UC Berkeley

The emerging electrical vehicle market and global decarbonization trend demand batteries with higher energy density, better safety, and longer cycle life. An all-solid-state battery has the potential to meet such requirements and is widely considered as the next-generation battery technology to replace the current Li-ion batteries. The remarkable success in the discovery of ceramic alkali superionic conductors has provided a wide selection of solid electrolytes, allowing the fabrication and testing of all-solid-state battery cells. However, the current solid-state battery still cannot compete with state-of-the-art Li-ion technology because of its poor cycle stability and low energy density.

One of the causes of the poor cycle stability in all-solid-state batteries is the lack of chemical stability between the solid electrolytes and electrodes. To efficiently and systematically investigate these chemical compatibility issues, a methodology was developed combining density functional theory calculations and simple experimental techniques such as X-ray diffraction, simultaneous differential scanning calorimetry, thermal gravimetric analysis, and electrochemistry. Two aspects of the chemical stability were investigated: the electrochemical stability of the solid-state conductor, which is relevant wherever the electrolyte contacts an electron pathway, and the electrochemical stability of the electrode/electrolyte interfaces. Computationally derived voltage stability windows are small for both Na3PS4 and Na3PSe4, which are confirmed experimentally. By combining the theory calculations and simple experimental techniques, the most stable system (NaCrO2|Na3PS4|Na–Sn) within a selected Na solid-state battery chemical space (more than 20 different combinations of electrodes and electrolytes) was efficiently found. Important selection criteria for the cathode, electrolyte, and anode in solid-state batteries are derived from this study. This method provides an essential guide for probing the chemical instability issue in the all-solid-state battery, and experimental results show the importance of interfacial engineering for the all-solid-state battery.

The mechanical degradation in all-solid-state batteries can also cause rapid capacity fade. To investigate the severity of the mechanical degradation after long-term cycling, the all-solid-state composite cathode morphology before and after long-term cycling was imaged using the focused ion beam scanning electron microscope tomography. A large amount of void volume (~ 12%) and contact loss area (> 10%) are found in the cathode composite after 50 cycles, revealing the severe mechanical degradation during cycling. The contact loss between the cathode and solid electrolyte is accompanied by the rapid capacity drop during battery cycling. This result suggests that mechanical degradation does not progress gradually starting from the first cycle. Instead, the majority of the void formation and contact loss likely occurs at later cycles and over a short time. This part of the study highlights the importance of mechanical stability for the all-solid-state battery.

Lastly, increasing the cathode active material loading is proposed as a method to increase the full cell energy density of the all-solid-state battery. Current low active material loading in the composite electrode is one of the main reasons for the low full cell energy density. In this part, modeling and experimental results show that a higher cathode loading and therefore an increased energy density can be achieved by increasing the ratio of the cathode to conductor particle size. The utilization of the cathode materials in the solid-state composite is shown to be highly dependent on the particle size ratio of the cathode to the solid electrolyte, especially in the regime of high cathode loading. These results are consistent with the ionic percolation being the limiting factor in cold-pressed solid-state cathode materials and provide specific guidelines on how to improve the energy density of composite cathodes for solid-state batteries. By reducing solid electrolyte particle size and increasing the active material particle size, over 50 vol% cathode loading with high cathode material utilization was experimentally demonstrated. This part of the work proves that a commercially-relevant, energy-dense cathode composite is achievable through simple mixing and pressing method.