Abstract
AbstractThe interfacial instability between a thiophosphate solid electrolyte and oxide cathodes results in rapid capacity fade and has driven the need for cathode coatings. In this work, the stability, evolution, and performance of uncoated, Li2ZrO3‐coated, and Li3B11O18‐coated LiNi0.5Co0.2Mn0.3O2 cathodes are compared using first‐principles computations and electron microscopy characterization. Li3B11O18 is identified as a superior coating that exhibits excellent oxidation/chemical stability, leading to substantially improved performance over cells with Li2ZrO3‐coated or uncoated cathodes. The chemical and structural origin of the different performance is interpreted using different microscopy techniques which enable the direct observation of the phase decomposition of the Li2ZrO3 coating. It is observed that Li is already extracted from the Li2ZrO3 in the first charge, leading to the formation of ZrO2 nanocrystallites with loss of protection of the cathode. After 50 cycles separated (Co, Ni)‐sulfides and Mn‐sulfides can be observed within the Li2ZrO3‐coated material. This work illustrates the severity of the interfacial reactions between a thiophosphate electrolyte and oxide cathode and shows the importance of using coating materials that are absolutely stable at high voltage.
Highlights
Direct Visualization of the Interfacial Degradation of Cathode Coatings in Solid State Batteries: A Combined Experimental and Computational Study
Known inorganic solid electrolytes (SEs), The interfacial instability between a thiophosphate solid electrolyte and thiophosphates offer the advantage of oxide cathodes results in rapid capacity fade and has driven the need for cathode coatings
We considered the chemical stability at the cathode/SE, cathode/coating, and coating/SE interfaces (Figure 1b)
Summary
The electrochemical stability window of a material and the decomposition products at a given Li chemical potential can be predicted using a first-principles methodology previously developed.[5,24,25] The thermodynamic stability window represents. A high reactivity of −414 meV per atom is predicted between discharged NCM523 and LPS, indicating a highly unstable interface upon direct contact. Both LZrO and LBO coatings exhibit significantly reduced reaction driving forces with NCM523 (0 meV per atom for LZrO and −27 meV per atom for LBO). For the LPS/coating interface, a non-negligible driving force exists between the LZrO coating and LPS (−111 meV per atom) to form Li3PO4, whereas LBO is. LBO is expected to act as a chemically more robust barrier layer at the NCM523/LPS interface than LZrO, and provides a wide stability window spanning the voltage range of typical NCM523 cathodes. A full list of the computed reactivities and the predicted reactions of all the reaction pairs is provided in Table S2 in the Supporting Information
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