METAL PHOSPHOROTHIOATES AND METAL-SULFUR ELECTROCHEMICAL SYSTEM CONTAINING THE SAME

Abstract

The disclosure relates to metal phosphorothioates, batteries comprising metal phosphorothioate, cells comprising metal phosphorothioate, and methods of making thereof.

Claims

1. A metal (M)-sulfur battery comprising: a cathode comprising an mP.sub.2S.sub.5-nM.sub.2S.sub.x complex; an anode comprising the metal, wherein the metal is passivated using an anode passivation solution comprising an aP.sub.2S.sub.5-bM.sub.2S.sub.y complex; and an electrolyte in contact with the cathode and the anode; wherein the metal (M) is lithium or sodium; wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.x (m:n) in the cathode is between 1:2 and 2:1, optionally the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.x (m:n) in the cathode is 1:2, 2:3, 1:1, 3:2, or 2:1; wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.y (a:b) in the anode passivation solution is between 1:2 and 2:1, optionally the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.y (a:b) in the anode passivation solution is 1:2, 2:3, 1:1, 3:2, or 2:1; and wherein x and y are independently an integer from 1 to 12.

2. The battery of claim 1, wherein M is sodium.

3. The battery of claim 1, wherein m:n is 1:1.

4. The battery of claim 1, wherein a:b is 1:1.

5. The battery of claim 1, wherein x is 8.

6. The battery of claim 1, wherein y is 1.

7. The battery of claim 1, wherein the battery further comprises a solid electrolyte interphase (SEI) on the anode, wherein the SEI mainly comprises Na.sub.4P.sub.2S.sub.7, Na.sub.4P.sub.2S.sub.6, Na.sub.2P.sub.2S.sub.6, Na.sub.3PS.sub.4 and NaPS.sub.3.

8. The battery of claim 1, wherein the electrolyte comprises NaPF.sub.6 in diglyme.

9. The battery of claim 1, further comprising a separator, wherein the separator keeps the cathode and the anode apart.

10. The battery of claim 1, wherein the battery is rechargeable.

11. The battery of claim 1, wherein the cathode is a liquid-phase cathode.

12. A metal (M)-sulfur cell comprising: a cathode comprising an mP.sub.2S.sub.5-nM.sub.2S.sub.x complex; an anode comprising the metal, wherein the metal is passivated using an anode passivation solution comprising an aP.sub.2S.sub.5-bM.sub.2S.sub.y complex; and an electrolyte in contact with the cathode and the anode; wherein the metal (M) is lithium or sodium; wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.x (m:n) in the cathode is between 1:2 and 2:1, optionally the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.x (m:n) in the cathode is 1:2, 2:3, 1:1, 3:2, or 2:1; wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.y (a:b) in the anode passivation solution is between 1:2 and 2:1, optionally the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.y (a:b) in the anode passivation solution is 1:2, 2:3, 1:1, 3:2, or 2:1; and wherein x and y are independently an integer from 1 to 12.

13. A method of manufacturing the metal (M)-sulfur cell of claim 12 comprising mixing a first stoichiometric ratio of metal sulfide (M.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder in a first organic solvent to form the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex; contacting a metal foil with the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex to form the passivated anode; mixing a second stoichiometric ratio of metal sulfide (M.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder in a second organic solvent to form the mP.sub.2S.sub.5-nM.sub.2S.sub.xcomplex; mixing the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex with electro-conductive carbon black and a salt to form the cathode; and contacting the electrolyte with the passivated anode and the cathode.

14. The method of claim 13, wherein the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex is formed by mixing the metal sulfide (M.sub.2S) and the sulfur (S) powder in the first organic solvent to provide a metal polysulfide (M.sub.2S.sub.y); and combining the metal polysulfide (M.sub.2S.sub.y) with the phosphorous pentasulfide (P.sub.2S.sub.5) to form the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex.

15. The method of claim 13, wherein the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex is formed via a one-step reaction by mixing the first stoichiometric ratio of the metal sulfide (M.sub.2S), the phosphorous pentasulfide (P.sub.2S.sub.5), and the sulfur (S) powder in the first organic solvent.

16. The method of claim 13, wherein the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex is formed by mixing the metal sulfide (M.sub.2S) and the sulfur (S) powder in the second organic solvent to provide a metal polysulfide (M.sub.2S.sub.x); and combining the metal polysulfide (M.sub.2S.sub.x) with the phosphorous pentasulfide (P.sub.2S.sub.5) to form the mP.sub.2S.sub.5-nM.sub.2S.sub.xcomplex.

17. The method of claim 13, wherein the mP.sub.2S.sub.5-nM.sub.2S.sub.xcomplex is formed via a one-step reaction by mixing the second stoichiometric ratio of the metal sulfide (M.sub.2S), the phosphorous pentasulfide (P.sub.2S.sub.5), and the sulfur (S) powder in the second organic solvent.

18. The method of any one of claim 13, wherein the first organic solvent and the second organic solvent are the same.

19. The method of claim 13, wherein the first and second organic solvents comprise diethylene glycol dimethyl ether (diglyme), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), 1,3-dioxolane (DOL), tetraethylene glycol dimethyl ether, or a combination thereof.

20. The method of claim 13, wherein the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex and the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex are formed at room temperature.

21. A metal phosphorothioate having the formula of cP.sub.2S.sub.5-dM.sub.2S.sub.z, wherein the metal (M) is lithium or sodium; the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.z (c:d) is between 1:2 and 2:1, optionally the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.z (c:d) is 1:2, 2:3, 1:1, 3:2, or 2:1; and z is an integer from 1 to 12.

22. The metal phosphorothioate of claim 21, wherein the metal is sodium.

23. The metal phosphorothioate of claim 21, wherein c:d is 1:1.

24. The metal phosphorothioate of claim 21, wherein z is 8.

25. The metal phosphorothioate of claim 21, wherein z is 1.

26. A method of preparing the metal phosphorothioate of claim 21 comprising: mixing a stoichiometric ratio of metal sulfide (M.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder in an organic solvent.

27. The method of claim 26, wherein the cP.sub.2S.sub.5-dM.sub.2S.sub.z complex is formed by mixing the metal sulfide (M.sub.2S) and the sulfur (S) powder in the organic solvent to provide a metal polysulfide (M.sub.2S.sub.y); and combining the metal polysulfide (M.sub.2S.sub.y) with the phosphorous pentasulfide (P.sub.2S.sub.5) to form the cP.sub.2S.sub.5-dM.sub.2S.sub.z complex.

28. The method of claim 26, wherein the cP.sub.2S.sub.5-dM.sub.2S.sub.z complex is formed via a one-step reaction by mixing the stoichiometric ratio of the metal sulfide (M.sub.2S), the phosphorous pentasulfide (P.sub.2S.sub.5), and the sulfur (S) powder in the organic solvent.

29. The method of claim 26, wherein the organic solvent is diglyme.

30. The method of claim 26, wherein the metal phosphorothioate is prepared at room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1A is a depiction of a sodium battery of the disclosure.

