Sodium-Ion Batteries: Current Understanding of the Sodium Storage Mechanism in Hard Carbons

In recent years, sodium-ion batteries (NIBs) have been greatly explored as an alternative technology to lithium-ion batteries (LIBs) due to their cost-effectiveness and promise in mitigating the energy crisis we currently face. Similarities between both battery systems have enabled a fast development of NIBs, however, their full commercialisation has been delayed due to the lack of an appropriate anode material. Hard carbons (HCs) arise as one of the most promising materials and are already used in the first generation of commercial NIBs. Although promising, HCs exhibit lower performance compared to commercial graphite used as an anode in LIBs in terms of reversible specific capacity, operating voltage, initial coulombic efficiency and cycling stability. Nevertheless, these properties vary greatly depending on the HC in question e.g. surface area, porosity, degree of graphitisation, defect amount, etc., which in turn are dependent on the synthesis method and precursor used. Optimisation of these properties will bring forward the widespread commercialisation of NIBs at a competitive level with current LIBs. This review aims to provide a brief overview of the current understanding of the underlying reaction mechanisms occurring in the state-of-the-art HC anode material as well as their structure-property interdependence. We expect to bring new insights into the engineering of HC materials to achieve optimal, or at least, comparable electrochemical performance to that of graphite in LIBs.