photocopying process took nearly a century from 1843 until the early 1940s, while the detailed crystal structure of PB was first confirmed as cubic by Ludi and co-workers in 1977, which is now widely accepted. [6] Remarkably, the past four decades have witnessed the exploration of PB in more and more new and totally different, but very promising application areas, reaching from rechargeable batteries [7] to catalysis [8] and biosensors, [9] from optically switchable films in electrochromic devices (smart windows) [10] to a helpful nanomaterial for cancer therapy. [11] Due to their excellent redox activity, low cost, and highly reversible phase transitions during the insertion/extraction process of certain cations, PB and PBAs have also been widely investigated as promising active materials for energy storage devices, especially for commercial sodium-ion batteries (SIBs) beyond other batteries system (potassium-ion batteries, [12,13] lithium-ion batteries (LIBs), [14] lithium-sulfur batteries (LI-S), [15] lithium-air batteries, [16] zinc-air batteries, [17] solid-state batteries, [18] etc.) in large-scale stationary energy storage systems in the near future. [19,20] The chemical formulas of PBAs could be represented asHere, A represents a single alkali metal or alkaline earth metal, or a mixture of these metals, while M 1 and M 2 typically are transition metals bonded by CN − bonds to form a 3D open structure with the capability to host element(s) A inside the crystal structure. □ represents the vacancy that is caused by the loss of an M 2 (CN) 6 group and the occupation by coordination water and interstitial water, the species and ionic radii of which are shown in Figure 2a. [21] With the different species and various ratios of A/M 1 /M 2 , the number of family members could reach more than 100, sharing different crystal phases, including monoclinic, [22,23] rhombohedral, [24,25] cubic, [26,27] tetragonal, [28] hexagonal, [29] etc. According to the amount of redox-active sites for battery application, PB and PBAs could be divided into dual-electron transfer type (DE-PBAs: M 1 and M 2 = Mn, Fe, Co) and single-electron transfer type (SE-PBAs: M 1 = Zn, Ni and M 2 = Fe, Co, Mn) with theoretical specific capacity of 170 and 85 mAh g −1 , respectively. [21] Taking the high average voltage and capacity of the DE-PBAs into consideration, they are promising and competitive, even to the level of LiFePO 4 (a well-known cathode material for the LIBs), for high energy density devices (≈450 Wh kg −1 on the material level). On the other hand, the negligible structural distortion and high conductivity of SE-PBAs make them desirable choices for fast-charging and long-life devices. [20,30] Prussian blue analogues (PBAs) have attracted wide attention for their application in the energy storage and conversion field due to their low cost, facile synthesis, and appreciable electrochemical performance. At the present stage, most research on PBAs is focused on their material-level optimization, whereas their properties in practical b...
