Studies of Spinel-to-Layered Structural Transformations in LiMn2O4

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Studies of Spinel-to-Layered Structural Transformations in LiMn2O4 Electrodes Charged to High Voltages Nicole Leifer,† Florian Schipper,† Evan M. Erickson,† Chandan Ghanty,† Michael Talianker,‡ Judith Grinblat,† Christian M. Julien,§ Boris Markovsky,*,† and Doron Aurbach† †

Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel § Sorbonne Universités, PHENIX, UPMC-Paris-6, Paris 75252, France ‡

ABSTRACT: This research sheds light on the intriguing phenomena of structural transformations that take place in spinel LiMn2O4 (space group Fd3m) when charged to high anodic potentials (4.3 to 5.1 V) in lithium cells. It was established from the XRD and electron diffraction analyses that the orthorhombic o-LiMnO2 (space group Pnma) is formed due to the partial transformation of spinel starting at 4.7 V. The Raman spectroscopy measurements of the LiMn2O4 samples charged to 5.1 V also provided evidence of o-LiMnO2, although its band overlaps with the delithiated spinel species. An additional important finding is that some layered-type LiMnO2 is formed upon cycling of the spinel material to 4.5 V, even upon the first charge. LiMnO2 exists as a stable component of the solid electrolyte interphase developed on these electrodes. We also concluded the formation of the layered LiMnO2 structure via solid-state 7Li NMR. The analysis of those results further indicated that the onset of the creation of the intermediate spinel-layered (“splayered”) phase clearly takes place between 4.5 and 4.7 V.

1. INTRODUCTION The large family of lithium intercalation cathode materials, which began with layered rhombohedral structure (R3̅m symmetry) lithium cobalt oxide, LiCoO2, now includes a variety of transition-metal oxides for positive electrodes of Liion batteries, each with its own advantages and shortcomings for practical use in electric vehicles and portable power devices.1−4 The manganese analog of the original material, LiMnO2,5 showed promise due to its lower cost, lower toxicity, higher safety, and comparable theoretical capacity (∼270 mAh g−1).6 However, this layered form proved to be metastable thermodynamically,7 converting to the cubic spinel structure LiMn2O4 (Fd3m symmetry) over electrochemical or chemical deintercalation/intercalation. This layered-to-spinel structural transformation was analyzed by Raman spectroscopy, demonstrating the evolution of the chemical bonding environment around Mn ions.8 The above transformation to a spinel-like cation ordering is possible if one-fourth of Mn ions migrate into the Li interlayer sites upon deintercalation. The layered lithium manganese oxides exhibit close-packed oxide ion layers stacked in an ABC sequence (cubic close packing) with alternate sheets of octahedral sites between the oxide ion layers occupied by Li and Mn.6 The spinel form possesses the same stacking of oxide ions, its structural reorganization involving only a redistribution of the cations.8 While initial research on these materials attempted to prevent this layered-to-spinel transformation, it was discovered that the spinel formed during cycling still © 2017 American Chemical Society

demonstrates good capacity retention and fading by 4.3 V), some transformation of spinel structure to a layered-type ordering has been detected, particularly at the outermost particle surface. The hypothesis is that the appearance of the layered-like structure is a consequence of the instability of the highly charged, delithiated (up to 5.1 V) spinel LiMn2O4. Delithiated layered Li1−xMnO2 is Received: January 29, 2017 Revised: April 5, 2017 Published: April 10, 2017 9120

