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Layer-structured materials with van der Waals gaps have long been utilized as active electrode materials in rechargeable Li ion batteries. However, the intrinsic electrical, optical, and structural properties are not well understood at a single flake scale. We developed a planar nano battery for in situ measurement of large-area, uniform, two-dimensional (2D) flakes during lithiation process. A huge, simultaneous increase of optical transmittance and conductivity was observed with Lithium insertion in graphene and MoS2 flakes. The excellent performance (up to 200% transmittance increase and two orders magnitude of resistance decrease) is studied and explained by the unique band structure of the 2D materials and in situ TEM measurements. The proposed methodology can be applied to study a broad range of nanomaterials electrodes by electrochemical intercalation, and the excellent property of lithiated 2D materials will inspire a variety of applications such as electrochromic devices and transparent electrodes.

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