All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite-, garnet- and NASICON-type solid electrolytes that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is notably larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.

Sulfide solid electrolytes possess high ionic conductivity and moderate dendrite suppression capability, but rather poor compatibility against oxide cathodes and metallic Li. Here, we report O-doped Li₆PS₅Br as solid electrolyte synthesized by a facile solid-state sintering. Different from other O-incorporated sulfides, the O atoms in Li₆PS_5-xO_xBr prefer to substitute the S atoms at free S²⁻ sites rather than those at the PS₄ tetrahedra. Remarkably, without deteriorating the ionic conductivity, this inorganic solid electrolyte with O doping exhibits comprehensively enhanced properties including excellent dendrite suppression capability, superior electrochemical and chemical stability against Li metal as well as high voltage oxide cathodes, and good air stability. Li(Ni_0.8Co_0.1Mn_0.1)O₂ and LiCoO₂-based all-solid-state batteries with Li₆PS_4.7O_0.3Br electrolyte deliver high specific capacity, superior rate capability, and outstanding cycling stability accompanied with low interfacial resistivity. This type of inorganic solid electrolytes is promising for all-solid-state batteries with high energy density.

The increased safety associated with all-solid-state batteries using inorganic ceramic electrolytes make it a promising technology, with potential to replace current commercial battery systems. The key challenges to realize this technology are the development of new solid electrolytes with high ionic conductivity and optimization of the ionic transport pathways across the multiple phases of the battery. In this study an optimal composition of the argyrodite i.e. Li₆PS₅Cl_0.5Br_0.5 is synthesized via the mechanical milling method. This material possesses a higher bulk ionic conductivity and reduced activation energy than the single halogen doped argyrodites i.e. Li₆PS₅X (X = Cl and Br), assessed by temperature-dependent impedance spectroscopy and Nuclear Magnetic Resonance (NMR) relaxometry. A combined X-ray and neutron diffraction analysis reveals an influence of the composition and distribution of halogen atoms on the Li-ion conductivity. All-solid-state batteries fabricated using Li₂S as cathode show a high reversible capacity of 820 mAh g⁻¹ for up to 30 cycles. In addition, the Li-ion diffusion across the interface between the Li₂S cathode and Li₆PS₅Cl_0.5Br_0.5 electrolyte is probed by exchange NMR spectroscopy. It reveals that Li-ion diffusion across this interface was the main factor limiting the performance of Li₆PS₅Cl_0.5Br_0.5 in the battery, despite its high bulk ionic conductivity.

It is a highly relevant topic concerning how to improve the electrochemical performance of anode materials for the next generation lithium-ion batteries (LIBs). Herein, for the first time, we report a facile and controllable approach to modify MoS ₂ nanosheets, a typical 2D material, as an anode material for advanced LIBs via oxygen plasma engineering. An oxygen plasma treatment not only generates vacancies/defects but also incorporates heteroatom doping to form Mo–O–C bonds. This unique hybrid specialty enables the oxygen-plasma-treated MoS ₂ to achieve superior electrochemical performance with high reversible capacities, a long-term cycle life, and good rate capabilities. The plasma-assisted modification is believed to be applicable for other 2D materials as an efficient anode for energy storages.

The high Li-ion conductivity of the Li₇P₃S₁₁ sulfide-based solid electrolyte makes it a promising candidate for all-solid-state lithium batteries. The Li-ion transport over electrode-electrolyte and electrolyte-electrolyte interfaces, vital for the performance of solid-state batteries, is investigated by impedance spectroscopy and solid-state NMR experiments. An all-solid-state Li-ion battery is assembled with the Li₇P₃S₁₁ electrolyte, nano-Li₂S cathode and Li-In foil anode, showing a relatively large initial discharge capacity of 1139.5 mAh/g at a current density of 0.064 mA/cm² retaining 850.0 mAh/g after 30 cycles. Electrochemical impedance spectroscopy suggests that the decrease in capacity over cycling is due to the increased interfacial resistance between the electrode and the electrolyte. 1D exchange ⁷Li NMR quantifies the interfacial Li-ion transport between the uncycled electrode and the electrolyte, resulting in a diffusion coefficient of 1.70(3)⋅10⁻¹⁴ cm²/s at 333 K and an energy barrier of 0.132 eV for the Li-ion transport between Li₂S cathode and Li₇P₃S₁₁ electrolyte. This indicates that the barrier for Li-ion transport over the electrode-electrolyte interface is small. However, the small diffusion coefficient for Li-ion diffusion between the Li₂S and the Li₇P₃S₁₁ suggests that these contact interfaces between electrode and electrolyte are relatively scarce, challenging the performance of these solid-state batteries.

