Visualization\nand Quantification of State of Charge-Dependent\nYoung’s Modulus of LiMn<sub>2</sub>O<sub>4</sub> Nanosized\nParticles by Bimodal AFM

Mechanical damage of LiMn<sub>2</sub>O<sub>4</sub> active\nmaterial\ncaused by volume change, phase transition, and lithium diffusion-induced\nstress is the main degradation mechanism in lithium-ion batteries.\nYoung’s modulus is a key parameter of mechanical property,\nand its variation with lithium content <i>x</i> or state\nof charge (SOC) at the nanoscale is an important issue because such\nvariation may have influences on the stress level and lithium-ion\ntransport. In this study, we successfully developed bimodal atomic\nforce microscopy (bimodal AFM) and related approaches to carry out\nsurface topography imaging and Young’s modulus mapping of Li<sub><i>x</i></sub>Mn<sub>2</sub>O<sub>4</sub> nanosized particles.\nIt was validated that the size of particles decreased with decreasing\nSOC due to delithiation during the charging cycle. The variation in\nYoung’s modulus with SOC was quantitatively determined using\nthe silicon material as a reference, and the trend of the variation\nis consistent with the reported results of molecular dynamics simulation.\nFurthermore, spatially nonuniform distribution of Young’s modulus\non the nanosized particle surface was found even upon completion of\ncharging. This phenomenon could be attributed to the coexistence of\ntwo phases during the charging process. Our experimental study reveals\nthe correlation between Young’s modulus of LiMn<sub>2</sub>O<sub>4</sub> and SOC at the nanosized particle level, and we believe\nthat the bimodal AFM will be widely used in the nanocharacterization\nof the electrode materials because lithium content- or SOC-dependent\nmechanical properties are common in battery electrode materials.