Kai Fang’s research while affiliated with Xiamen University and other places

Efforts to improve the specific capacity and energy density of lithium nickel-cobalt-manganese oxide (NCM) cathodes focus on operating at high voltages or increasing nickel content. However, both approaches necessitate a...

As the preferred anode material for sodium‐ion batteries, hard carbon (HC) confronts significant obstacles in providing a long and dominant low‐voltage plateau to boost the output energy density of full batteries. The critical challenge lies in precisely enhancing the local graphitization degree to minimize Na⁺ ad‐/chemisorption, while effectively controlling the growth of internal closed nanopores to maximize Na⁺ filling. Unfortunately, traditional high‐temperature preparation methods struggle to achieve both objectives simultaneously. Herein, a transient sintering‐involved kinetically‐controlled synthesis strategy is proposed that enables the creation of metastable HCs with precisely tunable carbon phases and low discharge/charge voltage plateaus. By optimizing the temperature and width of thermal pulses, the high‐throughput screened HCs are characterized by short‐range ordered graphitic micro‐domains that possess accurate crystallite width and height, as well as appropriately‐sized closed nanopores. This advancement realizes HC anodes with significantly prolonged low‐voltage plateaus below 0.1 V, with the best sample exhibiting a high plateau capacity of up to 325 mAh g⁻¹. The energy density of the HC||Na3V2(PO4)3 full battery can therefore be increased by 20.7%. Machine learning study explicitly unveils the “carbon phase evolution−electrochemistry” relationship. This work promises disruptive changes to the synthesis, optimization, and commercialization of HC anodes for high‐energy‐density sodium‐ion batteries.

Both LiFePO4 (LFP) and NaFePO4 (NFP) are phosphate polyanion‐type cathode materials, which have received much attention due to their low cost and high theoretical capacity. Substitution of manganese (Mn) elements for LFP/NFP materials can improve the electrochemical properties, but the connection between local structural changes and electrochemical behaviors after Mn substitution is still not clear. This study not only achieves improvements in energy density of LFP and cyclic stability of NFP through Mn substitution, but also provides an in‐depth analysis of the structural evolutions induced by the substitution. Among them, the substitution of Mn enables LiFe0.5Mn0.5PO4 to achieve a high energy density of 535.3 Wh kg⁻¹, while NaFe0.7Mn0.3PO4 exhibits outstanding cyclability with 89.6% capacity retention after 250 cycles. Specifically, Mn substitution broadens the ion‐transport channels, improving the ion diffusion coefficient. Moreover, LiFe0.5Mn0.5PO4 maintains a more stable single‐phase transition during the charge/discharge process. The transition of NaFe0.7Mn0.3PO4 to the amorphous phase is avoided, which can maintain structural stability and achieve better electrochemical performance. With systematic analysis, this research provides valuable guidance for the subsequent design of high‐performance polyanion‐type cathodes.

The initial Na loss limits the theoretical specific capacity of cathodes in Na-ion full cell applications, especially for Na-deficient P2-type cathodes. In this study, we propose a presodiation strategy for cathodes to compensate for the initial Na loss in Na-ion full cells, resulting in a higher specific capacity and a higher energy density. By employing an electrochemical presodiation approach, we inject 0.32 excess active Na into P2-type Na0.67Li0.1Fe0.37Mn0.53O2 (NLFMO), aiming to compensate for the initial Na loss in hard carbon (HC) and the inherent Na deficiency of NLFMO. The structure of the NLFMO cathode converts from P2 to P'2 upon active Na injection, without affecting subsequent cycles. As a result, the HC||NLFMOpreNa full cell exhibits a specific capacity of 125 mAh/g, surpassing the value of 61 mAh/g of the HC||NLFMO full cell without presodiation due to the injected active Na. Moreover, the presodiation effect can be achieved through other engineering approaches (e.g., Na-metal contact), suggesting the scalability of this methodology.