Locally provoked cell aging using different pressure profiles at 60% DOD

Volume expansion in lithium-ion batteries (LIBs) consists of irreversible and reversible dilation of graphite anodes. The former is because of the formation and growth of the solid electrolyte interface (SEI), while the latter occurs during battery aging. It is a known fact that during charging anodes in LIBs experience upto 10% volume expansion due to lithium ion intercalation. With inhomogeneous temperature-pressure distribution or current density gradients LIBs experience a space-dependent aging, leading to progressive irreversible volume changes. Hence, gradients in distribution of pressure, temperature and current density need to be understood in order to develop solutions for lifetime extension. The capacity loss in LIBs can be significantly reduced when the aging is homogeneous. Temperature gradients are counteracted by cooling and current density distribution is improved by electrode and cell optimization. Further amelioration of the temperature and current density distribution is geometrically limited. External bracings have shown to reduce contact resistance and delamination of electrode layers compared to unconstrained batteries. Moreover, the global and local pressure distribution is more variable and may even effect current density as well as temperature distributions indirectly. Therefore, it is of paramount importance to define an optimized level of external pressure that can be incorporated into the battery packs and investigate the aging pattern and capacity fade thereafter. The main focus of our research is the optimization of the pressure distribution. Bracing modules were developed for steering and monitoring the pressure distribution. The batteries are sandwiched between two rigid aluminium plates held together with screws. Pressure profiles are created by machining out height profiles with ten pressure zones between 50 to 200 kPa along the length of the battery. Ten pressure sensors, type Tekscan FlexiForce A201, are embedded in one of two aluminum plates (2618-T61). Two cushioning pads (CPD) are used as buffer layers (BISCO® HT-840 4.8mm) for the countersinked profiles of the plates preventing local pressure peaks on the sandwiched LIB. In order to provoke aging, the batteries were used in the upper voltage range (40-100% SOC/ 60% DOD). This would increase irreversible plating and the pressure distribution can influence lithiation locally. The results can be used for optimising pressure distribution in bracings as well as visualising pressure dependent degradation mechanisms. The experiments would reveal detailed local degradation attributed to local pressure zones and provoked cell aging. Visual analysis will indicate spatially distributed lithiation/SOC in the graphite electrodes which can be supported through coin cell capacity measurements. For SEM analysis the selected, unwashed electrodes are embedded in two layers of resin under vacuum. This ensures that the pores are filled with resin and the solid electrolyte interface (SEI) is preserved well. As a result locally provoked aging can be distinguished from general aging.