Localized superconcentrated electrolytes for silicon anodes

1. A system comprising:

an electrolyte consisting essentially of

an active salt, wherein the active salt is a lithium salt,

a nonaqueous solvent, wherein the nonaqueous solvent is (i) a carbonate other than fluoroethylene carbonate (FEC), (ii) a flame retardant compound, or (iii) both (i) and (ii), wherein the active salt is soluble in the nonaqueous solvent;

0-30 wt % FEC; and

a diluent, wherein the diluent is a fluoroalkyl ether, a fluorinated orthoformate, or a combination thereof, wherein the active salt has a solubility in the diluent at least 10 times less than a solubility of the active salt in the nonaqueous solvent; and

an anode comprising silicon.

2. The system of claim 1 , wherein the electrolyte includes from 2-30 wt % FEC.

3. The system of claim 1 , wherein the active salt has a molar concentration in the electrolyte within a range of from 0.5 M to 6 M.

4. The system of claim 1 , wherein the active salt is lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium bis(oxalato)borate (LiBOB), LiPF 6 , LiAsF 6 , LiBF 4 , LiCF 3 SO 3 , LiClO 4 , lithium difluoro oxalato borate anion (LiDFOB), LiI, LiBr, LiCl, LiSCN, LiNO 3 , LiNO 2 , Li 2 SO 4 , or any combination thereof.

5. The system of claim 1 , wherein the flame retardant compound is an organic phosphate, an organic phosphite, an organic phosphonate, an organic phosphoramide, a phosphazene, or any combination thereof.

6. The system of claim 5 , wherein the flame retardant compound is triethyl phosphate, trimethyl phosphate, tributyl phosphate, triphenyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) methyl phosphate; trimethyl phosphite, triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphite; dimethyl methylphosphonate, diethyl ethylphosphonate, diethyl phenylphosphonate, bis(2,2,2-trifluoroethyl) methylphosphonate; hexamethylphosphoramide; hexamethoxyphosphazene, hexafluorophosphazene, or any combination thereof.

7. The system of claim 1 , wherein the carbonate other than FEC is ethylene carbonate (EC) and ethyl carbonate (EMC), or EC and diethyl carbonate (DEC), or EC and dimethyl carbonate (DMC), or EC and any combination of EMC, DEC, DMC, and propylene carbonate (PC).

8. The system of claim 1 , wherein the diluent is bis(2,2,2-trifluoroethyl) ether (BTFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), methoxynonafluorobutane (MOFB), ethoxynonafluorobutane (EOFB), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), tris(hexafluoroisopropyl)orthoformate (THFiPO), tris(2,2-difluoroethyl)orthoformate (TDFEO), bis(2,2,2-trifluoroethyl) methyl orthoformate (BTFEMO), tris(2,2,3,3,3-pentafluoropropyl)orthoformate (TPFPO), tris(2,2,3,3-tetrafluoropropyl)orthoformate (TTPO), or any combination thereof.

9. The system of claim 1 , wherein the diluent is BTFE, TTE, TFEO, or any combination thereof.

10. The system of claim 9 , wherein the electrolyte consists essentially of:

1-3 M LiFSI;

EC-EMC in a ratio of from 4:6 to 2:8 by weight and 0-30 wt % FEC; and

the diluent, wherein a molar ratio of EC-EMC to the diluent is within a range of from 1-3.

11. The system of claim 9 , wherein the electrolyte consists essentially of:

1-3 M LiFSI;

triethyl phosphate (TEPa) and 0-30 wt % FEC; and

the diluent, wherein a molar ratio of TEPa to the diluent is within a range of 2-4.

12. The system of claim 1 , wherein the anode comprises a graphite/silicon composite.

13. The system of claim 12 , wherein the anode further comprises a lithium polyacrylate or polyimide binder.

14. The system of claim 13 , wherein the silicon is carbon-coated nano-silicon.

15. The system of claim 14 , wherein the binder comprises a polyimide.

16. The system of claim 15 , wherein the electrolyte consists essentially of:

1-3 M LiFSI:

triethyl phosphate (TEPa) and 0-30 wt % FEC; and

the diluent, wherein the diluent is BTFE, TTE, TFEO, or any combination thereof, and wherein a molar ratio of TEPa to the diluent is within a range of 2-4.

17. The system of claim 15 , further comprising a cathode, wherein the anode is prelithiated and the system has a capacity retention ≥80% after 100 cycles.

18. The system of claim 13 , wherein the binder comprises lithium polyacrylate, and the electrolyte consists essentially of:

1-3 M LiFSI;

EC-EMC in a ratio of 3:7 by weight and 0-30 wt % FEC; and

the diluent, wherein the diluent is BTFE, TTE, TFEO, or any combination thereof, and wherein a molar ratio of EC-EMC to the diluent is within a range of 1-3.

19. The system of claim 18 , further comprising a cathode, wherein the system has a capacity retention ≥70% after 100 cycles.

20. The system of claim 1 , further comprising a cathode, wherein the cathode comprises Li 1+w Ni x Mn y Co z O 2 (x+y+z+w=1, 0≤w≤0.25), LiNi x Mn y Co z O 2 (x+y+z=1), LiNi 0.8 Co 0.15 Al 0.05 O 2 , LiCoO 2 , LiNi 0.5 Mn 1.5 O 4 spinel, LiMn 2 O 4 , LiFePO 4 , Li 4-x M x Ti 5 O 12 (M=Mg, Al, Ba, Sr, or Ta; 0≤x≤1), MnO 2 , V 2 O 5 , V 6 O 13 , LiV 3 O 8 , LiM C1 x M C2 1-x PO 4 (M C1 or M C2 =Fe, Mn, Ni, Co, Cr, or Ti; 0≤x≤1), Li 3 V 2-x M 1 x (PO 4 ) 3 (M 1 =Cr, Co, Fe, Mg, Y, Ti, Nb, or Ce; 0≤x≤1), LiVPO 4 F, LiM C1 x M C2 1-x O 2 ((M C1 and M C2 independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1), LiM C1 x M C2 y M C3 1-x-y O 2 ((M C1 , M C2 , and M C3 independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1; 0≤y≤1; 0≤x+y≤1), LiMn 2-y X y O 4 (X=Cr, Al, or Fe, 0≤y≤1), LiNi 0.5-y X y Mn 1.5 O 4 (X=Fe, Cr, Zn, Al, Mg, Ga, V, or Cu; 0≤y≤0.5), xLi 2 MnO 3 .(1-X)LiM C1 y M C2 z M C3 1-y-z O 2 (M C1 , M C2 , and M C3 independently are Mn, Ni, Co, Cr, Fe, or mixture thereof; x=0.3-0.5; y≤0.5; z≤0.5), Li 2 M 2 SiO 4 (M 2 =Mn, Fe, or Co), Li 2 M 2 SO 4 (M 2 =Mn, Fe, or Co), LiM 2 SO 4 F (M 2 =Fe, Mn, or Co), Li 2-x (Fe 1-y Mn y )P 2 O 7 (0≤x≤1; 0≤y≤1), Cr 3 O 8 , Cr 2 O 5 , a carbon/sulfur composite, or an air electrode.