Electrolyte for sodium secondary battery and sodium secondary battery using thereof

1. A sodium secondary battery having an operation temperature of 120° C. to 200° C. and comprising:

a cathode containing a transition metal, and impregnated in an electrolyte having a melting point of 150° C. or less and comprising a molten sodium salt and an electrolyte additive made of an inorganic sodium salt represented by the following Chemical Formula:

Na x A

wherein A is at least one anion selected from the group consisting of S a O b 2− (1≤a<4, 1<b<9), a nitrate ion (NO 3 − ), and a combination thereof, and

x is an integer of 1 to selected depending on an ionic valence of A which is a counter ion;

an anode containing sodium; and

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

2. The sodium secondary battery of claim 1 , wherein A is one or more anions selected from the group consisting of a sulfate ion, which is any one selected from SO 4 2− , SO 3 2− , S 2 O 3 2− , S 2 O 5 2− , and S 2 O 7 2− , the nitrate ion (NO 3 − ), and the combination thereof.

3. The sodium secondary battery of claim 2 , wherein Na x A is one or more selected from the group consisting of sodium sulfate (Na 2 SO 4 ), anhydrous sodium sulfate (Na 2 SO 3 ), sodium thiosulfate (Na 2 S 2 O 3 ), sodium metabisulfite (Na 2 S 2 O 5 ), sodium pyrosulfate (Na 2 S 2 O 7 ), sodium nitrate (NaNO 3 ), and a combination thereof.

4. The sodium secondary battery of claim 1 , wherein the electrolyte contains 0.1 to 20 wt % of the electrolyte additive.

5. The sodium secondary battery of claim 1 , wherein the electrolyte has ion conductivity of 200 ms/cm or more at the time of being melted.

6. The sodium secondary battery of claim 1 , wherein the anode contains molten sodium.

7. The sodium secondary battery of claim 1 , wherein the cathode contains one or more metals selected from nickel (Ni), copper (Cu), iron (Fe), manganese (Mn), cobalt (Co), and an alloy thereof.