Selecting Substituent Elements for Li-Rich Mn-Based Cathode

Apr 8, 2015 - Shude Liu , Kwan San Hui , Kwun Nam Hui , Hai-Feng Li , Kar Wei Ng ... Minghao Zhang , Danna Qian , Bing-Joe Hwang , Ying Shirley Meng...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/cm

Selecting Substituent Elements for Li-Rich Mn-Based Cathode Materials by Density Functional Theory (DFT) Calculations Yurui Gao,† Xuefeng Wang,† Jun Ma,‡ Zhaoxiang Wang,*,† and Liquan Chen† †

Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China ‡ Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China S Supporting Information *

ABSTRACT: Li2MnO3 is known to stabilize the structure of the Li-rich Mn-based cathode materials xLi2MnO3·(1 − x)LiMO2 (M = Ni, Co, Mn, etc.). However, its presence makes these materials suffer from drawbacks including oxygen release, irreversible structural transition, and discharge potential decay. In order to effectively address these issues by atomic substitution, density function theory (DFT) calculations were performed to select dopants from a series of transition metals including Ti, V, Cr, Fe, Co, Ni, Zr, and Nb. Based on the calculations, Nb is chosen as an dopant, because Nb substitution is predicted to be able to increase the electronic conductivity, donate extra electrons for charge compensation and postpone the oxygen release reaction during delithiation. Moreover, the Nb atoms bind O more strongly and promote Li diffusion as well. Electrochemical evaluation on the Nb-doped Li2MnO3 show that Nb doping can indeed improve the performances of Li2MnO3 by increasing its electrochemical activity and hindering the decay of its discharge potential.

1. INTRODUCTION With decades of quick development, lithium-ion batteries (LIBs) have won great success in powering portable electronics and electric vehicles. The low capacity of traditional cathode materials (160 mAh g−1 or lower) has become the bottleneck to further increasing the energy density of the LIBs. In order to step over this challenge, it is critical to exploit cathode materials with higher energy densities.1 The Li-rich Mn-based layerstructured xLi2MnO3·(1 − x)LiMO2 (M = Ni, Co, Mn, etc.)2,3 are among the candidates, because of their high reversible capacity (over 250 mAh g−1). In these materials, the layerstructured Li2MnO3 phase acts as a Li-ion reservoir and structural stabilizer.2,3 However, the drawbacks of this building block are as clear, such as low conductivity, irreversible structural transition, and low Coulombic efficiency, because of the loss of oxygen in the initial delithiation process. These deteriorate the electrochemical performance and delay the commercialization of the Li-rich cathode materials. Elemental substitution has been proved an effective way to tune the structural and performances of the electrode materials.4−8 Previous efforts by Inaguma4 and Kim8 have managed to decrease the electric resistivity and improve the Liion diffusion by doping Ru and Cr in Li2MnO3, respectively. In addition, tracing back to the essential causes, it is believed that the performance degradation of Li2MnO3 is closely related to the layer-to-spinel structural transition, typically characterized with Mn migration into the Li layer.9−11 Our recent density © 2015 American Chemical Society

functional theory (DFT) simulation shows that Mn migration, by means of a series of MnO6 distortion, is involved in breaking the old Mn−O bonds and forming new ones. This indicates that the spinel-type transition can be suppressed by stabilizing the MnO6 (or M′X6, where M′ is a metal cation and X is an anion) octahedron. One way is to substitute Mn with transition-metal atoms that can compensate for charge variation during Li+ removal or can contribute extra electrons to O. In this way, the presence of O vacancy can be suppressed and the MnO6 distortion be alleviated. Another way is to dope atoms that can form stronger bonds with O to fix the O ions. The above two are cation substitution. On the other hand, substitution with anions that have higher electronegativity is also anticipated to be an effective way. These doped anions function by forming stronger bond with Mn and anchoring the transition-metal ions on their positions. In this work, based on DFT calculations, we extensively study the influences of cation substitution on the electronic and structural properties of Li2MnO3. Various transition metals (Ti, V, Cr, Fe, Co, Ni, Zr, and Nb) are considered as the potential substituent for Mn. The electronic structure, charge transfer, and Gibbs free energy of the oxygen evolution reaction are investigated. It shows that Nb substitution improves the Received: March 6, 2015 Revised: April 8, 2015 Published: April 8, 2015 3456

