Comprehensive Enhancement of Nanostructured Lithium-Ion Battery

Feb 27, 2017 - Efficient energy storage systems based on lithium-ion batteries represent a critical technology across many sectors including consumer ...
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Comprehensive Enhancement of Nanostructured Lithium-Ion Battery Cathode Materials via Conformal Graphene Dispersion Kan-Sheng Chen,† Rui Xu,‡ Norman S. Luu,† Ethan B. Secor,† Koichi Hamamoto,† Qianqian Li,†,§ Soo Kim,† Vinod K. Sangwan,† Itamar Balla,† Linda M. Guiney,† Jung-Woo T. Seo,† Xiankai Yu,† Weiwei Liu,† Jinsong Wu,†,§ Chris Wolverton,† Vinayak P. Dravid,†,§ Scott A. Barnett,† Jun Lu,‡ Khalil Amine,‡ and Mark C. Hersam*,†,∥,⊥,# †

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § The NUANCE Center, ∥Department of Chemistry, ⊥Department of Medicine, and #Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States ‡

S Supporting Information *

ABSTRACT: Efficient energy storage systems based on lithium-ion batteries represent a critical technology across many sectors including consumer electronics, electrified transportation, and a smart grid accommodating intermittent renewable energy sources. Nanostructured electrode materials present compelling opportunities for high-performance lithium-ion batteries, but inherent problems related to the high surface area to volume ratios at the nanometer-scale have impeded their adoption for commercial applications. Here, we demonstrate a materials and processing platform that realizes high-performance nanostructured lithium manganese oxide (nano-LMO) spinel cathodes with conformal graphene coatings as a conductive additive. The resulting nanostructured composite cathodes concurrently resolve multiple problems that have plagued nanoparticle-based lithium-ion battery electrodes including low packing density, high additive content, and poor cycling stability. Moreover, this strategy enhances the intrinsic advantages of nano-LMO, resulting in extraordinary rate capability and low temperature performance. With 75% capacity retention at a 20C cycling rate at room temperature and nearly full capacity retention at −20 °C, this work advances lithium-ion battery technology into unprecedented regimes of operation. KEYWORDS: Lithium manganese oxide, spinel, nanoparticle, high packing density, high rate capability, low temperature

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insertion/extraction processes.5,6 By reducing the characteristic dimensions of the active materials below their critical breaking size,7,8 this cracking issue can be circumvented, thereby allowing improved capacity retention.9 Moreover, nanostructured electrodes offer large active material/electrolyte contact area and short lithium ion/electron diffusion paths,1 which improves charging/discharging capacity and kinetics.10−12 Despite these compelling advantages, nanostructured electrodes possess several impediments that have hindered large-scale commercial adoption. In particular, increases in electrode/ electrolyte side reactions due to the large interfacial area deteriorates cycling and calendar life,1 while poor packing density and high additive content limit volumetric energy density.1−3 It is therefore a critical challenge to develop materials and processing strategies that maintain the benefits of

ne of the most pressing challenges facing modern society is the development of high-performance energy storage systems for applications ranging from portable electronic devices and emission-free transportation (e.g., electric vehicles) to emerging smart grids based on renewable energy sources. Thus far, lithium-ion batteries (LIBs) have been the most successful technology in fulfilling demanding energy storage requirements as a result of their high energy density and low carbon footprint. However, due to a variety of materials issues, LIBs have limited charge/discharge rate, stability, safety, and temperature range that have hindered their adoption in many sectors. In an effort to overcome these limitations, significant effort has been focused on nanostructured LIB electrode materials,1−4 which offer unique opportunities to introduce novel functionality and resolve issues associated with their bulk counterparts. A prominent example is the severe capacity fading of Li-alloying anode materials (e.g., Si) due to inherently large volumetric expansion and contraction, which causes cracking and pulverization of the active materials during repeated lithium © 2017 American Chemical Society

