Article Cite This: Chem. Mater. 2018, 30, 5061−5068
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Synthesis and Electrochemical and Structural Investigations of Oxidatively Stable Li2MoO3 and xLi2MoO3·(1 − x)LiMO2 Composite Cathodes Ethan C. Self,† Lianfeng Zou,∥ Ming-Jian Zhang,⊥ Richard Opfer,† Rose E. Ruther,‡ Gabriel M. Veith,† Bohang Song,§ Chongmin Wang,∥ Feng Wang,⊥ Ashfia Huq,§ and Jagjit Nanda*,†
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Materials Science and Technology Division, ‡Energy and Transportation Science Division, and §Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States ∥ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ⊥ Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *
ABSTRACT: The structural evolution of Li2MoO3 during electrochemical delithiation/lithiation is reported. Li2MoO3 undergoes an irreversible crystalline to amorphous transformation which starts during the first delithiation step and gradually proceeds throughout subsequent cycles. This observation is supported by complementary data obtained from X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The amorphization does not prevent reversible Li storage, and the Li2MoO3 cathodes exhibit initial capacities of 147 mAh/g (corresponding to 0.87 Li cycled per formula unit) at an average operating potential ∼2.5 V vs Li/Li+ with reasonably good cycling stability (81% capacity retention after 50 cycles). In-situ mass spectrometry studies reveal the amorphization of Li2MoO3 does not involve the release of oxygen gas even when charged to very positive potentials (i.e., 4.8 V vs Li/Li+). The Li storage properties and excellent oxidative stability of Li2MoO3 make it a promising candidate to improve the performance of traditional LiMO2 compounds in layered−layered composite cathodes with the general formula xLi2MoO3·(1 − x)LiMO2. Multiple synthesis routes were explored to prepare composites with x = 0.10−0.15, and their structure was characterized using XRD and time-of-flight neutron diffraction. Preliminary characterization of these materials shows that Li2MoO3 improves the cycling stability of an NMC cathode, presumably by mitigating detrimental structural rearrangement at high states of charge.
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INTRODUCTION One major technical barrier limiting the energy density of lithium-ion batteries is the lack of robust, high-capacity cathodes. When charged beyond ∼4.3 V vs Li/Li+, traditional layered LiMO2 cathodes (M = Mn, Co, Ni, etc.) undergo irreversible structural transformations with concomitant oxygen loss, leading to capacity and voltage fade upon cycling.1−3 Understanding and addressing these structural instabilities is vitally important to design next-generation cathodes which can better utilize their Li supply without sacrificing cycle life. Various cation substitutions have been explored to improve the performance of LiMO2 cathodes. Two notable examples include Ni-rich NMC (LiNixMnyCo1−x−yO2, x ≥ 0.5) and NCA (e.g., LiNi0.8Co0.15Al0.05O2) which have reversible capacities as high as 200 mAh/g.4−6 Despite their significant improvements over LiCoO2 (∼140 mAh/g), NMC and NCA still only utilize ∼70% of their Li reserve. An alternative strategy to stabilize the cathode structure at high states of charge is to design composite cathode structures with LiMO2 © 2018 American Chemical Society
as the primary Li storage site and with Li2M′O3 as a stabilizing unit. The most prominent among these materials are the layered−layered composites normally referred to as Li−Mnrich NMC (LMR-NMC, xLi2MnO3·(1 − x)LiNiyMnzCo1−y−zO2). After an electrochemical activation step in which Li and O are removed from the Li2MnO3, extremely high reversible capacities ∼250 mAh/g can be achieved.7−9 However, the viability of LMR-NMC cathodes is limited by Mn dissolution in the electrolyte and voltage fade which occurs during extended cycling.7,10−12 First-principles calculations have suggested that the voltage fade of these materials is caused by irreversible migration of Mn to the Li layer during the first charging cycle.13 Li2MoO3 has recently been proposed as a candidate to replace Li2MnO3 in layered−layered composite cathodes, Received: April 5, 2018 Revised: June 27, 2018 Published: June 29, 2018 5061
DOI: 10.1021/acs.chemmater.8b01408 Chem. Mater. 2018, 30, 5061−5068
