Solid electrolyte and lithium battery including the same

1. A solid electrolyte for an all-solid secondary battery, the solid electrolyte comprising a compound of Formula 1:

(Li 1-x M1 x ) 7-y PS 6-y M2 y   Formula 1

wherein, in Formula 1,

M1 is Na,

M2 is Cl, and

molar amounts of Li and M1 satisfy 0.01≤M1/(Li+M1)≤0.07, and

x and y satisfy 0<x≤0.07 and 0<y≤2, and

wherein the solid electrolyte has peaks at positions of 15.42°±0.50° 2θ, 17.87°±0.50° 2θ, 25.48°±0.50° 2θ, 30.01°±0.50° 2θ, and 31.38°±0.50° 2θ when analyzed by X-ray diffraction using CuKα radiation.

2. The solid electrolyte of claim 1 , wherein the solid electrolyte has an Argyrodite structure.

3. The solid electrolyte of claim 1 , wherein the solid electrolyte further comprises a first peak at a position of 30.01°±0.50° 2θ and at a second peak at a position of 33.65°±0.50° 2θ when analyzed by X-ray diffraction using CuKα radiation, and

wherein a ratio of an intensity of the second peak to an intensity of the first peak is less than about 0.1.

4. The solid electrolyte of claim 1 comprising (Li 5.6925 Na 0.0575 )PS 4.75 Cl 1.25 .

5. A secondary battery comprising:

a cathode comprising a cathode active material;

an anode comprising an anode active material; and

a solid electrolyte layer comprising the solid electrolyte of claim 1 .

6. The secondary battery of claim 5 , wherein the cathode active material comprises at least one of a lithium cobalt oxide, a lithium nickel oxide, a lithium nickel cobalt oxide, a lithium nickel cobalt aluminum oxide, a lithium nickel cobalt manganese oxide, a lithium manganese oxide, a lithium iron phosphate, a nickel sulfide, a copper sulfide, sulfur, an iron oxide, and a vanadium oxide.

7. The secondary battery of claim 6 , wherein the cathode active material further comprises the solid electrolyte of claim 1 and carbon nanofibers.

8. The secondary battery of claim 5 , wherein the anode active material comprises at least one of a carbon active material, a metal active material, an oxide active material.

9. The secondary battery of claim 5 , wherein an average particle diameter of the solid electrolyte layer is in a range of about 1 micrometer to about 10 micrometers.

10. The secondary battery of claim 5 , wherein a thickness of the solid electrolyte layer is in a range of about 10 micrometers to about 200 micrometers.

11. The secondary battery of claim 5 , wherein the secondary battery is an all-solid secondary battery.

12. A method of preparing the solid electrolyte of claim 1 , the method comprising:

mechanically milling a mixture comprising Li 2 S, P 2 S 5 , and M1 2 S and LiM2, or M1M2 to obtain a glass; and

heat-treating the glass at a glass transition temperature or greater to convert the glass into and ion conductive glass ceramic and obtain the solid electrolyte.

13. The solid electrolyte of claim 1 , wherein the solid electrolyte has an ion conductivity of greater than 2×10 −3 Siemens per centimeter.

14. A secondary battery comprising:

a cathode comprising a cathode active material able to reversibly intercalate lithium ions;

an anode comprising an anode active material comprising lithium; and

a solid electrolyte layer comprising a solid electrolyte of the formula (Li 1-x Na x ) 7-y PS 6-y Cl y ,

wherein

molar amounts of Li and Na satisfy 0.01≤M1/(Li+Na)≤0.07, and

x and y satisfy 0<x≤0.07 and 0<y≤2.