Received: January 20, 2017 Revised: February 21, 2017 Published: February 27, 2017 2539

DOI: 10.1021/acs.nanolett.7b00274 Nano Lett. 2017, 17, 2539−2546

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Figure 1. Schematic fabrication and characterization of n-LG cathode. (a) Schematic illustration of ethyl cellulose-stabilized nano-LMO and GNF dispersion (left) that forms high packing density n-LG (right). (b) Thermogravimetric analysis determines the composition ratio of nano-LMO and GNFs in n-LG. (c,d) SEM images of n-LG (c) and nano-LMO control (d). (e,f) FIB-SEM tomography of n-LG (e) and nano-LMO control (f) electrodes with the same loading of nano-LMO. The electrodes are characterized after being cycled in half cells 20 times at 55 °C. (g−i) TEM images of n-LG at different magnification.

effective strategy is therefore necessary to retain the primary nanoparticle structure of the active materials while promoting a dense network with efficient electronic and ionic conduction pathways. We present here a unique cathode slurry comprised of nano-LMO and GNF stabilized by ethyl cellulose that offers improved packing density of nanostructured LIB cathodes. Spinel LMO is selected as a model system due to its high power, low cost, environmental friendliness, and safety.13,14 GNFs offer advantages as a conductive additive compared to carbon black including improved electrical conductivity, mechanical resilience, and high aspect ratio.15,16 Moreover, graphene has previously been shown to stabilize the LMO/ electrolyte interface by forming a thin and stable SEI layer on graphene and suppressing Mn dissolution.17 In this work, the slurry is prepared with 66.7%, 7.3%, and 26% by weight of nano-LMO, GNF, and ethyl cellulose, respectively, using Nmethyl-2-pyrrolidone (NMP) as a solvent. Ultrasonication is performed to disperse the nano-LMO particles and GNFs with ethyl cellulose,18 which effectively prevents aggregation of the nano-LMO particles. Subsequently, the slurry is cast onto aluminum foil and dried in a vacuum oven at 110 °C for 12 h. As the NMP evaporates, the nano-LMO particles and GNFs remain well-dispersed to form a densely packed and uniform film as schematically illustrated in Figure 1a. Finally, ambient annealing at 285 °C for 3 h decomposes the ethyl cellulose (Figure 1b), resulting in a binder-free n-LG cathode containing

nanostructured electrodes while simultaneously resolving their persistent problems. Specifically, nanostructured battery electrodes require a method that (1) uses unaggregated primary nanoparticles for optimal kinetics; (2) achieves a high packing density and loading of active materials for improved volumetric/gravimetric energy density; and (3) effectively passivates active material surfaces to eliminate undesirable side reactions. Considering these key criteria, we have developed a novel processing strategy utilizing an ethyl cellulose-stabilized dispersion of primary nano-LMO particles and pristine graphene nanoflakes (GNF) to realize a nanoLMO/graphene composite (n-LG) cathode with substantially improved packing density and active material loading. Furthermore, due to the conformal graphene coating on the nano-LMO particle surfaces, this architecture suppresses deleterious electrode/electrolyte reactions and affords excellent cycling stability. The enhanced charge transfer resulting from nano-LMO and the highly conductive GNF network further yields excellent rate capability (∼75% capacity retention at 20C rate) and unprecedented electrochemical performance at low temperatures with nearly full capacity retention at −20 °C. Nanoparticles are prone to aggregation into highly porous microparticles, giving rise to large voids. In addition, interparticle space between secondary microparticles further reduces the packing density of electrodes,2 deteriorating the volumetric and gravimetric capacity of the resulting LIB. An 2540

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Figure 2. Galvanostatic characterization and stability evaluation of n-LG and nano-LMO control. (a,b) Voltage profiles of n-LG in charge and discharge cycles with 0.2C current rate (a) and corresponding dQ/dV curves (b). (c,d) Cycling stability comparison between n-LG and nano-LMO control in a half-cell geometry (c) and full-cell geometry (d).