Article
Chemistry of Materials
LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (3/7 by volume). The separator consisted of one sheet of Celgard 2325 and one sheet of glass microfiber (Whatman). After crimping, the cells were rested at open circuit for at least 1 h before electrochemical characterization. Galvanostatic charge/discharge experiments were conducted on a MACCOR Series 4000 battery tester by polarizing cells between either (i) 2.0−4.8 V at a current density of 10 mA/g (for Li2MoO3 cathodes) or (ii) 2.0−4.5 V at 20 mA/g (for NMC and composite cathodes). Cyclic voltammograms were collected between 2.0 and 4.8 V at a scan rate of 0.1 mV/s using a Biologic SP-200 potentiostat. Electrodes for post-mortem analysis were harvested from cycled cells which were disassembled inside an Ar-filled glovebox. The electrodes were rinsed several times with dimethyl carbonate and dried under vacuum overnight without exposing the electrodes to ambient conditions. Mass Spectrometry. In-situ mass spectrometry was used to monitor gas evolution from Li2MoO3 cathodes during battery operation. A commercial gas cell (EL-Cell, shown in Figure 3a) was prepared using a Li2MoO3 working electrode, Li counter/reference electrode, and Celgard 2325 and Whatman glass fiber separators. The electrolyte contained 1.2 M LiPF6 in a mixture of ethylene carbonate and ethyl methyl carbonate (3/7 by weight). The gas cell was connected to an Ametek Dymaxion mass spectrometer (0−200 amu), and He carrier gas (2 sccm) was flowed through the cell as it was cycled between 2.0 and 4.8 V at 20 mA/gLi2MoO3. O2 and CO2 levels (corresponding to m/z ratios of 32 and 44, respectively) were monitored as functions of time. Gas concentrations in the effluent stream were determined from 12 point calibration curves. Microscopy. Scanning electron microscopy (SEM) images of pristine Li2MoO3 powder were collected with a Zeiss Merlin SEM using a 1.0 kV accelerating voltage. The TEM and SAED experiments were performed on Titan 80−300 ETEM with an operating voltage of 300 keV. Electron diffraction patterns were simulated based on the standard Li2MoO3 structure (R3̅m space group) using SingleCrystal software. Raman Spectroscopy. Li2MoO3 cathodes were sealed in a hermetic optical cell (EL-Cell) in an Ar-filled glovebox to avoid air exposure. Raman spectra were acquired with an Alpha 300 confocal Raman microscope (WITec, GmbH) using a solid-state 532 nm excitation laser, a 20× objective lens, and a grating with 600 grooves per mm. The laser spot size and power were approximately 1 μm and 100 μW, respectively. Raman spectra were analyzed using WITec Project Plus software. Representative spectra collected over a large electrode area (at least 50 × 50 um2) are reported. X-ray and Neutron Diffraction. For X-ray diffraction (XRD) measurements, electrodes were covered with Kapton tape in an Arfilled glovebox to avoid air exposure. XRD was conducted on Scintag XDS 2000 and PANAlytical powder diffractometers with Cu Kα radiation (λ = 1.540562 Å) in the 2θ range of 10−80°. Ex-situ synchrotron XRD experiments were performed to assess how the cathode structure evolved during the first cycle. These experiments were performed at the F2 beamline of the Cornell High Energy Synchrotron Source (CHESS) at Cornell University. The wavelength of the X-ray beam was 0.295264 Å, and a 2D X-ray detector was deployed to collect the XRD patterns. To evaluate changes in Li2−xMoO3 crystallinity and cation mixing during cycling, XRD peak areas were determined by fitting data with the GaussAmp function in OriginLab. Time-of-flight (TOF) neutron powder diffraction experiments were also performed using the POWGEN instrument at the Spallation Neutron Source (Oak Ridge National Laboratory). The data were collected at 300 K using a wavelength of 0.7 Å. Rietveld refinement of the neutron diffraction data was performed using GSAS-II software.20
