Mesoparticle-Nanoparticle Size Relation for Improved Silicon-Carbon Composite Cycling Stability in Lithium-Ion Batteries

Energy storage is a topic that has become central to progress in applications ranging from cell phones to grid storage for renewable energies. As a result, any incremental progress in the field has the potential to instigate profound global impact and why so much effort is being put into improving the current industry leader of lithium-ion battery technology. Soon after the introduction of graphite as the standard anode material in the 1990’s, it was noted that silicon’s theoretical 4200 mAh/g capacity dwarfed graphite’s 372 mAh/g.^[1] Unfortunately, silicon’s problems of limited electrical conductivity, excessive solid electrolyte interphase (SEI) formation, and volume change during cycling have been problems preventing its commercial adoption. However, silicon’s volume expansion of ~300% upon lithiation has demonstrated that a critical scale of 150 nm, beyond which lithiation causes the particle to fracture itself.^[2] This learning has somewhat limited the structure of materials being researched to being below that scale. Some work has looked beyond these size restrictions with macrostructures,^[3] thus gaining some beneficial properties like reduced SEI formation afforded to larger structures. This naturally leads to the question of whether a macrostructure like a mesoparticle, particle made up of smaller particles, can benefit from both its large cumulative scale and the scale of its components.<br/>To perform this investigation, mesoparticles on the scale of 5 mm were produced through spray drying using a solution of 12% PVP to 88% Si by weight. The silicon particles themselves were selected to have 100 nm, 20 nm, and 10 nm in average diameter. After the spray drying process the mesoparticles underwent a CVD process developed by our lab to both coat the individual nanoparticles in a conformal carbon shell, and then graphitize the carbon shell without impacting the silicon or the macrostructure.^[1] The final result being identically produced mesoparticles of the same size, composed of individually graphite coated nanoparticles of three different sizes. For direct comparison between mesoparticles and single particles, the aforementioned CVD process was performed on the un-clustered silicon at each size. These materials were then analyzed through SEM, TEM, EDS, and XRD, while also undergoing standard half-cell testing and additional 3^rd party full cell testing as an additive to standard graphite anodes.<br/>Experimental results show both a clear beneficial impact of the mesoparticle macrostructure with regards to cycling stability, as well as an obvious influence in C-rate and general stability by the nanoparticle size. Less anticipated benefits of higher C-rate stability were observed with the mesoparticle structures being able to supply up to 4x the capacity of their un-structured counterparts when cycled at 0.5 C, despite near identical capacities when cycled at 0.1 C. The trend of general improvement in cycling stability was also observed in full-cell testing. Overall, we show that the mesoparticle macrostructure improves electrochemical stability in general, while simultaneously the size of its nanoparticle components express both the benefits and drawbacks of their size.