nanoparticles were used as the active material. Furthermore, while there have been several reports employing graphene21−26 or carbon27−29 coatings to improve the performance of nanostructured LMO cathodes, the volumetric energy densities of those cathodes at 0.2C current rate were either not provided or inferior to the value reported here. To inspect the structure of n-LG at the nanoscale, transmission electron microscopy (TEM) is performed, which shows that nano-LMO particles are evenly mixed with GNFs (Figure 1g) with graphene conformally coating the nano-LMO particle surfaces (Figure 1h). The higher-resolution TEM image in Figure 1i indicates intimate contact between nano-LMO particles and GNFs, along with the intact crystal structure of the individual nano-LMO particles. Electron diffraction for this particle (Figure S2) reveals that the particle has single-crystal structure, which can be indexed as the [110] direction of LMO phase. The graphene layer can be identified in the diffraction pattern by detecting the {002} diffraction ring of graphite. Density functional theory (DFT+U; U = 4.5 eV for Mn) calculations with the opt-type van der Waals functional30−33 also predict that the interaction between spinel LMO and graphene is thermodynamically favorable for both (001) and (111) LMO surfaces (i.e., energetically stable surface facets in LMO)34 with or without the presence of a graphene defect17 (Supporting Information). Finally, the intact and pristine structure of the graphene nanoflakes is confirmed by Raman spectroscopy (Figure S3). The electrochemical performance of the n-LG cathode is first evaluated in a half-cell configuration by galvanostatic measurements at a current rate of 0.2C (1C = 148 mA g−1). Charge/ discharge voltage profiles in Figure 2a show a pair of characteristic plateaus near 4 and 4.1 V in both charge and discharge cycles.35 A more clear and quantitative comparison can be performed when the curves are plotted as differential

90% wt active material loading and 10% wt conductive graphene additive. A scanning electron microscopy (SEM) image of the resulting n-LG film is shown in Figure 1c, confirming that interparticle aggregation is mitigated with only small gaps observed between the primary nano-LMO particles. Uniform distribution of the GNFs in the composite is also evident. In sharp contrast, a nano-LMO control (Figure 1d) prepared with nano-LMO, carbon black, and polyvinylidene fluoride (PVDF) binder in 8:1:1 weight ratio, respectively, shows substantial aggregation and uneven spacing between secondary clusters. More in-depth characterization of the electrode morphology is carried out using focused ion beam-SEM (FIB-SEM) tomography19 to investigate the n-LG and nano-LMO control electrodes following electrochemical cycling. As shown in Figure 1e,f, secondary backscattered electron signals indicate the spatial distribution of the nano-LMO (bright features), infiltrated epoxy (gray features), and conducting additives (dark features) in the n-LG and nano-LMO control, respectively. With the same loading of nano-LMO, it is evident that nanoLMO particles are much more uniformly and densely distributed in the n-LG than in the control, giving rise to markedly higher packing density of the active materials in n-LG. The homogeneous distribution of nano-LMO in n-LG also reveals that the integration of the electrode remains intact during coin cell assembly and electrochemical cycling, indicating the robustness of the binder-free n-LG. Quantitatively, by analyzing the FIB-SEM image of an as-prepared n-LG electrode (Figure S1), a 50% volume ratio of the nano-LMO in the electrode is determined. Given the mass density of LMO spinel (4.281 g cm−3), the active material packing density in nLG is 2.1 g cm−3, resulting in a volumetric energy density of 1030 Wh L−1 at 0.2C current rate, which is 30% higher than previous work20 (785 Wh L−1), where primary LMO 2541

