Operando Imaging of Over-Discharge-Induced Surface Morphology Evolutions of LiMn₂O₄ Submicron-Sized Particles by...

Spinel LiMn₂O₄ is a promising cathode material but suffers from severe capacity fading during battery operation. One of capacity fade mechanisms results from changes in its morphology and structure due to over-discharge. In this work, for the first time, we successfully tracked the morphologic evolution of LiMn₂O₄ submicron-sized particles during over-discharging by our home-made electrochemical high-speed atomic force microscopy (EC-HS-AFM). Seven hundred and sixty successive EC-HS-AFM images were stably captured at an imaging speed of ∼0.85 fps at corresponding potentials during over-discharging in ∼15 min, from which evolutions of nanoscale wrinkle-like and step-like structures on the particle surface were clearly observed. The phenomena could be resulted from the complex stresses due to structural distortion during the phase transformation from cubic (LiMn₂O₄) to tetragonal (Li₂Mn₂O₄), and the formation of the Li₂Mn₂O₄ phase was confirmed by ex situ XRD. Moreover, the particle surface area as a function of the potential was quantitatively extracted from the EC-HS-AFM images, revealing the irreversible expansion/contraction of the particles, and this finding obtained at the nanoscale was consistent with the macroscopic results tested by cyclic voltammetry and galvanostatic charge/discharge methods. These results demonstrate that the EC-HS-AFM is a powerful tool to establish the correlation between the over-discharge-induced surface morphology changes and irreversibility of the Li-ion insertion/extraction as well as capacity fading.