[0016] FIG. 1B is a photograph of traditional Na.sub.2S.sub.x and P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) complexes along with mP.sub.2S.sub.5-nNa.sub.2S.sub.8 (m:n=1:2, 2:3, 1:1, 3:2, 2:1) complexes.

[0017] FIG. 1C is a photo of mP.sub.2S.sub.5-nNa.sub.2S complexes in diglyme solvent.

[0018] FIG. 1D is a photo of mP.sub.2S.sub.5-nNa.sub.2S.sub.3 complexes in diglyme solvent.

[0019] FIG. 2A is a .sup.31P-NMR spectrum of P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) and mP.sub.2S.sub.5-nNa.sub.2S.sub.8 (m:n=2:3, 1:1, 3:2) complexes in diglyme solvent.

[0020] FIG. 2B is a Raman spectrum of P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) and mP.sub.2S.sub.5-nNa.sub.2S.sub.8 (m:n=1:1, 3:2) complexes in diglyme solvent.

[0021] FIG. 2C are Raman profiles of P.sub.2S.sub.5, S, and Na.sub.2S powders, and diglyme.

[0022] FIG. 3 is a .sup.31P-NMR spectrum of P.sub.2S.sub.5—Na.sub.2S.sub.8 in THF, diglyme, and DME solvents.

[0023] FIG. 4A is an .sup.1H-NMR spectrum of P.sub.2S.sub.5—Na.sub.2S.sub.8 in diglyme.

[0024] FIG. 4B is an .sup.1H-NMR spectrum of P.sub.2S.sub.5—Na.sub.2S.sub.8 in DME.

[0025] FIG. 4C is an .sup.1H-NMR spectrum of P.sub.2S.sub.5—Na.sub.2S.sub.8 in THF.

[0026] FIG. 5 is a comparison of the .sup.31P-NMR spectra of the P.sub.2S.sub.5—Na.sub.2S.sub.8 complex in diglyme taken within one day and after 30 days of storage.

[0027] FIG. 6 is a cyclic voltammetry trace of cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) as catholyte paired with passivated Na at a scan rate of 0.10 mV s.sup.−1 within the voltage range of 1.2-2.4 V.

[0028] FIG. 7A is a cyclic voltammetry trace of cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 as catholyte paired with passivated Na as anode at different scanning rates of 0.10, 0.15 and 0.20 mV s.sup.−1 in the voltage ranges of 1.2-2.4 V.

[0029] FIG. 7B is a cyclic voltammetry trace of cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 as catholyte paired with passivated Na as anode at different scanning rates of 0.10, 0.15 and 0.20 mV s.sup.−1 in the voltage ranges of 1.8-2.4 V.

[0030] FIG. 8 is a set of graphs depicting cyclic voltammetry peak current against the square root of scan rates for the anodic reaction (at ˜2.32 V) and the cathodic reaction (at ˜2.23 V).

[0031] FIG. 9A is a set of typical galvanostatic charge curves of the P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 catholyte cells at 0.5 C (1 C=1675 mA g.sup.−1) over the voltage range from 1.2 V to 2.4 V.

[0032] FIG. 9B is a galvanostatic discharge curve of the P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 catholyte cells at 0.5 C (1 C=1675 mA g.sup.−1) within the voltage range of 1.2-2.4 V.

[0033] FIG. 10A is a Raman profile at 2.4, 1.8 and 1.2 V for P.sub.2S.sub.5—Na.sub.2S.sub.8 catholyte cell.

[0034] FIG. 10B is a Raman profile at 2.4 V for Na.sub.2S.sub.8 catholyte cells cycled at 0.5 C (1 C=1675 mA g.sup.−1).

[0035] FIG. 11 is a graph depicting the galvanostatic cycling performance of cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 as catholyte paired with passivated Na at 0.5 C (1 C=1675 mA g.sup.−1) within the voltage ranges of 1.8-2.4 V and 1.2-2.4 V.

[0036] FIG. 12 is a graph depicting the galvanostatic cycling performance of cells consisting of P.sub.2S.sub.5 suspension as catholyte (same S concentration based on S in P.sub.2S.sub.5) paired with passivated Na at 0.5 C (1 C=1675 mA g.sup.−1) within the voltage range of 1.2-2.4 V.

[0037] FIG. 13 is a graph depicting the Coulombic efficiency of cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 as catholyte paired with passivated Na as anode at 0.5 C (1 C=1675 mA g.sup.−1) within the voltage ranges of 1.8-2.4 V and 1.2-2.4 V.

[0038] FIG. 14 is a graph evaluating the electrochemical cycling at different rates of 0.25 C, 0.5 C and 0.75 C (1 C=1675 mA g.sup.−1) for P.sub.2S.sub.5—Na.sub.2S.sub.8 catholyte cells within the voltage range of 1.8-2.4 V.

[0039] FIG. 15 is a graph depicting cycling performance of P.sub.2S.sub.5—Na.sub.2S.sub.8 catholytes over a range of active sulfur material loadings.

[0040] FIG. 16 is an electrochemical impedance spectrum of the P.sub.2S.sub.5—Na.sub.2S.sub.8 catholyte cells before cycling and after 50, 100, and 200 cycles at 0.5 C within the voltage range of 1.8-2.4 V and 1.2-2.4 V.

[0041] FIG. 17 is a set of scanning electron microscope image of the passivated Na surfaces of the P.sub.2S.sub.5—Na.sub.2S.sub.8 and the Na.sub.2S.sub.8 catholyte cells after 30 cycles.

[0042] FIG. 18 is a set of X-ray photoelectron spectra of (a) C.sub.1s, (b) P.sub.2p and (c) S.sub.2p on the passivated Na surface before cycling.

[0043] FIG. 19 is set of X-ray photoelectron spectra of C.sub.1s, P.sub.2p and S.sub.2p on the passivated Na surface for the P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 catholyte cells after 30 cycles.

[0044] FIG. 20A is an X-ray diffractogram of P.sub.2S.sub.5—Na.sub.2S.sub.xP.sub.2S.sub.5—Na.sub.2S.sub.3 and P.sub.2S.sub.5—Na.sub.2S.sub.8 passivated Na.

[0045] FIG. 20B is a Raman spectrum of P.sub.2S.sub.5—Na.sub.2S.sub.xP.sub.2S.sub.5—Na.sub.2S.sub.3 and P.sub.2S.sub.5—Na.sub.2S.sub.8 passivated Na.

[0046] FIG. 21A is a graph depicting the galvanostatic cycling performance of cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.8 as catholyte paired with P.sub.2S.sub.5—Na.sub.2S.sub.8 passivated at 0.5 C (1 C=1675 mA g.sup.−1) within the voltage range of 1.8-2.4 V.

[0047] FIG. 21B is a graph depicting the coulombic of cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.8 as catholyte paired with P.sub.2S.sub.5—Na.sub.2S.sub.8 passivated Na as anode at 0.5 C (1 C=1675 mA g.sup.−1) within the voltage range of 1.8-2.4 V.

