Controlling Void Space in Crumpled Graphene-Encapsulated Silicon Anodes using Sacrificial Polystyrene Nanoparticles

Silicon anodes have a theoretical capacity of 3590 mAh/g (for Li 15 Si 4 , at room temperature) which is tenfold higher than the graphite anodes used in current Li-ion batteries. This, and silicon’s natural abundance, makes it one of the most promising materials for next-generation batteries. Encapsulating silicon nanoparticles (Si NPs) in a crumpled graphene shell by spray drying or spray pyrolysis are promising and scalable methods to produce core-shell structures which buffer the extreme volume change (> 300 vol%) caused by (de)lithiaton of silicon. However, capillary forces cause the graphene-based materials to tightly wrap around Si NP clusters and there is little control over the void space required to further improve cycle life. Herein, we report a simple strategy to engineer void-space within the core by incorporating varying amounts of similarly sized polystyrene (PS) nanospheres in the spray drier feed mixture. The PS completely decomposes during thermal reduction of the graphene oxide shell and results in Si cores of varying porosity. We demonstrate that the best performance is achieved at a 1:1 ratio (PS/Si) leading to high capacities of 1638 mAh/(g Si+rGO ), 1468 mAh/(g Si+rGO ), and 1179 mAh/(g Si+rGO ) at 0.1 A/g, 1 A/g, and 4 A/g, respectively. Moreover, at 1 A/g, the capacity retention was 80.6% after 200 cycles. At a practical active material loading 2.4 mg/cm 2 , the electrodes could achieve an areal capacity of 2.26 mAh/cm 2 at 1A/g.