Composite of LiFePO4 with Titanium Phosphate ... - ACS Publications

Sep 18, 2013 - Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Ave., Argonne, Illinois ... 845 West Taylor St...
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Composite of LiFePO4 with Titanium Phosphate Phases as LithiumIon Battery Electrode Material Gary M. Koenig, Jr.,*,†,∥ Jiwei Ma,† Baris Key,† Justin Fink,† Ke-Bin Low,‡ Reza Shahbazian-Yassar,‡,§ and Ilias Belharouak*,† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Ave., Argonne, Illinois 60439, United States ‡ Research Resources Center - East (M/C 337), The University of Illinois at Chicago, 845 West Taylor St., Chicago, Illinois 60607-7058, United States § Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Dr., Houghton, Michigan 49931, United States S Supporting Information *

ABSTRACT: We report the synthesis of LiFePO4 (LFP) battery materials where during synthesis the iron has been substituted by up to 10 mol % with titanium. Analysis of the Ti-substituted materials revealed that at the substitution levels investigated, the Ti did not form a solid solution with the LFP, but rather minority phases containing Ti phosphates were formed and segregated at the nanoscopic scale. The minority phases were amorphous or not well-crystallized and accepted Li on first discharge in a lithium half cell, and solid state NMR spectra were consistent with one of the constituents being LiTi2(PO4)3. The Ti substituted materials had increased electrochemical capacities and discharge voltages relative to LFP prepared in an equivalent process, and the ability to accept Li on first discharge may find utility in using previously inaccessible capacity in battery cathode formulations and in balancing excess capacity from high energy cathode materials.

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(≤1% Fe substitution) levels.6−10 The exact cause of the conductivity improvements has been the focus of many research studies.11−14 In this work, we will describe LFP-based materials where we substituted the Fe in LFP for larger fractions, up to 10%, of Ti in an effort to determine the influence of potential Ti impurity phases on the electrochemical performance of Ti-substituted LFP. We hypothesized that at these high levels of substitution minority Ti-containing phases would form a composite with the host LFP material, and that at this level of substitution we would be able to identify the minority phases that formed. We also wanted to develop a single-step process, where the post treatment of the LFP composite material does not require nanosizing or carbon coating after furnace firing. The resulting materials reported herein were comprised of a composite of LFP and amorphous or not well-crystallized Ti phases, in contrast to a recent report of a solid solution at high Ti doping.15 The titanium phases and LFP phase showed evidence of segregating on a nanoscopic level. Our resulting material exhibited excellent rate capability as a cathode material relative to LFP synthesized under identical conditions, and did not require any milling or carbon coating treatment of the LFP-Ti

INTRODUCTION Two of the challenges of lithium-ion batteries for transportation include lowering the cost and increasing safety.1 One material that has shown great promise in potentially meeting these two challenges is LiFePO4 (LFP).2 The raw materials for LFP are relatively low cost, in particular because the redox-active transition metal is iron as opposed to other more expensive transition metals used in cathodes such as cobalt or nickel. LFP has also been reported to have very good resistance to oxygen release during thermal abuse, an important safety indicator.2 Although LFP has become a widely studied and commercially relevant material for battery applications, one of the drawbacks of this material is processing costs which are necessary to maximize rate capability. When LFP was first reported, it was noted that the material has low electronic conductivity.3 Following the discovery of LFP as a cathode material, further developments included nanosizing the particles via high energy milling4 and/or carbon coating to improve the electronic conductivity of LFP,5 and these efforts have resulted in the high performance LFP battery materials reported to date. The additional processing of LFP to improve the electrochemical performance adds to the overall material cost. One strategy that has been attempted to improve the electronic conductivity of LFP withough nanosizing or carbon coating steps is doping. Studies have investigated doping of the olivine LFP framework with other transition metals such as Zr, Ti, Nb, and Mg at low © 2013 American Chemical Society

Received: July 25, 2013 Revised: September 13, 2013 Published: September 18, 2013 21132

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Figure 1. (A) Plot of voltage as a function of time during the first two charge and discharge cycles of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ composite cathode material in a lithium half cell at a rate of 10 mA/g between 2.0 and 4.0 V. (B) Close up view of the same data from (A) during hour 25 to hour 30 which corresponds to the end of the first discharge and beginning of the second charge of the cell. (C) dQ/dV as a function of voltage calculated from the first discharge and second charge of the profile from (A). (D) Close up view of the dQ/dV profile from (C) restricted to between −2 and 2 mAh/V to highlight the peaks below 3.0 V.

