Sodium Formate as a Highly Efficient Sodium Compensation Additive for Sodium-Ion Batteries with a P2-Type Layered Oxide Cathode

P2-type manganese-based layered oxide is regarded as a promising candidate cathode material for sodium-ion batteries. However, when non-sodium-containing materials are used as the anode in a full cell, irreversible interfacial reactions occurring in non-sodium-containing anodes will significantly consume active Na+ ions in P2-type manganese-based oxides, leading to rapid capacity decay. In this work, the introduction of sodium formate is proposed as a sodium compensation additive into P2-type manganese-based layered oxide (Na0.66Ni0.26Zn0.07Mn0.67O2, NNZMO). The decomposition products of sodium formate can also act as a modifier for the surface of NNZMO particles. The results show that sodium formate added to NNZMO can significantly improve cycling performance for both the half cell and full cell. The capacity retention of the NNZMO cathode with 15 wt.% sodium formate is 88.6% after 100 cycles at 100 mA g-1 in a half cell, much higher than that (81.3%) of the NNZMO cathode without sodium formate. The full cell, based on a P2-type layered NNZMO cathode and a hard carbon anode with HCOONa as a sodium compensation additive, can provide 95.7 mAh g-1 first-cycle discharge specific capacity and 81.3% capacity retention after 100 cycles at 100 mA g-1, much higher than those (71.8 mAh g-1 and 55.7%) of the NNZMO cathode without sodium formate. This sodium compensation strategy provides an effective way to increase the discharge specific capacity and operational stability of the full cell and is also expected to be applicable to other cathode materials.