Compact Sn/SnO2 microspheres with gradient composition for high volumetric lithium storage

Volumetric performance of lithium ion batteries becomes increasingly significant in applications of miniaturized consumer electronics, such as cellphones. Herein, we report a successful design of compact Sn/SnO2 microspheres with a concentration gradient assembled by nanoparticles to realize high volumetric performance. In the structure, the secondary microsphere is able to offer more compact space in realizing higher volumetric capacity. The primary nanoparticles condense together, leading to a dense and porous structure, which can provide more efficient utilization of volume as well as effectively minimize volume variation. Due to the utilization of microstructure as well as the concentration gradient structure, the concentration gradient Sn/SnO2 microspheres deliver a high volumetric capacity (1593 mAh cm−3 at 200 mA g−1 after 200 cycles), outstanding rate capability (retaining a capacity of 1230 mAh cm−3), and stable cycling performance (a capacity retention of 97% after 900 cycles at 5 A g−1). This concentration gradient microsphere structure not only provide a high performance conversion-type composite anode, but also a strategy to engineer the energy density of anode materials in compact space towards high energy density.