Interface and Doping Effect on the Electrochemical Property of

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Interface and Doping Effect on the Electrochemical Property of Graphene/LiFePO

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Huayu Wang, Naiqin Zhao, Chunsheng Shi, Chunnian He, Jiajun Li, and Enzuo Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03449 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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The Journal of Physical Chemistry

Interface and Doping Effect on the Electrochemical Property of Graphene/LiFePO4 Huayu Wanga, Naiqin Zhaoa,b, Chunsheng Shia, Chunnian Hea, Jiajun Lia, Enzuo Liua,b*

a

Key Laboratory of Composite and Functional Materials, School of Materials Science

and Engineering, Tianjin University，Tianjin 300072，China

b

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin

University，Tianjin 300072，China

ABSTRACT Surface properties of olivine phosphate LiFePO4, as cathodes in lithium ion batteries, are of importance for overall performance as the nano scale of particles has become indispensable. Using the first-principles total energy calculations, the effects of graphene and/or Mn dopant on the electrochemical properties of graphene/LiFePO4 have been comprehensively investigated. It is revealed that the interfacial binding between graphene and LiFePO4 in parallel orientation is stable and improved in the process of doping. The Li adsorption energy at different sites elucidates the core-shell model in Li extraction/insertion process and indicates the 1

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anomalous Li storage in the interface between graphene and LiFePO4. Mechanisms underlying influences of adsorption site, Mn dopant and graphene modification on the Li adsorption energy are discussed through edge effect, doping stability and interfacial binding strength, respectively. The surface conductivity is improved in the presense of graphene and/or Mn dopant with respect to the bandlike electron transport.

1. Introduction Lithium ion batteries (LIBs) have been used as a key component of portable electronic devices due to its high energy density and long cycle life. As a promising cathode material having olivine structure, lithium transition metal phosphate LiFePO4 (LFPO) has inexpensive, inherently non toxic and stable nature, leading to massive research1. However, LFPO has inherently low electronic2 and ionic conductivity3. There are challenges to achieve high electrochemical performances, especially high rate capability4. Carbon coating is an efficient way to improve the electronic conductivity of LFPO cathode5. Besides the amount and distribution of carbon, the degree of graphitization is used to appraise the quality of carbon coating because of its dramatic influence on the conductivity and rate behavior of LFPO6. Unlike the morphology of carbon coating7-8, graphene sheets in graphene coated/modified particals have a regular stacking state and bind with LFPO surface in parallel orientation9-12. Due to the higher 2


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conductivity

and

structural

flexibility

of

graphene

sheets,

graphene

coating/modification of LFPO leads to outstanding electrochemical performance9-11. Moreover, LFPO modified with graphene has the specific capacity beyond the theoretical capacity10-11. The excess capacity originates near the cell voltage of 3.5V v.s. Li+/Li. Despite myriad studies on graphene modification, the interfacial binding between LFPO and graphene in parallel orientation, as well as the mechanism of the excess capacity of graphene modified LFPO, remains to be further studied. Isovalent substitution is another feasible method to improve the electrochemical performance of olivines. Substitution of Fe by Mn in LFPO (LFMPO) can promote the rate capability and cycling performance13-16. Slight Mn doping in LFPO reduces the band gap and enlarges the crystal space for Li diffusion, thus enhancing the electronic conductivity16 and Li ion diffusion rate14 respectively. Besides, surface redox potential is partially responsible for the capacity away from the voltage plateau in LFPO17. As the particle size at nano scale has been considered as a crucial parameter in preparation, the influence of surface properties on the overall electrochemical performance is of great importance. However, the effects of doping on the surface electron conductivity and surface redox potential have not been well clarified. In this paper, the influences of isovalent Mn doping and graphene modification on the electrochemical properties of LFPO have been comprehensively studied using first-principles calculations. The interfacial binding between graphene and LFPO, as 3



