Layered Oxide Material as a Highly Stable Na‐ion Source and Sink for Investigation of Sodium‐ion Battery Materials

Abstract Investigating Na‐ion battery (SIB) materials is complicated by the absence of a well‐performing (reference) electrode material since sodium metal cannot be considered as a quasi‐reference electrode. Taking advantage of the activity of both Ni and Mn, herein, the P2‐type and Mn‐rich Na 0.6 Ni 0.22 Al 0.11 Mn 0.66 O 2 (NAM) material, known to be an excellent positive electrode, is investigated as a negative electrode. To prove NAM stability as both positive and negative electrode, symmetric cells have been assembled without pre‐sodiation, which showed a reversible capacity of 73 mA h g −1 and a remarkable capacity retention of 82.6 % after 500 cycles. The outstanding cycling performance is ascribed to the high stability of the active material at both the highest and lowest Na‐ion storage plateaus and the rather limited electrolyte decomposition and solid‐electrolyte‐interphase (SEI) formation occurring. The long‐term stability of NAM at both electrodes enables its use as a “reference” electrode for the investigation of other positive and negative electrode materials for SIBs, resembling the role played by lithium titanate (LTO) and lithium iron phosphate (LFP) in LIBs.