Fast-Charging Capabilities of Hard Carbon Anodes in Sodium-Ion Batteries: Mechanisms, Strategies, and Prospects

Abstract

The transition toward sustainable energy systems has accelerated the development of sodium-ion batteries (SIBs) as promising alternatives to lithium-ion batteries (LIBs), owing to their abundant sodium resources and cost advantages. However, the commercialization of SIBs for electric vehicles (EVs) remains hindered by challenges in achieving fast-charging performance, particularly at the anode. Hard carbons (HCs), widely regarded as the most practical anode materials, face intrinsic rate limitations due to their complex sodium storage mechanism, which involves a sloping region (> 0.10 V vs. Na/Na⁺) and a plateau region (0.01–0.10 V vs. Na/Na⁺), the latter posing a risk of sodium metal plating during rapid charging. This review examines the structural and kinetic factors that govern the fast-charging performance of HC anodes. The relationships among HC formation, microstructure, and sodium storage mechanisms are highlighted to clarify how structural features dictate electrochemical behavior. Key factors limiting rate capability, including electronic and ionic conductivity, as well as Na⁺ desolvation and diffusion, are critically assessed. Recent advances in material design, such as precursor optimization, heteroatom doping, closed-pore structure regulation, metal atom modulation, and surface coating, are evaluated to identify strategies for enhancing Na⁺ transport. Progress in electrode–electrolyte interphase engineering, particularly through electrolyte optimization and HC surface modification for stable SEI formation, is also summarized. By linking fundamental kinetics with practical design, this review provides insights for developing high-performance HC anodes and accelerating the deployment of fast-charging SIBs in next-generation EVs.