Design and Properties Prediction of AMCO3F by First-Principles

Apr 10, 2017 - Gwenaelle Rousse , Hania Ahouari , Vladimir Pomjakushin , Jean-Marie Tarascon , Nadir Recham , and Artem M. Abakumov. Inorganic ...
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Design and Properties Prediction of AMCO3F by First-Principles Calculations Meng Tian,†,‡ Yurui Gao,§ Chuying Ouyang,∥ Zhaoxiang Wang,*,†,‡ and Liquan Chen†,‡ †

Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China § Department of Physics and Astronomy, California State University Northridge, Northridge, California 91330-8268, United States ∥ Department of Physics, Laboratory of Computational Materials Physics, Jiangxi Normal University, Nanchang, Jiangxi 330022, China S Supporting Information *

ABSTRACT: Computer simulation accelerates the rate of identification and application of new materials. To search for new materials to meet the increasing demands of secondary batteries with higher energy density, the properties of some transition-metal fluorocarbonates ([CO3F]3−) were simulated in this work as cathode materials for Li- and Na-ion batteries based on first-principles calculations. These materials were designed by substituting the K+ ions in KCuCO3F with Li+ or Na+ ions and the Cu2+ ions with transition-metal ions such as Fe2+, Co2+, Ni2+, and Mn2+ ions, respectively. The phase stability, electronic conductivity, ionic diffusion, and electrochemical potential of these materials were calculated by firstprinciples calculations. After taking comprehensive consideration of the kinetic and thermodynamic properties, LiCoCO3F and LiFeCO3F are believed to be promising novel cathode materials in all of the calculated AMCO3F (A = Li and Na; M = Fe, Mn, Co, and Ni). These results will help the design and discovery of new materials for secondary batteries. KEYWORDS: secondary batteries, first-principles calculations, AMCO3F, molecular dynamics, Li-ion diffusion

1. INTRODUCTION To date, most of the calculation studies are focused on explaining phenomena of experiments, but few studies have been conducted on structural design and performance prediction of new materials. Traditionally, new materials have been investigated empirically. For example, a layer-structured cathode material, Li9V3(P2O7)3(PO4)2, was obtained containing both [PO4]3− tetrahedra and [P2O7]4− groups based on pioneering work and experience.1 The process is slow for stumbling across empirical data and vast number of combinations, and the cost is high. It took years for Goodenough and his co-workers to find LiFePO4 as a cathode material of Li-ion batteries.2 In contrast, the development of efficient and accurate theoretical tools makes it possible to accelerate the pace of material discovery with the help of density functional theory (DFT) calculations.3−5 Ceder and his co-workers6 successfully designed and synthesized new materials LixNi2−4x/3Sbx/3O2. Though it may still take some more time for the material scientists to make the designed material a reality and to tailor its properties, computer simulation offers a good way to predict properties of new materials and provide guidelines for improvement of their performances. Recently, the materials genome initiative (MGI) (a combination of database, infrastructures, and computation) © 2017 American Chemical Society

was proposed to expand the research areas of the materials and speed the rate of discovery of new materials and predict their properties.7−9 In the field of power sources, quick development of portable electronics and electric vehicles provides not only great opportunities but also identifies urgent requirements for secondary batteries, especially in energy density and sustainable development of the field. However, the low specific capacity of the cathode material has become a bottleneck to the increase in energy densities. In addition, considering the rarity of cobalt (Co) and nickel (Ni) for conventional cathode materials, e.g., LiCoO2 and LiNi1/3Co1/3Mn1/3O2, it is essential to develop cathode materials containing abundant transition metals such as iron, for example.10−14 When factors such as the energy density, phase stability, safety, and cost are taken into account, polyanionic materials containing groups such as [SiO4]4− and [SO4F]3− are promising candidates of cathode materials. Lithium iron borate (LiFeBO3) shows a reversible capacity larger than 190 mAh g−1 at a C/20 rate between 1.5 and 4.5 V vs Li+/Li,15 but its high-rate performance is poor. The charge Received: March 7, 2017 Accepted: April 4, 2017 Published: April 10, 2017 13255

