Article pubs.acs.org/cm
Relationships between Mn3+ Content, Structural Ordering, Phase Transformation, and Kinetic Properties in LiNixMn2−xO4 Cathode Materials Hugues Duncan,†,‡ Bin Hai,§ Michal Leskes,∥ Clare P. Grey,∥ and Guoying Chen*,† †
Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Kinestral Technologies, Inc., 400 E. Jamie Court, South San Francisco, California 94080, United States § Chery Auto Company, Wuhu, Anhui Province China ∥ Chemistry Department, Cambridge University, Cambridge CB2 1EW, United Kingdom ‡
ABSTRACT: Micrometer-sized LiNixMn2−xO4 (0.3 ≤ x ≤ 0.5) single crystals with (111) surface facets were synthesized and characterized by 6Li magic angle spinning nuclear magnetic resonance, Fourier transform infrared spectroscopy, and electrochemical studies. All three techniques were sensitive to cation disorder and the corroborated results showed that structural ordering improves with x. The transition from the ordered to the disordered spinel was triggered by an increase in Mn3+ content, which was accomplished either by a change in chemical composition or postsynthesis thermal treatment. Disordering led to increased solid solution behavior, reduced two-phase transformation domains, and improved transport properties during Li extraction and insertion. Further increasing Mn3+ content in already disordered structure extends the solid solution domain and eliminates the presence of phase II; however, this has limited effect on rate capability. The study demonstrates the dominant role of structural ordering in morphology-controlled LiMn1.5Ni0.5O4, and it reveals that the kinetic significance of Mn3+ lies in its ability in triggering structural disordering. The rate performance of the spinels is not directly proportional to the Mn3+ content or the domain size of solid solution transformation in samples where two-phase transition is also present.
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INTRODUCTION
literature, which greatly contributes to the variation in reported experimental results. Highly disordered LMNO is typically obtained by solid-state synthesis at temperatures ranging from 400 to 900 °C and then followed by fast cooling. At temperatures above 700 °C, the formation of rock-salt-type impurities with proposed compositions of NiO and Li 1−x Ni x O in one study, 9 and (LixMn2/3Ni1/3)yO in a second,10 has also been reported. A significant amount of Mn3+ characterized by a plateau at 4.1 V on the charge/discharge voltage profile is often associated with disordered spinels, which may be a result of Ni/Mn offstoichiometry, and possibly oxygen vacancy formation. No evidence for oxygen vacancies was, however, found in a recent careful neutron diffraction study, and instead the generally observed Mn3+ in the disordered spinel phases was shown to be compensated by the (LixMn2/3Ni1/3)yO rock-salt phase.10 Highly ordered LMNO, on the other hand, is often obtained by post synthesis annealing near 700 °C followed by slow cooling.11,12 Cation ordering was found to be important in
There is an increasing demand for high-energy and high-power lithium batteries for applications in electric vehicles (EV) and plug-in electric vehicles (PHEV). While spinel LiMn2O4 operates at a voltage near 4.1 V, Ni-substituted LiNixMn2−xO4 (x ≤ 0.5) has an increased average voltage that is proportional to the Ni content.1−3 The much studied LiMn1.5Ni0.5O4 (LMNO) has a voltage plateau at 4.7 V and a high capacity of 147 mA h g−1, making it promising for high-energy applications. The complexity of the material, however, has demanded focused research on the relationships between performance and physical properties, such as particle microstructure, surface facets, impurities, Mn3+ content, and crystal structure.4−7 Depending on the distribution of the Ni and Mn atoms, LMNO can assume two crystal structures. In the ordered structure (space group P4332), Ni and Mn atoms occupy the 4a and 12d sites, respectively, while in the disordered structure (space group Fd3m ̅ ), both types of the atoms are randomly distributed in the octahedral 16d sites.8 In practice, however, perfectly ordered or disordered spinels are nearly impossible to synthesize. The terms of “ordered spinels” and “disordered spinels”, therefore, are not well-defined in the © XXXX American Chemical Society
Received: July 16, 2014 Revised: September 3, 2014
