A team comprising researchers from Samsung and academic colleagues in China and the US have determined the reason for voltage loss in Li-ion batteries using lithium-rich layered transition metal oxide cathodes.
Combining various X-ray spectroscopic, microscopic, and structural probes, they found that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. In an open-access paper in Nature Communications, the researchers propose that this mechanism is involved in stabilizing the oxygen redox couple, which they observed spectroscopically to persist for 500 charge/discharge cycles.
Lithium-rich layered oxide electrodes (Li1+xM1−xO2, 0 < x ≤ 1/3) have attracted interest as they offer access to substantially higher energy densities than conventional layered oxide electrodes owing to the presence of a reversible anionic redox couple in addition to the usual transition metal (TM) redox couples. Understanding the mechanism of anion redox in these systems is necessary to mitigate their unfavorable electrochemical properties, which include significant charge/discharge voltage hysteresis (several hundred millivolts) and long-term voltage fade, even when cycled at vanishingly low rates.
Establishing the oxygen redox mechanism in these electrode materials, however, has proved challenging. … a clear consensus on which mechanism prevails in which materials is lacking. A key factor giving rise to the confusion is the difficulty of probing anionic redox species with conventional spectroscopic techniques, given the heterogeneous nature of many Li-rich materials.
In this work, we address this challenge by combining scanning transmission X-ray microscopy and nanoscale XAS (STXM-XAS) with resonant inelastic X-ray scattering (RIXS) and various structural probes to investigate the anion redox mechanism in LMR-NMC.—Gent et al.
For their study, the material was prepared at Samsung, incorporated into battery cathodes, and analyzed. The analyses show that as lithium ions migrate from the cathode to the anode upon charging, transition metal ions move to fill the lithium vacancies—but not all of the metal ions move back during discharge. The incomplete ion shuttling leads to microscopic structural changes that alter oxygen’s bonding geometry, lowering oxygen’s redox potential and causing the drop in voltage.
We therefore reveal that anion redox chemistry in many Li-rich materials cannot be fully understood by study of only their as-synthesized structures. Likewise, variations in structural behavior must be considered when comparing anion redox chemistry and stability between materials. Our results suggest that it may be possible to tune the stability and voltage of anion redox through control of the TM migration pathways. Thus, we suggest a new strategy for designing Li-rich layered oxides with improved cycling performance whereby the oxygen redox chemistry is tuned through structural modifications rather than the more common covalency modifications, which typically require substitution with rare 4d and 5d elements. More broadly, we demonstrate that atomic and electronic structural evolution during (de)intercalation need to be considered when assessing anion and cation redox chemistry even in nominally topotactic intercalation electrodes.—Gent et al.
William E. Gent, Kipil Lim, Yufeng Liang, Qinghao Li, Taylor Barnes, Sung-Jin Ahn, Kevin H. Stone, Mitchell McIntire, Jihyun Hong, Jay Hyok Song, Yiyang Li, Apurva Mehta, Stefano Ermon, Tolek Tyliszczak, David Kilcoyne, David Vine, Jin-Hwan Park, Seok-Kwang Doo, Michael F. Toney, Wanli Yang, David Prendergast & William C. Chueh (2017) “Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides” Nature Communications 8, Article number: 2091 doi: 10.1038/s41467-017-02041-x