Article pubs.acs.org/JPCC
Li-Rich Li1+xMn2−xO4 Spinel Electrode Materials: An Operando Neutron Diffraction Study during Li+ Extraction/Insertion Matteo Bianchini,†,‡,§,∥ Emmanuelle Suard,§ Laurence Croguennec,†,∥,⊥ and Christian Masquelier*,‡,∥,⊥ †
CNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France Laboratoire de Réactivité et de Chimie des Solides, CNRS-UMR#7314, Université de Picardie Jules Verne, F-80039 Amiens Cedex 1, France § Institut Laue-Langevin, 71 Avenue des Martyrs, F-38000 Grenoble, France ∥ RS2E, Réseau Français sur le Stockage Electrochimique de l’Energie, FR CNRS#3459, F-80039 Amiens Cedex 1, France ⊥ ALISTORE-ERI, FR CNRS#3104, F-80039 Amiens Cedex 1, France ‡
S Supporting Information *
ABSTRACT: In situ neutron diffraction (ND) during battery operation is becoming a promising technique for the study of electrode materials in Li-ion batteries. We recently designed an electrochemical cell for operando ND studies and demonstrated that it can deliver powder patterns of good quality for Rietveld structural refinements. Herein we used such a cell to study the deintercalation process occurring in manganese spinels of general formula Li1+xMn2−xO4. Three samples with increasing Li/Mn ratio (x = 0, x = 0.05, and x = 0.10) were synthesized and measured on the D20 high-flux powder diffractometer at ILL. We found fundamental differences between the phase diagrams of the three samples, intimately related to their electrochemical features. Upon charge, the study revealed a sequence of two biphasic reactions for LiMn2O4 (with an intermediate phase of composition close to Li0.6Mn2O4), a solid solution followed by a biphasic reaction for Li1.05Mn1.95O4, and a full solid solution for Li1.10Mn1.90O4. Moreover, Rietveld refinement led to key parameters such as cell parameters, oxygen’s fractional atomic coordinates, and more importantly, lithium’s site occupancy factors, whose rate of variation is found to be related to the state of charge of the electrode.
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INTRODUCTION After 15 years of development, in situ neutron diffraction (ND) has finally become an important tool for the study of Li-ion batteries.1−5 The interest in such a technique lies in the properties of the neutron, whose atomic scattering mechanism allows being sensitive to lithium’s crystallographic information. Furthermore, neutrons are highly penetrating in matter, and they are a nondestructive probe, which makes them extremely valuable in the study of materials’ properties. In-situ ND during battery operation (commonly referred to with the Latin word operando) is becoming a technique of choice to provide significant results, as proved by the many different research groups that recently pushed the development of the technique.6−8 Not only reactor and spallation sources can now provide the necessary high neutron flux but also many different cells have been designed and produced to fulfill these studies. The technique can thus be applied to both commercial cells (for the study of the battery in its entirety, of failure mechanisms, fatigue, etc.9−13) and custom-made cells.6−8,14,15 In particular, we have recently built an in situ cell exploiting the properties of a neutron-transparent (Ti,Zr) alloy to obtain reliable electrochemical properties and ND patterns of high quality at the same time.6 This electrochemical cell allows charging/discharging high-loading electrodes (>200 mg of © 2014 American Chemical Society
powder) with small polarization and, importantly, to obtain good statistics on ND patterns that only show the Bragg peaks of the active electrode of interest. Rietveld structural refinements and extraction of the important physical/chemical parameters of the material are thus possible and reliable.6 In this work we present the use of such a cell to study the lithium extraction mechanisms from three manganese-containing spinels of compositions Li1+xMn2−xO4 (x = 0, 0.05, and 0.10). These are well-known positive electrode materials for Liion batteries despite serious capacity retention issues encountered in particular for stoichiometric LiMn2O4.16−21 Thorough control of synthesis conditions of these apparently simple oxides was revealed to be extremely important, and a slight departure from the original oxidation state of manganese (+3.5) toward the value +4 showed to dramatically bypass the capacity retention issues, at the expense of the theoretical capacity.22,23 Two main series of compositions were proposed by Thackeray19,22 and others:20,21,24,25 Li-rich stoichiometric spinels Li1+xMn2−xO4 (0 ≤ x ≤ 0.33) and cation-deficient spinels Li1−xMn2−2xO4 (0 ≤ x ≤ 0.11). Received: September 6, 2014 Revised: October 9, 2014 Published: October 17, 2014 25947
dx.doi.org/10.1021/jp509027g | J. Phys. Chem. C 2014, 118, 25947−25955
The Journal of Physical Chemistry C
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Table 1. Electrochemical and Crystallographic Characterization of Pristine Materials Li1+xMn2−xO4 (x = 0, 0.05, 0.10) from XRD, ND, and Galvanostatic Cyclinga pristine material S1: LiMn2O4 S2: Li1.05Mn1.95O4 S3: Li1.10Mn1.90O4
a = b = c (Å)
refined x (ND)
Mn oxid. state (calc.)