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Figure 3. Stability comparison of n-LG and nano-LMO control at 55 °C. (a) Capacity retention of n-LG and nano-LMO control. (b,c) Voltage profile evolution of n-LG (b) and nano-LMO control (c). (d) dQ/dV curves corresponding to the voltage profiles of n-LG (top) and nano-LMO control (bottom). (e) Electrochemical impedance spectra of n-LG and nano-LMO control before cycling (top) and after 20 cycles at 55 °C (bottom).

surface39 doping to reduce the amount of Mn3+ at the cathode/ electrolyte interface, and the application of protective surface coatings43−45 to minimize direct contact between LMO and the electrolyte. Meanwhile, it has previously been shown that single-layer graphene coatings on a LMO thin film can improve both cathode stability (capacity retention) and kinetics (rate capability).17 To investigate whether the results from the idealized graphene/LMO thin-film case apply to the n-LG composite, we assess the cycling performance of the n-LG composite in both half-cell and full-cell geometries. In Figure 2c, the n-LG half-cell, which employs Li metal as an anode, shows superior capacity retention of ∼95% after 350 cycles at 1C current rate at 25 °C, while the control exhibits 80% retention under the same conditions. The improved capacity retention of the n-LG half-cell is most likely due to surface protection by graphene.17 Confirming this conclusion, the differences in cycling life are even more pronounced for the full-cell geometry, as shown in Figure 2d. The capacity fade of the control full-cell is greater than 35% after 250 cycles, which is attributed to the additional capacity fade from the degraded graphite anode, whose impedance increases upon Mn2+ poisoning. On the other hand, the cell stability is nearly identical for the n-LG half-cell and full-cell, suggesting that migration of Mn2+ ions to the graphite anode is significantly suppressed. Further full-cell data

Table 1. Fitted Equivalent Circuit Parameters for the Impedance Spectra in Figure 3e RSEI RSEI RCT RCT

(1st cycle) (20th cycle) (1st cycle) (20th cycle)

n-LG (coin cell)

control (coin cell)

73.4 84.9 1.1 27.0

148.3 191.5 1.0 212.2

capacity (V versus dQ/dV), as shown in Figure 2b. The peak separation of 10 mV between charging and discharging confirms minimal electrode polarization and therefore efficient reaction kinetics. Historically, a major disadvantage of LMO cathode materials has been the relatively poor cycling performance resulting from disproportionation of Mn3+ that produces soluble Mn2+ at the cathode/electrolyte interface.36−38 As the Mn2+ ions deplete from the LMO surface, the impedance of the LMO cathode increases.39 Furthermore, Mn2+ ion migration to the graphite anode poisons the solid−electrolyte interface (SEI)40 on the anode surface, which also increases the impedance of the anode. Because the disproportionation reaction is surface-mediated, the inherently large surface area of nano-LMO particles can significantly increase the rate of this detrimental effect, leading to severe capacity fading following long-term cycling. Previous approaches to address Mn2+ dissolution include modification of the composition of the parent LMO crystals by bulk41,42 and 2542

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Figure 4. Rate capability comparison. (a) Rate capability measurements of n-LG and nano-LMO control with various current rates from 0.2C to 20C. (b,c) Voltage profiles at different discharge current rates for n-LG (b) and nano-LMO control (c).

