Understanding Capacity Loss in LFP/Graphite Pouch Cells at High Temperatures through Modelling

Lithium iron phosphate is one of the most heavily utilized cathode materials in lithium-ion batteries owing to its safety and low cost. Often applied in energy storage, lifetime improvement for lithium iron phosphate batteries over different temperatures is of great importance. This work focuses on understanding capacity loss in lithium iron phosphate cells through the modelling of capacity fade curves collected from LiFePO₄/graphite pouch cells cycled for up to two years under a variety of testing conditions. Capacity loss modelling was completed using a novel model which accounts for capacity loss incurred through lithium inventory loss and transition metal dissolution. Further, this work is presented in comparison to low voltage lithium nickel manganese cobalt oxide pouch cells. The results show a greater ability to predict capacity loss in lithium iron phosphate cells when the novel model is utilized as compared to other simple capacity fade models. The application of the model works well over a range of testing conditions and was validated through correlations made to physical cell parameters. From this work, solid electrolyte interphase growth behavior and iron dissolution are highlighted as some of the main causes of capacity loss at high temperatures for LFP/graphite cells.