To transition to renewable energy sources, developing sustainable and high-capacity energy storage is crucial. While Li-ion batteries are a major part of the solution, conventional graphite anodes limit their capacity. We are therefore exploring alternative materials, focusing on micro-sized silicon, which offers a theoretical capacity tenfold higher than graphite. The main challenge hindering silicon’s practical use is its massive volume expansion during battery cycling, which leads to mechanical failure and rapid performance decline.
In our group, we investigate a novel strategy to solve this: doping the silicon with trace amounts of aluminum. We use a combined theoretical and experimental approach. We used DFT simulations to show that aluminum atoms prefer to locate at the grain boundaries (GBs) of polycrystalline silicon. Our key finding is that the presence of aluminum at these boundaries significantly facilitates GB sliding, a crucial mechanism for relieving mechanical stress. Our simulations show that adding aluminum dramatically lowers the energy barrier for this sliding to occur. Using Crystal Orbital Hamilton Population (COHP) analysis, we determined the root cause: the Al-Si bonds at the GB are inherently weaker than the native Si-Si bonds, creating a “lubricating” effect that prevents stress accumulation.
Our experimental work strongly corroborates these computational findings. The Al-doped silicon anode we prepared showed significantly enhanced performance, with capacity retention of 43.22% after 50 cycles, compared to just 27.68% for undoped silicon. This demonstrates that incorporating trace aluminum is a viable and effective strategy to improve the structural integrity and cycling stability of silicon anodes for the next generation of Li-ion batteries.