Berberine promotes osteogenic differentiation of mesenchymal stem cells with therapeutic potential in periodontal regeneration

Journal of Alloys and Compounds 793 (2019) 360e368

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Selecting substituent elements for LiMnPO4 cathode materials combined with density functional theory (DFT) calculations and experiments Hongliang Zhang, Yang Gong, Jie Li, Ke Du, Yanbing Cao*, Jiaqi Li School of Metallurgy and Environment, Central South University, Changsha 410083, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 January 2019 Received in revised form 15 April 2019 Accepted 19 April 2019 Available online 19 April 2019

LiMnPO4 cathode material has a high voltage platform and matches the existing electrolyte window, and thus researchers are constantly shifting their focus from LiFePO4 to LiMnPO4. However, LiMnPO4 has lower electron (ion) conductivity than LiFePO4, and besides, its delithiated phase MnPO4 will suffer thermal decomposition at lower temperatures more easily than FePO4. In order to effectively solve the above problems by elements substitution, DFT calculations are employed to screen for suitable dopants from a series of transition metals including Fe, Mg, Ni, V, Nb, Ti. Properties such as electronic structure, atomic Bader charge, O2 evolution Gibbs free energy, average voltages, and lithium ion diffusion energy barrier were evaluated. Based on the calculation, Fe is the most effective doping element because Fe doping is able to reduce the band gap of the material and improve the electronic conductivity, suppress the O2 evolution reaction of the delithiation phase and improve the thermal stability. The reason for such a situation is that Fe can form a stronger covalent bond with the surrounding O atoms to bind the escape of O. Fe doping reduces ion diffusion energy barrier to promote lithium ion diffusion. Electrochemical tests show that Fe doping can improve the electrochemical properties of LiMnPO4. © 2019 Elsevier B.V. All rights reserved.

Keywords: LiMnPO4 cathode DFT calculation Electronic structure Thermal stability Diffusion energy barrier

1. Introduction Portable electronic devices, electric vehicles (EV), hybrid electric vehicles (HEV), and large-scale distributed energy storage systems have put forward higher energy density and safety requirements for rechargeable batteries, thus stimulating researchers' theory and experimentation research on lithium-ion batteries [1e8]. The LiFePO4 cathode material has been commercially exploited on a large scale since its first introduction due to it has a series of advantages such as excellent cycle stability, environmental friendliness, and low cost [9]. However due to its own operating voltage (~3.4 V vs Li/Liþ) limitations, its energy density is relatively low for EV and HEV applications and cannot meet the existing requirements. Therefore, researchers are focusing on LiCoPO4, LiNiPO4, LiMnPO4, which are also the same family of olivine structures. The development of the first two has a high voltage of ~4.8 V vs Li/Liþ and ~5.2 V vs Li/Liþ, which is much higher than the safe voltage that existing electrolytes can withstand. In addition to

* Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Cao). https://doi.org/10.1016/j.jallcom.2019.04.191 0925-8388/© 2019 Elsevier B.V. All rights reserved.

environmental friendliness and low price, LiMnPO4 material is more important with a voltage platform of 4.1 V vs Li/Liþ and is compatible with traditional electrolyte windows. Therefore, it is considered to have great potential as the next generation of cathode materials [10,11]. It's the same as phosphate cathode material, LiMnPO4 materials have low electronic (ion) conductivity due to their own structural restriction. Among all material modification methods, ion doping is one of the most effective methods. In the study of ion doping to improve the electrochemical performance of LiMnPO4 material, a series of doping elements including Fe, Zn, Ca, Mg, Ti, Zr [12e14] were doped in the Mn lattice sites experimentally. Other dopant ions, such as active center ions Ni and Co of LiNiPO4 and LiCoPO4, which have the same olivine structure as LiMnPO4, are also frequently selected dopants. For example, Yang [15] et al. synthesized Co doped LiMnPO4 by oleic acid-assisted method, and found that Co doping can increase the capacity but induce capacity decay. In addition, due to the difference in ionic radius, the volume of the unit cell shrinks after doping and leading to the chemical diffusion coefficient is greatly reduced. Wang [16] and others tried the equivalent ions such as Ni, Mg, Fe, Zn doped in the Mn position of

