Composite solid electrolyte, electrochemical cell including the same, and method of preparing the composite solid electrolyte

1. A composite solid electrolyte comprising:

a solid electrolyte having a NASICON type crystal structure and including elements of Li, Ti, Al, P, and 0 ; and

a protective layer, wherein the protective layer is on a surface of the solid electrolyte and comprises a compound represented by Formula 1

Li x Ml y M 2 z O t+x/2   Formula 1

wherein in Formula 1,

M1 has a Gibbs reaction energy of greater than 0 electron-volts per mole,

M2 is Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, Si, Al, W, Cd, Pd, Sn, Pb, Sc, Mg, Ca, Sr, Ba, or a combination thereof,

0< x< 1, 0< y< 1, 0< z< 1, and 0< t< 1,

provided that when M1 is Nb and z is 0, the compound represented by Formula 1 is Li 0.33 Nb 0.33 O, and when M1 is Zr or Al, z≠0.

2. The composite solid electrolyte of claim 1 , wherein M1 is an element having a Gibbs reaction energy value of about 0.3 electron-volts per mole or more.

3. The composite solid electrolyte of claim 1 , wherein the Gibbs reaction energy of M1 is obtained by subtracting a standard Gibbs energy of an anhydride of M1 as a reactant and a standard Gibbs energy of water or water vapor from a standard Gibbs energy of a hydroxide of M1 as a product, when the hydroxide of M1 is produced via a reaction between the anhydride of M1 and water or water vapor.

4. The composite solid electrolyte of claim 1 , wherein M1 is Pt, Eu, Hf, Zr, Ta, Ti, Ce, Nb, W, Mo, Co, Tl, Si, Sb, Ag, V, Sn, Sc, Ni, Mn, Fe, Be, Cu, Ge, Al, Au, C, Tm, Dy, Pb, or a combination thereof.

5. The composite solid electrolyte of claim 4 , wherein M1 is Zr, Nb, V, Ta or a combination thereof.

6. The composite solid electrolyte of claim 1 , wherein z≠0.

7. The composite solid electrolyte of claim 6 , wherein M2 is Zn, Mg, V, or a combination thereof.

8. The composite solid electrolyte of claim 1 , wherein the protective layer has a thickness of about 1 nanometer to about 200 micrometers.

9. The composite solid electrolyte of claim 8 , wherein the protective layer has a thickness of about 1 nanometer to about 1 micrometer.

10. The composite solid electrolyte of claim 1 , wherein the protective layer comprises Li 0.33 Nb 0.330 O, Li 0.08 Zr 0.17 V 0.25 O, Li 0.08 Zr 0.17 Ta 0.25 O, Li 0.83 Zr 0.17 Zn 0.25 O, Li 0.83 Zr 0.17 Mg 0.25 O, or a combination thereof.

11. The composite solid electrolyte of claim 1 , wherein the solid electrolyte has a thickness of about 1 micrometer to about 1,000 micrometers.

12. The composite solid electrolyte of claim 1 , wherein the composite solid electrolyte has an ionic conductivity of about 80% or greater than the ionic conductivity of a solid electrolyte without a protective layer at about 20° C.

13. A method of preparing a composite solid electrolyte, the method comprising:

providing a solid electrolyte having a NASICON type crystal structure and including elements of Li, Ti, Al, P, and O;

disposing a first layer comprising a first element on the solid electrolyte, wherein the first element has a Gibbs reaction energy of greater than 0 electron-volts per mole;

heat-treating the first layer to oxidize the first element and form an oxide comprising the first element;

depositing a metal on the oxide to form a current collector;

contacting the oxide, the current collector, or a combination thereof with a liquid electrolyte comprising lithium; and

applying a voltage to reduce the oxide to form a compound represented by Formula 1 to form a protective layer on the solid electrolyte and prepare the composite solid electrolyte of claim 1 .

14. The method according to claim 13 , wherein the oxide is represented by the formula M′1 y1 Ot1, wherein y1 and t1 are each independently greater than 0 and equal to or less than 1, and M′1 is the first element.

15. The method according to claim 13 , wherein the heat-treating comprises heat-treating in air.

16. The method of claim 13 , wherein the heat-treating comprises heat-treating at a temperature of about 500 ° C. or greater for about 1 hour or more.

