Micro-Sized Si-Ge Solid Solutions As Advanced Materials for High Capacity and Long-Cycle Life Anodes for Lithium Ion Battery
Authors: Mishra Kuber , Wu James J. , Zhou Xiao-Dong 
Published: February 2020

Mastery of strengthening strategies to achieve  high-capacity anodes for lithium-ion batteries can shed light on the understanding the nature of diffusion-induced stress and offer an approach to use micro-sized materials with an ultrahigh capacity for large-scale batteries. While the nano-sized materials as anodes show the promise, there is a need to utilize micro-sized materials for large-scale battery applications (e.g. electric vehicle) because of their cost-effective process, high package density, ease to scale-up, and the absence of undesirable side reactions between micro-sized anodes and the electrolyte. Consequently, Si-Ge alloys are of practical importance because the resident defects in solid-solutions can strengthen the materials, thus have a potential to alleviate diffusion-induced stress. Addition of a solute (Ge) into silicon creates a local stress field during the formation of solid solution, which interacts with resident dislocations and subsequently impedes their motion. As a result, an increase in yield stress in silicon-based anodes can be obtained, which are capable of withholding greater stresses to initiate slip and the attendant yielding.

Here, we report solute strengthening in a series of silicon (Si)-germanium (Ge) alloy particles, which have not been reported in open literature yet. Germanium is larger than that of silicon in size, hence when the larger solute atom (Ge) is added to the solvent atoms (Si), a compressive stress is generated in the vicinity of Ge atoms. This local stress field interacts with resident dislocations and subsequently impedes their motion to increase the yield stress in silicon-based anodes. Mechanical properties, e.g. Young’s modulus, of Si-Ge alloys have been studied, particularly in Yonenaga’s series of publications for the alloys at high temperatures (> 500oC). The stress-strain for germanium rich alloy was similar to Ge, likewise silicon-rich alloy similar with Si. The unique behavior occurs in compositions with x ~0.50 at which an athermal stress is in its maximum. Below the athermal stress, dislocations are pinned due to the dynamic interaction between a solute and surrounding dislocations, thus strengthening Si-Ge alloys. These results are consistent with our strain analysis and electrochemical measurements.

The synthesis process is facile, cost-effective and easily scalable. Si0.50Ge0.50 exhibits a stable capacity of 1320 mAh/g based on the total mass of the electrode at the current density of 500 mA/g with a capacity retention of 92% after 100 cycles. Isotropic structural changes in germanium and strengthening in solid solution contribute to the stable capacity cycling in micro-sized anodes. In comparison with silicon, the addition of Ge into Si substantially improves the capacity retention, particularly in Si0.50Ge0.50, which is consistent with literature reports that Si/Ge alloy showed a maximum yield stress in Si0.50Ge0.50. More importantly, our very recent work shows that the capacity retention can be further improved by designing the distribution of Ge into Si matrix and selecting proper solvent. When the anode particles have a proper structure and composition, they exhibit a very stable capacity up to a few thousand cycles. In Addition, the fluoroethylene carbonate based solvent plays an important role in yielding a better capacity retention.  We will summarize our work by analyzing the necessity for a stable mixed conducting structure in the anode to achieve high capacity and highly stable electrode for lithium ion batteries.