High‐Voltage Operation of All‐Oxide Solid‐State Sodium Batteries Using NASICON‐Related Materials

Abstract All‐oxide solid‐state sodium batteries (AOSSSBs), which are composed of oxide electrolytes and oxide active materials with sodium‐ion carriers, have attracted attention because of their high safety and material abundance. However, the operating voltage of AOSSSBs is inevitably low because the number of active materials available for use in AOSSSBs is restricted by undesirable reactions at electrolyte/electrode interfaces during high‐temperature fabrication. Herein, we fabricate AOSSSBs using a Na superionic conductor‐type solid electrolyte Na 3 Zr 2 (SiO 4 ) 2 (PO 4 ) (NZSP), a Na superionic conductor‐type negative active material NaTi 2 (PO 4 ) 3 , and a mixed‐phosphate cathode active material Na 4 Ni 3 (PO 4 ) 2 (P 2 O 7 ) by a Na 2 B 4 O 7 ⋅ 10H 2 O‐assisted low‐temperature fabrication technique. The obtained NaTi 2 (PO 4 ) 3 /NZSP/Na 4 Ni 3 (PO 4 ) 2 (P 2 O 7 ) batteries exhibit an average discharge voltage of 3.1 V, which is the highest voltage ever reported in AOSSSBs. In addition, Na 2 B 4 O 7 ⋅ 10H 2 O‐containing NZSP electrolytes show a wider electrochemical window in comparison with the pristine NZSP, enabling the batteries to endure a wide voltage operation (a capacity retention of 71 % after 10 cycles of charge/discharge in 0–5.1 V).