XRD: 8.2484(2) ND: 8.2487(1) XRD: 8.2307(4) ND: 8.2355(1) XRD: 8.2188(3) ND: 8.2218(1)
0.00(7)
+3.5
Li0.04Mn2O4/Li0.0Mn2O4
0.05(8)
+3.56
Li0.19Mn1.95O4/Li0.2Mn1.95O4
0.10(8)
+3.63
Li0.4Mn1.90O4/Li0.4Mn1.90O4
measured/expected fully oxidized composition
a Statistical errors are reported (3σ). For sample S1 the refined value of x is different than in Figure 1 because in that case oxygen’s SOF is also refined.
Diffraction experiments were initially performed using X-rays produced within a Bruker D8 diffractometer (Cu Kα1,2 wavelength, Bragg−Brentano geometry). Neutron diffraction was subsequently carried out on the D2B high-resolution neutron diffractometer at the Institut Laue-Langevin (ILL, Grenoble, France). Powder samples were put in cylindrical vanadium cans and measured in transmission geometry at λ = 1.5944 Å (calibrated using Na2Ca3Al2F14 as a reference). In situ (operando) experiments were performed on the D20 high-flux neutron diffractometer at ILL. For each of the three samples, a battery was prepared in our in situ cell.6 The electrode preparation followed the procedure described above, but with high loadings of about 200 mg. Standard electrolytes were replaced by a deuterated version, prepared with 1 M LiPF6 in d-EC:d-DMC (1:4 wt %), which had also been previously tested.6 No lithium enrichment was performed; i.e., lithium with (nominal) natural isotope abundance was used for all battery components. The batteries were measured operando continuously on the D20 diffractometer (λ = 1.5471 Å) in transmission geometry. Every pattern was acquired for 30 min while charging the battery at C/20 rate up to 4.5 V vs Li+/Li (4.6 V for the sample x = 0), corresponding to a variation of 0.025 Li/pattern. Data treatment and Rietveld refinements were performed using the FullProf Suite.39 We wish to emphasize that for every sample each operando ND pattern has been independently treated (i.e., no sequential refinements possible) due to the fact that lithium extraction results both in a decrease of the sample’s absorption (lithium is a neutron absorber) and in an increase of the background due to organic components of the battery (deuterated electrolyte) and other scattering elements (separator, carbon). It is important to mention that the variable absorption has been dealt with by using a decreasing μr coefficient, starting from about 0.5 for the pristine samples and decreasing by 0.0125/pattern to reach a value between 0.1 and 0.2 at the end of the charge, depending on the sample. For sample S1 (Table 1), which generates different phases upon electrochemical cycling, atomic displacement parameters (ADPs) were refined for the pristine, final, and intermediate compositions and then kept constant within the different domains during charge. For samples S2 and S3, which react through significant solid solution regions, a fixed ADP could not take into account the modifications occurring in these domains, and thus a Boverall factor was refined for every pattern. For S2 we observed a decrease of Boverall from 1.46 to 0.50 and for S3 from 1.26 to 0.53. One should note that although considering ADPs/Boverall is necessary to account for the data, it is detrimental for the value of the error bars obtained from refinement. This is particularly true for site occupancy factors (SOFs) since the parameters can be correlated. Moreover,
