The Effect of Fluoride Doping on Lithium Diffusivity in Layered

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The Effect of Fluoride Doping on Lithium Diffusivity in Layered Molybdenum Oxide Allison Wustrow, Justin C. Hancock, Jared T. Incorvati, John T. Vaughey, and Kenneth R. Poeppelmeier ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02141 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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ACS Applied Energy Materials

The Effect of Fluoride Doping on Lithium Diffusivity in Layered Molybdenum Oxide Allison Wustrow1,2, Justin C. Hancock1,2, Jared T. Incorvati1,2, John T. Vaughey1,3, Kenneth R. Poeppelmeier1,2,*

AUTHOR ADDRESS 1. Joint Center for Energy Storage Research (JCESR), 9700 S. Cass Av. Lemont, Illinois, 60439, United States, 2. Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, United States, 3. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois, 60439, United States

*[email protected]

KEYWORDS Fluorine Doping, Molybdenum Oxide, Lithium Intercalation, Galvanostatic Intermittent Titration Technique, Batteries

ACS Paragon Plus Environment

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ABSTRACT The effects of cation doping on cathode performance has been extensively studied, however the field of anion doping has historically received much less attention. Fluoride doping can greatly increase the initial diffusivity of the layered MoO3 system. The first discharge cycle of the layered α-MoO3 and MoO2.8F0.2 phases were investigated and compared using the galvanostatic intermittent titration technique (GITT) in a lithium ion cell. The analysis revealed that a slight reduction of the oxide by fluoride doping to form a disordered fluorobronze (MoO2.8F0.2) eliminated a slow electrochemical process observed in α-MoO3. Galvanostatic cycling studies show that while α-MoO3 has a higher initial capacity, it exhibits a first cycle coulombic efficiency of only 86% with rapid capacity fade which has been associated with lithium trapping within the MoO3 layer. In contrast, the Li+ intercalation process in the fluorobronze was found to have a 94% coulombic efficiency on the first cycle. By the third cycle coulombic efficiencies greater than 99% were observed for five cycles. A thorough investigation of the synthesis of MoO2.8F0.2 is also presented. Under mild hydrothermal conditions, the fluorination of α-MoO3 to form MoO2.8F0.2 is topotactic, while a competing reaction in solution forms MoO2.4F0.6 (ReO3 structure). Methods to prevent the solution phase reaction from occurring are discussed.


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Introduction

Lithium ion batteries are the preferred energy storage for high energy density applications and are currently used commercially in electric vehicles, medical devices and consumer electronics. With this success comes a need for research on next generation battery materials. Oxide cathodes, such as LiCoO21, LiMn2O42, and Li(Ni/Mn/Co)O23, are commonly used as commercial cathodes; however, oxyfluoride materials have recently been considered as viable alternatives, including Li2VxCr1-xO2F4, LiMn2-xLixO4-yFy5, Ti1xZrxO2-yFy

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and LiNiO2-xFx7. For common cathode materials, fluoride anion doping has

been studied as a method to create mixed valent materials. Various studies have shown that these disordered oxyfluorides have improved coulombic efficiency and reduced capacity fade on cycling. For example, it was found that by doping Li2VO3 to Li2VO2F, Li+ ion conduction could be significantly improved8. Fluorospinels have also been shown to have superior capacity retention at high cycling rates compared to oxide spinels5. The high reduction potential of fluorine allows for fluoride materials to act as stabilizing agents at a wide variety of voltage ranges, such as those experienced within an electrochemical


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cell. This property has been incorporated into electrolyte design, both in salts (e.g. LiPF6)9 and additives (e.g. 4-fluoroethylene carbonate (FEC)10) within the electrolyte. For electroactive materials, fluorides and oxyfluorides have been studied as conversion cathodes, such as CFx11 and iron oxyfluoride12-13. Fluorophosphates14 and fluorosulfates15 have been studied as cathodes for sodium batteries, with many displaying a small unit cell expansion upon intercalationand their insertion potentialis slightly higher than oxygenbased phosphate and sulfate phases. The rich chemistry of fluoride containing materials has great potential in the battery field.