DETAILED DESCRIPTION

[0048] In one aspect, the present disclosure provides a metal (M)-sulfur battery including a cathode including an mP.sub.2S.sub.5-nM.sub.2S.sub.xcomplex; an anode including the metal, wherein the metal is passivated/pretreated using an aP.sub.2S.sub.5-bM.sub.2S.sub.y complex; and an electrolyte in contact with the cathode and the anode; wherein the metal (M) is lithium or sodium; wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.x (m:n) in the cathode is various within a range from 1:2 to 2:1 (such as but not limited to 1:2, 2:3, 1:1, 3:2, or 2:1); wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.y (a:b) in the passivation solution is various within a range from 1:2 to 2:1 (such as but not limited to 1:2, 2:3, 1:1, 3:2, or 2:1); wherein x and y are independently an integer from 1 to 12.

[0049] In one embodiment, the metal is sodium. In another embodiment, m:n is 1:1. In another embodiment, a:b is 1:1. In another embodiment, x is 8. In another embodiment, y is 1. In another embodiment, the battery further includes a solid electrolyte interphase (SEI) on the passivated anode, wherein the SEI mainly includes Na.sub.4P.sub.2S.sub.7, Na.sub.4P.sub.2S.sub.6, Na.sub.2P.sub.2S.sub.6, Na.sub.3PS.sub.4 and NaPS.sub.3. In another embodiment, the SEI comprises one, two, three, or four compounds selected from the group consisting of Na.sub.4P.sub.2S.sub.7, Na.sub.4P.sub.2S.sub.6, Na.sub.2P.sub.2S.sub.6, Na.sub.3PS.sub.4 and NaPS.sub.3. In another embodiment, the electrolyte includes NaPF.sub.6 in diglyme. In another embodiment, the battery further includes a separator, wherein the separator keeps the cathode and the anode apart. In another embodiment, the battery is rechargeable. In another embodiment, the cathode is in the liquid phase.

[0050] In another aspect, the present disclosure provides a metal (M)-sulfur cell including a cathode including an mP.sub.2S.sub.5-nM.sub.2S.sub.xcomplex; an anode including the metal, wherein the metal is passivated/pretreated using an aP.sub.2S.sub.5-bM.sub.2S.sub.y complex; and an electrolyte in contact with the cathode and the anode; wherein the metal (M) is lithium or sodium; wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.x (m:n) in the cathode is various within a range from 1:2 to 2:1 (such as but not limited to 1:2, 2:3, 1:1, 3:2, or 2:1); wherein the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.y (a:b) is various within a range from 1:2 to 2:1 (such as but not limited to 1:2, 2:3, 1:1, 3:2, or 2:1); wherein x and y are independently an integer from 1 to 12.

[0051] In another aspect, the present disclosure provides a method of manufacturing the metal (M)-sulfur battery or cell disclosed herein including the steps of mixing an appropriate stoichiometric ratio of metal sulfide (M.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder in an organic solvent to form the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex; contacting a metal foil with the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex to form the passivated anode; mixing an appropriate stoichiometric ratio of metal sulfide (M.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder in an organic solvent to form the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex; mixing the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex with electro-conductive carbon black and a salt to form the cathode; and contacting the electrolyte with the passivated anode and the cathode. In some embodiments, the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex can be formed in two steps: i) mixing metal sulfide (M.sub.2S) and sulfur (S) powder in an organic solvent to provide a metal polysulfide (M.sub.2S.sub.y); ii) combining the metal polysulfide (M.sub.2S.sub.y) with phosphorous pentasulfide (P.sub.2S.sub.5) in a stoichiometric ratio to form the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex. In some embodiments, the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex can be formed in two steps: i) mixing phosphorous pentasulfide (P.sub.2S.sub.5) and metal sulfide (M.sub.2S) powder in a stoichiometric ratio in an organic solvent to provide the aP.sub.2S.sub.5-bM.sub.2S; adding a suitable amount of sulfur (S.sub.n, n=y−1) into aP.sub.2S.sub.5-bM.sub.2S to form the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex. In some embodiments, the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex can be formed via a one-step reaction by mixing appropriate the stoichiometric ratio of metal sulfide (M.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder in an organic solvent.

[0052] In some embodiments, the metal foil is a lithium foil. In preferred embodiments, the metal foil is a sodium foil.

[0053] In some embodiments, the salt is selected from the group consisting of lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium triflouromethanesulfonate (LiOTf), sodium perchlorate (NaClO.sub.4), sodium borofluoride (NaBF.sub.4), sodium hexafluorophosphate (NaPF.sub.6) sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(fluorosulfonyl)imide (NaFSI) and sodium triflouromethanesulfonate (NaOTf). In some embodiments, the salt is NaPF.sub.6.

[0054] In some embodiments, the organic solvent comprises diethylene glycol dimethyl ether (diglyme), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), 1,3-dioxolane (DOL), tetraethylene glycol dimethyl ether (TEGDME), or a combination thereof. In some embodiments, the organic solvent is a unary system (one solvent) and is selected from the group consisting of diglyme, DME, THF, DOL, and TEGDME. In some embodiments, the organic solvent is a mixture of two or three or four solvents selected from the group consisting of diglyme, DME, THF, DOL and TEGDME. Exemplary binary solvent (two solvent) systems include, but are not limited to, diglyme/THF, diglyme/DOL, DME/THF, DME/DOL, TEGDME/THF and TEGDME/DOL, wherein the two components are present in a volume ratio from 1:9 to 9:1. In some embodiments, the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex and the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex are formed at room temperature. In some embodiments, the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex and the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex are formed under heated conditions. In some embodiments, the concentration of the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex and the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex in the organic solvent does not exceed 50 wt %. In some embodiments, the concentration of the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex and the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex in the organic solvent is 20 wt %. In some embodiments, the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex and the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex are chemical stability in the solvent for a period of at least one month, at least two months, at least three months, at least four months, at least five months, or at least six months. In some embodiments, the aP.sub.2S.sub.5-bM.sub.2S.sub.y complex and the mP.sub.2S.sub.5-nM.sub.2S.sub.x complex are chemical stability in the solvent for a period of at least one month.

[0055] In some embodiments, the present disclosure provides a metal phosphorothioate having the formula of cP.sub.2S.sub.5-dM.sub.2S.sub.z, wherein the metal (M) is lithium or sodium; the ratio of P.sub.2S.sub.5 to M.sub.2S.sub.z (c:d) is various within a range from 1:2 to 2:1 (such as but not limited to 1:2, 2:3, 1:1, 3:2, or 2:1); and z is an integer from 1 to 12. In some embodiments, the metal is sodium. In another embodiment, c:d is 1:1. In another embodiment, z is 8. In another embodiment, z is 1.

[0056] In another aspect, the present disclosure provides a method of preparing the metal phosphorothioate disclosed herein including the steps of mixing an appropriate stoichiometric ratio of metal sulfide (M.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder in an organic solvent. In some embodiments, the organic solvent is diethylene glycol dimethyl ether (diglyme). In another embodiment, the metal phosphorothioate is prepared at room temperature.

[0057] In some embodiments, the present disclosure provides a catholyte (liquid-phase cathode) including active materials (i.e., the phosphorothioates disclosed herein), conductive agents (such as carbon black or ketjen black) and sodium salts (such as sodium hexaflourophosphate).

