, Lias, 2.5 M in 90 ODE: 10 TBF.

In addition to the above co-solvents, other saturated compounds have been found useful. They consist of the acyclic monoether which is dimethyl ether which leads to an average yield per cycle of 97.6% in mixture with 90% of ethyl ether; and the cyclic diether which is ethyltetrahydrofurfuryl ether which leads to an average yield per cycle of 97.6% in mixture with 90% ODE. there are also some unsaturated cosolvents which give good results. These are three unsaturated cyclic mono-ethers, 2-methyl furan which leads to an average yield per cycle of 98.5%, furan which leads to an average yield per cycle 98.0 ffi and 2,5-dimethyl furan which leads to an average yield per cycle of 97.7 ffi; and two unsaturated nitrogen heterocycles, the
N-methylpyrrole, which leads to an average yield per cycle of 98.2% and 3,5-dimethyl isoxazole which leads to an average yield per cycle of 98.0%; in all cases mixed with 90% ethyl ether and with a LiAsF6 concentration of 2.5 M.

The lithium salts which are preferred are those which correspond to the formula liXY6 where X represents As, P, Sb or
Bi and Y represents a halogen. A preferred example is Lias6. Other lithium salts may also be useful.

 The table below illustrates an unexpected phenomenon of stratification which is observed in an electrolyte in which the solvent mixture consists of 90 ODE: 10 DME (by volume). This phenomenon is an additional factor which must be taken into account when choosing the co-solvents, the co-solvent proportions and the salt concentrations.

Homogeneity of the solution as a function of the concentration of LiAsF6 in an electrolyte based on 90 ODE: 10 DME at 250C.

LiAsF6 (M; Physical aspect
1.50 two immiscible layers
1.75 two immiscible layers
2.00 homogeneous solution
2.25 homogeneous solution
It can therefore be seen that the electrolyte separates into two phases for a LiAsF6 concentration of less than 1.9 M while at a higher salt concentration, this undesirable phenomenon does not occur. Other binary or ternary electrolyte mixtures exhibit this phenomenon for salt concentrations below certain threshold values; two examples are shown below.

Minimum concentration of LiAsF to obtain a 6 Electrolyte mixture homogeneous solution at 250C (M)
1.85 75 ODE: 15 DME: 10 THF
0.70 90 ODE: 10 dimethyl-3,5
isoxazole
For the three mixtures studied in which a stratification is observed, the threshold values of the concentration of LiAsF6 increase when the proportion of ethyl ether decreases. Contrary to expectations, the addition of cosolvents such as TKF or 2-methyl furan does not prevent stratification. The critical parameter therefore seems to be a sufficient concentration of LiAsF6 to avoid phase separation.

 For unknown reasons, the stratification phenomenon does not appear regardless of the Lias6 concentration in the ODE: TBF, ODE: 2-methyl pyrrole or ODE: 2-methyl furan mixtures.

 As goes without saying and as it already follows from the foregoing, the invention is in no way limited to that of its modes of application, no more than to those of the embodiments of its various parts, having have been more particularly envisaged; on the contrary, it embraces all its variants.

Claims (23)

 1. Rechargeable electrochemical cell comprising an electrode containing lithium and an electrolyte, the electrolyte comprising ethyl ether, a co-solvent and a lithium salt, characterized in that the ethyl ether and the co-solvent are present in respective proportions greater than or equal to 70% and 5% by volume, these quantities being chosen so as to produce an increase in conductivity relative to that obtained with pure ethyl ether, proportionally equal to or greater than any decrease in charge performance -discharge compared to those obtained with pure ethyl ether.  2. Element according to claim 1, characterized in that the cosolvent is present in an amount which does not reduce the charge-discharge performance compared to that obtained with pure ethyl ether.  3. Element according to claim 2, characterized in that the cosolvent is present in an amount increasing the charge-discharge performance compared to those obtained with pure ethyl ether.  4. Element according to one of claims 1 to 3, characterized in that the cosolvent comprises a saturated compound.  5. Element according to claim 4, characterized in that the saturated compound comprises a saturated ether.  6. Element according to claim 5, characterized in that the saturated ether is tetrahydrofuran.  7. Element according to claim 5, characterized in that the saturated ether is dimethoxyethane.  8. Element according to claim 5, characterized in that the saturated ether is dioxolane-1,3.  9. Element according to claim 5, characterized in that the saturated ether is dimethyl ether.  10. Element according to claim 5, characterized in that the saturated ether is ethyltetrahydrofurfuryl ether.  11. Element according to one of claims 1 to 3, characterized in that the cosolvent comprises an unsaturated compound.  12. Element according to claim 11, characterized in that the co-solvent comprises furan.  13. Element according to claim 11, characterized in that the unsaturated compound is 2-methyl furan.  14. Element according to claim 11, characterized in that the unsaturated compound is 2,5-dimethyl furan.  15. Element according to claim 11, - characterized in that the unsaturated compound is N-methylpyrrole.  16. Element according to claim 11, characterized in that the unsaturated compound is 3,5-dimethyl. isoxazole.  17. Element according to one of claims 1 to 16, characterized in that the electrolyte additionally contains a second co-solvent.  18. Element according to claim 17, characterized in that the second cosolvent is a saturated ether.  19. Element according to one of claims 1 to 19, characterized in that the lithium salt is present in a sufficient concentration to avoid separation into layers of the electrolyte.  20. Element according to one of claims 1 to 19, characterized in that the ethyl ether is present in a proportion equal to or greater than 80% by volume.  21. Element according to one of claims 1 to 20, characterized in that the ethyl ether is present in a proportion equal to or greater than 90% by volume.  22. Element according to claim 6, characterized in that the ethyl ether is present in a proportion equal to or greater than 90% and the tetrahydrofuran is present in an amount equal to or less than 10%  23. Element according to one of claims 1 to 22, characterized in that the lithium salt is LiAsF6.