Heat generation rates of NaFePO4 electrodes for sodium-ion batteries and LiFePO4 electrodes for lithium-ion batteries

The heat generation connected to charging and discharging NaFePO4 (NFP) electrodes in sodium-ion batteries and LiFePO4 (LFP) electrodes in lithium-ion batteries is investigated. To guarantee good comparability, NFP based electrodes are prepared by an electrochemical displacement method using LFP electrodes as starting material resulting in identical particle size distribution, active material loading, binder, conductive additives, etc. Consequently, differences in the heat generation rates are exclusively determined by the substitution of the alkali metal cation. Irreversible heat generation rates are computed from galvanostatic intermittent titration technique measurements at different C-rates. Reversible heat generation rates are determined from the temperature dependence of the equilibrium potential. For both, NFP and LFP electrodes, the total heat generation increases with increasing C-rate. The reversible heat is found to be significant at low C-rate, whereas the irreversible heat dominates at high C-rate. For both NFP and LFP electrodes, differences in the total heat generation rates during charging and discharging are revealed mainly attributed to the reversible heat. The comparison between NFP and LFP reveals substantially larger heat generation rates for NFP electrodes, which is mainly caused by larger limitations of the insertion / extraction kinetics.