DOI: 10.1021/acs.jpcc.7b00929 J. Phys. Chem. C 2017, 121, 9120−9130

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The Journal of Physical Chemistry C theoretically more stable than delithiated spinel Li1−xMn2O4.21 Transformation from the spinel to the layered structure requires Mn migration from one octahedral site to another, through an intermediate tetrahedral site.22 This process has a high energy barrier and will not likely take place unless it is aided, for example, by charge disproportionation in the tetrahedral site. Such circumstances are unlikely in the bulk of the sample, in which most of the Mn ions are in the 4+ oxidation state, therefore explaining why the spinel structure retains its phase in the bulk. However, XPS analyses, as shown in Tang et al.’s study, indicated that near the particle surfaces the valence of Mn is lowered to Mn3+ and Mn2+, enabling the requisite charge disproportionation process and making manganese migration possible.20 Thus the results of these authors refer to the formation of a layered structure material on the surface of spinel particles. In another study, dynamic surface structural changes on spinel electrodes were shown (by in situ surface X-ray diffraction) to play an important role in the SEI formation, phase transitions, and Mn dissolution.23 Studying particle surfaces in Li-ion battery electrode materials has therefore become extremely important for both spinel and Nirich LiNi1−x−yCoxMnyO2 layered structures operating at high anodic potentials.24−27 It was demonstrated, for instance, that tailoring surfaces of the above materials mitigates the layeredto-rock-salt structural transformation on the surface, thus improving the electrode cycling behavior and energy density.26 Modifying cathode material surfaces by thin-metal oxides or salts (e.g., MgO, AlF3) layers or by doping with cations (e.g., Al3+, Zr4+) may increase their performance and prevent, to some extent, the structural layered-to-spinel transition upon cycling, as demonstrated by many groups.28−31 Previous studies have reported spinel-to-layered transformations also in the bulk of Mn3O4 spinel when the material was cycled in aqueous electrolyte solutions32−34 and more recently when the material was specifically inserted with crystal water.35 The latter study confirmed the layered structure via XRD, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). The analytical techniques used to study both the layered and the spinel materials, as well as the structural transitions, have included electrochemical methods, X-ray diffraction (XRD), electron diffraction (ED), transmission electron microscopy (TEM), and Raman vibrational spectroscopy. In fact, Raman spectroscopy was demonstrated as an original and appropriate tool to study the spinel-(Fd3m)-tolayered (R3̅m)-transformation in LiCoO2.36 X-ray absorption fine structure analysis (XAFS), neutron diffraction, and 6,7Li solid-state nuclear magnetic resonance (NMR) spectroscopy are also widely used.13,37−45 Solid-state NMR has proven to be an effective method in the examination of such materials due to the ability to probe the local lithium environments as well as the electronic states of nearby cations. The analytical tools used herein (XRD, TEM/ED, Raman spectroscopy, and solid-state NMR) were chosen due to their structural and surface sensitivity and their individual, yet complementary, advantages. The goal of this study was to understand more deeply the phenomenon of spinel-to-layered structural transformation during LiMn2O4 charge/discharge cycling to high anodic potentials (>4.3 V) and shed light on the nature of the layered structures formed. Because of the fact that both the spinel and layered structures are closely related to each other it can be suggested that partial spinel-to-layered transformation may take place on the electrode surface and in the bulk when the electrochemical parameters (anodic cutoff potential) of

LiMn2O4 electrodes exceed certain upper limits (e.g., 4.3 V). Past successes of other research groups in elucidating information about the structures and transformation processes of the LiMn2O4 spinel materials via 6,7Li NMR12,13,44−50 provided the motivation to apply the same techniques to this study, offering the potential for additional insight into the mechanisms taking place and the structures being formed. The primary investigative tool used in our work was solid-state NMR in combination with structurally sensitive long-range order (XRD, ED) and short-range order (Raman scattering) techniques to study LiMn2O4 spinel electrodes at various statesof-charge. We thus aimed to correlate the electrochemical data and structural (surface) responses of these electrodes charged to anodic potentials of 4.3 to 5.1 V.

