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the formation of GeS2-like species and Li2P2S6 has been observed in a cathode composite containing LGPS after extended cycling70,104. Recent studies showed that the decomposition of several sulfide SEs may be partially reversible or the decompo- sition products are redox-active, although it is unlikely that these processes contribute to the long-term cycling capacity of a battery. The association/dissociation of S–S bonds in a Li3PS4 glass+carbon cathode composite upon cycling between 0.6 and 3.6 V was observed by track- ing the XPS peak assigned to the bridging S–S bond102 (fig. 3c). The XPS result combined with Raman and X-ray absorption fine-structure data suggest that PS4 groups in the Li3PS4 glass undergo condensation upon charg- ing and that the process is partially reversible upon discharging102. This finding appears to be consistent with the reversible and potential-dependent change of the interfacial resistance of the cathode68. Using CV on a Li|β-Li3PS4|β-Li3PS4+C cell between 0 and 5 V, it was shown that the decomposition of β-Li3PS4 at 5 V is irreversible, but good reversibility is observed for subsequent cycles, indicating that the decomposition products are redox- active in this voltage range103. The same study further demonstrated that this redox activity is a superposition of that from elemental sulfur and phosphorus. Chemical mixing with oxide cathodes In addition to the electrochemical stability limitation of sulfides, the sulfide/oxide cathode interface suffers from degradation resulting from chemical mixing. As observed in the cross-sectional scanning TEM (STEM) image of a charged LiCoO2/Li2S–P2S5 interface and associated energy-dispersive X-ray spectroscopy line profile in fig. 3d, the interfacial layer contains Co, P and S, with Co diffusing into Li2S–P2S5 for over 50 nm (ref.47). Consistent with this observation, the computed driving force for chemical reaction between sulfides and oxide cathodes is large (>300 meV/atom), forming transition-metal sulfides (such as Co9S8 (ref.65), Mn2S3 (ref.64), Ni3S4 (ref.64) and CoNi2S4) and PO34− and SO24− polyanions64,65,95,105. The for- mation of PO34− and transition-metal sulfides results from the exchange of S2− in PS34− from the SE with O2− from the cathode. This exchange is energetically favourable, because the bond energy is significantly higher for a P–O bond than for a P–S bond but similar for transition-metal– sulfur and transition-metal–oxygen bonds106. Consistent with the thermodynamically predicted products, expli- cit modelling of the LiCoO2/β-Li3PS4 interface led to the observation that the energetically favourable exchange of Co and P leads to the formation of P–O and Co–S bonds86. Not surprisingly, when pairing sulfide SEs with sulfide cathodes containing the same S2− anion chemistry (such as LiCrS2, LiMnS2 or LiTiS2), the sulfide cathode/sulfide SE interfaces are much more stable than the oxide cathode/sulfide SE interfaces, as observed from chemical mixing calculations64 and in explicit interface calculations87. In the same spirit, thio-phosphate SEs were predicted to be chemically more compatible with LiFePO4 containing the same P5+ cation than with other oxide cathodes such as LiCoO2 and LiMn2O4 (ref.67). For experiments performed at room temperature, severe chemical mixing between sulfide SEs and oxide cathodes appears to occur only after charging and long cycling60. No reactivity has been observed between the as-prepared LiNi0.8Co0.1Mn0.1O2 and β-Li3PS4; however, after the first charge, PO xy − species were detected at the interface using XPS60. After 100 cycles, time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis revealed the formation of various PO xy − and SO xy − groups at the LiNi0.8Co0.15Al0.05O2/Li2S–P2S5 interface78. The effect of charging and cycling on chemical mixing may be explained by the fact that the computed chemical reactivity with a sulfide SE is even more pronounced for charged cathodes than for discharged compounds65. High-temperature processing can also promote the chemical mixing at the oxide cathode/sulfide SE interface. After heating the charged LiNi1/3Mn1/3Co1/3O2 with 75 Li2S–25 P2S5 glass above 300 °C, transition-metal sulfides MnS and CoNi2S4, and Li3PO4 were observed using synchrotron X-ray diffraction (XRD) and TEM107, in excellent agreement with the predicted reaction products at that interface. Similar products and the exchange between O2− and S2− (or Se2−) have been predicted by calculations on sodium sulfide and selenide SEs with oxide cathodes50. Indeed, sodium transition-metal sulfides (or selenides) and Na3PS3O have been observed using XRD at elevated temperature for a mixture of NaCrO2 and Na3PS4 (or Na3PSe4)50. Reduction stability with Li metal The reduction decomposition of sulfide SEs is typically initiated by the reduction of P5+ and other cations (such as Ge4+ and Sn4+) into phases including Li4P2S6 (ref.64), P (refs65,66) and Li2S. Upon contacting Li metal, they further decompose into a metal, Li-metal alloys and/or Li-containing binary compounds, such as Li3P (refs65,66). For example, Li3PS4 and Li7P3S11 have been predicted to decompose into Li3P and Li2S when in contact with a Li-metal anode64,84,95. The predicted decomposition is similar for LGPS, with additional germanium reduction to form Li15Ge4 (refs64,96). The formation of a metal or Li-metal alloy (as in the LGPS case) at the SE/Li interface is considered detrimental, as it makes the interphase an MCI, leading to the continued decomposition of the SE. The pronounced driving force to form these products makes them appear in ab initio molecular dynamics simulations of crystalline Li–P–S compounds or LGPS in contact with Li metal. Even within 20 ps at 300 K, the formation of LixS, LiyP and LizGe species is indicated by the lithium coordination numbers of sulfur, phos- phorus and germanium at the end of the simulation90. Indeed, Raman spectroscopy and XPS analyses have revealed the conversion of PS34− in β-Li3PS4 to P2S64− and Li2S at the β-Li3PS4/gold interface upon Li deposition, as well as partial reversibility upon Li stripping108. The detected P2S64− species is consistent with the predicted Li4P2S6 formation at the onset of reduction64. Li2S, Li3P and other reduced phosphorus species were detected at the Li7P3S11/Li interface using XPS and XRD109, and additionally reduced Ge (likely Li–Ge alloy or Ge) at the LGPS/Li interface51. As a result of the MCI forma- tion, the LGPS/Li interface suffers from the continuous decomposition and resistance growth51,110. A similar phenomenon has been reported for Li10SiP2S12 and Li10Si0.3Sn0.7P2S12 in contact with Li (ref.111), where the Reviews Nature reviews | MaterialsPDF Image | Understanding interface stability in solid-state batteries
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