Measuring T1 relaxation in paramagnetic solids with solid-state NMR: a case study on the milling induced phase transition in Li6CoO4

Solid-state NMR has been a vital tool for the study of structural evolution of cathodes in lithium-ion and sodium-ion batteries. However, the differentiation of relaxation parameters for certain sites is difficult owing to limited spectral resolution associated with strong anisotropic hyperfine interaction. Here we propose a novel IR-pjMATPASS method that can measure T₁ relaxation with site-specific resolution for paramagnetic solids. We apply this method to the characterization of ball-milling induced order-disorder phase transition in Li₆CoO₄ as a case study. The quasi-quantitate ⁷Li NMR enables the synthetic optimization of high energy ball-milling conditions to harvest a disordered cubic phase through site-specific ⁷Li T₁ measurements. The example study shown here provides a quantitative strategy for NMR studies of paramagnetic solids.

Cation-Disorder-Assisted Reversible Topotactic Phase Transition between Antifluorite and Rocksalt Toward High-Capacity Lithium-Ion Batteries

Multielectron reaction electrode materials using partial oxygen redox can be potentially used as cathodes in lithium-ion batteries, as they offer numerous advantages, including high reversible capacity and energy density and low cost. Here, a reversible three-electron reaction is demonstrated utilizing topotactic phase transition between antifluorite and rocksalt in a cation-disordered antifluorite-type cubic Li₆CoO₄ cathode. This cubic phase is synthesized by a simple mechanochemical treatment of conventionally prepared tetragonal Li₆CoO₄. It displays a reversible capacity of 487 mAh g^-1, a high value because of a reversible three-electron reaction using Co²⁺/Co³⁺, Co³⁺/Co⁴⁺, and O^2-/O₂^2- redox, occurring without O₂ gas evolution. The mechanochemical treatment is assumed to reduce its lattice distortion by cation-disordering and facilitate a reversible topotactic phase transition between antifluorite and rocksalt structures via a dynamic cation pushing mechanism.

A new sealed lithium-peroxide battery with a co-doped Li2O cathode in a superconcentrated lithium bis(fluorosulfonyl)amide electrolyte

We propose a new sealed battery operating on a redox reaction between an oxide (O²⁻) and a peroxide (O₂²⁻) with its theoretical specific energy of 2570 Wh kg⁻¹ (897 mAh g⁻¹, 2.87 V) and demonstrate that a Co-doped Li₂O cathode exhibits a reversible capacity over 190 mAh g⁻¹, a high rate capability, and a good cyclability with a superconcentrated lithium bis(fluorosulfonyl)amide electrolyte in acetonitrile. The reversible capacity is largely dominated by the O²⁻/O₂²⁻ redox reaction between oxide and peroxide with some contribution of the Co²⁺/Co³⁺ redox reaction.

Characterization of novel lithium battery cathode materials by spectroscopic methods: the Li5+xFeO₄ system

The novel, lithium-rich oxide-phase Li₅FeO₄ (LFO) could, in theory, deliver a specific capacity >900 mAh/g when deployed as a cathode or cathode precursor in a battery with a lithium-based anode. However, research results to date on LFO indicate that less than one of the five Li⁺ cations can be reversibly de-intercalated/re-intercalated during repetitive charging and discharging cycles. In the present research, the system Li5+xFeO₄ with x values in the range of 0.0-2.0 was investigated by a combination of Raman and X-ray absorption spectroscopic methods supported by X-ray diffraction (XRD) analysis in order to determine if the Li₅FeO₄ lattice would accommodate additional Li⁺ ions, with concomitant lowering of the valence on the FeIII cations. Both the Raman phonon spectra and the XRD patterns were invariant for all values of x, strongly indicating that additional Li⁺ did not enter the Li₅FeO₄ lattice. Also, Raman spectral results and high-resolution synchrotron XRD data revealed the presence of second-phase Li₂O in all samples with x greater than 0.0. Synchrotron X-ray absorption spectroscopy at the Fe kα edge performed on the sample with a Li-Fe ratio of 7.0 (i.e., x = 2.0) showed no evidence for the presence of FeII. This resistance to accepting more lithium into the Li₅FeO₄ structure is attributed to the exceedingly stable nature of high-spin FeIII in tetrahedral "FeIIIO₄" structural units of Li₅FeO₄. Partial substitution of the FeIII with other cations could provide a path toward increasing the reversible Li⁺ content of Li5xFeO₄-type phases.