Chemical delithiation of lithium-ion battery cathode materials produces new materials analogous to those generated in lithium-ion battery cathodes upon electrochemical cycling. However, the degrees to which chemically delithiated materials resemble their electrochemical counterparts are not well understood. Chemical delithiation enables the production of pure materials, uncontaminated by binders or conductive carbon that may confound spectroscopic studies. Chemical delithiation most often involves NO_x⁺-type chemical oxidants with very high potentials (> 4.6 V vs Li). However, NO_x⁺-type oxidants are non-innocent, requiring large excesses of oxidant that engage in side reactions not necessarily encountered during electrochemical cycling. These issues complicate attempts to tune the resulting material’s potential via stoichiometric control.

In contrast, reversible organic oxidants with more moderate oxidation potentials enable potentiometric control of the delithiation reaction. Most of these organic oxidants are stable under standard reaction conditions; titration of the residual oxidant may be used to quantify the amount of oxidant consumed, and therefore the amount of lithium extracted. By tuning the potential of the applied chemical oxidant, the potential of the resulting material may be tuned to achieve the desired chemical state.

This study investigates several chemical oxidants of varying oxidation potential and their effects on lithium-ion battery cathode materials that exhibit multi-step oxidations. Half-cells fabricated from these chemically delithiated cathode materials exhibit initial open-circuit voltages and initial discharge capacities that correlate with the measured oxidation potentials of the chemical oxidants.

Recently, several cathode materials have been reported that exhibit reversible capacities beyond those attributable to their redox-active transition metals alone; the implication is that partial oxidation of the oxygen sublattice occurs at high potentials.[1-3] Direct observation of the oxygen sublattice using synchrotron-based spectroscopic methods is impeded by the presence of electrolyte or carbon residue. Chemically delithiated samples of these cathode materials lack both electrolyte and carbon, and have been used to examine the oxygen sublattice at multiple potentials.

[1] Yabuuchi, Naoaki, et al. "High-capacity electrode materials for rechargeable lithium batteries: Li₃NbO₄-based system with cation-disordered rocksalt structure." Proceedings of the National Academy of Sciences, 112, 7650 (2015).

[2] McCalla, Eric, et al. "Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries." Science, 350, 1516 (2015).

[3] Urban, Alexander, and Gerbrand Ceder. "A disordered rock-salt Li-excess cathode material with high capacity and substantial oxygen redox activity: Li_1.25Nb_0.25Mn_0.5O₂." Electrochemistry Communications, 60, 70 (2015).

Acknowledgment

Support for this work from the Office of Vehicle Technologies of the U.S. Department of Energy, in particular, David Howell and Peter Faguy, is gratefully acknowledged. 'Sector 20 facilities at the Advanced Photon Source of Argonne National Laboratory, and research at these facilities, are supported by the U.S. DOE, Basic Energy Sciences, and National Sciences and Engineering Research Council of Canada and its founding institutions.

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