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spinning nuclear magnetic resonance (6Li MAS NMR) spectra were acquired on a 200 Bruker AVANCEIII 4.7T magnet using a 1.8 mm probe designed by Ago Samoson. A rotor-synchronized Hahn echo was used at a spinning frequency of 30 kHz, radio frequency amplitude of 83 kHz, and relaxation delay of 0.5 s. All spectra were referenced relative to a 1 M 6LiCl solution (set at 0 ppm) and fitted using the DMFIT software.17 Fourier transform infrared spectroscopy (FTIR) measurements were performed on KBr pellets using a Nicolet 6700 spectrometer in transmission mode with a spectral resolution of 4 cm−1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out on a simultaneous thermal analyzer (STA 449 F3, NETZSCH) under static air or Ar atmosphere. Typical sample loading in the platinum pan is 20 mg, and the data were collected between 30 and 1000 °C at the same heating and cooling rate of 3 °C/min. Composite electrodes were prepared by mixing 80 wt % of LiNixMn2−xO4 crystals, 10 wt % of polyvynilidenefluoride (PVdF) (Kynar 2801, Elf Atochem North America, Inc.), 5 wt % SFG-6 graphite (Timcal Ltd., Graphites and Technologies), and 5 wt % acetylene carbon black (Denka) in N-methyl pyrrolidone (NMP, EMD Inc.). The slurry was spread onto an aluminum foil current collector and dried at 100 °C under vacuum overnight. Cathode disks with an area of 1.6 cm2 were cut from the electrode sheets and assembled into 2023-type coin cells in an argon-filled glovebox (O2 < 1 ppm, H2O < 1 ppm). Lithium foil (Alfa-Aesar) was used as counter and reference electrodes, Celgard 2400 polypropylene membrane as separators, and 1 M LiPF6 in 1:1 ethylene carbonate (EC):diethylene carbonate (DEC) (Novolyte Technologies Inc.) as electrolyte. The cells were galvanostatically cycled between 3.0 and 5.0 V at various rates, using a multichannel cycler from Maccor Inc. In situ XRD studies were carried out at beamline 11-3 at the Stanford Synchrotron Radiation Lightsource (SSRL). Prior to cell assembly, a 1 mm sized hole was drilled through the center of the 2032 coin cell parts and Ni electrical leads were attached to both negative and positive ends. Binder and carbon free electrodes were prepared by pressing the crystals onto an Al current collector and then assembled into the coin cells. 1 M LiPF6 in EC:DEC was used as electrolyte and glass-fiber membranes (Whatman) as separators. The hole in the center was then covered by Kapton tape to conceal the cell components, which was followed by heat-sealing the entire coin cell into a pouch. The cells were cycled at C/10 rate in the voltage range from 3.0 to 4.9 V. After data collection, the 2θ scale was converted to the corresponding angles with λ = 1.5406 Å (wavelength of Cu Kα radiation) and the data were further analyzed by the Area Diffraction Machine software developed at SSRL.
influencing performance, with most reports concluding that the disordered structure delivers better rate capability and longterm stability,13,14 although higher rate capability in the ordered structure was also reported.15 It was proposed that the better performance in the disordered structure is related to the presence of the Mn3+ in the structure, which results in higher electronic conductivity and lithium diffusivity. The independent role of ordering/disordering and Mn3+ in kinetics is difficult to assess as they are often coupled in conventionally prepared samples. Moreover, our recent work demonstrated that particle morphology has a predominant role in the kinetic behavior of LMNO. Despite having a more ordered structure, a lower Mn3+ content, and significantly larger two-phase regions in the phase diagram, octahedron-shaped particles with (111) facets delivered better rate capability and had a much larger chemical diffusion coefficient than did the plates with prominent (112) facets.4 Similar results were reported by Manthiram and coworkers in recent publications.16 Their studies also showed that solid-state synthesized spinels, both ordered and disordered, are commonly accompanied by rock-salt impurities that reduce electrode capacity. Here, we report controlled studies performed on well-formed single crystals with the same morphology, surface facets, and nearly identical surface area for the first time. Unlike the conventional solid-state synthesis, the use of liquid reaction medium in the molten salt method allows us to eliminate the formation of rock-salt impurities and grain boundaries in the samples and, hence, ambiguity in the results. As oxygen vacancies are inherently difficult to control and characterize, we first vary the Mn3+ content by changing the Ni/Mn ratio in the LiNixMn2−xO4 (0.3 ≤ x ≤ 0.5) series, mimicking the scenario where Mn3+ in LMNO is created by off-stoichiometry through the extrusion of secondary rock-salt phases. Careful thermal studies were then performed on the series to probe the processes of structural ordering, oxygen loss, and impurity formation. We found that structural ordering was perturbed by both chemical composition and thermal treatment. The intricate relationships between ordering and Mn3+ content, as well as their effects on phase transitions and electrode performance, are revealed in this study.
