Design and scaling hybrid hard carbon electrodes to balance energy and power performance in Sodium-ion batteries

Abstract

Achieving both high energy and power density in a single Sodium-ion battery system remains a major challenge, as conventional designs often trade one for the other. This study addresses this gap by introducing a hybrid hard carbon (HHC) strategy that blends micro and nano-sized hard carbon particles to optimize electrode structure and performance. By systematically tuning particle size ratios, the work demonstrates how key electrode parameters, such as porosity and tortuosity, directly influence initial Coulombic efficiency, rate capability, and long-term stability. Among the various configurations, the HHC5050 electrode (50 % microparticles and 50 % nanoparticles, ∼2.5 mg/cm²) offers a well-balanced profile, delivering excellent reversibility (∼87.7 % capacity retention after 500 cycles), and favorable ion transport (theoretical tortuosity ∼1.25; experimental ∼10.3). It maintains ∼171.6 mAh g⁻¹ at 2C (∼54 % of the 0.1C capacity) and shows stable interfacial behavior confirmed by time-dependent electrochemical impedance and distribution of relaxation time analyses. When scaled to a high mass loading (∼13.6 mg/cm²), it retains ∼90 % of its initial capacity after 100 cycles with minimal performance loss. Importantly, the study bridges material-level optimization to practical implementation, demonstrating projected excellent full-cell performance with various cathodes. Paired with a Prussian blue analogue, the optimized HHC anode achieves projected energy densities of ∼453.2 Wh/kg (electrode level) and ∼175.8 Wh/kg (pouch cell level). These findings position the HHC5050 electrode as a viable and scalable anode design for future Sodium-ion batteries targeting mid-range electric vehicles and energy storage systems.