Scanning transmission electron microscopy was performed using a JEOL JEM-ARM200CF cold field-emission source operated at 200 kV with a probe aberration corrector. Samples were scanned with a probe size of about 3 Å and images were collected using a high-angle annular dark-field detector. The spherical aberration corrected electron probe provides a beam current of about 0.5 nA to the sample and EDXS elemental maps were obtained using an Oxford-Instrument X-Max 80 mm2 silicon drift detector. Dead-time of the detector was within 10−30% during mapping, which would ensure optimum energy resolution and peak position accuracy. Electrochemical testing was completed using 2032 coin cells at 25 °C. Lithium half cells consisted of a lithium foil metal anode, a porous polypropylene separator, and an electrolyte of 1.2 M LiPF6 dissolved in 3:7 ethylene carbonate:ethyl methyl carbonate. Full cells were made using graphite anodes with an electrode comprised of 92 wt % graphite and 8 wt % polyvinylidene difluoride (PVDF) binder pasted on Cu foil. All positive electrodes consisted of 80 wt % active material, 10 wt % carbon black, and 10 wt % PVDF coated on Al foil. All cells were assembled in a glovebox with a He environment.

composite material. We will also show that the Ti-rich phases can accept Li from the first discharge and may be used as an extra capacity source to aid in cell balancing. The ability of the titanium-substituted LFP composite to accept additional lithium shows promise as a potential route for balancing the capacity when used in combination with high energy caothode materials which have excess irreversible charging capacity and lithium on their first charge, such as Li- and Mn- enriched high energy materials.

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MATERIALS AND METHODS The precursors FeC2O4·2H2O (99% purity powder), TiOSO4 (synthesis grade powder), NH4H2PO4 (>99% purity powder), and Li2CO3 (>99% purity powder) were purchased from Sigma. The relative amounts of precursor used were chosen to match the Li, Ti, Fe, and P stoichiometry of the desired final material (e.g., LiFePO4 was 0.5:1:1 molar ratios of Li2CO3/ FeC 2 O 4 ·2H 2 O/NH 4 H 2 PO 4 ). After the precursors were weighed out, they were placed in a bottle and mixed on a roller with acetone and ZrO2 balls. The resulting mixture was dried and the powder was packed into an alumina boat and fired at 700 °C for 12 h in an inert argon atmosphere. No regrinding was performed. Powder X-ray diffraction (XRD) was performed using a Siemens D5000 with Cu Kα source radiation. Scanning electron microscopy (SEM) and SEM EDXS was done using a Hitachi S-4700-II with powders spread on double-sided carbon tape. Solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were performed at 202.4 MHz on a Bruker Avance III 500 MHz spectrometer using a 4 mm MAS probe. A rotor synchronized echo pulse sequence (π/2−τ−π− acq.), where τ = 1/νr (spinning frequency), was used to acquire the spectra at spinning speeds of 14 kHz. A π/2 pulse width of 3.5 μs was used with 1 s pulse recycle delay time. All spectra were referenced to 85% H3PO4, at 0 ppm.

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RESULTS AND DISCUSSION We synthesized powders of four different stoichiometries via solid state reaction of precursors of the stoichiometry Li:(Fe/ Ti):P 1:1:1 using an identical procedure (see Materials and Methods for details). This procedure did not include any milling or carbon coating steps after firing the material. We made one sample of pure LFP and three of increasing levels of Ti substitution of 1.5, 5, and 10 mol % for Fe in the stoichiometry of the precursors used for synthesis. We refer to these powders in the manuscript as (1 − x)LiFePO4· LixTix(PO4)δ,where x is the mol % of Fe substituted by Ti in the precursor stoichiometry. We refer to our powders using this stoichiometry for two reasons. First, the crystalline phase that is detected for all 4 samples from X-ray diffraction spectra (Figure 21133

