a-SiN_x as a Next Generation Anode Material for Li-Ion and Solid State Batteries

Silicon-based materials offer a wide range of benefits for battery applications. In Comparison to carbon-based anodes, higher energy densities can be reached, and more independent supply chain can be established. Compared to lithium metal, silicon-based materials offer a reduced dendrite formation at similar capacity, which can enable fast-charging, and significantly easier processing suitable for most existing production lines. Looking at solid state batteries, challenges of current silicon technologies are the stability during cycling due to the volume expansion of the silicon causing loss of electrical conductivity and potentially inconsistent cell pressure. Furthermore e.g. for sulphur-based solid electrolytes, a loss of lithium inventory due to reactions on the Si-SE interface is limiting cycle life. Amorphous SiN_x (a-SiN_x) is a promising next-generation anode material which tackles some of those issues from a materials perspective. It is a conversion material which forms a matrix phase in the first cycle, consisting mainly of Li₂SiN₂ and Li₃N. Submicron scale SiN_x can be produced from the gas phase in a hot-wall reactor from monosilane (SiH₄) and ammonia (NH₃). Stoichiometry, size and morphology can be tuned by process conditions. We have demonstrated the scale up from a lab-scale equipment (10 g/h) to a pilot scale equipment (1 kg/h) while still obtaining the desired product and controlling nitrogen distribution in the particles with core-shell type or more homogenous distributions possible. Amorphous SiN_x Powders with a nitrogen content up to 30 wt.%, a specific surface area above 10 m²/g and Si crystallinity below 5 % can be obtained. a-SiNx is a conversion material, which means that part of the lithium is converted into an intentionally formed matrix phase, affecting the first cycle Coulombic efficiency (FCE). However, it shows greatly enhanced cycle stability for hundreds of full cycles at capacities around 1400 mAh/g. The matrix phase formed in the first cycle dampens electrode swelling, acts as an intrinsic solid electrolyte and prevents excessive SEI growth. The battery performance in respect to nitrogen content and particle size is reported and an optimum in terms of capacity, stability and FCE is proposed.