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EXPERIMENTAL SECTION
RESULTS AND DISCUSSION Characterization of Mn3+ Content and Ordering in the Spinels. Chemical compositions of the LiNixMn2−xO4 (0.3 ≤ x ≤ 0.5) single-crystal samples synthesized by the molten salt method were carefully verified by inductively coupled plasma analysis. Scanning electron microscopy studies showed that all crystal samples adapted the same octahedron shape with an average size of 2 μm similar to the octahedrons obtained in our previous report.4 Figure 1a compares the XRD patterns of the samples. No impurities were detected and all peaks were indexed to the cubic spinel structure with the space group Fd3̅m. The peaks shifted toward higher angles as x increases, corresponding to a smaller lattice at higher Ni content. The lattice parameters obtained from full-pattern Rietveld refinement, as well as the theoretical Mn3+ content in the samples, are plotted as functions of Ni content x in Figure 1b. Consistent with previous reports, decreasing Ni2+ content increases the Mn3+/Mn4+ ratio, leading to a slight expansion in the cubic lattice.18 This is a direct result of the differences in the ionic radii of octahedral-coordinated Ni2+, Mn3+, and Mn4+, which are 0.83, 0.785, and 0.67 Å, respectively. At x ≤ 0.45,
LiNixMn2−xO4 (0.3 ≤ x ≤ 0.5) single crystals were synthesized by a molten salt method previously described.4 Stoichiometric amounts of Ni(NO3)2·6H2O and Mn(NO3)2·4H2O (Sigma-Aldrich, 97%) were dissolved in a minimum amount of deionized water and then added to a eutectic mixture of LiCl−KCl flux (molar ratio = 0.59:0.41). The molar ratio between the flux and the total transition metals (denoted as R) was fixed at 50. The mixture was ground in a mortar, dried at 180 °C for 3 h, and then milled in a mixer/mill (8000M, SPEX SamplePrep) for 30 min. After transferring to a covered alumina crucible, the reaction mixture was heated to 650 °C at a rate of 3 °C/ min, held at 650 °C for 8 h, and then cooled to room temperature at a rate of 1 °C/min. The final product was thoroughly washed with warm deionized water to remove the flux salts and then dried at 60 °C in a vacuum oven overnight. To disorder the sample with x = 0.5, postsynthesis treatment was performed by heating the sample at 720 °C for 5 min and then cooling to room temperature at 3 °C min−1 in air. X-ray diffraction (XRD) patterns were collected between 10° and 80° (2θ) using a Panalytical X’Pert Pro diffractometer equipped with monochromatized Cu Kα radiation. The scan rate was 0.0025°/s and the step size was 0.01°. Lattice parameters and phase ratios in the samples were determined by full-pattern Rietveld refinement using Riqas software (Materials Data, Inc.). Solid-state 6Li magic angle B
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Figure 1. (a) XRD patterns of LiNixMn2−xO4 (0.3 ≤ x ≤ 0.5) crystal samples synthesized by a molten salt method and (b) the relationships between Ni content (x), lattice parameters, and theoretical Mn3+ content in the structure.
