Performance evaluation of sodium-ion batteries for microgrid and off-grid energy storage applications

Introduction Sodium-ion batteries are increasingly considered for stationary energy storage; however, their application-level performance depends on how experimentally measured electrochemical behavior translates into realistic grid-connected and standalone operating conditions. Methods This study evaluated 24 5.0-Ah sodium-ion pouch cells through baseline electrochemical characterization, identification of a second-order Thevenin electrochemical-thermal model, annual system simulation, and application-specific aging tests. Discharge behavior was characterized at rates of 0.20–2.00 C and temperatures of 10–40 °C. Pulse and impedance measurements were used to parameterize the model across 10%–90% state of charge. The calibrated model was scaled to a 504-kWh grid-connected microgrid and a 120.96-kWh standalone off-grid system, both simulated over 365 days at 15-min resolution. Results Increasing discharge rate reduced usable capacity and energy efficiency, whereas Coulombic efficiency remained above approximately 99%. The model reproduced measured voltage responses with millivolt-scale prediction errors and captured state-of-charge- and temperature-dependent resistance trends. The grid-connected microgrid achieved 91.6% round-trip efficiency, an 18.7-kW tracking root-mean-square error, and 4.3% renewable curtailment. The standalone system achieved 89.8% round-trip efficiency, a 1.6% annual loss-of-power-supply probability, and 6.4% curtailment. After 500 equivalent full cycles, capacity state of health was 85.2%, 81.9%, and 79.8% under reference, microgrid, and off-grid cycling, respectively, with corresponding resistance growth of 39%, 58%, and 74%. Discussion These findings demonstrate that application-specific duty cycles materially influence the efficiency, reliability, and degradation of sodium-ion battery storage systems. Integrating experimentally calibrated electrochemical-thermal models with application-level simulations provides a practical basis for evaluating battery performance under realistic stationary energy-storage conditions.