P2-type manganese-based cathode materials are notable for their high theoretical capacity and open prismatic channels. However, rapid capacity decay caused by harmful irreversible phase transformations (P2-O2) and oxygen release in the high-voltage region (>4.2 V) limits their commercial application. Here, a co-doping strategy with lithium and niobium is proposed to enhance the structural stability at both room-temperature (RT) and low-temperature (LT) over extended cycles. The X-ray absorption near-edge spectra and first-principles calculations reveal that Li doping elevates the valence states of transition metals, thereby enhancing the reversibility of anionic redox reactions. Additionally, Nb doping leads to the formation of a rock-salt-like phase on the surface, which improves the stability of the electrode/electrolyte interface. In situ X-ray diffraction and differential electrochemical mass spectrometry show that Li and Nb co-doping prevents the P2-O2 phase transition, suppresses gas release, and enhances long-cycle stability. The P2-Na 0.67 Ni 0.24 Mn 0.64 Li 0.1 Nb 0.02 O 2 (NNMLNb) exhibits a reversible specific capacity of 127.4 mAh g −1 and 101.5 mAh g −1 at -20 °C and -30 °C, respectively. Remarkably, the sample cycles stably for 1200 cycles with a discharge capacity of 85 mAh g −1 , corresponding 90 % capacity retention. This work provides a novel strategy to improve the cycling life of sodium-ion batteries at both RT and LT.
The application of sodium-based batteries in grid-scale energy storage requires electrode materials that facilitate fast and stable charge storage at various temperatures. However, this goal is not entirely achievable in the case of P2-type layered transition-metal oxides because of the sluggish kinetics and unfavorable electrode|electrolyte interphase formation. To circumvent these issues, we propose a P2-type Na 0.78 Ni 0.31 Mn 0.67 Nb 0.02 O 2 (P2-NaMNNb) cathode active material where the niobium doping enables reduction in the electronic band gap and ionic diffusion energy barrier while favoring the Na-ion mobility. Via physicochemical characterizations and theoretical calculations, we demonstrate that the niobium induces atomic scale surface reorganization, hindering metal dissolution from the cathode into the electrolyte. We also report the testing of the cathode material in coin cell configuration using Na metal or hard carbon as anode active materials and ether-based electrolyte solutions. Interestingly, the Na||P2-NaMNNb cell can be cycled up to 9.2 A g −1 (50 C), showing a discharge capacity of approximately 65 mAh g −1 at 25 °C. Furthermore, the Na||P2-NaMNNb cell can also be charged/discharged for 1800 cycles at 368 mA g −1 and −40 °C, demonstrating a capacity retention of approximately 76% and a final discharge capacity of approximately 70 mAh g −1 .
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