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Li-ion batteries have revolutionized the portable electronics industry and empowered the electric vehicle (EV) revolution. Unfortunately, traditional Li-ion chemistry is approaching its physicochemical limit. The demand for higher density (longer range), high power (fast charging), and safer EVs has recently created a resurgence of interest in solid state batteries (SSB). Historically, research has focused on improving the ionic conductivity of solid electrolytes, yet ceramic solids now deliver sufficient ionic conductivity. The barriers lie within the interfaces between the electrolyte and the two electrodes, in the mechanical properties throughout the device, and in processing scalability. In 2017 the Faraday Institution, the UK’s independent institute for electrochemical energy storage research, launched the SOLBAT (solid-state lithium metal anode battery) project, aimed at understanding the fundamental science underpinning the problems of SSBs, and recognising that the paucity of such understanding is the major barrier to progress. The purpose of this Roadmap is to present an overview of the fundamental challenges impeding the development of SSBs, the advances in science and technology necessary to understand the underlying science, and the multidisciplinary approach being taken by SOLBAT researchers in facing these challenges. It is our hope that this Roadmap will guide academia, industry, and funding agencies towards the further development of these batteries in the future.

high energy density and long cycle life. [2] However, LIBs suffer from severe safety issues caused by lithium dendrites and flammable organic electrolytes, hindering their wide application in large-scale energy storage systems. [3] Additionally, the limited lithium reserves and their uneven distribution may further increase the cost of LIBs. [4] Therefore, it is extremely urgent to explore novel battery technologies with higher energy density, enhanced safety, and lower cost. [5] To replace or complement lithium-based energy storage systems, various batteries based on sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), and aluminum (Al) have received extensive attention for use as next-generation electrochemical storage systems. [6] Among the metal-based cells, Al-based batteries (ABs) are one of the most promising alternatives for energy storage due to the high specific capacity, low cost, abundant reserves, and light weight of the Al anode. [7] Al can offer a theoretically gravimetric capacity of 2980 mAh g −1 , which is much higher than that of Mg (2200 mAh g −1 ) or Zn (820 mAh g −1 ) and only lower than that of Li (3860 mAh g −1 ). From a volume standpoint, Al has a larger volume capacity (8040Ah cm −3 ) than lithium (2060 Ah cm −3 ). [1,6a,8] Considering the combination of the specific capacity and earth reserves, Al can beat all other metals (Figure 1), demonstrating a promising candidate for developing advanced energy storage systems with high energy densities and low-cost. [9] ABs are a class of energy storage systems in which Al metals are used as anodes in the form of plates, foils, or particles. Al was initially used as an anode in the Buff cell coupled with a carbon cathode in 1857. [10] Later on, Al-MnO 2 batteries and Al-O 2 batteries were in succession demonstrated and explored in 1960s. [11] Since then, extensive efforts have been devoted to exploring more advanced Al batteries for clean and sustainable energy. According to the electrolyte characteristics, ABs can be briefly classified into aqueous Al batteries and non-aqueous Al batteries, including primary Al-air batteries, rechargeable Al-O 2 batteries, Al-ion batteries (AIBs), Al-graphite dual-ion batteries (DIBs), Al-S batteries, and others for various applications (Figure 2).Although enormous research progress on Al-based batteries has been achieved, there are still some challenges that need to be addressed, especially on the Al anode side. PrimaryThe ever-growing market of electric vehicles and the upcoming grid-scale storage systems have stimulated the fast growth of renewable energy storage technologies. Aluminum-based batteries are considered one of the most promising alternatives to complement or possibly replace the current lithium-ion batteries owing to their high specific capacity, good safety, low cost, light weight, and abundant reserves of Al. However, the anode problems in primary and secondary Al batteries, such as, self-corrosion, passive film, and volume expansion, severely limit the batteries' practical performanc...

All-solid-state batteries with a Li anode and ceramic electrolyte have the potential to deliver a step change in performance compared with today's Li-ion batteries 1,2 . However, Li dendrites (filaments) form on charging at practical rates, penetrate across the ceramic electrolyte leading to short-circuit and cell failure 3,4 . Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip [5][6][7][8][9] . Here we show that initiation and propagation are separate processes.

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The present work describes the features of the synthesis and physicochemical and electrical properties of a new Dy-doped BaCeO3–BaZrO3 proton-conducting electrolyte as well as its application in a reversible solid oxide fuel cell.

Solid‐state lithium batteries will revolutionize the lithium‐ion battery and energy storage applications if certain key challenges can be resolved. The formation of lithium‐protrusions (dendrites) that can cause catastrophic short‐circuiting is one of the main obstacles, and progresses by a mechanism that is not yet fully understood. By utilizing X‐ray computed tomography with nanoscale resolution, the 3D morphology of lithium protrusions inside short‐circuited solid electrolytes has been obtained for the first time. Distinguishable from adjacent voids, lithium protrusions partially filled cracks that tended to propagate intergranularly through the solid electrolyte, forming a large waved plane in the shape of the grain boundaries. Occasionally, the lithium protrusions bifurcate into flat planes in a transgranular mode. Within the cracks themselves, lithium protrusions are preferentially located in regions of relatively low curvature. The crack volume filled with lithium in two samples is 82.0% and 83.1%, even though they have distinctly different relative densities. Pre‐existing pores in the solid electrolyte, as a consequence of fabrication, can also be part‐filled with lithium, but do not have a significant influence on the crack path. The crack/lithium‐protrusion behavior qualitatively supports a model of propagation combining electrochemical and mechanical effects.

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