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well as the effect of Mn dopant on it, is studied. The Li ion storage improvement at graphene/LFPO interfaces and the surface redox potenial changes due to the presence of graphene and the dopant are investigated. The effects of graphene and the dopant on the surface conductivity have also been analyzed. 2. Methodology Calculations are performed using the projector augmented wave (PAW) formalism of density functional theory as implemented in the Vienna ab initio Simulation Package (VASP)18. The spin polarized generalized gradient approximation (GGA) with the parameterization of Perdew-Burke-Ernzerhof (PBE)19 was used for exchange and correlation energy of interacting electrons. With the consideration of the strong electron correlation within d state of transition metals (TMs), the GGA+U approach20 was adopted to accurately calculate the electron properties of transition metal phosphates. The rotationally invariant scheme as presented by Dudarev21 was employed, with U-J of 4.3 eV for both Fe and Mn17; 22, the parameterization of which has been justified by the good agreement between redox potentials and cell voltages of bulk olivines (see Table S1). In the energy and electronic properties calculations, the Gaussian smearing method23 was used, and the width of smearing was chosen as 0.05 eV. A plane-wave basis with a kinetic-energy cutoff of 500 eV was chosen. The DFT-D2 method of Grimme24 with default parameters was applied to describe long-range weak interactions at interfaces and surfaces, the implementation of which

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is validated by its versatile applications in surface systems25-27 and interface systems comprising of graphene and metals/metal chalcogenides28-32. An antiferromagnetic (AFM) Fe and Mn high-spin states were assumed, because AFM state of bulk LFPO and LiFe0.75Mn0.25PO4 is more stable than ferromagnetic (FM) and nonmagnetic states according to this work (see Table S2 and Figure S1) and other studies33-34. Bulk LFPO has an orthorhombic structure with a space group of Pnma. Calculated lattice parameters of LFPO, LiFe0.75Mn0.25PO4 and graphene are listed in Table S2, in good agreement with previous studies. Since (010) surfaces of LFPO have the smallest surface energy according to first-principles calculations17, and (010) surface is the largest facet of LFPO particles according to the experimental observation35, (010) surface was chosen to be analyzed. Initial surface structures were carved out of the relaxed bulk crystals. Interface structures are presented in Figure 1. In order to minimize the lattice mismatch between graphene and LFPO along a and c axis, a 2 × 1 supercell of LFPO (010) surface containing 20 Li, 20 Fe, 20 P and 80 O atoms and a 5 × 2 graphene supercell containing 40 C atoms were employed. The lattice of graphene is modified to match the surface lattice of LFPO, with the mismatch less than 5% as shown in Table S1. In the b direction, atoms beneath the third Li layer from surface were fixed to simulate the bulk crystal. In order to study the effect of doping on surface and interface properties, one outermost Fe ion in surface or interface structures was substituted with one Mn ion, respectively, as shown in Figure 1. The selection of doping site is based 5



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on the facts that Mn dopants observed in experiments homogeneously distribute both at the surface and in the bulk, with the Mn/Fe mole ratio widely ranging from 0.5% to 90%14-15,

36-39

, and that the surface is important for overall electrochemical

performance when the volume ratio of surface to bulk for nano-sized particles is not low. The Brillouin zone was sampled using Monkhorst-Pack scheme40 with 1 × 1 × 3 k-point grid. For all models, geometry optimization was not finished until the Hellmann-Feynman forces were less than 0.03 eV/Å. Structure and electronic visualization and analysis were performed using VESTA41.

Figure 1. Interfaces between graphene and (a) intact LFMPO surface, (b) LFMPO surface after the delithiation of the outermost layer of Li atoms, and (c) LFMPO surface after the adsorption of two Li atoms at the interface. B and c axes represent [010] and [001] directions, respectively. 6


