Formation of Perpendicular Graphene Nanosheets on LiFePO4: A

Aug 2, 2012 - Coating carbon layers on LiFePO4 nanoparticles used in Li-ion batteries greatly increase the electron transport in this cathode material...
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Formation of Perpendicular Graphene Nanosheets on LiFePO4: A First-Principles Characterization W. T. Geng* National Institute for Materials Science, Tsukuba 3050047, Japan School of Materials Science & Engineering, University of Science and Technology Beijing, Beijing 100083, China

D. H. Ping, J. Nara, and T. Ohno† National Institute for Materials Science, Tsukuba 3050047, Japan ABSTRACT: Coating carbon layers on LiFePO4 nanoparticles used in Li-ion batteries greatly increase the electron transport in this cathode material. Using first-principles density functional theory calculations, we have theoretically investigated the interaction energies of graphene lying parallel and perpendicular to a LiFePO4 (010) surface and have found that the perpendicular orientation is energetically favorable. Our computations suggest that graphene nanosheets prefer standing vertically on LiFePO4 (010) using C−O and C−Fe bonding rather than spreading on it, as has been widely assumed. The interfacial chemical bonding both fastens the coated carbon layer and facilitates electron conductivity across the interface. By contrast, encapsulating LiFePO4 with parallel graphene sheets does not increase electron conductivity across the interface.

1. INTRODUCTION The discovery of phosphor-olivine LiFePO4 (LFPO), a low cost, nontoxicity, and thermally stable oxide, as a potential positive electrode for lithium rechargeable batteries by Padhi et al.1 has since inspired tremendous efforts in attempts to optimize its performance. In addition to particle size reduction,2 cation doping,3 noncarbonaceous interganular network conduction,4 carbon addition to the LFPO matrix,5 and especially thin-layer carbon coating to microscale6 and nanoscale LFPO particles7 have also been shown effective to increase both the electronic conductivity and discharge capacity of this material. It is generally believed that a more connective distribution of carbon leads to better electrochemical performance through minimizing the Li-insertion induced polarization of LFPO particles,8 as is evident from the observation that a core−shell LFPO/C nanostructure9 rendered improved electrode kinetics. The sp2-coordinated (graphitized and hence layered) carbon atoms that supply both better electron transportation and lower diffusion barrier for lithium, rather than the sp3-coordinated carbon atoms, are responsible for the improved performance of LFPO.10 Interestingly, it was found that thinner coating yields increased the reversible capacity of LFPO, due to a more uniform distribution of small carbon particles.11 Well-formed carbon layers have been made as thin as only several nanometers (nm)12 or even just as thin as 1.3 nm without remarkable capacity fading after 1000 cycles.13 These observations strongly suggest that thinner coating is correlated with more robust structure of carbon atoms, especially those at the core/shell interfaces. © 2012 American Chemical Society

Enormous endeavors to optimize carbon coating notwithstanding,14 information of the LFPO/carbon interfacial structure in the atomic scale is still lacking. Understanding of the effect of carbon coating is therefore limited to an ambiguous picture in which C atoms are required both to be mostly sp2-bonded to each other and to adhere to the LFPO surface so as to assist electron transportation from the active particle to the carbon network. Julien et al. are the first to address the structure of the LFPO/carbon interface in great detail using many characterization tools, including both Raman scattering spectroscopy and high-resolution transmission electron microscopy (HRTEM).15 Their analyses on thick carbon layers (30 nm) suggested both that the ratios of sp3- to sp2-bonded C atoms and of H to C are very small and also that carbon atoms are amorphous with low hardness, which are beneficial to Li diffusion. In a more recent work, the same group has investigated the structural transformation during carbon-coating of LFPO at elevated temperatures using HRTEM and found that at 600 °C carbon layers started to form and 700 °C is the optimum synthesis temperature for a homogeneous painting of thin (3 nm) carbon layers,16 which is close to the temperature adopted in some other independent studies.17 Again, the coated carbon atoms were judged to be in the amorphous state at the sharp interface. One point that we believe is extremely important but unfortunately has not been Received: May 18, 2012 Revised: July 10, 2012 Published: August 2, 2012 17650