Molybdenum oxide, α-MoO3 is a naturally layered d0 material which has been studied as a lithium-ion cathode, and has been shown to intercalate up to 1.5 Li atoms per Mo atom16. The intercalation process is partially irreversible, owing to Li inserting into the layers of molybdenum octahedra, outside the Van der Waals gap limiting the coulombic efficiency17. MoO3 has also been studied as a multivalent cathode for both magnesium18 and calcium19 ions. MoO3 can be fluoride doped to form molybdenum fluorobronze, MoO2.8F0.2 (α-Mo(O/F)3). α-Mo(O/F)3 is structurally similar to the parent


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oxide, and although a slight change in the centering of the molybdenum atom changes the space group from Pbnm to Cmcm, the two structures can be thought of as isostructural to a first approximation. Although fluoride was initially thought to be randomly distributed throughout the three anion sites within the structure, a recent computational study by Wan et. al. found that the fluoride preferentially resides on the anion site within the layer while the terminal anions within the Van der Waals gap are primarily oxygen20. In 2015, Opra et. al. showed that lithium could intercalate into the material and concluded that the higher electric conductivity of the material contributed to its superior electrochemical activity over the parent oxide21. Incorvati et. al. later showed that while bulk MoO3 had limited Mg2+ intercalation capacity, the fluorine doped α-Mo(O/F)3 could intercalate up to 0.2 Mg2+ per molybdenum atom, compared to no measurable capacity of MoO3 under similar conditions22, demonstrating that fluoride doping can also improve multivalent electrochemistry.

This paper will discuss the selective hydrothermal synthesis of both molybdenum fluorobronzes, α-Mo(O/F)3 and β-Mo(O/F)3 (MoO2.4F0.6), a cubic ReO3 material which is


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more reduced than α-Mo(O/F)3 and which adopts a structure closely related to β-MoO3 23.

The factors which control the extent of reduction under hydrothermal conditions will be

discussed in detail. In addition, the Li intercalation electrochemistry of α-MoO3 and αMo(O/F)3 are compared using galvanostatic cycling. Li diffusion during the first discharge process is compared using the galvanostatic intermittent titration technique (GITT)24. Results from GITT are compared to the Li+ intercalation behavior reported in this paper as well as to the previously reported Mg2+ intercalation behavior seen by Incorvati et. al22.

Experimental

Caution: Hydrofluoric acid is toxic and corrosive! It must be handled with extreme caution and the appropriate protective gear and training25-27.

Synthesis: In a typical synthesis, MoO3 (1 g, 6.9 mmol) and Mo metal (25 mg, 0.26 mmol) were combined in a mortar and pestle and 1 mL of hydrofluoric acid (49% HF in water by weight) was added. For microwave reaction conditions, the starting material was placed into a Teflon tube, and the mixture was heated in a SynthWAVE single reaction chamber microwave synthesis system made by Milestone. In a typical heating


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profile, the reaction was heated to 240 oC over 10 minutes, allowed to dwell at temperature for one hour, before being cooled for one hour, however the reactions which most consistently yielded α-Mo(O/F)3 had a maximum temperature of 160 oC. For traditional hydrothermal reactions, the reactants were sealed in a Teflon pouch, loaded into a 125 mL Parr acid digestion vessel, heated at 5 oC/min to 240 oC, then held at 240 oC

for 48 hours before being cooled to room temperature at 10 oC/min. The resulting

mixture was filtered and rinsed with deionized water. Powder x-ray diffraction (PXRD) measurements were taken using an Ultima IV x-ray diffractometer (Rigaku) with Cu radiation from 10o to 55o. Scanning electron microscopy (SEM) measurements were taken using a Hitachi SU8030 microscope with a 5 kV accelerating voltage.

Electrochemistry: Both MoO3 and α-Mo(O/F)3 were cycled against lithium metal. In a typical test, a slurry consisting of 80% active material, 10% Timcal C45 carbon, and 10% polyvinylidene fluoride by mass was laminated onto steel foil. Lithium foil was used as the anode. A 1.2 M solution of LiPF6 in a 3:7 mixture of ethylene carbonate and ethyl methyl carbonate (Gen 2) was used as the electrolyte. 2032-type coin cells were


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assembled in an argon glovebox with a glass fiber separator (Whatman, GF/F). A typical cell had an active mass of ~5 mg. The cells were cycled in a MACCOR cycler at constant current mode at C/20 where C=186 mA/g (1 Li+ intercalated per Mo atom) within the voltage window of 1.5 V – 3.3 V vs Li metal for 20 cycles. The galvanostatic intermittent titration technique (GITT) was also performed on these cells. For these experiments, a 0.012C current was applied to the cell for 30 minutes, with the voltage being measured every 30 seconds. The current was then turned off and the system was allowed to return to its equilibrium voltage with the voltage being measured every 3 minutes. The relaxed voltage was determined by averaging the last 5 voltage measurements in this window.

