Method of improving the charge/discharge cycle life and safety of an alkali metal-sulfur secondary battery

1. A method of improving a cycle-life of a rechargeable alkali metal-sulfur cell, said method comprising implementing an electronically non-conducting anode-protecting layer between an anode active material layer and a cathode active material without using a porous separator in said cell, by depositing a layer of an elastomer onto one primary surface of the anode active material layer to form a protected anode, wherein said anode-protecting layer has a thickness from 1 nm to 100 μm and the elastomer having a fully recoverable tensile elastic strain from 2% to 1,000%, a lithium ion or sodium ion conductivity from 10 −8 S/cm to 5×10 −2 S/cm, and an electronic conductivity less than 10 −4 S/cm when measured at room temperature;

wherein said elastomer contains a material selected from a non-sulfonated or sulfonated version of metallocene-based poly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer, or a combination thereof.

2. The method of claim 1 , wherein said cathode active material layer comprises a sulfur-containing material selected from a sulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphene hybrid, conducting polymer-sulfur hybrid, metal sulfide, sulfur compound, or a combination thereof.

3. The method of claim 2 , wherein said sulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphene hybrid, or conducting polymer-sulfur hybrid is a mixture, blend, composite, chemically or physically bonded entity of sulfur or sulfide with a carbon, graphite, graphene, or conducting polymer material.

4. The method of claim 1 , wherein said cell contains a non-solid-state electrolyte in ionic contact with said anode active material layer and said cathode active material layer.

5. The method of claim 4 , wherein said non-solid-state electrolyte is selected from an organic liquid electrolyte, ionic liquid electrolyte, polymer gel electrolyte, quasi-solid electrolyte having a lithium salt dissolved in an organic or ionic liquid with a lithium salt concentration higher than 2.0 M, or a combination thereof.

6. The method of claim 1 , wherein said anode active material layer, said anode-protecting layer, and said cathode active material layer are laminated together in such manner that the battery cell is under a compressive stress or strain when the cell is made.

7. The method of claim 2 , wherein said sulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphene hybrid, or conducting polymer-sulfur hybrid is a mixture, blend, composite, chemically or physically bonded entity of sulfur or sulfide with a carbon, graphite, graphene, or conducting polymer material.

8. The method of claim 2 , wherein said graphene comprises single-layer graphene or few-layer graphene, wherein said few-layer graphene is defined as a graphene platelet formed of 10 or less than 10 graphene planes.

9. The method of claim 2 , wherein said metal sulfide contains M x S y , wherein x is an integer from 1 to 3 and y is an integer from 1 to 10, and M is a metal element selected from an alkali metal, an alkaline metal selected from Mg or Ca, a transition metal, a metal from groups 13 to 17 of the periodic table, or a combination thereof.

10. The method of claim 9 , wherein said metal element M is selected from Li, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al.

11. The method of claim 2 , wherein said carbon or graphite material in said cathode active material layer is selected from mesophase pitch, mesophase carbon, mesocarbon microbead (MCMB), coke particle, expanded graphite flake, artificial graphite particle, natural graphite particle, highly oriented pyrolytic graphite, soft carbon particle, hard carbon particle, carbon nanotube, carbon nanofiber, carbon fiber, graphite nanofiber, graphite fiber, carbonized polymer fiber, an activated version thereof, activated carbon, carbon black, acetylene black, or a combination thereof.

12. The method of claim 2 , wherein said conducting polymer-sulfur hybrid contains an intrinsically conductive polymer selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof.

13. The method of claim 1 , wherein said elastomer further contains from 0.1% to 50% by weight of a lithium ion-conducting additive or sodium ion-conducting additive dispersed therein.

14. The method of claim 13 , wherein said lithium ion-conducting additive is selected from Li 2 CO 3 , Li 2 O, Li 2 C 2 O 4 , LiOH, LiX, ROCO 2 Li, HCOLi, ROLi, (ROCO 2 Li) 2 , (CH 2 OCO 2 Li) 2 , Li 2 S, Li x SO y , or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.

15. The method of claim 13 , wherein said lithium ion-conducting additive is dispersed in said elastomer and is selected from lithium perchlorate (LiClO 4 ), lithium hexafluorophosphate (LiPF 6 ), lithium borofluoride (LiBF 4 ), lithium hexafluoroarsenide (LiAsF 6 ), lithium trifluoro-methanesulfonate (LiCF 3 SO 3 ), bis-trifluoromethyl sulfonylimide lithium (LiN(CF 3 SO 2 ) 2 ), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF 2 C 2 O 4 ), lithium nitrate (LiNO 3 ), Li-fluoroalkyl-phosphate (LiPF 3 (CF 2 CF 3 ) 3 ), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

16. The method of claim 13 , wherein said lithium ion-conducting additive is selected from poly(ethylene oxide) (PEO), polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, or a combination thereof.