A comparative study of Fd-3m and P4332 “LiNi0.5Mn1.5O4”
Research highlights
► High voltage spinel "LiNi0.5Mn1.5O4"are prepared and thoroughly characterized. ► The formation of the rock salt impurity is explained through a peritectoid phase transition and the electrochemical de-insertion mechanism was investigated. ► We present the electrochemical deinsertion mechanism and discuss it in relation with recent reports on the effect of structural strains.
Introduction
Within the present energetic context, mastering energy storage and conversion has become primordial. Lithium-ion batteries are leaders for the storage due to their high energy densities compared to other technologies. During the last years, much attention has been paid to the search for materials with improved performances in terms of power and energy densities [1]. Increasing this latter can be realized either through a capacity increase or by using a material which can be oxidized at higher voltage. Besides numerous studies, this remains a challenging task. As an example, Dominko et al. recently reported on the reactivity of Li2MnSiO4 from which an oxidation process with two-electrons per manganese was expected [2]. However, XAS measurements showed that the reaction proceeds only within the Mn(III)/Mn(II) couple and that the evolution was not fully reversible. Another hope comes from the possibility to use high potential spinel structures. Indeed, LiMn2 − xMxO4 (M = Ni, Cr, Fe, Co, Cu), showing a high voltage plateau at around 5 V and delivering a high capacity, have been reported as attractive cathode materials [3], [4], [5], [6], [7]. Among them, LiMn1.5Ni0.5O4 spinel is one of the most promising candidates with namely a high operating voltage at ~ 4.7 V versus Li+/Li, good cycling performances without Jahn–Teller effect related to the presence of Mn3+ and good thermal stability [8].
Two types of phases have been reported for LiNi0.5Mn1.5O4 [9]. A low temperature ordered structure, with P4332 space group, can be obtained by using synthesis processes under oxygen [10] or post-annealing in air below 700 °C [11]. In this structure the Li atoms are located at 8c sites, Ni atoms at 4a sites, Mn atoms at 12d sites, and O atoms at 8c and 24e sites. For preparation routes employing higher synthesis temperature (over 700 °C) the preparation of “LiNi0.5Mn1.5O4” sample with Fd-3m structure is generally accepted [12]. This “high temperature polymorph” is always accompanied with a rock salt impurity phase, which has been attributed either to NixO [5], LixNi1 − xO [13], or (LiNiMn)xO [14] and appearing concomitantly with oxygen deficiency within the spinel. It is thus abusive to really define it as a polymorph of LiNi0.5Mn1.5O4 since the stoichiometry is different from the ordered phase (oxygen vacancies and nickel content lower than 0.5). Within this structure, Ni and Mn atoms are randomly distributed in the 16d sites. Furthermore, this is a complicated system as the prepared spinel LiNi0.5Mn1.5O4 samples have been sometimes reported as a combination of the two phases [13] using both neutron and X-ray diffraction for characterization. Indeed, this latter is not sufficient to discriminate the two phases as the scattering factors of Ni and Mn are similar. Some extra peaks linked to a superstructure should in principle be distinguished for the ordered phase. These related peaks are usually of low intensity and difficult to evidence. Recently, an alternative to this two-phase model, namely the partial ordering of nickel within the P4332, was proposed [15].
The lithium deintercalation/uptake mechanisms within these two phases were also previously studied using ex-situ X-ray diffraction [9]. While delithiation of the cathode material with Fd-3m space group undergoes a two-phase transition, P4332 ordered phase react through a three-phase transition.
In the present work, we synthesized well defined and characterized samples which crystallized either with Fd-3m or P4332 space group. They were clearly identified using Raman and Fourier transform infrared (FTIR) spectroscopy and electrochemical properties. We thus report on their fine structural characterization together with their electrochemical properties. In a second step, we further compared these two phases through the in-situ study of the lithium extraction mechanism, as a first step to better understand the differences in their electrochemical behavior.
Section snippets
Synthesis
The Fd-3m and P4332 samples were obtained by using a ceramic route. Namely, stoichiometric amounts of lithium (AR, 99%, Alfa), manganese (Mn(NO3)2·4H2O, AR, 98%, Alfa) and nickel (Ni(NO3)2·6H2O, AR, 98%, Alfa) nitrates were dissolved in ethanol. After evaporation of the solvent under continuously stirring at 80 C, the resulting paste was preheated at 500 °C for 2 h, and further annealed at 800 °C for 8 h. From this temperature, the sample was cooled naturally within the furnace which was switched
Structure
The first step was to carefully characterize the structure, i.e. the space group, of the formed phases. The XRD diagrams are consistent with well crystallized samples (Fig. 1). The refinements in a full pattern matching mode, allowed determining the a cell parameter as 8.1713(1) Å, for C800 while it is 8.1688(1) Å for C600. As usually reported [14] we can notice that both samples have a very small amount of a rock salt phase (attributed to NixO or LixNi1 − xO or LixNiyMnzO) with Bragg Peaks at 2θ =
Conclusions
Since the demonstration of the electroactivity of LiNi0.5Mn1.5O4 at high voltage, numbers of papers have been reported, for instance on the effect of the preparation route on the electrochemical properties. However, it is still unclear as the synthesis route can play on different characteristics of the formed phase. In the present work, we prepared two samples with the same stoichiometry presenting with Fd-3m (C800) and P4332 (C600) phases. As usually reported these non-stoichiometric phases
Acknowledgments
Financial supports from CAS project (KJCX2-YW-W26) and NSFC project (50730005) are acknowledged. Liping Wang thanks French Government Scholarship (dossier no 2008741).
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