𝑑𝐸 𝑑𝛿

was determined by a least squares regression of the 5 𝑑𝐸 𝑡

nearest data points in the plot of the relaxed voltage against x in LixMo(O/F)3. 𝑑

was

found by a least squares regression of the voltage while current was being applied across the cell. The first voltage taken after the current was applied was not used owing to ohmic shift. The diffusion coefficient was determined by equation 124, where Vm is the molar volume of the material, I is the current applied, S is the surface area of the


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cathode, F is faradays constant, and D is the diffusion coefficient. In cases where GITT was performed on a charging cycle of the material, it was immediately proceeded by a discharge to 1.5 V at a current of C/20. Cathode surface area was determined by SEM as detailed in the supporting information. Particles were assumed to be flat plats, and from representative images, the surface area of a gram of material was determined.

𝑑𝐸 𝑑 𝑡

2𝑉𝑚𝐼 𝑑𝐸 𝐷𝜋 𝑑𝛿

(1)

= 𝑆𝐹

Results and Discussion

Synthesis of α-Mo(O/F)3 and β-Mo(O/F)3

Both α-Mo(O/F)3 and β-Mo(O/F)3 can be synthesized using the same basic hydrothermal synthesis procedure, which uses Mo metal as a reducing agent and HF as a fluorinating agent. These experiments were performed both with a traditional hydrothermal method inside a sealed acid digestion vessel, as well as in a microwave hydrothermal reactor. Both methods yielded similar results, however, reactions could be performed in the microwave much more quickly than in a traditional oven set up. To


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obtain a phase pure sample of either phase, it is important to control the extent of reduction of the α-MoO3 starting material. Increasing the amount of Mo metal added to the reaction increases reduction leading to the formation of β-Mo(O/F)3 (Figure 1), however, decreasing the amount of Mo metal in the reaction vessel limits the yield of the reaction. In order to obtain α-Mo(O/F)3 in high purity and yield, several additional parameters must be tuned.

Figure 1: Powder X-Ray Diffraction Pattern of fluorine doped MoO3 synthesized with increasing molybdenum metal content. The reaction temperature strongly affects the distribution of products. Increasing the temperature to 200oC favors β-Mo(O/F)3, while lower temperatures limit the extent of reduction, leading to α-Mo(O/F)3, and if the reaction temperature is low enough, unreacted α-MoO3 (Figure 2). At higher temperatures, a lower yield is observed. As


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temperature increases, the solubility of the reagents increases as well. Because yield decreases as more of the reagents dissolve, it can be concluded that the reaction happens on the surface of undissolved α-MoO3 rather than in the aqueous phase. Further support of this claim can be made by comparing syntheses using α-MoO3 as received to reactions where α-MoO3 was ball milled prior to use. As-received α-MoO3 consists of large plates on the order of 30 µm, which can be ball milled into splintered rods of approximately 1 µm in diameter. α-Mo(O/F)3 samples synthesized using these two starting materials maintain the morphology of α-MoO3 used (Figure 3), indicating that the starting material is never fully dissolved, although all of the α-MoO3 has been reacted. Furthermore, α- Mo(O/F)3 does not form when the α-MoO3 reagent is replaced with the more soluble molybdic


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Figure 2: Powder X-Ray Diffraction Pattern of fluorine doped MoO3 synthesized at increasing temperatures.

Figure 3: SEM images of α-Mo(O/F)3 synthesized from A.) ball milled α-MoO3 and B.) asreceived α-MoO3.


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acid (H2MoO4). These observations are consistent with a topotactic mechanism where the reaction begins on the surface of the oxide and propagates inwards.

As the reduction of α-MoO3 to form either α-Mo(O/F)3 occurs primarily on the surface of the oxide, it is possible to limit reduction of the material by decreasing the effective surface area of the reagent. This was achieved by loading greater amounts of solid precursor into the reaction vessel. As the reaction vessel used in microwave reactions was a Teflon tube approximately ½″ in diameter, adding additional solid to the vessel did not affect the interfacial area between solid and liquid. Therefore, high reagent loading limits reduction, allowing for the formation of α-Mo(O/F)3, despite the solution being more reducing as a higher amount of Mo metal is dissolved in the same volume of HF (Figure 4). Furthermore, because only a certain amount of the solid reagents can dissolve in a given volume, the yield of the reaction increases with higher loading. α-Mo(O/F)3 can be synthesized in high purity and yield by using a moderate temperature (⁓160 oC) with a high reagent loading and minimal Mo metal added as a reducing agent


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Figure 4: Diffraction patterns of fluorobronze materials synthesized at different reagent loadings.

