Lithium-substituted sodium layered transition metal oxide fibers as cathodes for sodium-ion batteries

Na layered transition metal oxide fibers with/without partial Li substitution were prepared by the combination of a novel centrifugal spinning method and thermal treatment. Compared to the Li-free fibers, the Li-substituted Na layered transition metal oxide fiber cathodes exhibit higher capacities, better cycling stability and enhanced rate capability. Among the studied Li-substituted Na layered transition metal oxide fibers, Na_0.8Li_0.4Ni_0.15Mn_0.55Co_0.1O₂ demonstrates the best overall electrochemical performance. The highest capacity of 138 mA h g^-1 is achieved at 15 mA g^-1. As the current density increased to 75 and 300 mA g^-1, Na_0.8Li_0.4Ni_0.15Mn_0.55Co_0.1O₂ fibers still deliver high capacities of 113 and 94 mA h g^-1, respectively. This specific fiber composition also yields stable cycling performance and superior rate performance at various current densities between 15-600 mA g^-1. The results suggest that partial Li substitution is an effective method to stabilize the structure of the Na layered transition metal oxide cathodes and hence enhance the electrochemical performance. It is also demonstrated that centrifugal spinning can be an attractive technology for mass production of micro-sized, fibrous electrodes.