Study of the correlation between electric and mechanical/thermal parameters in a graphite - LiFePO4 pouch cell

Abstract

The correlation of thermodynamic parameters with the commonly studied electric parameters in grahite/LFP lithium-ion battery cells is thoroughly analyzed. From the comparison of stored charge, incremental capacity peak intensity and position and state of health against pressure it was revealed that there is a strong connection between pressure and peak intensity. The evolution of cell pressure with cycles justifies this relation, however, the cell temperature obeys the joule effect following the current variations. An almost linear dependence of pressure on state of charge, for 1 C current rates, throughout the scale presents a more accurate means of state of charge determination. In a first approximation, using a linear behavior, the variation of the normalized pressure with state of charge has been found to follow a slope of 0.67 in the first part of charging and 0.36 in the rest part of charging. By correlating the pressure to stored capacity a 0.294 kPa of pressure increase for every mAh of capacity loss has been determined which is translated in terms of state of health to a mean of 0.01 kPa for each 1% drop in cell health. Furthermore, signs of potential deterioration of cell performance could be reflected on pressure data as early as 140 cycles earlier than the generally accepted 80% end of life limit.