Novel Symmetric Cell Design for Analyzing All-Solid-State Battery Electrode

All-solid-state lithium batteries (ASSBs) with sulfide-based solid electrolytes (SSEs) have drawn much attention recently for the next-generation batteries. But there are some obstacles to overcome for their practical use. The SSE|cathode interface with poor contact and (electro)chemical instability is the subject of special interest, since it critically aggravates the core performance of battery, including cycling stability and rate capability. In addition to the interfacial property, it has been widely known that Li + transport characteristics inside the cathode is another key factor in terms of fast and efficient operation of all the lithium battery systems, including the ASSBs. Thus, it goes without saying that the accurate characterization of the interfacial and bulk properties of the cathode is essential to develop the ASSBs. There are limited numbers of test cells to analyze the interface and bulk properties of the battery. First, a conventional half-cell includes Li metal as a counter electrode (CE), which is assumed to have a negligible resistance compared to that of the working electrode (i.e., cathode). It has been reported, however, that the effect of Li CE on total electrochemical signal cannot be disregarded. Moreover, the relative contribution of Li CE to total signal is increasing with the use of the state-of-the-art cathode, whose resistance is getting dramatically lower with technological progress. That is, it is getting difficult to obtain a reliable cathode response by using the two-electrode half-cell configuration. Second, a three-electrode test cell provides the signal of each electrode separately and has been extensively used for the electrochemical analysis. When one uses it for the ASSBs, however, the signals can be erroneous due to the structural distortion caused by the reference electrode (RE) insertion and its own chemical instability, which is especially severe in ASSBs with SSEs. Finally, an alternative cell design to avoid the above problems is a symmetric cell including two identical electrodes facing each other. A general symmetric cell is fabricated by charging/discharging two unit-cell, disassembling them, and re-assembling the identical electrodes to face each other. Unfortunately, a series of such procedure, esp. disassembling/re-assembling, is not applicable to the ASSB systems. In this study, a novel non-destructive potential-controllable symmetric cell (PCSC) is proposed for ASSBs. It features a porous temporary CE (TCE) in parallel between two symmetric electrodes. TCE works as an acceptor or provider of Li to charge or discharge, thereby controls the electrode potential. Symmetric electrodes are then evaluated electrochemically after disconnecting the TCE. As a case study, we investigated and comparatively analyze the electrochemical characteristics of an ASSB cathode, LiNbO 3 -coated LiNi 0.8 Co 0.1 Mn 0.1 O 2 , obtained from the conventional test cells and PCSC. Figure presents the impedance spectra of test cells. The two-electrode half-cell impedance is noticeably affected by Li CE over a wide frequency range and the three-electrode cell impedance suffers from severe noise probably caused by RE in the low frequency region. This indicates that both test cells are not suitable for interpreting reliably the interfacial and bulk properties of the cathode. In contrast, the impedance spectra from the PCSC have similar trend to that of three-electrode cell, but provides much more stable impedance spectra, allowing more accurate estimation of interface and bulk characters. In this presentation, the results obtained from DC techniques, such as galvanostatic method, are also comparatively analyzed with conventional test cells and our PCSC. Moreover, the reliability of the estimated values from the PCSC will be discussed by the comparison with those of the previous works. Figure 1