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3. Results and Discussion 3.1 Interfacial binding between graphene and LFPO To ascertain the interfacial binding between graphene and iron based olivine phosphates, the interfacial binding energies Ebin were calculated, which is defined as Ebin = Eint – Esur – Egra. Esur, Egra and Eint denote the total energies of olivine phosphate surface, isolated single layer graphene with lattice mismatch considered, and graphene/olivine phosphate interface, respectively. Compared to the interfacial binding between graphene and TMs or TM oxides which is typical physical adsorption31, 42, the Ebin per carbon atom and the interface space d between graphene and olivine phosphates shown in Table 1 are at the same order of magnitude ( about 40 meV for Ebin per carbon atom and about 3 Å for d), suggesting the nature of physical adsorption. This is in agreement with the estimation of the G/LFPO interface binding strength in Geng’s study8. The electronic structure at the interface has been systematically studied to further understand the interfacial binding. The charge density differences are shown in Figure 2 (a). In both G/LFPO and graphene/LFMPO (G/LFMPO) interfaces, electrons transfer from C atoms in graphene to the four outermost O atoms in olivines, consistent with the Bader analysis43 summarized in Table S3. Since the distances (about 2.8 Å) and electron transfers between C and O atoms (less than 0.15 e/ C-O couple) in both interfaces are larger and smaller than the usual ionic bonds, respectively, the interaction induced by electron transfer belongs to the electrostatic 7



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interaction rather than ionic bonding. In addition, there is electron accumulation between the outermost Li atoms and its nearest C atoms as shown in Figure 2 (a), and the lengths of Li-C in G/LFPO (2.68 Å) and G/LFMPO (2.67 Å) interfaces roughly approach the Li-C bond length (2.24 Å) when one Li ion is chemically adsorbed on single layer graphene44. It is advantageous to investigate whether covalent bonds form between Li and C through the electron localization function (ELF) analysis. ELF is a position-dependent function with 0 < ELF < 1, and the value of ELF = 0.5 corresponds to the electron-gas like pair probability45. As shown in Figure 2 (b), ELF is very small between graphene and LFMPO surface, demonstrating the absence of any directional covalent bonds31-32. The same conclusion is reached from projected DOS in Figure 2 (c) where no hybridization of electronic states between Li and its nearest C is observed. The electron accumulation between Li atoms and C atoms is supposed to be the charge redistribution of graphene induced by corrugation (see section 3.2). The interfacial binding strength with the unit of meV/A2 is extensively resorted for various systems and is compared with the typical vdW binding energy, which are in small energy interval of 13-21 meV/Å2, in order to clarify the type of interface interaction46-50. The Ebin per unit area of G/LFPO and G/LFMPO interfaces listed in Table 1 reaches and mildly exceeds the upper limit, respectively, manifesting the main contribution of vdW interaction and slight contribution of other binding types to the interfacial binding strength. Based on the aforementioned discussion, it is concluded that the interfacial binding belongs to the physical adsorption caused 8


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mainly by vdW interaction and slightly by electron transfer induced electrostatic attraction, the complex nature of which is common in weak-interaction interface systems comprising of graphene and metals/metal chalcogenides51-54.

Table 1. Calculated interfacial binding energies Ebin per carbon atom (meV), Ebin per unit area (meV/ Å2) and the interface spaces d (Å) of graphene on LFPO and LFMPO surfaces. Interface

G/LFPO

G/LFMPO

Ebin per carbon atom

-49.81

-53.65

Ebin per unit area

-20.18

-21.74

d

2.88

2.86

9



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Figure 2. (a) Charge density difference of G/LFMPO interface, where charge density difference is obtained by subtracting charge density of isolated graphene and that of LFMPO surface from that of G/LFMPO interface. Blue and red colors indicate the electron accumulation and depletion, respectively. The isosurface contours correspond to 2 × 10-3 e/Å3. (b) Spin polarized projected DOS of the outermost Li and its nearest C of G/LFMPO interface which are pointed by arrow in Figure 1 (a). The Fermi level is set as zero. (c) Electron localization function (ELF) of G/LFMPO interface. The isosurface contours correspond to 0.5. Symbols of elements are in accordance with Figure 1. Counterparts of G/LFPO interface are similar with those of G/LFMPO interface.

