The demand to increase energy density of rechargeable batteries for portable electronic devices and electric vehicles and to reduce the cost for grid-scale energy storage necessitates the exploration of new chemistries of electrode materials for rechargeable batteries. The open framework-structure of Prussian-blue materials has recently been demonstrated as a promising cathode host for a variety of monovalent and multivalent cations with the tunable working voltage and discharge capacities. Recent progress toward the application of Prussian-blue cathode materials for rechargeable batteries is reviewed, with special emphasis on charge-storage mechanisms of different insertion species, factors influencing electrochemical performances, and possible approaches to overcome their intrinsic limitations.

In recent years, considerable attention has been focused on the development of sodium-ion batteries (SIBs) because of the natural abundance of raw materials and the possibility of low cost, which can alleviate the concerns of the limited lithium resources and the increasing cost of lithium-ion batteries. With the growing demand for reliable electric energy storage devices, requirements have been proposed to further increase the comprehensive performance of SIBs. Especially, the low-temperature tolerance has become an urgent technical obstacle in the practical application of SIBs, because the low operating temperature will lead to sluggish electrochemical reaction kinetics and unstable interfacial reactions, which will deteriorate the performance and even cause safety issues. On the basis of the charge-storage mechanism of SIBs, optimization of the composition and structure of electrolyte and electrode materials is crucial to building SIBs with high performance at low temperatures. In this review, the recent research progress and challenges were systematically summarized in terms of electrolytes and cathode and anode materials for SIBs operating at low temperatures. The typical full-cell configurations of SIBs at low temperatures were introduced to shed light on the fundamental research and the exploitation of SIBs with high performance for practical applications.

The V 4+ /V 3+ (3.4 V) redox couple has been welldocumented in cathode material Na 3 V 2 (PO 4 ) 3 for sodium-ion batteries. Recently, partial cation substitution at the vanadium site of Na 3 V 2 (PO 4 ) 3 has been actively explored to access the V 5+ /V 4+ redox couple to achieve high energy density. However, the V 5+ /V 4+ redox couple in partially substituted Na 3 V 2 (PO 4 ) 3 has a voltage far below its theoretical voltage in Na 3 V 2 (PO 4 ) 3 , and the access of the V 5+ /V 4+ redox reaction is very limited. In this work, we compare the extraction/insertion behavior of sodium ions from/into two isostructural compounds of Na 3 VGa(PO 4 ) 3 and Na 3 VAl(PO 4 ) 3 , found that, by DFT calculations, the lower potential of the V 5+ /V 4+ redox couple in Na 3 VM(PO 4 ) 3 (M = Ga or Al) than that in Na 3 V 2 (PO 4 ) 3 is because of the extraction/insertion of sodium ions through the V 5+ /V 4+ redox reaction at different crystallographic sites, that is, sodium ions extracting from the Na(2) site in Na 3 VM(PO 4 ) 3 while from the Na(1) site in Na 3 V 2 (PO 4 ) 3 , and further evidenced that the full access of the V 5+ /V 4+ redox reaction is restrained by the excessive diffusion activation energy in Na 3 VM(PO 4 ) 3 .

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