Electrolyte for rechargeable electrochemical battery cells

Abstract

An electrolyte for a rechargeable non-aqueous electrochemical battery cell having a negative electrode and a positive electrode is described. The electrolyte contains sulfur dioxide and comprises a conducting salt, and a battery cell therefore has almost no capacitance loss over the cycles. Furthermore, a corresponding battery cell and a method for producing the electrolyte are described.

Claims

1. A liquid sulfur-dioxide-containing electrolyte for a rechargeable non-aqueous electrochemical battery cell comprising: at least a first conducting salt with the stoichiometric formula K(ASX.sub.2).sub.p, wherein K denotes a cation from the group of the alkali metals with p=1, the alkaline earth metals with p=2 or the zinc group with p=2, wherein A denotes an element from the third main group of the Periodic Table, S denotes sulfur, X denotes a halogen, and wherein the first conducting salt is dissolved in liquid sulfur dioxide.

2. The electrolyte according to claim 1, wherein a concentration of the first conducting salt in the liquid sulfur dioxide is at least 10.sup.−4 mol/l.

3. The electrolyte according to claim 1, wherein the first conducting salt has the stoichiometric formula LiAlSCl.sub.2.

4. The electrolyte according to claim 1, wherein said electrolyte further comprises a second conducting salt with the stoichiometric formula K(AX.sub.4).sub.p, wherein K, A, X, and p are as defined in claim 1, and wherein the second conducting salt is dissolved in the liquid sulfur dioxide.

5. The electrolyte according to claim 1, wherein said electrolyte comprises a further conducting salt with the stoichiometric formula K(AOX.sub.2).sub.p, wherein K, A, X, and p are as defined in claim 1, and wherein the further conducting salt is dissolved in the liquid sulfur dioxide.

6. The electrolyte according to claim 1, wherein said electrolyte is free of substances with the stoichiometric formula KAX.sub.4, wherein K, A, and X are as defined in claim 1.

7. A rechargeable non-aqueous electrochemical battery cell, comprising a negative and a positive electrode and an electrolyte according to claim 1.

8. The rechargeable non-aqueous electrochemical battery cell, comprising an electrolyte according to claim 7, wherein the positive electrode has a porosity of less than 25%.

9. The rechargeable non-aqueous electrochemical battery cell comprising an electrolyte according to claim 7, wherein the negative electrode has a porosity of less than 25%.

10. The rechargeable non-aqueous electrochemical battery cell comprising an electrolyte according to claim 7, wherein the negative electrode has a porosity of less than 20%.

11. The rechargeable non-aqueous electrochemical battery cell comprising an electrolyte according to claim 7, wherein the negative electrode has a porosity of less than 15%.

12. The rechargeable non-aqueous electrochemical battery cell comprising an electrolyte according to claim 7, wherein the negative electrode has a porosity of less than 12%.

13. The rechargeable non-aqueous electrochemical battery cell comprising an electrolyte according to claim 7, wherein the positive electrode has a porosity of less than 20%.

14. The rechargeable non-aqueous electrochemical battery cell comprising an electrolyte according to claim 7, wherein the positive electrode has a porosity of less than 15%.

15. The rechargeable non-aqueous electrochemical battery cell comprising an electrolyte according to claim 7, wherein the positive electrode has a porosity of less than 12%.

16. A method for the production of a sulfur-dioxide-containing electrolyte for a rechargeable electrochemical battery cell, comprising: at least the production of LiAlSCl.sub.2 according to the reaction equation
Li.sub.2S+LiAlCl.sub.4.fwdarw.LiAlSCl.sub.2+2LiCl, wherein the reaction takes place in liquid sulfur dioxide.

17. The method according to claim 16, wherein the method is carried out at a temperature of 10° C. to 50° C. and under a pressure at which the SO.sub.2 remains liquid at these temperatures.

18. The method according to claim 16, wherein the method is carried out at a temperature of greater than 50° C. and under a pressure at which the SO.sub.2 remains liquid at this temperature.

19. A method for the production of a sulfur-dioxide-containing electrolyte for use in a rechargeable battery cell, comprising: at least the production of LiAlSCl.sub.2 according to the reaction equation
Li.sub.2S+AlCl.sub.3.fwdarw.LiAlSCl.sub.2+LiCl, wherein the reaction takes place in liquid SO.sub.2.

