lightweight, and compact and allow for versatile device geometries. They must also be scalable and offer high energy density to provide improved packing efficiency and longer device operation. Although both Ni-MH batteries and LIBs have been commercialized since the 1990s, [1] LIBs possess twice the gravimetric/volumetric energy density (250 Wh kg −1 /700 Wh L −1 vs 170 Wh kg −1 /350 Wh L −1 ), [2] higher battery voltage (3.7 V vs 1.2 V), and longer cycle life with lower self-discharge, [3] contributing tremendously to the proliferation of portable electronic devices (e.g., mobile phones, laptops, cameras, tablets) as well as emerging new technologies such as wearable electronic devices (e.g., smart watches and sport-related tracking devices). Their high gravimetric/ volumetric energy density, [2] excellent cycle life (thousands of cycles), and lack of the memory effect have positioned LIBs as state-of-the-art power sources and one of the greatest successes of modern electrochemistry, revolutionizing the way we acquire, process, transmit, and share information globally. Nevertheless, advances in battery energy density, safety, costs, and flexibility in shape and size are still needed to keep up with the rapidly growing demand for devices with even longer runtime as well as real-time data collection and transmission capabilities in addition to increasingly energy-demanding applications such as electric vehicles (EVs) and electricity grid storage. Even though LIBs were first commercialized in all electric vehicles (EVs) in 2010 and also emerged for grid application in the same time frame, the low energy density (≈250 Wh kg −1 ) and high average cost (≈$156 kWh −1 in 2019) of conventional LIBs do not meet the requirements for advanced EVs and grid-scale energy storage. [4][5][6] Specifically, the driving range per charge (miles), which is related to the energy density of each cell, and the cost are important parameters for EVs. For example, one 85 kWh battery pack in a Tesla Model S requires 7104 LIB cells, with an energy density of 265 Wh kg −1 , providing an average range of 250 miles, which is ahead of the range of other EVs but still behind the target of 375 miles. [4] In grid-scale applications, LIBs can be used for various tasks: frequency regulation, peak shaving, load leveling, and large-scale integration of renewable energies, with specific properties generally required for each task. For frequency regulation, LIBs need to provide a fast response, high rate performance, and high-power capability,The introduction of new, safe, and reliable solid-electrolyte chemistries and technologies can potentially overcome the challenges facing their liquid counterparts while widening the breadth of possible applications. Through tech-historic evolution and rationally analyzing the transition from liquidbased Li-ion batteries (LIBs) to all-solid-state Li-metal batteries (ASSLBs), a roadmap for the development of a successful oxide and sulfide-based ASSLB focusing on interfacial challenges is introduced, while accounting ...

The introduction of new, safe, and reliable solid‐electrolyte chemistries and technologies can potentially overcome the challenges facing their liquid counterparts. In article number 2002689, Jennifer L. M. Rupp and co‐workers provide a thorough understanding of the major challenges in materials and interfaces, and mitigation strategies associated with all‐solid‐state Li metal batteries, where either oxides or sulfide‐based solid electrolytes are in the spotlight.

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Introduction Oxide-based all-solid-state batteries (SSBs) are candidates for the next-generation of rechargeable batteries because of their advantages in terms of both high energy density and safety. However, a significant disadvantage of this type of batteries is large contact resistance between active materials and solid electrolytes, which decreases Li+ conductivity in electrodes. To solve this problem, we prepared composite powders of 5V-class active material: LiNi1/2Mn3/2O4 (LNM) and a Li+-conductive solid electrolyte (Ohara Inc. -prepared proprietary composition of Li2O-P2O5-Nb2O5-B2O3-GeO2). We deposited these composite powders on Pt substrates by an Aerosol Deposition (AD) method. AD is a powerful approach to produce composite films at room temperature [1] while suppressing reactions of active materials with solid electrolytes. After preparation, we performed electrochemical measurements of the resultant composite film electrodes.   Experimental Composite powders of LNM (Toshima D50=10-15 µm) and Li2O-P2O5-Nb2O5-B2O3-GeO2 solid electrolyte (Ohara Inc.) were prepared by a dry-powder mixer (NOB-MINI, Hosokawa Micron Co.). The mixing ratio of LNM to Li2O-P2O5-Nb2O5-B2O3-GeO2 solid electrolyte was 20:1, weight basis. The obtained composite powders were deposited on Pt substrates by AD to prepare the composite electrodes. Pure LNM powder was also deposited on Pt by AD to compare to the composite electrodes. AD films were characterized by Raman spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry (EDX). The amounts of active materials in the composite electrodes were estimated by Image-J image processing software based on cross-sectional-SEM images. Weights of LNM in AD films were estimated by using the volume and density of LNM. In this procedure, we assumed that films were considered fully densified in these estimates. The resultant films were covered by LiPON solid electrolyte films by radio frequency magnetron sputtering. Subsequently, Li anode films were deposited on the top of LiPON films by vacuum evaporation. The characterization studies of fabricated SSBs (Pt | LNM or the composite electrodes | LiPON | Li) included cyclic voltammetry (1.0 mV sec-1), impedance spectroscopy (frequency range: 10 mHz to 500 kHz), and galvanostatic charge-discharge measurements (1.0 μA cm-2). All electrochemical measurements were carried out at 25 °C in an Ar filled glovebox.     Results and Discussions The SEM images of the composite powder showed that the solid electrolyte powder uniformly distributed on LNM particle surfaces. In Figure 1, the cross-sectional SEM images of AD films showed that AD films have dense structures, and the thicknesses were more than about 1 μm. In the case of AD films of the composite electrode, LNM and Li2O-P2O5-Nb2O5-B2O3-GeO2solid electrolyte were uniformly distributed with layer-like structures. Nyquist plots at 4.7 V (Li/Li+) showed that the charge transfer resistance was lower for composite cathodes compared to pure LNM cathodes. Figure 2 shows the first charge-discharge curves of SSBs. The SSB with a composite electrode had a specific capacity of 62.7 mAh g-1 which was higher value compared to the SSB with pure LNM electrode (4.3 mAh g-1). Approximately 43% of the theoretical specific capacity of LNM (147 mAh g-1) could be obtained by fabricating the composite electrolyte. It is thought that Li2O-P2O5-Nb2O5-B2O3-GeO2 solid electrolyte filled in space among the LNM particles produce Li+conductive pathways, which increased the amounts of effective active materials, and reduced apparent resistance. In this way, the discharge capacity for the composite electrodes increased. Conclusion As a result, for electrodes fabricated by an AD technique, the composite electrodes had lower charge transfer resistance and higher discharge capacity compared to pure LNM electrodes. Acknowledgement This work is financially supported by NEDO-Post LiEAD. Figure 1