Graphite has been currently considered as a promising cathode material in dual ion batteries (DIBs) due to its unique features of sp 2 hybridized carbon and stacked two-dimensional layered structures. However, unexpected volume/thickness changes in the graphite cathodes, induced by the intercalation/deintercalation of anions with large molecular size have been known to be a critical problem in designing DIB cells. To understand the volume/thickness changes in the DIB electrodes, in operando optical observing apparatus has been employed to observe the cross-section view of a graphite-based cathode upon cycles in the present work. The observation suggests that the cathode initially presented a huge irreversible thickness change (60%), and such thickness variation was prone to reduce and remain <20% in the following cycles. The results from both in operando observation and electrochemical characterizations collectively indicate that the greater thickness variation at initial cycle should be attributed to both anion intercalation into graphite-based cathodes and irreversible decomposition of chemical components in the DIB system. The method here highlights a universal route for fundamentally understanding the electrodes of huge volume variation.