[0058] In another embodiment, the present disclosure provides active materials (i.e., sodium/lithium phosphorothioates) which could be synthesized through complexation reactions among sodium/lithium sulfide, sulfur and phosphorous pentasulfide in the corresponding stoichiometric ratio in solvents (such as diethylene glycol dimethyl ether, 1,2-dimethoxyethane or tetrahydrofuran) without external heating process.

[0059] In another embodiment, the present disclosure provides a passivation solution which can effectively generate solid electrolyte interphase (SEI) on sodium/lithium metal anode to stabilize battery performance.

[0060] In some embodiments, the present disclosure provides sodium/lithium phosphorothioate species, which may be employed as a catholyte for room temperature sodium/lithium batteries. In some embodiments, synthesis of the sodium/lithium phosphorothioates is low-cost without the need for external heating process. In some embodiments, the sodium/lithium phosphorothioates may be used as a catholyte for sodium/lithium sulfur batteries, and show superior electrochemical performance.

[0061] In some embodiments, the present disclosure provides a method of using the sodium/lithium phosphorothioates as a catholyte for sodium/lithium sulfur batteries, which may prevent precipitation of low-order sodium/lithium polysulfide, stabilize the metallic anode, and lead to highly-stable long-term battery cycling. In some embodiments, using the sodium/lithium phosphorothioates as a catholyte may provide a higher sodium/lithium-ion diffusion rate, improve electrochemical kinetics, and lead to high-rate performance. In some embodiments, using the sodium/lithium phosphorothioates as a catholyte may provide significantly enhanced electrochemical potentials and facilitate high energy/power density. In some embodiments, using the sodium/lithium phosphorothioates as passivation solution to metal anode may effectively alleviate side reactions and enhance stability of metallic sodium/lithium performance.

[0062] In some embodiments, using the sodium/lithium phosphorothioates as a catholyte may enable sodium/lithium-sulfur electrochemistry, which may enhance energy density, stabilize long-term cycling, and reduce energy cost compared to current lithium-ion technologies. In some embodiments, using the sodium/lithium phosphorothioates as a catholyte may enable a much faster voltage rise in charge processes and thus a better Coulombic efficiency behavior compared to traditional sodium/lithium-sulfur system. In some embodiments, using the sodium/lithium phosphorothioates as a catholyte may enable higher active-sulfur loadings simply by using a higher concentration of the mP.sub.2S.sub.5-nM.sub.2S.sub.xcomplex and/or adding a larger volume of the complex solution on carbon current collector.

[0063] The present disclosure provides a new series of sodium phosphorothioates via the interaction between sodium polysulfide (Na.sub.2S.sub.x) and phosphorothioates, where S.sub.x chains of different lengths are interconnected by phosphorothioate derivatives, forming uniform soluble species. The present disclosure provides sodium phosphorothioates with a longer S.sub.x chain, which possess higher reactivity, and could serve as catholytes (liquid-phase cathode) for Na batteries. Phosphorothioates with short S.sub.x chains (i.e., those with low “x” values), though possessing no electrochemical reactivity, can generate an effective solid electrolyte interphase (SEI) on metallic Na, which would result in a highly reactive anode and requires protection from direct contact with electrolytes/polysulfides to alleviate side reactions in Na batteries.

[0064] The present disclosure provides a novel Na battery electrochemistry with a greatly reactive catholyte and a pretreated Na anode, which solves intrinsic issues of the presence of unanchored reactive intermediate polysulfides, the formation of undissolved discharged sulfides and the repeated liquid-solid phase transition. By addressing the above issues, the batteries disclosed herein exhibit superior electrochemical performance of an initial capacity of 440 mAh g.sup.−1 with a high retention of 80% even after over 400 cycles. The sodium phosphorothioates disclosed herein provide batteries with high energy, low cost and long lifespan.

[0065] Comprehensive characterization tools, such as nuclear magnetic resonance (NMR), Raman, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), were employed to investigate the molecular structure and chemistry involved in the molecules and complexes disclosed herein. In some embodiments, the present disclosure provides molecules/complexes which possess short-range order phosphorous groups connected with sulfur chain in different lengths due to different mixing molar ratios.

[0066] Electrochemical evaluations, such as cyclic voltammetry (CV), galvanostatic charge-discharge, long-term cycling and electrochemical impedance spectroscopy (EIS), were performed to investigate electrochemistry and demonstrate high-performance sodium/lithium sulfur battery applications. Sodium/lithium sulfur electrochemistry was identified, and the mechanism of superior electrochemical performance was revealed.

[0067] In another aspect, sodium (Na), as used in the present disclosure, possesses certain chemical/physical properties similar to Li, but Na also has several advantages over Li. By way of example, Na's first ionization energy of 495.8 kJ mol.sup.−1 is lower than that of Li (520.2 kJ mol.sup.−1), leading to improved kinetics in chemical reactions. As an earth-abundant element, Na is over 1000 times more abundant than Li in the earth crust. The cost of Na raw materials (carbonate salt) is more than 100 times less expensive than that of Li.

EXAMPLES

Example 1

A New Family of Sodium Phosphorothioates (mP.SUB.2.S.SUB.5.-nNa.SUB.2.S.SUB.x.) for Na Battery Chemistry

[0068] mP.sub.2S.sub.5-nNa.sub.2S.sub.x complexes were obtained through the reactions among precursors of sodium sulfide (Na.sub.2S), sulfur (S) and phosphorous pentasulfide (P.sub.2S.sub.5) in diglyme solvent at room temperature, in which no external heating is required. This new family of complexes mP.sub.2S.sub.5-nNa.sub.2S.sub.xcould be tailored in two dimensions: length of S.sub.x chain (x) and the ratio of P.sub.2S.sub.5 to Na.sub.2S.sub.x (m:n), showing unique electrochemical characteristics for Na battery chemistry, which can contribute to the construction of a novel semi-solid Na battery (FIG. 1A). The battery configuration is illustrated in FIG. 1A, where the catholyte is stored in a current collector while metallic Na anode is protected with a solid electrolyte interphase (SEI) layer on the surface. It is found that, with respect to the catholyte side, longer S chain complexes possess highly interconnected structures, showing higher electrochemical reactivity compared to shorter S chain catholytes in relatively isolated status. Such longer S chain systems could be employed as high-capacity catholytes for Na batteries. Regarding the anode side, shorter S chain complexes could serve as passivation reagents to generate a uniform protective SEI on metallic Na anode, while the longer S chain ones inhomogeneously corrode the anode surface. Both longer and shorter S chain complexes show distinctive and irreplaceable functions, where combining a longer S chain catholyte and a shorter S chain pretreated Na anode creates novel Na battery chemistry showing superior electrochemical performance.