The ultrafast ionic conductivity of Li ₆ PS ₅ Br, which is higher than 1 mS cm ^-1 at room temperature, makes it an attractive candidate electrolyte for the all-solid-state Li-S battery. A simple synthesis route with an easy scale up process is critical for practical applications. In this work, the highest room temperature ionic conductivity (2.58 × 10 ^-3 S cm ^-1 ) of Li ₆ PS ₅ Br is obtained by an optimal annealing temperature in a simple solid-state reaction method. Neutron diffraction and XRD show that the origin of the highest ionic conductivity is due to the higher purity, smaller mean lithium ion jumps and the optimal Br ordering over 4a and 4c sites. All-solid-state Li-S batteries using a S-C composite cathode in combination with the optimized Li ₆ PS ₅ Br electrolyte and Li-In anode show high (dis)charge capacities. Different cycling modes (charge-discharge and discharge-charge) reveal that the capacity of the S-C-Li ₆ PS ₅ Br/Li ₆ PS ₅ Br/Li-In battery arises from both the active S-C composite and the Li ₆ PS ₅ Br in the cathode mixture. The contribution of the latter is verified from all-solid-state batteries using Li ₆ PS ₅ Br and its analogues as active materials. Ex situ XRD and electrochemical performance results show that the contribution of capacity from Li ₆ PS ₅ Br in the cathode mixture may be associated with the decomposition product Li ₂ S, while the Li ₆ PS ₅ Br in the bulk solid electrolyte layer is stable during cycling.

The development of high-performance all-solid-state batteries relies on charge transport in solid electrolytes, where transport across grain boundaries often limits their bulk conductivity. The argyrodite Li ₆ PS ₅ X (X = Cl, Br) solid electrolyte has a high conductivity; however, macroscopic diffusion in this material involves complex jump processes, which leads to an underestimation of the activation energy. Using a comprehensive frequency- and temperature-dependent analysis of the spin-lattice relaxation rates, a complete estimation of Li self-diffusion is demonstrated. Another experimental challenge is quantifying the impact of grain boundaries on the total bulk conductivity. Li ₆ PS ₅ Cl and Li ₆ PS ₅ Br have identical crystalline structures, but with ₆ Li MAS NMR, their resonance peaks have different chemical shifts. Exploiting this with two-dimensional ₆ Li- ₆ Li exchange NMR on a mixture of Li ₆ PS ₅ Br and Li ₆ PS ₅ Cl, we observe Li exchange between particles of these two materials across grain boundaries, allowing direct and unambiguous quantification of this often limiting process in solid-state electrolytes.

High grain-boundary resistance, Li-dendrite formation, and electrode/Li interfacial resistance are three major issues facing garnet-based solid electrolytes. Herein, interfacial architecture engineering by incorporating 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP-TFSI) ionic liquid into a garnet oxide is proposed. The “soft” continuous BMP-TFSI coating with no added Li salt generates a conducting network facilitating Li⁺ transport and thus changes the ion conduction mode from point contacts to face contacts. The compacted microstructure suppresses Li-dendrite growth and shows good interfacial compatibility and interfacial wettability toward Li metal. Along with a broad electrochemical window larger than 5.5 V and an Li⁺ transference number that practically reaches unity, LiNi_0.8Co_0.1Mn_0.1O₂/Li and LiFePO₄/Li solid-state batteries with the hybrid solid electrolyte exhibit superior cycling stability and low polarization, comparable to those with commercial liquid electrolytes, and excellent rate capability that is better than those of Li-salt-based ionic-liquid electrolytes.