2. EXPERIMENTAL SECTION 2.1. LiMn2O4 Spinel Material. The spinel LiMn2O4 (Nanomyte BE-30) was purchased from NEI Corporation and used as is. The chemical analysis of the material was carried out using the inductive coupled plasma technique (ICP-AES, spectrometer Ultima-2 from Jobin Yvon Horiba) and confirmed a Li:Mn ratio of 1:2. Its specific surface area was ∼1.6 to 2.4 m2 g−1 and average particle size was around 6 to 7 μm. 2.2. Electrochemical Measurements. Electrochemical tests of spinel LiMn2O4 electrodes were carried out in twoelectrode pouch-type cells with lithium foil as counter electrodes and Celgard polypropylene separators. The working electrodes comprised LiMn2O4, carbon black (CB), and polyvinylidene difluoride (PVdF) binder (80:10:10 by weight) pasted on aluminum foil (∼20 μm thick, from Strem). Typical loading of spinel electrodes was 4 to 5 mg cm−2, and counter Li metal electrodes were ∼200 μm thick plates. Electrochemical cells were assembled in a glovebox filled with highly pure argon (VAC). The electrolyte solutions (high purity, Li battery grade, HF ≈ 30 ppm, H2O ≈ 10 ppm) comprised ethyl-methyl carbonate (EMC) and ethylene carbonate (EC) (weight ratio of 7:3) and 1 M LiPF6. After assembly, the electrochemical cells were stored at room temperature overnight to ensure a complete impregnation of the electrodes and the separators with the electrolyte solution. The measurements were performed using a Solartron BTU-1470 battery test unit and an Arbin multichannel battery tester. The cells were cycled from 3.0 to 4.3, 4.5, 4.7, or 5.1 V for five to eight cycles at a C/ 15 rate (1 C = 120 mAh/g during 1 h) and terminated at either discharged (3.0 V) or charged states at the above potentials before disassembly. The electrochemical measurements were performed at 30 °C in thermostats. After testing, the cells were disassembled in a glovebox filled with pure argon; LiMn2O4 electrodes were washed with dimethyl carbonate (DMC) and dried under vacuum overnight. 2.3. X-ray Diffraction, Transmission Electron Microscopy, and Electron Diffraction Measurements. X-ray powder diffraction measurements were performed using an AXS D8 Advance diffractometer from Bruker (Germany) in the 2θ range from 10 to 110°, with a step size of 0.02°, at 15 s/step rate. The analysis of the XRD patterns was carried out as described elsewhere.19,38 HR-TEM examinations of LiMn2O4 samples were performed with a JEOL JEM-2100 (LaB6) highresolution electron microscope, and convergent beam electron diffraction (CBED) technique (4−7 nm probe size) was employed for structural characterization. Samples for the TEM 9121

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Figure 1. (a) Voltage profiles and (b) their corresponding differential capacity dQ/dV plots of LiMn2O4 electrodes measured at the third cycle at different cutoff voltages of 4.3, 4.5, 4.7, and 5.1 V (as indicated) at 30 °C, C/15 rate (1C rate calculated as 130 mAh g−1). Intensities of the peaks I (∼4.025 V) and II (∼4.14 V) related to two-step spinel behavior are shown partially to maintain the correct scale for the minor peaks in the 4.5 to 5 V domain. (c) Differential capacity dQ/dV plots of LiMn2O4 electrodes cycled from 3.0 to 5.1 V for 5 cycles at 30 °C (C/15 rate). (d) Evolution of the voltages related to two-step behavior (peaks I and II) of LiMn2O4 electrodes upon cycling to various cutoff potentials, as indicated, for five cycles at 30 °C (C/15 rate).

kHz on a 4 mm Bruker probe and 18−24 kHz on a 3.2 mm Bruker probe. The cycled samples were scratched from the current collectors and packed into the NMR rotors in a controlled atmosphere (argon) glovebox. The measured 90° pulse widths were 1.9 and 2.1 μs, respectively, on the two probes. Spectra were acquired using single-pulse and variable-τ Hahnecho ((90)x − τ − (180)y − τ − acquire) sequences. Samples were collected either under ambient (i.e., no temperature control) or controlled temperature conditions, as indicated. It is known that in high-speed spinning probes, because the thermocouple probe is located outside of the sample rotor, the sample itself may be at a different temperature than what is indicated by the probe thermometer. For this reason, a series of experiments were conducted using a known “sample thermometer”, PbNO3(s), to calibrate the spinning speeds according to temperature, as has been conducted in previous studies.53

studies were prepared by the methodology described in another study.51 2.4. Raman Spectroscopy. Micro-Raman spectroscopy measurements of LiMn2O4 electrode samples were performed ex situ at room temperature using a micro-Raman spectrometer from Renishaw inVia (United Kingdom) equipped with a 514 nm laser, a CCD camera, and an optical Leica microscope. The diameter of the laser beam was ∼1 μm. Raman spectra were collected at least from 10−15 arbitrary locations on a sample. To prevent local decomposition of the sample, we kept the laser power at a low level of ∼0.23 mW, as measured by using a Nova II detector (Ophir Photonics Group). The samples were transferred from the glovebox to the microscope in sealed vials (under argon); they were covered with thin borosilicate glass pieces upon measurements or tested in special pouch cells for in situ Raman spectroscopy studies.52 The spectrum of silicon (peak at 520 cm−1) was used a standard reference. The data analysis and spectra deconvolution were performed using Renishaw WiRE 3.3 software. 2.5. Solid-State Nuclear Magnetic Resonance (NMR). 7 Li nuclei were probed on a Bruker 200 MHz at an operating frequency of 77.77 MHz and externally referenced to LiCl(aq) at 0 ppm. The spectra were collected at spinning speeds of 10−15

