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Similar to LLT and LGPS, the Ti4+ in LATP and Ge4+ in LAGP are expected to undergo facile reduction by Li metal. DFT calculations predict the reduction of LATP and LAGP below 2.17 V (or 2.7 V) and below 2.7 V (or2.9V),respectively64,66,formingLi2Ti2(PO4)3 (ref.64), P, LiTiPO5, AlPO4 and Li3PO4 (ref.66) for LATP, and Ge, GeO2, Li4P2O7 and AlPO4 (ref.66) for LAGP. The fully reduced products by Li metal are predicted to be Li2O, Li3P, Ti–Al, Li–Al and Li–Ge alloys66. Clearly, the direct contact between LAT(G)P and Li metal cannot lead to stable solid-state cells. A slight but noticeable reduction of LAGP at 0.85 V has been cap- tured by CV197. XPS analysis on the surface of LAGP and LATGP (a commercial NASICON-type glass-ceramic containing both Ti and Ge) after Li deposition revealed Ti4+ reduction to Ti3+ in LATGP and Ge4+ reduction to elemental Ge in LAGP198, similar to findings for LATGP after cycling in a Li symmetric cell199. After contact- ing LAGP with molten Li, Li–Ge alloy formation has been observed by XPS200; this is one of the fully reduced products predicted by DFT. However, Al3+ remains in its trivalent oxidation state, in contrast to the DFT pre- diction66,198. The presence of electron-conductive phases such as the Li–Ge alloy at the LAT(G)P/Li metal inter- face leads to the formation of an MCI, explaining the continuous increase of the impedance of a Li symmetric cell using a LAGP or LATGP SE198,199. Further evidence of the reduction decomposition of LAGP was provided201 using in situ TEM, ex situ XRD, SEM and Raman spec- troscopy, which showed that a thick amorphous inter- phase was formed between Li and LAGP. In addition, the large expansion (130%) of the LATP layer resulting from Li insertion was observed to induce crack initiation and widening in the LAGP pellet near the LAGP/Li inter- face200–202. Such continuous interfacial-reaction-driven chemomechanical degradation, rather than the inter- phase formation itself, was claimed to be the primary cause for the observed impedance growth202. On the cathode side, LAGP was initially reported to be stable up to 6 V based on CV measurements203; however, DFT calculations suggested lower oxidation limits of 4.21 V (or 4.8 V) for LATP and 4.27 V (or 4.5 V) for LAGP, above which O2 gas and phosphates would form64,66. It should be noted that the predicted oxida- tion stability of LATP and LAGP above 4 V is the high- est among all the SEs covered in this Review. The high voltage stability can be attributed to the strong P–O hybridization that prevents oxygen oxidation67. For LATP in contact with LiCoO2, a mild driving force (~50 meV/atom) is predicted to delithiate LiCoO2 to Li0.5CoO2 and form Li3PO4, in addition to Co3O4, LiAl5O8 and TiO2 (ref.65). The tendency to form Li3PO4 when a compound with PO4 groups is in contact with a cathode was recently studied in detail67. In experiments, the LATGP (or LATP)/LiCoO2 interface remained stable at 500 °C, as indicated by high-resolution TEM analysis204; however, interdiffusion occurred at higher temperature, forming a porous amorphous layer. Such high-temperature reactivity has also been observed at LATP/spinel cathode interfaces. XRD was used to study the chemical reactivity of mixtures of LATP with diffe- rent spinel cathodes (Li2NiMn3O8, Li2FeMn3O8 and LiCoMnO4) at high temperature80. Decomposition products including Li3PO4, AlPO4, TiO2, Co3O4, MnFeO3 and LiMnPO4 were detected above 600 °C, in good agreement with the DFT-predicted products at this temperature80. These results suggest that, similar to garnets, NASICON SEs also suffer from severe interface decomposition during the co-sintering process. Under battery-operating conditions, no noticeable intermix- ing was observed at the LiCoO2/LATP interface after 50 cycles in a LiCoO2|LATP|LiPON|Li cell204, consistent with the calculated zero reaction driving force between LATP and half-lithiated Li0.5CoO2 or fully delithiated CoO2 (ref.65). A recently developed NASICON-type conductor, LiZr2(PO4)3, exhibits good ionic conductivity of ~10−4 S cm−1 at 80 °C (ref.205). At the LiZr2(PO4)3/Li interface, a thin amorphous layer containing Li3P and Li8ZrO6 forms, which likely functions as an SEI, owing to its poor electronic conductivity205,206, in contrast to the MCI layers formed at LATP/Li and LAGP/Li inter- faces. This comparison highlights the effect of non-Li cations on the character of the SE/Li interface, which is detailed in Table 1. In addition, LiZr2(PO4)3 exhibited compatibility with LiFePO4 in a Li|LiZr2(PO4)3|LiFePO4 solid-state cell, with a high coulombic efficiency over 40 cycles205. Indeed, LiZr2(PO4)3 was calculated to be stable up to 4.60 V and chemically stable with LiFePO4 because of their same anion chemistry. Inorganic coatings Direct contact between the SE and electrode can be avoided by applying a coating layer, which acts as an artificial SEI that permits conduction of Li ions but not of electrons, thus expanding the practical stability window of the SE. The thickness of the coating can be controlled to be between 1 and 10 nm (refs47,56,207), which is generally thinner than an in situ-formed SEI47,79,156,201. The essential requirements for the coating material are chemical stability with both the SE and relevant elec- trode and electrochemical stability over the operating voltage range of the relevant electrode. Therefore, the composition of the cathode and anode coating should be optimized differently according to the specific SE–electrode combination. Cathode coatings In an early coating demonstration, a Li4Ti5O12 coating was applied on LiCoO2 to improve the capacity, cycla- bility and power density56. The application of LiPON coatings on LiCoO2, Li-rich NCM and LiNi0.5Mn1.5O4 cathodes has also been demonstrated to be effective in enhancing the cyclability at high C-rates and high voltage208–210. In addition, LiNbO3 and LiTaO3 are fre- quently used coating materials because they are relatively easy to coat and exhibit reasonable ionic conductivity when amorphous57,58. In fact, LiNbO3, LiTaO3 and LiNb0.5Ta0.5O3 have all shown promise in protecting thio-LISICONs and LGPS from reacting with LiCoO2 and NCM cathodes32,58,70,211. Varying degrees of success have also been achieved in SSBs with sulfide SEs using other oxide coatings, including Li2O–ZrO2 (ref.212), Li2SiO3 (refs213,214), Li3BO3–Li2CO3 (ref.215), Li3PO4 (ref.216), Reviews Nature reviews | MaterialsPDF Image | Understanding interface stability in solid-state batteries
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