LAUSR

Hard carbons, an important category of amorphous carbons, are non-graphitizable and are widely accepted as the most promising anode material for the emerging sodium-ion batteries (SIBs) because of their changeable low-potential charge/discharge plateaus. However, their microstructures are not fixed as layer-structured graphitic materials do and are difficult to accurately demonstrate. The successful use of hard carbons in SIBs revives the interests to give a clear picture of their complicated microstructures that are in close relevance to the sodium storage. In this review, we start by revisiting the past definitions and structural models of hard carbons, and then present our renewed understanding of their sodium storage mechanism, focusing especially on the critical low-potential plateaus. 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 an ideal configuration, namely sieving carbons with tightened pore entrance and enlarged pore body, is finally proposed for superhigh sodium storage. We expect this review to offer a clear picture of hard carbons, and help realize a truly rational design of high-capacity carbon anodes engining the industrialization of SIBs, and more promisingly open up a window for exploring their possible new uses. This article is protected by copyright. All rights reserved.