Exploring a New, Scalable Synthesis Route to Disordered Rock Salt (DRX) Cathode Materials

Abstract

The global demand for lithium-ion batteries (LIBs) is ever-increasing as many nations seek to achieve a net carbon neutrality by the late 2030s. LiNi x Mn y Co1–x–y O2, (NMC) is one of the most widely-used Li-ion cathode active materials. Despite its excellent performance (e.g., reversible capacities up to ~220 mA h g–1 and 500+ cycles), NMC contains both Ni and Co, which are expensive and have vulnerable supply chains. Li-excess disordered rock salt (DRX) materials are promising new cathode materials that utilize earth-abundant transition metals such as Mn and Ti. Through a combination of cationic and anionic charge compensation, DRX materials can attain specific energies ≥700 Wh kg–1. Doping F– into the DRX structure (Li1+x Mn y Ti2–1–x–y O2–z F z ) has been reported to improve the material’s cycling stability. Despite their promising properties, most DRX cathodes are prepared through solid-state synthesis routes which require high-energy milling, provide little-to-no control over the particle morphology, and are difficult to scale.In this work, we developed a scalable combustion synthesis route (the glycine nitrate process) to produce high purity, high performance DRX cathodes. More specifically, we prepared two DRX precursors with nominal compositions of Li1.2Mn0.5Ti0.3O1.95 and Li1.2Mn0.7Ti0.1O1.85. When the precursors were heated under Ar to 1000 °C for 1 – 4 h, only 50% Li1.2Mn0.5Ti0.3O1.95 adopted a DRX structure, and no DRX formed for Li1.2Mn0.7Ti0.1O1.85. However, adding LiF to the precursors facilitates DRX phase formation during annealing and yields high purity DRX powders with the nominal compositions Li1.25Mn0.5Ti0.3O1.95F0.05 and Li1.35Mn0.7Ti0.1O1.85F0.15. In situ X-ray diffraction was employed to study the synthesis process, revealing DRX begins to form at just 600 °C, which is much lower than traditional solid-state routes. Furthermore, electrochemical tests on the Li1.35Mn0.7Ti0.1O1.85F0.15 cathode reveal these materials attain excellent performance with initial reversible capacities up to 215 mA h g–1 and stable cycling performance. These promising results demonstrate that combustion synthesis is a viable method for the scale-up of DRX materials.This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and was sponsored by the Vehicle Technologies Office (VTO) under the Office of Energy Efficiency and Renewable Energy (EERE). Some measurements were conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.