Operando Imaging of Over-Discharge-Induced Surface Morphology Evolutions of LiMn2O4 Submicron-Sized Particles by Electrochemical High-Speed Atomic Force Microscopy.

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.

Bimodal AFM-Based Nanocharacterization of Cycling-Induced Topographic and Mechanical Evolutions of LiMn2O4 Cathode Films.

Evolution of LiMn2O4 mechanical property during charge/discharge cycles is a critical issue because it is closely related to the performance of lithium-ion batteries. Extensive studies have been conducted by first-principles calculations/molecular dynamics simulation at the atomic level and by the nanoindentation technique at the micron scale. In this study, cycling-induced topographic and mechanical evolutions of the LiMn2O4 films are investigated at the nanoscale using the bimodal atomic force microscopy (AFM), which provides a complementary approach to bridge the gap between atomic-level calculation and micron-scale measurement. The topographic change and elastic modulus degradation of the LiMn2O4 films during the charge/discharge cycles are found to occur simultaneously and irreversibly. Moreover, a dramatic decrease in the elastic modulus of the films takes place at the first 10 cycles, which is consistent with the significant loss of the capacity and the change of the Coulombic efficiency measured by the galvanostatic method. By considering the nanoscale phenomena and the macroscopic measurement results, the reasons for the elastic modulus degradation are discussed. This study would be a valuable addition to a better understanding of the degradation mechanisms of this cathode material.

Visualization of Electrochemical Cycling-Induced Dimension Change in LiMn2O4 Nanoparticles by High-Speed Atomic Force Microscopy.

Exploring dynamic dimension change and lithium-ion diffusion kinetics of active nanoparticles is important to further improve the qualities of lithium-ion batteries (LIBs), such as the cycle life and charge rate. For advancing such research, an imaging technique that is capable of operating in an electrochemical environment with high spatial and temporal resolutions is really needed. In this work, we successfully developed electrochemical high-speed atomic force microscopy (EC-HS-AFM), which enabled nanoscale imaging at the rate of ∼1 frame/s during electrochemical cycling. The dimensional evolutions of LiMn2O4 single nanoparticles accompanying an insertion/extraction reaction of lithium ions were visualized. The surface area-potential hysteresis loops of the single nanoparticles at different sweep rates were quantitatively extracted from the successive HS-AFM images or video. The first-order derivative of the hysteresis loop was interestingly similar to the cyclic voltammetry (CV). Moreover, the EC-HS-AFM experiments confirmed that the utilization of the nanoparticles in the cathode can indeed improve the rate performance of the LIBs. These results demonstrated that EC-HS-AFM would be a promising tool to study dimensional evolutions and lithium-ion diffusion kinetics at a nanoscale.

Development of electrochemical high-speed atomic force microscopy for visualizing dynamic processes of battery electrode materials.

Development of lithium ion batteries with ultrafast charging rate as well as high energy/power densities and long cycle-life is one of the imperative works in the field of batteries. To achieve this goal, it requires not only to develop new electrode materials but also to develop nano-characterization techniques that are capable of investigating the dynamic evolution of the surface/interface morphology and property of fast charging electrode materials during battery operation. Although electrochemical atomic force microscopy (EC-AFM) holds high spatial resolution, its imaging speed is too slow to visualize dynamics occurring on the timescale of minutes. In this article, we present an electrochemical high-speed AFM (EC-HS-AFM), developed by addressing key technologies involving optical detection of small cantilever deflection, dual scanner capable of high-speed and wide-range imaging, and electrochemical cell with three electrodes. EC-HS-AFM imaging from 1 fpm to ∼1 fps with a maximum scan range of 40 × 40 µm2 has been stably and reliably realized. Dynamic morphological changes in the LiMn2O4 nanoparticles during cyclic voltammetry measurements in the 0.5 mol/l Li2SO4 solution were successfully visualized. This technique will provide the possibility of tracking dynamic processes of fast charging battery materials and other surface/interface processes such as the formation of the solid electrolyte interphase layer.

In situ probing behaviors of single LiNiO2 nanoparticles by merging CAFM and AM-FM techniques.

Probing single active nanoparticles of Li-ion battery electrodes is challenging but important to reveal their behaviors including morphology, mechanical properties and electrochemical reactions with an electrolyte. In this work, we in situ investigated voltage-induced behaviors of single LiNiO2 nanoparticles by merging conductive atomic force microscopy (CAFM) and amplitude modulation-frequency modulation (AM-FM) techniques. The former was used to apply a voltage between a selected single nanoparticle and a substrate through its tip, while the latter was done for imaging. Evolution in the morphology and stiffness of the nanoparticles induced by different voltages under air and dried argon atmospheres was tracked, respectively. The evolution mechanisms related to electrochemical reactions were discussed in detail. These results suggest that the merged techniques would provide an indirect and effective approach to study the behaviors and electrochemical reactions of electrode materials on the nanometer scale and even single nanoparticles.