3. RESULTS AND DISCUSSION 3.1. Electrochemical Testing of Spinel Electrodes: Polarization to High Anodic Potentials. In Figure 1a,b, we demonstrate, respectively, the voltage profiles and differential capacity dQ/dV plots of LixMn2O4 electrodes measured during 9122

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The Journal of Physical Chemistry C the third charge/discharge cycles from 3.0 V to various anodic cutoff potentials of 4.3, 4.5, 4.7, and 5.1 V versus Li+/Li0 at C/ 15 rate (as indicated). Discharge capacities of approximately 112−120 mAh g−1 were measured from these electrodes, in agreement with the data of the producer (NEI). Spinel electrodes show a typical two-stage behavior at ∼4.05 to 4.1 V due to ordering of the Li+ ions on one half of the tetrahedral 8a sites. In a fully charged state (x close to 0), λ-MnO2 is formed.54,55 The electrode LiMn2O4 samples charged to 4.7 and 5.1 V displayed additional short voltage plateaus (or minor redox peaks on dQ/dV plots) at ∼4.5 V that are intrinsic to the spinel and can be ascribed to the presence of Mn ions in the tetrahedral sites.56 These authors also observed the reversible redox couple at 4.9 V during cycling of LiMn2O4. It should be noted that an electrochemical process at 4.5 V was detected by Bates et al.57 for thin-film LiMn2O4 electrodes prepared via ebeam evaporation or magnetron sputtering and for those produced by solid-state spinel synthesis as well. In our experiments, however, only two oxidation peaks were detected at ∼4.8 V (only in the first cycle) and at ∼5.0 V for the first few charge/discharge cycles (Figure 1c), likely due to different synthetic and annealing conditions of various spinel materials studied. The above oxidation peaks are attributed to anodic decomposition of solution species (EC) and of CB, with CO2 evolution at potentials 4.7 to 4.9 V, as studied by differential electrochemical mass spectrometry.58,59 These processes may result in partial reorganization and stabilization of the cathodic SEI formed during initial cycles. During polarization to high anodic potentials, the oxidation currents at ∼5.1 V continuously decrease with cycling, as seen from Figure 1c, indicating stability of the passive surface films. Indeed, it has been shown recently from impedance measurements of LiMn2O4 electrodes that the SEI resistance becomes constant first at potentials 4.1 to 4.2 V and even decreases at 4.5 V, while the charge-transfer resistance stabilizes at the end of charge to 4.5 V.60 However, prolonged cycling of LiMn2O4 to high anodic potentials ∼5 V demonstrates fast capacity fade and low coulombic efficiency.20,38,61 This can be attributed to sluggish kinetics of Li+ extraction/insertion of LiMn2O4 electrodes as a result of unstable conditions developed at high cutoff voltages. Our findings clearly show that peak potentials I (∼4.025 V) and II (∼4.140 V) of the characteristic two-stage spinel behavior remain constant during cycling if the cutoff voltage does not exceed 4.3 to 4.7 V (Figure 1d). In contrast, they shift to more positive values and increase when LiMn2O4 is charged past 5.0 V, the potential domain of extremely high electrochemical and structural instability. For that reason, we tested spinel electrodes over only a few cycles, ensuring stable electrochemical behavior irrespectively of the applied anodic cutoff voltage. The electrodes thus cycled were terminated either in discharged (3.0 V) or in charged states up to 5.1 V and subjected to structural and surface analyses by NMR, TEM, ED, XRD, and Raman spectroscopy. 3.2. Analyses of the Structural Data Obtained: Identification of Highly Anodically Polarized Spinel and New Phases Formed. 3.2.1. XRD and TEM Studies. Figure 2 shows the XRD patterns recorded from LiMn2O4 electrode samples in charged state that were cycled to different cutoff potentials, up to 5.1 V. XRD patterns from the discharged samples did not indicate any measurable differences to the profiles and therefore are not shown. The XRD profiles of cycled electrodes are compared with the original spinel pristine material. For clarity, the profiles are divided into two

