X-ray emission spectroscopy (XES) is a chemical state analysis method. It is possible to show the change in a compound’s bonding state profile by measuring fluorescent X-rays with high energy resolution. Recently, the XES method has been evaluated in the field of advanced materials such as battery materials and catalysts, and the need for laboratory spectrometers is increasing. We are evaluating the applicability of Si-based negative electrode materials and next-generation battery materials for lithium-ion batteries using double-crystal spectroscopy with high-energy resolution. Quantitative analysis results of Li–Si alloy composition and side reaction products were reported based on changes in the Si Kβ spectral profile during electrochemical charging and discharging. It is known that the X-ray emission spectrum changes depending on the chemical state of the material, but the technique is not actively applied to chemical state analysis currently. In this paper, as the basis for describing the X-ray emission analysis method, we explain the optical system required to obtain high resolution, describe the interpretation of the X-ray emission spectrum and spectral changes due to the chemical state, and introduce application examples.