Toward High-Quality Sulfide Solid Electrolytes: A Liquid-Phase Approach Featured with an Interparticle Coupled Unification Effect.

Sulfide solid electrolytes (SSEs) are highly wanted for solid-state batteries (SSBs). While their liquid-phase synthesis is advantageous over their solid-phase strategy in scalable production, it confronts other challenges, such as low-purity products, user-unfriendly solvents, energy-inefficient solvent removal, and unsatisfactory performance. This article demonstrates that a suspension-based solvothermal method using single oxygen-free solvents can solve those problems. Experimental observations and theoretical calculations together show that the basic function of suspension-treatment is "interparticle-coupled unification", that is, even individually insoluble solid precursors can mutually adsorb and amalgamate to generate uniform composites in nonpolar solvents. This anti-intuitive concept is established when investigating the origins of impurities in SSEs electrolytes made by the conventional tetrahydrofuran-ethanol method and then searching for new solvents. Its generality is supported by four eligible alkane solvents and four types of SSEs. The electrochemical assessments on the former three SSEs show that they are competitive with their counterparts in the literature. Moreover, the synthesized SSEs presents excellent battery performance, showing great potential for practical applications.

Synthesis of Deliquescent Lithium Sulfide in Air.

In the field of lithium-sulfur batteries (LSBs) and all-solid-state batteries, lithium sulfide (Li2S) is a critical raw material. However, its practical application is greatly hindered by its high price due to its deliquescent property and production at high temperatures (above 700 °C) with carbon emission. Hereby, we report a new method of preparing Li2S, in air and at low temperatures (∼200 °C), which presents enriched and surprising chemistry. The synthesis relies on the solid-state reaction between inexpensive and air-stable raw materials of lithium hydroxide (LiOH) and sulfur (S), where lithium sulfite (Li2SO3), lithium thiosulfate (Li2S2O3), and water are three major byproducts. About 57% of lithium from LiOH is converted into Li2S, corresponding to a material cost of ∼$64.9/kg_Li2S, less than 10% of the commercial price. The success of conducting this water-producing reaction in air lies in three-fold: (1) Li2S is stable with oxygen below 220 °C; (2) the use of excess S can prevent Li2S from water attack, by forming lithium polysulfides (Li2Sn); and (3) the byproduct water can be expelled out of the reaction system by the carrier gas and also absorbed by LiOH to form LiOH·H2O. Two interesting and beneficial phenomena, i.e., the anti-hydrolysis of Li2Sn and the decomposition of Li2S2O3 to recover Li2S, are explained with density functional theory computations. Furthermore, our homemade Li2S (h-Li2S) is at least comparable with the commercial Li2S (c-Li2S), when being tested as cathode materials for LSBs.

Influence of Al doping on the structure and electrochemical performance of the Co-free LiNi0.8Mn0.2O2 cathode material.

The transformation from LiNi1-x-yCoxMnyO2 (NCM) cathodes to Co-free LiNi1-xMnxO2 (NM) cathodes is considered as an effective solution for the electric vehicle (EV) industry to deal with the high cost of cobalt. However, severe Li/Ni disorder, structural instability and poor cycling stability are the main obstacles to their practical application. Al doping has proven to be an effective method to improve the electrochemical performance of Ni-rich NCMs. However, with regard to Ni-rich Co-free NM cathodes, the influence of Al doping on the structural stability and electrochemical performance of NM cathodes is still not clear. In this work, Al doped LiNi0.8Mn0.2-xAlxO2 cathodes are designed and their structural stability and electrochemical performance are investigated by a combination of XRD, SEM, TEM, CV, GITT, cycling testing and EIS techniques. As a result, Al doping can effectively inhibit Li/Ni disorder and improve the structural and thermal stability. In detail, 5% is the optimal doping amount for LiNi0.8Mn0.2O2 cathodes to obtain the best electrochemical performance and the LiNi0.8Mn0.15Al0.05O2 cathode shows an excellent capacity retention of 91.97% after 300 cycles at 3.0-4.3 V. This work provides an effective strategy for the development of Ni-rich Co-free NM cathodes.

Green Synthesis for Battery Materials: A Case Study of Making Lithium Sulfide via Metathetic Precipitation.

For some future clean-energy technologies (such as advanced batteries), the concept of green chemistry has not been exercised enough for their material synthesis. Herein, we report a waste-free method of synthesizing lithium sulfide (Li2S), a critical material for both lithium-sulfur batteries and sulfide-electrolyte-based all-solid-state lithium batteries. The key novelty lies in directly precipitating crystalline Li2S out of an organic solution after the metathetic reaction between a lithium salt and sodium sulfide. Compared with conventional methods, this method is advantageous in operating at ambient temperatures, releasing no hazardous wastes, and being economically more competitive. To collect the valuable byproduct out of the liquid phases, a "solventing-out crystallization" technique is employed by adding an antisolvent (AS) of low boiling point. The subsequent distillation of the new solution under vacuum evaporates off the AS rather than the high-boiling-point reaction solvent (RS), saving a lot of energy. Consequently, the separated AS and RS containing the unreacted lithium salt can be directly reused. For industrial production, the entire process may be operated continuously in a closed loop without discharging any wastes. Moreover, Li2S cathodes and sulfide-electrolyte Li6PS5Cl derived from the synthesized Li2S show impressive battery performance, displaying the great potential of this method for practical applications.

A Low-Cost Liquid-Phase Method of Synthesizing High-Performance Li6PS5Cl Solid-Electrolyte.

Li6PS5Cl is an extensively studied sulfide-solid-electrolyte for developing all-solid-state lithium batteries. However, its practical application is hindered by the high cost of its raw material lithium sulfide (Li2S), the difficulty in its massive production, and its substandard performance. Herein we report an economically viable and scalable method, denoted as "de novo liquid phase method", which enables in synthesizing high-performance Li6PS5Cl without using commercial Li2S but instead in situ making Li2S from cheap materials of lithium chloride (LiCl) and sodium sulfide. LiCl, a raw material needed for making both Li2S and Li6PS5Cl, can be added at a full-scale in the beginning and unrequired to separate when making the intermediate Li3PS4. Such a consecutive feature makes this method time-efficient; and the excess amount of LiCl in the step of making Li2S also facilitates removing the byproduct of sodium chloride via the common ion effect. The materials cost of this method for Li6PS5Cl is ∼ $55/kg, comparable with the practical need of $50/kg. Moreover, the obtained Li6PS5Cl shows high ionic conductivity and outstanding cyclability in full battery tests, that is, ∼2 mS/cm and >99.8% retention for 400+ cycles at 1 C, respectively. Thus, this innovative method is expected to pave the way to develop practical sulfide-solid-electrolytes for all-solid-state lithium batteries.