Visualizing fast interlayer anisotropic lithium diffusion via single crystal microbattery

Ion insertion in host layered materials is the foundation of energy storage mechanism of commercial lithium-ion batteries. Contrary to general view that small lithium (Li) diffuses freely between large interlayer, here, we in situ visualize Li diffusion pathways into the layered germanium phosphide (GeP) crystal flake along different directions via a microbattery device. Surprisingly, a more preferential Li diffusion pathway along zigzag [010] direction than armchair [102] direction is observed, demonstrating a strong in-plane diffusion anisotropy up to 7.0. And an interlayered Li diffusion coefficient of 3.4 x 10(-7) cm(2) s(-1) is obtained for GeP flake, over 1,000 times faster than that measured for graphite flake (2.4 3 10(-11) cm(2) s(-1)), suggesting a high potential of GeP as an attractive fast-charging material. The visualizing and quantitative strategy for the intrinsic diffusion process provides solid and scientific guidance on high -through material screening for fast-charging batteries.