Figure 2. XRD powder profiles recorded from the pristine LiMn2O4 electrode samples and those cycled to 4.3, 4.5, 4.7, and 5.1 V and terminated at these potentials, as indicated. 2θ ranges (a) from 17.5 to 20.5° and (b) from 32 to 62°. “s”, “*”, and “o” denote LiMn2O4 spinel, delithiated spinel-type phase containing protons, and the orthorhombic o-Li0.5MnO2 phase, respectively.

parts, which are presented in Figure 2a,b and correspond, respectively, to a 2θ range of 17.0−20.7° and 32−64°. As expected, the observed diffraction pattern of the pristine spinel LiMn2O4 could be readily indexed to a cubic unit cell (space group Fd3m), with the lattice constant a = 8.239 Å calculated by least-squares fit to indexed spinel peaks. The main change that occurs in the profiles of the cycled samples is a shift of spinel peaks to higher angles, thus indicating that the lattice parameter “a” of the cubic spinel decreases. This implies that although the cubic structure of the spinel is retained, lithium ions are removed from their tetrahedral sites in the Mn2O4 framework of the spinel structure, leading to contraction of the unit-cell dimension to the value of ∼8.08 Å. It can therefore be said that the cycled material can be considered as delithiated spinel Li1−xMn2O4. As follows from Figure 2, in addition to the major peaks of the delithiated spinel, a faint peak (marked by asterisk) appears at 2θ ≈ 44.2° in XRD profiles of the cycled material. Keeping in view that during charging Li+−H+ ion exchange may take place at high anodic potentials,20,62 it seems reasonable to attribute this peak to the protonated spinel-type phase such as lithium manganese oxide hydroxide. Like spinel, this phase is described by the space group Fd3m, its lattice parameter is 8.196 Å, and its strong (004) peak matches the position at 2θ ≈ 44.2°.63 The analysis of the XRD patterns shows another structural change occurring in the material when it is cycled to 5.1 V: additional peaks marked with an “o” appear in the corresponding XRD profiles. These peaks were identified as related to the orthorhombic phase Li0.5MnO2 belonging to space group 62 (Pnma). The amount of this phase can be estimated as 3−5%. Its cell parameters, as obtained by a 9123

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Figure 3. Examples of convergent beam electron diffraction patterns measured from pristine spinel LiMn2O4 electrode (a) and from those cycled in the potential ranges 3.0 to 4.3 V (b) and 3.0 to 5.1 V (c) and terminated at charged states of 4.3 and 5.1 V, respectively.

standard least-squares refinement procedure, were a = 9.577 Å, b = 2.758 Å, and c = 5.082 Å. It is believed that formation of this phase is due to the fact that charging to voltage 5.1 V enhances the extraction of lithium from tetrahedral sites in the spinel structure, thus promoting a Jahn−Teller distortion effect and leading to a cubic-to-orthorhombic transition. The reverse phenomenon, in which orthorhombic LiMnO2 (o-LiMnO2) undergoes partial structural transformation to spinel Li1−xMn2O4, has been reported to occur when cycled to 4.0 V.64 The results of TEM examination of LiMn2O4 samples are consistent with the structural information obtained by XRD analysis. Examples of CBED patterns demonstrating the presence of spinel LiMn2O4 in the sample from an electrode cycled to 4.3 V and of delithiated spinel Mn2O4 in sample from an electrode cycled to 5.1 V are shown, respectively, in Figure 3a,b. The measured interplanar distance for (111) reflection in diffraction in Figure 3b is equal to 4.64 Å, which uniquely corresponds to the distance d(111) in the delithiated spinel Mn2O4, while in the original spinel material this distance should be d(111) = 4.757 Å. Pattern in Figure 3c was indexed in terms of the orthorhombic unit cell of Li0.5MnO2, thus illustrating the presence of this phase in the material charged to 5.1 V. The high-resolution image in Figure 4 provides an additional evidence of coexistence of both orthorhombic Li0.5MnO2 and cubic spinel structures in the material charged to 5.1 V. These phases were identified on the basis of indexed Fourier transform patterns (see inserts “a” and “b”) taken, respectively, from areas “a” and “b”. 3.2.2. Raman Spectroscopic Studies. Typical Raman spectra of both pristine and cycled LiMn2O4 electrodes are shown in Figure 5a−c. The spectrum of the pristine electrode indicates broad bands at ∼625 cm−1 and ∼475−480 cm−1 assigned to A1g and T2g(2) symmetries, respectively and a shoulder at ∼580 cm−1 that mainly originates from the stretching vibration of the Mn4+−O bond.65−67 A band located at ∼370 cm−1 is related to T2g(1) Raman active species. The ratio of the integral intensities of the main peak and the shoulder I625/I580 is 6.2 for the pristine sample, as calculated by fitting the experimental spectrum. Figure 5a also represents the spectrum of the electrode cycled in the potential range of 3.0 to 4.3 V and terminated in a discharged state (∼75% Li+ was extracted). This spectrum resembles that of the pristine electrode, although both peaks are broader in the cycled sample, while the ratio of their intensities is 6.4, similar to the pristine electrode. This is likely due to minor changes in the Mn3+ and Mn4+ proportions as a function of x in Li1−xMn2O4 upon charge/discharge cycling.66 Figure 5a indicates a spectrum of the electrode

