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Abstract
In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO4 (space group Pnma) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li2/3(Co2+)2/3(Co3+)1/3PO4 phase. Two resonances are observed for Li2/3CoPO4 with intensity ratios of 2:1 and 1:1 in the 31P and 7Li NMR spectra, respectively. An ordering of Co2+/Co3+ oxidation states is proposed within a (a × 3b × c) supercell, and Li+/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co3+ ions and Li vacancies are found to order along the b-axis. Two other low energy Li+/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO4–CoPO4 convex hull and they may be readily interconverted by Li+ hops along the b-direction.
Highlights
IntroductionOlivine-type LiFePO41 has been extensively studied as a promising cathode material for Li-ion batteries, due to its good reversibility, safe operating voltage (3.45 V vs Li/Li+) and high reversible capacity[2] of 160 mAh/g (compared to the practical capacity of 140 mAh/g for LiCoO2,3 the commonly used cathode in the portable communications industry).the low operating voltage of LiFePO4 leads to an energy density that is considered low for use in electric transportation
Olivine-type LiFePO41 has been extensively studied as a promising cathode material for Li-ion batteries, due to its good reversibility, safe operating voltage (3.45 V vs Li/Li+) and high reversible capacity[2] of 160 mAh/g.the low operating voltage of LiFePO4 leads to an energy density that is considered low for use in electric transportation
P local environments and their relative populations, predicted using the model outlined above and the experimental 7Li and 31P nuclear magnetic resonance (NMR) data obtained for the intermediate structure, led us to propose an intermediate phase with stoichiometry Li2/3(Co2+)2/3(Co3+)1/3PO4, and with a superstructure obtained by tripling the unit cell in the b-direction
Summary
Olivine-type LiFePO41 has been extensively studied as a promising cathode material for Li-ion batteries, due to its good reversibility, safe operating voltage (3.45 V vs Li/Li+) and high reversible capacity[2] of 160 mAh/g (compared to the practical capacity of 140 mAh/g for LiCoO2,3 the commonly used cathode in the portable communications industry).the low operating voltage of LiFePO4 leads to an energy density that is considered low for use in electric transportation. Olivine-type LiFePO41 has been extensively studied as a promising cathode material for Li-ion batteries, due to its good reversibility, safe operating voltage (3.45 V vs Li/Li+) and high reversible capacity[2] of 160 mAh/g (compared to the practical capacity of 140 mAh/g for LiCoO2,3 the commonly used cathode in the portable communications industry). LiMnPO4, LiCoPO4, and LiNiPO47,8 have more sluggish kinetics than isostructural LiFePO4, and are more difficult to fully lithiate and delithiate. LiCoPO4 has the lowest hole polaron migration barrier of the high voltage olivines and subsequently has the highest electronic conductivity among LiMnPO4, LiCoPO4 and LiNiPO4.9−12 the high redox potential of the Co2+/Co3+ couple means that LiCoPO4 has a high theoretical energy density of 800 Wh/kg, as compared with 580 Wh/kg for LiFePO4, but is not as affected as LiNiPO4 by the electrolyte decomposition that occurs at high voltages.[13,14]. Loss of long-range order was observed, which agrees with the chemical delithiation results of Wolfenstine et al.[17,18]
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