near linear relationships between x and both lattice parameter and Mn3+ content were observed, suggesting that lattice parameters may be used to estimate the Mn3+ content in this region. As the differences in the X-ray diffraction from P4332 and Fd3̅m structures are inherently subtle, powder XRD is known to be insensitive in detecting cation ordering.19 Additional techniques, therefore, were explored to further evaluate the crystal structure of the spinels. 6 Li MAS NMR is well-known for its sensitivity to the local environments surrounding Li atoms. The lithium NMR shifts of these paramagnetic compounds are mainly affected by the Fermi contact interactions with the unpaired electrons of the transition metals in the Li ions local coordination shell.20 For the perfectly ordered LiM1.5Ni0.5O4 spinel, the Li environment contains 3 Ni2+ and 9 Mn4+ cations in the first cation coordination shell (namely Li(ONi2+)3(OMn4+)9, this notation indicates that the 12 cations (M) are connected to Li via Li− O−M pathways), giving rise to a single resonance in the 6Li NMR spectrum. In contrast, a random distribution of Mn and Ni atoms in the disordered structure can lead to an increased number of Li environments and consequently more complex spectra.21 A single peak at 1040 ppm was observed here for the x = 0.5 sample (Figure 2), indicating the presence of a near perfectly ordered structure. The presence of Mn3+ shifts the resonances to lower ppm values. For example, in the case of LiMn2O4 spinel, where the Li nuclei are surrounded by both Mn3+ and Mn4+ ions (with fast electron hopping resulting in an average oxidation state of Mn3.5+), a 6,7Li resonance of approximately 510 ppm is observed. Figure 2 shows the
Figure 2. 6Li MAS NMR spectra of LiNixMn2−xO4 crystal samples.
NMR spectra collected for the whole LiNixMn2−xO4 series. The 1040 ppm peak dominates the spectra of the x = 0.5, 0.45, and 0.4 samples with a slight shift for the x = 0.4 sample. This shift can be due to changes in transition-metal ordering in the second coordination shell of the Li ions or due to small variation in the MAS frequency between samples leading to differences in the sample temperature and therefore the Fermi contact shift. A second peak is observed at 940 ppm, which increases in intensity from x = 0.45 to x = 0.4. A third weak peak at 840 ppm is seen for x = 0.4. These peaks are likely a result of Mn3+ inclusion in the Li first cation coordination sphere. Further reducing the Ni content led to a shift in the center of gravity toward lower frequency and an increase in the resonance complexity, both of which suggest reduced cation ordering in the structure. The spectra for x = 0.3 and 0.35 are radically different from those of x = 0.4, 0.45, and 0.5. A broad line shape consisting of multiple peaks centered at 900 ppm indicates that multiple Li+ environments are present, typical of a highly disordered structure. The samples, therefore, can be divided into two structural groups: cation ordered (x = 0.4, 0.45, and 0.5) and disordered (x = 0.3 and 0.35). FTIR has been previously used to distinguish the ordered P4332 and disordered Fd3m ̅ structures in the spinel. The former has a lower symmetry, which results in eight well-defined absorption bands at 432, 468, 478, 503, 557, 594, 621, and 648 cm−1.22 The spectrum from the disordered, on the other hand, C
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is broader with the peaks at 432, 557, and 648 cm−1 largely absent.23 The bands at 594 and 621 cm−1 were attributed to Ni−O and Mn−O vibrations, respectively, and their intensity ratio has been previously used as a qualitative measure for transition-metal ordering in the spinel.24 Figure 3 compares the
Figure 3. FTIR spectra of LiNixMn2−xO4 crystal samples.
