No abstract

The application of lithium metal as a negative electrode in all-solid-state batteries shows promise for optimizing battery safety and energy density. However, further development relies on a detailed understanding of the chemo-mechanical issues at the interface between the lithium metal and solid electrolyte (SE). In this study, crack formation inside the sulfide SE (Li3PS4: LPS) layers during battery operation was visualized using in situ X-ray computed tomography (X-ray CT). Moreover, the degradation mechanism that causes short-circuiting was proposed based on a combination of the X-ray CT results and scanning electron microscopy images after short-circuiting. The primary cause of short-circuiting was a chemical reaction in which LPS was reduced at the lithium interface. The LPS expanded during decomposition, thereby forming small cracks. Lithium penetrated the small cracks to form new interfaces with fresh LPS on the interior of the LPS layers. This combination of reduction–expansion–cracking of LPS was repeated at these new interfaces. Lithium clusters eventually formed, thereby generating large cracks due to stress concentration. Lithium penetrated these large cracks easily, finally causing short-circuiting. Therefore, preventing the reduction reaction at the interface between the SE and lithium metal is effective in suppressing degradation. In fact, LPS-LiI electrolytes, which are highly stable to reduction, were demonstrated to prevent the repeated degradation mechanism. These findings will promote all-solid-state lithium-metal battery development by providing valuable insight into the design of the interface between SEs and lithium, where the selection of a suitable SE is vital.

The dynamic changes of ionic conduction path in the cross-sectional graphite composite electrodes of bulk-type all-solid-state lithium batteries have been monitored using operando confocal microscopic observations for color changes of graphite in response to their stage structures. The ionic conduction path decreased in the cross-sectional direction as cycle numbers increased, with simultaneous capacity degradation. The local reactivity of lithiation and delithiation was evaluated by image analysis considering state-of-charge (SOC) values. Electrode thickness changes were examined from the confocal microscope images obtained in the operando observations. The results revealed that voids and cracks were formed during cycle tests and that the thickness gradually increased. These cracks and voids were one of the main contributors to the limitation of ionic conduction paths in the depth direction. Operando microscopic observation and subsequent image analysis elucidated not only the morphological changes of active materials but also the differences in local SOC changes in the electrode.

Li metal is very attractive as a negative electrode material for high-energy-density all-solid-state batteries owing to its high specific capacity and low electrochemical potential. However, short-circuiting of batteries upon formation of Li dendrites is a serious issue that hinders its successful practical application. An advantage of all-solid-state batteries is their inherent safety at high temperatures. In this study, the high-temperature performance of all-solid-state Li-metal batteries containing sulfide-glass electrolytes was investigated. Symmetric cells with Li 3 PS 4 electrolytes exhibited better Li dissolution-deposition performance at 100 • C than at 25 • C. In addition, inserting Au thin films at the Li/Li 3 PS 4 interface enabled stable operation of the symmetric cells at high current density (1.3 mA cm −2 ) and large areal capacity (6.5 mAh cm −2 ) without short-circuiting. All-solid-state Li-metal batteries with Au thin films (Li/Au/Li 3 PS 4 /LiNi 1/3 Mn 1/3 Co 1/3 O 2 ) exhibited high rate performance at 2.4 mA cm −2 and long lives of over 200 cycles at 100 • C. The dissolution of the Au thin films into the Li metal is a possible reason for the enhanced electrochemical performance. These results indicate that interface modification and optimizing operating temperature are promising strategies to achieve all-solid-state batteries with high energy densities.

No abstract