While optical microscopy of single particle electrochemistry has proven insightful for future nanoparticle-based batteries, little is explored for micron-sized particles of more practical interest. This is largely hindered by the currently limited methodology. Accordingly, we report transmission optical microscopy using near-infrared light for accessing intra-particle electrochemistry in virtue of strong light penetration as compared to visible light. Using near-infrared (λ > 730 nm) bright-field microscopy, the redox electrochemistry of single LiCoO₂ microparticles can be readily measured based on the measurements of optical contrast changes during electrochemical cycling. Further using the established methodology, we discover that the solid-state diffusion inside most single microparticles is distinctly directional, instead of in an isotropic manner from outer to inner as observed for the other particles. This phenomenon is also observed using dark field scattering microscopy with near-infrared light, suggesting non-uniform crystal inner structures responsible for the geometrically asymmetric heterogeneity of charge transfer kinetics within each single particle. These results indicate potential opportunities offered by the near-infrared optical methodology for operando studying practical battery materials.

中文翻译：

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电池单微粒电化学的近红外可视化

虽然单颗粒电化学的光学显微镜已被证明对未来基于纳米颗粒的电池具有深刻的洞察力，但对具有更实际意义的微米级颗粒的探索却很少。这在很大程度上受到当前有限方法的阻碍。因此，我们报告了使用近红外光的透射光学显微镜，由于与可见光相比具有较强的光穿透性，因此可以访问颗粒内电化学。使用近红外（λ > 730 nm）明场显微镜，可以根据电化学循环过程中光学对比度变化的测量，轻松测量单个LiCoO ₂微粒的氧化还原电化学。进一步使用已建立的方法，我们发现大多数单个微粒内部的固态扩散具有明显的方向性，而不是像其他颗粒所观察到的那样以各向同性的方式从外到内。使用近红外光暗场散射显微镜也观察到这种现象，表明不均匀的晶体内部结构导致每个颗粒内电荷转移动力学的几何不对称异质性。这些结果表明近红外光学方法为研究实际电池材料提供了潜在的机会。