| Description |
Silicon anodes can theoretically enable lithium-ion batteries with 10x the capacity of current graphite anodes for electric vehicle applications. Silicon anodes traditionally have suffered from over 300% expansion during lithiation, leading to mechanical damage. However, volume expansion problems have largely been overcome through the use of nanomaterials. Conversely, the mechanical and chemical failure of a passivation layer, known as the solid electrolyte interphase (SEI), under cycling and calendar conditions remains poorly understood. Understanding and mitigating these passivation problems is the key to solving calendar life aging problems recently identified for silicon anodes. This work first investigated potentiostatic holds as a qualitative accelerated stage-gate for evaluating calendar aging and identified the required experimental process to do such an experiment reliably. To gain more insight into the mechanical failure of the SEI during cycle and calendar aging, the use of moir´e microscopy was evaluated for making in situ strain measurements of the SEI, but low resolution due to electrode bowing from gas generation limited this technique. Instead, the relationship between chemical and mechanical degradation is explored through scanning electrochemical microscopy imaging on model silicon thin films as a function of potential and time at open circuit potential. Silicon is found to be better passivated in the lithiated state rather than the delithiated state, and within resolution limits, the change in passivation is global rather than the formation of discrete cracks. Passivation is found to decrease with rest time. The role of SEI mechanics in calendar life measurements is further elucidated through specially designed protocols for full cells to understand decay from cycling reference performance tests versus decay from calendar aging. For all rest durations explored, graphite aging was found to be driven by the time since assembly, whereas silicon degradation was dependent on cycling and rest duration where longer rests with the same amount of cycling led to time since assembly dominating degradation. |