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368

LiMnPO4, and found that the Ni, Mg, Fe doping capacity increased by 30%, while Zn doping made its capacity performance is lower. In addition, modification methods such as ion doping and coating are often used simultaneously to improve the electrochemical properties of the material. For example, Liang et al. [17] subtly constructed and synthesized composite materials LiMnPO4@NaTi2(PO4)3@C/3D Graphene by Ti doping and 3D graphene coating, benefiting from synergetic contributions from these design rationales, the integrated TLMP@NTP@C/3D-G cathode yields high-rate capability and long cycle life. As a battery material, energy density and safety are often contradictory, and it is critical to seek balance among them. LiFePO4 material has been proven to be very safe because it's delithium phase FePO4 is decomposed to release O2 at temperature above 500e600  C [18]. While LiMnPO4 provides a high voltage platform of 4.1 V, this material has poor thermal stability. LiMnPO4 material is a two-phase reaction in the process of assembly into a battery for charging and discharging, in which LiMnPO4 and MnPO4 coexist. LiMnPO4 is thermodynamically stable above 400  C or higher, however, recent investigations found that delithium phase MnPO4 decomposes at 120e210  C to release O2 [19,20]. In addition, S.P. Ong et al. [21] calculated and evaluated the thermal stability of MnPO4 and FePO4 by first-principles phase diagram calculations, and results found that the thermal stability of MnPO4 was far worse than that of FePO4, which released O2 at a lower temperature and could be reduced to Mn2P2O7 to form a new phase. In order to inhibit the O2 evolution of the delithiated phase MnPO4, the researchers conducted a large number of doping studies on the material. For example, J. Kim et al. [22] improved the O2 evolution behavior by mixing the metal Fe into the lattice Mn site to make the delithiation phase more stable. DH Snydacker et al. [23] studied the O2 evolution of different ions doped delithiated phases Mn1xMxPO4 (M ¼ Fe, Ni, Al, Mg) by DFT phase diagram calculation. The phase diagram calculation found that Fe substitution increases the initial temperature of O2 release and reduces the cumulative amount of O2 release, while Al slightly reduces the O2 release amounts due to the inactive composition of AlPO4, however, the Mg and Ni doping decreases the initial temperature of O2 release and destroys the thermal stability of the material. Doping transition metal ions have two important functions. One is that the transition metal ions compensate for the charge change during delithiation or transfer additional electrons to O. The other is that the transition metal ions can form strong bonds with the surrounding O and fixed O. There has been a large amount of theoretical research on LiFePO4, however, little or no calculation computational investigations on insights into the thermal stability of ions doped LiMnPO4. In order to search for ideal substituent elements and understand the influence mechanism of ion doping on electron (ion) conductivity and thermal stability from a microscopic point of view, in this work, a series of transition metal ion doping at the Mn site was investigated based on DFT calculations. The electronic structure, atomic Bader charge, thermal stability, average voltage and ion diffusion were evaluated.