DOI: 10.1021/acs.chemmater.5b00875 Chem. Mater. 2015, 27, 3456−3461

Article

Chemistry of Materials

On the other hand, when lower-valenced metal ions are substituted for Mn4+, our calculations show that it is almost impossible to form O vacancies, because the formation energy of O vacancy is very high (0.72 eV for Co substitution and others ranging between 1.37 eV and 2.13 eV). In this case, the lower-valenced guest ions exist in a form such as Mn, Co, and Ni in LiMnO2, LiCoO2, and LiNiO2, respectively, where the Li content decreases in the transition-metal layer and O vacancies are not formed. In addition, when the actual content of the substitution guests is low, vacancies may not be formed, although the host structure contains local strains. First-principles molecular dynamics (FPMD) calculations are carried out within a 2 × 2 × 1 supercell (96 atoms wherein). The temperature is tuned by the Nosé−Hoover thermostat22,23 with a time step of 1 fs. To achieve high efficiency, two simulation temperatures, 2500 and 3000 K, are employed, based on our simulation test result that, at a temperature of 3000 K, Li ions diffuse but all the other ions keep vibrating around their thermodynamics equilibrium positions in a time scale of 10 ps. The Li diffusion coefficient (DLi) is obtained by linear least-squares fitting to the slope of the mean square displacements (MSDs), according to

electronic conductivity and postpones the oxygen release during delithiation. In addition, Nb atoms can bind O more strongly, contribute extra electrons for charge compensation, and promote Li+ diffusion. Electrochemical evaluation demonstrates that Nb doping improves the performance of Li2MnO3 and hinders the decline in its delithiation potential.

2. METHODS 2.1. DFT Calculations. Spin-polarized DFT12,13 calculations are implemented in the Vienna Ab-initio Simulation Package (VASP) with pseudopotentials established by the projector-augmented wave (PAW)14 method and the Perdew−Burke−Ernzerh (PBE)15 exchange-correlation functional. The Hubbard-type U correction for the strong-correlation d-electrons of transition metals are taken into account (see Table 1). The plane-wave basis is cut off by 500 eV.

Table 1. Hubbard U Values for Different Transition Metalsa (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Zr and Nb)

a

TM

U (eV)

ref(s)

TM

U (eV)

ref(s)

Ti V Cr Mn Fe

2.5 3.1 3.5 4.9 4.9

16, 17 18 18 19 19

Co Ni Zr Nb

5.37 6.0 NAb 1.5

19 19 20, 21 21

DLi =

⇀ 1 ⎛⎜ 1 ⎞⎟ 1 lim ⟨|∑ δ r (t )|2 ⟩ ⎝ ⎠ 6 N t →∞ t i

(1)

2.2. Experiments. Pristine and Nb-doped Li2MnO3 were prepared by calcination of a mixture of stoichiometric amounts of Li2CO3 (5 at. % excess) and MnCO3 (and Nb2O5) at 800 °C for 20 h in air. The structure of the as-prepared samples was characterized on an X’Pert Pro MPD X-ray diffractometer (XRD, Cu Kα1, λ = 1.5418 Å) calibrated with Cu. The chemical states of the Nb and O ions were identified by X-ray photoelectron spectroscopy (XPS) (ESCALAB250, Thermo, USA) calibrated with C1s (284.8 eV) from the contaminated carbons. The Fourier-transformed infrared (FTIR) transmittance spectra were tested by irradiating the sample (dispersed in KBr) disk in the vacuum chamber of a Vertex 70v spectrometer (Bruker Optics, Germany). The electrode sheet was fabricated by spreading the slurry of the asprepared powder (Li2MnO3 or Li1.95Mn0.95Nb0.05O3), carbon black and polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidone (NMP) at a weight ratio of 8:1:1 onto an aluminum foil. The electrode sheets were then dried under vacuum at 100 °C for 8 h. Half cells were assembled with fresh lithium foil as the counter electrode, Celgard 2400 as the separator, and 1 mol L−1 LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v) as the electrolyte, in an argon-filled glovebox (MBraun, Lab Master 130). The test cell was galvanostatically cycled at a current density of 10 mA g−1 between 2.0 V and 4.8 V vs Li+/Li on a Land BA2100A battery tester (Wuhan, China).

