Hard carbons, an important category of amorphous carbons, are non-graphitizable and are widely accepted as the most promising anode materials for emerging sodium-ion batteries (SIBs)， because of their changeable low-potential charge/discharge plateaus. However, their microstructures are not fixed and are difficult to accurately demonstrate as graphites do. The successful use of hard carbons in SIBs revives the interest to clearly picture their complicated microstructures that are in close relevance to sodium storage. In this review, the past definitions and structural models of hard carbons are revisited first, and a renewed understanding of their sodium storage is presented. Three critical structural features are highlighted for hard carbons, namely crystallites, defects, and nanopores, which are directly responsible for the presence of the low-potential plateaus and their reversible extension. The impact of these structural features upon the sodium storage is then deeply discussed and sieving carbons is finally proposed as an ideal configuration of carbon anode for superhigh sodium storage. This review is expected to offer a clear picture of hard carbons, and help realize a truly rational design of high-capacity carbon anodes, driving the industrialization of SIBs, and more promisingly open up a window for exploring their possible new uses.
Keywords: crystallites; defects; hard carbons; nanopores; sodium-ion batteries.
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