Cdl represents the double-layer capacitance that relates to the roughness of the cathode particle surface. WS is the Warburg diffusional impedance corresponding to the slope of the lowfrequency region, which relates to the Warburg diffusion of Li ions in the solid phase of the cathode particles. As shown in Figure 3e and Table 1, RSEI (solid−electrolyte interface resistance) of the nano-LMO electrode is decreased and stabilized by substituting carbon black in the cathode with graphene because the electrode/electrolyte interfacial layer forming on graphene is thinner and more well-defined.17 After 20 cycles, RSEI of the nano-LMO control increases 30% from 148.3 to 191.5 Ω, caused by the change of the interfacial layer at the cathode.40 In contrast, RSEI of n-LG only increased from 73.4 to 84.9 Ω, as graphene nanoflakes in the cathode hinder Mn dissolution, thus alleviating surface degradation of the interfacial layer. Moreover, graphene in the n-LG cathode also significantly mitigates RCT (charge transfer resistance) increases during cycling. Although the initial RCT of the two cells were very similar, a substantial divergence in impedance changes after 20 cycles is observed between the cells. In particular, the RCT of the nano-LMO control increases from 1.0 to 212.2 Ω, while that of the n-LG cell only changes from 1.1 Ω to 27.0 Ω. The significant increase in RCT of the nano-LMO control is believed to account for the cell polarization illustrated in the voltage profiles (Figure 3c) and is likely caused by a structural change of the cathode surface and cathode/electrolyte interface due to Mn dissolution.39 On the other hand, comparatively stable RCT in the n-LG cell is due to the high electrical conductivity and conformal coating of graphene, which promotes efficient charge transfer, in addition to its mitigating effect on Mn dissolution. A direct measurement using inductively coupled plasma mass spectrometry (ICP-MS) of dissolved Mn for the n-LG and control at 55 °C reveals an average of two times lower Mn concentration in the electrolyte

including voltage profiles and Coulombic efficiency are provided in Figure S4. Because elevated temperatures are known to accelerate the degradation mechanisms of LMO, cells are commonly cycled at 55 °C as an additional test of stability. Under these conditions, as depicted in Figure 3a, n-LG exhibits superior cycling stability relative to the control. While n-LG retains 80% of its first cycle capacity at the 200th cycle, the control retains only 55%. To provide further characterization of the electrodes during cycling, voltage profile curves are shown in Figure 3b,c for the n-LG and control, respectively, which clearly illustrate the disparity in cathode evolution. For n-LG, the voltage plateaus for charge/discharge cycles stay at constant positions despite a slight increase in their slopes, whereas for the control the plateaus shift up and down, respectively, with more pronounced increase in slopes. The dQ/dV curves (Figure 3d) further confirm that the positions of the charge/discharge peaks remain constant for n-LG, while showing an increased splitting for the control. These results indicate a significant increase of polarization in the control cathode due to increased LMO resistance.46 In contrast, as a result of the high electrical conductivity and enhanced electrochemical stability afforded by the GNF network, the electrode polarization for n-LG is small and relatively invariant during high temperature cycling, giving rise to improved capacity retention and energy efficiency47 (Supporting Information). Electrochemical impedance spectroscopy (EIS) (Figure 3e) taken at 50% depth of discharge before and after battery cycling at 55 °C corroborates this conclusion. The equivalent circuit shown in the inset of Figure 3e is used to fit the impedance spectra, where RS is the electrolyte resistance and RSEI and CSEI reflect the resistance and geometric capacitance, respectively, of the anode/cathode solid−electrolyte interphase (SEI). RCT is assigned to the charge-transfer resistance of the cathode, and 2543

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Figure 5. Low-temperature performance of n-LG cathode. (a,b) Voltage profiles of n-LG and nano-LMO control, respectively, with current rate of 0.2C at 25 °C, 0 °C, and −20 °C. (c,d) Rate capability measurements of n-LG and nano-LMO control, respectively, carried out at 25 and 0 °C. (e) Direct rate capability comparison between n-LG and nano-LMO at −20 °C. (f) Electrochemical impedance spectra of n-LG and nano-LMO control at 25 °C (top), 0 °C (middle), and −20 °C (bottom).