361

used to correct the strong correlation of d electrons in transition metal ions. The U values associated with different transition metal ions are listed in Table 1. All calculated plane wave cutoff energies are above 620 eV to ensure smooth convergence of the system until energy<10^-5 eV/atom, Max. force <0.03 eV/Å, Max. stress <0.05Gpa and Max. displacement<0.001 Å. The MonkhorstePack scheme for 2*4*3 k-point meshes in the Brillouin zone (BZ) was selected [34]. The density of states is calculated using a Gaussian smearing of 0.05 eV. The atomic charge calculates by the Bader charge analysis method [35]. In order to reasonably simplify the calculation model and make the calculated concentration closer to the experimental doping concentration, in this work, all calculations were carried out in the 1  1  2 LiMnPO4 supercell (according to the cell lattice parameters, the supercell is as square as possible to reduce the periodic interaction of atoms), which contains 8 LiMnPO4 formula units. In a series of elements TM (TM ¼ Fe, Mg, Ni, V, Nb, Ti) doped in the Mn site models, the lattice Mn position in the unit cell LiMnPO4 is replaced by substituted elements TM, as shown in Fig. 1 (The structure schematic was drawn in the VESTA software [36]). Although ions (TM) doped with higher Mn2þ may produce Li vacancy defects, the effects of those vacancy defects can be eliminated during the discharge process. Therefore, we have reason to believe that LiMn0.875TM0.125PO4 (TM ¼ Fe, Mg, Ni, V, Nb, Ti) can be commonly accepted for different ion doping at the Mn site [37,38].

2.2. Experimental methods In the preparation of LiMnPO4 and LiMn0.875Fe0.125PO4, stoichiometric ratio of Mn powder (99.34% þ pure), LiH2PO4 (99.7% þ pure), PVA (99% þ pure, GJ29-JX09, the length (n) is 2400e2500, 5 g PVA/0.1 mol Mn) and FeC2O4$2H2O (99.5% þ pure) were ball milled in a liquid environment. A 250 ml zirconia bottle was used for powder milling in a planetary ball mill, and the mixture was ball milled thoroughly with zirconia balls at 500 r/min for 4 h at room temperature, and the resulting slurry was then dried at 60  C for 10 h. The obtained dried product was calcined at 650  C for 5 h in an inert Ar atmosphere, the heating rate was controlled at

Table 1 Hubbard U value for different transition metals. TM

U(eV)

Ref (s)

TM

U(eV)

Ref (s)

Mn Ni Fe

4.5 6.5 4.0

[28,29] [28,29] [29]

Nb Ti V

1.5 2.5 3.1

[30] [31,32] [33]

2. Methods 2.1. Computational details The DFT calculation was implemented in the Cambridge Sequential Total Energy Package (CASTEP) module under the Material Studio software [24]. The spin-polarized pseudopotentials plane wave method [25] was employed and the generalized gradient approximation (GGA) [26] based on PBE [27] was used to deal with exchange-related functions. The Hubbard-U types was

Fig. 1. LiMnPO4 schematic diagram of a polyhedral model in which Mn is replaced by TM (TM ¼ Fe, Mg, Ni, V, Nb, Ti) elements.

