Despite the efforts devoted to the identification of new electrode materials with higher specific capacities and electrolyte additives to mitigate the well-known limitations of current lithium-ion batteries, this technology is believed to have almost reached its energy density limit. It suffers also of a severe safety concern ascribed to the use of flammable liquid-based electrolytes. In this regard, solid-state electrolytes (SSEs) enabling the use of lithium metal as anode in the so-called solid-state lithium metal batteries (SSLMBs) are considered as the most desirable solution to tackle the aforementioned limitations. This emerging technology has rapidly evolved in recent years thanks to the striking advances gained in the domain of electrolyte materials, where SSEs can be classified according to their core chemistry as organic, inorganic, and hybrid/composite electrolytes. This strategic review presents a critical analysis of the design strategies reported in the field of SSEs, summarizing their main advantages and disadvantages, and providing a future perspective toward the rapid development of SSLMB technology.

Despite the potential advantages promised by solid-state batteries, the success of solid-state electrolytes has not yet been fully realised. This is due in part to the lower ionic conductivity of solid electrolytes. In many solid superionic conductors, grain boundaries are found to be ionically resistive and hence contribute to this lower ionic conductivity. Additionally, in spite of the hope that solid electrolytes would inhibit lithium filaments, in most scenarios their growth is still observed, and in some polycrystalline systems this is suggested to occur along grain boundaries. It is apparent that grain boundaries affect the performance of solid-state electrolytes, however a deeper understanding is lacking. In this perspective, the current theories relating to grain boundaries in solid-state electrolytes are explored, as well as addressing some of the challenges which arise when trying to investigate their role. Glasses are presented as a possible solution to reduce the effect of grain boundaries in electrolytes. Future research directions are suggested which will aid in both understanding the role of grain boundaries, and diminishing their contribution in cases where they are detrimental.

While all-solid-state batteries have built global consensus with regard to their impact in safety and energy density, their anode-less versions have attracted appreciable attention because of the possibility of further lowering the cell volume and cost. This perspective article summarizes recent research trends in anode-less all-solid-state batteries (ALASSBs) based on different types of solid electrolytes and anticipates future directions these batteries may take. We particularly aim to motivate researchers in the field to challenge remaining issues in ALASSBs by employing advanced materials and cell designs.

Solid polymer electrolytes (SPEs) possess several merits including no leakage, ease in process, and suppressing lithium dendrites growth. These features are beneficial for improving the cycle life and safety performance of rechargeable lithium metal batteries (LMBs), as compared to conventional non-aqueous liquid electrolytes. Particularly, the superior elasticity of polymeric material enables the employment of SPEs in building ultra-thin and flexible batteries, which could further expand the application scenarios of high-energy rechargeable LMBs. In this perspective, recent progresses on ion transport mechanism of SPEs and structural designs of electrolyte components (e.g. conductive lithium salts, polymer matrices) are scrutinized. In addition, key achievements in the field of single lithium-ion conductive SPEs are also outlined, aiming to provide the status quo in those SPEs with high selectivity in cationic transport. Finally, possible strategies for improving the performance of SPEs and their rechargeable LMBs are also discussed.

Garnet-type solid-state electrolytes (SSEs) are particularly attractive in the construction of all-solid-state lithium (Li) batteries due to their high ionic conductivity, wide electrochemical window and remarkable (electro)chemical stability. However, the intractable issues of poor cathode/garnet interface and general low cathode loading hinder their practical application. Herein, we demonstrate the construction of a reinforced cathode/garnet interface by spark plasma sintering, via co-sintering Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) electrolyte powder and LiCoO₂/LLZTO composite cathode powder directly into a dense dual-layer with 5 wt% Li₃BO₃ as sintering additive. The bulk composite cathode with LiCoO₂/LLZTO cross-linked structure is firmly welded to the LLZTO layer, which optimizes both Li-ion and electron transport. Therefore, the one-step integrated sintering process implements an ultra-low cathode/garnet interfacial resistance of 3.9 Ω cm² (100 °C) and a high cathode loading up to 2.02 mAh cm⁻². Moreover, the Li₃BO₃ reinforced LiCoO₂/LLZTO interface also effectively mitigates the strain/stress of LiCoO₂, which facilitates the achieving of superior cycling stability. The bulk-type Li|LLZTO|LiCoO₂-LLZTO full cell with areal capacity of 0.73 mAh cm⁻² delivers capacity retention of 81.7% after 50 cycles at 100 μA cm⁻². Furthermore, we reveal that non-uniform Li plating/stripping leads to the formation of gaps and finally results in the separation of Li and LLZTO electrolyte during long-term cycling, which becomes the dominant capacity decay mechanism in high-capacity full cells. This work provides insight into the degradation of Li/SSE interface and a strategy to radically improve the electrochemical performance of garnet-based all-solid-state Li batteries.

