An effect of crystallinity of graphite on formation of graphite intercalated compounds (GICs) and reversibility in K cells was studied by comparing that of lithium-ion batteries. Though high reversible capacities and coulombic efficiencies of graphite electrodes in K cells were achieved during initial cycles regardless of the crystallinity, high crystallinity graphite demonstrated less potential-hysteresis and superior capacity retention to low crystallinity graphite. Operando XRD measurement confirmed similar staging process of K-GICs for both graphite, however, high crystallinity graphite was transformed into higher crystallinity of K-GIC as well as higher reversibility of potassium de-/intercalation crystallinity of graphite, which includes coherence length and the degree of random stacking, is found to be a predominant factor for highly reversible potassium intercalation, which differs from the lithium case. We concluded that the high crystallinity is of importance for the application of graphite to long-life potassium-ion batteries. The working electrodes consisting of 89 wt% graphite powder (supplied by Showa K. K.), 6 wt% conductive carbon (C45, TIMCAL), and 5 wt% carboxymethyl cellulose sodium salt (Daicel Miraizu Ltd.) as a binder were prepared as reported in our paper. 16 An optimal amount of deionized water was used to prepare the slurry, and then the slurry was coated on a Cu foil for charge-discharge tests and an Al foil for operando XRD measurements. The electrode was dried at 373 K under vacuum before cell assembling. The mass loading of active material was 1.2 mg cm -2 for charge-discharge tests and 6 mg cm -2 for operando XRD. The electrolyte was prepared by dissolving the appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI, Kanto Chemical) or potassium bis(fluorosulfonyl)imide (KFSI, Solvionic) in the mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at 1 : 1 v/v (battery grade, Kishida Chemical). The salts and solvent were used as received without any drying and purification. Chemical) electrolyte decomposition and SEI formation on the operando XRD measurements. Charge-discharge tests of operando XRD were performed in only CC mode during both charge and discharge in the potential range of 0 – 2 V vs. K + /K at a current density of C/30. carbon-to-carbon bonds. 31 This domain boundary is not necessarily parallel to the stacking direction of the carbon layer, disturbing the long-range periodicity of the carbon and potassium layers. We recently reported that high-stage K-GICs have a high degree of Daumas-Herold defects with large FWHM values of 00 l reflections and the defects almost disappear by complete potassium intercalation to form stage-1 KC 8 . 16 Thus, larger FWHM values of the 00 l peaks were observed for the higher-stage K-GICs in both graphite electrodes, indicating a higher degree of stacking faults including the Daumas-Herold defects and the original turbostratic disorder. In stage-3 K-GICs, which is a higher stage than stage-2 and -1, the FWHM value of L-Graphite (0.912°) is quite larger than 0.435° of H-Graphite ( Fig. 3(a) and (b) ). The difference in FWHM of the two graphite samples became small by transitions to lower-stages of stage-2 and -1, but the FWHM values for the L-Graphite electrode are always larger than those for the H-Graphite electrode at the same stage. The original turbostratic disorder in the pristine graphite may increase the number of the Daumas-Herold defects, resulting in large FWHM values.