The interest in the manganese spinel class of materials lies also in the fact that it possesses an extremely rich set of physical and chemical properties, at the origin of many debates and doubts in the scientific community, not all clarified yet. Typical examples include the presence of two electrochemical features at 3.3 and 4.6 V vs Li+/Li26−28 or the role played by a possible oxygen under stoichiometry.24,25,29,30 In addition, the crystal chemistry of lithium manganese spinels is extremely varied, with phase transitions at various temperatures31 and for different composition ranges.16 We focused here on the family of Li-rich Li1+xMn2−xO4 (0 ≤ x ≤ 0.33) compositions, for which the manganese oxidation state can be increased from 3.5+ to 4+ by substituting part of the manganese in the 16d crystallographic sites with lithium. This reduces the usable capacity but also eliminates significantly capacity fading upon cycling thanks to the lower amount of Mn3+ (which is at the origin of cooperative Jahn−Teller distortion and/or disproportionation to yield soluble Mn2+ species32−34). Few studies have addressed the mechanism of Li+ extraction/insertion from these compositions by means of ex situ and in situ X-ray diffraction,35−38 none of them in a systematic way and for comparison purposes for different Li/ Mn ratios. We therefore decided to perform an operando neutron diffraction study during the extraction of lithium from Li1+xMn2−xO4 spinels, with the goal of giving quantitative information about the different phases involved and with special attention being paid to lithium’s site occupancy factors (SOFs). To this end, three samples were synthesized, of nominal stoichiometries LiMn2O4 (x = 0), Li1.05Mn1.95O4 (x = 0.05), and Li1.10Mn1.90O4 (x = 0.10).
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EXPERIMENTAL SECTION Three samples of the spinel positive electrode material Li1+xMn2−xO4 were prepared for x = 0, x = 0.05, and x = 0.10. To this end, as described in ref 24, stoichiometric amounts of (1 + x)/2Li2CO3 and (2 − x)MnOOH were carefully mixed, heated at 400 °C for 2 h, cooled, and mixed at RT prior to being heated for 20 h at 800 °C in air and rapidly cooled (∼10 °C/min) down to RT. For electrochemical testing, the active materials were mixed with carbon Super P in 90:10 wt % ratios and ground in a mortar. No binder was added to the mixture. Swagelok-type cells were used to prepare lithium half cells. Glass fiber separators (Whatman) and LP30 electrolyte (Aldrich) were used. The half cells were cycled at C/20 rate between 3.2 and 4.5 V vs Li+/Li, using small amounts of powders (∼20−25 mg) as positive electrodes. An ICP-OES spectrometer (Varian 720ES Optical Emission Spectrometer) was also used to obtain the Li/Mn ratio after complete dissolution of the samples into a hydrochloric acid solution. 25948
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Figure 1. Top: Rietveld refinement of neutron diffraction data recorded using the D2B high-resolution diffractometer at ILL for LiMn2O4 (left), Li1.05Mn1.95O4 (center), and Li1.10Mn1.90O4 (right). Bottom: Respective voltage−composition curves recorded vs Li+/Li during galvanostatic cycling at the C/20 rate (left) and their inverse derivative curves (right), showing the two electrochemical features typical of spinel materials.