Development of low-resistance electrode/electrolyte interfaces is key for enabling all-solid-state batteries with fast-charging capabilities. Low interfacial resistance and high current density were demonstrated for Na-β''-alumina/sodium metal interfaces, making Na-β''-alumina a promising solid electrolyte for high-energy all-solid-state batteries. However, integration of Na-β''-alumina with a high-energy sodium-ion intercalation cathode remains challenging. Here, we report a proof-of-concept study that targets the implementation of a Na-β''-alumina ceramic electrolyte with a slurry-casted porous NaCrO₂ cathode with infiltrated sodium hydroborates as secondary electrolyte. The hydroborate Na4(B12H12)(B10H10) possesses similar sodium-ion conductivity of 1 mS cm^-1 at room temperature as Na-β''-alumina and can be fully densified by cold pressing. Using the Na₄(B₁₂H₁₂)(B₁₀H₁₀) secondary electrolyte as interlayer between Na-β''-alumina and NaCrO₂, we obtain a cathode-electrolyte interfacial resistance of only 25 Ω cm² after cold pressing at 70 MPa. Proof-of-concept cells with a sodium metal anode and a NaCrO2 cathode feature an initial discharge capacity of 103 mAh g^-1 at C/10 and 42 mAh g^-1 at 1 C with an excellent capacity retention of 88% after 100 cycles at 1 C at room temperature. Ion-milled cross-sections of the cathode/electrolyte interface demonstrate that intimate contact is maintained during cycling, proving that the use of hydroborates as secondary electrolyte and as an interlayer is a promising approach for the development of all-solid-state batteries with ceramic electrolytes.

Lithium phosphorus oxygen nitrogen (LiPON) as solid electrolyte discovered by Bates et al in the 1990s is an important part of all-solid-state thin-film battery (ASSTFB) due to its wide electrochemical stability window and negligible low electronic conductivity. However, the ionic conductivity of LiPON about 2 × 10−6 S cm−1 at room temperature is much lower than that of other types of solid electrolytes, which seriously limits the application of ASSTFBs. This review summarizes the research and progress in ASSTFBs based on LiPON, in the solid-state electrolyte of LiPON-derivatives with adjustable chemical compositions of the amorphous structure for the improvement of the ionic conductivity and electrochemical stability, in the critical interface issues between LiPON and electrodes, and in preparation methods for LiPON. This review is helpful for people to understand the interface characteristics and various preparation methods of LiPON in ASSTFBs. The key issues to be addressed concern how to develop solid-state electrolyte films with high conductivity and high-quality interface engineering as well as large-scale preparation technology, so as to realize the practical application of highly integrated ASSTFBs.

Moist air is a great challenge for manufacturing sulfide-based all-solid-state lithium batteries as the water in air will lead to severe decomposition of sulfide electrolytes and release H₂S gas. However, different with direct reaction with water, short-period air exposure of Li₁₀GeP₂S₁₂ sulfide electrolyte with controlled humidity can greatly enhance the stability of Li₁₀GeP₂S₁₂ against lithium metal, thus realizing stable Li₁₀GeP₂S₁₂ based all-solid-state lithium metal batteries. During air exposure, partial hydrolysis reaction occurs on the surface of Li₁₀GeP₂S₁₂ pellets, rapidly generating a protective decomposition layer of Li₄P₂S₆, GeS₂ and Li₂HPO₃ in dozens of seconds. This ionically conductive but electronically insulation protecting layer can effectively prevent the severe interface reaction between Li10GeP2S12 and lithium metal during electrochemical cycling. The Li/40s-air-exposed Li₁₀GeP₂S₁₂/Li cell shows long cycling stability for 1000 h. And the LiCoO₂/40s-air-exposed Li₁₀GeP₂S₁₂/Li batteries present good rate capability and long cyclic performances, showing capacity retention of 80% after 100 cycles.