FTIR spectra of the LiNixMn2−xO4 crystal samples. With decreasing x, Ni2+ is replaced by the smaller sized Mn3+, which leads to stronger Ni−O bonds and therefore a blue shift of the peak at 594 cm−1. The peak ratio of 594/621 also decreased, consistent with reduced ordering at lower Ni content. Peaks at 432, 557, and 648 cm−1 were only present when Ni content was above 0.40, and overall broader features were observed in the spectra from the samples with x ≤ 0.35. These results confirm that ordering/disordering transition in the spinel is influenced by chemical composition, and ordering in LiNixMn2−xO4 improves with x. Further structural assessment was achieved by electrochemical studies. Composite electrodes prepared from the LiNixMn2−xO4 crystals were assembled in coin cells and galvanostatically cycled between 3.0 and 5.0 V. Figure 4a shows the typical integrated capacity and voltage (dQ/dV) profiles. As previously reported, the peaks around 4.7 V can be attributed to the Ni2+/Ni3+ and Ni3+/Ni4+ redox couples and the broad peak centered at 4.1 V to the Mn3+/Mn4+ redox couple (inset). The separation between the Ni redox couples was determined to be 20, 21, and 27 mV for x = 0.5, 0.45, and 0.4, respectively, which increased to 60 mV for x = 0.35 and 63 mV for x = 0.3. These values are consistent with the reported values of 20 and 60 mV for the ordered and disordered LMNO, providing further evidence that the transition from the highly ordered to highly disordered occurred rapidly in a narrow composition window of 0.35 < x < 0.4. The amount of Mn3+ in the samples was estimated by peak integration of the Mn3+/Mn4+ redox couple at 4.1 V, which produced 19.3, 15, 9, 4, and 1.5% for x = 0.3, 0.35, 0.4, 0.45, and 0.5, respectively. Considering the relatively large error involved in this method, the values are in good agreement with the calculated theoretical content of 23.5, 18.2, 12.5, 6.45, and 0%. Figure 4b compares the relationships between x, the estimated Mn3+ content, and the voltage gap (ΔV) of the Ni redox peaks determined from the dQ/dV profiles. While the Mn3+ content exhibits a near linear relationship with x, changes in ΔV are relatively small in the regions where x is below 0.35 or above 0.4. This suggests that the order/disorder transition is
Figure 4. (a) dQ/dV profiles of LiNixMn2−xO4 crystal samples, (b) the relationships between Ni content (x), ΔV of the Ni redox peaks, and Mn3+ content estimated from the dQ/dV profiles, and (c) rate capability comparison of LiNixMn2−xO4 crystal samples.
associated with a narrow Mn3+ content window, and structural ordering is somewhat insensitive to the Mn3+ content within the ordered and disordered domains. The transport properties of the LiNixMn2−xO4 crystals were compared in the varying-rate cycling of half-cells between 3.0 and 5.0 V, and the results are shown in Figure 4c. The disordered spinels consistently delivered higher capacities than the more ordered ones, clearly demonstrating the critical role of structural ordering in kinetics. Among the three ordered spinels, the sample with x = 0.5 and the lowest Mn3+ content showed the best performance at nearly all rates tested, suggesting that transport properties are not directly proportional to the Mn3+ content. It is worth noting that, on the dQ/ dV profiles, the redox couples at 4.7 V were largely asymmetric in x = 0.3 and 0.35, with significant peak broadening observed in the lower voltage peak. This suggests that the phase D
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secondary phases. For the ordered samples with x = 0.40, 0.45, and 0.50, an additional peak corresponding to structural disordering was also observed at 720 °C, providing evidence that order/disorder transition is initiated by the change in Mn3+ content in the structure. This was observed as sharp endothermal peaks at 720 °C on the heating DSC traces (Figure 5b), suggesting that structural disordering in the spinels is a first-order transition.26 Structural reordering, however, was not observed on the cooling DSC traces, likely due to the relatively fast cooling rate used in our experiments. The reversible loss of oxygen between 700 and 950 °C in all samples was further shown as broad peaks on both heating and cooling DSC traces. In addition, a pair of sharp peaks centered at 830 °C on heating and 810 °C on cooling was observed, suggesting the presence of an additional reversible process. This was attributed to the extrusion/incorporation of the cubic rock-salt phases with space group Fm3̅m, traditionally assigned as NiO or LixNi1−xO. Recent studies, however, indicated that Mn may also be present in the structure.10 Figure 