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Tao et al. Energy Mater 2022;2:200036 https://dx.doi.org/10.20517/energymater.2022.46 Page 11 of 35

potential, conductivity, mechanical properties (e.g., Young’s and shear moduli), thermal stability, defect
density and the concentration of elements/charge carriers are the key determinants of interface
formation [45,83-85] . The electrochemical processes mainly occur at the multi-phase interface of the solid
components in the composite cathode during cycling. The formed interfaces between the cathode and
electrolyte differ from an interface of active materials/electronic conductors to an interface of active
materials/Li-ion conductors. A number of issues may simultaneously coexist at the cathode/electrolyte
interface, including incomplete interfacial contact, interfacial decomposition, space-charge layers,
polarization and chemical interdiffusion, which are regarded as the major reasons for the increased
interfacial resistance.

Volume changes
Generally, a significant volume change (up to ~80%) occurs between the sulfur and Li S during battery
2
cycling; thus, regardless of the type of SSE, the cathode/SSE interfaces normally suffer from contact failure.
In the case of cathode composites, the volume ratio and geometric arrangement between different solid
components are critical factors that affect the volume change, thereby determining the structural stability of
the cathodes.

Interfacial reactions
Cathode composite systems consisting of several components contain multi-interfaces and spontaneously
evolve towards a nonequilibrium thermodynamic state and a chemical potential gradient exists across the
interfacial interactions because, in most cases, the electrochemical windows of SSEs cannot adequately
match the chemical potential of cathode (μ ) materials [86,87] . The electrochemical windows of SSEs are
c
completely dependent on the energy separation E between the conduction band or lowest unoccupied
g
molecular orbital (LUMO) and valence band or highest occupied molecular orbital (HOMO) [Figure 8].
The thermodynamic stability of an interface is strongly determined by the Fermi energy of the electrodes. If
μ (μ for cathode) is below the HOMO of the SSEs or μ (μ for anode) > LUMO, the interface is
c
c
a
a
thermodynamically unstable. The nonequilibrium thermodynamic state can provide a thermodynamic
driving force at an interface, causing the interfacial reactions between the two components to form the
cathode/electrolyte interphases.
In comparison with oxide cathodes, sulfide-based cathodes exhibit more intimate interfacial contact and
similar chemical potentials with sulfide electrolytes; thus, the key issue increasing the cathode/electrolyte
interfacial resistance could be the stress/strain induced because of the volume change of the sulfide
cathodes. However, it has been found that the chemical interaction and interfacial decomposition that
simultaneously occur at the interface may be non-negligible disadvantages that restrict the stability of
cathode/electrolyte interfaces. SSEs have limited thermal stability during high temperature or pressure
processing and weak electrochemical stability during cycling, with the induced formation of an interphase
resulting from nonequilibrium side reactions controlled by the kinetic process. In addition, the interfacial
reactions between sulfide-based SSEs and sulfur are also observed and form undesired insulating by-
products, which can be attributed to the reaction of the functional groups with sulfur. As a result, the
undesired insulating by-products caused by the side interfacial reactions are detrimental to the interfacial
ion conductivity.

In spite of tremendous efforts to investigate the interfaces between solid electrolytes and electrodes, a
comprehensive understanding of their role has not yet been achieved. A deep understanding of these
complex interfaces, including the formation mechanisms of the interphase, components, microstructures,
pathways of Li-ion diffusion and dynamic characteristics, is crucial to developing future ASSLSBs. The