Garnet-type Li₇La₃Zr₂O₁₂ solid electrolytes were commonly prepared by two steps solid-state reaction method, which undergoes high temperature over 1000 °C and thus inevitable for lithium volatilization and formation of secondary phases. Here, we propose a new intergrain architecture engineering of a solution method, to avoid high temperature sintering for preparing lithium halide (LiX) coated garnet-type solid electrolytes, which contain Al and Ta co-doped Li₇La₃Zr₂O₁₂ (Li_6.75La₃Zr_1.75Ta_0.25O₁₂, LLZTO) synthesized at 900 °C with cubic structure. Owing to the increased relative density, the improved formability, and the altered ion transport mode from point to face conduction by LiX coating on LLZTO grains, LiX-coated LLZTO samples demonstrate a good Li dendrite suppression ability and a high ionic conductivity that is three orders of magnitude higher than pristine LLZTO. In another way, this result demonstrates the critical role of the grain boundaries on the ion transport for oxide superionic conductors. The present coating method provides a new strategy to prepare brittle solid electrolytes avoiding high temperature sintering.

Li₆PS₅X (X = Cl, Br, I) argyrodites possess high ionic conductivity but with rather scattered values due to various processing conditions. In this work, Li₆PS₅X solid electrolytes were prepared by solid-state sintering or mechanical alloying and optimized with or without excess Li₂S. Solid-state sintering prefers excess Li₂S, whereas mechanical alloying prefers stoichiometric Li₂S to synthesize high-purity samples with high ionic conductivity. Solid-state sintering is also more suitable than mechanical milling for high ionic conductivity. Li₆PS₅Cl with the highest ionic conductivity among Li₆PS₅X was comprehensively characterized for electrochemical performance and air stability. MoS₂/Li₆PS₅Cl all-solid-state batteries assembled with Li₆PS₅Cl-coated MoS₂ as cathode and with Li₆PS₅Cl as solid electrolyte demonstrate high capacity and good cycling stability.

Exploration of advanced anode materials is a highly relevant research topic for next generation lithium-ion batteries. Here, we report severe lattice distorted MoS₂ nanosheets with a flower-like morphology prepared with PEG400 as additive, which acts not only as surfactant but importantly, also as reactant. Notably, in the absence of a carbon-related incorporation/decoration, it demonstrates superior electrochemical performance with a high reversible capacity, a good cycling stability, and an excellent rate capability, originated from the advantages of synthesized MoS₂ including enlarged interlayer spacing, 1T-like metallic behavior, and coupling of Mo–O–C (and Mo–O) hetero-bonds. PEG-assisted synthesis is believed applicable to other anode materials with a layered structure for lithium-ion batteries.

The high Li-ion conductivity of the argyrodite Li₆PS₅Cl makes it a promising solid electrolyte candidate for all-solid-state Li-ion batteries. For future application, it is essential to identify facile synthesis procedures and to relate the synthesis conditions to the solid electrolyte material performance. Here, a simple optimized synthesis route is investigated that avoids intensive ball milling by direct annealing of the mixed precursors at 550 °C for 10 h, resulting in argyrodite Li₆PS₅Cl with a high Li-ion conductivity of up to 4.96 × 10^-3 S cm^-1 at 26.2 °C. Both the temperature-dependent alternating current impedance conductivities and solid-state NMR spin-lattice relaxation rates demonstrate that the Li₆PS₅Cl prepared under these conditions results in a higher conductivity and Li-ion mobility compared to materials prepared by the traditional mechanical milling route. The origin of the improved conductivity appears to be a combination of the optimal local Cl structure and its homogeneous distribution in the material. All-solid-state cells consisting of an 80Li₂S-20LiI cathode, the optimized Li₆PS₅Cl electrolyte, and an In anode showed a relatively good electrochemical performance with an initial discharge capacity of 662.6 mAh g^-1 when a current density of 0.13 mA cm^-2 was used, corresponding to a C-rate of approximately C/20. On direct comparison with a solid-state battery using a solid electrolyte prepared by the mechanical milling route, the battery made with the new material exhibits a higher initial discharge capacity and Coulombic efficiency at a higher current density with better cycling stability. Nevertheless, the cycling stability is limited by the electrolyte stability, which is a major concern for these types of solid-state batteries.