Yiyang Li2 1 Jongwoo Lim2 3 Saiful Islam4 Martin Bazant5 William C. Chueh2

2, Stanford University, Stanford, California, United States
1, Sandia National Laboratories, Livermore, California, United States
3, Seoul National University, Seoul, , Korea (the Republic of)
4, University of Bath, Bath, , United Kingdom
5, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

LiXFePO4 is a model phase-separating material for Li-ion batteries. While its tendency to phase-separate between particles at the electrode length scale is well-documented, the phase separation mechanism within individual particles has been highly controversial, with experimental observations of both intra-particle phase separation and solid solution. Phase separation not only creates large elastic strains that degrade the material, but also prevents us from accessing and studying a large lithium composition space in the miscibility gap. Understanding the phase transformation mechanisms are therefore crucial for developing safe and long-life batteries.

To understand whether LiFePO4 undergoes intra-particle phase separation, we developed a microfluidic liquid platform that enables us to track in real time the migration of lithium within individual particles during lithiation and delithiation. We directly visualize phase separation at low cycling rates and solid solution behavior at high rates of cycling, with a spatial resolution as low as 10 nm. To explain this behavior, we use both experiment and simulations to identify lithium surface diffusion at the particle/electrolyte interface as the dominant pathway by which lithium migrates within individual particles during phase separation. Thus, solid solution arises at high rates when the rate of lithium insertion is faster than the rate of surface diffusion. Stabilizing solid solution within individual particles not only reduces elastic strain during charge and discharge, but also provides new avenues towards the study of the defect-chemical properties in the previously-inaccessible miscibility gap.