The influence mechanism of Mn dopant on interfacial binding strength is discussed. As illustrated in Figure 3 (a), the relation of Ebin between G/LFPO and G/LFMPO interfaces depends on Δ1 and Δ2. Δ1 and Δ2 represent the energy variation induced by Mn substitution at LFPO surface and G/LFPO interface, respectively. It is defined as EΔ = Edop – Eund, where Edop denotes the energy of the structure with one metal ion substituted by a dopant, and Eund denotes the energy of the corresponding structure without doping. The EΔ at surface and interface are -1.54 and -1.70 eV respectively. To explain what causes the discrepancy between EΔ at the surface and at interface, the bond length of Mn-O bond is measured. The average Mn-O bond length in G/LFMPO interface and LFMPO surface is 2.092 Å and 2.096 Å respectively, for 10


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Mn-O1 is much shorter in the former than in the latter, as depicted in Figure 3 (b) and (c). The shrinkage of Mn-O1 in G/LFMPO interface is caused by electron accumulation near O1 in the process of interfacial binding which enhances the electrostatic attraction between Mn and O, as demonstrated in Figure 2 (a) and Table S3. Since the stronger bond correlates to the higher doping stability55, the smaller average bond length indicating higher bond strength explains the difference of EΔ at the surface and the interface and the stronger binding energy of G/LFMPO interface than that of G/LFPO interface.

Figure 3. (a) Schematic representation of the relative energies of different systems in the calculation of Ebin. (b) Crystal structure of MnO6 octahedrons in G/LFMPO interface. (c) Crystal structure of MnO6 octahedrons in LFMPO surface. Symbols of elements are in accordance with Figure 1. 11



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Therefore, the parallel binding between graphene and iron based olivine phosphates is stable and belongs to physical adsorption, with electron transferring from C atoms in graphene mainly to the outermost O atoms in olivine surface. The interfacial binding energy of G/LFMPO interface is stronger than that of G/LFPO interface, which results from the higher stability of Mn dopant at the interface than at the LFPO surface, induced by the shrinkage of Mn-O bond after electron transfer from C to O atoms. 3.2 Li adsorption behavior at the interface Optimized configuration of G/LFPO interface after the delithiation of the four outermost Li atoms is shown in Figure 1.(b). For Li adsorption at surface/interface, two Li atoms are symmetrically put at the surface/interface. Probable adsorption sites are chosen based on the charge density of G/LFPO in Figure S2, and sites with lower charge density are more likely to be stable for Li adsorption56. The most stable adsorption site is the bridge site above the two outermost O atoms, and the optimized configuration of G/LFPO with two Li atoms adsorbed is presented in Figure 1. (c) as an instance. The calculated adsorption energies are listed in Table 2. The adsorption energy57 Eads is defined as Eads = (E0 + n*ELi – ET)/n, where ET denotes the energy of the structure with Li adsorbed or inserted, E0 denotes the energy of the corresponding structure without Li adsorbed or inserted, ELi denotes the energy per atom in metallic Li bcc phase, and n is the number of Li atoms adsorbed or inserted.

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Table 2. Eads (eV) for Li adsorption at the surfaces and interfaces (Ads) and for the delithation of the outermost Li atom layer at the surfaces and interfaces (Dli). LFPO

LFMPO

G/LFPO

G/LFMPO

Dil

4.18

4.17

4.20

4.28

Ads

3.61

3.42

3.67

3.60

The relation between Eads for the delithiation of the outermost Li atoms at surface and bulk redox potential sheds light on the shape of charge/discharge profile and Li extraction/insertion mechanism. Given that Eads for the delithiation of the outermost Li atoms of LFPO surface is larger than the cell voltage of LFPO, Li atoms are inserted on the surface earlier than in the bulk upon discharging and extracted later upon charging by definition. As a result, Li storage at the outermost layer, besides the surface amorphization58, contributes to the specific capacity above the bulk redox potential in LFPO (3.43V v.s. Li+/Li ). Moreover, the relation mentioned above supports the “new core-shell” model proposed by Laffont et al59 to explain the Li insertion/extraction process observed in experiments, according to which LFPO/FPO system always keeps a structure with a shell of LFPO and a core of FPO due to the successive Li migration. Eads for Li adsorption at the surfaces and interfaces reveals the possibility of a kind of anomalous Li storage mechanism. The Eads for Li adsorption at the interface of G/LFPO approximates to the cell voltage of LFPO with the discrepancy of 7.0%. 13



