Lithium-7 MAS NMR and electrochemical studies of spinel-based positive electrode materials for lithium rechargeable batteries.

摘要

Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and electrochemical techniques have been used to determine dominant failure modes of LiMn₂O₄-based positive electrode materials for lithium rechargeable batteries, and to elucidate the role of metal-for-manganese substitution in the stabilization of the material towards electrochemical cycling on the 4V plateau.; Metal substituted spinels, LiM_xMn_2−xO ₄ (M = Li, Zn, Ni, Al, Co, Cr) have been synthesized by traditional ceramic techniques, and characterized with Xray Diffraction (XRD), Super Quantum Interference Device (SQUID) magnetometry, electrochemical techniques, and NMR spectroscopy. The 7Li NMR spectra of metal-substituted spinels are characterized by a large peak at ∼500ppm, arising from “normal” lithium, surrounded by 12 manganese nearest-neighbors, and smaller peaks spanning 530–650ppm, arising from “near-defect” lithium, with one or more lattice defects or substituting metals in the coordination sphere. As the level of substitution increases, the relative intensity of the near-defect peak increases, and both peaks broaden. It is determined that the residual MAS linewidth arises from chemical shift dispersion. The increase in the linewidth for the normal lithium peak upon substitution therefore suggests that substitution affects bond lengths, angles, and strengths throughout the material, not just in the vicinity of the substituting atom.; Various compositions were cycled galvanostatically (C/15 rate) 100 times from 3.3V to 4.4V vs. Li metal at room temperature using a 1M LiPF₆ in EC/DMC (1:2) electrolyte. A rapid capacity fade was observed for LiMn ₂O₄, which was mitigated to varying extents by substitution. For spinels that displayed high initial capacity, the Cr-substituted spinels showed the best performance. After cycling, the 7Li MAS NMR spectra of the spinels changed in characteristic ways: the peaks broadened, and the near-defect peaks increased in relative intensity. These changes were most pronounced for the poor-performing LiMn₂O₄, and were almost undetectable in the most robust compositions.; To determine the cause of the NMR-detectable structural change upon cycling, and thus the dominant mode of failure, the results for the cycled electrodes were compared to results for electrodes which had been exposed to model degradation processes. These processes were chosen so as to introduce specific damage into the spinel structure, mimicking failure by one of the many possible mechanisms proposed in the literature. This comparison provided substantial evidence that manganese dissolution and concomitant Li-for-Mn ion exchange at the end-of-discharge is the dominant mode of failure on the 4V plateau. Cr-substitution effectively mitigated this failure mode.