Data taken from refs 16−21. bNA = not available.

In order to reasonably simplify the model, all the calculations are carried out in a primitive cell of Li 2 MnO 3 (C2/c) with 4Li2Mn0.75M0.25O3 (M = Ti, V, Cr, Fe, Co, Ni, Zr, and Nb) in it. A doping content of 1/4 Mn (Li2Mn0.75M0.25O3) can mostly maintain the main structure and component of Li2MnO3 and by conducting a detailed investigation into Li2Mn0.75M0.25O3, the relative characteristics (electronic and binding properties, etc.) of Li2MnO3 doped with different cations also can be revealed. Therefore, the Li2Mn0.75M0.25O3 configuration is adopted for calculations in this work. All the atoms are relaxed, using a 5 × 5 × 3 Γ-centered k-mesh, until the Hellman− Feynman force reaches 0.01 eV Å−1. As for the calculations of the density of states (DOS), a denser k-mesh (7 × 7 × 4 k-mesh) is adopted. Although the substitution of guest ions (M) with a higher valence than Mn4+ may generate Li vacancies in Li2MnO3, these vacancies will be eliminated during discharging or the pristine Li2MnO3 will inevitably undergo this lithiation state during charging. Therefore, we believe that different atom substitutions should be compared at the same lithiation state and, as a result, Li2Mn1−xMxO3 could be adopted.

Figure 1. (a) Calculated density of states (DOS) of Li2MnO3 and Li2Mn0.75M0.25O3 (M = Ti, V, Cr, Fe, Co, Ni, Zr, and Nb); (b) charge distribution scheme for the new states with energies ranging from −0.5 eV to 0.0 eV in LiMn0.75Nb0.25O3. All energies refer to the Fermi energy. 3457

DOI: 10.1021/acs.chemmater.5b00875 Chem. Mater. 2015, 27, 3456−3461

Article

Chemistry of Materials

Figure 2. (a) Calculated Gibbs free energy for O2 release from different delithiation states, LiyMn0.75M0.25O3 (y = 2, 1.75, 1.5, 1.25, and 1), and (b) binding energy of O to M of Li2Mn0.75M0.25O3.

Table 2. Extra Charge Transferred from M to the Six Nearest O Atoms Obtained by Bader Atomic Analysis of Li2Mn1−xMxO3a No.

Fe

Co

Ni

Ti

V

Zr

Nb

Cr

1 2 3 4 5 6 average

−0.011 −0.011 0.077 −0.010 −0.011 0.077 0.019

0.042 0.048 0.047 0.042 0.048 0.047 0.045

0.099 0.112 0.093 0.099 0.112 0.093 0.101

−0.084 −0.091 −0.089 −0.084 −0.091 −0.089 −0.088

−0.050 −0.042 −0.039 −0.049 −0.042 −0.039 −0.043

−0.230 −0.224 −0.227 −0.230 −0.224 −0.227 −0.227

−0.069 −0.048 −0.066 −0.069 −0.048 −0.066 −0.061

−0.022 0.032 −0.033 −0.021 0.032 −0.033 −0.007

Note: The extra charge ρ(n,M) = ρ1 − ρ2, where ρ1 and ρ2 are the Bader charge of O(n) in MO6 (x = 0.25) and in MnO6 (x = 0), respectively, and n refers to the order of O atoms in MO6 octahedron and the MnO6 octahedron. Since the coordination number for M or Mn is 6, Nos. 1−6 refer to the six first-coordinated O. a