low packing density,2 more conductive carbon additive is required to form a sufficient percolating network to achieve equally high electrical conductivity and comparable power performance to n-LG. Indeed, most reported high-power nanostructured cathodes contain only 75% wt or less of active material,20,26,28,48 which compromises volumetric and gravimetric energy density when considering a full cell. The achievement of superlative power performance with 90% active material loading, enabled by the high electrical conductivity and favorable morphology (i.e., high aspect ratio) of pristine GNFs in binder-free n-LG, offers a considerable advantage for wideranging applications. Despite their widespread use across many industries, LIBs suffer a significant drawback of poor low-temperature performance.49−51 For many applications, such as personal mobile electronics, electric vehicles, and high altitude aircraft, the additional weight and energy consumption of onboard heating elements for temperature regulation are prohibitive. Given the excellent power performance of the n-LG cathode, rate

for n-LG (Figure S5), further confirming the impedance spectroscopy analysis and capacity retention studies. The rate capability of n-LG and nano-LMO control are presented in Figure 4a for which specific discharge capacities are measured at current rates ranging from 0.2C to 20C. During the tests, charge and discharge currents are increased concurrently. As shown by the discharge voltage profiles in Figure 4b,c, n-LG exhibits excellent power performance. In particular, with respect to that obtained at 0.2 C, n-LG retains 100%, 95%, 90%, 85%, and 75% capacity at 1C, 5C, 10C, 15C, and 20C, respectively. To rule out a possible electrodethickness effect in the rate performance comparison, we also performed the rate capability test for the nano-LMO control (Figure S6) with one-third mass loading to match the thickness of the n-LG electrode. The controls in either case (same nanoLMO mass loading or electrode thickness), however, show significantly inferior power performance, likely due to the much lower electrical conductivity of the electrode. Because reduced LMO particle size induces large interparticle resistance due to 2544

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specific capacity in addition to retaining high rate capability at 0 °C. Overall, the versatile process proposed here substantially alleviates the disadvantages of nano-LMO (i.e., low packing density and poor cycling stability) and magnifies its advantages (i.e., fast kinetics) for outstanding power performance and unprecedented low-temperature operation. The generality of this approach further offers compelling opportunities for alternative LIB chemistries and electrochemical systems.

performance tests were performed at low temperature to assess whether this advance will translate to improved performance at 0 °C and −20 °C. Here, the same electrolyte was used without adding any specialized solvents or additives.52 Figure 5a,b shows the voltage profiles of n-LG and nanoLMO control, respectively, taken at a current rate of 0.2C at 25 °C, 0 °C, and −20 °C. In the case of n-LG, only minor changes in the shape and plateau position are observed for the voltage curves at 25 °C, 0 °C, and −20 °C, indicating that the n-LG maintains appreciably fast kinetics at low temperature. On the other hand, the voltage profiles of the nano-LMO control at low temperatures (Figure 5b) show much more pronounced polarization in the cathode electrode. In Figure 5c, it is evident that n-LG has equivalent rate capabilities at 25 and 0 °C up to 6C, with only slight differences at 8C and 10C. Under the same measurement conditions, the nano-LMO control shows clearly inferior rate performance at 0 °C (Figure 5d). Furthermore, at −20 °C (Figure 5e), n-LG retains a remarkable 96% of its room-temperature capacity at 0.2C, and even exhibits excellent capacity/retention (105 mAhg−1/88%) at 1C, while the nanoLMO control retains only 85% at 0.2C and 51% at 1C, respectively. It has previously been suggested that the electrolyte− electrode interface is one of the dominant factors that limits battery kinetics,53 which would explain why the GNF surface modification in n-LG yields superior low-temperature performance compared to the nano-LMO control. To further elucidate the underlying role of GNF in n-LG cathodes, temperaturedependent electrical conductivity measurements of both n-LG and nano-LMO control were carried out at a temperature range from 6 to 300 K (Figure S7). It is evident that both cathodes have nearly constant electrical conductivity in the entire temperature range, whereas the value for the nano-LMO control is 1 order of magnitude lower than that for n-LG. Because the electrical conductivity of LMO decreases by 3 orders of magnitude from 25 °C to −20 °C,54 it is plausible that the local charge transfer efficiency from the conducting agents (i.e., graphene or carbon black) to the nearly insulating LMO surfaces is the limiting factor for cathode performance at low temperatures. Therefore, as suggested by Figure 1h,i, the large and intimate contact between nano-LMO and GNFs in n-LG offers a more desirable morphology to promote efficient charge transfer, thereby giving rise to the observed superior lowtemperature performance. This conclusion is further supported by the temperature-dependent EIS measurements shown in Figure 5f, where the impedance difference (diameter of the semicircle) between the n-LG and nano-LMO control becomes larger as the temperature decreases. In summary, we have developed a high-performance composite cathode comprised of nano-LMO particles and GNFs. As a result of the unique properties of the ethyl cellulose-stabilized slurry, the robust and binder-free composite possesses significantly improved packing density with individually dispersed nano-LMO primary particles and GNFs. The cycling life of the n-LG cathode is superior to that of the nanoLMO control at both 25 and 55 °C because the GNFs effectively suppress Mn dissolution and lead to relatively invariant impedance of the electrode. Benefiting from the reduced size of nano-LMO and highly conductive pristine GNFs, the n-LG cathodes exhibit excellent power performance during both charge/discharge cycles up to 20C at room temperature. Moreover, in the extreme condition of −20 °C, the n-LG cathode maintains 96% of its room-temperature

