Sodium secondary battery

1. A sodium secondary battery comprising:

an anode containing sodium;

a cathode containing a transition metal and an alkali metal halide; and

a sodium ion conductive solid electrolyte provided between the anode and the cathode,

wherein the cathode is impregnated in a molten salt electrolyte represented by the following Chemical Formula 1.

NaM(X 1 ) n (X 2 ) 4−n   Chemical Formula 1

In Chemical Formula 1,

M is an element selected from metal and metalloid groups having an oxidation number of 3;

X 1 and X 2 are each independently selected from halogen elements; and

n is more than 0 but less than 4 (0<n<4).

2. The sodium secondary battery of claim 1 , wherein n is 0.2 or more but 3.8 or less (0.2≦n≦3.8).

3. The sodium secondary battery of claim 1 , wherein X 1 is chlorine (Cl), and X 2 is iodine (I), and

n is 2.0 or more but 3.8 or less (2.0≦n≦3.8).

4. The sodium secondary battery of claim 1 , wherein X 1 is chlorine (Cl), and X 2 is bromine (Br), and

n is 0.2 or more but 3.8 or less (0.2≦n≦3.8).

5. The sodium secondary battery of claim 1 , wherein M is boron, aluminum, gallium, or indium.

6. The sodium secondary battery of claim 1 , wherein the molten salt electrolyte has a melting point of 150° C. or less.

7. The sodium secondary battery of claim 1 , wherein the molten salt electrolyte further contains an electrode active material made of a sodium halide salt.

8. The sodium secondary battery of claim 1 , wherein the anode contains a sodium metal or a sodium alloy.

9. The sodium secondary battery of claim 1 , wherein it has an operation temperature of 100 to 200° C.