20. The method according to claim 19, wherein the method is carried out at a temperature of 10° C. to 50° C. and under a pressure at which the SO.sub.2 remains liquid at these temperatures.

21. The method according to claim 19, wherein the method is carried out at a temperature of greater than 50° C. and under a pressure at which the SO.sub.2 remains liquid at this temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a general diagram of a rechargeable battery.

DETAILED DESCRIPTION OF THE INVENTION

(2) FIG. 1 shows a general diagram of a rechargeable battery 1 with a housing 2 and at least one battery cell 3, which has a positive electrode 4 and a negative electrode 5. Here, the electrodes 4, 5 are connected via dissipation elements, optionally via electrode terminals commonly used in battery technology, with terminal contacts 7, 8 via which the battery can finally be charged or discharged.

(3) Lithium metal oxides, such as LiCoO.sub.2, LiNiFeCoO.sub.2 or Li.sub.3V.sub.3O.sub.8, or alternatively lithium metal phosphates such as LiFePO.sub.4 are preferably used as active positive materials. The active negative material can preferably be graphite, another type of carbon, lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12, LTO), or silicon (Si).

(4) In the battery 1, a sulfur-dioxide-containing electrolyte is used that comprises at least a first conducting salt with the stoichiometric formula K(ASX.sub.2).sub.p. Here, the abbreviation K denotes a cation from the group of the alkali metals (in particular Li, Na, K, Rb, Cs) or the alkaline earth metals (in particular Be, Mg, Ca, Sr, Ba) or the zinc group (i.e. the twelfth group of the periodic table, in particular Zn, Cd, Hg). If K is selected from the group of the alkali metals, p=1. If K is selected from the group of the alkaline earth metals or the zinc group, p=2. The abbreviation A denotes an element from the third main group of the periodic table, in particular boron, aluminum, gallium, indium, and thallium, and the abbreviation X denotes a halogen, in particular fluorine, chlorine, bromine, or iodine. S denotes sulfur. K preferably denotes Li. Particularly preferably, the first conducting salt has the stoichiometric formula LiAlSCl.sub.2, i.e. the first conducting salt is preferably lithium sulfodichloroaluminate.

(5) The electrolyte according to the invention comprises a first conducting salt dissolved in sulfur dioxide as an “SO.sub.2-containing” electrolyte. Here, this term refers to an electrolyte that does not comprise sulfur dioxide as an additive in a low concentration, but in which the mobility of the ions of the first conducting salt, which is contained in the electrolyte and effects charge transfer, is at least partially ensured by the SO.sub.2.

(6) An advantage of this non-aqueous inorganic SO.sub.2-containing electrolyte is that—in contrast to the organic electrolyte solutions of the lithium-ion cells in common practical use—it is not flammable. The known safety risks of lithium-ion cells are caused in particular by their organic electrolyte solution. When a lithium-ion cell catches fire or even explodes, the organic solvent of the electrolyte solution is the flammable material. An electrolyte according to the invention is preferably substantially free of organic materials, wherein “substantially” is to be understood as meaning that the amount of any organic materials present is so low that they constitute no safety hazard.

(7) It has been found to be favorable if the electrolyte according to the invention is substantially free of substances that corrode, dissolve, or otherwise decompose or damage the desired lithium dithionite layer. The term “substantially free” is to be understood here as meaning that the substance is at most present in such a small amount that it does not decompose/damage the lithium dithionite layer. Examples of such substances that should not be present are oxidants such as chlorine, thionyl chloride, and sulfuryl chloride.

(8) The electrolyte according to the invention comprises the first conducting salt dissolved in SO.sub.2 of the formula K(ASX.sub.2).sub.p, described above in greater detail. Here, SO.sub.2 can be used in the purest form possible, i.e. with the lowest possible content of impurities.

(9) In a preferred embodiment, a concentration of the first conducting salt in SO.sub.2 is at least 10.sup.−4 mol/l, in particular at least 10.sup.−3 mol/l, in particular at least 10.sup.−2 mol/l, in particular at least 10.sup.−2 mol/l, and in particular at least 1 mol/l.