Rietveld refinements of the crystal structures of the three pristine samples were performed from the ND data, using the structures reported in the literature as initial models (ICSD 40485 and ICSD 16674640,41). LiMn2O4 crystallizes in the Fd3̅m space group, with Mn in octahedral 16d sites and oxygen in close-packed 32e sites, building a framework of MnO6 edgesharing octahedra. Lithium is found in the 8a tetrahedral position. The substitution of Li for Mn in Li1+xMn2−xO4 occurs on the 16d site, therefore generating Li+ in the octahedral oxygen environment,19 written as Li8a[LixMn2−x]16dO4. Our Rietveld refinements confirmed that the oxygen’s fractional atomic coordinates are close to x = y = z = 0.2368(2) for all the samples. Atomic displacement parameters (ADPs) and, more importantly, lithium−manganese exchange ratios on the 16d site were also refined. Table 1 reports the obtained values, in good agreement with nominal compositions. For sample S1 we also tried to refine oxygen’s site occupancy factors (SOFs) together with Li−Mn exchange, but this did not result in any significant deviation from the full occupancy of the 32e site, confirming the good stoichiometry of the sample. To further prove this, we verified that S1 undergoes a structural transition from cubic to orthorhombic symmetry below 285 K, due to charge ordering, as previously described31,42,43 (Figure S1, Supporting Information). Contrary to what was reported by Berg-Thomas and Sharma,10,44 we did not find any occupation of the 16c site by Li+. In the case of Sharma et al. in particular, an in situ ND study is reported, but comparison with our data is difficult since they provide no information on the sample in its pristine form, received as deintercalated and characterized as [Li0.11(6)]8a[Li0.23(11)]16cMn2O4. Inspection of the reported cubic cell parameter reveals that upon cycling it never exceeds
lithium and manganese have poor contrast for neutrons (they have both negative scattering lengths, bLi = −1.90 fm and bMn = −3.75 fm), which explains why error bars for SOFs are far more important than for every other parameter. For this reason such error bars will often be reported with two digits.
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RESULTS AND DISCUSSION Pristine Materials. The three samples were initially characterized in their pristine form. The neutron diffraction patterns collected using the D2B high-resolution neutron diffractometer at ILL are gathered in Figure 1. The obtained cell parameters are reported in Table 1, in remarkable agreement with those found in the literature19,24,25 when lithium substitutes part of manganese with concomitant oxidation of the remaining Mn: from X-ray diffraction we obtain a = 8.2484(2) Å for LiMn2O4 (Sample S1), a = 8.2307(4) Å for Li1.05Mn1.95O4 (S2), and a = 8.2188(3) Å for Li1.10Mn1.90O4 (S3). It should be kept in mind that since the oxidation state of manganese increases with increasing x in Li1+xMn2−xO4 the total amount of lithium that can be extracted from the spinel structure decreases accordingly. More precisely, simple calculations show that the total amount of extractable lithium decreases as 1 − 3x (0 ≤ x ≤ 0.33), and therefore the amount of lithium that remains in the structure at the end of charge increases as 4x (i.e., reaching the fully oxidized MnIV compositions Li0Mn2O4 for sample S1, Li0.2Mn1.95O4 for S2, and Li 0.4 Mn 1.90 O 4 for S3). The values we observed experimentally from galvanostatic electrochemical Li+ extraction agree with these considerations and are reported in Table 1. The decrease of cell parameters and of Mn−O distances (Figure 1) is also in good agreement with an increase in the manganese oxidation state. 25949
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a = 8.218 Å, which is typical of nonstoichiometric spinels as discussed above (Li−Mn exchange for example). The nonstoichiometry of the sample of Sharma et al. could also explain the fact that they observe the (dis)charge as a solid solution, while on the contrary we observe biphasic domains for the stoichiometric sample S1 (Li/Mn = 1/2). The galvanostatic electrochemical data, gathered in Figure 1, show the expected voltage−composition features, i.e., two “plateaus” located around 4.05 and 4.15 V vs Li+/Li. Besides the marked differences in first charge capacities (discussed above), the dissimilarity between the three samples stands in the shape of the derivative curves (Figure 1). Both “plateaus” give rise to peaks that are strongly composition-dependent in intensity (as expected from the different capacity) and shape, indicating a possible different lithium extraction mechanism. As a general trend, peaks are sharper for S1, and they become broader and broader from S2 to S3, i.e., as the average oxidation