Comprehensive characterisation of cell to cell variation in commercial sodium ion 18650 battery cells

With the first commercial sodium-ion (Na-ion) cells entering the battery market and several manufacturers announcing upcoming Na-ion production, understanding their cell-to-cell variation is becoming essential for the safe and reliable deployment in energy storage systems. This work presents a comprehensive characterisation of electrical and electrochemical variability in a batch of 100 fresh commercial 18650 Na-ion cells. Disparities in nominal capacity, open-circuit voltage (OCV) profiles, electrochemical impedance spectroscopy (EIS) features, incremental capacity analysis (ICA) peaks, and internal resistance (IR) are evaluated, and their mutual correlations are discussed. The nominal capacity exhibits a coefficient of variation (CV) of 2.14%, demonstrating excellent homogeneity that is comparable to, or even surpassing, that of commercial 18650 Li-ion cells. In contrast, EIS features show notably higher variability, which may reflect the novelty and ongoing optimisation of current Na-ion manufacturing. Irregularities in OCV profiles limit the accuracy of state-of-charge (SOC) estimation, with errors approaching ∼2% in the low-SOC region, while remaining below ∼1% at mid- and high-SOC levels. IR values are comparable to those reported for Li-ion batteries, and IR profiles remain relatively uniform while correlating strongly with nominal capacity ( r = −0.87), identifying IR as a promising SOH indicator. ICA reveals five peaks, three of which are consistently recognisable across the entire population. These peaks show strong correlations with several key battery parameters, highlighting their diagnostic relevance. Overall, this work establishes a comprehensive baseline for understanding cell-to-cell variation in commercial Na-ion cells and underlines the need to incorporate such variability into diagnostic methods, battery models, and BMS strategies for future Na-ion battery systems. • A batch of 100 commercial 18650 Na-ion cells was characterised, providing a baseline of their cell-to-cell variation • Correlations between EIS, GITT, and ICA features, nominal capacity, OCV values, and other parameters were analysed • Nominal capacity exhibited excellent homogeneity, highly comparable to that of commercial Li-ion cells • Internal resistance and ICA peaks B-D demonstrate strong diagnostic relevance for future SOH evaluation protocols • Low-SOC OCV variation limits the accuracy of OCV-based SOC estimation to ∼2%, highlighting a key challenge for Na-ion BMS