although the synthesis of these Mo-containing composites has not been reported to date.14−16 Li2MoO3 has R3̅m symmetry and thus is expected to coherently blend with traditional layered LiMO2 materials. Furthermore, Mo can access multiple oxidation states (e.g., Mo4+−Mo6+), allowing for reversible Li storage in both the Li2MoO3 and LiMO2 moieties. Finally, Li2MoO3 may have improved oxidative stability compared to Li2MnO3 due to reversible anionic charge compensation.14 As reported by Ceder and co-workers,17 oxygen redox activity in Li-excess layered materials originates from unique Li−O−Li configurations in which electrons can be extracted from unhybridized O 2p states. To aid the development of composite xLi2MoO3·(1− x)LiMO2 cathodes, detailed structural analysis of the Li2MoO3 component must be performed. Previous works14,18 in this area have emphasized that Li2MoO3 undergoes a reversible phase transformation during the first charge/ discharge cycle. However, these studies fail to recognize an important phenomenon shown here that Li 2 MoO 3 is irreversibly converted to an amorphous phase during extended cycling without any significant loss in its electrochemical capacity. The present work details this transformation using Raman spectroscopy, synchrotron X-ray diffraction (XRD), and transmission electron microscopy (TEM). In-situ mass spectrometry is also used to evaluate the stability of the oxygen network in Li2MoO3. Motivated by the unique properties of Li2MoO3, synthesis and electrochemical characterization of composite cathodes with the general formula xLi2MoO3·(1 − x)LiMO2 (M = Ni, Mn, and Co) are reported for the first time.
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EXPERIMENTAL SECTION
Synthesis of Li2MoO3. Li2MoO3 was synthesized by reducing Li2MoO4 under flowing Ar/H2 (96/4) at 675 °C for either 12 or 48 h. The resulting powder was immediately transferred to an Ar-filled glovebox for storage. Electrode slurries were prepared by mixing Li2MoO3, Super P Li carbon black, and poly(vinylidene fluoride) (PVDF) (80/10/10 weight ratio) in N-methyl-2-pyrrolidone (NMP). The slurry was cast onto a carbon-coated Al foil current collector and dried overnight under vacuum before preparing electrochemical cells. Synthesis of NMC and Mo-Containing Composites. NMC cathode powders (target composition: LiNi1/3Mn1/3Co1/3O2) were synthesized using a sol−gel procedure previously reported19 where Li(CH3COO)·2H2O, Ni(OCOCH3)2·4H2O, Mn(CH3COO)2·4H2O, Co(CH3COO)2·4H2O, and citric acid were dissolved in deionized water in a molar ratio of 3.15/1/1/1/3. The solution was gently heated while stirring to slowly evaporate the water and produce a solid precursor which was heated at 400 °C (5 °C/min ramp rate) for 4 h in air followed by a final heat treatment at 850 °C (5 °C/min) for 15 h in air. To prepare composite cathode powders with the formula xLi2MoO3·(1 − x)LiNi1/3Mn1/3Co1/3O2, appropriate amounts of Li2MoO3 and NMC powders were mixed using high energy ball milling (SPEX SamplePrep 8000M) for 1 h, and the resulting powder was heated at 850 °C (5 °C/min) for 15 h under flowing Ar. A onepot sol−gel synthesis route was also explored to prepare composite Mo-substituted NMC cathodes (target composition of x = 0.15). In this approach, Li(CH3COO)·2H2O, (NH4)6Mo7O24·4H2O, Ni(OCOCH3)2·4H2O, Mn(CH3COO)2·4H2O, Co(CH3COO)2·4H2O, and citric acid were dissolved in deionized water, and the solution was slowly evaporated while stirring. The resulting solid was heat treated at 400 °C (5 °C/min ramp rate) for 4 h in air followed by a final heat treatment at 850 °C (5 °C/min) for 15 h in air. Electrochemical Characterization. CR2032 half cells were constructed in an Ar-filled glovebox using a slurry cast cathode and a Li metal counter/reference electrode. The electrolyte was 1.2 M
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RESULTS AND DISCUSSION Li2MoO3 was synthesized by reducing Li2MoO4 powder at 675 °C for 12 or 48 h under flowing Ar/H2. XRD measurements (see Figure 1a) indicated the shorter reaction time yielded 5062
DOI: 10.1021/acs.chemmater.8b01408 Chem. Mater. 2018, 30, 5061−5068
Article
Chemistry of Materials
The electrochemical properties of Li2MoO3 cathodes were evaluated in half cells through cyclic voltammetry and galvanostatic charge/discharge experiments. The cyclic voltammograms in Figure 2a show that irreversible redox
Figure 1. (a) X-ray diffraction (XRD) patterns for the Li2MoO4 precursor and the product obtained after heating at 675 °C for 12 and 48 h under flowing Ar/H2. (b) Time-of-flight neutron diffraction (ND) pattern and Rietveld refinement for the synthesized phase-pure Li2MoO3 powder. (c) Scanning electron microscopy (SEM) image of the Li2MoO3 particles.