3. RESULTS AND DISCUSSION 3.1. Electronic Structure. Figure 1a shows the calculated density of states (DOS) of Li2Mn0.75M0.25O3. The band gap of Li2MnO3 is ∼1.70 eV. Substitution of Ti or Zr does not change the band gap of Li2MnO3. V or Nb substitution introduces new electrons in the gap and makes the band gap of Li2MnO3 decrease. The charge distribution for these new states, with energies from −0.5 eV to 0 eV of Li2Mn0.75Nb0.25O3 (Figure 1b), reveals that these electrons are originated from Nb but are transferred to O. Because of the strong interaction between Mn and O, shown from the strong binding of O to Mn (Figure 2b) and the Bader atomic analysis of Li2MnO3 (Table S1 in the Supporting Information), some of these electrons are, in turn, transferred to Mn from O. That is, charge transfer from Nb to Mn is conducted by way of the O atom. Therefore, the Mn and O atoms around Nb obtain extra electrons from Nb, and the valences of these Mn and O atoms decrease slightly in comparison to that in Li2MnO3. On the other hand, Cr, Co or Ni doping creates electronic holes in the gap, mainly originated from the doped transition metals. Their substitution also narrows the band gap. Fe substitution introduces new electron states and electronic holes in the gap and renders the band gap smaller (0.40 eV). Therefore, substitution of Mn with V, Cr, Nb, Fe, Co, or Ni is expected to improve the electronic conductivity of Li2MnO3. Bader atomic charge analysis on Li2Mn0.75M0.25O3 is conducted to evaluate the charge transference in the Mdoped Li2MnO3. The extra charge transferring to O (refer to our previous paper24 for details) is obtained, as displayed in Table 2. It is shown that the average extra electrons for Fe, Co, and Ni are positive. That is, fewer electrons are transferred to O from Fe, Co, or Ni than from Mn. In contrast, Ti, V, Zr, and

Nb substitution introduces extra electrons to O. In addition, the number of the electrons transferred from Cr to O is almost the same as that from Mn. 3.2. Oxygen Release. It is believed that the O2− ions are oxidized upon Li+ ion removal from pristine Li2MnO3.25 Bruce et al.26 and Nakamura et al.27 detected O2 experimentally in the charged Li-rich oxides (see ref 26 for Li[Ni0.2Li0.2Mn0.6]O2 and ref 27 for Li2MnO3). Oxygen evolution is believed to be one of the most important reasons for the irreversible structural evolution of Li2MnO3.5,26 Therefore, stabilizing the structural O and postponing oxygen release are of crucial importance in improving the electrochemical performance of Li-rich oxides and designing superior Mn-based layered materials. Herein, we evaluate the influence of M substitution on the O2 release reaction by comparing their Gibbs free energy. Assuming that the O2 release reaction at various delithiation states, LiyMn0.75M0.25O3 (y = 2, 1.75, 1.5, 1.25, and 1), can be expressed as Li yMn 0.75M 0.25O3 = Li yMn 0.75M 0.25O3 − δ +

δ O2 2

(2)

where δ is the amount of O removed from the lattice. The reaction enthalpy can be calculated according to ΔH =

E(Li yMn 0.75M 0.25O3 − δ ) + 0.5δE(O2 ) − E(Li yMn 0.75M 0.25O3) 0.5δ

(3)

where E is the corresponding calculated energy per formula unit (f.u.) if the temperature effects are ignored. The oxygen-deficient structure LiyMn0.75M0.25O3−δ is obtained by removing the O atom with the lowest Bader charge. Taken into account the entropy of O2 at the standard state (−TΔS = −0.63 eV),6 the Gibbs free energy ΔG (Figure 2a) 3458

