Oxygen Loss in Layered Oxide Cathodes for Li-Ion Batteries: Mechanisms, Effects, and Mitigation

Abstract

Layered lithium transition metal oxides derived from LiMO 2 (M = Co, Ni, Mn, etc.) have been widely adopted as the cathodes of Li-ion batteries for portable electronics, electric vehicles, and energy storage. Oxygen loss in the layered oxides is one of the major factors leading to cycling-induced structural degradation and its associated fade in electrochemical performance. Herein, we review recent progress in understanding the phenomena of oxygen loss and the resulting structural degradation in layered oxide cathodes. We first present the major driving forces leading to the oxygen loss and then describe the associated structural degradation resulting from the oxygen loss. We follow this analysis with a discussion of the kinetic pathways that enable oxygen loss, and then we address the resulting electrochemical fade. Finally, we review the possible approaches toward mitigating oxygen loss and the associated electrochemical fade as well as detail novel analytical methods for probing the oxygen loss.

Gas Generation in Lithium Cells with High-Nickel Cathodes and Localized High-Concentration Electrolytes

“…High-nickel cathodes experience runaway reactions at lower temperatures and with greater heat release, exacerbating the issue. Similarly, during typical high-voltage operation or in the event of overcharging, gases are released through the reaction of the electrolyte with the cathode. , Gas bubbles can accumulate in the cathode and inhibit the transport of lithium ions, can depressurize pouch cells and increase contact resistance, and in extreme cases can cause cell rupture.…”

“…In total, the cell with LHCE generated roughly 170 μmol m –2 of gas compared to ∼300 μmol m –2 for the cell with LP57, a factor of ∼2 difference. For both electrolytes, these gases were composed mostly of CO 2 , with a small portion of O 2 and minor quantities of CO. CO 2 generation, at least in typical carbonate electrolytes, is mostly associated with solvent decomposition, with some debate over the contributions of the binder, conductive carbon, and other species, such as residual lithium carbonate. − While the chemical origins of CO 2 at high voltages are complex, O 2 can be readily associated with the loss of lattice oxygen from the layered oxide cathode at high states of charge. , Interestingly, the electrolyte with less total gas generation, the LHCE, sees a greater proportion of O 2 relative to the other gases. The greater proportion of O 2 generation also leads to a greater amount of O 2 relative to the carbonate electrolyte.…”

“…19−22 While the chemical origins of CO 2 at high voltages are complex, O 2 can be readily associated with the loss of lattice oxygen from the layered oxide cathode at high states of charge. 7,23 Interestingly, the electrolyte with less total gas generation, the LHCE, sees a greater proportion of O 2 relative to the other gases. The greater proportion of O 2 generation also leads to a greater amount of O 2 relative to the carbonate electrolyte.…”

Oxygen-redox activity in non-Li-excess W-doped LiNiO2 cathode

“…[4] In practice, this often requires high voltage operation to facilitate increased delithiation, which induces various degradation modes namely structural instabilities, parasitic electrode-electrolyte reactions, surface reconstructions and oxygen loss, within the cathode that drastically reduce their cycle life. [5][6][7] To mitigate such processes, and therefore, maintain high energy densities over long-term cycling, we need to understand the structural and electronic changes in these cathodes at extreme states of delithiation and high voltages (>4.5 V vs. Li + /Li). [1] A key challenge here is to characterize the charge compensation mechanisms operating at these voltages.…”

Spontaneous Lithiation of Binary Oxides during Epitaxial Growth on LiCoO2

“…In the reversible range, the delithiation of LiCoO 2 to form Li x CoO 2 (0.5 < x < 1) is expected to oxidize Co above 3+. However, it is widely known that Li x CoO 2 becomes increasingly unstable as x decreases, and releases oxygen at elevated temperatures, leading to reduce Co below 3+. , In addition, over delithiation can lead to the reduction of Co 3+ as a result of spinel Co 3 O 4 or rock-salt CoO formation. , LiCoO 2 regions not covered by Li 2 WO 4 islands grown on STO(001) experience less Li loss as illustrated in Figure a and thus are expected to have better thermal stability. On the other hand, LiCoO 2 regions not covered by Li 2 WO 4 islands readily contribute Li during the formation of Li 2 WO 4 when growing on STO(111) (Figure b), which is more susceptible to reduce Co due to oxygen release at high temperature.…”