(10) In a preferred embodiment, in addition to the first conducting salt, the sulfur-dioxide-containing electrolyte can further comprise a second conducting salt with the stoichiometric formula K(AX.sub.4).sub.p and/or a further conducting salt with the stoichiometric formula K(AOX.sub.2).sub.p. The second conducting salt and/or the third conducting salt are/is preferably dissolved in SO.sub.2. For the letters K, A, X and p used here as abbreviations, one may insert elements according to the above-described selection criteria. In this case, it is preferable, but not required, to select the same elements for all of the conducting salts used. O denotes oxygen.

(11) In particular, an embodiment is preferred in which the sulfur-dioxide-containing electrolyte is free of substances with the stoichiometric formula KAX.sub.4, in particular free of LiAlCl.sub.4, wherein the letter abbreviations K, A and X again refer to elements according to the above-described element groups. The sulfur-dioxide-containing electrolyte should preferably be free of all substances that fulfill the stoichiometric formula KAX.sub.4 in any combination of the elements described by the above abbreviations. Alternatively, the sulfur-dioxide-containing electrolyte should preferably at least be free of the substance with the stoichiometric formula KAX.sub.4 obtained by inserting the elements selected for the first conducting salt.

(12) However, if a small amount of KAX.sub.4 is present in the sulfur-dioxide-containing electrolyte, this will be consumed after the above-described self-discharge reaction.

(13) Remarkably, if a substance KAX.sub.4, in particular LiAlCl.sub.4, is not present in the electrolyte, a self-discharge according to the above-described equations or an analogous equation does not take place with or in the electrolyte according to the invention, provided that the letters K, A and X do not denote lithium or aluminum or chlorine. In this case, the electrolyte is not consumed. Moreover, no consumption of lithium ions or electric charge takes place, nor do any sparingly soluble or precipitating salts form. Consequently, even for long-term operation of the battery cell, it is sufficient if the cell is initially filled only with a significantly reduced amount of electrolyte compared to conventional battery cells filled with SO.sub.2-containing electrolytes. Compared to conventional battery cells filled with SO.sub.2-containing electrolytes, the amount of electrolyte to be used in production of the cell can be reduced to one-third.

(14) With the new electrolyte, a reaction according to the above-discussed equation (Eq. IV) does not take place. Advantageously, therefore, the additional introduction of an electric charge or an amount of lithium ions in order to compensate for the self-discharge according to the equation (Eq. IV) can be dispensed with. Accordingly, the capacitances of the electrodes can be dimensioned in a more customized manner. Moreover, the amount of the electrolyte with which the battery is to be filled can be correspondingly reduced, as it is no longer consumed and poorly soluble salts such as lithium chloride no longer precipitate, and accordingly no longer plug the pores of the negative electrode and thus increase the internal resistance.

(15) The amount of the ion-conducting electrolyte involved in the charging and discharging processes is thus kept almost completely constant during the entire service life of the battery cell. In particular, a reduction in the amount of electrolyte can be achieved in the preferred embodiment, in which the positive electrode has a porosity of less than 25%, less than 20%, less than 15%, and alternatively, in particular less than 12%. Alternatively or additionally, it is preferred in a further embodiment for the negative electrode to have a porosity of less than 25%, less than 20%, less than 15%, and alternatively, in particular less than 12%.

(16) A reduction in the porosity of an electrode corresponding to the reduced amount of electrolyte can be achieved in particular by proportionally adding to the respective electrode, which is preferably composed of particles of diameter R, particles of the same material but a smaller diameter, in particular R/3. This causes the smaller particles to be placed in the interstices between the larger particles. In addition to the lower porosity, such electrodes can also show higher mechanical stability.

(17) By using the electrolyte described, and by decreasing the porosity from e.g. 30% to 12%, the specific energy and the energy density of the battery cell can be increased from the 65 Wh/kg or 200 Wh/l of a conventional precycled battery cell to over 155 Wh/kg or over 470 Wh/l. The nominal capacity for a prismatic cell with the external dimensions of 130 mm×130 mm×24.5 mm can thus be increased, e.g. from the approximately 22 Ah of a conventional precycled battery cell to over 61 Ah.

(18) The capacity decrease with the number of cycles is sharply reduced by using the electrolyte according to the invention. In this manner, a self-discharge is suppressed to such an extent that it is practically no longer detectable.

