Structural and vacancy assisted engineering of cobalt selenide for ultrahigh energy density sodium ion pouch cell†

Cobalt selenide (CoSe) exhibits potential as an anode material in sodium-ion batteries (SIBs), but challenges remain in achieving stable Na⁺ storage and high energy density full cells by controlling CoSe. In this work, multi-scale modulation of CoSe was achieved through structural and vacancy engineering. Specifically, a phosphorus-doped Co_0.85Se@nitrogen-doped carbon hollow nanobox (P-Co_0.85Se@PNC) was constructed by optimizing pyrolysis of chemically-modified ZIF-67 templates followed by selenization and in situ P doping. The P-Co_0.85Se@PNC prepared by the multi-step method possesses a homogeneous, hollow structure, effectively mitigating the volume stress caused by sodium ion extraction during cycling. The effective doping of P elements in Co_0.85Se@NC introduces vacancies and increases the lattice spacing, facilitating Na⁺ transport. During sodium ion half-cell performance evaluation, the P-Co_0.85Se@PNC material demonstrates robust electrochemical behavior, showcasing a consistent and reversible specific capacity of 351.52 mA h g⁻¹ over 100 cycles at 1 A g⁻¹. Moreover, it exhibits remarkable cycling stability, experiencing only a negligible 0.075% capacity decay after 1000 cycles at a high current density of 10 A g⁻¹. Detailed kinetic analysis of the P-Co_0.85Se@PNC, along with dynamic crystalline phase/morphological changes during charge and discharge processes, elucidated its Na⁺ extraction mechanism. In order to broaden the utilization of P-Co_0.85Se@PNC anode materials in SIBs, a pouch cell assembly incorporating P-Co_0.85Se@PNC and NaNi_1/3Fe_1/3Mn_1/3O₂ was employed. Examination revealed the attainment of an extraordinarily high energy density, reaching 205.63 W h kg⁻¹ (power density: 330 W kg⁻¹), concomitant with flexible attributes. This study provides a blueprint for material optimization and high-energy density device applications based on cobalt selenide sodium-ion battery anodes.