362

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368

4  C/min, and the black powder was cooled to room temperature. The final product is LiMnPO4/C and LiMn0.875 Fe0.125PO4/C. X-ray diffraction (XRD) analysis was performed on an X-ray diffractometer characterized (XRD, D/max-r A type CuKa, 40 kV, 300 mA, 10e80 ). Scanning electron microscopy (SEM JEOL JSM6360LV) was performed for surface morphology and particle size analysis. The charge-discharge properties of LiMnPO4/C and LiMn0.875Fe0.125PO4/C were tested refer to our previous work [39]. 3. Results and discussion 3.1. Electronic structure Fig. 2 (a) and (b) show the total density of states (DOS) and the partial density of states (PDOS) of LiMnPO4 and its different element dopants LiMn0.875M0.125PO4 (M ¼ Ni, Mg, Fe, Nb, V, Ti). For the calculation, the energy of antiferromagnetic (AFM) LiMnPO4 is lower than ferromagnetic (FM) LiMnPO4 about 8.2 meV/f. u.In experiments, G. Rousse [40] et al. also proved that the LiMPO4 magnetic sequence is AFM by neutron diffraction experiments, so we chose AFM magnetic sequence as the subsequent relevant calculation setting. It is also apparent from the DOS diagram of Fig. 2 (a) that the spin-up and the spin-down electronic state are symmetric along the energy axis. Table 2 lists band gap of different ions doped compounds. It can be seen that the calculated band gap of LiMnPO4 is 3.948 eV, which is very close to the others works (4.0eV [41], 3.96eV [42]). It is worth noting that the band gap of LiFePO4 is 3.7 eV [41], which explains why the electronic conductivity of LiMnPO4 is lower than LiFePO4. For non-transition metal Mg doping, no impurity band is introduced between the band gaps, and the band gap remains substantially unchanged. For Fe doping, a new electronic state occurs between the band gaps, which reduces the bandgap of the material from 3.948eV to 3.577eV. From Fig. 2. (b), it can be seen that the new electronic state is derived from the Fe3d electronic state of the dopant. For Ni, Nb, Ti, V substituted Mn of LiMnPO4, it can be clearly seen that a new band appears between the band gaps, so that the band gap of the compounds is greatly reduced from 3.948 to 2.949e0.208eV. Similar to the case of Fe doping, the appearance of new electronic states between the gaps is due to the d electrons of the transition metal ions, however, it is worth noting that the Nb doping makes the Mn3d unoccupied orbital closer to the Fermi level. For the transition metal (TM) occupied the center of the octahedron, the classic crystal field induce the 3d orbital of the transition metal ions splitting into t2g and eg characters [43,44]. For the representative transition metals Mn(3d-t2g)3 and (3d-eg)2, Fe(3d -t2g)4 and (3d-eg)2, Ni(3d-t2g)6 and (3d-eg)2, Nb(3d-t2g)6 and (3deg)4 bands. To further explore the mechanism of band gap reduction, We calculated the representative orbital charge distribution of LiMn0.875TM0.125PO4(TM ¼ Mn,Fe,Ni,Nb), the charge distribution images of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown in Fig. 2 (c), (d), (e), (f). For LiMnPO4, the charge of HOMO is derived from the hybridization of Mn (3d-t2g) and O2p, while the LUMO orbital is derived from the hybridization of Mn (3d-eg) and O2p. For Fe and Ni doping, the HOMO orbital charge is mainly derived from the TM (3d-t2g) orbital, the O2p orbital contributes a small amount of charge, and the LUMO orbital is all contributed by the TM (3d-eg) orbital. In addition, for Nb doping, both HOMO and LUMO orbital electrons are hybrid orbitals of Nb3d and O2p. In summary, the doped TM metal can improve the electronic conductivity of the LiMnPO4 material to varying degrees by introducing impurity band of hetero-atoms between the band gaps. The density of states and the orbital charge density images further explain that the non-pure bands between the band gaps are derived from TM (3d-t2g) and TM

(3d-eg).

3.2. Thermal stability Although the LiMnPO4 cathode material has a high energy density, its delithiated phase MnPO4 decomposes at a temperature higher than 200  C, and then convert into a new phase Mn2P2O7 (MnPO4 /Mn2 P2 O7 þ O2 ) [20,21,23,45]. The phase transition temperature is much lower than that of FePO4 (500  C), indicating that LiMnPO4 is used in the process of power battery, in the case of thermal runaway, the temperature of the battery module rises sharply, finally, the MnPO4 phase decomposes and releases O2, igniting the organic electrolyte and even causing an explosion. We believe that O in the crystal lattice of LiMnPO4 material will partially oxidize during the Li removal process (The average Bader charge of O atom is from 7.858 decrease to 7.728). O2 evolution is considered to be the most important cause of poor thermal stability of MnPO4 materials. Therefore, it is particularly important to suppress the O2 release reaction and stabilize the O in the crystal lattice to improve the electrochemical performance of the LiMnPO4 positive electrode material. In this part, we predict and evaluate the effect of different element doping on the O2 release reaction and fixing the O in the lattice by DFT calculation. Although MnPO4 eventually releases O2 and then transforms into Mn2P2O7, we still believe that the O2 release reaction is that O is first removed from the crystal lattice and then following reaction occurs:

x MnPO4 /MnPO4x þ O2 2 In the formula, x is the amount of O that escapes from the lattice. The total energy can be calculated by DFT, and the enthalpy of reaction (DH) can be obtained according to the following formula:

