Guidelines for All-Solid-State Battery Design and Electrode Buffer Layers Based on Chemical Potential Profile Calculation

Protective coatings
on cathode active materials have become paramount
for the implementation of solid-state batteries; however, the development
of coatings lacks the understanding of the necessary coating properties.
In this study, guidelines for the design of solid electrolytes and
electrode coatings in all-solid-state batteries are proposed from
the viewpoint of the steady-state Li chemical potential profile across
the battery cell. The model calculation of the (electro)­chemical potential
profile in all-solid-state batteries is established by considering
the steady-state mixed ionic and electronic conduction in the solid
electrolyte under the assumption of local equilibrium. For quantitative
discussion, the potential profiles within oxygen ion conductors are
calculated instead of Li/Na ion conductors as their partial electronic
conductivities have not been reported so far in sufficient detail.
Based on the calculated chemical potential profile, two main conclusions
are obtained: (1) the decisive factor for the formation of the chemical
potential profile of the neutral mobile component (e.g., oxygen or
lithium) in the solid electrolyte is its electronic conductivity (and
the activity dependence), and (2) a particularly large potential drop
is formed in a region where the electronic conductivity becomes small.
While these conclusions are valid and general for any solid electrolyte
device, they are particularly important for the design of protective
coatings and the understanding of the functionality of self-assembled
solid electrolyte interphases in all-solid-state batteries. To protect
the solid electrolyte from decomposition by reduction/oxidation at
the anode/cathode interfaces, a sufficient chemical potential drop
is necessary within the coating layer or directly at the interphase
layer. To achieve this situation, the coating/interphase materials
need to have a lower electronic conductivity than the solid electrolyte.