Table 1.

Interfacial challenges exist in cathode-solid electrolyte systems according to the different characteristics of the four types of solid electrolytes and the corresponding solutions, recent advances and limitations still exist.

Interfaces	Interfacial mechanical performance	Interfacial chemical stability	Solutions and advances	Limitations still exist
Cathode-Solid polymer electrolyte interface	Excellent elasticity and deformability promote favorable interface contact Poor strength cannot block Li dendrites	PEO-based SPE is not stable above 4.0 V (Croce et al., 2001)	(a) Optimization of Li salts (Zhang et al., 2011; Ma et al., 2016b) (b) Polymer matrix modification such as copolymerization, branching and crosslinking (Tong et al., 2014; Porcarelli et al., 2016) (c) Gel/ plasticized polymer electrolyte (Manuel Stephan, 2006; Zewde et al., 2018) (d) Cathode coating (Ma et al., 2017; Yang et al., 2018)	(a) Performance matched with high-voltage cathode such as LiCoO2 and Li2MnO4 is still poor (b) The weaknesses of liquid electrolyte still exist in gel system, such as flammable property (c) Short-circuit concerns
Cathode-solid oxide electrolyte interface	High strength properties can partially block dendrite Poor flexibility lead to a poor solid-solid contact Dendrite can grow along grain boundaries	Stable up to 6V (Li et al., 2015; Thangadurai et al., 2015) High-temperature handling (>500°C) to may lead to elements interdiffusion and form transition layer	(a) Surface coating (Han et al., 2017) (b) Co-sintering (Wakayama et al., 2016) (c) In-situ synthesized electrolyte layer (Yoshima et al., 2016; Kazyak et al., 2017) (d) Interface softening (Seino et al., 2011; Sakuda et al., 2012; Liu et al., 2016) (e) Interface buffer layers (Kato et al., 2014; Park et al., 2016) (f) Amorphous cathode (Matsuyama et al., 2016; Nagao et al., 2017)	(a) Even intimate contact could be achieved at pristine state, contact loss will happen upon cycling during to the rigid ceramic nature (b) High interface resistance prohibits thicker cathode layer and high capacity battery as a result
Cathode-solid sulfide electrolyte interface	Reasonable strength and decent deformability Poor elasticity lead to contact loss upon periodic cathode expanding and shrinking	High Li chemical potential leads to a space charge layer when matched with oxide cathodes Electrochemically unstable when contacted with high-voltage cathode	(a) Interface buffer layer to mitigate SCL (Haruyama et al., 2014; Koerver et al., 2017) (b) Hot press, in-situ synthesis and sulfide electrolyte coating onto active materials to obtain intimate contact (Kitaura et al., 2011; Yao et al., 2016; Ito et al., 2017)	Contact loss upon cycling is still an unsolved problem which makes external pressure necessary
Cathode /solid composite electrolyte interface	Combine the virtues of both polymer and ceramic with both reasonable strength and flexibility, promising to obtain favorable contact	By adding inorganic fillers in PEO based solid electrolyte, the anti-oxidation property at high voltage is still under discussion even various studies reported high electrochemical window	By regulating the composition of composite electrolyte, solid electrolytes with different performance will be obtained to adapt to different requirements	Drawbacks exist in single solid electrolyte system such as poor stability of SPE, SCL in solid sulfide electrolyte and poor flexibility of solid oxide may still exist when these components contact cathode A novel solid electrolyte with high ionic conductivity, chemical stability Compatibility with cathode is still a long way to go