DOI: 10.1021/acsami.7b03304 ACS Appl. Mater. Interfaces 2017, 9, 13255−13261

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ACS Applied Materials & Interfaces

account the radius and valence of the transition-metal ions (Fe2+, Mn2+, Co2+, and Ni2+), the typical structures of two real compounds, KCuCO3F and KCaCO3F,21,22,24,25 were adopted. KCaCO3F is known to have a hexagonal structure (P6̅m2),22,24 while KCuCO3F is orthorhombic (Pmc21).21 These two materials share some common characteristics. Their carbon, oxygen, and transition-metal atoms are in the BC-plane, while the Li+ and F− ions are in another layer. As the radius of Cu2+ (0.62 Å) is close to those of Fe2+, Mn2+, Co2+, and Ni2+ (0.61, 0.67, 0.65, and 0.69 Å, respectively), the Cu2+ ions in KCuCO3F were substituted with Fe2+, Mn2+, Co2+, or Ni2+ ions, and the K+ ions were replaced with Li+ or Na+ ions to get an orthorhombic AMCO3F. The length of the M−O (M = Fe, Mn, Co, and Ni) and M−F bonds in AMCO3F (Figure 1 and

capacity of lithium iron silicate (Li2FeSiO4) reaches 165 mAh g−1, higher than that of many other polyanionic materials.16,17 Unfortunately, its capacity fades quickly due to poor structural stability. Similarly, the capacity of LiFeSO4F fades quickly.18−20 Fluorocarbonates are another family of polyanionic material but have not been widely studied as a cathode material. Members of them such as KCuCO3F and KCaCO3F21,22 share some common structural characteristics with carbon and oxygen ions in one plane and alkali metal and fluorine ions in the other plane.21−25 These materials are stable below 700 °C.22 Due to their low chemical formula mass, attractive highcapacity cathode materials may be obtained for advanced secondary batteries with the replacement of potassium and bivalence metal by an alkali metal and transition metal, respectively. We hereby design and predict the properties of ([CO3F]3−) based material AMCO3F (A for Li and Na; M for Fe, Mn, Co, and Ni) using first-principles calculations. The structure of AMCO3F was obtained by substituting the alkali ions and the transition-metal atoms in the stable structures such as KCuCO3F and KCaCO3F with A and M, respectively. The kinetic (electronic conductivity and Li-ion diffusion coefficient) and thermodynamic (phase stability and electrochemical potential) properties of the materials were predicted on the basis of first-principles calculations.

2. METHODS The calculations were carried out with the Vienna ab initio simulation package (VASP).26 We employed the generalized gradient approximation (GGA) with a parametrized exchange correlation functional according to Perdew-Burke and Ernzerhof (PBE).27 The Hubbard U correction was used to accurately describe the transition metal for the strong correlation of d-electrons.28 The U values were chosen according to the scheme and previous theoretical studies, as listed in Table 1.29,30 Spin-polarized calculations were adopted. Figure 1. Structure of LiMCO3F.

Table 1. U Values for Different Transition Metals metal

Fe

Mn

Co

Ni

U (eV)

3.9

4.0

5.6

6.0

Table 2) are similar to those in LiMSO4F (M = Fe, Co, and Ni).33 In other words, the valence of the transition metal is +2. Table 2. List of Bond Lengths for M−O and M−F in LiMCO3F

The total energy of each unit cell was carefully examined. The potentials of Fe_pv, Mn_pv, Co, and Ni_pv were used for considering the 3p electrons. To eliminate the error due to employment of different potentials, the same potential for each transition metal was used throughout the calculations. All of the AMCO3F compounds were calculated in both ferromagnetic (FM) states and antiferromagnetic (AFM) state with a 7 × 7 × 9 κ-point mesh generated with Γpoint centered method with a supercell A16M16C16O48F16 containing 112 atoms. Calculations show that the AFM configuration was more stable with energy lower than that of the FM configuration. The cutoff energy of the planar waves was chosen to be 550 eV. The electronic wave functions became converged when the energy variation was less than 1 × 10−5 eV. The positions of the atoms in the designed materials were fully relaxed until the Hellman−Feynman force became less than 0.002 eV Å−1. Gaussian smearing method with broadening width of 0.02 eV was applied. To calculate the electrochemical potential and Liion diffusion energy barrier (NEB), vacancy defect was used in the calculations, which is the major type of point defect in the compounds. First-principles molecular dynamics (FPMD) was used to investigate the dynamic properties of the ions in the AMCO3F.31,32

crystal

bond

d (Å)