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S1, Supporting Information) is not distinguishable from the pure LFP sample and were consistent with previous XRD patterns for olivine-structured LFP.3,6 The lattice parameters (Table S1) determined from the XRD patterns also did not have significant changes upon Ti incorporation into the synthesis. Second, the results described below will provide evidence that there are titanium phosphate phases whose precise relative amounts cannot be distinguished. The material that will be the focus of the rest of the manuscript will be of the stoichiometry 0.9LiFePO4·Li0.1Ti0.1(PO4)δ. We focused on the 10 mol % titanium substitution to maximize our opportunity to identify the minority Ti-containing phases and because this material had the most promising rate capability of the materials synthesized. Figure 1A displays the voltage as a function of time during the first 2 charge and discharge cycles of a Li/0.9LiFePO4· Li0.1Ti0.1(PO4)δ coin cell. Most of the reversible capacity of the material was achieved at ∼3.4 V with low polarization loss, consistent with the majority of the material being comprised of a LFP phase.3 However, as can more clearly be seen in the voltage regions highlighted by Figure 1B, reversible capacity was also provided in the material at voltages below 3.0 V. dQ/ dV plots further confirm the dominant contribution of LFPtype features (Figure 1C) and the reversible low voltage features (Figure 1D). The three reversible low voltage features occurred at ∼2.30, ∼2.40, and ∼2.80 V. These voltage features are all within the range calculated for phosphates with the Ti4+/ Ti3+ redox couple, which led us to speculate that titanium phosphate phases were responsible for the reversible capacity at these potentials.16 In particular, TiP2O7 and LiTi2(PO4)3 have previously been synthesized and potentials near these values have been measured.17 The discharge profiles and capacities were measured for LFP (Figure 2A) and 0.9LiFePO4·Li0.1Ti0.1(PO4)δ (Figure 2B) in lithium half cells after equivalent electrode processing and at the same gravimetric current densities. The 0.9LiFePO4· Li0.1Ti0.1(PO4)δ material had substantially better performance than the pure LFP material at every rate measured, both with regards to overall capacity and the average voltage during discharge (additional data for the capacity of 10 cycles each at increasing discharge rates can be found in Supporting Information, Figure S2). The performance at the highest rate, 1000 mA/g, was especially striking. Not only is the capacity of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ composite 77% greater than the pure LFP (94 mAh/g compared to 53 mAh/g), but the average discharge voltage is 17% higher (2.89 vs 2.47 V). Also, batteries typically have a practical cutoff voltage below which they are not operated and thus any capacity below this voltage will not be used.18 At 1000 mA/g, the capacity above 2.8 V is 73 mAh/g for the composite and only 1 mAh/g for the pure LFP, meaning that a practical cutoff of 2.8 V or higher would result in an even more significant energy advantage for the composite. To put the composite LFP material in the context of rate capability relative to other LFP materials reported in the literature, we have compared the composite material‘s performance to the best LFP materials prepared via different routes reported in a recent review publication.19 The LFP composite has rate capability comparable to the best materials prepared via doping, though not at the level of the best LFP materials reported to date via any synthesis method. The striking improvements in the electrochemical performance of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ composite relative to the pure

Figure 2. Discharge curves for (A) LiFePO4 and (B) 0.9LiFePO4· Li0.1Ti0.1(PO4)δ at discharge rates of 10 (, I), 100 (· · ·, II), 500 (− − −, III), and 1000 (− · −, IV) mA/g.