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that yields uniform carbon shell wrapping of the LFPO particles. An intriguing question that now arises is, If carbon atoms at the C/LFPO are in the form of GNSs, do they bond with the LFPO surface or simply wrap these particles with van der Waals-like weak interaction? Apparently, if GNSs are standing on the particle, then electron transfer to and from the particle will probably be very easy, using chemical bonds as conduction channels. Besides, lithiation and delithiation in the charge− discharge circles will also be easier when GNSs connect vertically to the LiFePO4 particles instead of lying on the particle surface, thereby blocking the diffusion channels. Apparently, the C/LiFePO4 interfacial structure is of great relevance to the electrochemical performance of this cathode material. The widely accepted model of carbon black is a spherical carbon particle (sp3-bonded) wrapped by concentrically aligned graphite sheets (sp2-bonded) from the surface to the center.27 This model has recently been supported by atomistic computational studies of porous carbon black.28 In this picture, the carbon atoms in the spherical particle are amorphous and those forming the graphitic blankets are partially ordered. Connections between these graphitic fragments are mainly of grain boundary type (still mostly sp2-bonded), and at the core/ shell interface the two parts are mainly sp3-bonded. Since the sp2 type C−C bonding is predominate in carbon layers coated on LiFePO4 particles, we can view this substance as being composed of only microscale graphitic sheets but not sp3bonded carbon particles, connecting each other either by sp2bonded grain boundaries, sp3-bonded intersections, or, less likely, point-type edge−edge contacts with dangling bonds on both edges. The ratio of sp2 to sp3 bonds depends mainly on the average size of graphitic fragments. Tilt grain boundaries are common extended defects in graphite, especially in highly oriented pyrolytic graphite (HOPG), due to its polycrystalline character, and have been observed by scanning tunneling microscopy (STM) nearly a quarter of century ago.29 The details of the intersections of nonparallel graphitic sheets, on the other hand, remain hardly, if not at all, explored due to less common appearance in HOPG. In Figure 1, we illustrate schematically how GNSs could possibly be distributed and connected with the LFPO particles

paid attention to is that although studies carried out by different authors or even the same group of authors all suggest carbon at the surface of LFPO particles is noncrystalline, the degree of amorphousness varies with the preparation conditions of the cathode material, as is evident from close examination of the HRTEM images of the C/LiFPO4 interface from refs 13 and 16−18. Very interestingly, a comparison of those images shows that when the annealing time increases from 5,16 7,18 8,14 10,17 12,19 to 15 h,9 the degree of ordering increases monotonically. In fact, Fey and Lu claimed in an earlier work that they observed, for the first time, crystalline carbon near the LFPO particles in the intergranular region (Figure 6c in ref 19). Since the seemingly crystalline carbon particle has blurred interfaces with the neighboring LFPO particles, however, this claim has neither drawn much attention nor been supported by other authors. Regarding the direct C−LFPO interaction, Pan et al.18 proposed that there is C−O chemical bonding at the interface based on their X-ray absorption spectroscopy analyses and speculated that it is the C−O bonding that induces the change in spin-polarization of interfacial Fe atoms, a surface effect observed by Zaghib et al.20 We recall that, if treated simply by heating, disordered carbon will crystallize in the temperature range 700−3000 °C.21 However, when in contact with certain metals, graphitization of carbon films can be greatly enhanced.22 This knowledge has recently been applied to the growth of graphene on metals (650−950 °C).23 Although C cannot penetrate into the oxide LFPO,15 chemical bonding between C and LFPO, if it exists, could also possibly foster the crystallization of carbon. To enhance the discharge capacity and rate capability of nanostructure electrode materials, a new approach in carbon addition into LFPO nanoparticles reported recently is using graphene nanosheets (GNSs) as direct additives to offer stably structured carbon,24 instead of using other organic materials to produce amorphous carbon.6 Upon optimizing (stabilizing) the graphene/LFPO interface, Zhou et al.25 improved the performance of the graphene coating, though it is not superior to the amorphous carbon coating,13 and proposed that the GNSs are generally larger than the diameters of LFPO particles with particle-on-sheet rather than sheet-encapsulating-particle type contacts. Since a major part of the surfaces of active particles is not covered by carbon, the efficiency of GNS additives is bound to be limited. Addressing the same issue, Su et al. argued that LFPO particles were wrapped homogeneously and loosely by GNSs based on their analyses of HRTEM images.26 More importantly, they made a comparison of GNS/LiFePO4 composite with amorphous C/LFPO and graphene modified C/LFPO composites, and they found that the ordering of carbon at the LFPO surface depends strongly on the preadded graphene. It is worth noting that the HRTEM image for the amorphous C/LFPO interface is very similar to that in ref 16, a structure obtained also with the five-hour annealing; the image for graphene modified C/LFPO resembles that in ref 17 with an annealing time of 8 h, and strikingly, the ripple-like morphology for the claimed GNS/LFPO interface looks just like the 10-h annealed sample in ref 17 which was taken by the authors as amorphous. The astonishing similarity of HRTEM images obtained for assumed GNS/LFPO and amorphous C/LFPO interfaces strongly suggests they are in fact for similar structures. That is, the amorphous carbon near the LFPO surface has been crystallized (graphitized) to some extent after long time annealing at high temperature. It is probably the crystallization

