N-doped carbon encapsulated CoMoO4 nanorods as long-cycle life anode for sodium-ion batteries.

Volume expansion and poor conductivity result in poor cyclability and low rate capability, which are the major challenges of transition-metal oxide as anode materials for sodium-ion batteries (SIBs). Herein, N-doped carbon encapsulated CoMoO4 (CoMoO4@NC) nanorods are developed as excellent anode materials for SIBs with long-cycle life. The N-doped carbon shells serve as buffer to accommodate severe volume changes during sodiation/desodiation, and at the same time improve electronic conductivity and activate surface sites of CoMoO4. The optimized composite presents rapid reaction kinetics and excellent cycle stability. Even at a high current density of 1 A g-1, it still shows long-cycle life and maintains specific capacity of 190 mAh g-1 after 3200 cycles. Furthermore, CoMoO4@NC anode is applied to match with Na3V2(PO4)3 cathode to assemble full-cells, in which it accomplishes reversible capacity of 152 mAh g-1 after 100 cycles, with capacity retention of 75% at a current density of 1 A g-1, highlighting the practical application for SIBs.

Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi0.45- xMn0.25Ti0.3Co xO2 for Sodium-Ion Storage at High Voltage.

O3-type NaNiO2-based cathode materials suffer irreversible phase transition when they are charged to above 4.0 V in sodium-ion batteries. To solve this problem, we partially substitute Ni2+ in O3-type NaNi0.45Mn0.25Ti0.3O2 by Co3+. NaNi0.45Mn0.25Ti0.3O2 with co-substitution possesses an expanded interlayer and exhibits higher rate capability, as well as cyclic stability, compared with the pristine cathode in 2.0-4.4 V. The optimal NaNi0.4Mn0.25Ti0.3Co0.05O2 delivers discharge capacities of 180 and 80 mA h g-1 at 10 and 1000 mA g-1. At 100 mA g-1, NaNi0.4Mn0.25Ti0.3Co0.05O2 exhibits 152 mA h g-1 in the initial cycle and maintains 91.4 mA h g-1 after 180 cycles. Through ex situ X-ray diffraction, co-substitution is demonstrated to be effective in enhancing the reversibility of P3-P3″ phase transition from 4.0 to 4.4 V. Electrochemical impedance spectroscopy indicates that higher electronic conductivity is achieved by co-substitution. Moreover, cyclic voltammetry and the galvanostatic intermittent titration technique demonstrate faster kinetics for Na+ diffusion due to the co-substitution. This study provides a reference for further improvement of electrochemical performance of cathode materials for high-voltage sodium-ion batteries.