Elevated temperatures can extend the life of lithium iron phosphate cells in hybrid electric vehicles

This study investigates the effects of elevated temperature on commercially available high power graphite/LiFePO4 cells using a temperature dependent, electrolyte enhanced, single particle model (ESPM-T) coupled with a Solid Electrolyte Interphase (SEI) layer growth aging model. The ESPM-T is capable of simulating up to 25C and 10 sec charge-discharge pulses within a 35- 65% SOC window and 25°C to 40°C temperature range with less than 1% voltage error, so it is suitable for hybrid electric vehicle (HEV) applications. The aging model is experimentally validated with an aggressive HEV cycle running for 4 months with less than 1% error. Instead of defining battery End of Life (EOL) as an arbitrary percent of capacity loss, we use the cycle number when the battery voltage hits 3.6V/2V (maximum/minimum) voltage limits. This is the practical limit of operation without reduced performance. Simulations show that operating cells at 35oC increases their life by 45% compared to room temperature operation. If the cell temperature is increased stepwise, then battery life is increased 85% more with a 50oC cell temperature at EOL. Battery initial size can be reduced by 24% using this temperature set-point strategy.