The Effect of Stress and Fracture on Lithiation Kinetics in Silicon Nanoparticles
Matthew McDowell (Stanford University), Seok Woo Lee (Stanford University), Ill Ryu (Stanford University), Chongmin Wang (Pacific Northwest National Laboratory), William Nix (Stanford University), Yi Cui (Stanford University)
Lithium ion batteries: When Chemistry meets Mechanics
Mon 9:00 - 10:30
Salomon 003
Silicon is an attractive high-capacity anode material for Li-ion batteries, but to design better-performing Si electrodes, it is necessary to develop a comprehensive understanding of both the fundamental nature of the Li-Si reaction and the effects of the ~300% volume change that accompanies lithiation. Here, in situ transmission electron microscopy (TEM) is used to observe the reaction of crystalline Si nanoparticles in real time. The experiments reveal that the lithiation reaction slows dramatically as the reaction front progresses into particles of all sizes. Analysis of the reaction front trajectories suggests that the reaction slows because large hydrostatic stresses in the vicinity of the reaction front diminish the driving force for the reaction. Interestingly, this leads to faster lithiation in larger particles that fracture compared to smaller particles that do not fracture because fracture results in stress relaxation. Finally, this behavior is compared to the lithiation of amorphous Si nanospheres, which are observed to be lithiated by a similar two-phase mechanism but do not exhibit reaction front slowing and have a larger critical size for fracture. Overall, these experimental results suggest that stress plays a central role in governing the reaction kinetics in this unique large-volume change reaction. These findings inform our understanding of the rate performance of real Si anodes, and the observed dependence of reaction rate on size and fracture characteristics is important for designing electrode architectures with optimized rate capability.