LiFeCO3F (U = 3.9 eV)

Fe2−O3 Fe2−O5 Fe2−F Fe2−O1 Fe2−F Mn1−O4 Mn1−O6 Mn1−F Mn1−O2 Mn1−F Co1−O2 Co1−O4 Co1−F Co1−O6 Co1−F Ni1−O4 Ni1−O2 Ni1−F Ni1−O6 Ni1−F

2.19580 1.97532 2.02369 2.18810 2.02369 2.22283 2.08797 2.05188 2.20826 2.05188 2.04875 2.17681 1.98091 2.01067 1.98091 2.05050 2.05320 1.96490 1.99527 1.96490

LiMnCO3F (U = 4.2 eV)

LiCoCO3F (U = 5.6 eV)

LiNiCO3F (U = 6.1 eV)

3. RESULTS AND DISCUSSION 3.1. Structures and Phase Stability. As the AMCO3F are currently only virtual compounds, some rational structures have to be assigned to them before the calculations. Taking into 13256

DOI: 10.1021/acsami.7b03304 ACS Appl. Mater. Interfaces 2017, 9, 13255−13261

Research Article

ACS Applied Materials & Interfaces Calculations show that the Bader charges of the alkali metals (Li and Na), transition metals (Fe, Mn, Co, and Ni), O, and F in this compound are +1, + 2, −2, and −1, respectively (Table S1). Simple calculations show that the optimized orthorhombic LiCoCO3F has formation energies lower than the hexagonal LiCoCO3F does. The lattice parameters of the orthorhombic LiCoCO3F are a = 3.492 Å, b = 4.596 Å, and c = 9.622 Å; the volume of its optimized unit cell is 154.39 Å3. The structures of the other AMCO3F compounds are optimized with the same initial structure of LiCoCO3F (Figure 2a). It shows that these polyhedron-based structures consist of

The positive decomposition energies indicate that the structure of AMCO3F is thermodynamically stable at 0 K. All other decomposition reaction energies of AMCO3F are positive except for Edecomp1 of LiNiCO3F (−0.03 eV; Table 3). This means that LiNiCO3F can be decomposed to LiF and NiCO3 as a result of the lower energy of NiCO3. According to the relative value of the decomposition energy, the order of phase stability of these fluorocarbonates is NaFeCO3F > NaCoCO3F > NaMnCO3F ≈ NaNiCO3F > LiFeCO3F > LiCoCO3F > LiMnCO3F > LiNiCO3F. 3.2. Electrochemical Potentials. The electrochemical potential is an important parameter of an electrode material. It is the energy required to deintercalate or intercalate one removable ion (e.g., the Li or Na ions) from the lattice of an electrode material AM CO3F → A1 − x M CO3F + xA

(5)

This energy can be determined by first-principles calculations. Because the first-principles calculations were performed at 0 K, the change of the entropies of AMCO3F is negligible. In addition, the PΔV, around 10−5 eV, is negligible in solid state reactions. On determining the impact of Gibbs free energy on the above reaction, the changes of the entropy (ΔS) and volume (ΔV) were negligible. Therefore, we have The electrochemical potential can be calculated as V=−

a CO3 triangle and MO3F2 hexahedron with three M−O bonds and two M−F bonds. The lattice parameters of LiFeCO3F and LiNiCO3F are also very similar to those of LiCoCO3F but distinct from those of LiMnCO3F (with the largest c-axis parameter) (Figure 2b). That might be due to the larger radius of Mn2+ and weaker Li−O bonds along the c-axis in LiMnCO3F. All of the studied NaMCO3F (M = Fe, Mn, Co, and Ni) have similar a, b, and c lattice parameters. The dependence of their cell parameters on the atomic radius of the transition metal is consistent with that of the orthorhombic LiMPO4 and Li2MPO4F34,35 (Figure 2b). The decomposition energy of the materials is calculated to evaluate the relative phase stability of the AMCO3F materials. Two typical decomposition paths are considered, including AM CO3F → A F + M CO3

and 1 (A 2 CO3 + M F2 + M CO3) (2) 2 The decomposition energies by reactions 1 and 2 are calculated as AM CO3F →

