An all-electrochem-active silicon anode enabled by spontaneous Li-Si alloying for ultra-high performance solid-state batteries

Li-Si alloys are considered as key anode materials for advanced silicon-based solid-state batteries (Si-ASSBs) due to their high ionic/electronic conductivity. However, the high content of Li in Li-Si alloys increases the risk of Li dendrite growth and soft short circuits during battery charging. Herein, an all-electrochem-active Si/Li21Si5 composite anode is developed with the rational architecture and optimized ratio of Li21Si5 and pure Si. By utilizing defects in the oxide layer of micron Si, the Li21Si5 alloy is first synthesized separately by a quick and spontaneous Li-Si alloying reaction, followed by its mixing with Si particles by cold-pressing. In this composite anode, the Li21Si5 grains are uniformly distributed among the Si particles, and serve as a soft buffer for Si. Due to the limited self-discharge between Li21Si5 grains and Si particles, the Li21Si5 alloy maintains a lithium-saturated state and functions as a lithium supplement. Meanwhile, it constructs a conductive network in the anode, which can facilitate the transport of lithium ions and induce their storage in Si particles, thus avoiding dendrite formation. Consequently, the Si/Li21Si5-ASSB can achieve an ultra-high initial Coulombic efficiency (ICE) of 97.8% at 25 degrees C, as well as a low expansion rate of 18.9% at a surprisingly large areal capacity of 17.9 mA h cm-2. This work provides a new method and valuable insight into Li-Si alloy synthesis, and the obtained Si-based anode shows great promise for application in ASSBs. An all-electrochem-active Si/Li21Si5 anode was designed, in which the Li21Si5 served as efficient ionic/electronic channel and expansion buffer. It achieved an ultra-high ICE of 97.8% and a low expansion rate of 18.8% at a capacity of 17.9 mA h cm-2.