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Thereby, the interfacial Li storage may contribute to the outstanding capacity beyond theoretical capacity of LFPO reported in the studies of Zhu et al11 and Hu et al10. Although the redox reaction of graphene is supposed to account for the excess capacity in those studies, it is unable to strictly explain the reason why the excess capacity is mainly from the cell voltage. According to the calculation results above, interfacial Li storage is probable and accurately contributes to the capacity near the plateau voltage. Furthermore, anomalous Li storage mechanism is possible for the structures without graphene decoration. However, due to the poor kinetics of olivine phosphates without carbon coating, theoretical capacity is hard to be achieved so that direct evidence of anomalous Li storage needs meticulous characterization. It is important to investigate the origin of the discrepancy of Eads at different sites. As shown in Table 2, most Li atoms at the outermost layer and at the surfaces/interfaces have higher Eads while the Eads for Li atoms in bulk LFPO is only 3.43 eV. The higher Eads for Li atoms at the edge sites, including the outermost layer and the surfaces or interfaces, than in the bulk are attributed to the edge effect common in graphene60 and MoS261 where C and S at edge sites are chemically more active than those at inner sites and thus, more favorable for Li binding. The activity of O atoms at the LFPO surface is substantiated through the projected DOS of the O atoms, as shown in Figure S3. Compared with the bulk FePO4 that is well known semiconductor with a band gap62 of 3.6 eV, the 2p projected DOS of O at both LFPO surface and partially delithiated LFPO surface are not zero at Fermi level and have 14


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peaks in the region barely above Fermi level, indicating better accessibility of charge transfer from Li and stronger binding energy of Li adatom. Other systems have similar properties. Moreover, the higher Eads for Li atoms at the outermost layer than that for Li atoms at the surfaces/interfaces result from the two more O atoms binding with Li atoms at the outermost layer. In order to investigate the effect of Mn dopant on Eads, similar with the aforementioned discussion on the effect of Mn dopant on Ebin, Δ1 and Δ2 are analyzed, which are corresponding to the energy variations induced by Mn substitution EΔ of the systems with and without Li atoms adsorbed, respectively. E Δ in different systems and the corresponding average Mn-O bond length in MnO6 octahedron L are listed in Table 3. In the surface systems, the conspicuous correlation between L and EΔ reveals that smaller bond lengths accompany more stable dopants. This is also consistent with the charge densities of Mn-O bonds shown in Figure S4, where shorter bond has larger charge density leading to higher bond strength33. Therefore, Mn dopant reduces Eads for Li atoms at edge sites of surfaces. In the interface systems, same regularity that smaller L is correlated to more negative EΔ is revealed. However, there is an exception between G/LFMPO interface and the equivalent with the outermost Li atoms delithiated that the former has larger L and still more negative EΔ than the latter. With the consideration that graphene is the only structural difference between the interface and surface configurations, the structural and energetic properties of graphene in the interfaces are investigated, as shown in Table S4. In 15



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doping process, the graphene maximum corrugation is alleviated in G/LFPO interface, but deteriorates in partially delithiated G/LFPO interface. Furthermore, the graphene relative energy is elevated as corrugation enlarges, in agreement with previous report30. It is explained why partially delithiated G/LFMPO interface with shorter Mn-O bonds has less negative E Δ than G/LFMPO interface. Thus, Mn dopant decreases Eads for Li atoms at the interface but increases Eads for Li atoms at the outermost layer of interfaces.

Table 3. The average bond length of Mn-O of MnO6 octahedron L (Å) and corresponding EΔ (eV). Adsorbing structures denote those with two Li adsorbed at surface/interface, and delithiated structures denote those after delithiation of the four outermost Li.

