scarce resources, uneven distribution, and arduous recycling of lithium. Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) operating with similar mechanism to that of LIBs are considered as affordable alternatives, [2] as a result of the desirable performances as well as much abundant resources of sodium and potassium. [3] The performances of the alkali metal-ion batteries depend much on the cathode and anode materials. Various types of cathode materials based on the reversible insertion/ extraction of alkali metal ions including transition metal oxides, fluorides, phosphates, hexacyanoferrates, and sulfates have been developed, and plenty of them exhibit desirable energy density and cycling performances. [4] Progress on the research for anode materials is relatively slow, however, as compared with their cathode counterparts. [5] Based on the reaction mechanisms, the anodes generally fall into three categories: insertion based, conversion based, and alloying based. [6] The conversion-based materials exhibit high theoretical specific capacities derived from the conversion reactions during the uptake of alkali metal ions. [7] Due to the large volume variations during charge/discharge, however, the conversion-based anodes exhibit rapid capacity fading. The alloyingbased materials deliver high specific capacity by the alloying reaction, but the material pulverization derived from repeated volume changes results in poor reversibility. [8] Insertion-based materials include titanium-based oxides and carbonaceous materials. Although the small volume change, high rate capability, and good cycling stability of titanium-based oxides are desirable, their high working voltages and low specific capacities are detrimental to the power density of the full cells. [9] Carbonaceous materials, including graphite, carbon nanotubes (CNTs), graphene, soft carbon (SC), hard carbon (HC), etc., are promising anode candidates for alkali metal-ion batteries. [10] Graphite has been developed as a practical anode for commercial LIBs. They have steady discharge curves and low operation potential (≈0.1 V vs Li + /Li), and the formation of stable graphite intercalation compounds (GICs) LiC 6 delivers a moderate theoretical intercalation capacity of 372 mAh g −1 . [11] While the intercalation capacities of graphite anodes for SIBs and PIBs are not satisfactory, delivering 35 mAh g −1 for SIBs with NaC 64Hard carbon (HC) is recognized as a promising anode material with outstanding electrochemical performance for alkali metal-ion batteries including lithium-ion batteries (LIBs), as well as their analogs sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Herein, a comprehensive review of the recent research is presented to interpret the challenges and opportunities for the applications of HC anodes. The ion storage mechanisms, materials design, and electrolyte optimizations for alkali metal-ion batteries are illustrated in-depth. HC is particularly promising as an anode material for SIBs. The solid-electrolyte interph...

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 CN − 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...

No abstract