Figure 1. Schematic drawing of possible C/LiFePO4 interfacial structures. Note that a graphene nanosheet is generally bent when connecting either an oxide surface or another graphene nanosheet. 17651

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at the immediate interface. Here, to make the computational effort affordable, we assume the coated carbon is composed of two-dimensional GNSs (i.e., monolayer graphitic fragments) with regular shapes, partially ordered but not, in general, parallel to each other. It is worth noting that tilt grain boundaries in single-layer graphene have been under intensive study30 due to the potential application of graphene in electronics and spintronics. Although the honeycomb structure is destroyed, each C atom can still have three nearest-neighbors at the grain boundaries; hence, sp3 bonding remains dominant. Since sp2-type C−C bonds are stronger than both their sp3 counterparts and presumably the C−LFePO4 interaction, structure D should probably have the lowest free energy due to having the fewest sp3 bonds. However, the chances for all the GNSs to match perfectly through grain boundaries with no need of translational adjustment are by all means extremely low, and structures A, B, and C should have good chances to be present, as is the same reason for the stability of carbon black. By comparison, transformation of the local structures in A, B, and C involves only the rotation of the GNSs, making regulation of alignment much easier. If the C-LFPO bonding is significantly stronger than the GNS−GNS intersection bonding, then structure A will more likely be the C-LFPO interfacial geometry, whereas if the latter is much stronger than the former, structure B will be prevalent; and if the two are comparable, structure C, a compromise of A and B, will have more chance to appear. With these analyses, elucidation of the C-LFPO interfacial structure in the atomic scale now comes down to the determination of the binding strength of (1) graphene grain boundaries, (2) graphene−graphene intersections, (3) graphene standing on LFPO, and (4) graphene spreading on LFPO. In a very recent work, Liu et al. conducted a series of hybrid molecular dynamic simulations on the structure, energy, and structural transformations of the symmetric tilt grain boundaries (without dangling bonds) in graphene.31 The calculated formation energies for grain boundaries with different misorientation angles fall within 0.25−0.55 eV·Å−1. It is reasonable to estimate the average formation energies for more generally tilted grain boundaries to have similar values. As for the graphene−graphene intersections, there has been no reported work on either the experimental or theory side, to the best of our knowledge. To meet the requirement of lattice match, there should be only two types of junctions when two graphene sheets are connected. One is zigzag-to-zigzag (left), and the other is armchair-to-armchair (right), as shown in Figure 2. To make the computation affordable, we have let the edge of a GNS other than the one forming an interface or an intersection be free and passivated with H. Note that, along the intersection line, the two sheets are infinitely long and have a periodicity of 2.46 and 4.26 Å, respectively; and in the other two dimensions, they have finite size, which is large enough to minimize the interaction of free ends and interfaces. According to density functional theory (DFT) calculations by Wang et al.,32 the (010) surface is one of those having the smallest formation energy, in agreement with the observation that (010) is the large facet of the noncoated LFPO crystals.33 Besides, LFPO [010] is the direction along which Li ions are intercalated into and dis-intercalated out of the particles, first predicted by theory34,35 and then confirmed by experiment.36 We therefore choose this surface on which to put a GNS (see Figure 3). The experimental lattice constants for LFPO are a = 10.33 Å, b = 6.01 Å, and c = 4.69 Å.1 However, to start the

