In Situ TEM for Dynamic Materials Behaviors in Li-Ion and Beyond-Li Batteries

In situ and operando transmission electron microscopy (TEM) has gained rapid growth in the past decade and attracted tremendous attention in diverse scientific research because it can acquire dynamic material behaviors along with simultaneous property measurements of the target material system undergoing various controlled physical and chemical processes. Rechargeable batteries are essential for energy storage that underpins the advancement of consumer electronics and automobile propulsion technologies and links renewable energy resources to viable power-grid applications. To meet the increasing demand for improved energy density, lifetime, and safety in large-scale and power-intensive devices, the fundamental understanding of the structure-property relationship of new battery materials with novel structures is critically needed. Specifically, in situ TEM has been widely used to obtain useful information on the atomic to nanoscale by offering spatiotemporal characterization of materials transformation during electrochemical reactions.<br/>Here, we report the application of in situ TEM in mechanistic understanding of reaction mechanisms and kinetics for electrochemical reactions of two-dimensional bismuth telluride with lithium, sodium, and potassium, respectively. We built nanoscale battery cells to include active Bi2Te3 nanoflakes and alkali metals using in situ dry-cell setup and performed cyclic discharge and charge processes in potentiostatic mode. We found that both lithiation and sodiation were carried out through two-step conversion and alloying reactions whereas potassiation would involve an extra intercalation reaction at the beginning, leading to distinct electrochemical pathways and different intermediate products during phase transitions. It is also worth noting that sodiation exhibited unexpected high reaction kinetics than lithiation and potassiation, in accordance with its superior high-rate performance in coin-cell tests. This phenomenon was further explained by first-principles calculations and electro-chemo-mechanical modeling, which suggested the critical interfacial stress effect induced by the geometric dependence of reaction front propagation. The comparative investigation of electrode material behaviors in Li-ion, Na-ion, and K-ion batteries provides valuable insights into understanding the alkali-ion electrochemistry and kinetics that can be further leveraged as guiding principles for new design and development of Li-ion and beyond-Li battery technologies.