[0069] Various S.sub.x chain length (x=1, 2, 3, 4, 6, 8) and P.sub.2S.sub.5 to Na.sub.2S.sub.x ratios (m:n=1:2, 2:3, 1:1, 3:2, 2:1) in mP.sub.2S.sub.5-nNa.sub.2S.sub.x are exemplified herein (FIGS. 1B, 1C, and 1D). It is noted that solid S alone cannot complex with P.sub.2S.sub.5 in diglyme solvent. The solid S needs to react with Na.sub.2S to form Na.sub.2S.sub.x before complexing with P.sub.2S.sub.5. The differences between P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) complexes and the corresponding Na.sub.2S.sub.x can be visually distinguished. Before reacting with P.sub.2S.sub.5, the appearance of Na.sub.2S.sub.xvaries based on the length of the S.sub.x chain. Shorter-chain Na.sub.2S.sub.x (x=1, 2, 3) are insoluble in diglyme; while, Na.sub.2S.sub.4 shows limited solubility and Na.sub.2S.sub.6/Na.sub.2S.sub.8 can be fully dissolved and exhibits a dark red brown color. Both insoluble and soluble Na.sub.2S.sub.x can complex with P.sub.2S.sub.5 to form transparent and homogeneous solutions. The colors of P.sub.2S.sub.5—Na.sub.2S.sub.x change from orange to light yellow and further to deep yellow with the increase of S.sub.x chain lengths. When tailoring the P.sub.2S.sub.5 to Na.sub.2S.sub.x ratio m:n with a fixed x dimension (x=8) in mP.sub.2S.sub.5-nNa.sub.2S.sub.8 system (m:n=1:2, 2:3, 1:1, 3:2, 2:1) in FIG. 1B, homogeneous systems are formed in the ratio range from 2:3 to 3:2 with color showing as dark red, yellow and deeper yellow, respectively, while both 1:2 and 2:1 ratios result in nonhomogeneous systems (solids at the bottom of vials in FIG. 1B). The similar m:n ratio effect was also investigated and confirmed in mP.sub.2S.sub.5-nNa.sub.2S and mP.sub.2S.sub.5-nNa.sub.2S.sub.3 (m:n=3:2, 1:1 and 2:3) systems (FIGS. 1C and 1D).

Methods and Materials

[0070] New family complexes preparation: Sodium sulfide (Na.sub.2S), phosphorous pentasulfide (P.sub.2S.sub.5) and sulfur (S) powder (Sigma-Aldrich) were used as starting materials to synthesize mP.sub.2S.sub.5-nNa.sub.2S.sub.x complexation systems. P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) complexes were prepared in different stoichiometric ratios of Na.sub.2S, S and P.sub.2S.sub.5 (in 1:x-1:1 molar ratio) in diethylene glycol dimethyl ether (diglyme, anhydrous, Sigma-Aldrich) solvent, which were stirred to form a solution without heating in a solid-to-liquid ratio of 20 wt %. Similar to the synthesis process of P.sub.2S.sub.5—Na.sub.2S.sub.xcomplexes, Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) were obtained by mixing Na.sub.2S and S powder (in 1:x-1 molar ratio) in diglyme with a solid-to-liquid ratio of 20 wt %. As to mP.sub.2S.sub.5-nNa.sub.2S.sub.8, precursors were calculated and mixed stoichiometrically to get m:n of 1:2, 2:3, 1:1, 3:2 and 2:1 in diglyme. The photo (FIG. 1B) shows the appearance of systems after same synthesis time around 1 to 2 hours. Same synthesis method was employed to mP.sub.2S.sub.5-nNa.sub.2S and mP.sub.2S.sub.5-nNa.sub.2S.sub.3 (m:n=2:3, 1:1, 3:2) systems.

[0071] Catholyte preparation: P.sub.2S.sub.5—Na.sub.2S.sub.x, Na.sub.2S.sub.8 and P.sub.2S.sub.5 were mixed with 1 vol % Ketjen Black (AkzoNobel, EC-600JD) and 0.5 M sodium hexafluorophosphate (NaPF.sub.6, Sigma-Aldrich, 98%) to prepare as catholytes. As-prepared catholytes (˜16 ul) were then dropped onto the commercial carbon fiber paper (FuelCellStore) disk with a diameter of 12 mm. In P.sub.2S.sub.5—Na.sub.2S.sub.x and Na.sub.2S.sub.8 catholytes, S concentrations (based on the S in Na.sub.2S.sub.x) were kept as around 1.44 M. The active material loading (based on the S content) is 0.74 mg cm.sup.−2 in FIG. 11. Several other loadings, up to 2.25 mg cm .sup.−2, have been tested as shown in FIG. 15. As the control group of P.sub.2S.sub.5 suspension, the S concentration was based on S in P.sub.2S.sub.5 , where insoluble solids were well dispersed in diglyme.

[0072] Sodium metal anode passivation: Commercial sodium metal cube (Na, 99.9%, Sigma-Aldrich) was manufactured into Na foil with a diameter of 12 mm. The efficient agent used to stabilize Na foil was 10 wt % P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1). Na foil was immersed in the passivation complex overnight. The control group of passivation solution is 10 wt % P.sub.2S.sub.5—Na.sub.2S.sub.x (x=8), which was used to prepare P.sub.2S.sub.5—Na.sub.2S.sub.8 passivated Na following the same procedure.

[0073] Electrochemical measurements: Electrochemical performance was tested in 2032-type coin cells assembled with as-prepared catholyte on carbon fiber paper (CFP), passivated Na metal anode and separator (Celgard 2400) soaked with electrolyte, which consisted of 1.0 M NaPF.sub.6 in diglyme. The assembled batteries were galvanically charged and discharged at 0.5 C (1 C=1675 mA g.sup.−1) at voltage range of 1.2-2.4 V and 1.8-2.4 V using standard testing system (CT 2001A, Wuhan LAND Electronics Co., Ltd). Electrochemical Impedance Spectroscopy (EIS) was performed using electrochemical workstation (VMP3, Biologic Science Instruments) at a scanning frequency from 900 KHz to 100 mHz. Cyclic voltammetry was performed using electrochemical workstation (VMP3, Biologic Science Instruments) at scanning rates of 0.1, 0.15 and 0.20 mV s.sup.−1 at voltage range of 1.2-2.4 V and 1.8-2.4 V.

[0074] Materials characterizations: Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) were carried out using field emission gun environmental scanning electron microscope (XL 300 ESEM-FEG). Focus ion beam (FIB) was performed to reveal cross-section SEM image using FEI Scios2 LoVac dual beam FEG/FIB SEM. Laser Raman Spectroscopy (LRS) was measured using Horiba labRAM HR Evolution operating at wavelength of 532 nm, where double pass macro cuvette holder was used for solution analysis. Raman shifts were calibrated using silicone reference with a very sharp silicon mode at 520 cm.sup.−1. .sup.31P and .sup.1H Nuclear Magnetic Resonance (NMR) was carried out using Bruker 600 MHz Advance III HD with a 5 mm probe of Bruker BB(F)O. Chemical shifts in .sup.31P NMR were calibrated to 85% phosphoric acid solution (H.sub.3PO.sub.4, δ=0 ppm). X-ray photoelectron spectroscopy (XPS) analysis was conducted on PHI Versaprobe II scanning XPS microprobe with 0.47 eV system resolution using monochromatic 1486.7 X-ray source. Notably, the samples were transferred into the XPS chamber via a sealed Argon-filled vessel to avoid the exposure to air.

