LAUSR

Manganese-based spinel cathode materials for sodium-ion batteries (SIBs) are promising candidates for next-generation batteries. Unfortunately, most SIB cathode materials exhibit inferior electrochemical properties and are less understood owing to their lower performance. Na[Ni0.5Mn1.5]O4 (NNMO) should be a promising candidate because of its high operating voltage and firm octahedral host structure. Compared with Li[Ni0.5Mn1.5]O4, first-principle calculations are conducted to elucidate the reasons for the low performance of NNMO and determine the requirements for realizing high-performance cathode materials for SIBs. Owing to the Ni2+/Ni4+ double redox, NNMO could operate at a high voltage; however, the large Na+ increases the local site energy of the redox center, promoting electron extraction from the redox center, leading to unexpected voltage loss. Additionally, the homogeneous free energy confirms that NNMO would undergo phase separation into fully intercalated and deintercalated phases, inducing lattice misfits along the interfaces of the two phases. Particularly, higher phase transition barrier and large Na+ cause fast phase separation, inducing increased polarization and severe stress field upon cycling. Strategies to improve manganese-based spinel cathode materials of SIBs, especially NNMO, must be differentiated by considering the effects of thermodynamic stability, structural evolution, and electronic structure on the electrochemical and cycle performance of cathode materials.