L

EΔ

LFMPO

2.096

-1.54

Adsorbing LFMPO

2.126

-1.16

Delithiated LFMPO

2.081

-1.59

G/LFMPO

2.092

-1.70

Adsorbing G/LFMPO

2.114

-1.54

Delithiated G/LFMPO

2.083

-1.38

In order to elucidate the effect of graphene modification on Eads, similar analysis is conducted. Δ1 and Δ2 correspond to the interfacial binding energies Ebin of systems 16


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with and without Li atoms adsorbed, respectively. It is revealed that graphene modification increases Eads at edge sites, which relates to the higher Ebin of the systems with Li atoms adsorbed than those of the systems without Li atoms adsorbed. Ebin and the interface spaces (d) of different systems are presented in Table 4. As expected, Ebin increases as d decreases. The charge density differences of G/LFPO and G/LFMPO interfaces with Li atoms adsorbed are shown in Figure 4. Electrons transfer from Li adatoms to neighbor C atoms and O atoms during adsorption, which is confirmed by the Bader analysis in Table S5. In addition, the minimal distances of Li-O and of Li-C in Li adsorbed G/LFPO and G/LFMPO interfaces listed in Table S5 approach the ionic Li-O and Li-C bond lengths. Furthermore, Ebin of interfaces with Li atoms adsorbed (-21.53 meV/Å2 of Li adsorbed G/LFPO interface and -25.36 meV/Å2 of Li adsorbed G/LFMPO interface) beyond the range of typical vdW binding energies implies the contribution of other binding types. Therefore, ionic bonds between Li adatoms and the nearest O and C atoms are supposed to form, leading to strong Coulomb interaction between adatoms and graphene, as well as the surfaces, and accounting for the more negative Ebin of interfaces with Li atoms adsorbed than those of original interfaces, in agreement with the analysis of Li intercalated graphene/MoS2 interface30. Li adsorption at the interface results in more stabilization of the interface interaction, and thus, higher Eads is obtained. On the other hand, the less negative Ebin of partially delithiated interfaces than those of the original interfaces are ascribed to two factors. One is aforementioned severe graphene 17



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corrugation in partially delithiated interfaces, which destabilizes the interfaces. The other is the influence of delithiation on the interfacial electron transfer which is analyzed through Bader charge and summarized in Table S3. In partially delithiated interfaces, electron transfer not only occurs between graphene and the outermost O atoms, but between graphene and relatively inner atoms as well, for the M (M=Fe, Mn) and O atoms at other edge sites are oxidized after the removal of the outermost Li atoms and thus more inclined to capture electrons. Then, larger distances and weaker electrostatic interaction and between charged graphene and charged atoms in partially delithiated interfaces than those in original interfaces are obtained, leading to less negative Ebin. Consequently, graphene modification increases the Eads for Li atoms both at the outermost layer and at the interfaces.

Table 4. Ebin (eV) and interface spaces d (Å) of different systems. Delithiated denotes the delithiation of the outermost Li atoms, adsorbing denotes the adsorption of two Li atoms at interface. Delithiated

Original

Adsorbing

Ebin

d

Ebin

d

Ebin

d

LFPO

-1.91

2.99

-1.99

2.88

-2.13

2.27

LFMPO

-1.70

3.00

-2.15

2.86

-2.50

2.12

18


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Figure 4. Charge density differences of Li adsorption in (a) G/LFPO interface, (b) G/LFMPO interface, where charge density difference is obtained by subtracting charge densities of interfaces without Li atoms absorbed and those of isolated Li atoms from those of Li adsorbed interfaces. Blue and red colors indicate the electron accumulation and depletion, respectively. The isosurface contours correspond to 3 × 10-3 e/Å3. Symbols of elements are in accordance with Figure 1. Therefore, the adsorption energies of Li atoms at different sites illustrate to some extent the shape of charge/discharge profile, Li extraction/insertion mechanism and outstanding specific capacity beyond theoretical limit. The dependence of Li adsorption energy on factors including adsorption site, Mn dopant and graphene modification is elaborated through edge effect, doping stability and interfacial binding strength, respectively.