LFP led us to further explore the structural characteristics of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ material. The XRD patterns of LFP and 0.9LiFePO4·Li0.1Ti0.1(PO4)δ indicated that the crystalline phase within the two materials was very similar, thus we explored if the morphology differed between of the two electrode powders and contributed to the electrochemical performance discrepancy. The surface area of the LFP and 0.9LiFePO4·Li0.1Ti0.1(PO4)δ materials determined via BET analysis were 19.5 and 17.0 m2/g, respectively. Not only are the surface areas of the two materials comparable, but the pure LFP surface area was greater, indicating that the improved rate capability of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ cathode material relative to the pure LFP was not the result of a larger relative surface area. Surface morphologies were also confirmed to be very similar between the two materials via SEM (Figure S3). After confirming that the two powders had relatively similar surface morphologies, we next investigated the composition of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ material using energy dispersive X-ray spectroscopy (EDXS) with both a scanning electron microscope (SEM) and a scanning transmission electron microscope (STEM). SEM/EDXS had peaks identified as corresponding to the elements Fe, Ti, P, and O within the sample (Figure S4). This spectra is consistent with the elemental composition of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ material and was collected and averaged over hundreds of particles. STEM imaging further confirmed the sample morphology from SEM, with aggregates of primary particles in the size range of a couple hundred nanometers (Figure 3A). EDXS mapping using the STEM allowed us to investigate if the Ti and Fe elements were homogeneously distributed within 21134

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minority phases indicated that these phases were amorphous or not well-crystallized. In an effort to identify these phases, we performed 31P solid state NMR experiments to further investigate the Ti minority phases in our 0.9LiFePO4· Li0.1Ti0.1(PO4)δ composite material. We identified three primary compounds that were likely candidates for the Ti minority phase(s): LiTiOPO4, TiP2O7, and LiTi2(PO4)3.17,20 We synthesized phase-pure samples of these compounds via the same precursor reagents and synthesis procedure as used for the LFP and 0.9LiFePO4·Li0.1Ti0.1(PO4)δ materials. The 31P MAS NMR spectra for the three Ti compounds, as well as the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ material, can be found in Figure 4. The NMR spectra reveals that the

Figure 4. 31P solid state magic angle spinning nuclear magnetic resonance spectra of single phase titanium phosphate compounds (LiTiOPO4, TiP2O7, LiTi2(PO4)3) and the composite 0.9LiFePO4· Li0.1Ti0.1(PO4)δ material.

0.9LiFePO4·Li0.1Ti0.1(PO4)δ material does not have any peaks or pronounced shoulders at −10 ppm or in the range of −41 ppm to −50 ppm, which would have indicated the presence of LiTiOPO 4 and TiP 2 O 7, respectively. The 0.9LiFePO 4 · Li0.1Ti0.1(PO4)δ material has one broad peak centered at ∼ − 30 ppm. Note that very broad resonance(s) was also observed around 3500 ppm due to the LiFePO4 phosphorus component, but these resonances are not shown because the spectra have been truncated for clarity. Resonances were also observed in the same range for pure LiFePO4, as has previously been reported in the literature.21 LiTi2(PO4)3 also has a single resonance located within this region (at −28 ppm) which suggests that an amorphous or not well-crystallized LiTi2(PO4)3 is likely to be present within the composite. The relatively broad character of this resonance in the composite with respect to the crystalline model material spectrum is presumably due to the amorphous or not well-crystallized nature of this phosphorus containing phase. While our three phase pure polyanion Ti species are not an exhaustive search of this family, the NMR spectra indicates that LiTiOPO4 and TiP2O7 are not present in our material, while LiTi2(PO4)3 is a likely candidate for the minority phase. We also note that another potential compound in the composite outside of the Ti polyanions, Li3PO4, does not have a peak near −30 ppm (resonance peak is near +10 ppm). Previous studies have investigated the structure and electrochemistry of LiTi2(PO4)3.17,20 This material forms crystals with

Figure 3. (A) Scanning transmission electron microscope image of 0.9LiFePO4·Li0.1Ti0.1(PO4)δ powder. EDXS mapping was performed on the same region shown in (A), with the intensity of the color shown corresponding to the relative intensity of the Kα peak detected in that same region selected for the elements of Fe (B, purple), Ti (C, blue), and P (D, green).

these individual primary particles, as opposed to the SEM/ EDXS that was collected as an average over many particles. As shown in Figure 3B and C, there is a region within the material where there is significantly pronounced segregation of the Ti and less Fe. The results shown in Figure 3 were consistent with other regions observed within the material. The EDXS maps in Figure 3B and C provide evidence that the Ti does not form a solid solution with the LFP phase at these concentrations, but rather segregates into a Ti-enriched phase. The presence of P distributed throughout the material (Figure 3D) led us to speculate that the Ti-enriched phase contained phosphate (and thus the Li0.1Ti0.1(PO4)δ in our composite designation). Unfortunately, our XRD patterns (Figure S1) did not contain any impurity peaks that would help in identifying these phosphate phases. The lack of XRD peaks from the Ti-enriched 21135

