Synthesis, Structural and Electrochemical Investigation of Disordered Rock Salt Compounds Based on Ni and Ti

Abstract

Li-ion batteries achieved in the last 3 decades extremely high energy densities, making them the energy storage devices of choice for the electrification of our society. Despite this improvement, their energy density is still low as compared to gasoline, motivating further research towards materials with better performances. State of the art positive electrodes are nowadays layered materials with a ratio Li:M of 1:1, with M being one or multiple transition metals. Disordered rock salt (DRX) materials without a layered structure were thought to be electrochemically inactive. With the development of the percolation theory, 1(Urban, Lee, and Ceder) it was demonstrated that lithium-rich materials (Li:M>1) with a cation disordered rock-salt structure (DRX) can achieve high reversible specific capacities. The general formula of these lithium-rich compounds can be written Li1+y(MM′)1−yO2 where the Li content ranges usually from 1.05 to 1.33, essential to promote Li percolation pathways. With M is indicated a redox active specie (Mn, Ni, Fe, Cr, V) and M’ is typically a d0 element, which is redox inactive during charge and discharge and therefore stabilizes the disordered structure and acting as charge compensator.2 (Chen, Ahn, and Chen)In this work a class of Li-rich compounds with the chemical composition Li2yNiyTi2-3yO2 has been investigated (y = 0.50, 0.55, 0.60, 0.67). These samples were prepared via solid-state reaction at three different temperatures (700 °C, 800 °C, 900 °C). X-ray diffraction has been used to determine the crystal structure of the samples, revealing a predominant rock salt-type phase. However, by decreasing the synthesis temperature and increasing the Li content, phase separation occurs into a rock salt-type and a Li2TiO3 phase. The synthesis has been also verified by DFT and by in situ X-ray diffraction. Neutron diffraction experiment have been carried out to fully understand the cationic distribution in the samples. Finally, the electrochemical performances of the samples have been investigated in Li half cells, revealing a strong voltage hysteresis between charge and discharge.3 (Li et al.) Bibliography Urban, Alexander, Jinhyuk Lee, and Gerbrand Ceder. “The Configurational Space of Rocksalt-Type Oxides for High-Capacity Lithium Battery Electrodes.” Advanced Energy Materials 4, no. 13 (September 2014): 1400478. https://doi.org/10.1002/aenm.201400478.Chen, Dongchang, Juhyeon Ahn, and Guoying Chen. “An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes.” ACS Energy Letters, March 17, 2021, 1358–76. https://doi.org/10.1021/acsenergylett.1c00203.Li, Biao, Khagesh Kumar, Indrani Roy, Anatolii V. Morozov, Olga V. Emelyanova, Leiting Zhang, Tuncay Koç, et al. “Capturing Dynamic Ligand-to-Metal Charge Transfer with a Long-Lived Cationic Intermediate for Anionic Redox.” Nature Materials 21, no. 10 (October 2022): 1165–74. https://doi.org/10.1038/s41563-022-01278-2