LiMO2 Composite Cath

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Synthesis, 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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01408 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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Chemistry of Materials

Synthesis, Electrochemical, and Structural Investigations of Oxidatively Stable Li2MoO3 and xLi2MoO3•(1-x)LiMO2 Composite Cathodes Ethan C. Self1, Lianfeng Zou2, Ming-Jian Zhang3, Richard Opfer1, Rose E. Ruther4, Gabriel M. Veith1, Bohang Song5, Chongmin Wang2, Feng Wang3, Ashfia Huq5, and Jagjit Nanda1* 1

Materials Science and Technology Division, 4 Energy and Transportation Science Division, and 5 Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 2 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352 3 Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, NY 11973 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.

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 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-Mn-rich NMC (LMRNMC, 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 LMRNMC cathodes is limited by Mn dissolution in the electrolyte and voltage fade which occur 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, although the synthesis of these Mo-containing composites has not been reported to date.14-16 Li2MoO3 has R3m 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 coworkers17, 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•(1x)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

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shown here that Li2MoO3 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 Xray 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.

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 one-pot 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 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 hour 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

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were collected between 2.0 – 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 – 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 (R3m 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 20x 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 x 50 um2) are reported. X-Ray and Neutron Diffraction: For X-ray diffraction (XRD) measurements, electrodes were covered with Kapton tape in an Ar-filled 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 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

RESULTS AND DISCUSSION


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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 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 R3m 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.

Table 1. Rietveld refinement results for Li2MoO3 neutron diffraction data. Atom Li (3a) Li (3b) Mo (3b) O (6c)

x 0 0 0 0

Position y 0 0 0 0

z 0 0.5 0.5 0.2445(1)

Fractional Occupancy 0.961(18) 0.321(4) 0.679(4) 1

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.

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 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/gLi2MoO3 (corresponding to 0.87 Li cycled per Mo) at an average 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 < 3.5 V vs. Li/Li+ due to the oxidation/reduction of the Mo redox center. The absence of a flat voltage plateau suggests that the lithiation/delithiation proceeded through a disordered phase with a distribution of reaction site energies. After 50 cycles, the electrode had a reversible capacity of 119 mAh/g with moderate voltage fade. Overall, the results in Figure 2 are consistent with previous reports16, 21-23 on the electrochemical properties of Li2MoO3.



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not observed here. Possible explanations for this result are: (i) the electrolyte decomposed to form solid products which were not detectable with the mass spectrometry setup and/or (ii) electrolyte oxidation to O2/CO2 was kinetically slow on the Li2MoO3 cathode.

Figure 3. (a) Diagram of the electrochemical cell (image adapted with permission from EL-Cell GmbH) used for in-situ mass spectrometry studies. (b) Measured O2 and CO2 profiles evolved from the cell during charge and discharge.

Figure 2. Electrochemical characterization of Li2MoO3 cathodes in half cells cycled between 2.0 – 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.

Under the aggressive cycling tests used in Figure 2 (i.e., charging to 4.8 V vs. Li/Li+), the lattice oxygen in most conventional cathodes (e.g., LiMO2) becomes unstable, resulting in gas evolution and poor cycling stability.24 Although Li2MoO3 is not a high voltage cathode by itself (as is evident from Figure 2), knowing the oxidative stability at high voltage is a useful design rule to determine whether Li2MoO3 is a good candidate for a structural subunit in high voltage composite cathodes. To evaluate the oxidative stability of Li2MoO3, in-situ mass spectrometry was used to monitor O2 and CO2 gas evolution (two common decomposition products of Li-ion battery cathodes and electrolytes) during electrochemical delithiation/lithiation. A picture of the gas cell is shown in Figure 3a, and the corresponding potential and gas profiles are given in Figure 3b. After a 1 h rest at open-circuit, the background O2 and CO2 levels due to electrolyte evaporation were 0.23 and 0.16%, respectively. Upon galvanostatically charging/discharging the cell, the O2/CO2 gas levels remained at or below their baseline values, demonstrating that the oxygen network in Li2MoO3 was stable up to 4.8 V vs. Li/Li+. Interestingly, the liquid carbonate electrolyte may be expected to produce O2 and/or CO2 gases beyond its thermodynamic stability window (approximately 1.3 - 4.5 V vs. Li/Li+)25, but this was

