Ping Hu

Na4Co3(PO4)2P2O7 is a representative mixed polyanionic cathode with a high operating voltage and high energy density. However, insufficient cycling stability is the core bottleneck constraining the widespread application of sodium-ion batteries (SIBs). Herein, we propose a synergistic strategy that combines non-stoichiometry with Fe2+ doping to construct Na3.4Co2.4–xFex(PO4)1.4P2O7 (NCFPP-x), which not only enhances phase purity but also improves material stability. Through ex situ electrochemical impedance spectroscopy-distribution of relaxation times and galvanostatic intermittent titration technique analyses, it is demonstrated that Fe doping leads to the reduced charge transfer impedance and enhanced Na+ diffusion coefficient. The in situ XRD characterization results demonstrate a highly reversible incomplete solid-solution reaction mechanism during the Na+ intercalation and deintercalation processes. The NCFPP-1.2||Hard carbon (HC) full cell assembled with an HC anode delivers an energy density of 325 Wh kg–1, retaining 80.0% after 200 cycles. The present work offers important theoretical basis and experimental support for the advancement of high-voltage polyanion-type cathode materials.

To enhance the energy density of batteries and explore intrinsic charge storage mechanism of the active materials, it is important to reduce or eliminate the use of non‐active materials in electrodes, such as binder and conductive additives. Herein, free‐standing Na₄MnV(PO₄)₃@C (F‐NMVP@C) fiber membrane is fabricated and directly used as a sodium‐ion battery (SIB) cathode. In situ X‐ray diffraction reveals that the V³⁺/V⁴⁺ redox reaction occurs through a solid‐solution reaction while a two‐phase Mn²⁺/Mn³⁺ redox reaction is identified, and both are highly reversible. Meanwhile, ex situ electrochemical impedance spectroscopy reveals that both the ion diffusion coefficient and charge transfer resistance of F‐NMVP@C change reversibly during the Na⁺ intercalation/de‐intercalation. Battery full cells are assembled based on the free‐standing F‐NMVP@C cathodes and F‐Sb@C anodes, which manifests a high energy density (293 Wh kg⁻¹) and good cyclability (87.5% after 100 cycles at 1 C). The high‐performance free‐standing cathodes and anodes shed light on the development of flexible SIBs.

Aluminum-ion batteries have attracted great interest in the grid-scale energy storage field due to their good safety, low cost and the high abundance of Al. However, Al anodes suffer from severe dendrite growth, especially at high deposition rates. Here, we report a simple strategy for constructing a highly reversible, dendrite-free, Al-based anode through directly introducing a solid-solution-based metal coating to a Zn foil substrate. Compared with Cu foil substrates and bare Al, a Zn foil substrate shows a lower nucleation barrier of Al deposition due to the intrinsic, definite solubility between Al and Zn. During Al deposition, a thin, solid-solution alloy phase is first formed on the surface of the Zn foil substrate and then guides the parallel growth of flake-like Al on Zn substrate. The well-designed, Zn-coated Al (Zn@Al) anode can effectively inhibit dendrite growth and alleviate the corrosion of the Al anode. The fabricated Zn@Al–graphite battery exhibits a high specific capacity of 80 mAh·g⁻¹ and an ultra-long lifespan over 10,000 cycles at a high current density of 20 A·g⁻¹ in low-cost molten salt electrolyte. This work opens a new avenue for the development of stable Al anodes and can provide insights for other metal anode protection.