Example 2

Characterizations of mP.SUB.2.S.SUB.5.-nNa.SUB.2.S.SUB.x

[0075] To facilitate in-depth understanding of the molecular structures, both nuclear magnetic resonance (NMR) spectroscopy and Laser Raman spectroscopy were performed for P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) and mP.sub.2S.sub.5-nNa.sub.2S.sub.8 (m:n=3:2, 1:1 and 2:3), as shown in FIGS. 2A and 2B. The existence of P.sub.2S.sub.5 derivatives, known as P.sup.n short range orders (SROs), can be identified by .sup.31P chemical shifts observed in NMR profiles. P.sup.n SRO denotes a P group with n number of bridging S atoms, across P.sub.2S.sub.5—Na.sub.2S.sub.x forming ranges, which have been discovered in glassy phosphorothioates solid-state materials..sup.1-6 It is noted that no P.sub.2S.sub.5 solution NMR profile is presented due to its insolubility in diglyme. .sup.31P-NMR profiles of P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) (FIG. 2A) solutions display 5 major distinctive chemical shifts. The signals at 49, 63, 118, 127 and 30 ppm are assigned to P.sup.2′, P.sup.3, P.sup.2, P.sup.0 and P.sup.2″ (a derivative of P.sup.2′) SRO units, respectively..sup.1-6 All assignments were assigned based on density functional theory (DFT) and previous studies in literature. The spectroscopic data indicate that highly branched network structures are preferred for longer S chain system, while shorter S chain systems tend not to exhibit this property. As to mP.sub.2S.sub.5-nNa.sub.2S.sub.8 (m:n=3:2, 1:1, 2:3) systems, .sup.31P-NMR results reveal the existence of P.sup.2′, P.sup.3 and P.sup.2 SROs in P.sub.2S.sub.5—Na.sub.2S.sub.8, while P.sup.2NMR resonance disappears in 3P.sub.2S.sub.5-2Na.sub.2S.sub.8, possibly due to the interaction of excessive P.sub.2S.sub.5 with P.sup.2 (linear connectivity) in forming P.sup.3 (network connectivity)..sup.1-6 Nevertheless, NMR resonances are barely observed except for P.sup.3 in the 2P.sub.2S.sub.5-3Na.sub.2S.sub.8, which is possibly ascribed to the network connectivity destruction by excess Na.sub.2S.sub.8.

[0076] Raman profiles in FIG. 2B reveal chemical interactions among precursors of Na.sub.2S, P.sub.2S.sub.5 and S during the complexation process. Before reaction, Na.sub.2S.sub.xP.sub.2S.sub.5 and S precursors show distinctive Raman shifts (FIG. 2C). After reaction, all characteristic peaks of precursors disappear while new peaks occur due to the formation of mP.sub.2S.sub.5-nNa.sub.2S.sub.x. Raman profile of P.sub.2S.sub.5—Na.sub.2S shows both distinctive and similar shifts to those in P.sub.2S.sub.5—Na.sub.2S.sub.x (x=2, 3, 4, 6, 8), where the peaks at 388 and 418 cm.sup.−1 indicate the presence of P.sup.2″ SRO while the peak at 482 cm.sup.−1 is ascribed to the symmetric stretching modes of —S—S— between P.sup.n SROs..sup.1, 7-14 In P.sub.2S.sub.5—Na.sub.2S.sub.x (x=2, 3, 4, 6, 8) profiles, the band at 200˜215 cm.sup.−1 indicates the presence of P.sup.2/P.sup.3 SROs while peaks at 386 and 493 cm.sup.−1 reveal the modes in P.sup.2 SRO. Besides, the peak at 575 cm.sup.−1, assigned to T.sub.2 asymmetric stretching of tetrahedral structure in P.sup.n SROs,.sup.1, 7-14 presenting lower-energy Raman shift with the increase of S.sub.x chain length, which indicates the vibrations of P.sup.n SROs are affected by the chain length..sup.14 For P.sub.2S.sub.5—Na.sub.2S.sub.x (x=4, 6, 8), two more peaks at 239 and 258 cm.sup.−1 are observed, which indicate the presence of P.sup.2′ SRO..sup.1, 7-14 The observations in Raman profiles are consistent with NMR results, which together reveal the structure of P.sub.2S.sub.5—Na.sub.2S.sub.x as network connectivity of P.sup.n SROs with S.sub.x in-between. As to mP.sub.2S.sub.5-nNa.sub.2S.sub.8 (m:n=3:2, 1:1) systems, similar Raman profiles are observed, while the intensity of 200˜215 cm.sup.−1 band (P.sup.2/P.sup.3 SROs) decreases for 3P.sub.2S.sub.5-2Na.sub.2S.sub.8, which corresponds to the disappearance of P.sup.2 SRO observed in corresponding NMR profiles. Besides diglyme, other solvents including dimethoxyethane (DME) and tetrahydrofuran (THF) were also evaluated to prepare P.sub.2S.sub.5—Na.sub.2S.sub.8. .sup.31P-NMR profiles of P.sub.2S.sub.5—Na.sub.2S.sub.8 in three different solvents (FIG. 3) show three similar characteristic resonances with chemical shifts observed, indicating the existence of similar P.sub.2S.sub.5—Na.sub.2S.sub.8 complex in different solvents. It is noted that the chemical shifts in these three NMR profiles is due to the solvent effect, where the polarity of three solvents in ascending order is diglyme>THF>DME. .sup.1H-NMR profiles of P.sub.2S.sub.5—Na.sub.2S.sub.8 in diglyme, DME and THF along with corresponding blank solvents are shown, respectively, in FIGS. 4A, 4B, and 4C, which exclude any reactions between P.sub.2S.sub.5—Na.sub.2S.sub.8 and solvents..sup.14 Also, the .sup.31P-NMR spectrum (FIG. 5) shows the P.sub.2S.sub.5—Na.sub.2S.sub.8 complex to be stable after storage for 30 days in diglyme.