DOI: 10.1021/acs.chemmater.5b00875 Chem. Mater. 2015, 27, 3456−3461

Article

Chemistry of Materials

Figure 3. (a) Charge compensation in Li2Mn0.75Nb0.25O3 (color legend: olive green, O; blue, Mn; red, Nb; magenta, Li; and black, the total DOS). Also shown are mean square displacements (MSDs) of Li in Li2MnO3 and in Li2Mn0.75Nb0.25O3 simulated by FPMD at (b) 2500 K and (c) 3000 K, respectively. (d) XRD patterns of pristine and Nb-doped Li2MnO3 (the inset shows selected XRD patterns between 2θ = 50° and 2θ = 60°).

can be obtained based on the reaction enthalpy. The ΔG value of Li2MnO3 reaches zero when y = 1.5, while that of the Ti-, V-, Cr-, Co-, Ni-, and Zr-doped Li2MnO3 reaches zero before y = 1.5. Therefore, the substitutions, especially the Ni and Co substitution, cannot delay the O2 release reaction. However, the ΔG of the Fe- or Nb-doped Li2MnO3 does not reach zero until more than 0.5 Li/f.u. is removed. This indicates that Fe and Nb doping suppresses the O2 release reaction. In addition, the binding energy of O to metal M is half the ΔH value determined using eq 3. But, herein, the oxygendeficient structure LiyMn0.75M0.25O3−δ is achieved by removing the O atom nearest to M, different from the case in the calculation of ΔG. Although Ti, V, Cr, and Zr substitution does not delay the O2 release reaction, they can form stronger M−O bonds, as seen from the binding energy of O to M (Figure 2b). Therefore, they are expected to stabilize the local structural oxygen and hinder the structural transition of Li2MnO3 during delithiation. It was reported that Ti and Cr doping increases the stability of the structural oxygen of Mn-based layer-structured materials.8,28,29 However, Co and Ni substitution promotes the O2 release reaction as they contribute fewer electrons to O than the Mn atom does. The weaker binding of O to Co and Ni is another important reason (Figure 2b). This is also in agreement with the relatively lower formation energy of O vacancy in Cosubstituted Li2MnO3 (ca. 0.72 eV). It was reported that Co substitution for Mn0.5Ni0.5 in Li[Li0.2Mn0.60Ni0.20]O2 increases the oxygen loss, evidenced with increased capacity at the plateau region.29 Although the Gibbs free energy profile indicates that Fe substitution suppresses the O2 release reaction, the Bader analysis shows that Fe does not contribute more electrons than Mn does. Therefore, we do not try to

synthesize and evaluate the Fe-doped Li2MnO3. In contract, Nb doping is an effective way to improve the electrochemical performance of Li2MnO3 as it transfers extra electrons to O and postpones the O2 release reaction. Moreover, it binds O more strongly. We will further examine the related properties of Nbsubstituted Li2MnO3 in the following. 3.3. Property Evaluation for Li2Mn0.75Nb0.25O3. Nb substitution (Li2Mn0.75Nb0.25O3) increases the cell volume of Li2MnO3 by 4.3%. The cell volume changes slightly during delithiation (see Table S2 in the Supporting Information). The delithation potential, evaluated by eq 4, is found to decrease with Nb doping. Vave = −

E(Mn1 − xNbx O3) − E(Li 2Mn1 − xNbx O3) + 2E(Li) − 2e (4)

3.3.1. Charge Compensation. Oxygen evolution is closely involved in the charge compensation in the Li2MnO3-related cathode materials. The charge transference in pristine Li2MnO3 is mainly compensated by the oxidation of O during delithiation.30 Herein, we investigate the charge compensation during delithiation from Li2Mn0.75Nb0.25O3, based on the calculated DOS (Figure 3a) and the Bader charge analysis (Table S3 in the Supporting Information). When 0.25 Li/f.u. is removed from Li2Mn0.75Nb0.25O3, the states of O and Mn near the Fermi level decrease. These states, introduced due to Nb substitution and with higher energy, are different from those of the pristine Li2MnO3. In the delithiated phase, Li1.75MO3, the states near the Fermi level are mainly the contribution of the O 2p orbitals, while Mn 3d states mostly remain beneath the O 2p. As more Li ions are removed, the charge compensation is mainly contributed from O, the same as in Li2MnO3. The 3459

