Abstract Solid-state batteries (SSBs) with Li metal anodes offer higher energy density than conventional lithium-ion batteries, but Li anodes present both manufacturing and subsequent cycling challenges. Anode-free configurations mitigate some of the manufacturing issues by plating Li onto a current collector to form the anode during the first charge; however, any uneven Li deposition and subsequent stripping leads to low Coulombic efficiency during cycling and promotes Li dendrite formation and associated early cell failure. We study Li plating and stripping in an anode-free SSB configuration using an interlayer composed of a 100 nm Ag film sputtered onto a 1 μm particulate carbon layer on a Cu current collector, easily manufactured using laboratory standard equipment and materials. During charging, Ag reduced the overpotential for the onset of Li plating and facilitated Li deposition at the Cu/C interface. Cross-sectional microscopy tracked the evolution of the anodic interface during cycling and demonstrated the formation of a dense Li layer during charging but incomplete stripping during discharge, resulting in micro-cavity formation and decreased Li utilization. Anode-free full cells using the sputtered Ag/C bilayer, a Li–Ni–Mn–Co oxide (NMC) cathode, and a sulfide electrolyte had a capacity >200 mA h/g, representing a 25% increase over a C-only interlayer, and an initial Coulombic efficiency of 86%. These findings highlight the catalytic efficiency of Ag and C even in thin but readily scalable and easily prepared layers.

Sulfide solid electrolytes have high ionic conductivities necessary to achieve high-rate solid-state cathodes at room temperature and low pressure. Cathode active materials generally require coatings to avoid deleterious oxidative decomposition reactions with the electrolyte. Coatings add cost and complexity to the manufacture. Here we decouple the effect of double and triple phase boundaries on the decomposition in the thick (i.e., ∼110 μm) uncoated solid state cathode. We show that more severe oxidative decomposition of solid electrolytes occurs when the cathode active materials, carbon, and the solid electrolyte coexist, highlighting the importance of the triple phase boundary concerning the decomposition. By regulating the electronic pathways at the triple phase boundary, a thick uncoated electrode at 1 mA cm-2 and 2 MPa stack pressure, delivers an initial areal capacity of ∼4.6 mAh cm-2 at 30 °C and ∼85% capacity retention after 500 cycles.

Solid-state batteries can outperform lithium-ion batteries in energy per unit mass or volume when operating with a Li metal anode. However, Li anodes pose significant manufacturing challenges. Anode-free cells avoid these challenges by plating metallic Li at the anode on the first charge, but subsequent nonuniform cyclic stripping and plating decrease the Coulombic efficiency and encourage Li dendrites and early cell failure. We report a new spray-printed nanocomposite bilayer of silver/carbon black (Ag/CB) between anodic current collectors and a Li6PS5Cl solid electrolyte comprising an Ag-rich region at the current collector and a CB-rich region at the solid electrolyte. Compared with previous Ag/CB mixtures, this bilayer promoted more uniform Li anode plating and improved cycling. Cells with a high-Ni oxide cathode had an initial discharge capacity of >190 mAh/g and a Coulombic efficiency of >98% over 100 cycles. Improved Li plating uniformity with the structured Ag/CB interlayer was confirmed by using secondary-ion mass spectrometry (SIMS) imaging.

The fraction of polytetrafluoroethylene (PTFE) binder in solvent-free Li-ion battery electrodes is shown to have a dramatic impact on their processability, microstructural evolution and elec- trochemical performance. We show experimentally that increasing binder fraction from 0.5 to 4 wt.% transformed the electrode microstructure from an efficient, open structure containing PTFE nano-fibrils to a cemented morphology with blocked porosity. The solvent-free electrodes showed a classical visco-elastic response during compression, comprising three distinct regions of deformation. The electrode stiffness and yield/flow strength increased non-linearly with binder fraction such that for higher binder contents (>2wt.%), there was extensive LiNi0.6Co0.2Mn0.2O2 (NMC) particle fracture during the calendering process, with cracks propagating along the grains of polycrystalline NMC particles. Conversely at lower binder fraction (<2wt.%), the PTFE readily fibrillated into highly textured (100) crystalline nano-fibrils and NMC particles remained largely intact. These electrodes showed superior electrochemical performance due to higher ionic mobility through the open nano-fibrillar microstructure and intact NMC particles.

Many battery applications target fast charging to achieve an 80 % rise in state of charge (SOC) in < 15 min. However, in the case of all-solid-state batteries (SSBs), they typically take several hours to reach 80 % SOC while retaining a high specific energy of 400 W h kg(cell)(-1). We specify design strategies for fast-charging SSB cathodes with long cycle life and investigate the fast-charging capability of a sulfide-based single crystal Li-Ni-Mn-Co oxide composite cathode. At 30 degrees C and charging at 15 mA cm(-2), a specific capacity of 150 mA h g(-1) was achieved in similar to 8 min, with 81 % capacity retention after 3000 cycles. Critically, a 3-electrode arrangement was used to avoid the common problem of overcharging at high current densities. By following the design strategy and optimized manufacturing, a 210 mu m thick cathode was able to be charged at an extraordinary current density of 50 mA cm(-2) to reach an areal capacity of 8 mA h cm(-2) in only 10 min, suggesting practical cathodes for SSBs with 400 W h kg(cell)(-1) may be within reach.

