Experimental investigation of lithium-ion cells ageing under isothermal conditions for optimal lifetime performance

Lithium-Ion cell ageing is sensitive to cell temperature. Previous studies have investigated ageing under adiabatic or controlled environmental temperature (i.e., isoperibolic) conditions. Notably, these conditions do not impose a uniform cell surface temperature (i.e., isothermal condition) or a controlled cooling rate, as an active Thermal Management System (TMS) would. This leads to a clear interdependence between charge/discharge rates and cell temperature, due to the uncontrolled cell temperature history. Consequently, the separate influence of these variables on the cell performance cannot be investigated. In this study, the ageing of a 300 mAh Lithium Cobalt Oxide (LCO Li-Ion) pouch cell under isoperibolic and isothermal conditions in the range of 0 °C - 40 °C is investigated. Each cycle comprises a CC-CV (constant current-constant voltage) charge of 1C and a CC discharge of 2C. Similar average ageing rates for isoperibolic and for isothermal conditions but at different reference temperatures were found. For example, an isoperibolic temperature condition of 25 °C yielded a similar degrading rate as an isothermal condition at 30 °C. This is mainly due to the effect of the cell self-heating (Joule heating) which increases the median operating temperature above that of the surroundings. These findings emphasise that uncontrolled cell thermal conditions lead to overall performance strongly dissimilar and randomly dependent on the transient heat transfer coefficient of isoperibolic TMS. Finally, an optimal isothermal condition that maximises the cell electrochemical efficiency and minimises its ageing is identified in the range of 25 °C-35 °C.