DOI: 10.1021/acs.chemmater.5b00875 Chem. Mater. 2015, 27, 3456−3461

Article

Chemistry of Materials above result is in good agreement with the Bader charge analysis. 3.3.2. Li Diffusion. The diffusion of the Li+ ions in Li2Mn0.75Nb0.25O3 is investigated according to the FPMD simulations (see Figures 3b and 3c). When the simulation temperature is 2500 K, the Li+ ions in Li2MnO3 are not found diffusing within the simulation time (10 ps). However, Li diffusion is observed in Li 2 Mn 0.75 Nb 0.25 O 3 . When the simulation temperature rises to 3000 K, Li diffusion can be found in both Li2MnO3 and Li2Mn0.75Nb0.25O3, with a coefficient of 1.167 × 10−5 cm2/s in Li2MnO3 and 2.183 × 10−5 cm2/s in Li2Mn0.75Nb0.25O3. These indicate that Nb substitution enhances the Li+ ion diffusion. Therefore, Nb is chosen as a substituent to improve the performance of Li2MnO3 in reality. 3.4. Characteristics and Electrochemical Performance of Nb-Doped Li 2 MnO 3 . The Nb-doped Li 2 MnO 3 (Li2Mn0.95Nb0.05O3) is characterized by XRD (Figure 3d), FTIR and XPS spectra (see Figure S1 in the Supporting Information). The XRD pattern shows that the as-prepared Li1.95Mn0.95Nb0.05O3 is phase-pure. The diffraction peaks shift to lower angles after Nb substitution because Nb doping expands the cell, in good agreement with the DFT calculations. When the content of Nb reaches 10 at. %, a new phase Li 3 NbO 4 appears (see Figure S1 in the Supporting Information). The XPS spectrum (Figure S1 in the Supporting Information) shows that the valence of Nb in the phase-pure Li1.95Mn0.95Nb0.05O3 is +5, which is different from that of the Nb4+ cation in the above computational model. Of course, Li2Mn0.95Nb0.05O3 (Nb4+) may be experimentally obtained by changing the precursors and optimizing the synthesis conditions. However, from the viewpoint of electrochemical dynamics, Li1.95Mn0.95Nb0.05O3 (Nb5+) is an intermediate stage that Li2Mn0.95Nb0.05O3 (Nb4+) inevitably undergoes upon delithiation, as mentioned in the Methods section. Moreover, the chemically synthesized Li1.95Mn0.95Nb0.05O3 (Nb5+) probably contains some Li vacancies distributed more uniformly than that formed via electrochemical delithiation. Therefore, Li1.95Mn0.95Nb0.05O3 (Nb5+) is still acceptable for the following electrochemical evaluation and the above computational results are still applicable. On the other hand, the presence of Nb5+ is also expected to affect the electrochemical properties. Introduction of a small number of Li+ vacancies lowers the capacity but promotes the Li+ diffusion. Moreover, it enhances the metal−oxygen bond, stabilizing the MO6 octahedra and thus suppressing Mn migration into the Li layer. The broad peaks between 400 and 700 cm−1 in the FTIR spectrum (Figure S1 in the Supporting Information) are assigned to the vibration of the Mn−O and/or Li−O bonds.31,32 After Nb substitution, these peaks become wider, in good agreement with the calculations that the length of the Mn−O bond varies in a wider range (1.923−1.949 Å). From the structural viewpoint, Nb substitution makes the nearby Mn−O bonds longer but those far from it shorter, leading to the wider variation of the Mn−O bond length. In addition, Nb substitution pushes the Li and even the Mn atoms farther away and, thus, some Li−O and even Mn−O interaction becomes weaker. This is reflected with the red-shifting of the vibration peak at 556 cm−1 after Nb substitution. Moreover, Nb doping enhances the vibration peak at 630 cm−1. The potential profiles of Li2MnO3 and Li1.95Mn0.95Nb0.05O3 between 2.0 V and 4.8 V are shown in Figure 4. The delithiation potential decreases and the polarization in the first

