Aqueous zinc ion batteries (A-ZIBs) have been used as new alternative batteries for grid-scale electrochemical energy storage because of their low cost and environmental protection. Finding a suitable and economical cathode material, which is needed to achieve high energy density and long cycle stability, is one of the most important and arduous challenges at the present stage. Potassium manganese hexacyanoferrate (KMHCF) is a kind of Prussian blue analogue. It has the advantages of a large 3D frame structure that can accommodate the insertion/extraction of zinc ions, and is nontoxic, safe, and easy to prepare. However, regularly synthesized KMHCF has higher water and crystal defects, which reduce the possibility of zinc ions' insertion/extraction, and subsequently the discharge capacity and cycling stability. In this work, a KMHCF material with less water and low defects was obtained by adding polyvinylpyrrolidone during the synthesis process to control the reaction process. The KMHCF serves as the cathode of A-ZIBs and exhibits an excellent electrochemical performance providing a specific capacity of 140 mA h g −1 for the initial cycle at a current density of 100 mA g −1 (1 C). In particular, it can maintain a reversible capacity of 85 mA h g −1 , even after 400 cycles at 1 C. Moreover, unlike the traditional zinc storage mechanism of A-ZIBs, we found that the KMHCF electrode undergoes a phase transition process when the KMHCF electrode was activated by a small current density, which is attributed to part of the Mn on the lattice site being replaced by Zn, thus forming a new stable phase to participate in the subsequent electrochemical reaction.

Sodium-ion batteries (SIBs) are on the verge of achieving practical applications, and the key is to find suitable electrode materials. The polyanionic iron-based material Na 3.12 Fe 2.44 (P 2 O 7 ) 2 (NFPO) possesses an open three-dimensional framework structure with good thermal stability and is regarded as an outstanding cathode material for SIBs. Nevertheless, its poor electrical conductivity, problems with erosion of electrolytes, and structural deterioration during cycling still need to be urgently addressed. Here, we first design a Mg 2+ -doped NFPO (NFPO-Mg) material with a dual-action effect. On the one hand, Mg 2+ improves the intrinsic conductivity of the NFPO material, and on the other hand, Mg 2+ promotes the formation of a homogeneous and stable cathode− electrolyte interphase film during the cycling process, which results in a superior rate performance and cycling stability. A capacity of 68.6 mAh g −1 was achieved at 50C (1C = 117.4 mAh g −1 ), and a capacity retention of 79.1% was maintained after 3000 cycles at 20C. More impressively, NFPO-Mg exhibits outstanding high-temperature electrochemical performance, with a capacity retention of 95.3% after 400 cycles at 10C at 60 °C (much higher than the 54.2% for the NFPO). This paper explores an effective method for improving the electrochemical performance of cathode materials, which may prove instrumental in guiding the design of more high-performance cathode materials in the future. KEYWORDS: sodium-ion battery, cathode material, Na 3.12 Fe 2.44 (P 2 O 7 ) 2 , Mg 2+ doping, cathode−electrolyte interphase

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