Manganese vacancy motivated structural disorder-to-order transformation to boost fast-charging and long-lasting sodium-ion battery P2-type layered cathode

Abstract

In this study, manganese vacancy and Li-ion are introduced into Na_2/3[Ni_1/6Mn_5/6]O₂ to tailor the relationship between the structure and property. Neutron and X-ray diffraction confirm a structural transition from disordering to ordering in the transition-metal layers to form an in-plane honeycomb structure in the designed Na_5/6[Ni_1/6Li_1/6□_1/18Mn_2/3]O₂ (V-NNM, □ = Mn vacancy) material, which, for the first time, benefits from the Mn vacancies in transition-metal sites. Besides, neutron diffraction also uncovers that V-NNM adopts the P2 structure with the P6₃ space group, different from the P6₃/mmc space group by X-ray diffraction (XRD), and doped Li ions mainly enter the transition-metal sites in V-NNM. Electrochemical measurements demonstrate that V-NNM delivers a reversible specific capacity close to the theoretical value and long-term cycling stability (71.6% capacity retention after 1000 cycles). V-NNM also exhibits the excellent high-rate performance, such as the reversible capacities of 72.5%, 63.7%, 57.1% and 50.5% relative to the theoretical value at 10 C, 15 C, 20 C and 25 C, respectively, indicating the fast Na-storage. Theoretical calculations and molecular dynamics simulations further validate the structural disorder-to-order transformation, decreased band gap and enhanced Na⁺ diffusion kinetics in V-NNM, which are responsible for the electrochemical performance. In addition, in situ XRD experiments disclose a complete solid-solution reaction layered cathode accompanied by near zero-strain characteristic upon charging and discharging, ensuring the structural integrity and stability, as well as the resulting electrochemical performance. This work paves the way for comprehending and optimizing the structure-property relationship of layered materials for sodium-ion batteries.