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...
