nanomaterials Review Sulfide and Oxide Inorganic Solid Electrolytes for All-Solid-State Li Batteries: A Review Mogalahalli V. Reddy 1 , Christian M. Julien 2,* , Alain Mauger 2 and Karim Zaghib 3,* 1 Centre of Excellence in Transportation Electrification and Energy Storage (CETEES), Institute of Research Hydro-Québec, 1806, Lionel-Boulet Blvd., Varennes, QC J3X 1S1, Canada; [email protected] 2 Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université, UMR-CNRS 7590, 4 place Jussieu, 75252 Paris, France; [email protected] 3 Department of Mining and Materials Engineering, McGill University, Wong Building, 3610 University Street, Montreal, QC H3A OC5, Canada * Correspondence: [email protected] (C.M.J.); [email protected] (K.Z.) Received: 17 July 2020; Accepted: 11 August 2020; Published: 15 August 2020 Abstract: Energy storage materials are finding increasing applications in our daily lives, for devices such as mobile phones and electric vehicles. Current commercial batteries use flammable liquid electrolytes, which are unsafe, toxic, and environmentally unfriendly with low chemical stability. Recently, solid electrolytes have been extensively studied as alternative electrolytes to address these shortcomings. Herein, we report the early history, synthesis and characterization, mechanical properties, and Li+ ion transport mechanisms of inorganic sulfide and oxide electrolytes. Furthermore, we highlight the importance of the fabrication technology and experimental conditions, such as the effects of pressure and operating parameters, on the electrochemical performance of all-solid-state Li batteries. In particular, we emphasize promising electrolyte systems based on sulfides and argyrodites, such as LiPS5Cl and β-Li3PS4, oxide electrolytes, bare and doped Li7La3Zr2O12 garnet, NASICON-type structures, and perovskite electrolyte materials. Moreover, we discuss the present and future challenges that all-solid-state batteries face for large-scale industrial applications. Keywords: electrolytes; solid state; nanomaterials; sulfides; oxides; all-solid-state batteries; energy storage; composites 1. Introduction Inorganic oxide and sulfide materials have recently been studied as solid electrolytes for all-solid-state batteries (ASSBs) owing to their high safety profile, wide temperature window, and better mechanical properties than those of liquid electrolytes. Solid-state electrolytes (SSEs) can be widely used for solid-state Li batteries [1,2], sensors [3,4], fuel cells [1], Li-air [1,5,6], and Li-S [7] batteries. Although solid-state electrolytes can be used for all these different applications, we focused mainly on electrolytes for all-solid-state Li batteries. Recently, Reddy et al. [8] summarized the early history of Li batteries. In brief, a Li battery consists of a cathode (positive electrode), an electrolyte (Li ionic conductor), a separator, and an anode (negative electrode). The cathode material consists of either LiCoO2 (LCO), Li(NixMnyCoz)O2 (NMC), LiFePO4 (LFP), or LiMn2O4 (LMO), and in some cases intercalated binary oxides, whereas Li metal, Li-In alloys, graphite, Li4Ti5O12 (LTO), or Si, Sn-Co-C mixed composites are used as anode materials [2]. In addition, Li batteries use liquid [9], gel polymer [10–12], or combinations of polymer and solid electrolytes. The electrode preparation techniques for all-solid-state lithium batteries (ASSLBs) differ from those of commercial Li batteries. Furthermore, the fabrication technologies of oxide and sulfide electrolyte-based ASSBs are different. Nanomaterials 2020, 10, 1606; doi:10.3390/nano10081606 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1606 2 of 80 Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 79 For example, carbon is is used as a conductive additi additiveve during the fabrication of sulfide sulfide electrolytes but not for the fabrication of oxide electrolytes. Moreover, depending on the mechanical properties of sulfide sulfide electrolytes, a suitable stack pressure is required for the assembly of ASSBs.ASSBs. Oxide solid electrolytes require high-temperature ( >700700 °C)◦C) sintering sintering to to improve the particle-particle contact between electrode and electrolyte. The The general general schema schematictic diagram diagram of of ASSBs ASSBs is is presented presented in in Figure Figure 1.1. The ideal electrolyte materials for ASSBs should fe featureature the following important properties: (i) High 3 1 8 1 ionicionic conductivity ofof 1010−−3 S cm −-1 atat room room temperature, temperature, (ii) low electronic conductivityconductivity of of< <1010−−8 S cm−−1,, which prevents their self-discharge, (iii) wide electrochemical potential potential window, window, (iv) good chemical stabilitystability over over the the operating operating temper temperatureature range and and toward toward the the electrodes, electrodes, (v) transference transference number of approximately 1, (vi) matching thermal expansion expansion co coeefficientsfficients with the cathode materials, (vii) good chemicalchemical stability; stability; no no crystal crystal structure structure phase phase transformation should should occur occur for for the electrode active materials up toto/near/near theirtheir sintering sintering temperatures, temperatures, (viii) (viii) their their sintering sintering temperature temperature should should match match that thatof the of electrodethe electrode active active materials, materials, and (xv)and (xv) low toxicitylow toxicity and costand ecostffective effective [13]. [13]. Figure 1.1. SchematicSchematic diagram diagram of theof fabricatedthe fabricated electrolyte electrolyte for all-solid-state for all-solid-state Li batteries Li and batteries its cross-sectional and its cross-sectionalscanning electron scanning micrograph. electron Reproduced micrograph. with Repr permissionoduced fromwith [permission13]. Copyright from 2018 [13]. Royal Copyright Society 2018of Chemistry. Royal Society of Chemistry. Many researchersresearchers have have investigated investigated new new solid electrolytessolid electrolytes to replace to flammablereplace flammable liquid electrolytes liquid electrolytesor improve theor performanceimprove the of existingperformance solid electrolytesof existing and solid elucidate electrolytes their fundamental and elucidate properties their and technological developments. Huggins (1977) [14], Weppner (1981) [15], Kulkarni et al. (1984) [16], fundamental properties and technological developments. Huggins (1977) [14], Weppner (1981) [15], KulkarniMinami (1985)et al. (1984) [17], [16], Pardel Minami and Ribes (1985) (1989) [17], Pardel [18], Adachiand Ribes et al.(1989) (1996) [18], [Adachi19], Owens et al. (1996) (2000) [19], [20], OwensThangadurai (2000) and [20], Weppner Thangadurai (2002) and [21], Weppner Knauth (2009) (2002) [ 22[21],], and Knauth Fergus (2009) (2010) [22], [23 and] published Fergus reviews(2010) [23] on publishedsolid electrolytes. reviews The on journalsolid electrolytes.Solid State IonicsThe journaldevoted Solid to these State materialsIonics devoted was created to these in materials 1980. This was has createdbeen considered in 1980. 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