KVOPO

1. An electrode comprising:

NaVOPO 4 having orthorhombic crystalline symmetry and space group Pna2 1 , as an active intercalation host material,

wherein the electrode is capable of electrochemical insertion and release of greater than one sodium ion per vanadium,

wherein the NaVOPO 4 is formed by a solid phase synthesis process from a heated powdered mixture of ammonium metavanadate, ammonium phosphate monobasic, and potassium carbonate, to yield KVOPO 4 having corner-sharing VO 6 octahedra and PO 4 tetrahedra, defining two types of tunnels comprising a first type of tunnel formed of rings of two PO 4 tetrahedra and a second type of tunnel formed of rings of three PO 4 tetrahedra and three VO 6 octahedra, followed by substitution of the potassium ions with sodium ions.

2. The electrode according to claim 1 , wherein the electrode is a cathode material within a sodium ion rechargeable battery, further comprising an anode and an electrolyte, the cathode material having a discharge capacity of greater than 158 mAh/g.

3. The electrode according to claim 1 , in combination with a sodium donor anode.

4. The electrode according to claim 3 , in combination with a sodium ion transport electrolyte.

5. The electrode according to claim 1 , further comprising an insoluble conductive additive.

6. The electrode according to claim 5 , wherein the conductive additive comprises a conductive carbon additive.

7. The electrode according to claim 1 , wherein the powdered mixture is heated at a temperature of between 600-800° C.

8. The electrode according to claim 7 , wherein the solid phase synthesized KVOPO 4 is mixed with carbon black and milled.

9. The electrode according to claim 1 , further comprising poly(vinylidene fluoride) binder.

10. The electrode according to claim 1 , in combination with a sodium-containing anode, and a sodium transport electrolyte, together comprising a battery having a peak open circuit voltage of at least 3 volts.

11. The electrode according to claim 10 , wherein the electrode has a capacity of at least C=158 mAhg −1 .

12. The electrode according to claim 10 , wherein a discharge voltage curve of the battery comprises two major plateau regions.

13. The electrode according to claim 12 , wherein the two major plateau regions comprise a higher voltage plateau region having a voltage comprising about 3.8 V, and a lower voltage plateau region having a voltage comprising about 2 V.

14. The electrode according to claim 1 , in combination with at least:

a sodium donor anode; and

an electrolyte comprising sodium ions; and

wherein the electrode further comprises a current collector,

to thereby form a reversible sodium battery.

15. A reversible sodium battery comprising:

a sodium donor anode;

a sodium intercalation cathode comprising NaVOPO 4 , having a volume greater than 90 Å 3 per VOPO 4 , orthorhombic crystalline symmetry, and space group Pna2 1 ,

the NaVOPO 4 being formed by a solid phase synthesis process from a heated powdered mixture of ammonium metavanadate, ammonium phosphate monobasic, and potassium carbonate, to yield KVOPO 4 having corner-sharing VO 6 octahedra and PO 4 tetrahedra, defining two types of tunnels comprising a first type of tunnel formed of rings of two PO 4 tetrahedra and a second type of tunnel formed of rings of three PO 4 tetrahedra and three VO 6 octahedra, followed by substitution of the potassium ions with sodium ions;

a current collector configured to collect current from the sodium intercalation cathode; and

an electrolyte comprising sodium ions.

16. The reversible sodium battery of claim 15 , wherein the sodium intercalation cathode is capable of electrochemical insertion and release of more than one sodium ion per vanadium atom.

17. The reversible sodium battery of claim 15 , wherein the cathode further comprises a binder selected from one or more of the group consisting of a polyvinylidenefluoride (PVDF), a polytetrafluoroethylene (PTFE), a styrene butadiene rubber (SBR), and a polyimide.

18. The reversible sodium battery of claim 15 wherein the current collector comprises conductive carbon.

19. The reversible sodium battery of claim 18 , wherein the conductive carbon comprises carbon black.

20. An electrode for a reversible sodium battery, comprising NaVOPO 4 , having orthorhombic crystalline symmetry and space group Pna2 1 as an active ingredient capable of electrochemical insertion and release of sodium ions, having two well-separated voltage plateaus in a discharge curve of a battery formed with the electrode as cathode corresponding to at least two redox couples of vanadium cations,

the NaVOPO 4 being formed by a process comprising the steps of:

milling a mixture of ammonium metavanadate, ammonium phosphate monobasic, and potassium carbonate;

heating the milled mixture to a reaction temperature, and holding the reaction temperature until a solid phase synthesis of KVOPO 4 occurs;

milling the KVOPO 4 together with conductive particles to form a conductive mixture of fine particles;

adding binder material to form a conductive cathode material; and

electrochemically removing potassium from the KVOPO 4 ,

wherein:

said heating is at a temperature of between 600° C. and 800° C. for about 10 hours;

the conductive particles comprise carbon particles; and

the binder material comprises poly vinylidene fluoride.