state of Mn in the pristine sample increases, indicating the possible change of the transition nature from first order to second order. Operando Neutron Diffraction during Li+ Extraction. After the above-described characterizations of the samples’ composition and cation distribution between different sites, operando neutron diffraction experiments upon charge were performed in a systematic way. A lithium half cell was prepared in our in situ cell and charged at C/20 rate while recording ND patterns continuously every 30 min. Figure 2 (and Figure S2, Supporting Information) is reported to give an overall idea of the quality of the ND data collected operando during charge (Li+ extraction) from the three samples. A great advantage of ND that is readily obvious from these figures is the possibility to exploit high angular range regions (up to 120°) that allow clearly separating and visualizing peak shifts. The delithiation of sample S3 seems to result in a smooth peak shift from lower to higher angular 2θ values, while sample S2 displays a similar behavior only until the voltage composition plateau at 4.15 V vs Li+/Li is reached. There, a biphasic reaction occurs since the initial peaks disappear and new ones at higher angles concurrently take their place. Sample S1 also shows a a twophase reaction associated with the “plateau” at 4.15 V vs Li+/Li, as well as a second one at ∼4.05 V, better visualized through the derivative curve of Figure 1. All these “visual” considerations have been carefully analyzed by means of Rietveld refinements. Details about the refinement procedure are given in the Experimental Section. Figure 3 summarizes the obtained phase diagrams (vs y, global Li content) as gathered from Rietveld refinement of our operando ND data, subsequently discussed in more detail in the second part of this paper for each of the three samples. Phase transitions are highlighted, and the corresponding miscibility gaps can be evaluated. 2.1. Li+ Extraction from Li1.10Mn1.90O4 (Sample S3). In Li1.10Mn1.90O4, Mn is in the average oxidation state +3.63, and therefore only 0.7 Li+ can be theoretically extracted, leading to the final composition Li0.40Mn1.90O4 ([Li0.30]8a[Li0.10Mn1.90]16dO4). The use of 200 mg of positive electrode at the C/20 rate resulted in the effective extraction of 0.6 Li+, satisfactory but nonetheless incomplete. We indeed observed that part of the pristine powder (∼10−15%) did not react; while the main diffraction peaks of the active material were shifting from the original position to higher angular positions, a shoulder remained fixed close to the original position (Figure S3, Supporting Information). All ND patterns were therefore refined keeping a “pristine-like” inactive phase.
Figure 2. Contour plot of neutron diffraction patterns recorded operando upon charge (Li+ extraction at C/20 rate) for LiMn2O4 (top), Li1.05Mn1.95O4 (middle), and Li1.10Mn1.90O4 (bottom). Focus on the 90°−120° 2θ angular range. Every scan was recorded for 30 min; therefore, the scale on the y axis can be read as scan number but also as time (hours), divided by 2.
The relative weight ratio of the two phases is shown in Figure 4, with all important refined parameters. It should be mentioned that the phase quantification is possible since the different phases have the same symmetry, contain approximately the same chemical elements, and have the same morphology, but it is nonetheless semiquantitative (no reference material is present in the sample). The cubic unit-cell parameter of the main phase decreases from the initial value of a = 8.2225(4) Å to a final one of a = 8.0776(3) Å, i.e., a 5% unit cell volume shrinkage from 555.93(4) to 527.04(4) Å3, in a solid solution manner. The value of the unit-cell parameter of the final member confirms the presence of remaining lithium in the structure (λ-MnO2 was reported to be cubic with a = 8.03 Å45). The oxygen x = y = z fractional atomic coordinate was also refined, revealing a weak dependence on the state of charge. Indeed, it only increased from 0.2362(6) to 0.2370(5). This will remain in general true 25950
dx.doi.org/10.1021/jp509027g | J. Phys. Chem. C 2014, 118, 25947−25955
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Figure 3. Phase diagram and observed miscibility gaps for LiMn2O4 (top), Li1.05Mn1.95O4 (middle), and Li1.10Mn1.90O4 (bottom). In red is given the y value at the end of the in situ charge. For every sample, two bars are reported to account for the bias given by the unreacting part of the electrode. The top bar represents the observed phase diagram and miscibility gaps, while the bottom one represents those corrected considering the full capacity Ctheor. of the materials (i.e., every observed composition interval is divided by the ratio Cobtained/Ctheor.,
