Additives for lithium batteries and methods
Lithium tetrafluoro(malonato)phosphate compounds are useful as additives in lithium ion battery applications. The compounds are represented by Formula (I): MPF 4 [—O(C═O)—(CX′X″)—(C═O)O—]; wherein M is Li or Na; each X′ and X″ independently is selected from the group consisting of H, alkyl, fluoro-substituted alkyl, and F; or wherein the X′ and X″ together are —CR 2 —(CR′ 2 ) m —CR″ 2 —; each R, R′ and R″ independently is selected from the group consisting of H, methyl, trifluoromethyl, and F; and m is 0 or 1. These compounds can be prepared in high purity and a high yield by reaction of a metal hexafluorophosphate with a bis-silyl malonate compound. A similar oxalato compound, lithium tetrafluoro(oxalato)phosphate), can be made in the same manner, but using a bis-silyl oxalate in place of the bis-silyl malonate. Advantageously, the compounds can be formed, in situ, in a LiPF 6 -containing electrolyte solution.
1. A salt of Formula (I): MPF 4 [—O(C═O)—(CX′X″)—(C═O)O—]; wherein M is Li or Na; each X′ and X″ independently is selected from the group consisting of H, alkyl, fluoro-substituted alkyl, and F; or wherein the X′ and X″ together are an alkylene group of formula: —CR 2 —(CR′ 2 ) m —CR″ 2 —; each R, R′ and R″ independently is selected from the group consisting of H, methyl, trifluoromethyl, and F; and m is 0 or 1; wherein at least one of X′ and X″ is F.
2. The salt of claim 1 , wherein M is Li.
3. The salt of claim 1 , wherein at least one of X′ and X″ is alkyl.
4. The salt of claim 1 , wherein at least one of X′ and X″ is C 1 to C 4 alkyl.
5. The salt of claim 1 , wherein X′ is F and X″ is methyl.
6. An electrolyte for a lithium ion electrochemical cell, the electrolyte comprising a non-aqueous organic solvent containing a lithium electrolyte salt at a concentration of about 0.5 to about 3 M, and an additive at a concentration of about 0.25 to about 5 percent by weight (wt %); wherein the additive comprises a compound of Formula (I): MPF 4 [—O(C═O)—(CX′X″)—(C═O)O—]; wherein M is Li; each X′ and X″ independently is selected from the group consisting of H, alkyl, fluoro-substituted alkyl, and F; or wherein the X′ and X″ together are an alkylene group of formula: —CR 2 —(CR′ 2 ) m —CR″ 2 —; each R, R′ and R″ independently is selected from the group consisting of H, methyl, trifluoromethyl, and F; and m is 0 or 1; wherein at least one of X′ and X″ is F.
7. The electrolyte of claim 6 , wherein the lithium electrolyte salt comprises 1 to 2 M LiPF 6 .
8. The electrolyte of claim 6 , wherein the non-aqueous solvent comprises at least one organic carbonate solvent.
9. The electrolyte of claim 6 , wherein at least one of X′ and X″ is C 1 to C 4 alkyl.
10. The electrolyte of claim 6 , wherein X is F and X″ is methyl.
11. The electrolyte of claim 6 , wherein the compound of Formula (I) is present in the electrolyte at a concentration in the range of about 0.5 to about 2 wt %.
12. The electrolyte of claim 6 , further comprising a silyl fluoride of formula FSiZ 3 wherein each Z independently is alkyl.
13. A lithium electrochemical cell comprising a cathode, an anode, a lithium-porous separator between the cathode and the anode, and an electrolyte of claim 6 contacting the cathode and the anode.
14. The electrochemical cell of claim 13 , wherein the cathode comprises a lithium nickel-manganese-cobalt oxide compound.
15. The electrochemical cell of claim 13 , wherein the anode comprises graphite.
16. The electrochemical cell of claim 13 , wherein the compound of Formula (I) is present in the electrolyte at a concentration in the range of about 0.5 to about 2 wt %, and wherein the electrolyte salt is LiPF 6 .
17. A battery comprising a plurality of the electrochemical cell of claim 13 electrically connected together.
18. A method of preparing the electrolyte of claim 6 in which the lithium salt is LiPF 6 , comprising the steps of adding a bis-silyl ester compound to an electrolyte comprising the LiPF 6 in the non-aqueous organic solvent, and stirring the resulting mixture at ambient temperature for at least about 24 hours to form the salt of Formula (I), in situ; wherein the bis-silyl ester is a compound of Formula (II): Z 3 Si—O(C═O)—(CX′X″)—(C═O)O—SiZ 3 ; each X′ and X″ independently is selected from the group consisting of H, alkyl, fluoro-substituted alkyl, and F; or wherein the X′ and X″ together are an alkylene group of formula: —CR 2 —(CR′ 2 ) m —CR″ 2 —; each R, R′ and R″ independently is selected from the group consisting of H, methyl, trifluoromethyl, and F; m is 0 or 1; and each Z independently is alkyl.
19. The method of claim 18 , wherein byproduct FSiZ 3 that is formed by the reaction of the LiPF 6 with the compound of Formula (II) is retained in the electrolyte.