Sodium-ion batteries (SIBs) are required to possess long cycle life when used for large-scale energy storage. The polyanionic Na₄MnV(PO₄)₃ (NMVP) reveals good cyclic stability due to its unique three-dimensional (3D) frame structure, but it still faces the challenge of interfacial degradation in practical applications. In this work, NASICON-type Na_1.3Al_0.7Ti_1.3(PO₄)₃ (NATP) was deposited on the surface of NMVP to promote interface stability by surface modification and gradient doping. As a result, the optimized NMVP@2%NATP released a capacity retention of 44.8% after 1000 cycles at 5 C, much higher than that of the initial NMVP (28.9%). The enhanced electrochemical performance was mainly attributed to NATP coating acting as a fast ion transport carrier and physical barrier, significantly facilitating the Na⁺ diffusion and isolating side reaction at the electrode/electrolyte interface. On the other hand, Ti⁴⁺ and Al³⁺ cations from the NATP were partially doped inside the NMVP surface to boost the transport of Na⁺, and the perfect lattice matching of NVMP and NATP improved the interface and structural stability accompanying long cycling. This work demonstrated the effectiveness of surface modification with high lattice match material and provided new perspectives for high energy density solid-state SIBs.

Considering limited energy density of current lithium metal batteries (LMBs) due to low capacity of traditional intercalation-type cathodes, alternative high-energy cathodes are eagerly demanded. In this regard, conversion-type metal fluoride/sulfide/oxide cathodes have emerged great attention owing to their high theoretical specific capacities, supplying outstanding energy density for advanced LMBs. However, their low ionic/electrical conductivities, huge volume changes, sluggish reaction kinetics, and severe side reactions result in quick capacity fading and poor rate capability of LMBs. Recent research efforts on the conversion-type cathodes have brought new insights, as well as effective approaches toward realizing their excellent electrochemical performances. Here, the recent discoveries, challenges, and optimizing strategies including morphology regulation, phase structure engineering, surface coating, heterostructure construction, binder functionalization, and electrolyte design, are reviewed in detail. Finally, perspectives on the conversion-type metal fluoride/sulfide/oxide cathodes in LMBs are provided. It is believed that the conversion-type cathodes hold a promising future for the next-generation LMBs with high energy density.