A series of ex-situ synchrotron X-ray diffraction (XRD) experiments were performed to assess how the cathode structure evolved during the first cycle. In these studies, the cathodes were cycled to a predetermined voltage (see Figure 4a) at 10 mA/gLi2MoO3 followed by a 5 h potential hold. The harvested cathodes were thoroughly rinsed with dimethyl carbonate and dried in the glovebox prior to characterization. The resulting XRD patterns at various states of charge throughout the first cycle are shown in Figure 4b with particular regions of interest emphasized through contour plots shown in Figures 4d and 4e. The intensity of the Li2MoO3 diffraction peaks decreased throughout the first cycle. One possible explanation for this observation is loss of crystallinity due to formation of an amorphous phase (note that this interpretation is shown to be correct using Raman spectroscopy and selected area electron diffraction (SAED) as is discussed later in the text). To qualitatively determine the extent of amorphization, the integrated area of the Li2MoO3 (003) peak was divided by that of the Al current collector’s (111) peak. Since all samples had approximately the same loading (2.77 ± 0.80 mgLi2MoO3/cm2), decreases in the (003)Li2MoO3/(111)Al ratio correspond to reduced crystallinity of the Li2-xMoO3 phase. As shown in Figure 4c, the pristine electrode had a (003)Li2MoO3/(111)Al ratio of 0.67 which rapidly decreased to 0.12 at the end of the first charge. Upon lithiation, this ratio did not change significantly, indicating the Li2MoO3 irreversibly lost much of its crystallinity during the first cycle. The XRD patterns of the cycled electrodes also showed changes in the relative intensity of the (003)/(104) peaks which is related to the degree of cation mixing. The pristine electrode had a (003)/(104) peak ratio of 1.26, indicating the presence of a well-ordered R3m structure with Mo primarily located in the 3b sites.26 This conclusion is supported by the Rietveld refinement results shown in Table 1. The (003)/(104) ratio decreased substantially to 0.83 after the first


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cycle, demonstrating the Li2MoO3 became more disordered upon electrochemical cycling. These results contradict a recent report by Ma et al.14 who claimed that Li2MoO3 undergoes a reversible phase transformation involving reversible Mo-ion migration to/from the Li vacancies. The irreversible structural transformation which occurred during the first cycle was also explored using ex-situ Raman spectroscopy. Figure 5 shows Raman spectra for Li2MoO3 cathodes harvested at various stages during the first cycle. The pristine Li2MoO3 powder and uncycled electrode exhibited several distinct Raman bands which agree well with previous reports.15, 27 The bands located at 286, 364, and 921 cm-1 are assigned to Mo=O vibrational modes whereas the bands at 493 and 729 cm-1 are due to symmetric and antisymmetric Mo-OMo stretches, respectively.28, 29 The Raman signature was unchanged when charging up to 3.75 V vs. Li/Li+. However, when charging beyond 4.0 V (corresponding to a capacity of 113 mAh/g and a cathode stoichiometry of Li1.33MoO3), dramatically different spectra were recorded. Specifically, two broad bands appeared which were centered near 300 and 880 cm-1 and contained multiple convoluted peaks. These features changed subtly throughout the delithiation/lithiation process, but the electrode clearly did not return to the original structure when discharged to 2.0 V. In general, the broadening of the Raman bands indicates a loss of symmetry in the Li2-xMoO3 cathode structure due to formation of a disordered, amorphous phase, a conclusion which is consistent with the XRD data presented in Figure 4. Similar band broadening has also been observed for nanostructured V2O5 cathodes which undergo a crystalline to amorphous transformation during lithiation.30 To understand how the structure evolved beyond the first cycle, additional XRD data (using Cu Kα radiation, λ = 1.540562 Å) and Raman spectra were collected on cathodes in either the charged or discharged states after 10 cycles. As shown in Figure 6a, the diffraction peaks of the cycled elec-

trodes were very weak compared to that of the pristine sample. Note that the broad peaks near 2θ = 20° are due to the Kapton tape which was used to avoid air exposure during the XRD measurements. The Raman spectra of the cycled electrodes (see Figure 6b) were similar to those obtained during the first charge/discharge cycle (Figure 5) with two very broad peaks centered around 300 and 880 cm-1. Interestingly, there was no notable difference in the Raman spectra of cathodes in either the charged or discharged states after 10 cycles. To further study the nature of the Li2-xMoO3 phase which formed during cycling, ex-situ TEM images were collected for a pristine cathode and cathodes in the discharged state after 1, 2, and 50 cycles. The morphology and corresponding SAED patterns of these samples are shown in Figures 7(a-d) and 7(eh), respectively. The pristine sample showed the presence of a highly crystalline Li2MoO3 phase as indicated by the periodic spacing of the diffraction spots collected from the (100) zone axis. After 1 and 2 cycles, the SAED patterns contained a few, low intensity spots corresponding to a minor crystalline phase with R3m symmetry. The predominantly diffuse patterns indicates an amorphous phase nucleated throughout the material, a result which is consistent with the XRD and Raman data presented in Figures 4-6. After 50 cycles, the SAED pattern (Figure 7h) was very diffuse without discernible diffraction spots, which demonstrates the cathode became completely amorphous after extended cycling. This cycling-induced transformation is presumably caused by non-uniform stresses that disrupt the periodicity of the Li2-xMoO3 structure as lithium is extracted and reinserted.31 These effect of these stresses is cumulative, meaning the amorphization occurs gradually over many cycles. Note that this phenomenon is not unique to Li2MoO3; similar behavior has been reported with other Li-ion cathode chemistries including V6O13 and Li2MnxFe1-xSiO4.32, 33

