A novel strategy for capacity judgement of hard carbon in sodium-ion batteries: Ensuring the consistency of the available anode capacity between half-cell and full-cell

Hard carbon, with its abundant resources, low cost, and high specific capacity, is wildly accepted as the most promising anode material of sodium-ion batteries (SIBs). However, current half-cell test method for assessing the available specific capacity of hard carbon faces challenges. Typically, the constant voltage or low current discharge strategy is typically adopted at the end of the discharge process to minimize the influence of polarization. Unfortunately, owing to the potential of constant voltage or low current discharge stage is close to the sodium deposition potential, there is a risk of sodium plating on hard carbon anode, making the sodiation capacity from half-cell test is difficult to be fully employed in full-cell system. In light of this, this work proposes an improved half-cell test strategy that fits the dQ/dV curve of the desodiation process of hard carbon at over-sodiated states, allowing for a more accurate determination of the available capacity of the hard carbon. Moreover, the reliability of this strategy is verified in 21700 cylindrical cells through different anode capacity design. This work provides a reliable methodology for capacity design of hard carbon anode in practical full-cell system and accelerates the commercialization of SIBs.