In recent years, solid-state battery (SSB) performance has steadily improved with the use of sulfide solid electrolytes (SEs). However, most research has focused on small (diameter <10 mm), thick (separator >500 μm) pellet-type cells that use non-scalable manufacturing routes and yield a low cell energy density. Technical applications require thinner and larger sheet-type cells made by scalable techniques. We examine the applicability of a scalable layer-by-layer spray printing approach for manufacturing sheet-type SSB components. Sprayed sulfide SE separators with thickness as thin as 10 μm and high ionic conductivity of 1 mS cm−1 are fabricated, along with a sprayed composite cathode that delivered a capacity retention of 63% after 800 cycles. Finally, the flexibility of spray printing for process integration is demonstrated by the fabrication of an anode-free cell consisting of a sprayed Ag-C layer and a sprayed SE layer.

Abstract Solid‐state lithium batteries may provide increased energy density and improved safety compared with Li‐ion technology. However, in a solid‐state composite cathode, mechanical degradation due to repeated cathode volume changes during cycling may occur, which may be partially mitigated by applying a significant, but often impractical, uniaxial stack pressure. Herein, we compare the behavior of composite electrodes based on Li4Ti5O12 (LTO) (negligible volume change) and Nb2O5 (+4% expansion) cycled at different stack pressures. The initial LTO capacity and retention are not affected by pressure but for Nb2O5, they are significantly lower when a stack pressure of <2 MPa is applied, due to inter‐particle cracking and solid‐solid contact loss because of cyclic volume changes. This work confirms the importance of cathode mechanical stability and the stack pressures for long‐term cyclability for solid‐state batteries. This suggests that low volume‐change cathode materials or a proper buffer layer are required for solid‐state batteries, especially at low stack pressures.

Ag-carbon composite interlayers have been reported to enable Li-free (anodeless) cycling of solid-state batteries. Here, we report structural changes in the Ag-graphite interlayer, showing that on charge, Li intercalates electrochemically into graphite, subsequently reacting chemically with Ag to form Li-Ag alloys. Discharge is not the reverse of charge but rather passes through Li-deficient Li-Ag phases. At higher charging rates, Li intercalation into graphite outpaces the chemical reactions with Ag, delaying the formation of the Li-Ag phases and resulting in more Li metal deposition at the current collector. At and above 2.5 mA·cm−2, Li dendrites are not suppressed. Ag nanoparticles do not suppress dendrites more effectively than does an interlayer of graphite alone. Instead, Ag in the carbon interlayer results in more homogeneous Li and Li-Ag formation on the current collector during charge.

Many studies of solid-state battery cathodes employ high stack pressures and low current densities. In practice, cells operating at current densities in the mA cm(-2) range at stack pressures of a few MPa are required. Here, we show the influence of the composite cathode components LiNi0.83Mn0.06Co0.11O2, Li3InCl6, and carbon nanofibers, operating at 2-MPa stack pressure and find that the overall composite cathode capacity is determined primarily by the conductivity of the solid electrolyte. Higher conductivities reduce the mass of the solid electrolyte required to access a high capacity from the active material (high utilization), enabling higher active material loadings and higher overall capacities. Cycling between 2.6 and 4.2 V rather than 4.4 V reduces the LiNi0.83Mn0.06Co0.11O2 volume change from 6% to 2.5%, achieving 94% rather than 65% capacity retention after 50 cycles for a reduction in capacity of only 14%.

To match the high capacity of metallic anodes, all-solid-state batteries (ASSBs) re- quire high energy density, long-lasting composite cathodes such as Ni-Mn-Co (NMC)- based lithium oxides mixed with a solid-state electrolyte (SSE). However in practice, cathode capacity typically fades due to NMC cracking and increasing NMC/SSE in- terface debonding because of NMC pulverization, which is only partially mitigated by the application of a high cell pressure during cycling. Using smart processing proto- cols we report a single crystal particulate LiNi0.83Mn0.06Co0.11O2 and Li6PS5Cl SSE composite cathode with outstanding discharge capacity of 210 mAh g−1 at 30 °C. A first cycle coulombic efficiency of >85%, and >99% thereafter, was achieved despite a 5.5% volume change during cycling. A near-practical discharge capacity at a high areal capacity of 8.7 mAh cm−2 was obtained using a novel asymmetric anode/cathode cycling pressure of only 2.5 MPa/0.2 MPa.

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.

Sulfide-based Li+ conducting solid electrolytes, such as argyrodite, Li6PS5Cl, for all-solid-state batteries can have comparable ionic conductivities with liquid electrolytes. However, the interface between sulfide containing solid electrolytes and Li metal and Li-ion positive electrodes has been found to be unstable, leading to poor cell performance and cycling. Understanding the in situ evolution of interfacial layers between the electrolyte and both electrodes is of paramount importance for designing stable and long-life solid-state batteries. Here, in situ Raman microscopy was used to study the interface between Li6PS5Cl electrolyte and metallic Li and LiCoO2. Under potential control, Raman microscopy identified the appearance of degradation products (Li2S, P2Sx and polysulfides) at the electrode/solid electrolyte interface.

The use of thicker electrodes can contribute to a reduction in cell costs. However, the properties of the electrode must be kept in view to be able to meet the performance requirements. Herein, the possibility of simultaneous multilayer slot die coating is investigated to improve the electrode properties of medium‐ and high‐capacity anodes. The stable coating window of the two‐layer slot die coating process is investigated to produce property‐graded multilayer electrodes. Electrodes with different styrene–butadiene rubber (SBR) gradients are investigated with regard to adhesive force and electrochemical performance. An increase in the adhesive force of up to 43.5% and an increase in the discharge capacity is observed.