DH ¼

EðMnPO4x Þ þ 0:5xEðO2 Þ  EðMnPO4 Þ 0:5x

In the formula, E is the total energy of the corresponding structure calculated by DFT. The O deficient structure MnPO4-x is obtained by removing the O atom having the lowest Bader charge. Taken into account the entropy of O2 at the standard state (-TDS ¼ 0.63 eV) [20], the Gibbs free energy change DG (as shown in Fig. 3) can be calculated ðDG ¼ DH  0:63eVÞ. Fig. 3 is the Gibbs free energy change obtained by DFT calculation of O2 evolution reaction of different metal elements substitution LiMnPO4. It can be seen from Fig. 3 that the DG of MnPO4 is 4.195 eV, indicating that the reaction is non-spontaneous and endothermic. Further, for Ni and Mg doping, the DG of O2 evolution reaction is lowered, so that the decomposition temperature of the reaction is reduced, the thermal stability is poor than MnPO4, however, the Fe doping is reversed, and thus the initial temperature at which the decompose reaction occurs is increased. The calculation results of the calculation models is consistent with others work by first principle phase diagram calculation [23]. In addition, in a series of substituted elements, the effect of Fe and Nb doping is most significant, which improve the thermal stability of MnPO4, while other doping elements such as Ni, Mg, V, Ti make the thermal stability of the material lower. Interestingly, the Gibbs free energy of the material obtained by Ti doping is negative, indicating that the O2 evolution reaction is spontaneous. On the other hand, O in the crystal lattice of Mn1-xMxPO4 material forms a PO4 tetrahedron with P, and forms an MO6 octahedral structure with Mn or a substitute metal ion M. Therefore, it is considered that the bond length of the MeO bond can reflect the local stability of doped ions to stabilize O to a certain extent. As

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368

363

Fig. 2. Different elements doped at Mn sites of LiMnPO4 near the Fermi surface (10e10eV) (a) total density of states (DOS) and (b) partial density of states (PDOS); (c), (d), (e), (f) represent the HOMO (Left) and LUMO (Right) orbital charge distributions of LiMnPO4, Fe, Ni, Nb doping, respectively; the Fermi level is set to 0 eV (the vertical black dotted line in the figure).

364

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368

Table 2 Calculated band gap values of different LiMn0.875M0.125PO4 (M ¼ Ni, Mg, Fe, Nb, V, Ti).

ions

doping

compounds

Doped Elements

Mn

Fe

Mg

Ni

V

Nb

Ti

Band Gap (eV)

3.948

3.577

3.940

2.949

0.208

1.170

1.536

relative to the O2p bands [47]. During the charging process, Liþ removal causes one electron transition from below the Fermi surface to above the Fermi surface, at the same time, the ions in the material are then oxidized. For Fig. 4 (a), in addition to the oxidation of Mn2þ, a large amount of O2 is also oxidized, so that the Bader charge value of O in the MnPO4 material is lower (7.728), and then it is more easily decomposed by further thermal decomposition to release O2. Moreover, in the part below the Fermi surface, the O2p state is mainly dominant. For Fe doping, mainly Mn2þ is oxidized, a small amount of O2 are slightly oxidized, and the average Bader charge of O in Fe-doped MnPO4 material is higher than that of undoped MnPO4 material (7.732), and further, below the Fermi surface, Fe3d orbital are closer to the Fermi surface than O2p. The electron preferential transition on the Fe3d orbital is more than the O2p orbital, so that the Fe doping inhibits the oxidation degree of O, thereby increasing the initial temperature of the O2 evolution reaction and improving the thermal stability of the material.

