Microstructure dynamics of rechargeable battery materials studied by advanced transmission electron microscopy

The ever-growing energy requirements, the decreasing fossil fuel resources and the urgent need for environmental protection have spurred the search for sustainable energy alternatives, including both renewable energy sources and efficient storage technologies. Lithium/sodium (Li/Na)-ion batteries (LIBs/SIBs) are the most attractive and promising energy storage devices in the consumer market. The primary progress made by using advanced transmission electron microscopy (TEM) characterization and its close correlation with the battery properties are reviewed here. In addition, the atomic structure and chemistry of electrode materials are illustrated with respect to the surface reconstruction and the interface structure. The phase transformation and defect evolution are then discussed with respect to (1) the intermediate state of LiFePO4 that results in a high rate capability; (2) the in situ electrochemical reaction on nanomaterials; and (3) the alloying reaction for the high-energy-density silicon (Si) anode. The fundamental science underlying the microstructure evolution has been explored in depth to the atomic level and is discussed further in the context of electronic structure theory. The microstructure dynamics of electrode materials during battery cycling is dictated by the coupling between their lattice, charge and orbital characteristics, as shown in the schematic. TEM monitoring enables tracking lattice and chemical bonding at high spatial resolution, and thereby establishes the relationship between structure and performances. Using transmission electron microscopy (TEM) to capture atom-resolved images of batteries in action can help perfect high-capacity anodes. To realize lightweight, inexpensive batteries with long lifespans, researchers are adding metals and semimetals such as sodium or silicon to lithium anodes. Lin Gu from the Chinese Academy of Sciences and colleagues review efforts to better understand the relationship between the structure and properties of these hybrid electrodes with in situ TEM studies. This technique uniquely identifies changes to the crystal phase that occur during recharging of low-cost sodium–lithium anodes, for example, and degrade their performance. Spotting how lithium atoms are squeezed and transformed into silicon alloys, and the routes positive ions and negative electrons take in lithium–manganese oxides can help battery designers appreciate how electronic structure influences electrochemical outputs.