Na 3 V 2 (PO 4 ) 2 F 3 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 Na 3 V 2 (PO 4 ) 2 F 3 as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution 23 Na and 31 P solid-state nuclear magnetic resonance (NMR). The 23 Na 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 Na 3 V 2 (PO 4 ) 2 F 3 electrode upon charge have been observed, where a (= b) continues to increase and c 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 23 Na 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 31 P 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 (V 4+ ) 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 Na 3 V 2 (PO 4 ) 2 F 3 when used in a NIB.

3.8 V (vs Li + /Li), making them a class of promising cathode material and attracting considerable attention. Nevertheless, the inferior cycling stability and poor rate capability of Ni-rich oxide cathode materials have to be overcome before they can compete in practical implementation. [2,4] These drawbacks are largely attributed to their spherical micrometer-sized secondary particles aggregated densely by many randomly oriented primary nanoparticles, [4][5][6] as shown in Figure 1a. On the one hand, concomitant with this structure, the surface of secondary particles is terminated with random crystal planes. As Li + can only diffuse along the 2D {010} plane in the hexagonal-layer structure of NCM materials, [4,7] the randomly exposed crystal planes (not solely the active {010} plane) may substantially hinder the Li + exchange at the electrode/electrolyte interface. Meanwhile, the randomly oriented primary nanoparticles induce a prolonged and mazy Li + diffusion pathway inside the secondary particles, because Li + ions have to migrate across the grain boundaries, especially between the grains with inconsistent crystal planes. On the other hand, the successive phase transition accompanied by repeated Li + insertion/extraction would result in anisotropic variation of the lattice parameters, and such variation is severely aggravated with the increase of Ni content. [8] Accordingly, in the Ni-rich oxide cathode materials, the substantial anisotropic lattice expansion/contraction would result in drastic microstrains at the boundaries of randomly oriented primary particles due to the asynchronous volume Ni-rich Li[Ni x Co y Mn 1−x−y ]O 2 (x ≥ 0.8) layered oxides are the most promising cathode materials for lithium-ion batteries due to their high reversible capacity of over 200 mAh g −1 . Unfortunately, the anisotropic properties associated with the α-NaFeO 2 structured crystal grains result in poor rate capability and insufficient cycle life. To address these issues, a micrometersized Ni-rich LiNi 0.8 Co 0.1 Mn 0.1 O 2 secondary cathode material consisting of radially aligned single-crystal primary particles is proposed and synthesized. Concomitant with this unique crystallographic texture, all the exposed surfaces are active {010} facets, and 3D Li + ion diffusion channels penetrate straightforwardly from surface to center, remarkably improving the Li + diffusion coefficient. Moreover, coordinated charge-discharge volume change upon cycling is achieved by the consistent crystal orientation, significantly alleviating the volume-change-induced intergrain stress. Accordingly, this material delivers superior reversible capacity (203.4 mAh g −1 at 3.0-4.3 V) and rate capability (152.7 mAh g −1 at a current density of 1000 mA g −1 ). Further, this structure demonstrates excellent cycling stability without any degradation after 300 cycles. The anisotropic morphology modulation provides a simple, efficient, and scalable way to boost the performance and applicability of Ni-rich layered oxide cathode materials.

No abstract

(17)O-(1)H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross-polarization magic angle spinning (CPMAS) NMR, CP rotational-echo double resonance (CP-REDOR) NMR, and heteronuclear correlation (HETCOR) NMR spectroscopy to distinguish between Brønsted acid sites with different O-H distances. The accessibility of the various Brønsted acid sites was quantified by adsorbing the basic probe molecule trimethylphosphine in known amounts. On the basis of these experiments, locations of different Brønsted acid sites in H-Mordenite (H-MOR) were proposed. The use of (17)O chemical shift correlations to help assign sites is discussed.