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Prussian blue analogues (PBAs) have been regarded as prospective cathode materials for sodium-ion batteries due to tunable chemical composition and structure. Herein, a high-performance rhombohedral nickel hexacyanoferrate is synthesized via a controllable low-temperature reaction process. It can deliver impressive capacity retention of 87.8% after 10 000 cycles at 10C and high rate discharge capacity of 53 mAh g–1 at 40C. According to the structural evolution and lattice water movement, superior electrochemical performance is ascribed to small lattice alteration and high reversibility of rhombohedral–cubic transition upon Na+ insertion/extraction. The environment information of local- and long-range structure evolution is revealed by ex situ X-ray absorption spectroscopy (XAS) and in situ X-ray diffraction (XRD). Importantly, lattice water movement during cycling by Fourier transform infrared (FTIR) measurements offers an experimental validation about Na+ nonlinear migration path, as well as the accumulative lattice distortion effect from large-size Na­(OH2)+ unit. The revealed mechanism points out the modified path for PBAs.

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We have performed a systematic study of the layered ternary carbides Ti 2 AlC, V 2 AlC, and Cr 2 AlC using 27 Al NMR spectroscopy. The quadrupole splittings, Knight shifts, as well as spin-lattice relaxation times on each material have been identified. The sign of the isotropic Knight shift varies from positive for Ti 2 AlC and V 2 AlC to negative for Cr 2 AlC, attributed to the enhancement of hybridization with increasing valence electron count in the transition metal. Universally long relaxation times are found for these alloys. Results provide a measure of Al-s Fermi-level density of states N s ͑E F ͒ for Ti 2 AlC and V 2 AlC. In addition, the evidence that N s ͑E F ͒ correlates with the transition metal d-electron count has been explored in the present NMR investigation.

V NMR measurements were performed on the quasi-one-dimensional compound BaCu 2 V 2 O 8 . Two 51 V NMR lines that are associated with two vanadium crystallographic sites have been resolved. For each site the temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a broad maximum at T max Ӎ 280 K. Below T max , the NMR shifts and spin-lattice relaxation rates clearly indicate activated behavior, confirming the spin gap formation in BaCu 2 V 2 O 8 . While the alternating-chain model has been proposed to account for the observed spin gap characteristics, our NMR results suggest that the nearly isolated dimer picture could be an alternative scenario for the understanding of the gap feature in this material.

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