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Reviews electronically conductive Li17Sn4 and Li21Si5 phases are predicted to form34. By contrast, many Li–non-metal binary phases are stable against Li metal64. In principle, these binaries are good candidates for passivating the SE/Li interface if they are ionically conductive but electronically insulat- ing. For example, to stabilize the Li2S–P2S5 glass/Li inter- face, LiI was added to the glass SE112, enabling the stable cycling of a Li symmetric cell8,110,112. A similar effect has been achieved in Na3PS4, where Cl doping has been shown to improve capacity retention by introducing the electron insulator NaCl at the Na3PS4/Na interface113. Another Na-ion conductor, Na3SbS4, has been predicted and experimentally verified to form Na2S and Na3Sb at its interface with Na metal, making the interphase an MCI. One solution might come from the observation that, after purposely exposing Na3SbS4 to air to generate a hydrated Na3SbS4·8H2O phase on its surface and con- tacting Na metal, Na-stable compounds NaH and Na2O were produced with good ionic conductivity and high electronic resistivity114. This hydration process has been shown to effectively passivate the SE/Na interface and enable more stable cycling of a Na symmetric cell. These findings highlight the effectiveness of introducing ionic-conductive but electronic-insulating phases to the SEI, as well as the importance of predictive calculations in the reverse design of battery interfaces. Argyrodites Argyrodites with the general chemical formula Li6PS5X (X=Cl, Br, I) are another class of sulfide ionic conduc- tors37,38 that are predicted to have a similar electro- chemical window, chemical reactivity with cathodes and decomposition products to other sulfides64,115. Consistent with the predictions64,66,115, elemental sul- fur, lithium polysulfide, P2Sx and LiCl have been observed to be the oxidation decomposition prod- ucts116,117. For argyrodites in contact with Li metal, the decomposition products Li2S and Li3P have been detected by XPS118. Recently, the interface between LiNi0.6Co0.2Mn0.2O2 and Li6PS5Cl has been investigated using XPS and ToF-SIMS119. Similar to the observations at the LiNi0.8Co0.1Mn0.1O2/β-Li3PS4 interface78, increased amounts of PO xy − and SO xy − species were detected upon cycling119. The presence of the halide anion also leads to the generation of LiX (X=halogen) binaries upon decomposition, which may assist in passivating the inter- faces with the electrode as for Cl-doped Na3PS4 (ref.113). Indeed, good capacity retention over 300 cycles has been reported in a LiNi1/3Co1/3Mn1/3O2|Li6PS5Cl|Li–In cell117. Doping Li6PS5Br with O has also been shown to improve the stability against Li-metal and oxide cathodes64–66,120. Summary In summary, although sulfide materials combine excel- lent mechanical processability and ionic conductivity, experimental and theoretical investigations indicate that their chemical and electrochemical stability are severely limited. First, the facile oxidation of S2− results in poor electrochemical stability, limited to approximately 2.5 V in the cathode composite. S2− oxidation leads to conden- sation of PS4 units with a general decrease of lithium content and, ultimately, even to the formation of ele- mental sulfur. Such oxidation decomposition is con- sidered one of the main causes of the large first-cycle capacity loss in a high-voltage solid-state cell60. Although this degradation is mostly considered to occur at the cathode/SE interface, it occurs even at non-functional interfaces such as the carbon/SE and current collector/SE interfaces. This degradation reduces the effective ionic conductivity in the cathode composite. Because the SE decomposition products that form at high voltage are generally highly oxidized and alkali-deficient, they may retard further decomposition; however, the extent to which these decomposition products are passivating requires further investigation. Second, when oxide cathodes are in contact with sulfide SEs, there is a fur- ther driving force for degradation via the exchange of S2− and O2−, leading to the formation of PO34− polyanions and transition-metal sulfides. This effect leads to both impedance growth and capacity loss. Against the Li-metal anode, reduction of all but just a few metal or metalloid ions occurs, creating electron- ically conducting products that form an MCI. This phenomenon is a particular problem for some highly conducting sulfides that contain Ge, Si, Sn and Sb. The addition of halogens, such as I and Cl, may contribute to the formation of a passivating SEI containing Li halides that prevents further reduction. Oxides Oxide-based SEs include garnets, thin-film LiPON, perov- skites, antiperovskites and NASICONs. They exhibit higher oxidation stability and improved chemical stabil- ity with oxide cathodes compared with sulfide SEs64–66. However, the room-temperature bulk ionic conductivity of oxide SEs is generally lower than that of sulfides, and their large grain-boundary resistance further restricts the total ionic conductivity14,121–124. Because of the mechanical rigidity of oxides, high-temperature sintering is usually required to produce a dense SE pellet and to achieve inti- mate contact between the SE and the electrode within the electrode composite27,54,123,125. The high processing temperature can degrade electrode materials such as LiNixCoyMn1−x−yO2 (NCM)126 and LiCoO2 (ref.127), and promote the chemical reactivity at the SE/electrode inter- face80,128. The difficulty of cell manufacturing with oxide SEs results in limited reports on the performance of full solid-state cells with a thick electrode composite layer and a dense oxide SE pellet129,130, yielding fewer experimental data on the interfacial stability of oxide SEs under battery operating conditions than those available for sulfide SEs. Garnets Among oxide SEs, Li garnets have been widely studied because of their high ionic conductivity (10−4–10−3 S cm−1)24,25,27,131, apparent stability against Li metal and wider electrochemical windows than sulfides25–27,132,133. The first reported Li-ion conducting garnets had the composition Li5La3M2O12 (M=Nb, Ta) (LLNbO, LLTaO)134. Since then, strategies to increase the Li concentration via aliovalent doping have been used to achieve higher ionic conductivity in garnets, including subvalent doping with a 2+ ion on the La3+ site or another transition-metal www.nature.com/natrevmatsPDF Image | Understanding interface stability in solid-state batteries
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