The composition, morphology, and evolution of the solid electrolyte interphase (SEI) formed on hard carbon (HC) electrodes upon cycling in sodium-ion batteries are investigated. A microporous HC was prepared by pyrolysis of d-(+)-glucose at 1000°C followed by ball-milling. HC electrodes were galvanostatically cycled at room temperature in sodium-ion half-cells using an aprotic electrolyte of 1 m sodium bis(trifluoromethanesulfonyl)imide dissolved in propylene carbonate with 3 wt % fluoroethylene carbonate additive. The evolution of the elec-trode/electrolyte interface was studied by impedance spectroscopy upon cycling and ex situ by spectroscopy and microscopy. The irreversible capacity displayed by the HC electrodes in the first galvanostatic cycle is probably due to the accumulation of redox inactive Na x C phases and the precipitation of a porous, organic-inorganic hybrid SEI layer over the HC electrodes. This passivation film further evolves in morphology and composition upon cycling and stabilizes after approximately ten galvanostatic cycles at low current rates.[a] Dr.
LiCoPO4 (LCP) is a challenging high voltage positive electrode material for next-generation secondary Li-ion cells. Doping the LCP olivine lattice with iron and annealing the material at high temperature result in improved and stable performances in lithium cells. Here we investigate the structural effects of iron doping and annealing at high temperature by advanced synchrotron X-ray techniques (X-ray diffraction and absorption) in close comparison with the corresponding performances in lithium cells (lithium de-insertion/insertion) and the ionic diffusion coefficients evaluated by galvanostatic intermittent titration tests. The partial substitution of cobalt ions in the olivine lattice with iron ions, 2+ or 3+, strongly affects the long range crystal structure as well as the short range atomic coordination. These structural changes alter the concentration of anti-site defects, the natural concentration of lithium vacancies, and the size of the lithium diffusion channels along the [010] direction as well as their local distortion. The balancing between these competitive effects modulate the lithium transport properties in the lattice
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