Doyle, S.

Ehrenberg, H.

Indris, S.

Kaus, M.

Mangold, S.

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KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Electrochemical Delithiation/Relithiation of LiCoPO4: A Two-Step Reaction mechanism Investigated by in situ XRD, in situ XAS, and ex situ 7Li/31P NMR spectroscopy Sylvio Indris1, Maximilian Kaus1, Stephen Doyle2, Stefan Mangold2, Helmut Ehrenberg1 1Institute for Applied Materials – Energy Storage Systems, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany 2ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany Karlsruhe Institute of Technology Institute for Appllied Materials – Energy Storage Systems LiCoPO4 was synthesized by a solid-state method. In situ XANES experiments were evaluated based on full-cycle experiments confirming the predominantly reversible character of the redox reaction. In situ XRD measurements were performed to follow structural changes during cycling indicating the existence of an intermediate phase upon cycling. The local phosphorous and lithium environments were studied by ex situ 31P and 7Li NMR at different states of charge proving the existence of an intermediate phase of the composition Li2/3CoPO4. Based on these findings a model of the (de)lithiation process of LiCoPO4 is developed and discussed. Summary In situ XRD, ex situ 7Li/31P NMR spectroscopy, and to some extent in situ XAS all reveal the existence of two two-phase redox reaction steps upon the (de)lithiation process of LiCoPO4. For the first time the lithiation state of the intermediate phase is determined by ex situ 7Li and 31P NMR spectroscopy to be Li2/3CoPO4 which is in good agreement with our and previous XRD and neutron diffraction measurements. The reversibility of the delithiation/relithiation, as observed by XRD, XAS, and NMR, is high. The poor cycling stability has to be ascribed to side reactions of the electrolyte. Synthesis and Characterization by XRD, SEM, and TEM  LiCoPO4 was synthesized by a solid-state reaction by mixing stoichiometric amounts of (NH4)Co(PO4) · H2O and Li2CO3 which were then ground in a mortar. The powder was calcined in air for 16 hours at 600°C, reground, and again calcined in air for 8 hours at 600°C.  TEM XRD reveals the presence of an intermediate phase during delithiation and relithiation. The composition of this intermediate phase is difficult to estimate from the applied charge due to side reaction of the electrolyte. Overall, a two-step reaction with two subsequent two-phase steps is observed. The volume changes of all three phases during cycling are small. Electrochemical Characterization 7Li and 31P Mas NMR reveal an intermediate phase with two Li environments (relative intensity of 1:1) and two P environments (2:1) References XRD  improved cyclic stability by addition of HFiP  two plateaus in charge profile  strong capacity fading during cycling - olivine structure - narrow size distribution - minor impurity of Co2P2O7 (4 wt%) comparison with pure SnO2 [1] N. N. Bramnik, K. Nikolowski, C. Baehtz, K. G. Bramnik, H. Ehrenberg, Chem. Mater. 19 (2007), 908. In Situ XAS (XAS beamline, ANKA) Ex Situ 7Li and 31P NMR In Situ XRD  (PDIFF beamline, ANKA)   Space group a (Å) b (Å) c (Å) V (Å3) Phase ‘A’  Pnma 10.20048(17) 5.92127(10) 4.69949(9) 283.848(9) Phase ‘B’ Pnma 10.08597(71) 5.85849(30) 4.71872(46) 278.823(37) Phase ‘C’ Pnma 9.63743(286) 5.81302(159) 4.73839(124) 265.456(128)   Space group  Pnma a (Å) 10.20275(5) b (Å) 5.92186(3) c (Å) 4.69982(3) V (Å³) 283.960(3) RBragg 7.32 RF 7.12 ²  5.54 d50 (µm) 0.577 From the lattice constants, a composition of about Li0.7CoPO4 ist estimated for the intermediate phase. The presence of two different Li environments and two different P environments in the intermediate phase can be explained by the different Co2+/Co3+ environments. phase evolution In situ XAS reveals a good reversibility of the Co2+/Co3+ oxidation/reduction. Pricincipal component analysis hints at a two-step reaction. SEM 

Electrochemical delithiation/relithiation of LiCoPO₄: A two-step reaction mechanism investigated by in situ XRD, in situ XAS, and ex situ ⁷Li/³¹P NMR spectroscopy

https://publikationen.bibliothek.kit.edu/220097892/3816674

Electrochemical delithiation/relithiation of LiCoPO₄: A two-step reaction mechanism investigated by in situ XRD, in situ XAS, and ex situ ⁷Li/³¹P NMR spectroscopy

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