Edecomp2 =

(3)

1 (E A 2CO3 + EM F2 + EM CO3) − EAM CO3F 2

[EAM CO3F − E A1−xM CO3F − xEA ] (7)

xe

(versus Li and Na metal for LiMCO3F (M = Fe, Mn, Co, and Ni) and NaMCO3F, respectively). The calculated delithiation (or desodiation) potentials of AMCO3F are also listed in Table 3. It indicates that the electrochemical potentials of these compounds are in the order of LiFeCO3F < LiMnCO3F < LiNiCO3F < LiCoCO3F, similar to that of some orthorhombic materials (LiMPO4, LiMSiO4, and Li2MPO4F)29,35,36 and NaFeCO3F < NaMnCO3F < NaCoCO3F ≈ NaNiCO3F. 3.3. Electronic Conductivity. The electronic and ionic conductivity of the electrode significantly affects the rate performance of an electrode material. Here, we evaluated the electronic conductivity of these fluorocarbonates by calculating their density of states (DOS) (Figure 3) and band structure (Figure 4). Calculations show that the band gap of AMCO3F varies from 2.6 eV (NaFeCO3F) to 3.7 eV (NaCoCO3F). In comparison, the band gap of LiFeCO3F is even smaller than that of some polyanion cathode materials such as LiFePO4 (3.72 eV),37 Li2FePO4F (3.46 eV),38 and Li2FeSiO4 (2.75 eV).39 In addition, the band gap of LiCoCO3F is also smaller than that of LiCoPO4 (4.15 eV),40 Li2CoPO4F (about 3.80 eV),38 and Li2CoSiO4 (3.75 eV).39 Smaller band gap means that it is easy for the electrons to jump from the valence band into the conduction band. Therefore, the electronic conductivity of LiMCO3F is expected to be higher than that of some polyanion cathode materials containing the same

(1)

Edecomp1 = EA F + EM CO3 − EAM CO3F

(6)

ΔG = ΔE + P ΔV − T ΔS ≈ ΔE

Figure 2. Optimized structure viewed along the a-axis (a) and lattice parameters of orthorhombic AMCO3F (A = Li and Na; M = Fe, Mn, Co, and Ni) (b).

(4)

Table 3. Decomposition Energy and Electrochemical (EC) Potential of AMCO3F AMCO3F

LiFe

LiMn

LiCo

LiNi

NaFe

NaMn

NaCo

NaNi

Edecomp1 (eV) Edecomp2 (eV) EC potential (V)