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3.3 Surface electron conductivity and Li diffusivity To investigate the surface electron conductivity of doped/undoped LFPO surface and G/LFPO interface, the electronic structures of MO6 (M=Fe, Mn) coordination octahedrons in the outermost and second layers are investigated, since the electronic states of the olivine phosphate surfaces near the Fermi level mainly consist of M-3d (M=Fe, Mn) and O-2p states. M-3d and surface total states near the Fermi level are plotted in Figure 5. Electronic structures are hardly impacted by vdW correction (see Figure S6), in accordance with previous studies31, 63. As shown in Figure 5, the surface total DOS at the Fermi level in all surfaces/interfaces are much larger than zero, indicating good electron conductivity. They are comprised of the states of MO6 octahedrons at the outermost layer, as shown in Figure 6. However, the DOS near the Fermi level in FeO6 octahedrons at the second layer is smaller (see Figure S7). The good electron conductivity is mainly caused by the coordination loss of Fe at surface, especially at the outermost layer. At the LFPO surface, the effective coordination number64 of Fe at the outermost layer is 2.04, and those of Fe at the second layer and in bulk are 5.59 and 5.68 respectively, demonstrating that the distortion of coordination polyhedrons recedes with the depth of surface. There are similar trend in other structures. The good surface conductivity is one of the reasons why the rate capability of LFPO is improved by reducing the particle size4.

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Figure 5. Spin-polarized surface total DOS and local DOS projected on M-3d of (a) LFPO surface, (b) LFMPO surface, (c) G/LFPO interface and (d) G/LFMPO interface. For each structure, states of four coordination octahedrons in the rectangle in Figure 1.(a) are chosen to plot the local DOS projected on M-3d and to approximate the surface total DOS by the summation of M-3d and O-2p therein. The Fermi level is set as zero.

In order to analyze the effect of Mn dopant and graphene modification on the surface electron conductivity, the integration of the total surface DOS near the Fermi level65-66 is listed in Table 5. Mn dopant, graphene modification and their coexistence all improve the surface electron conductivity of LFPO, which is further confirmed by 21



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comparing the contours of the surface total DOS. The surface total DOS of LFMPO surface from -1.0 to -0.7 eV is larger than that of LFPO surface, as presented in Figure 5 (b). Therefore, the valence band maximum (VBM) of LFMPO surface is connected more closely with the part below, leading to better electron conductivity, corresponding to previous experiment13. The total surface DOS of G/LFPO interface at the Fermi level presented in Figure 5 (c) is larger than that of LFPO surface, directly leading to better electron conductivity, in agreement with experiments12, 67. The G/LFMPO interface has advantages of both LFMPO surface and G/LFPO interface. It is significant to elucidate the origin of the better surface electron conductivity with the presence of Mn dopant and graphene modification. As shown in Figures 6 (a), (b) and (c), the local DOS in MnO6 octahedron and FeO6 octahedron at the outermost layer nearest to the Mn dopant of LFMPO surface account for its larger surface total DOS from -1.0 to -0.7 eV and from -0.3 to 0.5 eV than that of LFPO surface. On the other hand, given the less valence electron in Mn2+ (3d5) compared to Fe2+ (3d6), Mn doping is p-type doping and gives rise to an acceptor level at 1.0-1.7 eV contributed mainly by Mn-3d and its hybridizing O-2p (see Figure 6 (c)) and slightly by the nearest Fe-3d and its hybridizing O-2p at the second layer (see Figure S7 (b)), leading to a lower band gap and the better conductivity than LFPO surface. This is in agreement with previous calculation of bulk LFPO16. As shown in Figures 6 (a), (d) and S7 (a), (c), the local DOS in FeO6 octahedrons at the outermost layer and second 22


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layer of G/LFPO interface account for its larger surface total DOS from -0.5 to 0 eV than that of LFPO surface, indicating that the not only the outstanding conductivity of graphene, but the improved surface conductivity of LFPO contributes to the good rate capability of graphene modified LFPO. The local DOS of G/LFMPO interface has the advantages of both LFMPO surface and G/LFPO interface.