Electrochemistry of α-Mo(O/F)3

Li insertion into α-Mo(O/F)3 was compared to lithium insertion in α-MoO3. The oxide has been well studied in the literature and has been reported to intercalate up to 1.5 lithium ions per molybdenum atom in the structure16, consistent with capacities observed in this study. The fluorobronze exhibited a much lower capacity at the same rate, only producing LixMoO2.8F0.2 with x as high as 1.2 (Figure 5). A lower capacity is to be expected in the fluoride doped material, owing to the partial reduction of molybdenum in the charged state to Mo5.8+. After the first discharge to 1.5 V, both the


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pristine and the fluoride doped oxide had reached a Mo oxidation state of Mo4.6+. Capacity, however, was retained between cycles much more effectively in α-Mo(O/F)3. After 10 cycles at a rate of C/20, the doped material maintained 80% of its original capacity, while the oxide had only 53% of its original capacity (Figure 6). The high coulombic efficiency of α-Mo(O/F)3 indicates that no irreversible side reactions, including the formation of LiF are occurring. Despite an initially lower capacity, the high capacity retention of the fluorobronze allows it to exhibit capacities significantly greater than its parent oxide after around 5 cycles.. A closer look at the charge and discharge curves of these two materials helps shed some light on this difference.


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Figure 5: The discharge (A) and charge (B) curves for the first cycle of α-MoO3 and αMo(O/F)3.


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Figure 6 Capacity vs cycle of α-Mo(O/F)3 (blue) and α-MoO3 (red) cycled against lithium at a rate of C/20, as well as the coulombic efficiencies of α-Mo(O/F)3 (cyan) and α-MoO3 (orange) While the discharge behavior of the pristine and fluoride doped samples are mostly analogous at low rates, the charging cycles are vastly different. At low rates, the fluoride doped material’s charging is dominated by a long extremely flat plateau at 2.4V. In contrast, the voltage of the oxide changes more drastically at all points as lithium is removed. While both the pristine and doped oxide exhibit voltage plateaus indicative of a phase transformation on the first discharge, this process appears to be reversible only in α-Mo(O/F)3. The plateau associated with the phase transformation is absent in the second discharge curve of α-MoO3 and in all subsequent discharges. The dQ/dV plots of the first and fifth cycle of both cathodes show that while there are no irreversible


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processes in the first cycle of α-Mo(O/F)3, the two sharp peaks that appear in the first discharge of α-MoO3 do not appear in its first charge cycle (Figure 7). In subsequent cycles α-Mo(O/F)3 maintains the same major processes, while the shape of the α-MoO3 curve is significantly changed, further showing the initial intercalation processes are not sustained.

Figure 7: dQ/dV plots of the first cycle of lithium intercalation/deintercalation in α-MoO3 (A) and α-Mo(O/F)3 (B).


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The chemical diffusion coefficient (Dchem) for the first discharge and the first charge cycle of both α-MoO3 and α-Mo(O/F)3 was determined by GITT (Figure 8). Dchem is the diffusion coefficient of the intercalation species, in this case a lithium ion and the corresponding electron28. Dchem is related to DLi by equation (2) where δ is

𝑑𝐸

(2)

𝐷𝑐ℎ𝑒𝑚 = DLi(1 ― 𝛿)𝛿 𝑑𝛿 /𝜅𝑇

the molar fraction of Li within the cathode,

𝑑𝐸 𝑑𝛿

is the change in the equilibrium voltage as

lithium is inserted, κ is the hopping rate constant, and T is the temperature. An increase in Dchem therefore indicates either an increase in DLi or a reduction of κ29. Obtaining numerical values for these constants would require measurements at multiple temperatures, which was not done in this study. However, it can be stated that a higher Dchem corresponds to a more mobile lithium species. It should be noted that the initial electrical conductivity of the fluorobronze (1.8·10-6 S/cm) has been shown in the literature to be much higher than that of the oxide (4.4·10-9 S/cm )21. The lower electrical conductivity is a contributor to the initial difference in Dchem seen between the parent oxide and the fluoride doped sample, as it limits the rate at which the electrochemical


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process of a lithium ion and a charge balancing electron inserting into the cathode can occur.

