Redox mediators for metal-sulfur batteries

1. An energy storage device comprising:

an anode;

a cathode comprising:

a lithium sulfide M x S y , wherein M is lithium, subscript x is from 0 to 2 and y is from 1 to 8,

a dissolved redox mediator, wherein the dissolved redox mediator is a benzoperyleneimide (BPI) of Formula II:

wherein

each R 1 is independently selected from the group consisting of halogen, C 1-20 alkyl, C 1-20 haloalkyl, C 1-20 alkoxy, C 1-20 heteroalkyl, C 3-20 carbocycle, C 3-20 heterocycle, C 6-20 aryl, C 5-20 heteroaryl, —N(R 3 )(R 4 ), —OR 3 , —C(O)R 3 , —C(O)OR 3 , —OC(O)R 3 , —C(O)N(R 3 )(R 4 ), —N(R 3 )C(O)R 4 , —N(R 3 )C(O)N(R 4 )(R 5 ), —OC(O)N(R 4 )(R 5 ), —N(R 3 )C(O)OR 4 , —SR 3 , —S(O)R 3 , —S(O) 2 R 3 , —N 3 , —B(OR 3 ) 2 , and —SeR 3 ,

alternatively, two R 1 groups on adjacent ring atoms can be combined to form —O(CH 2 CH 2 ) m O—, wherein subscript m is an integer from 3 to 10,

each R 3 , R 4 and R 5 is independently selected from the group consisting of H and C 1-20 alkyl, and

subscript n is from 1 to 5,

the BPI having a redox potential suitable for reducing or oxidizing lithium sulfide M x S y or reducing elemental sulfur, and

an electrolyte;

a membrane separator between the anode and the cathode; and

a current collector in electrical contact with the anode and cathode.

2. The energy storage device of claim 1 , wherein the anode comprises lithium.

3. The energy storage device of claim 1 , wherein the lithium sulfide comprises at least one of Li 2 S 8 and Li 2 S 6 .

4. The energy storage device of claim 1 , wherein the electrolyte comprises a metal salt.

5. The energy storage device of claim 4 , wherein

the cation of the metal salt is selected from the group consisting of lithium and sodium; and

the anion of the metal salt is selected from the group consisting of bis(trifluoromethyl)sulfonimide, trifluoromethylsulfonate, fluorosulfonimide, perchlorate, tetrafluoroborate, hexafluorophosphate, nitrate, fluoride, chloride, bromide, and iodide.

6. The energy storage device of claim 1 , wherein

the electrolyte comprises at least one of diglyme, PGMEA, dimethoxyethane, triglyme, tetraglyme, dioxolane, THF, propylene carbonate, dimethylcarbonate, ethylene carbonate, ethyl methyl sulfone (EMS), propyl methyl sulfone (PMS), water, poly(ethylene oxide) and copolymers thereof, dimethylsulfoxide, N-methylpyrrolidinone, and acetonitrile.

7. The energy storage device of claim 1 , wherein the cathode further comprises a conductive additive.

8. The energy storage device of claim 7 , wherein the conductive additive comprises carbon.

9. The energy storage device of claim 1 , wherein the current collector comprises at least one of carbon cloth, carbon felt, carbon paper, carbon particles, carbon nanomaterial, metal chalcogenide, metal, and metal oxide.

10. The energy storage device of claim 1 , wherein:

the anode comprises lithium;

the lithium sulfide is Li 2 S 8 ,

the BPI has the structure:

and

the electrolyte comprises diglyme, lithium bis(trifluoromethyl)sulfonamide, and lithium nitrate.

11. An electrode composition, comprising:

a lithium sulfide M x S y , wherein M is metal lithium, subscript x is from 0 to 2 and y is from 1 to 8;

a redox mediator, wherein the redox mediator is a benzoperyleneimide (BPI) of Formula II:

wherein

each R 1 is independently selected from the group consisting of halogen, C 1-20 alkyl, C 1-20 haloalkyl, C 1-20 alkoxy, C 1-20 heteroalkyl, C 3-20 carbocycle, C 3-20 heterocycle, C 6-20 aryl, C 5-20 heteroaryl, —N(R 3 )(R 4 ), —OR 3 , —C(O)R 3 , —C(O)OR 3 , —OC(O)R 3 , —C(O)N(R 3 )(R 4 ), —N(R 3 )C(O)R 4 , —N(R 3 )C(O)N(R 4 )(R 5 ), —OC(O)N(R 4 )(R 5 ), —N(R 3 )C(O)OR 4 , —SR 3 , —S(O)R 3 , —S(O) 2 R 3 , —N 3 , —B(OR 3 ) 2 , and —SeR 3 ,

alternatively, two R 1 groups on adjacent ring atoms can be combined to form —O(CH 2 CH 2 ) m O—, wherein subscript m is an integer from 3 to 10, each R 3 , R 4 and R 5 is independently selected from the group consisting of H and C 1-20 alkyl, and

subscript n is from 1 to 5,

the BPI having a redox potential suitable for reducing or oxidizing M x S y ; and

an electrolyte.