3.3. Property evaluate

Fig. 3. Gibbs free energy change DG of lattice O evolution reaction in delithium phase Mn0.875TM0.125PO4 (TM ¼ Mn, Ni, Mg, Fe, Nb, V, Ti).

shown in Table 3 below, although in the Gibbs free energy calculation of the reaction, it is indicated that only Fe and Nb can increase the initial temperature of O2 evolution, but for other dopant ions, the bond length with the surrounding O is significantly smaller than MneO bond, indicating that the doping ions can form a strong MeO bond with the surrounding O, stabilizing the local structure of the material. In addition, the presence of doping ions makes the difference in bond length between MeO gradually shrink, indicating that doping will moderate the John-Teller effect to a certain extent, reduce lattice distortion, and improve structural stability. For the battery cathode material, the removal of one Liþ by one molecular unit of LiMnPO4 during charging is inevitably accompanied by oxidation of other ions in the material. In order to further explore the mechanism of Fe doped inhibit O2 evolution and improve the electrochemical performance of LiMnPO4, we calculated the PDOS of (Li)MnPO4 and its Fe-doped (Li)MnPO4 compound by DFT. As shown in Fig. 4. In specific, we used the HSE06 hybrid functional [46] as implemented in the CASTEP program. Although the use of this functional has an adverse effect on the exponential growth of the computational amount, it can accurately reproduce the position of the transition metal d orbital bands

LiMnPO4 material is highly valued by researchers because of its high voltage platform (4.1 V vs/Li) and matching with existing conventional electrolyte windows. Therefore, the effect of ion doping on the battery voltage platform is particularly important. Here, we calculated the voltage platform of LiMn1-xMxPO4 (M ¼ Mn, Fe, Ni, Mg, Nb, V, Ti) by DFT þ U, which is a method that can be used by researchers to accurately predict the voltage of electrode materials [29,49e52]. Average open circuit voltage calculation formula as follows:

EðLiMn1  xMxPO4Þ  EðMn1  xMxPO4Þ  EðLiÞ V¼ e In the formula, E(LiMn1-xMxPO4) and E(Mn1-xMxPO4) represent the total energy of one formula unit of the lithiated state LiMn1xMxPO4 and the delithiated phase Mn1-xMxPO4 (M ¼ Fe, Mg, Ni, V, Nb, Ti), respectively, E(Li) represents and the energy of a single Li atom. In the calculation, we corrected the strong correlation interaction between the transition metal ion d electrons through the Hubbard þ U model, and finally the calculated average voltage of LiMnPO4 (4.14 V) and the experimental measured voltage platform (4.1 V) is very close, the error is less than 0.1 V, indicating that the relevant parameters of the model and calculation settings are reasonable. As shown in Fig. 5 below, among all the doping ions, Mg, Ni, and Ti doping increase the voltage platform, especially Ti doping. The V and Nb doping causes the voltage platform to drop. The Fe-doped voltage platform has a slight decrease compared to LiMnPO4, but we still think it has a higher voltage compared LiFePO4.

Table 3 Details of TM-O distance and average TM-O distance. TM

TM-O distance(Å)a

Average TM-O Distance (Å)a

Average MneO distance (Å)b

Mn Fe Ni Mg Nb V Ti

1.947,1.933,1.920,2.864*3 1.936,1.947,2*2.030,2*2.355 1.935,1.945,2*2.015,2*2.641 1.983,2.010,2.093*2,2.321*2 1.924,1.927,2.038*2,2.090*2 1.819,1.836,2.052*2,2.118*2 1.957,1.991,1.999*2,2.116*2

2.398 2.109 2.199 2.137 2.018 1.999 2.030

2.246 2.216 2.166 2.225 2.034 2.175 2.227

Note. a Average TM-O of a local representative octahedron TMO6. b Average MneO distance over full supercell MnO6.

