Defect and Surface Area Control in Hydrothermally Synthesized LiMn_0.8Fe_0.2PO₄ Using a Phosphate Based Structure Directing Agent

Abstract

As confirmed by ion coupled plasma-mass spectrometry and powder X-ray diffraction, stoichiometrically pure samples of olivine type LiFe 0.2 Mn 0.8 PO 4 in the presence of a phosphoric acid ester based structure directing agent have been prepared. A Rietveld analysis of X-ray and n 0 diffraction data suggests that the solids are largely free of defect occupation of Fe at the lithium sites. This is confimed by 57 Fe-Mößbauer spectroscopic investigations. The use of the structure directing surfactant results in significantly higher specific surface areas (SSA) and smaller particle sizes as compared to samples prepared without using a surfactant. In using NO 3 -salt educts or different surfactants a further increase in SSA but at a cost of lower stoichiometric lithium contents can be achieved. INTRODCUTIONOlivine type lithium transition metal phosphates LiMPO 4 (M = Fe, Mn, Co, Ni) are amongst the most promising candidates for cathode materials in next-generation rechargeable lithium-ion batteries. For this reason they have been in the focus of intense research activities in the past one and a half decades. 1─4 One of the most important development goals is to improve the charge/discharge capacity by a proper choice of the composition and the structure of the battery materials. The theoretical maximum capacity of LiFePO 4 as a cathode material is 170 mAh/g. 5 Values closely approaching this upper limit have been reported for materials which have been prepared via solid state reaction or with hydrothermal synthesis methods. 4 In contrast partially manganese substituted materials of type LiFe 1-x Mn x PO 4 prepared in hydrothermal reactions often show disappointing electrochemical properties significantly below this value. 6 This is often attributed to lattice defects, like Fe/Li interchange (which is often also called an 'anti-site defect') or an surplus of M 2+ -ions (M = Me, Fe) at lithium sites (which are also called M1 sites in the olivine structure, as compared to the positions of the transition metal which are called M2 sites) (see figure 1). Such defects presumably block Li ions in the diffusion pathway. 7 Theoretical considerations and model calculations suggest that the most favourable defect occurring in LiMPO 4 (M= Fe, Mn, Co, Ni) olivines is the Li + /M 2+ interchange (anti−site defect), with an occurrence lower than 2%. 8 Hereby is the actual amount expected to largely depend on synthesis conditions especially the reaction temperature. Systems with M = Fe or Co, are expected to be especially prone to Li + /M 2+ interchange defects. 9 Whereas the influence of experimental conditions on structural defects for the end members LiMnPO 4 and LiFePO 4 of the solid solution series LiMn 1-x Fe x PO 4 have already been investigated in detail 8-12 much less is documented for mixed stoichiometries such as LiMn 1-x Fe x PO 4 . 13 In a very recent work Jensen et al. have investigated the structural nature of these lattice defects in LiFe 1-x Mn x PO 4 materials. 6 From combined X-ray diffraction (XRD), neutron-diffract...

High performance nano-sized LiMn1−xFexPO4cathode materials for advanced lithium-ion batteries

“…1e and Table S2 †). 28,30,31 Fe substituting usually happens in 4c-site which is occupied by Mn ion. However, the cation antisite defects could be arisen, so that a slight amount of Fe/Mn ions (<1%) could occupy 4a site, while Li ions occupy 4c site.…”

Recent Advances of Mn‐Rich LiFe1‐yMnyPO4 (0.5 ≤ y < 1.0) Cathode Materials for High Energy Density Lithium Ion Batteries

“…Majority of the previously structural studies on LiFe 1‐ y Mn y PO 4 solid solutions, have shown Fe substitution at the Mn site by a systematic decrease in the lattice parameters, that is, the lattice parameters linearly increases with increasing Mn content in the LiFe 1‐ y Mn y PO 4 solid solutions. In fact, in the preparation of metal phosphates (such as LiFePO 4 , LiMnPO 4 and LiFe 1‐ y Mn y PO 4 ), different synthetic methods or minor changes of experimental conditions in the preparation will result in different optimal Fe/Mn ratios and crystal structures.…”

“…The local valence states of Fe and Mn had tiny fluctuations around their standard valence values. It is worth pointing out that a recent work by Schoiber and co‐workers has shown that pure samples of olivine type LiFe 0.2 Mn 0.8 PO 4 prepared by a hydrothermal route in the presence of structure directing agent are largely free of defect occupation of Fe at the lithium sites, basing on a Rietveld analysis of X‐ray and neutron diffraction and 57 Fe‐Mössbauer spectroscopy. However, the sample with the same Li/Fe antisite defect was obtained if they used a similar synthetic strategy (without a structure directing agent) as previously reported .…”

Crystal and Morphology Design of Dittmarite-Type Ammonium Iron–Manganese Phosphates, NH4Mn1–xFexPO4·H2O, as Precursors for Phospho-olivine Electrodes

“…Whereas the antisite defects at the single LiFePO 4 and LiMnPO 4 have been widely studied theoretically and experimentally using diffraction and microscopy methods, much less data for defect chemistry of the mixed LiMn 1– x Fe x PO 4 compositions have been reported. − Based on theoretical calculations it has been predicted a higher Li/M migration energy for mixed LiMn 0.5 Fe 0.5 PO 4 compared to single LiFePO 4 suggesting that occurrence of antisite defects in the mixed compositions would have a greater blocking effect on lithium diffusion kinetics. For hydrothermally synthesized LiMn 1– x Fe x PO 4 ( x = 0.5; 0.75; 1), Jensen et al have established that only Li/Fe defects are present instead of Li/Mn defects .…”