5c compares the DSC profiles during heating and cooling of the sample with x = 0.5 in both air and Ar. On the heating trace collected under Ar, both peaks from oxygen loss and structural disordering shifted nearly 50 °C toward lower temperature, suggesting the close association of the two processes as well as their sensitivity to the oxygen partial pressure. In the absence of oxygen, the weight loss is irreversible as the sample decomposes upon heating to 1000 °C. The study reveals the important role of Mn3+ content in regulating the order/disorder transition and rock-salt formation. LMNO remains ordered at low Mn3+ content but disorders when Mn3+ increases to a critical level. Further increasing Mn3+ leads to the exclusion of Ni-rich rock-salt phases and transition-metal nonstoichiometry in the spinel phase, as LMNO with the Ni/Mn ratio of 1:3 can no longer accommodate the large amount of Mn3+ in the structure. LiNi x Mn 2−x O 4 spinels with x < 0.5 are capable of accommodating a higher Mn3+ content and, therefore, less likely to be contaminated by rock-salt impurities during synthesis, an observation that was recently made by Song et al.16 The involvement of nonstoichiometry in transition metals and possibly oxygen, however, largely complicates the relationship between Mn3+ content and ordering schemes in LMNO. Their direct correlation in solid-state synthesized samples has been shown to be difficult by several groups. Effect of Mn3+ Content and Structural Ordering on Phase Transformation and Kinetic Properties. A recent computational study has shown the importance of Ni/Mn ordering in influencing structural evolution during charge and discharge, 27 but this correlation is largely unclear in experimental results. Most studies reported the presence of two topotactic two-phase transitions involving three cubic phases in the ordered spinels, but the overlap between the three phases as well as the size of the solid solution domain vary significantly.28−31 Furthermore, transformations through both two and three cubic phases were reported in the disordered samples.32−34 The discrepancies may be related to the subtle differences in the ordering scheme of the samples, but Mn3+ content in the structure, either created by oxygen vacancies or Ni/Mn nonstoichiometry, may also play a role. In an effort to probe these relationships and further understand the impact of structural ordering and Mn3+ content in phase transformation and transport properties, the ordered sample with x = 0.5 was disordered by thermal treatment at 720 °C followed by fast
transformation mechanism in these samples are likely different than that of x = 0.4, 0.45, and 0.5. While the structurally more ordered spinels transform through two two-phase transitions, evidenced by the two sets of sharp redox peaks at 4.7 V, the disordered is likely to have a much larger presence of solid solution at low state of charge (SOC). The details on phase transition mechanism are further discussed later in the paper. Changes in Mn 3+ Content and Order/Disorder Transition Driven by Thermal Treatment. It has been shown that thermal treatment perturbs structural ordering and the transition between order/disorder can be achieved by postsynthesis annealing, but the mechanism is largely unknown. To this end, thermal behavior of the LiNixMn2−xO4 series was investigated using a simultaneous TGA and DSC thermal analyzer under both heating and cooling conditions. When heated in air at a rate of 3 °C/min, significant weight loss occurred around 700 °C in all samples. Depending on the initial Mn3+ content, the total mass loss at 1000 °C ranged from 5.8 to 6.5% (Figure 5a). The lost weight was mostly regained
Figure 5. (a) TGA and differntial TGA spectra, (b) DSC spectra of the LiNixMn2−xO4 crystal samples, and (c) comparison of DSC spectra collected in air and Ar (x = 0.5).
upon cooling, indicating excellent reversibility of the process. The broad reversible peak developed at 700 and centered at 830 °C on the differential TGA heating traces was attributed to the loss of oxygen.25 This reaction generates Mn3+ in the spinel, which is compensated by either oxygen vacancy formation or Ni/Mn off-stoichiometry along with the formation of rock-salt E
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cooling. Comparison studies on LiNixMn2−xO4 crystal samples with x = 0.3 and 0.5 (before and after annealing) were then performed, and the FTIR spectra as well as the dQ/dV profiles are shown in Figure 6. In the FTIR spectra (Figure 6a), both x
Figure 6. (a) FTIR spectra and (b) dQ/dV profiles of LiNixMn2−xO4 crystal samples with x = 0.3 and x = 0.5 (before and after annealing).