Figure 2. Electrochemical characterization of Li2MoO3 cathodes in half cells cycled between 2.0 and 4.8 V. (a) Cyclic voltammograms collected at a scan rate of 0.1 mV/s. (b) Galvanostatic charge/ discharge curves and (c) differential capacity plots collected at a current density of 10 mA/gLi2MoO3.
mixed phases of Li2MoO4 and Li2MoO3, whereas the product obtained after 48 h was phase-pure Li2MoO3 (used for all subsequent experiments in this study). Since XRD is not sensitive to light elements such as Li, a detailed structural analysis of Li2MoO3 was performed using time-of-flight neutron diffraction (ND, see Figure 1b). Rietveld refinement of the ND data showed the Li2MoO3 had R3̅m symmetry wherein Li was located in both the 3a and 3b crystallographic sites (see Table 1). The 3b site was comprised of 67.9% Mo and 32.1% Li. A scanning electron microscopy (SEM) image in Figure 1c shows the Li2MoO3 powder consisted of irregularly shaped particles which were on the order of 1−10 μm in size.
processes occurred at potentials >3.8 V vs Li/Li+ during the first oxidation scan. The peak near 3.8 V may be due to a structural rearrangement of the Li2MoO3 cathode, whereas the small peak and tail near 4.7 V are likely caused by electrolyte oxidation. During subsequent scans, a reversible redox reaction was observed at ∼2.5 V vs Li/Li+, and the electrolyte decomposition was diminished, suggesting formation of a passive film on the electrode surface. During the first galvanostatic charge cycle (see Figure 2b), a sloping potential profile over 3.6−4.8 V vs Li/Li+ was observed with a corresponding delithiation capacity of 223 mAh/gLi2MoO3 (i.e., capacity normalized to the mass of the active material). The theoretical capacity of Li2MoO3 assuming complete Li extraction is 340 mAh/g, indicating ∼34% of the Li remained in the cathode structure after charging to 4.8 V. During the subsequent lithiation cycle, a dramatically different profile was observed with a reversible capacity of 147 mAh/gLi 2MoO 3 (corresponding to 0.87 Li cycled per Mo) at an average
Table 1. Rietveld Refinement Results for Li2MoO3 Neutron Diffraction Data position atom
x
y
z
fractional occupancy
Li (3a) Li (3b) Mo (3b) O (6c)
0 0 0 0
0 0 0 0
0 0.5 0.5 0.2445(1)
0.961(18) 0.321(4) 0.679(4) 1 5063
DOI: 10.1021/acs.chemmater.8b01408 Chem. Mater. 2018, 30, 5061−5068
Article
Chemistry of Materials operating potential ∼2.5 V vs Li/Li+. The large hysteresis between the first charge and discharge steps suggests that an irreversible Li2−xMoO3 phase transformation occurred. Upon subsequent cycles, the cathode showed gradual changes to the voltage profile (also highlighted through differential capacity plots in Figure 2c) where most of the electrochemical activity occurred at potentials