Figure 4. High-resolution TEM image measured from a LiMn2O4 electrode cycled in the range of 3.0 to 5.1 V and terminated in the charged state of 5.1 V. It demonstrates coexistence of both orthorhombic Li0.5MnO2 and cubic spinel structures in the material. These structures were identified on the basis of indexed Fourier transform patterns (insets “a” and “b”) taken, respectively, from areas “a” and “b”.

subjected to charge/discharge cycles (3.0 to 4.3 V) and terminated in the charged state at 4.3 V. The amount of Li calculated to remain is x ≈ 0.37 in LixMn2O4. This spectrum is characterized by a strong band around at 590−600 cm−1, a peak at 495 cm−1, and a shoulder at 630−640 cm−1, in qualitative agreement with previous results from delithiated spinel electrodes.66 Note that on this delithiated LiMn2O4 sample, along with representative spectra, a few locations were detected demonstrating Raman responses similar to those of the lithiated electrode with the I625/I580 ratio of 5.8. This may relate to the inhomogeneous state-of-charge of the LiMn2O4 particles on the electrode surface, which has been previously shown44 and ascribed to slow phase transitions throughout the electrode bulk.45 Interesting features were identified in Raman spectra of LiMn2O4 electrodes cycled from 3.0 to 5.1 V and measured in a discharge state (Figure 5b). Approximately 90% of the Li+ ions can be extracted from spinel upon charge to 5.1 V, as indicated in the voltage profiles in Figure 1a. The spectra of the material at this state of discharge (Figure 5b) indicate three main bands at 625−630, 580−590, and 480 cm−1; however, arbitrarily chosen locations on the electrode additionally also demonstrate a shoulder at higher wavenumbers: ∼655−660 cm−1. The average I632/I590 ratio is ∼0.9, which implies significant changes 9124

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Figure 5. (a) Raman spectra of pristine LiMn2O4 electrode and those cycled to 4.3 V and terminated in discharged (3.0 V) and charged (4.3 V) states, as indicated. (b) Raman spectra of LiMn2O4 electrodes cycled from 3.0 to 5.1 V and terminated in discharged (3.0 V) and charged (5.1 V) states, as indicated. Two typical arbitrary locations on the electrode sample measured in discharged state are shown. (c) Raman spectra of LiMn2O4 electrodes cycled to 4.5 and 4.7 V and terminated in charged states at these potentials, as indicated. (d) Changes of the integral intensity of 580 cm−1 shoulder in Raman spectra during cycling spinel electrodes to various cutoff potentials. Electrodes were terminated in the charged (delithiated state) at 4.3, 4.5, 4.7, and 5.1 V.

Figure 6. Solid-state 7Li MAS NMR spectra of cycled spinel LiMn2O4 electrodes terminated in the charged states (a) 4.3 V (at spinning speed 22 kHz), (b) 4.5 V (at spinning speed 15 kHz), (c) 4.7 V (at spinning speed 15 kHz), and (d) 5.1 V (spinning speed 22 kHz). Main isotropic peaks are indicated by shifted vertical lines, connecting the common features in the different spectra. As discussed in the text, these shifts are artifacts of the sample collection conditions, dependent on the different temperatures (due to differences in the spinning speeds applied). * indicate spinning sidebands.

