Visualization and Quantification of State of Charge-Dependent Young's Modulus of LiMn2O4 Nanosized Particles by Bimodal AFM.

Mechanical damage of LiMn2O4 active material caused by volume change, phase transition, and lithium diffusion-induced stress is the main degradation mechanism in lithium-ion batteries. Young's modulus is a key parameter of mechanical property, and its variation with lithium content x or state of charge (SOC) at the nanoscale is an important issue because such variation may have influences on the stress level and lithium-ion transport. In this study, we successfully developed bimodal atomic force microscopy (bimodal AFM) and related approaches to carry out surface topography imaging and Young's modulus mapping of LixMn2O4 nanosized particles. It was validated that the size of particles decreased with decreasing SOC due to delithiation during the charging cycle. The variation in Young's modulus with SOC was quantitatively determined using the silicon material as a reference, and the trend of the variation is consistent with the reported results of molecular dynamics simulation. Furthermore, spatially nonuniform distribution of Young's modulus on the nanosized particle surface was found even upon completion of charging. This phenomenon could be attributed to the coexistence of two phases during the charging process. Our experimental study reveals the correlation between Young's modulus of LiMn2O4 and SOC at the nanosized particle level, and we believe that the bimodal AFM will be widely used in the nanocharacterization of the electrode materials because lithium content- or SOC-dependent mechanical properties are common in battery electrode materials.

Accurate detection of subsurface microcavity by bimodal atomic force microscopy.

Subsurface detection capability of bimodal atomic force microscopy (AFM) was investigated using the buried microcavity as a reference sample, prepared by partially covering a piece of highly oriented pyrolytic graphite (HOPG) flake with different thickness on a piece of a cleaned CD-R disk substrate. This capability can be manifested as the image contrast between the locations with and without the buried microcavities. The theoretical and experimental results demonstrated that the image contrast is significantly affected by the critical parameters, including the second eigenmode amplitude and frequency as well as local structural and mechanical properties of the sample itself. Specifically, improper parameter settings generally lead to incorrect identification of the buried microcavity due to the contrast reduction, contrast reversal and even disappearance. For accurate detection, the second eigenmode amplitude should be as small as possible on the premise of satisfying the signal-to-noise ratio and second eigenmode frequency should be close to the resonance frequency of the cantilever. In addition, the detectable depth is closely related to microcavity dimension (thickness and width) of the HOPG flake and local stiffness of the sample. These results would be helpful for further understanding of the detection mechanism of bimodal AFM and facilitating its application in nano-characterization of subsurface structures, such as the micro-/nano- channels to direct the flow of liquids in lab-on-a-chip devices.

Tracking Electrochemical-Cycle-Induced Surface Structure Evolutions of Cathode Material LiMn2O4 with Improved Operando Raman Spectroscopy.

Linking surface structure evolution to the capacity fading of cathode materials has been a problem in lithium ion batteries. Most of the strategies used to solve this problem are focused on the differences between the unaged and aged materials, leading to the loss of intermediate dynamic change information during cycling. Raman spectroscopy is a convenient, nondestructive, and highly sensitive tool for characterizing the surface/near-surface region structure. In this work, we improved an operando Raman system, which is able to record in situ and in real time a series of Raman spectra during charging/discharging cycles and is even able to record very weak Raman peaks without the use of SRES enhancement, which facilitates sample preparation. These series of Raman spectra revealed an inherent correlation between the electrode potential/Li content and the surface structure changes of the as-prepared pure LiMn2O4 film, including the biphase reaction, the evolution of the peroxo O-O bond, and the formation of the Mn3O4 surface phase. They were the first to show that the number of peroxo O-O bonds was decreased with an increasing number of cycles and that this decrease was accompanied by an increase in the Mn3O4 phase. With the help of the data measured by XPS, c-AFM, electrochemical testing equipment, and the calculation based on density functional theory, the causes of the capacity fading of the material are discussed. This work not only showed a direct correlation between the surface structure evolution and the capacity fading of the LiMn2O4 but also could provide an alternative operando Raman system that could be widely used for the in situ characterization of battery electrode materials.