ϵ-VOPO
The epsilon polymorph of vanadyl phosphate, ε-VOPO 4 , made from the solvothermally synthesized H 2 VOPO 4 , is a high density cathode material for lithium-ion batteries optimized to reversibly intercalate two Li-ions to reach the full theoretical capacity at least 50 cycles with a coulombic efficiency of 98%. This material adopts a stable 3D tunnel structure and can extract two Li-ions per vanadium ion, giving a theoretical capacity of 305 mAh/g, with an upper charge/discharge plateau at around 4.0 V, and one lower at around 2.5 V.
1. A method of making a lithium ion battery cathode, comprising:
forming ε-VOPO 4 particles having a diameter of 100-200 nm;
coating the ε-VOPO 4 particles with conductive carbon, to form coated ε-VOPO 4 particles;
adding a binder to the coated ε-VOPO 4 , particles to form a mixture;
depositing the mixture on a current collector,
wherein the deposited mixture has a capacity of at least 275 mAh per gram of Li 2 VOPO 4 at a discharge rate of C/20.
2. The method according to claim 1 , wherein the ε-VOPO 4 particles are solvothermally generated.
3. The method according to claim 1 , wherein the ε-VOPO 4 particles are hydrothermally generated.
4. The method according to claim 1 , wherein said coating comprises mixing the ε-VOPO 4 particles with graphene nanoplatelets.
5. The method according to claim 4 , wherein the graphene nanoplatelets have a surface area of at least 100 m 2 /g.
6. The method according to claim 1 , wherein the binder comprises polyvinylidene fluoride.
7. The method according to claim 1 , wherein the ε-VOPO 4 particles are coated with carbon nanotubes.
8. The method according to claim 1 , wherein the coated ε-VOPO 4 particles have a coating thickness of 10 nm.
9. The method according to claim 1 , wherein the ε-VOPO 4 particles of the lithium ion battery cathode comprises a vanadium which is adapted to undergo a change in oxidation state of two between a charged state filled with intercalated lithium ions and a discharged state depleted of intercalated lithium ions.
10. The method according to claim 9 , wherein the lithium ion battery cathode has a current-voltage profile which displays voltage plateaus on discharge at a discharge rate of C/20, at about 2.1 V, 2.25 V, 2.5 V, and 3.9 V.
11. The method according to claim 10 , wherein the ε-VOPO 4 particles have a capacity of at least 305 mAh per gram of Li 2 VOPO 4 at a discharge rate of C/20.
12. The method according to claim 10 , wherein the lithium ion battery cathode has a discharge capacity of at least 90% of a theoretical value for the discharge capacity of the ε-VOPO 4 particles.
13. The method according to claim 9 , wherein the ε-VOPO 4 particles have a capacity of at least 290 mAh per gram of Li 2 VOPO 4 at a discharge rate of C/20.
14. The method according to claim 1 , wherein the ε-VOPO 4 particles have an energy capacity of at least 850 mWh/g.
15. The method according to claim 1 , wherein the conductive carbon comprises graphene particles, and the mixture comprises between 5% and 15% by weight of the graphene particles.
16. The method according to claim 1 , wherein the mixture comprises at least 75% by weight ε-VOPO 4 particles, at least 5% by weight graphene nanoplatelets, and at least 5% by weight of a binder.
17. The method according to claim 1 , wherein the lithium ion battery cathode has at least two states, comprising:
a first state in which at least 80 mol % of vanadium of the ε-VOPO 4 particles is oxidized in a first oxidation state and associated with two lithium ions per vanadium, and
a second state in which at least 80 mol % of the ε-VOPO 4 particles is oxidized in a second oxidation state which differs by two from the first oxidation state.
18. The method according to claim 1 , further comprising forming a lithium ion battery comprising:
the lithium ion battery cathode;
a lithium or lithium ion anode;
an electrolyte adapted to operate at a battery potential of at least 4.5 V; and
a supporting lithium salt.
19. A method of making a lithium ion battery cathode, comprising:
combining ε-VOPO 4 , particles having a diameter of 100-200 nm coated with electrically conductive carbon particles, and a binder to form a mixture; and
coating a current collector with a slurry of the mixture,
wherein the ε-VOPO 4 has a dual lithium ion exchange characteristic, having a capacity of about 125 mAh per gram of Li 2 VOPO 4 at a discharge rate of C/20 while maintaining a voltage exceeding 3.7 V and a capacity of at least 275 mAh per gram of Li 2 VOPO 4 at a discharge rate of C/20 while maintaining a voltage exceeding 1.6 V.
20. A lithium ion battery cathode, comprising a mixture of ε-VOPO 4 particles having a size of 100-200 nm, coated with electrically conductive graphene, and a binder, on a current collector, having a capacity of at least 275 mAh per gram of Li 2 VOPO 4 at a discharge rate of C/20.