The next automobile you purchase could be powered by rechargeable lithium-ion batteries (LIBs). Thanks to new technologies that replace traditional liquid electrolytes with solid-state ceramic materials, LIBs can now be made smaller, cheaper, lighter and safer than ever before. However, solid-state LIBs still lack the power output of their wet-cell cousins because of the large resistance to ion transfer at the battery’s electrode/electrolyte interface.

Kazuo Yamamoto and co-workers from the Japan Fine Ceramics Center in collaboration with scientists from Shizuoka University, Kyoto University and Chubu Electric Power in Japan1 have now used a technique called electron holography to capture the first images of chemical diffusion inside a solid-state LIB as it undergoes a charge–discharge reaction. “Many researchers and companies are studying how to improve LIB performance,” says Yamamoto, “but so far, nobody has observed directly how batteries work inside.”

“Lithium is a very light element,” explains Yamamoto, “making it very difficult to detect by common transmission electron microscope imaging.” The researchers instead observed the interference pattern of a pair of coherent transmission electron beams. This holography approach provided detailed high-resolution images of the spatial distribution of electric potential inside the batteries. Using these images, the team were able to measure ion transport inside a working LIB.

Fig. 1: When lithium-ion batteries are charged, ions are extracted from the positive electrode (left) and transferred to the negative electrode (toward the right) through the solid electrolyte. Electron holography images (colorscale of potential from white (1 V) to dark blue (0 V)) corresponding to the transmission electron microscopy image (grayscale) show the formation of an electrical double layer around the electrode/electrolyte interface as the battery is charged.Adapted from Ref. 1. Reproduced with permission. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA

The holography measurements revealed that as an LIB is charged, lithium ions are rapidly extracted from the electrode/electrolyte interface. However, ions located far from the interface do not move within the electrode at the same rate. Furthermore, a micrometer-sized interfacial region called an electrical double layer is formed in the battery while charging (Fig. 1). In combination, these phenomena disturb smooth ion transfer and increase battery resistance.

Observing this electrical double layer was quite surprising, says Yamamoto, because it extended over a much larger region than predicted by classical models for electrode/liquid-electrolyte interfaces. “This result will be useful for developing new theories about solid-state LIBs,” he says. Furthermore, the unprecedented information available through electron holography should help identify the best materials for overcoming interfacial resistance problems. “Our results and techniques make it possible to easily compare good batteries with bad ones, providing clear answers that will lead to rapid development of high-performance batteries,” states Yamamoto.