Figure 4. Selected (a) charge/discharge profiles and (b) normalized voltage profiles of Li2MnO3 and Li1.95Mn0.95Nb0.05O3 between 2.0 V and 4.8 V vs Li+/Li. The peak of the first charging profile at the initial ca. 20 mAh/g is assigned to the electrolyte.

cycle becomes less severe after Nb doping. The first charge (discharge) capacity is 272 (165) mAh g − 1 for Li1.95Mn0.95Nb0.05O3, while that for Li2MnO3 is only 60 (39) mAh g−1. This agrees with the FPMD simulation result that Nb doping promotes Li diffusion so that more Li ions can be extracted. On the other hand, Nb5+ doping introduces some Li vacancies, beneficial for Li ion diffusion. Moreover, the reversible capacity remains 135 mAh g−1 after 10 cycles. The discharging potential remains stable and decreases less than that of the Li2MnO3, demonstrating that Nb doping suppresses the fading of the lithiation potential. This agrees with the above Gibbs free-energy calculations and supports our previous attribution that the O vacancy and Mn migration are responsible for decay of the lithiation potential.

4. CONCLUSIONS In this article, density functional theory (DFT) calculations are conducted to investigate the doping effects of different elements on the physical and electrochemical performances of Li2MnO3. The following observations were made: (1) V, Cr, Fe Co, and Ni substitution for Mn decreases the band gap of Li2MnO3; (2) Ti, V, Zr, and Cr doping transfers extra electrons to O; (3) Fe doping suppresses the oxygen release reaction according to the Gibbs free-energy calculations and, thus, promisingly increases the electrochemical stability of oxygen; and (4) Ti, V, Cr, and Zr binds O more strongly than Mn and promisingly increases the stability of oxygen in the lattice. However, Nb seems to be the best substituent element in Li2MnO3, because it narrows the band gap, donates extra 3460

DOI: 10.1021/acs.chemmater.5b00875 Chem. Mater. 2015, 27, 3456−3461

Article

Chemistry of Materials

(17) Lutfalla, S.; Shapovalov, V.; Bell, A. T. J. Chem. Theory Comput. 2011, 7, 2218. (18) Wang, L.; Maxisch, T.; Ceder, G. Phys. Rev. B 2006, 73, 195107. (19) Zhou, F.; Cococcioni, M.; Marianetti, C.; Morgan, D.; Ceder, G. Phys. Rev. B 2004, 70, 235121. (20) Duan, Y. J. Renewable Sustainable Energy 2011, 3, 013102. (21) Hautier, G.; Ong, S. P.; Jain, A.; Moore, C. J.; Ceder, G. Phys. Rev. B 2012, 85, 155208. (22) Nosé, S. J. Chem. Phys. 1984, 81, 511. (23) Hoover, W. G. Phys. Rev. A 1985, 31, 1695. (24) Gao, Y. R.; Ma, J.; Wang, X. F.; Lu, X.; Bai, Y.; Wang, Z. X.; Chen, L. Q. J. Mater. Chem. A 2014, 2, 4811. (25) Koyama, Y.; Tanaka, I.; Nagao, M.; Kanno, R. J. Power Sources 2009, 189, 798. (26) Armstrong, A. R.; Holzapfel, M.; Novák, P.; Johnson, C. S.; Kang, S.-H.; Thackeray, M. M.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 8694. (27) Yu, D. Y. W.; Yanagida, K.; Kato, Y.; Nakamura, H. J. Electrochem. Soc. 2009, 156, A417. (28) Xiao, P.; Deng, Z. Q.; Manthiram, A.; Henkelman, G. J. Phys. Chem. C 2012, 116, 23201. (29) Deng, Z. Q.; Manthiram, A. J. Phys. Chem. C 2011, 115, 7097. (30) Xiao, R. J.; Li, H.; Chen, L. Q. Chem. Mater. 2012, 24, 4242. (31) Gao, Y.; Wang, Z.; Chen, L. J. Power Sources 2014, 245, 684. (32) Pasquier, A. D.; Orsini, F.; Gozdz, A. S.; Tarascon, J.-M. J. Power Sources 1999, 81−82, 607.

