Is There Hope for Ni-Based Disordered Rock Salt Cathode Materials?
Authors: Reitano Agnese , Kunz Sylvia , Bianchini Matteo 
Published: August 2024

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. LiNiO2 is a noteworthy example. Disordered rock salt (DRX) materials were instead thought to be electrochemically inactive until the development of a percolation theory,1 which demonstrated that lithium-rich materials (Li:M>1.1) 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 as Li1+y(MM′)1−yO2 where the Li content ranges usually from 1.05 to 1.33. 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 well accommodates the disordered structure.2 Materials with such structures have been shown to deliver extremely high specific capacities, resulting in equally high specific energies.3

In our recent work we investigated two DRX compounds based on Ni-redox. First, Li2NiO3, which is layered (with monoclinic distortion), but that decomposes to a disordered rock salt structure with cycling. The material displays interesting defect chemistry but fast capacity fading.4

More recently, we developed a class of Li-rich compounds with the chemical composition Li2yNiyTi2-3yO2 (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). By X-ray and neutron diffraction, we demonstrated a pure phase solid solution at high temperature, yet, by decreasing the synthesis temperature and increasing the Li content, phase separation occurs into a rock salt-type and a Li2TiO3 phase. The synthesis process has been also verified by DFT and by in situ X-ray diffraction. In all DRX materials synthesized, the electrochemical performances revealed a strong voltage hysteresis between charge and discharge and fast capacity fade.5,6 This raises then the question of whether any Ni-based DRX material can provide high capacity, high rate capability and stability, while avoiding the commonly encountered large hysteresis, or whether this can only be achieved in Mn-based compounds. In this presentation, by discussing our own work and recent literature reports, we aim at answering this question.

Bibliography

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Chen, Dongchang, Juhyeon Ahn, and Guoying Chen. “An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes.” ACS Energy Letters, 2021, 1358–76.

Clément et al., Cation-disordered rocksalt transition metal oxides and oxyfluorides for high energy lithium-ion cathodes, Energy & Environmental Science, 2020, 13 (2), 345-373

Bianchini et al., From LiNiO2 to Li2NiO3: Synthesis, Structures and Electrochemical Mechanisms in Li-Rich Nickel Oxides, Chem. Mater. 2020, 32, 9211−9227

Reitano, et al. Manuscript submitted, 2024.

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 Materials21, 10: 1165–74.