0.33 2.15 3.0

0.03 3.12 3.5

0.12 1.25 4.5

−0.03 0.45 4.6

0.99 0.98 2.9

0.60 0.96 3.4

0.67 0.99 4.4

0.60 0.96 4.4

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ments (MSD) of all the atoms in AMCO3F are shown as a function of time in Figure 5. It appears clearly that the migration of the Fe, C, O, and F ions in LiFeCO3F (Figure 5a) and LiCoCO3F (Figure 5b) is negligible; only the Li ions can actually diffuse in them. These meet the critical requirements to the intercalation electrode materials. For better comparison with LiCoCO3F, the MD simulation time for LiFeCO3F was also set to be 10 ps. The slope of Li-ion diffusion in LiFeCO3F is 0.137 Å2 ps−1, smaller than that in LiCoCO3F (0.162 Å2 ps−1), obtained by the linear least-squares fitting. In contrast, the migration of C, O, F, and the transition-metal ions in LiMnCO3F (Figure 5c) and LiNiCO3F (Figure 5d) is significant. Therefore, these two materials cannot be used as an electrode material of a secondary battery because irreversible structural changes will occur in them upon lithium extraction or insertion. None of the NaMCO3F materials (Figures 5e−h) can be used as electrode materials because the diffusion of their Na ions is negligible, and the migration of the other ions is obvious. Taking consideration of their molecular dynamics, we believe that LiCoCO3F (better) and LiFeCO3F can be promising novel cathode materials among all the calculated AMCO3F. The MSD of LiCoCO3F was projected along the three Cartesian axes to determine the diffusion of lithium ions in more detail (Figure 5i). This method has been employed to accurately predict the Li-ion dynamics in, for example, LiFePO4 and Li2MnO3.41,42 It shows that the Li ions diffuse faster along the a-axis direction than along the b- or c-directions in LiCoCO3F. As the displacement of the Li-ions along the b-axis and c-axis is negligible, the Li ions are supposed to move by a one-dimensional mechanism in LiCoCO3F. The Li diffusion coefficient D (D = x2/(2tN), where x2 is MSD, t is time, and N is the diffusion dimension, is calculated to be 8.1 × 10−5 cm2 s−1, larger than that of LiFePO4 (about 10−14 cm2 s−1).43 This means that the diffusion of the Li ions in LiCoCO3F is facile. To better compare the Li-ion diffusion in LiCoCO3F and LiFeCO3F, nudged elastic band calculations were implemented. The Li-ion diffusion energy barrier in LiCoCO3F is 0.7 eV, smaller than that of LiFeCO3F (Figure 6). The Li ions have better dynamic properties in LiCoCO3F. These results are consistent with those found by FPMD methods.

Figure 3. Comparison of the total density of states of AMCO3F.

transition metal. From the band gaps of AMCO3F, the order of electronic conductivity is NaFeCO 3 F > LiFeCO 3 F > NaMnCO3F > LiCoCO3F ≈ LiNiCO3F > NaMnCO3F > LiMnCO3F > NaCoCO3F. 3.4. Dynamics of the Ions. Because the kinetic process of the Li ions is quite an important factor to be considered in the application of the electrode materials, the dynamics of the A ions in AMCO3F was investigated by the first-principles molecular dynamics calculations. The mean square displace-

Figure 4. Calculated band structure of AMCO3F. 13258

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Figure 5. MSD of the A, M, C, O, and F ions in AMCO3F (a−h) and the projected MSD of the Li ions in LiCoCO3F on three Cartesian axes (i).

materials, and the Li-ion diffusion energy barrier. In addition, the first-principles molecular dynamics simulations show that the diffusion of the Li ions of LiCoCO3F is one-dimensional, and the diffusion coefficient reaches 8.1 × 10−5 cm2/s. We hope that these computations will motivate materials scientists to synthesize and test these promising cathode properties and accelerate the development of secondary batteries.



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03304. Charge state in AMCO3F by Bader charge calculations (PDF)

Figure 6. Calculated energy barriers for Li-ion diffusion in LiCoCO3F and LiFeCO3F.



4. CONCLUSIONS In this work, a series of metal fluorocarbonates AMCO3F was designed and comprehensively modeled as new cathode materials for Li- and Na-ion batteries by first-principles calculations with GGA+U methods. Calculations on the decomposition energies indicate that AMCO3F is thermodynamically stable. The delithiation potential of these sodium transition-metal fluorocarbonates is ca. 0.3 V lower than that of their lithium transition phosphate counterparts, but the electronic conductivity of the fluorocarbonates is expected to be higher than that of the phosphates. LiCoCO3F and LiFeCO3F are expected to be two promising novel cathode materials for Li-ion batteries after comprehensive consideration of the electrochemical potential, band gap, decomposition energy, alkali-ion diffusion coefficient of all the AMCO3F

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel. & Fax: +86 10 82649050. ORCID

Yurui Gao: 0000-0001-7486-8134 Zhaoxiang Wang: 0000-0002-1123-6591 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Dr. R. J. Xiao in this group is greatly acknowledged. This work was financially supported by the MOST of China (Grants 2016YFB0100400 and 2015CB251100) and the National Natural Science Foundation of China (NSFC, Grants 51372268 and 11234013). 13259

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DOI: 10.1021/acsami.7b03304 ACS Appl. Mater. Interfaces 2017, 9, 13255−13261