Lithium sulfur batteries and electrolytes and sulfur cathodes thereof
Lithium sulfur battery cells that use water as an electrolyte solvent provide significant cost reductions. Electrolytes for the battery cells may include water solvent for maintaining electroactive sulfur species in solution during cell discharge and a sufficient amount of a cycle life-enhancing compound that facilitates charging at the cathode. The combination of these two components enhances one or more of the following cell attributes: energy density, power density and cycle life. For instance, in applications where cost per Watt-Hour (Wh) is paramount, such as grid storage and traction applications, the use of an aqueous electrolyte in combination with inexpensive sulfur as the cathode active material can be a key enabler for the utility and automotive industries, for example, providing a cost effective and compact solution for load leveling, electric vehicles and renewable energy storage. Sulfur cathodes, and methods of fabricating lithium sulfur cells, in particular for loading lithium sulfide into the cathode structures, provide further advantages.
1. An aqueous sulfur battery cell, comprising:
an anode structure comprising a lithium electroactive material;
a cathode comprising a solid electron transfer medium;
an aqueous electrolyte solution comprising an aqueous solvent system and electroactive sulfur species dissolved therein, the aqueous solvent system comprising water solvent for maintaining electroactive sulfur species in solution during cell discharge and a cycle life enhancing compound, that facilitates charging at the cathode;
wherein the aqueous electrolyte solution is in electron transfer contact with the cathode but isolated from direct contact with the lithium electroactive material.
2. The aqueous lithium sulfur battery cell of claim 1 , wherein the water serves as a reactive solvent for maintaining electroactive sulfur species in solution during cell discharge.
3. The aqueous lithium sulfur battery cell of claim 1 , wherein the aqueous electrolyte solution has an active stoichiometric ratio represented by the chemical formula Li 2 S x , with x having a value between 1 and 7.
4. The aqueous lithium sulfur battery cell of claim 3 , wherein the cycle life enhancing compound is present in an amount sufficient to support an aqueous electrolyte solution having an active stoichiometric ratio represented by the chemical formula Li 2 S x , wherein x is greater than 5.
5. The aqueous lithium sulfur battery cell of claim 1 , wherein the cycle life enhancing compound is an organic compound.
6. The aqueous lithium sulfur battery cell of claim 5 , wherein the organic compound is a protic nitrogen containing organic compound.
7. The aqueous lithium sulfur battery cell of claim 5 , wherein the organic compound is a primary amine.
8. The aqueous lithium sulfur battery cell of claim 5 , wherein the organic compound is a secondary amine.
9. The aqueous lithium sulfur battery cell of claim 5 , wherein the organic compound is a tertiary amine.
10. The aqueous lithium sulfur battery cell of claim 5 , wherein the organic compound is represented by the general formula:
wherein R 1 is H or an organic radical (i.e., functional group), R 2 is H or an organic radical, and R 3 is H or an organic radical.
11. The aqueous lithium sulfur battery cell of claim 1 , wherein the cycle life enhancing compound is ammonia.
12. The aqueous lithium sulfur battery cell of claim 11 , wherein the amount of ammonia in the aqueous solvent is greater than 5% (m/m).
13. The aqueous lithium sulfur battery cell of claim 11 , wherein the amount of ammonia in the aqueous solvent is between 5 to 35% (m/m).
14. The aqueous lithium sulfur battery cell of claim 11 , wherein the amount of ammonia in the aqueous solvent is between 14 to 33% (m/m).
15. The aqueous lithium sulfur battery cell of claim 11 , wherein the pH of the electrolyte absent the ammonia is greater than 12.
16. The aqueous lithium sulfur battery cell of claim 1 , wherein the volume percent of water in the aqueous solvent system is at least 10%.
17. The aqueous lithium sulfur battery cell of claim 1 , wherein the cell is a secondary battery.
18. The aqueous lithium sulfur battery cell of claim 5 , wherein the amount of said organic compound dissolved in the electrolyte is sufficient to yield a cell whereby the total capacity is provided by dissolved active sulfur species, and the delivered capacity of said dissolved sulfur species corresponds to a value that is greater than 700 mAh/gram of dissolved sulfur over at least 2 cycles.
19. The aqueous lithium sulfur battery cell of claim 1 , wherein the aqueous solvent system provides a medium for twofold solvolysis of electroactive sulfur species, a firstfold solvolysis taking place during discharge and a secondfold solvolysis taking place during charge, the water serving to solubilze, via hydrolysis, active sulfur species on discharge and the cycle life enhancing compound serving as a reactive solvent for solubilizing electroactive sulfur species on charge.
20. The aqueous lithium sulfur battery cell of claim 19 , wherein the aqueous electrolyte solution has an active stoichiometric ratio of greater than 5.