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contact between the SE and planar electrode. When the reaction only forms a thin layer on the surface of the planar electrode, this may not be detectable under typical CV test conditions. For an oxidation reaction of Li3PS4 occurring over a 1-V window at a sweep rate of 0.1 mV s−1, forming a 10-nm-thick layer on the pla- nar electrode, the calculated CV current is ~0.3 μA cm−2, on the same order of magnitude as the values measured in the CV of a Li|LGPS|Pt cell97 and of a Li|Li3|PS4|C cell103. To capture the redox of the SE from such a small current, high-sensitivity CV measurements are needed. Such measurements have indicated oxidation decomposition currents of a Na-ion conductor Na2(B12H12)0.5(B10H10)0.5 on the nA cm−2 scale to begin at 3 V (ref.236), which is significantly lower than previous CV results46,237. Alternatively, mixing electronically conductive particles such as carbon with the SE to form a composite working electrode (WE) has been shown to increase the oxidation and reduction current by several orders of magnitude in the CV of a Li|SE|WE|semi-blocking electrode cell, giv- ing rise to more visible oxidation and reduction signals for voltage-stability measurements97,103,144. Even with the use of a composite WE, choosing a cut- off current criterion for CV to determine the oxidation/ reduction limit of the SE is difficult, because the CV current strongly depends on the experimental setup and procedures238. Instead, these limits should be determined by the potential at which the oxidation/reduction current increases drastically during the sweeps. The occurrence of the reduction peak of SE oxidation products was also used to help determine the oxidation limits of several sulfide SEs238. In addition, a Li electrode is often used as the counter and reference electrode for CV26,32,239, but it may react with the SE; also, a true reference electrode is needed to accurately determine the applied potential on the WE240. These issues may be mitigated by using a three-electrode setup with a non-Li counter electrode such as In (refs238,241) or Au (ref.101) and a non-Li refer- ence electrode such as In (refs238,241) or a Ag3SI/Ag mix- ture101. Because CV is an indirect method to characterize interfacial reactions, we believe that it is good practice to supplement CV with other interface-characterization techniques such as TEM and XPS97,99,238 to confirm the voltage-stability window and to capture detailed information on the reaction products. In general, the use of high-sensitivity instruments, the magnification of the reaction signal (for example, by increasing the reaction region, temperature or time) and the combination of various complementary characteri- zation techniques are effective ways for characterizing interfacial reactions in experiments. Performance metrics for SSBs It is important to re-evaluate the commonly used per- formance metrics created for Li-ion batteries and con- sider their applicability to SSBs. In Li-ion batteries, the inventory management of Li ions is particularly impor- tant, because the only Li that cycles in the cell originates from the cathode. Therefore, the coulombic efficiency of Li-ion batteries must be very high242,243 and the Li loss during the formation of the SEI layer should be min- imized. Similarly for SSBs, ideally the cell would start ‘anode-less’, with all the Li starting in the cathode and plating and stripping as Li metal at the anode. However, in typical lab solid-state cells, ‘extra’ Li is available from a Li-metal anode or from the breakdown of the Li-containing SE. For example, the oxidation decompo- sition of the SE can provide extra Li ions and electrons during charging. In some reports102,103, oxidation decom- position products have provided extra reversible capac- ity over a few cycles. However, given that these capacities correspond to conversion reactions, they are unlikely to contribute to stable long-term cycling; additionally, the conversion reaction may occur below the cathode cutoff voltage, thereby limiting the reversibility. We note that decomposition reactions of the SE can have a complex contribution to the temporary capacity of the battery, making it difficult to rely on coulombic efficiency alone to gauge the stability of the SSB. For example, when discharged to a low voltage, the SE on the cathode side can be reduced and contribute to the discharge capacity, resulting in a coulombic efficiency sometimes higher than 100%69,98. Therefore, one can, in principle, cycle the cell with a high coulombic efficiency and limited capacity fade even when serious SE degra- dation occurs. At some point, the SE degradation will, however, increase the impedance to the point where the capacity loss at the imposed current rates outweighs the capacity contribution from the SE decomposition. Reviews Ionic conductivity Reduction Oxidation stability stability Ionic coSnudlfiudcteisvity Ionic conductivity Li3AIN2 Li3N Sulfides Li+ (M–X)a– ab High Li content Polarizable X (e.g. Se2– >S2– >O2–) No M X = N Covalent M–X bonding (e.g. P–O, B–O) Garnet LixA3B2O12 Li–Si–O Li–Si–O system system LGPS family L10MP2S12 Li3PO4 (vs Li3PS4) Li3OCI Li3N Li3OCI Li3N LATP Fig. 7 | trade-offs between ionic conductivity and electrochemical stability upon tuning the solid electrolyte composition. The pie charts represent the effect of individual strategies on the ionic conductivity, reduction stability and oxidation stability of the solid electrolyte. Green indicates that the corresponding property is enhanced by the strategy, whereas red indicates an impairment. White means that the effect of the strategy is not clear. Examples of systems demonstrating the effect of individual strategies are provided in the corresponding sectors. LATP, Li1+xAlxTi2−x(PO4)3; LGPS, Li10GeP2S12; M, non-Li cation(s); X, anion. Enhanced Impaired Nature reviews | MaterialsPDF Image | Understanding interface stability in solid-state batteries
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