Promising Nanometric Spinel Cobalt Oxides for Electrochemical Energy Storage: Investigation of Li and H Environments by NMR

Spinel-type cobalt oxides with formula H_<i>x</i>Li_<i>y</i>Co_3−δO₄ exhibit interesting properties for various electrochemical energy storage applications thanks to their attractive electronic properties, due to the presence of H and Li ions in their structure as well as their nanometric dimensions. The effect of temperature on the H and Li environments is studied by investigating materials heat-treated at temperatures ranging from 25 to 650 °C by means of NMR spectroscopy. Two types of proton are evidenced: one bonded to oxygen atoms belonging to the network (hydroxyl group) and the other one involved in the H₂O molecule. This configuration is in agreement with IR spectroscopy measurements, revealing the absence of free −OH groups, which mean that protons in the structure are involved in hydrogen bonds. After heat treatments at increasing temperature, NMR confirms that hydrogen is released, which induces first the migration of Li ions beyond 200 °C (probably from the 8a to the 16c sites), followed by a progressive reorganization of the structure with formation of HT-LiCoO₂ beyond 400 °C

Local Structure and Dynamics in the Na Ion Battery Positive Electrode Material Na₃V₂(PO₄)₂F₃

Na₃V₂(PO₄)₂F₃ is a novel electrode material that can be used in both Li ion and Na ion batteries (LIBs and NIBs). The long- and short-range structural changes and ionic and electronic mobility of Na₃V₂(PO₄)₂F₃ as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution ²³Na and ³¹P solid-state nuclear magnetic resonance (NMR). The ²³Na NMR spectra and XRD refinements show that the Na ions are removed nonselectively from the two distinct Na sites, the fully occupied Na1 site and the partially occupied Na2 site, at least at the beginning of charge. Anisotropic changes in lattice parameters of the cycled Na₃V₂(PO₄)₂F₃ electrode upon charge have been observed, where <i>a</i> (= <i>b</i>) continues to increase and <i>c</i> decreases, indicative of solid-solution processes. A noticeable decrease in the cell volume between 0.6 Na and 1 Na is observed along with a discontinuity in the ²³Na hyperfine shift between 0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement of unpaired electrons within the vanadium t_2g orbitals. The Na ion mobility increases steadily on charging as more Na vacancies are formed, and coalescence of the resonances from the two Na sites is observed when 0.9 Na is removed, indicating a Na1–Na2 hopping (two-site exchange) rate of ≥4.6 kHz. This rapid Na motion must in part be responsible for the good rate performance of this electrode material. The ³¹P NMR spectra are complex, the shifts of the two crystallograpically distinct sites being sensitive to both local Na cation ordering on the Na2 site in the as-synthesized material, the presence of oxidized (V⁴⁺) defects in the structure, and the changes of cation and electronic mobility on Na extraction. This study shows how NMR spectroscopy complemented by XRD can be used to provide insight into the mechanism of Na extraction from Na₃V₂(PO₄)₂F₃ when used in a NIB