Both the oxide and the fluorobronze had maximum Dchem of 10-12 cm2/sec. Typical maximum diffusion coefficients can range from 10-14 cm2/sec (LiFePO430) to 10-8 cm2/sec (Li2MnO331). However, as GITT measurements on the same phase can often differ by as much as an order of magnitude32-33 between papers owing to different approximations used to measure surface area, it is difficult to compare the values found in this work to the literature in a meaningful fashion. The oxide and fluorobronze studies presented here were done in the same laboratory under the same conditions, so direct comparison is possible. It was found that in the early stages of lithium intercalation, Dchem of the oxide begins at 10-16 cm2/sec and increases steadily until 0.2 Li per formula unit have been intercalated into the material, after which it remains at 10-12 cm2/sec until an additional 0.2 Li have been intercalated. In contrast, the first intercalation process in the fluorobronze has a Dchem of 10-12 cm2/sec for the first 0.4 Li that intercalate into the material into the material. Fluoride-doping and thus partial reduction of the sample


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allows the sluggish first intercalation step to be bypassed. This observation of mobility with respect to lithium intercalation may have implications pertaining to the intercalation of less mobile cations into layered molybdenum oxide species. Previous work from Incorvati et. al. has shown that Mg2+ intercalates far more readily into α-Mo(O/F)3 than into α-MoO322. Mg2+ has only been reported to intercalate into undoped α-MoO3 when the cathode is a thin film18. These observations can be attributed to the initial diffusion coefficient, which is 4 orders of magnitude lower in the oxide. Magnesium is effectively unable to intercalate into the oxide owing to the low rate, so no capacity is observed. Conversely, in α-Mo(O/F)3 the magnesium intercalation process has a maximum capacity of 75 mAh/g (y = 0.2 for MgyMoO2.8F0.2). In the GITT of lithium intercalation, Dchem of α-Mo(O/F)3 decreases rapidly by an order of magnitude starting at 75 mAh/g (x = 0.4 for LixMoO2.8F0.2) indicative of a sudden drop in the mobility of the cation. While the softer Li+ ion is still mobile enough in the lattice to continue intercalation, the more highly charged Mg2+ cation gets trapped, and electrochemical intercalation cannot continue. A more generalized relationship between the lithium intercalation chemistry and the magnesium intercalation chemistry will be a problem for future study, however,


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GITT may provide a facile way to screen potential multivalent cathodes and to determine the extent to which they must be doped before intercalation can occur.

Figure 8: Diffusion coefficients during the first discharge cycle of α-MoO3 (red) and αMo(O/F)3 (blue) obtained using GITT. Conclusions

Both α-Mo(O/F)3 and β-Mo(O/F)3 can be controllably synthesized using a microwave hydrothermal reaction. Because of the topotactic nature of this reaction, the scale at which the reaction was performed strongly affected the obtained product. The Li intercalation processes of both α-Mo(O/F)3 and the structurally similar α-MoO3 were compared.α-MoO3 had an initial discharge capacity of 1.5 Li atoms per formula unit, while the partially reduced α-Mo(O/F)3 intercalated only 1.2 Li atoms under the same


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conditions. However, it was found that by fluorine doping α-MoO3, the Li+ intercalation process became reversible, and over 80% of the initial capacity was retained after 10 cycles . GITT was performed on both systems, and it was shown that the diffusion coefficients for both systems had a maximum of 10-12 cm2/sec. The GITT data of the first discharge process correlated well with previously reported Mg2+ intercalation studies. Lithium GITT may be a facile way to screen potential multivalent cathodes across a wide range of doping levels with a single experiment.

AUTHOR INFORMATION

Corresponding Author *Kenneth R. Poeppelmeier * [email protected]

Associated Content

Supporting Information

S1, S2: Galvanostatic cycling of α-MoO3 and α-Mo(O/F)3 at C/20. S3: Galvanostatic cycling of β-Mo(O/F)3 at C/20. S4: Determination of active surface area. S5-S8:


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Supplemental figures relating to GITT. S9: Dchem of the first charge cycle for α-MoO3 and α-Mo(O/F)3.

Author Contributions A.W. performed the electrochemical experiments and most of the synthetic experiments, and wrote the manuscript with input from all of the authors. J.C.H. and J.T.I. assisted in performing many of the synthetic experiments. J.T.V and K.R.P. supervised the electrochemical and synthetic work respectively. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. This material is available free of charge via the internet at http://pubs.acs.org.

Funding Sources ACKNOWLEDGMENT This work was supported as a part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This work made use of the Jerome B. Cohen X-Ray Diffraction


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Facility supported by the MRSEC program of the National Science Foundation (DMR1720139) at the Materials Research Center of Northwestern University and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205.), as well as the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.

1.

Mizushima, K.; Jones, P.; Wiseman, P.; Goodenough, J. B., LixCoO2 (0< x

The Effect of Fluoride Doping on Lithium Diffusivity in Layered

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