Example 3

Electrochemical Evaluation on Battery Chemistry

[0077] To demonstrate the relationship between chain length and reactivity, cyclic voltammetry (CV) was employed on the cells consisting of P.sub.2S.sub.5—Na.sub.2S.sub.x (x=1, 2, 3, 4, 6, 8) as catholyte and passivated Na metal as anode along with 1.0 M NaPF.sub.6 in diglyme as electrolyte at a scan rate of 0.10 mV s.sup.−1 within a voltage range of 1.2-2.4 V, as shown in FIG. 6. The preparation of catholyte, passivated anode and electrolyte as well as the cell configuration have been detailed in the experimental methods. In CV profiles, P.sub.2S.sub.5—Na.sub.2S and P.sub.2S.sub.5—Na.sub.2S.sub.2, catholytes barely present any oxidation peaks, indicating no reversible electrochemical reactivity, while longer S.sub.x chain P.sub.2S.sub.5—Na.sub.2S.sub.x (x=3, 4, 6, 8) show obvious oxidation and reduction peaks, which indicate the presence of reversible redox reactions. Noting that longer S.sub.x chains possess higher electrochemical activity, P.sub.2S.sub.5—Na.sub.2S.sub.8 (the longest S.sub.x chain in the family) was further investigated and compared with a traditional Na.sub.2S.sub.8 counterpart, where coin cells were assembled in the configuration of P.sub.2S.sub.5—Na.sub.2S.sub.8, Na.sub.2S.sub.8 and P.sub.2S.sub.5 as catholyte paired with passivated Na metal anode in 1.0 M NaPF.sub.6 diglyme electrolyte. CV profiles of P.sub.2S.sub.5—Na.sub.2S.sub.8 reveal the same electrochemistry as that of Na.sub.2S.sub.8 (FIGS. 7A and 7B), where the CV profiles of P.sub.2S.sub.5—Na.sub.2S.sub.8 cells exhibits more pairs of redox peaks compared to those in Na.sub.2S.sub.8 cells, indicating that P.sub.2S.sub.5—Na.sub.2S.sub.8 possesses higher electrochemical kinetics and could anchor/stabilize intermediate polysulfide via P.sup.n units. The electrochemical kinetics is positively correlated to Na.sup.+ ion diffusion rate, which has been evaluated through CV measurements at different scan rates of 0.10, 0.15 and 0.20 mV s.sup.−1 for both P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 catholyte cells. According to classical Randles Sevcik equation, all cathodic and anodic peak currents are linear with the square root of scan rates, where the slopes of curves are positively correlated to the corresponding Na.sup.+ ion diffusion, plotted in FIG. 8..sup.15 The higher Na.sup.+ ion diffusion rate of P.sub.2S.sub.5—Na.sub.2S.sub.8 reveals that the highly interconnected network structure could facilitate Na.sup.+ ion transfer, improving the kinetics of Na battery chemistry.

[0078] As revealed in the galvanostatic charge profiles (FIG. 9A), the Na.sub.2S.sub.8 catholyte shows a slow rise of charging voltage to ˜2.28 V. In contrast, the P.sub.2S.sub.5—Na.sub.2S.sub.8 catholyte exhibits a much faster voltage rise. The corresponding galvanostatic discharge profiles of P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 catholyte cells at 1.2-2.4 V are presented in FIG. 9B, where P.sub.2S.sub.5—Na.sub.2S.sub.8 cells display both higher discharge electrochemical potentials and higher discharge capacity compared to those of Na.sub.2S.sub.8 cells, leading to higher specific energy delivery. The discharge curve of Na.sub.2S.sub.8 shows three plateaus at ˜2.23, 1.90 and 1.70 V, corresponding to the reduction peaks in CV profile, which reveals the electrochemical reactions from S.sub.8 to Na.sub.2S, in well consistent with previous reports. As to P.sub.2S.sub.5—Na.sub.2S.sub.8 catholyte cells, the discharge curve shows four plateaus, corresponding to CV reduction peaks at ˜2.23, 2.11, 1.93 and 1.70 V, which are assigned to the gradual reactions from S.sub.-.sup.2− to S.sub.2.sup.2− species, confirmed in Raman (FIG. 10A). S.sub.-.sup.2−, S.sub.3.sup.− and S.sub.2.sup.2− exhibit as the main products, excluding the formation of solid S at 2.4 V. In contrast, the Raman profile (FIG. 10B) of Na.sub.2S.sub.8 cells at 2.4 V indicates the presence of solid S and a great amount of untransformed S.sub.3species, which confirms its low kinetic, leading to heavy residual of non-fully charged species. Another important observation in Raman is that P.sup.2′ trends to transform to P.sup.2 at lower discharge potential, accommodating the low-solubility S.sub.2.sup.2− species into the network structure built upon P.sup.n SROs.

Example 4

Long-Term Cycling Evaluation on Catholyte Cells

[0079] Galvanostatic cycling (FIG. 11) at 0.5 C (1 C=1675 mA g.sup.−1) was performed at 2.4-1.8 V and 2.4-1.2 V to evaluate long-term electrochemical stability for battery chemistries using both P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 catholytes. Mere P.sub.2S.sub.5 suspension as catholyte does not deliver any capacity (FIG. 12), which indicates that the electrochemical reactivity in P.sub.2S.sub.5—Na.sub.2S.sub.x catholyte is contributed by S.sub.x from Na.sub.2S.sub.x. Despite voltage ranges, Na.sub.2S.sub.8 cells quickly decay due to both precipitation of low-order sodium polysulfide on the current collector and severe side reactions with anode, leading to poor Coulombic efficiency (FIG. 13). In contrast, P.sub.2S.sub.5—Na.sub.2S.sub.8 cells present good cycling stability and retention as well as high Coulombic efficiency at 1.8-2.4 V, where an initial capacity delivery of 440 mAh g.sup.1 is achieved and a capacity of 352 mAh g.sup.−1 can be retained even after over 400 cycles. The stable performance at different C-rates (FIG. 14) further demonstrates that P.sub.2S.sub.5—Na.sub.2S.sub.8 can serve as high performance catholyte. At higher active material loadings up to 1.57 mg cm.sup.−2, P.sub.2S.sub.5—Na.sub.2S.sub.8 cells also exhibit steady cycling performance (FIG. 15). At very high loadings up to 2.25 mg cm.sup.−2, where high complex concentrations were employed, an initial capacity decay was observed, possibly due to the parasitic reactions on sodium metal anodes. When operating within 1.2-2.4 V, the P.sub.2S.sub.5—Na.sub.2S.sub.8 cells deliver a higher initial capacity of 468 mAh g.sup.−1 with a stable performance for about 130 cycles, and slowly decay after that.

[0080] To investigate the capacity decay mechanism of P.sub.2S.sub.5—Na.sub.2S.sub.8 cells cycled at 1.2-2.4 V, electrochemical impedance spectroscopy (EIS) of the P.sub.2S.sub.5—Na.sub.2S.sub.8 cells cycled at 0.5 C in 1.8-2.4 and 1.2-2.4 V were compared (FIG. 16). Nyquist plots (before cycling, after 50 cycles, and after 100 cycles) show two semi-circles, where the high-frequency semi-circle is attributed to the interfacial resistance/capacitance of the catholyte and the low-frequency semi-circle is ascribed to the charge-transfer resistance/pseudocapacitance of the catholyte..sup.16 At the voltage range of 1.8-2.4 V, no obvious increment of impedance is observed over cycling, indicating good cycling reversibility and stability. As to the voltage range of 1.2-2.4 V, before 200 cycles, the impedance shows similar values as those at 1.8-2.4 V. However, the high-frequency semi-circle greatly increases after 200 cycles due to great accumulation of electrochemically inactive P.sub.2S.sub.5—Na.sub.2S.sub.2, which is hard to convert back to active P.sub.2S.sub.5—Na.sub.2S.sub.x (x=3, 4, 6, 8) species, leading to heavy increase of charge-transfer resistance.

