Designing Capacitive Contribution in Hard Carbon Materials for Balancing Energy and Power under High Current Density for Sodium-Ion Batteries

Developing high power density sodium-ion batteries by exploiting the high power nature of capacitive behavior has been a hot topic in recent years. However, the improvement in power density of sodium-ion batteries usually comes at the cost of a loss in energy density, so a trade-off between power and energy densities is required. Herein, we innovatively establish a connection between the capacitive contribution in the electrode material and the energy and power densities of sodium-ion batteries. The energy and power densities of sodium-ion batteries at high current densities are equilibrated by tuning the capacitive contribution in the hard carbon materials. First, it is proved that the power and energy densities are a joint function of the current density and the capacitive contribution by theoretical analysis. Then, hard carbon materials are designed and fabricated by tuning the capacitive contribution guided by the theoretical analysis to equilibrate energy and power densities of sodium-ion batteries at high current densities. Finally, the variation of the power and energy densities of the sodium-ion batteries with the current density and capacitive contribution is obtained. The results indicate that at low current densities (<1 A/g), the sodium-ion batteries employed with hard carbon anode with low capacitive contribution have a similar power density but higher energy density compared with those with high capacitive contribution. When the current density reaches or exceeds 1 A/g, the sodium-ion batteries employed with hard carbon anode with high capacitive contribution reveal both higher power and energy densities (power and energy densities are 8,316.66 Wh/kg and 251.81 W/kg at 3 A/g, respectively). These results are attributed to the various capacity decay rates of the battery and capacitive parts at different current densities, and the energy density provided by the capacitive part is limited. This work provides guidance on the introduction of capacitive contribution in electrode materials for ion batteries from a full-battery perspective.