Densification of composite cathodes in all-solid-state Li-ion batteries via sheet metal hydroforming pressing

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Densification of composite cathodes in all-solid-state batteries (ASSBs) was performed via sheet-metal-type hydroforming pressing. The modified system enables pre-heated, “quasi” isostatic pressing under fluid-free conditions by applying the pressure thorough a rubber diaphragm. The structures of the composite electrodes before and after densification were examined using synchrotron X-ray computed tomography and compared with those obtained using other conventional pressing methods. After pressing the composite cathodes at 140 MPa and room temperature, the normalized void ratio, estimated from the ratio of the number of voxels for carbon/voids to the number of voxels for the active materials, decreased by 50 %. This reduction is equivalent to that obtained by cold isostatic pressing under the same conditions. For uniaxial pressing, the disappearance of large spotty-like voids was observed; however, their contribution to densification was limited. An increase in the pressing temperature effectively promoted the densification of the composite cathodes. The void ratio was reduced by a factor of eight when pressing was performed at 150 °C. Owing to its fluid-free nature, the proposed method could be applied to ASSB manufacturing processes requiring severe dew point control.