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would not contribute to its deliverable energy (and thus would bring down the overall cell energy density and/or possibly lead to lithium plating at the anode side). Balancing of a battery is complicated by irreversible capacity losses of the active material, which must be accounted for in determining the usable capacity of the electrode. A very promising family of battery cathode materials that frequently has significant irreversible capacity losses with its first charge cycle is the socalled Li-and Mn-enriched high energy materials.25−27 These materials have reported capacities in excess of 250 mAh/g and have shown promise as the next generation of cathode materials for electric vehicles. One of the challenges of these materials is that on the first charge the Li2MnO3 phase integrated within this material releases Li and/or oxygen, and the capacity and Li associated with this process typically is not fully recovered on subsequent cycles. The fate of the Li lost from the cathode material is still a subject of study, however, we explored the possibility of recovering some of the irreversible capacity lost from a Li-and Mn-enriched cathode using the low voltage Liaccepting activity of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ composite. The Li-and Mn-enriched cathode material that was used has had its synthesis and electrochemical properties reported in a previous work.28 The overall material stoichiometry was Li1.2(Mn0.62Ni0.38)0.8O2, and the material had a first charge cycle capacity of 327 mAh/g when charged to 4.9 V and a first discharge cycle capacity of 194 mAh/g when discharged to 2.0 V with the charge and discharge being performed at a rate of ∼C/10. We note that this first cycle irreversible charge capacity of 40.7% is relatively high and 194 mAh/g discharge capacity is relatively low for this family of materials, however, the high energy material was chosen to provide plenty of excess Li on charge to establish the feasibility of reinserting this excess Li into the low voltage features of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ composite on discharge. A cathode was made using the procedure detailed in the Materials and Methods where the active material consisted of a 50:50 wt % blend of 0.9LiFePO4 ·Li0.1 Ti0.1 (PO 4) δ and Li1.2(Mn0.62Ni0.38)0.8O2. Graphite was used as the active material in the negative electrode. The first two charge/discharge curves for this cell can be found in the Supporting Information (Figure S5). The dQ/dV plot of the third charge/discharge cycle of this cell reveals that the reversible low voltage features are still present for the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ material in the full cell with a graphite anode (Figure 5B). The full cell does not have a source of lithium at the anode, thus the presence of the low voltage features supports the hypothesis that some of the excess lithium extracted irreversibly from the Li1.2(Mn0.62Ni0.38)0.8O2 material in the cathode provided the lithium that was necessary to be subsequently inserted into the low voltage minority phase of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ material in the cathode. To our knowledge, this is the first report of using low voltage features in a secondary cathode additive to attempt to help balance the irreversibly lost lithium from a Li- and Mnenriched high energy cathode material. We note that we cannot yet confirm the source of the Li inserted into the low voltage features. Other sources, for example, could include Li that typically is not accessible for the high energy material after being lost to the graphite but was not from the Li2MnO3 decomposition. The source of the lithium inserted at lower potentials will be the subject of future studies. We anticipate that significant amounts of optimization will be necessary to determine the utility of blending a (1 − x)LiFePO4·LixTix(PO4)δ composite with a Li- and Mn-enriched

the NASICON (name is a shortened version of sodium (Na) Super Ionic Conductor) structure with a network of tetrahedra that share corners with octahedra, all oriented along the caxis.17,22 The pure phase material reversibly intercalates Li ions at ∼2.5 V (vs Li metal) with a flat plateau on charge and discharge. While this voltage is in the range of the three low voltage features observed for the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ composite (Figure 1), it is not clear that any or all of the these features can be conclusively assigned to LiTi2(PO4)3. In particular, the evidence presented above is consistent with the minority phase(s) possessing amorphous or not well-crystallized structure, and it is not known how this would influence the electrochemistry of these materials. Amorphous battery materials can have significantly different electrochemical properties than their crystalline counterparts.23 An important electrochemical characteristic of LiTi2(PO4)3 (as well as other Ti-containing phosphates with Ti initially in the Ti4+ oxidation state) is that it accepts Li upon assembly in a Li half cell and thus a Li/LiTi2(PO4)3 cell is typically initiated with a discharge cycle. Figure 5A shows the first 20 mAh/g of capacity of the