Figure 2. Atomic structure at the zigzag- (a) and armchair-type (b) graphene intersections. The free edges of the graphene sheets are passivated by hydrogen. Dimension b is infinite for both sheets. Shown here are already optimized configurations. Large and small circles represent C and H atoms, respectively. We use different colors for C atoms in distinct sheets, in order to better distinguish the two sheets at the interface.

Figure 3. Side view of a graphene nanosheet adsorbed parallel to (c) or perpendicularly on a LiFePO4 (010) surface with the intersection along the [100] (a) or [001] (b) directions. The free ends of the GNSs in parts a and b are passivated by hydrogen. The green, brown, purple, and red circles repersent Li, Fe, P, and O atoms. Shown here are optimized configurations.

computation, we should first optimize them using a firstprinciples method. On the zigzag edge, the periodicity of a graphene sheet is dzz = 2.46 Å; and on the armchair edge, it is dac = 4.26 Å. Obviously, to put a graphene sheet standing on the (010) surface (a−c plane), the smallest supercell can be built up with the approximations c = 2dzz (left) and a = 4dzz (middle); to put it lying on the surface, the smallest supercell can be built up with the approximation 2a = 5dac and c = 2dzz (right).

2. COMPUTATIONAL DETAILS In this article, we use first-principles calculations to study the binding of graphene−graphene sheets and a graphene−LFPO surface. The computational technique is the DFT based Vienna ab initio simulation package.37 The electron−ion interaction was described using the projector augmented wave (PAW) method.38 The exchange correlation between electrons was treated with the generalized gradient approximation (GGA) in the Perdew−Burke−Ernzerhof (PBE) form.39 The on-site Coulomb interaction of Fe-3d electrons was described by a Hubbard U introduced by Dudarev et al.40 We chose the same value of U = 4.7 eV and the double counting form J = 1.0 eV as in refs 32 and 35. We used an energy cutoff of 500 eV for the 17652

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Table 1. Calculated Binding Strength (eV·Å−1) of a Graphene Sheet on the (010) Surface of LFPO Based on Density Functional Theory (DFT), in Both Parallel and Perpendicular Orientationsa interface

GNS−LFPO parallel

DFT

0.02

vdW-corrected

0.1−0.2

GNS−LFPO perpendicular along a

GNS−LFPO perpendicular along c

GNS−GNS intersection

graphene GB

graphene perfect

0.22

0.52

0.30 (zz) 0.32 (ac) 0.4

0.6−0.9

1.16

a

Those for graphene intersections (both zigzag (zz) and armchair (ac) type), graphene grain boundaries (GB),30 and the cleavage energy of perfect graphene are also listed for comparison. Note that the van der Waals (vdW) correction is estimated based on the reported vdW-DFT study on the interlayer binding of graphene sheets.41.

