Degradation Behavior of Layered Structure Cathode in Sulfide Based All-Solid-State Batteries
Authors: Shin Hyeon-Ji , Kim Jun Tae , Jung Hun-Gi 
Published: September 2023

With declaration of carbon zero in response to climate change, the importance is also being bandied about developing large-scale energy storage system, especially for electric vehicle (EV). Commercialized EV with a highest development lithium-ion batteries (LIB) could operate over 300 miles per single charging, but liquid electrolyte-based LIB gives rise to a huge risk concerning about ignition and explosion. All-solid-state batteries (ASSBs) is considered to alternative next-generation battery that have high safety advantageous with removal the risk of ignition by applying non-combustible inorganic solid-electrolytes (SEs). In case of sulfide-based ASSBs, there were progressed in performance itself at cell capacity increase and cycling characteristic due to advance in cell configuration, composite cathode, and cell fabrication technique compared to early works. Nevertheless, it is still demanded improvement related to the cathode-electrolyte interface stability, and its dominant causes are electrochemical- and chemo-mechanical problems. Therefore, most of researchers have tried to figure out the behavior of all-solid-state batteries by providing external pressure, and then resolve degradation issues including contact loss and resistive layer formation. However, in practical and commercialization aspects, the external pressured cell can be a stumbling block to scale-up from lab to large-scale ASSBs stage.

In this study, we evaluated electrochemical performance using by general coin cell to investigate behavior of ASSBs, which is consisted with LiNi0.5Co0.2Mn0.3O2/Li6PS5Cl/VGCF|Li6PS5Cl|In-Li. The ASSB coin cell showed a significant capacity degradation having 59 % of capacity retention in 50th cycles, as expected. Herein, the main cause of the rapid capacity decrease is likely to be due to the accelerated physical contact loss. In addition to this, we have expanded the scope of analysis to investigate if there are other causes entire range including cell- and material level. The degradation in cell level is the inhomogeneous electrochemical reaction, that is within composite cathode, resulting from analysis of electrochemical impedance spectroscopy, cross-sectional scanning electron microscopy, x-ray diffraction, and X-ray photoelectron spectroscopy. This induces more severe degradation on close to SE separator, resulting in intensification of SE deterioration accompanied by damage implying a phase transformation of the active material. Furthermore, even though the contact loss problem has improved through re-pressing, the capacity was not recovered, which is because the material itself has deteriorated.

Moving on to the material level, many cracks are observed inside of the active material, and sulfur infused into grain-boundary of active materials that confirmed by energy dispersive X-ray spectroscopy. In addition, through analyzing transmission electron microscopy, the phase transformation both mixed and rock-salt phase was confirmed the cathode surface of not only secondary particle but also primary particle of interior cathode. For more specifically affirmed this, various analysis including Raman spectroscopy and inductively coupled plasma optical emission spectroscopy were conducted, and we confirmed that the major cause is due to depletion of the Li source. Therefore, the loss of active Li by infused sulfur caused acceleration of deterioration cathode making poor capacity retention. This study is meaningful in providing another guideline that should be regarded for practical all-solid-state batteries through a new consideration in cathode degradation.