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Reviews It is worthwhile to mention that energy efficiency is being increasingly used as a metric for Li-ion bat- teries244,245. Besides incorporating coulombic efficiency, energy efficiency also captures the voltage losses in discharge due to the impedance growth. Hence, it is critical to directly measure the cell impedance and its growth rate. These measurements are particularly important at high temperatures and high state-of-charge; thus, a calendar-life test should be performed, during which the impedance growth is monitored over long-term storage of the charged bat- tery, and the discharge capacity should be measured before and after the calendar-life testing. Such tests can reveal the effect of even minor interfacial reactions on the impedance growth and cell performance. In addi- tion, to achieve high energy density in SSBs, high cath- ode and low SE loading within the cathode composite are required, making the negative effect of SE decomposition on the cell performance even more pronounced. Future perspectives The mechanisms underlying the high ionic conductiv- ity of Li-ion conductors are reasonably well established. Polarizable anions such as S2− can shield the electro- static interactions between the host structure and the migrating Li ions48. The topology of the host structure can be optimized to keep the coordination of Li as con- stant as possible246. In addition, a high Li content can create frustrated Li arrangements and force Li to reside in high-energy sites, from which migration is easier247,248. These insights have led to the rapid development of new superionic conductors. The next important task in SSB development is the reduction of the high interfacial reactivity and resistance. Commercial SSBs will require a high loading density of active material with a low SE content in the cathode composite and a thin separator, which will require careful management of the reactivity of the SE to minimize the increase in resistivity along the Li-ion-transport path. Based on the available exper- imental and theoretical results, it appears unlikely that any SE material in use today is absolutely stable against high-voltage cathodes as well as Li metal; thus, either the use of stable coatings or the formation of stable pas- sivation layers will be required. Hence, characterizing the passivation interphases between SEs and electrodes and their growth should be a priority for the SSB field. Although it may be possible to develop conductors that are thermodynamically stable against both Li metal and high-voltage cathodes, many of the factors that enhance Li-ion conductivity (more polarizable anions, high Li content, reducible cations) narrow the electrochemical stability window. The recent advances in the modelling and charac- terization of interfaces in SSBs have greatly narrowed the gap between experimental observations and com- putational predictions. For example, the low calculated oxidation stability limits for sulfides (<2.5 V) and oxides (<5 V) based on thermodynamic models contrasted sharply with early claims of >5-V stability for many SEs. More careful CV and direct characterizations in recent studies have resolved these discrepancies and validated the computational results97,102. High-throughput com- puting249,250 and the establishment of large databases of ab initio phase diagrams, such as the Materials Project83, have made it fairly straightforward to compute the ther- modynamic reaction products that will form at an inter- face. Many of these predicted decomposition products have been confirmed using advanced characterization techniques, including XPS, Raman spectroscopy, XRD, TEM/STEM, EDS, electron energy loss spectroscopy and ToF-SIMS. Even when the predicted interphases are not observed in experiments, the computational results often capture the qualitative features of the interfacial reactions, such as the redox centre driving the electro- chemical decomposition, the preferred bond formation upon chemical mixing and the formation of a stable interface, an MCI or an SEI. The predictive power of these interface models can effectively guide the reverse engineering of interfaces in SSBs, as recently demon- strated in the stabilization of the Na3SbS4/Na interface by hydration114. Nevertheless, factors such as the rate of elemental dif- fusion, new phase nucleation and whether new phases formed at the interface will be amorphous or crystal- line are difficult to predict using current computational methods. The time scale relevant to experimental obser- vations cannot be achieved in explicit interface model- ling using ab initio techniques. Further development of these models should aim to include kinetic factors to predict, for example, the most likely reaction pathways and products (including amorphous phases), stricter bounds for kinetic stabilization and the upper bound of the processing temperature. On the experimental side, efforts should be made to elucidate the composition and structure of individual interfaces and interphases under processing and battery-cycling conditions and the way they individually affect the cell performance. Stable interfaces should be distinguished from inter- faces at which passivation slows down the reaction. This task requires the development of non-destructive, spatially resolved characterization techniques, as well as in situ or operando techniques that can reveal the com- positional and structural evolution of the interface. Such experimental data can be used synergistically with com- putational modelling to shed light on the mechanisms and kinetic pathways of interfacial reactions. Published online xx xx xxxx 1. Arora, P., White, R. E. & Doyle, M. Capacity fade mechanisms and side reactions in lithium-ion batteries. J. Electrochem. Soc. 145, 3647–3667 (1998). 2. Vetter, J. et al. Ageing mechanisms in lithium-ion batteries. J. Power Sources 147, 269–281 (2005). 3. Li, J., Ma, C., Chi, M., Liang, C. & Dudney, N. J. Solid electrolyte: the key for high-voltage lithium batteries. Adv. Energy Mater. 5, 1401408 (2015). 4. 5. 6. 7. Lin, D., Liu, Y. & Cui, Y. Reviving the lithium metal anode for high-energy batteries. Nat. Nanotechnol. 12, 194–206 (2017). Xu, W. et al. Lithium metal anodes for rechargeable batteries. Energy Environ. Sci. 7, 513–537 (2014). Bhattacharyya, R. et al. In situ NMR observation of the formation of metallic lithium microstructures in lithium batteries. Nat. Mater. 9, 504–510 (2010). Epelboin, I., Froment, M., Garreau, M., Thevenin, J. & Warin, D. Behavior of secondary lithium and aluminum-lithium electrodes in propylene carbonate. J. Electrochem. Soc. 127, 2100–2104 (1980). 8. Han, F., Yue, J., Zhu, X. & Wang, C. Suppressing Li dendrite formation in Li2S–P2S5 solid electrolyte by LiI incorporation. Adv. Energy Mater. 8, 1703644 (2018). 9. Yu, X., Bates, J. B., Jellison, G. E. & Hart, F. X. A stable thin-film lithium electrolyte: lithium phosphorus oxynitride. J. Electrochem. Soc. 144, 524–532 (1997). www.nature.com/natrevmatsPDF Image | Understanding interface stability in solid-state batteries
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