Figure 4. (a) Charge/discharge curve of a Li2MoO3 cathode with the corresponding (b) ex-situ synchrotron XRD patterns. (c) Ratio of integrated peak areas for (003)Li2MoO3/(111)Al and (003)/(104) at various cutoff potentials during the first charge/discharge cycle. Closed and



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open symbols represent ex-situ XRD samples which were collected during charging and discharging, respectively. The (003)/(104) peak analysis was conducted on electrode powders which were removed from the current collector to improve the signal-to-noise ratio. (d-e) Contour plots highlighting changes in the relative intensity of the (003) and (104) peaks compared to the (111) peak from the Al current collector.

Figure 7. (a-d) Transmission electron microscopy (TEM) images and (e-h) corresponding selected area electron diffraction (SAED) patterns collected from the (1 0 0) zone for a Li2MoO3 cathode before cycling and after 1, 2, and 50 cycles.

Figure 5. Ex-situ Raman spectra collected for Li2MoO3 cathodes at various stages during the first cycle. Note that electrodes were characterized at the same states of charge used for the ex-situ synchrotron XRD experiments shown in Figure 4.

Figure 6. (a) Ex-situ XRD and (b) ex-situ Raman spectra for Li2MoO3 cathodes before cycling and after 10 cycles in both the charged (4.8 V) and discharged (2.0 V) states.

The Li storage properties and excellent oxidative stability of Li2MoO3 make it a promising candidate as a structural stabilizer in composite xLi2MoO3•(1-x)LiMO2 cathodes. As an initial investigation of these new materials, composite cathodes containing Li2MoO3 and LiNi1/3Mn1/3Co1/3O2 (NMC) subunits with x = 0.10 – 0.15 were prepared by ball-milling the two endmembers followed by heat treatment under Ar. The NMC powder used in the composites was synthesized via a sol-gel route and was phase-pure with R3m symmetry as indicated by the XRD and ND data in Figures 8a and 8b, respectively. The ball-milled composite consisted of separate Li2MoO3 and NMC phases with a cubic (space group Fm3m) Li3MoO4 impurity phase (present at ca. 5%) as determined from refinement of ND data (Figure 8c). The presence of a multi-phase structure was confirmed by XRD data which showed splitting of the (0 0 3) reflection (see Figure S1). A one-pot sol-gel synthesis route using metal acetate and ammonium molybdate precursors was also explored to prepare the Mo-substituted NMC. However, this approach resulted in over-oxidation of the Mo to the 6+ state (i.e., formation of Li2MoO4 as shown in Figure S2) and was thus not further explored. Figure 9 compares the electrochemical properties of a baseline NMC cathode (x = 0) with that of composite cathodes (x = 0.10) prepared by ball-milling NMC and Li2MoO3 endmembers followed by either: (i) no heat treatment or (ii) heat treatment at 850°C for 15 h under Ar (i.e., the same conditions presented in Figure 8). The NMC showed a high initial capacity of 177 mAh/g with an average operating potential of 3.9 V vs. Li/Li+ when cycled between 2.0 – 4.5 V (see Figures 9a and 9b). The composite which was not heat treated exhibited a plateau ~3.9 V and a sloping profile in the range of 2.0 3.5 V. This profile is essentially a superposition of the charge/discharge behavior of the two endmembers, indicating the unheated composite consisted of sequestered Li2MoO3 and NMC domains. On the other hand, the composite which was heated at 850°C showed a single voltage plateau ~3.9 V with no appreciable capacity below 3.5 V. This result suggests Ni/Mn/Co were the main redox centers of the heated composite, and the Mo provided negligible charge compensation despite comprising 10% of the structure. Compared to NMC, the heated composite had a 24% lower reversible capacity


