Metal-air batteries, utilizing the reduction of ambient oxygen, have the highest energy density because most of the cell volume is occupied by the anode while the cathode active material is not stored in the battery. Lithium metal is a tempting anode material for any battery because of its outstanding specific capacity (3842 mA h g(-1) for Li vs. 815 mA h g(-1) for Zn). Combining the high energy density of Li with ambient oxygen seems to be a promising option. Specifically, in all classes of electrolytes, the transformation from Li-O2 to Li-air is still a major challenge as the presence of moisture and CO2 reduces significantly the cell performance due to their strong reaction with Li metal. Thus, the quest for electrolyte systems capable of providing a solution to the imposed challenges due to the use of metallic Li, exposure to the environment and handling the formation of reactive discharged product is still on. This extended Review provides an expanded insight into electrolytes being suggested and researched and also a future vision on challenges and their possible solutions.

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 ...

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This review reports on the most updated technological aspects of Li-air battery cathode materials. It provides the reader with recent developments, alongside critical views. The requirements for air-cathodes, as well as the classification and characterization of carbon-based and carbon-free air cathodes, are listed. The effects of two major substituent groups of materials, namely carbon and advanced materials (metals, metal-oxides, metal-carbides, and metal-nitrides) aimed at replacing carbon, are discussed in terms of their chemical and electrochemical stability. The report covers aspects of surface chemistry and structure influence on the electrolyte and discharge products stability. The review also reports on the efforts to suppress side reactions and deterioration of the polymeric binders (if a composite electrode is being considered). This is recognized as a means to enhance Li-air battery performance. The report concludes with an outlook and perspective, providing the readers with some insight on other factors and their impact on the long road toward a viable air-cathode suitable for Li-air battery operations.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201808303.respectively. [1] The configuration of an aprotic LOB is based on coupling lithium metal anode and air-breathing cathode in a non-aqueous electrolyte system. The electrochemical reactions occurring during discharge are complex multistep reactions and may involve dissolved and/or adsorbed species alongside parasitic reactions. [2][3][4][5] Oxygen reduction reaction during discharge largely depends on the cathode and electrolytes characteristics; yet, a dominant process involving a two-electron oxygen reduction reaction (ORR), with the formation of Li 2 O 2 , is reported, according to the following reaction [6] The reverse process, namely lithium peroxide oxidation, occurs upon charging. The formation/decomposition of Li 2 O 2 during discharge/charge is often accompanied by parasitic side reactions, due to instability of the major battery components (cathode, electrolyte, and anode) under operating conditions. [7][8][9][10][11][12] While lithium metal anode corrosion can be mitigated by a protective solid electrolyte (SE) membrane, [13][14][15] the electrolyte and cathode are considered as the most challenging components of LOB. Much efforts have been placed on understanding the role of the electrolyte, its degradation, on Li-O 2 reaction mechanism and battery performance. [16][17][18][19][20] Nonetheless, the high discharge/ charge overpotential and severe capacity loss in the course of cycling are also largely related to the air-breathing cathode. A stable, high surface area, cost-efficient air-electrode with excellent catalytic activities toward oxygen reduction and oxidation is crucial for the development of practical LOB. Cathode materials should be stable, durable, and hold interfaces with minimum surface oxide layers, which can inhibit charge transfer during the charging p...

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.