electrons to O, suppresses the oxidation of O upon Li removal, and binds O more tightly than Mn in Li2MnO3. In addition, further study shows that Nb substitution contributes to the charge compensation by means of transferring electrons to Mn and O and promotes Li diffusion as well. Based on the above DFT screening and evaluations, Nb substitution promisingly improves the electrochemical performance of Li2MnO3. Therefore, phase-pure Li1.95Mn0.95Nb0.05O3 is successively prepared and identified by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) tests. The electrochemical evaluation shows that Nb doping increases the reversible capacity of Li2MnO3 and hinders the decay of its discharge potential. This study will be helpful for guiding the construction of high-performance Li-rich Mn-based cathode materials.

■

ASSOCIATED CONTENT

S Supporting Information *

Bader atomic analysis of Li2MnO3, volume change and Bader atomic analysis of Li2Mn0.75Nb0.25O3 during delithiation, as well as experimental characterizations (XRD, FTIR, and XPS spectra) of Nb-doped Li2MnO3. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-10-82649050. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS ZXW appreciates the financial support of the National 973 Program of China (No. 2015CB251100) and the National Natural Science Foundation of China (NSFC, Nos. 51372268 and 11234013).

■

REFERENCES

(1) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359. (2) Thackeray, M. M.; Kang, S.-H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A. J. Mater. Chem. 2007, 17, 3112. (3) Yabuuchi, N.; Yoshii, K.; Myung, S. T.; Nakai, I.; Komaba, S. J. Am. Chem. Soc. 2011, 133, 4404. (4) Mori, D.; Sakaebe, H.; Shikano, M.; Kojitani, H.; Tatsumi, K.; Inaguma, Y. J. Power Sources 2011, 196, 6934. (5) Robertson, A. D.; Bruce, P. G. Chem. Commun. 2002, 2790. (6) Hummelshoj, J. S.; Blomqvist, J.; Datta, S.; Vegge, T.; Rossmeisl, J.; Thygesen, K. S.; Luntz, A. C.; Jacobsen, K. W.; Norskov, J. K. J. Chem. Phys. 2010, 132, 071101. (7) Ma, J.; Zhou, Y. N.; Gao, Y. R.; Kong, Q. Y.; Wang, Z. X.; Yang, X. Q.; Chen, L. Q. Chem.Eur. J. 2014, 20, 8723. (8) Kim, S.; Noh, J.-K.; Yu, S.; Chang, W.; Chung, K. Y.; Cho, B.-W. J. Electroceram. 2013, 30, 159. (9) Zheng, J. M.; Gu, M.; Xiao, J.; Zuo, P. J.; Wang, C. M.; Zhang, J. G. Nano Lett. 2013, 13, 3824. (10) Ghanty, C.; Basu, R. N.; Majumder, S. B. Solid State Ionics 2014, 256, 19. (11) Johnson, C. S.; Li, N. C.; Lefief, C.; Vaughey, J. T.; Thackeray, M. M. Chem. Mater. 2008, 20, 6095. (12) Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, B864. (13) Kohn, W.; Sham, L. J. Phys. Rev. A 1965, 140, A1133. (14) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953. (15) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (16) Hu, Z.; Metiu, H. J. Phys. Chem. C 2011, 115, 5841. 3461

DOI: 10.1021/acs.chemmater.5b00875 Chem. Mater. 2015, 27, 3456−3461