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b00274. Details on experimental methods, supporting characterization data (TEM, Raman spectroscopy, FIB-SEM, ICPMS, temperature-dependent conductivity), supporting electrochemical data (voltage profiles and Coulombic efficiency for full cells), and further discussion (DFT of graphene/LMO interaction and energy efficiency of battery cells at high temperatures) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vinod K. Sangwan: 0000-0002-5623-5285 Itamar Balla: 0000-0002-9358-5743 Chris Wolverton: 0000-0003-2248-474X Jun Lu: 0000-0003-0858-8577 Khalil Amine: 0000-0001-9206-3719 Mark C. Hersam: 0000-0003-4120-1426 Present Address

(K.H.) Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology, Nagoya, Aichi 463−8560, Japan. Author Contributions

K.-S.C. and R.X. contributed equally. K.-S.C and M.C.H. conceived the idea; K.-S.C., E.B.S., N.S.L., and J.-W.T.S. prepared the nano-LMO/graphene dispersion; K.-S.C., X.R., N.S.L., and W.L. performed and analyzed electrochemical experiments; K.H. and X.Y performed and analyzed the FIBSEM measurements; Q.L. and J.W. performed and analyzed the TEM measurements; S.K. conducted the DFT calculation of the LMO/graphene interaction; K.-S.C. and V.K.S. performed and analyzed the low-temperature conductivity measurements; K.-S.C. and I.B. performed and analyzed the Raman spectroscopy measurements; K.-S.C. and L.M.G. performed and analyzed the ICP-MS measurements; M.C.H. oversaw the development and execution of the research. K.-S.C., E.B.S., and M.C.H. cowrote the manuscript. All coauthors discussed the results and contributed to the editing of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was primarily supported by the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences (DE-AC02-06CH11357) and made use of facilities at the NUANCE Center, the Quantitative 2545

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Letter

Nano Letters

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Bio-Element Imaging Center, and the National Energy Research Scientific Computing Center at Northwestern University, which have received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205), the MRSEC program (NSF DMR1121262), the International Institute for Nanotechnology (IIN), the Keck Foundation, the State of Illinois, NASA Ames Research Center (NNA06CB93G), and Office of Science of the United States Department of Energy (DE-AC0205CH11231). This research was also supported in part through the computational resources and staff contributions of the Quest High Performance Computing Facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology. K.-S.C. would like to thank Dr. Laila Jaber-Ansari for helpful guidance and discussions at the initial stage of the research. S.K. acknowledges support from Northwestern University-Argonne Institute of Science and Engineering (NAISE). K.H. acknowledges partial support from the JSPS Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers.

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DOI: 10.1021/acs.nanolett.7b00274 Nano Lett. 2017, 17, 2539−2546