intersection, the sp3 C−C bonds are longer than the standard sp2 C−C bonds. In the zigzag case, the length of the three sp3 C−C bonds is 1.56 Å for the vertical one and 1.52 Å for the horizontal two, whereas they are 1.55 Å and 1.53 Å in the armchair case. We note that, in each case, the average sp3 C−C bond length is about the same as the standard bond length in diamond, 1.54 Å. The binding energy for a graphene intersection is defined as the energy gain when the vertical and horizontal sheets get connected; i.e., Eb = Etot − (Ev + Eh). Our DFT calculations show that the binding in the zigzag-tozigzag type intersection is 0.75 eV per C−C bond, i.e., 0.30 eV·Å−1. Without spin-polarization, it is 0.23 eV·Å−1. In the armchair-to-armchair case, no magnetic moments are found for either edge or intersection carbon atoms, and the binding strength is 0.69 eV per C−C bond (0.32 eV·Å−1). When we amend the vdW correction discussed above, these binding strengths are about 0.40 eV·Å−1. 3.2. Graphene/LFPO Interface. We list in Table 1 the calculated binding energy of a graphene sheet on the (010) surface of LFPO, in both parallel and perpendicular orientations. The optimized atomic structures of various graphene/LFPO interfaces are shown in Figure 3. As expected from the stoichiometric feature and the small surface energy of LFPO (010), a parallel graphene layer stays rather distantly (2.97 Å in average) from and thus binds very weakly to the oxide surface, 0.0015 eV·Å−2. When corrected by the vdW effect, this binding strength is estimated to be 0.02 eV·Å−2, equivalent to 0.2 (along c) and 0.1 eV·Å−1 (along a) for a perpendicular sheet. For the perpendicular case, we have searched the most stable adsorption position for arrays along both a and c axes (Figure 3). It turns out that graphene sheets prefer standing along the c direction, with the most stable position shown in Figure 3b. We note that the dimensions for our supercell are a = 10.42 Å, b = 6.01 Å, and c = 4.75 Å, and the periodicity in graphene along the zigzag-type edge is 2.46 Å. Thus, the lattice mismatch is smaller when a graphene sheet stands along the c axis (3.5%) than along the a axis (5.9%). Our first-principles calculations thus demonstrate that a GNS binds perpendicularly more strongly to the LFPO (010) surface (along c) than perpendicularly to another GNS, and less strongly than to another GNS through the coplanar grain boundary. This means geometry C is energetically more favorable than geometry B in the models shown in Figure 1. Now, we look at the chemical bonding at the C/LFPO interface. We draw in Figure4 the calculated density of valence electrons on two planes intersecting the interface; one contains interfacial C and Fe atoms (a), and the other contains interfacial C and O atoms. From this charge distribution, it is clearly seen that there is a strong chemical bond between C and O across the interface, with a bond length of 1.41 Å, which is about the same value as a typical single C−O bond, 1.43 Å.47

plane wave basis set for all systems to ensure equal footing. The Brillouin-zone integration was performed within the Monkhorst−Pack scheme using k meshes of (1 × 6 × 1), (1 × 8 × 1), (2 × 1 × 6), and (1 × 1 × 6) for the geometries described in Figure 2a, Figure 2b, Figure 3a,b, and Figure 3c, respectively. The energy relaxation for each strain step is continued until the forces on all the atoms are converged to less than 1 × 10−2 eV·Å−1. It has to be pointed out that, when dealing with graphite, the van der Waals (vdW) interaction between graphene layers has to be treated properly. First-principles calculations based on vdW-DFT yield a stronger interlayer binding of about 0.05 eV per C atom than the conventional GGA approximation.41 What we consider here are graphene fragments which most likely have sizes of several (800 °C) as to readily crystallize carbon into graphite such as in the case of carbon-coated Li4Ti5O12,48,49 one could end up with graphene layers in parallel to the particle surfaces, because connecting small graphene sheets into large sheets is almost always energetically favorable. Alternatively, depending on the competition of binding strength between the graphene− graphene intersection and graphene-oxide surface, the orientation of nanoscale graphene sheets may vary with surface directions and materials. In view of the similarity of the interfacial structure of carbon/LiFePO4 yielded by distinct reactants, amorphous and graphene sheets, one can reach the conclusion that partial crystallization of carbon occurs in the vicinity of the carbon/LiFePO4 interface. As a consequence, using graphene sheets as ingredients instead of carbon black might not be optimal in consideration of its high cost. More importantly, by virtue of this atomic resolution, we shall be able to judiciously seek elements or compound additions that can strengthen the carbon coating and simultaneously improve the interfacial conductivity in similar or more general cases, in an effort to improve the electrochemical performance of these electrode materials.50

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. † E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the support of the MEXT Program for Development of Environment Technology using Nanotechnology.

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REFERENCES

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