Figure 5. (A) First two discharge/charge cycles of a Li/0.9LiFePO4· Li0.1Ti0.1(PO4)δ cycled at a rate of 10 mA/g. The cell was started on a discharge cycle and cycled between 2.0 and 4.0 V. Only the first 20 mAh/g of capacity are shown. (B) dQ/dV as a function of voltage calculated from the third charge and third discharge of a cell comprised of a graphite anode and a cathode where the active material was mixture of 50 wt % 0.9LiFePO4·Li0.1Ti0.1(PO4)δ and 50 wt % Li1.2(Mn0.62Ni0.38)0.8O2. The cell was cycled at a rate of 20 mA/g.

first two charge and discharge cycles of a Li/0.9LiFePO4· Li0.1Ti0.1(PO4)δ cell where the cell was started with a discharge cycle. The low voltage features are clearly present on the initial discharge, consistent with materials such as LiTi2(PO4)3 which accept Li initially on discharge in a Li half cell. The activity of the low voltage features on the initial discharge led us to investigate the possibility of taking advantage of this electrochemical activity in a full cell. One important aspect of a battery is balancing the cell.24 The usable capacity of the active anode material should match as close as possible to the usable capacity of the cathode material in the full cell to avoid having unusable active material on either side of the cell, which would be excess weight in the battery that 21136

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high energy material, however, we believe this could be a valuable strategy. Blending of high energy materials with even pure LFP that has excellent rate capability may be valuable for transportation applications.29 By blending a (1 − x)LiFePO4· LixTix(PO4)δ composite material with excellent rate capability in place of pure LFP, the low voltage features may help with gaining extra capacity in the full cell and utilizing the irreversible capacity that is lost, in addition to the pulsing advantages provided by the high rate material.

CONCLUSIONS The synthesis and characterization of composite (1 − x)LiFePO4·LixTix(PO4)δ materials has been described, in particular 0.9LiFePO4·Li0.1Ti0.1(PO4)δ. This material has excellent rate capability as a lithium-ion cathode material, especially relative to LFP synthesized in an equivalent process, and does not require a separate nanosizing milling or carbon coating step. The material has capacity at below 3 V, which we have attributed to minority Ti phosphate phases that are amorphous or not well-crystallized in character and segregated at the nanoscopic particle level from the dominant LFP material rather than forming a solid solution. NMR provided evidence that the minority phase contains LiTi2(PO4)3 and does not contain LiTiOPO4 nor TiP2O7. Due to the nature of the low voltage features as accepting Li on the first discharge of the 0.9LiFePO4·Li0.1Ti0.1(PO4)δ cathode material, the composite was blended with a high energy material that had significant irreversible loss of Li on the first charge cycle. The blend of 0.9LiFePO4·Li0.1Ti0.1(PO4)δ and the high energy material had capacity below 3 V when paired with graphite in a full cell, indicating that (1 − x)LiFePO4·LixTix(PO4)δ composites may potentially be used to help balance the irreversible capacity loss of some high energy Li-ion battery materials that irreversibly lose Li on their first charge. ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, calculated lattice parameters, SEM images, cycle life data, SEM EDXS spectra, and full cell charge/discharge profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Authors

*Tel: 630-252-4450. E-mail: [email protected]. *Tel: 434-982-2714. E-mail: [email protected]. Present Address ∥

G.M.K.: Department of Chemical Engineering, University of Virginia, 102 Engineers Way, Charlottesville, VA 22903. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the U.S. Department of Energy, Freedom CAR, and Vehicle Technologies Office. The electron microscopy was accomplished at the Electron Microscopy Center for Materials Research at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under Contract DE-ACOZ-06CH11357. 21137

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