Mn4+ :Mn3+ :Mn2+ were shown in the charged LiMn2 O 4 electrodes in Tang’s work via XPS.20 The Raman responses of the samples cycled from 3.0 to 4.5, 4.7, and 5.1 V are shown in Figure 5b,c. As expected, these Raman spectra differ from those of the LiMn2O4 electrodes in a charged state of 4.3 V (Figure 5a). First of all, they display characteristic features

in the proportion of Mn ions as compared with that seen in the pristine electrode and in the electrode cycled to 4.3 V (discharged state). It is suggested that some Mn2+ ions form on the electrode surface due to Mn dissolution that increases at high anodic potentials: 2Mn3+ → Mn4+ + Mn2+.68 The presence of an increased amount of Mn2+ and different proportions of 9125

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mixed-valence state, containing both Mn3+ and Mn4+ ions, which may induce several different paramagnetic shifts depending on the configuration of neighboring Mn ions around different Li centers in the 7Li NMR spectrum. However, because LiMn2O4 is also a hopping semiconductor, in which the electrons hop between the eg orbitals of the Mn ions, and the time scale of this hopping is fast in comparison with the NMR time scale (∼10−5 s), the Li ions “feel” only the average Mn oxidation state (+3.5) and indicate only one magnetically inequivalent lithium site.44 Data collected at a B0 field of 4.7 T at ∼50 °C indicated this resonance between 498 and 520 ppm, depending on sample synthesis temperature.46 Although samples synthesized at all temperatures indicate this single main resonance, the samples synthesized at lower temperatures (4.3 V) potentials. First, we conclude that the high state-of-charge is not favorable for LiMn2O4 electrodes in lithium batteries, from the viewpoint of surface stability, intense reactions with electrolyte solution species, and the formation of new phases at 4.5 to 5.1 V. Evidence of new phases formed upon cycling to these potentials were detected by all three structurally (and surface) sensitive techniques, used in complement with one another. The XRD and ED analyses revealed the formation of orthorhombic o-LiMnO2 due to the structural transformation of cubic spinel starting at 4.7 V. The Raman spectroscopic studies of the 5.1 V sample also indicated evidence of o-LiMnO2, although its band overlaps with the spinel species, Mn2O4. The 7Li NMR data also possibly indicated evidence of (delithiated) LiMnO2 species, in the same shift range as the delithiated layered materials, although the identification of o-LiMnO2 specifically, was not unequivocal. Another important finding from the diffraction studies was evidence that proton-containing delithiated spinel was formed at high anodic potentials (above 4.3 V), presumably due to Li+−H+ exchange. The protons are formed as a result of the decomposition of the electrolyte solution species at these 9127

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The Journal of Physical Chemistry C voltages.76 The Raman data additionally indicated evidence of a layered-type LiMnO2 formed upon cycling of the spinel material to 4.5 V and above. This layered structure species is indicated even upon the first charge to 5.1 V, as part of the electrode’s surface film formed. It was also indicated that these features are retained upon aging the cell in a charged state for several days, indicative of the high stability of this new layered phase. Direct and indirect evidence of the creation of a layered LiMnO2 phase was strongly suggested by the solid-state NMR results as well, in which the intermediate “splayered” phase was clearly identified, by itself implying the occurrence of at least the onset of a transformation of the spinel species. As such, the detection of “splayered” intermediate phase formation in the samples provides more certainty in the assignment of the 720 ppm (710 ppm) peak indicated in the 4.7 and 5.1 V sample spectra to a delithiated layered or orthorhombic LiMnO2 phase. In addition, the energy barrier for the transformation from “splayered” to layered (or possibly o-LiMnO2) has been calculated to be within the range of expected temperatures reached during battery cycling.72 This work can help in better understanding the complicated mechanisms of Li- and Mn -rich high-capacity cathodes, whose operation requires high anodic voltages and involves several types of phase transitions. It is warranted to conclude that this research will also stimulate further investigations of phase transformations in spinel LiMn2O4 electrodes in broader aspects, with respect to the relation of this phenomenon to practical lithium battery applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: + 972 (3) 531 8832. ORCID

Nicole Leifer: 0000-0002-1708-8585 Doron Aurbach: 0000-0002-1151-546X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Gil Goobes for useful discussions regarding the NMR interpretations and reviewing the final MS and Dr. Ortal Haik for valuable discussions regarding setup and procedures of the electrochemistry measurements. This work was funded by the INREP project (supported the Israel Committee of High Education).

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