Figure 6. Spin-polarized local DOS projected on M-3d and O 2p in (a) one FeO6 octahedron at the outermost layer of LFPO, (b) the FeO6 octahedron at the outermost layer nearest to Mn dopant of LFMPO, (c) the MnO6 octahedron of LFMPO and (d) one FeO6 octahedron at the outermost layer of G/LFPO. The Fermi level is set as zero. 23



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Since bulk olivine phosphates are wide band gap materials exhibiting polaronic rather than bandlike transport68-69, the hole polaron formation at surface is investigated. When a Li ion is removed from LFPO, the valence state of Fe changes from +2 to +3, resulting in a localized hole and induced lattice distortion, which is called small hole polaron70. The stabilization of small hole polarons is necessarily related to the electronic structure where the VBM consist predominantly of the highly localized 3d states69. On one hand, as presented in Figure 5 and Table 5, all surfaces have more difficulty in stabilizing small hole polarons than the bulk, for the O dominance at the VBM makes the hole created by electron withdrawal delocalized and the polaron unstable70. The percentage of 3d character in band of G/LFPO and G/LFMPO interfaces does not take states of graphene into account, leading to a somewhat overestimation. The poor stability of polaron and the nonzero DOS at the Fermi level indicate that the bandlike rather than polaronic transport is dominant at the surfaces. On the other hand, Mn dopants have positive impacts on polaron stabilization.

Table 5. Percentage of TM 3d character in band from -1 to 0 eV and the integration of the total DOS from -0.5 to 0.5 eV at the surfaces/interfaces. States of four innermost O atoms in the two octahedrons at the second layer are neglected to keep the composition stoichiometric in the calculation of the percentage of M-3d character in band. The percentage of TM character in band nearest to the VBM in bulk LFPO71 is listed for comparison. 24


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LFPO

LFMPO

G/LFPO

G/LFMPO

Surface percentage

37.55%

44.36%

32.43%

40.19%

Bulk percentage

85%

N/A

N/A

N/A

Integration

3.99

4.70

4.85

4.60

In short, surface conductivity is enhanced in the presence of Mn dopant, graphene modification and their coexistence with respect to bandlike electron transport, which is confirmed by the higher integration of the surface total DOS near the Fermi level. The dominance of the bandlike rather than polaronic transport at the surfaces is substantiated by the nonzero DOS near the Fermi level and the poor stability of polaron. Additionally, though the adsorption energies for Li atoms at the outermost layer of the surfaces/interfaces are large, the energy barriers of Li diffusion shown in Figure S5 and the discussion therein demonstrate that both the graphene modification and the coexistence of Mn dopant and graphene modification significantly improve the Li diffusion kinetics on the phoshpor-olivine surfaces. 4. Conclusion The effects of graphene modification and Mn dopant on the interface interaction and electrochemical properties of graphene/LiFePO4 have been systematically investigated based on the first-principles total energy calculations. It is revealed that graphene binds stably with the surface of iron based olivine phosphates in parallel orientation through physical adsorption with electron transfer from graphene to the surface of olivines. The outermost Mn dopant enhances the interfacial binding due to 25



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the higher doping stability at interface than at surface. The adsorption energy of Li at different sites explains to some extent the shape of charge/discharge profile, Li extraction/insertion mechanism and outstanding specific capacity beyond theoretical limit. The dependence of Li adsorption energy on factors including adsorption site, Mn dopant and graphene modification is elaborated through edge effect, doping stability and interfacial binding strength, respectively. The surface conductivity is improved in the presence of Mn dopant, graphene modification and their coexistence with respect to bandlike electron transport, which is attributed to the larger DOS near the Fermi level. The dominance of the bandlike rather than polaronic transport at the surfaces is substantiated by the nonzero DOS near the Fermi level and the poor stability of polaron. Both the graphene modification and the coexistence of Mn dopant and graphene modification significantly improve the Li diffusion kinetics on the phoshpor-olivine surfaces. This work underlines the importance of surface properties for overall electrochemical performance particularly in nanoparticles.

ASSOCIATED CONTENT

Supporting information The properties of surface/interface models, the electron transfer in the process of interfacial binding, the details about Li adsorption behavior, the details and discussion

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about Li diffusivity, and the density of states relevant to surface electron conductivity. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Enzuo Liu. E-mail: [email protected]. Tel. & Fax. : +86 22 27891371. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 11474216, 51472177), and the Innovation Foundation of Tianjin University. The work was carried out at National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1(A).

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