All-solid-state batteries (ASSBs) containing solid electrolytes (SEs) are considered promising for use as next-generation energy storage devices in electric vehicles and portable electronics. Apart from safety benefits, in principle, they offer both higher energy and power densities than conventional lithium-ion batteries (LIBs) using a liquid carbonate-based electrolyte. However, ASSBs will only become competitive in the near future if some major problems are solved. These concern mainly technical functionality aspects, SE conductivity, and the stability of the electrode-solid electrolyte interface. The latter is not only crucial in terms of enabling the use of a Li-metal anode, but also at the cathode side, where a stable and robust contact between the SE and the cathode active material (CAM) is vital for long-term cycling performance. For example, this is the case when using layered Ni-rich oxide CAMs such as Li_1+x(Ni_1-y-zCo_yMn_z)_1-xO₂ (NCM) with high specific capacity and thiophosphate-based solid electrolytes.

Here we report on the strong impact of both CAM particle size and external applied pressure on the capacity of bulk-type ASSBs. Our model cell system comprised compacted pellets of NCM622 (60% Ni) as CAM, ß-Li₃PS₄ or Li₆PS₅Cl as SE, and In-metal as anode. In particular, we demonstrate the benefit of using small-size cathode particles (≤ 5 μm) to achieve virtually full theoretical specific capacity at low external pressure. This finding is rationalized through complementary galvanostatic cycling and X-ray diffraction on operating cells (in situ) as well as on harvested CAM/SE composites (ex situ). Besides, we reveal the significance of considering electrochemically inactive CAM in ASSBs, the proportion of which is equally increased for large particles and low applied pressure. Using charge transport measurements, we provide evidence that the presence of inactive CAM is due to a lack of electronic contact; ionic percolation seems not to be an issue for the two-phase cathode composites employed here. Overall, the results emphasize the importance of using size-tailored CAM, allowing for good electronic percolation, and thus conductive additives, which may cause SE degradation upon cycling, can be avoided.