= 0.3 and 0.5 (annealed) showed the signature disordered features with largely broadened peaks at 594 and 621 cm−1, which were drastically different from that of x = 0.5 before annealing. On the dQ/dV profiles (Figure 6b), the separation for the peaks at 4.7 V was determined to be 60, 58, and 20 mV for x = 0.3, 0.5 (annealed), and 0.5, respectively, with the area integration of the broad peaks at 4.1 V estimated 19.3, 5.8, and 1.5% Mn3+. The effort to disorder x = 0.5 without creating additional Mn3+ was unsuccessful, suggesting that under these conditions more than 1.5% Mn3+ is needed to randomize previously ordered cations in LMNO. Structural evaluation during galvanostatic charge and discharge of the spinels were compared by synchrotron in situ XRD studies. The full XRD patterns as well as the (331), (333), and (531) reflections collected on x = 0.5 are shown in Figure 7a, with the detailed phase composition at a given Li content obtained from full-pattern Rietveld refinement shown in Figure 7b. Both charge and discharge involved two twophase transitions between three distinct cubic phases. The initial lattice parameters of the cubic phases were determined to be 8.169 (phase I), 8.089 (phase II), and 8.008 Å (phase III), respectively. On charge, phase I had a slight decrease in lattice parameter but remained as a single phase at low SOC. Phase II appeared at a Li content near 0.59, which coexisted with phase I between the Li content of 0.59 and 0.29. Phase III appeared near Li0.29 and coexisted with phase II until Li0.19, which then
Figure 7. (a) XRD patterns and expanded regions of the XRD patterns and (b) lattice parameters (top) and percentage of the phases present (bottom) during synchrotron in situ charge (left) and discharge (right) of the ordered sample with x = 0.5. * indicate peaks from the background. Phase I (square), phase II (circle), and phase III (diamond).
became the single phase at Li contents below 0.19. Three phases also briefly coexisted in a small window between Li content of 0.2 and 0.3. The phase transformation was largely reversible during the discharge of electrode. Similarly, the evolution of the XRD patterns as well as the (331), (333), and (531) reflections during the galvanostatic charge and discharge of x = 0.5 (annealed) are shown in Figure 8. During charge, the peaks gradually shifted toward higher angles, suggesting lithium extraction through a phase I solid solution with decreasing lattice parameters (Figure 8a). Phase II appeared at Li content near 0.6 and only briefly coexisted with phase I. Between Li content of 0.55 and 0.35, phase II existed as F
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III maintained near constant values at 8.090 and 8.020 Å, respectively. The existence of phase II as a single phase not only reduced the size of the two-phase domains, but also eliminated the domain where the three cubic phases coexisted as the case for the ordered x = 0.5. The results for the disordered x = 0.3 are shown in Figure 9. Unlike the samples with x = 0.5, only two cubic phases were observed during both charge and discharge. An extensive solid solution region dominated nearly the entire charging process, with the detection of phase III and coexistence of two phases only observed below Li0.2 (Figure 9a). On discharge, however,
Figure 8. (a) XRD patterns and expanded regions of the XRD patterns and (b) lattice parameters (top) and percentage (bottom) of the phases present during synchrotron in situ charge (left) and discharge (right) of the disordered sample with x = 0.5 (annealed).
a single phase with nearly constant lattice parameter. Phase III, which appeared near Li0.35, coexisted with phase II until Li0.15 and then became the single phase at low Li contents. Upon discharge the process was largely reversed, with a slight increase in the two-phase domains between phases II and III resulting from an increased presence of phase II. Figure 8b summarizes the variations in the lattice parameters of the three phases as well as the phase diagrams during the charge and discharge of the disordered x = 0.5. The lattice parameter of phase I decreased from 8.152 to 8.112 Å on charge, while phases II and
Figure 9. (a) XRD patterns and expanded regions of the XRD patterns and (b) lattice parameters (top) and percentage (bottom) of the phases present during synchrotron in situ charge (left) and discharge (right) of the disordered sample with x = 0.3. G
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phase III had a much larger presence and it coexisted with the phase I until the Li content reached 0.6, suggesting large hysteresis. The lattice parameter of phase I decreased from 8.190 to 8.080 Å, while phase III remained nearly constant at 8.020 Å (Figure 9b). Phase II was not detected during charge or discharge. The observed phase transformation behavior is consistent with the results from the dQ/dV profiles (Figure 4a) where the broad feature of the lower voltage peak in x = 0.3 suggested dominating solid solution behavior in the sample. We wish to point out that the Li content in our phase diagrams was determined from the total amount of charge passed during the in situ experiment, which did not account for the inevitable side reactions occurring at the high operating voltage. The inaccuracy in this approach as well as the relatively low frequency in data collection resulted in the qualitative nature of our phase diagrams. The transport properties of the LiNixMn2−xO4 crystals (x = 0.3 and 0.5 before and after annealing) were compared in the varying-rate cycling of half-cells between 3.0 and 5.0 V (Figure 10). The plateau area around 4.1 V is consistent with the estimated Mn3+ content in the samples. At C/22 rate, the disordered spinels with x = 0.3 and 0.5 delivered a discharge capacity close to the theoretical value of 147 mA h g−1, while the ordered x = 0.5 with the dominant two-phase transitions delivered a slightly lower capacity of 139 mA h g−1. The disordered spinels consistently outperformed the ordered x = 0.5 at all rates tested (Figure 10b), confirming the importance of structural ordering in kinetic properties. Compared to the sample with x = 0.3, slight improvement was also observed on disordered x = 0.5 despite its lower Mn3+ content. This suggests that once the structure is disordered, further increasing Mn3+ content has limited impact on Li transport. In samples where both solid solution and two-phase transformations are involved, direct correlation between kinetics and the extent of solid solution transformation is not found.