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during the first cycle (135 vs. 177 mAh/g for NMC) but showed improved cycling stability with 84% capacity retention over 20 cycles (compared to only 76% for NMC). The unheated composite showed the worst cycling behavior with the lowest capacity and capacity retention after 20 cycles. These results demonstrate Li2MoO3 can effectively stabilize conventional LiMO2 cathodes when incorporated into a composite structure. The improved cycling stability of the composite may be due to either: (i) the presence of the Li-rich Li2MoO3 phase or (ii) the lower degree of delithiation when charging the NMC subunit. Future work on these Mocontaining composites will focus on optimizing the cathode’s performance by exploring various synthesis routes and degrees of Mo-substitution. A fundamental understanding of how Li2MoO3 affects the cathode’s structural stability at high states of charge will also be developed. Figure 9. Electrochemical performance of half-cells containing either an NMC (LiNi1/3Mn1/3Co1/3O2) or ball-milled composite 0.10Li2MoO3•0.90LiNi1/3Mn1/3Co1/3O2 cathode. Charge/discharge curves collected during (a) cycle 1 and (b) cycle 10. (c) Cycling stability and (d) capacity retention over 20 cycles. The ball-milled composites were prepared either with or without an 850°C heat treatment as indicated. Cells were cycled galvanostatically at 20 mA/g between 2.0 – 4.5V.

CONCLUSIONS

Figure 8. (a) XRD and (b-c) ND of (b) NMC and (c) composite 0.15Li2MoO3•0.85LiNi1/3Mn1/3Co1/3O2 cathode powders. The composite was prepared by ball milling followed by heat treatment at 850°C for 15 h in Ar.

The structural evolution of Li2MoO3 during electrochemical delithiation/lithiation is reported. An irreversible crystalline to amorphous transformation was initiated when the cathode was charged to potentials ≥ 4.0 V vs. Li/Li+. This amorphization reaction proceeded with further cycling, and there was no apparent crystallinity remaining in the cathode after 50 cycles. These conclusions are supported by complementary information obtained from XRD, Raman spectroscopy, and TEM. The significant hysteresis in the initial charge/discharge curves of Li2MoO3 provides further proof that an irreversible structural rearrangement occurred. These findings present strong evidence against the reversible phase transformation mechanism proposed by Ma et al.14 Interestingly, the amorphization of Li2MoO3 did not prevent reversible Li storage from occurring. Li2MoO3 cathodes exhibited an initial reversible capacity of 147 mAh/gLi2MoO3 with an average operating potential ~2.5 V vs. Li/Li+. The cathodes retained 81% of their initial capacity after 50 cycles. In-situ mass spectrometry studies showed that the oxygen network in Li2MoO3 was stable even when charged to very positive potentials (i.e., 4.8 V vs. Li/Li+). The excellent oxidative stability of Li2MoO3 combined with its Li storage properties can potentially be used to stabilize traditional LiMO2 materials in high energy density layered-layered composite cathodes with the general formula xLi2MoO3•(1-x)LiMO2. Highenergy ball milling of Li2MoO3 and NMC was explored as a viable strategy to produce these composite structures. Compared to a baseline NMC cathode, a composite with 10% Mo substitution had lower initial capacity (135 vs. 177 mAh/g) but improved the cycling stability (e.g., 84 vs. 76% capacity retention after 20 cycles between 2.0 – 4.5 V). Future studies will investigate the effect of different Mo content on the structure and electrochemical properties of this new class of composite cathodes.



ASSOCIATED CONTENT Supporting Information. Additional X-ray and neutron diffraction patterns for composite Mo-NMC cathodes. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION

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(8) Wu, Y.; Manthiram, A., High Capacity, SurfaceModified Layered Li[Li(1−x)⁄3Mn(2−x)⁄3Nix⁄3Cox⁄3]O2 Cathodes with Low Irreversible Capacity Loss. Electrochem. Solid State Lett. 2006, 9, A221. (9) Kim, S.; Aykol, M.; Hegde, V. I.; Lu, Z.; Kirklin, S.; Croy, J. R.; Thackeray, M. M.; Wolverton, C., Material design of high-capacity Li-rich layered-oxide electrodes: Li2MnO3 and beyond. Energy Environ. Sci. 2017, 10, 22012211.

Corresponding Author * Email: [email protected]

ACKNOWLEDGMENT This research is supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) Program. X-ray diffraction and scanning electron microscopy were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Neutron diffraction at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, DOE.

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LiMO2 Composite Cath

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