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between the coating and the electrode material and the other between the coating and the SE. Because the coating acts as a second electrolyte, it must be stable at the electrode voltage and resist chemical reactions with both the electrode and the SE. However, if coating-layer imperfections leave part of the electrode surface in con- tact with the SE, as shown in fig. 1, unfavourable inter- facial reactions still occur in the coated electrode system. On the other hand, these coating imperfections may be necessary for the electron transport between the coated electrode and current collector, posing a paradox in the current coating strategy67. Among the remaining interfaces in cathode com- posites, decomposition of the SE can also occur at the current collector/SE and carbon/SE interfaces, where the SE is subjected to the working lithium or sodium chemical potential68–70. Although neither ion nor electron transport across these interfaces is required for battery cycling, such decomposition unavoidably compromises the high bulk ionic conductivity of the SE over time. On the alkali-metal-anode side, the instability of the SE arises from its reduction by metallic lithium or sodium. If the SE contains a metal or metalloid ele- ment(s), such reduction often leads to the generation of electron-conductive products at this interface, render- ing it a detrimental MCI that continuously consumes the SE51,63,64. Interface models Direct experimental probing of buried solid/solid inter- faces is fundamentally challenging, as it is difficult to separate the solids for experimental characterization without damaging their surfaces71. Focused-ion-beam milling has been used to create cross sections of such interfaces for characterization with transmission elec- tron microscopy (TEM) or energy-dispersive X-ray spectroscopy analysis47,72. The decomposition of a perov- skite SE or LiPON during Li deposition has also been successfully investigated using in situ X-ray photo- electron spectroscopy (XPS)63,73. The experimental difficulty of characterizing the interface has motivated the computational modelling of these interfaces using density functional theory (DFT). These computa- tional methods differ in the kinetic limitations they impose, the assumptions made about the effects of external conditions (such as electrochemical cycling or high-temperature processing) and the extent of inter- mixing possible at the interface. In this section, we dis- cuss the various levels at which interface stability can be modelled, because they can give insight into the products experimentally observed at the interfaces. Electrochemical stability The electrochemical stability window, or voltage stabil- ity window, of an SE describes its ability to resist oxi- dation or reduction through the extraction or insertion of alkali ions and electrons. Because a high operating voltage is desirable for batteries with high energy den- sity, the SE must be stable over a wide voltage window. It should be noted that although the electrochemical stability window is an intrinsic property of the bulk SE rather than of the interface, it is critical to the interface stability because the electrochemical decomposition of the SE typically occurs at its interface with an electron source, where the SE directly experiences the applied voltage V. The applied voltage can be directly converted to an alkali (for example, Li) chemical potential μLi using equation 1 (ref.74) neglecting overpotential effects, where μ 0 is the lithium chemical potential in Li metal and e the Li elementary charge: μ =μ0−eV. (1) Li Li Hence, at the cathode side, the SE experiences a very low Li chemical potential and is subject to decomposi- tion by Li extraction. Formally, such stability can be eval- uated by calculating the grand potential Φ of the material using equation 2, where c is the composition of the mate- rial, E[c] the enthalpy and nLi[c] the Li concentration of composition c: Φ[c, μLi] = E[c] − nLi[c]μLi =E[c]−n [c]μ0 +n [c]eV. (2) Li Li Li The grand potential convex hull at a given voltage is formed by the grand potentials of a set of phases and their linear combinations that minimize the grand potential at each composition c − nLi that excludes Li. The electrochemical stability window of a material cor- responds to the range of voltages over which it is stable (exactly on the grand potential convex hull). As an exam- ple, three grand potential convex hulls containing the SE β-Li3PS4 at different voltages are presented in fig. 2a. It can be observed that β-Li3PS4 is thermodynamically stable at 2.1V but not at 0V and 3V. Decomposition of an SE yields new phases, which may require an activated process such as nucleation and, thus, an overpotential. For instance, the breakdown of an SE at high voltage (that is, decomposition by oxidation) is predicted to form phases with lower Li content (such as P2S5 for β-Li3PS4 at V = 3 V). Therefore, the stability estimated from this grand potential convex hull method represents the worst-case scenario (no kinetic stabiliza- tion) for the SE. Although it is difficult to directly predict such nucleation overpotentials, they should be similar to those observed in conversion electrodes (typically no more than a few hundred millivolts)75,76. Topotactic stability Although the thermodynamic approach in the previous section provides the narrowest electrochemical stability window, the maximum voltage limits for an SE can be estimated from the potentials at which an electron and an alkali ion can be topotactically removed or added, as this process is expected to have no kinetic limitations: electron extraction/addition should be facile at the interface and an SE has, by definition, high bulk ionic mobility. The calculation of this topotactic stability win- dow is analogous to the calculation of battery voltages in intercalation electrodes74,77. An example of the calcula- tion of the topotactic extraction voltage (Vtopo,ext) for the Na SE Na3PSe4 is presented in fig. 2b, where the voltage to extract the most unstable Na atom from the SE was Reviews Nature reviews | MaterialsPDF Image | Understanding interface stability in solid-state batteries
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