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CONCLUSIONS In this study, impurity-free, well-formed single-crystal samples with the same morphology, surface facets, and nearly identical surface area were used to probe the intricate relationships between spinel physical properties, phase transformation, and transport behavior. A small amount of lattice Mn3+ was accommodated even in the near perfectly ordered LMNO with the P4332 space group. Both chemical composition and thermal treatment were found to drive the order/disorder transition, triggered by a change in Mn3+ content through Ni/Mn ratio adjustment in the former and oxygen loss and/or Ni/Mn offstoichiometry in the latter. When samples with a combination of Mn3+ content and ordering scheme are used, their roles in phase transition mechanisms, particularly with regard to the overlapping between the three cubic phases (phases I, II, and III) and solid solution domain size, were revealed. Structural ordering was found most critical in regulating LMNO properties and behavior, with the randomization of the transition metals leading to an increase in solid solution behavior, a reduction in the two-phase transformation domains, possible elimination of the domain where three phases coexist, and an improvement in charge and discharge kinetics. Direct correlation between rate capability and Mn3+ content or solid solution domain size was not found. The effect of phase boundary, however, is not clear from this study and it requires further investigation. It is possible that better kinetics may be
Figure 10. (a) Voltage profiles and (b) rate capability comparison of LiNixMn2−xO4 crystal samples.
achieved by eliminating two-phase transition altogether, in other words, transforming through solid solution throughout the entire Li content range in the LiNixMn2−xO4 spinels.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. H
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(28) Arunkumar, T. A.; Manthiram, A. Electrochem. Solid-State Lett. 2005, 8, A403. (29) Wang, L.; Li, H.; Huang, X.; Baudrin, E. Solid State Ionics 2011, 193, 32. (30) Xiao, J.; Yu, X.; Zheng, J.; Zhou, Y.; Gao, F.; Chen, X.; Bai, J.; Yang, X.-Q.; Zhang, J.-G. J. Power Sources 2013, 242, 736. (31) Ariyoshi, K.; Iwakoshi, Y.; Nakayama, N.; Ohzuku, T. J. Electrochem. Soc. 2004, 151, A296. (32) Kim, J.-H.; Myung, S.-T.; Yoon, C. S.; Kang, S. G.; Sun, Y.-K. Chem. Mater. 2004, 16, 906. (33) Kim, J.-H.; Yoon, C. S.; Myung, S.-T.; Prakash, J.; Sun, Y.-K. Electrochem. Solid-State Lett. 2004, 7, A216. (34) Rhodes, K.; Meisner, R.; Kim, Y.; Dudney, N.; Daniela, C. J. Electrochem. Soc. 2011, 158, A890.
ACKNOWLEDGMENTS The authors acknowledge the support of Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. We thank Drs. Jordi Cabana and Chunjoong Kim for assisting with the synchrotron experiments. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (G.C., H.D., and B.H.) and Subcontract No. 6517749 (C.P.G. and M.L.).
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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on September 10, 2014, with incorrect versions of Figures 7 and 8. The corrected article was published on September 11, 2014.
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