Coupled ion/electron hopping in Li(x)NiO2: a 7Li NMR study.
Deintercalated "Li(x)NiO2" materials (x = 0.25, 0.33, 0.50, 0.58, and 0.65) were obtained using the electrochemical route from the Li0.985Ni1.015O2 and Li0.993Ni1.007O2 compounds. Refinements of X-ray diffraction data using the Rietveld method show a good agreement with the phase diagram of the Li(x)NiO2 system studied earlier in this laboratory. Electronic conductivity measurements show a thermally activated electron-hopping process for the deintercalated Li0.5NiO2 phase. In the Li(x)NiO2 materials investigated (x = 0.25, 0.33, 0.50, and 0.58), 7Li NMR shows mobility effects leading to an exchanged signal at room temperature. A clear tendency for Li to be surrounded mainly by Ni3+ ions with the 180 degree configuration is observed, particularly, for strongly deintercalated materials with smaller Li+ and Ni3+ contents, even upon heating, when this mobility becomes very fast in the NMR time scale. This suggests that Li/vacancy hopping does occur on the NMR time scale but that Ni3+/Ni4+ hopping does not occur independently. The position of Li seems to govern the oxidation state of the Ni around it at any time; the electrons follow the Li ions to satisfy local electroneutrality and minimal energy configuration. The observed NMR shifts are compatible with the Li/vacancy and Ni3+/Ni4+ ordering patterns calculated by Arroyo y de Dompablo et al. for x = 0.25 and x = 0.50, but not for x = 0.33 and x = 0.58.
Chazel
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Institut de Chimie de la Matière Condensée de Bordeaux-CNRS and ENSCPB, Université Bordeaux 1, 87 Av. Schweitzer, 33608 PESSAC Cedex, France.
Ménétrier
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Croguennec
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Delmas
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D004583:Electrons; D007477:Ions / Q000737:chemistry; D007554:Isotopes; D008094:Lithium / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008280:Magnetics; D009532:Nickel / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D012680:Sensitivity and Specificity; D014961:X-Ray Diffraction
D007477:Ions; D007554:Isotopes; D009532:Nickel; D008094:Lithium; D010100:Oxygen