Si anodes for Li+ batteries: what is the ideal structure?

Significant research is focused on improving the performance of Li ion batteries (LIBs) by using Si in anodes due to its ten times higher theoretical capacity when compared with that of graphite, and it is also naturally abundant. However, there are many issues in using high Si content due to nearly threefold expansion of Si upon lithiation, leading to mechanical breakdown and loss of performance within a few cycles. This leads to the challenge of simultaneously optimising the stability, capacity, lithiation rates and solid electrolyte interphase (SEI) formation. It is unclear what the ideal porous structure is to meet the desired performance specifications. This perspective article proposes quantitative governing equations to correlate structural features with performance, which can provide guiding principles to design (meso)porous silicon (p-Si) structures. The structural features include nano-scale primary particles, and their meso-scale connectivity, degree of aggregation and tortuosity. We hypothesize that hierarchical aggregates of sub-micron networked particles will have an enhanced rate of lithiation yet with mechanical stability to withstand swelling via accordion expansion or disinterspersion (D.I.). Preliminary evaluation of these principles is presented, and future research avenues are outlined. To test these hypotheses, experimental investigations are needed to identify and compare various hierarchical structures that can maximise the criteria described above. These guiding principles will prove valuable for developing Si for LIB anodes, thereby accelerating their commercial uptake.