LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) | SiOx-graphite (SiO-Gr.) battery chemistry is of intensive attention because its achievable practical energy density is approaching impressively 300 Wh Kg⁻¹. However, it still suffers rapid capacity fades during repeated cycles, both chemical, electrochemical and mechanical irreversibility contribute. A comprehensive understanding behind the fading behavior of the cell chemistry is required before fully realize the benefits of this chemistry. Herein, the in-situ thickness variation is introduced as a diagnostic technique and is performed on 5–55 Ah NCM811|SiO-Gr-Gr cells. With the help of Li reference electrode and in-situ X-ray diffraction device, the correspondence between thickness variation and the electrode potential is carefully investigated. Firstly, the NCM811|SiO-Gr-Gr cell is characterized with the maximum cell thickness at around 80% state-of-charge (SOC) in the discharge process, rather than at 100% SOC. Secondly, the electrochemical behaviors during rate charge/discharge are diagnosed, and a Li platting signal is resolved from thickness variation profile at 2C. This work confirms that the thickness monitoring is a nondestructive and informative complement to conventional diagnostic techniques for failure analysis of pouch cells.