Influence of Particle Size and Mass Loading of Hard Carbon on Sodium Ion Battery Rate Performance in Industrially Relevant Full Cells

Sodium-ion batteries (SIBs) represent a great opportunity for stationary storage, back-up power supply and light electric vehicle applications. Energy and power requirements for these applications needs to be satisfied. Beside material improvement, electrode microstructure plays a critical role. Herein, the influence of hard carbon (HC) particle size and mass loading on the rate performance have been comprehensively investigated. This work analyses the relative contribution of ionic resistance within the porous electrode (R_ion) in SIB full cells employing relevant mass loadings. R_ion was obtained by applying electrochemical impedance spectroscopy (EIS) to symmetrical cells and by fitting a transmission line model (TLM) under “blocking” conditions. The relative contribution of R_ion arising from HC electrodes was further compared to the charge transfer resistance (R_CT) in full cell configuration utilising a three-electrode cell under “non-blocking” conditions. Results illustrate mass loading to be as far more influential on cell rate performance compared to the two particle sizes analyzed. The three-electrode study reveals the contribution of R_ion to be greater than that of R_CT at high mass loadings and high states of charge for the HC electrode and elucidates that the gravimetric and volumetric discharge capacity of cells is limited by the layered oxide cathode and HC electrode respectively.