[0081] No beneficial P interactions exist in the case of mere Na.sub.2S.sub.8 catholyte cells. Both P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 cells performed within 1.8-2.4 V at 0.5 C for 30 cycles were dissembled and characterized to evaluate the contribution of P interactions to stabilize Na—S battery chemistry. The catholyte current collectors for P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 cells after cycling were subjected to SEM and corresponding energy-dispersive X-ray spectroscopy (EDS) on C, Na and S elements. Compared to clean commercial carbon paper, the catholyte current collector dissembled from P.sub.2S.sub.5—Na.sub.2S.sub.8 cell showed a very similar clean matrix without solid precipitation observed, where Na and S elements could be detected, originating from the soluble species of P.sub.2S.sub.5—Na.sub.2S.sub.x. Nevertheless, the catholyte current collector dissembled from the Na.sub.2S.sub.8 cell displayed a great solid agglomeration of insoluble low-order sodium polysulfide, which explains the poor electrochemical performance of Na.sub.2S.sub.8 cells. The SEM and EDS results indicate that mere Na.sub.2S.sub.8 could lead to the formation of insoluble discharge products, while new molecules of P.sub.2S.sub.5—Na.sub.2S.sub.8 could accommodate the insoluble products within the network of P″ SROs, leading to superior electrochemical performance.

Example 5

Characterization on Passivated Na Over Cycling

[0082] The metallic Na anodes employed in the cells were pretreated before being paired with catholytes, using a passivation solution of P.sub.2S.sub.5—Na.sub.2S.sub.xwhich generates a protective solid electrolyte interphase (SEI) layer on metallic Na surface. The surface and cross-section morphologies of P.sub.2S.sub.5—Na.sub.2S passivated Na were analyzed and revealed nanostructured-petal cluster surface morphology with petal thickness of 500 nm and SEI with a thickness of 2 μm (outlined in the inset). Further x-ray photoelectron spectroscopy (XPS) of C.sub.1s, P.sub.2p and S.sub.2p profiles, in FIG. 18 unveil the compositions of passivation layer. The binding energies of all elements were calibrated with respect to C.sub.1s at 284.8 eV..sup.17-19In C.sub.1s XPS profile, binding energy at 286.4 eV is assigned to C—O in RCH.sub.2ONa, which is due to the decomposition of electrolyte..sup.17-19 During XPS peak-fitting for P.sub.2p and S.sub.2p, 2p.sub.3/2 to 2p.sub.1/2 area ratio is fixed at 2:1 according to the ratio of degeneracy. The doublet separation of 2p.sub.3/2 and 2p.sub.1/2 for P.sub.2p is 0.87 eV while the one for S.sub.2p is 1.18 eV. The high-resolution P.sub.2p XPS spectrum exhibits three doublets, at 132.01, 132.92 and 133.74 eV (based on the 2p.sub.3/2), respectively, ascribed to the presence of P.sup.1P, P.sup.1 and P.sup.0 SROs..sup.14, 20-22 The high-resolution of S.sub.2p XPS presents three doublets, at 161.49, 162.17 and 162.99 eV (based on the 2p.sub.3/2), respectively, assigned to Na—S—P, P═S and P—S—P in P.sup.n SROs, which are in well consistent with results of P.sub.2p XPS..sup.14, 20-22 Paired with such passivated Na, Na.sub.2S.sub.8 catholyte cells show poor electrochemical performance, while the P.sub.2S.sub.5—Na.sub.2S.sub.8 cells demonstrate superior stability.

[0083] To explain the difference, the passivated Na surfaces from dissembled P.sub.2S.sub.5—Na.sub.2S.sub.8 and Na.sub.2S.sub.8 cells after 30 cycles were characterized via SEM and corresponding XPS, shown in FIGS. 17 and 19. The passivated Na from P.sub.2S.sub.5—Na.sub.2S.sub.8 cells and from Na.sub.2S.sub.8 cells (FIG. 17) show different surface morphology after cycling, where the P.sub.2S.sub.5—Na.sub.2S.sub.8 cells display a smooth surface while the Na.sub.2S.sub.8 cells exhibit gully-like topography due to severe side-reactions. Additionally, shown in FIG. 19, C.sub.1s and S.sub.2p XPS of passivated Na for P.sub.2S.sub.5—Na.sub.2S.sub.8 cells after cycling show similar profiles to those before cycling, while P.sup.0 disappears in the P.sub.2p XPS profile with appearance of —PF.sub.6 (137.29 eV),.sup.17-19 which is due to the stabilization of the passivation layer and to the decomposition of NaPF.sub.6 salt, respectively. Nevertheless, the passivated Na from Na.sub.2S.sub.8 cells after cycling shows tremendously different C.sub.1s, P.sub.2p and S.sub.2p XPS profiles. In addition to C—C and C—O, C═O (288.34 eV) and O—C═O (289.93 eV).sup.17-19 are detected, which are possibly due to the reaction of fresh Na with electrolyte after the original protection layer is etched and destroyed by Na.sub.2S.sub.8. The severe side reactions of Na.sub.2S.sub.8 with original passivation layer is confirmed by the disappearance of both P.sup.0 and P.sup.1 in P.sub.2p XPS profile as well as the presence of S.sup.2− (159.52 eV) doublet.sup.17, 18 detected in S.sub.2p XPS. The SEM and XPS results indicate that mere Na.sub.2S.sub.8 could heavily react with the original P.sub.2S.sub.5—Na.sub.2S passivation layer on the anode, while P.sub.2S.sub.5—Na.sub.2S.sub.8 could stabilize the original passivation layer, showing good compatibility. The formation of robust SEI using short S chain P.sub.2S.sub.5—Na.sub.2S is unique and due to chemical reactions between P.sub.2S.sub.5—Na.sub.2S and metallic Na. P.sub.2S.sub.5—Na.sub.2S.sub.3 and P.sub.2S.sub.5—Na.sub.2S.sub.8 were selected from the P.sub.2S.sub.5—Na.sub.2S.sub.x family and evaluated due to the presence of different S valence compared to that (−2) in P.sub.2S.sub.5—Na.sub.2S. Both candidates showed aggressive reactions with Na metal, leading to porous and ravined surface morphology. With higher S valence, P.sub.2S.sub.5—Na.sub.2S.sub.8 passivation solution showed even more severe side reactions compared to P.sub.2S.sub.5—Na.sub.2S.sub.3 one. It is noted that all SEI layers via three passivation solutions show similar amorphous phase and compositions (FIGS. 20A and 20B). Besides Na.sub.4P.sub.2S.sub.7, Na.sub.4P.sub.2S.sub.6, Na.sub.2P.sub.2S.sub.6 and Na.sub.3PS.sub.4 compounds, the SEI layer of P.sub.2S.sub.5—Na.sub.2S passivated Na mainly contains NaPS.sub.3, which is not a main component detected in SEI layers of P.sub.2S.sub.5—Na.sub.2S.sub.3 and P.sub.2S.sub.5—Na.sub.2S.sub.8 passivated Na. Such NaPS.sub.3 compound is compatible with long S chain P.sub.2S.sub.5—Na.sub.2S.sub.8 catholyte, leading to stable battery chemistry realization. In contrast, pairing with P.sub.2S.sub.5—Na.sub.2S.sub.8 passivated Na metal anode without NaPS.sub.3 component, P.sub.2S.sub.5—Na.sub.2S.sub.8 catholyte cells show poor cycling performance with low Coulombic efficiency (FIGS. 21A and 21B), due to geographical inhomogeneity and chemical incompatibility of SEI generated using high S chain P.sub.2S.sub.5—Na.sub.2S.sub.xcomplexes.

REFERENCES

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