(19) A further aspect of the invention relates to a method for the production of a sulfur-dioxide-containing electrolyte according to the invention for use in a battery cell, comprising at least the production of LiAlSCl.sub.2 according to the reaction equation
Li.sub.2S+Li.sup.++AlCl.sub.4.sub.−−>Li.sup.++AlSCl.sub.2.sub.−+2LiCl,  (Eq. V)

(20) wherein the reaction takes place in liquid SO.sub.2 and preferably at a temperature in the range of −20° C. to −5° C., in particular in the range of −15° C. to −7° C., in particular at −10° C. At such a low temperature, and under normal pressure conditions, SO.sub.2 is usually liquid.

(21) The reaction described by the equation (Eq. V) is preferably carried out by addition of fine-grained, in particular anhydrous, Li.sub.2S to LiAlCl.sub.4 dissolved in SO.sub.2, which is preferably stirred during this process. The reaction is exothermic. Preferably, Li.sub.2S is added in a substance amount that is equimolar, i.e. identical to the substance amount of the LiAlCl.sub.4 present. If more Li.sub.2S is added, LiAlS.sub.2 precipitates as a dark sediment. Conversely, if less Li.sub.2S is added, this yields a mixture of LiAlSCl.sub.2 and LiAlCl.sub.4. The ratio of the dissolved LiAlSCl.sub.2 and dissolved LiAlCl.sub.4 can therefore be adjusted by means of the amount of lithium sulfide, Li.sub.2S, added. After completion of the reaction, the LiCl, which precipitates as a white sediment, is preferably filtered off.

(22) A further aspect of the invention relates to a further, alternative method for the production of a sulfur-dioxide-containing electrolyte according to the invention for use in a battery cell, comprising at least the production of LiAlSCl.sub.2 according to the reaction equation
Li.sub.2S+AlCl.sub.3−>Li.sup.++AlSCl.sub.2.sub.−+LiCl,  (Eq. VI)

(23) wherein the reaction takes place in liquid SO.sub.2 and preferably at a temperature in the range of −20° C. to −5° C., in particular in the range of −15° C. to −7° C., in particular at −10° C.

(24) The production of the sulfur-dioxide-containing electrolyte can also take place at temperatures above −10° C. provided that the SO.sub.2 is placed under corresponding pressure, as SO.sub.2 is no longer liquid at temperatures of greater than −10° C. under normal pressure, i.e. at a standard atmospheric pressure of 1013.25 hPa. As a higher temperature increases the reaction rate according to Equation V or Equation VI below, the reaction can take place more rapidly under simultaneous pressurization of the reactants, wherein the pressure must be high enough that the SO.sub.2 remains liquid at the given temperature in question. In this manner, production of the electrolyte can be carried out more rapidly. In particular, production of the electrolyte, i.e. the reaction according to Equation V or VI, can be carried out at temperatures between 0° C. and 50° C. or above and at a respectively corresponding suitably high pressure. In particular, production can thus be carried out at a temperature of 0° C. and a pressure of greater than 1.8 bar, at a temperature of 20° C. and a pressure of greater than 3.6 bar, at a temperature of 40° C. and a pressure of greater than 6 bar, at a temperature of 60° C. and a pressure of greater than 10 bar, or at even higher temperatures and a suitably high pressure. In this case, the temperature conditions and respective pressures given here are to be understood as guideline values, wherein the person having ordinary skill in the art knows that the conditions also depend on further components.

(25) The reaction described by the equation (Eq. VI) is preferably carried out by adding fine-grained, in particularly anhydrous, Li.sub.2S to a suspension of AlCl.sub.3 in liquid SO.sub.2, which is preferably stirred during this process. Preferably, Li.sub.2S is added in a substance amount that at most is equal to the substance amount of the AlCl.sub.3 present, in particular exactly equal to the substance amount of the AlCl.sub.3 present. After completion of the reaction, the LiCl, which precipitates as white sediment, is preferably filtered off.

(26) Instead of filtering off the LiCl, AlCl.sub.3 can also be added. This gives rise to LiAlCl.sub.4 as a reaction product, wherein the ratio of dissolved LiAlSCl.sub.2 to dissolved LiAlCl.sub.4 can be adjusted based on the amount of AlCl.sub.3 added.