17. The method of claim 13 , wherein the protective layer has a thickness of about 1 nanometer to about 1 micrometer.

18. The method of claim 13 , wherein the metal deposited on the oxide to form the current collector is silver.

19. A method of preparing a composite solid electrolyte, the method comprising:

providing a solid electrolyte having a NASICON type crystal structure and including elements of Li, Ti, Al, P, and O;

disposing a first layer comprising a first element on the solid electrolyte, wherein the first element has a Gibbs reaction energy of greater than 0 electron-volts per mole;

disposing a second layer comprising a second element of Groups 2 to 14 on the solid electrolyte;

heat-treating the first layer and the second layer to form an oxide comprising the first element and the second element;

depositing a metal on the oxide to form a current collector;

contacting the oxide, the current collector, or a combination thereof with a liquid electrolyte comprising lithium; and

applying a voltage to reduce the oxide to form a compound represented by Formula 3-1 to form a protective layer on the solid electrolyte and prepare the composite solid electrolyte of claim 1 .

20. The method of claim 19 , wherein the first layer is deposited before the second layer.

21. The method of claim 19 , wherein the second layer is deposited before the first layer.

22. The method of claim 19 , wherein the first layer is between the solid electrolyte and the second layer.

23. The method of claim 19 , wherein the second layer is between the solid electrolyte and the first layer.

24. The method of claim 19 , wherein the oxide is represented by the formula M″ 1 y2 M″2 z2 O t2 , and

wherein

M″1 is the first element;

M″2 is the second element; and

y2, z2, and t2 are each independently greater than 0 and equal to or less than 1.

25. The method of claim 19 , wherein the heat-treating comprises heat-treating in air.

26. The method of claim 19 , wherein the heat-treating comprises heat-treating at a temperature of about 500° C. or more for about 1 hour or longer.

27. The method of claim 19 , wherein the protective layer has a thickness of about 1 nanometer to 1 micrometer.

28. The method of claim 19 , wherein the metal deposited on the oxide to form the current collector is silver.

29. An electrochemical cell comprising:

a negative electrode;

an electrolyte comprising a composite solid electrolyte; and

a positive electrode having a NASICON type crystal structure and including elements of Li, Ti, Al, P, and O, wherein the composite solid electrolyte comprises a solid electrolyte, and a protective layer, wherein the protective layer is on a surface of the solid electrolyte and comprises a compound represented by Formula 1

Li x M 1 y M 2 z O t+x/2   Formula 1

wherein in Formula 1,

M1 has a Gibbs reaction energy of greater than 0 electron-volts per mole,

M2 is Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, Si, Al, W, Cd, Pd, Sn, Pb, Sc, Mg, Ca, Sr, Ba, or a combination thereof,

0< x< 1, 0< y< 1, 0< z< 1, and 0< t< 1,

provided that when M1 is Nb and z is 0, the compound represented by Formula 1 is Li 0.33 Nb 0.33 O and when M1 is Zr or Al, z 0, and

wherein the protective layer of the composite solid electrolyte is between the solid electrolyte and the positive electrode.

30. The electrochemical cell of claim 29 , wherein the negative electrode comprises lithium metal or a lithium alloy.

31. The electrochemical cell of claim 29 wherein the composite solid electrolyte is crack-free.

32. The electrochemical cell of claim 31 , wherein the composite solid electrode is crack-free in the presence of LiOH.

33. The electrochemical cell of claim 29 , wherein the composite solid electrolyte is crack-free at an interface with the positive electrode at an operating voltage of about 2 volts to about 4.5 volts vs Li/Li+.

34. The electrochemical cell of claim 29 , wherein the positive electrode is configured to use oxygen or water as a positive active material.

35. The electrochemical cell of claim 29 , wherein the electrolyte further comprises a liquid electrolyte, a gel electrolyte, a polymeric ionic liquid, or a combination thereof.

36. The electrochemical cell of claim 35 , wherein the liquid electrolyte comprises LiSCN, LiN(CN) 2 , Li(CF 3 SO 2 ) 3 C, LiC 4 F 9 SO 3 , LiN(SO 2 CF 2 CF 3 ) 2 , LiCl, LiF, LiBr, LiI, LiB(C 2 O 4 ) 2 , LiBF 4 , LiBF 3 (C 2 F 5 ), lithium bis(oxalato) borate, lithium oxalyldifluoroborate, lithium difluoro(oxalato)borate, lithium bis(trifluoro methanesulfonyl)imide, lithium bis(fluorosulfonyl)imide , LiCF 3 SO 3 , LiAsF 6 , LiSbF 6 , LiClO 4 , or a combination thereof.

37. The electrochemical cell of claim 35 , wherein the liquid electrolyte comprises an organic solvent, an ionic liquid, an oligomer, or a combination thereof.