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368

365

Fig. 4. DFT-HSE06 functional calculation was used to obtain the PDOS (a) (Li)MnPO4 and (b) Fe doping (Li)MnPO4 near Fermi surface (10e10 eV) A partial enlargement (1~1eV) of PDOS is shown below in (c) and (d), respectively. The black dotted line represents Fermi surface.

Fig. 5. Open circuit voltage of different element doped LiMn1-xMxPO4 calculated by DFT þ U.

effect of doping on Li-ion diffusion is evaluated and predicted herein. The elastic band method (NEB) can accurately calculate the diffusion energy barrier of lithium ions, and the calculated amount is greatly reduced relative to the first-principles molecular dynamics. Further, according to the fact that LiMnPO4 and LiFePO4 have the same olivine structure, we believe that the diffusion of LiMnPO4 is mainly spread by the single vacancy in the (010) direction [53e56]. The diffusion energy barrier of different ion doping calculated by NEB method is shown in Fig. 6. The calculation results show that the diffusion energy barrier of LiMnPO4 (1.09eV) is higher than that of LiFePO4 (0.60eV [54], 0.55eV [57], 0.54eV [58]), indicating that the lithium ion diffusion rate of pure LiMnPO4 is lower than that of LiFePO4, and the results are consistent with the experiment. For Fe doping, the calculated diffusion energy barrier is 0.66 eV, and the activation energy barrier is greatly reduced relative to the un-doped material, indicating that doping Fe can effectively increase the lithium ion diffusion rate. In addition to Nb doping, other ion doping can promote lithium ion diffusion to varying degrees, and the promoting order is V, Ni, Ti, Mg, diffusion energy barrier as a function diffusion path are depicted in Fig. 6.

3.4. Li diffusion

3.5. Characteristics and electrochemical performance of Fe-Doped LiMnPO4/C

In rechargeable lithium ion batteries, high power requires that Li diffusion in and out of the electrode materials takes place fast enough to supply the electric current. The kinetic properties play a crucial role in the electrochemical performance of rechargeable lithium ion batteries. However, for the LiMnPO4 material, there is a problem that the intrinsic lithium ion diffusion rate is low, and the

The Fe-doped LiMnPO4/C sample (LiMn0.875Fe0.125PO4/C) was characterized by XRD (Fig. 7 (a)) and SEM (Fig. 7 (b)). The XRD pattern showed that the prepared LiMn0.875Fe0.125PO4/C showed no impurity phase, and the diffraction peak shifted to a higher angle, there is a voltage platform about 3.5 V in Fig. 7 (d), which indicating that Fe was successfully doped into the LiMnPO4 lattice to form a

366

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368

Fig. 6. Energy barrier of lithium ion diffusion as a function of diffusion path.

solid solution. The scanning electron microscope of LiMn0.875Fe0.125PO4/C is shown in Fig. 7 (b). The SEM image shows that the LiMn0.875Fe0.125PO4/C powders consists of small-sized particles of less than 100 nm and particles uniform distribution. The rate performance of LiMnPO4/C and LiMn0.875Fe0.125PO4/C was investigated.

The coin-types cell was cycled between 2.5 V and 4.5 V at a rate of 0.1C, 0.2C, 0.5C, 1C, 2C, 3C, (Fig. 7(c) and (d)) are discharge curves of LiMnPO4/C and LiMn0.875Fe0.125PO4/C, respectively. Both samples showed a stable discharge platform of approximately 4.0 V, which is basically consistent with the average voltage calculated by our DFT þ U calculation. Compared with the LiMnPO4/C material, in Fig. 7(d), the bend curve observed between 3.4 V and 3.7 V could be ascribed to the Fe2þ/Fe3þ redox reaction, which is characteristic redox plateau of Fe2þ in the olivine structure. Within the 3.8 V and 4.3 V range, the LiMn0.875Fe0.125PO4/C cathode showed a clear redox potential plateau around 4.15 V (for charge) and 3.98 V (for discharge), which corresponding to the Mn2þ/Mn3þ redox reaction. In addition, after Fe doping, the polarization can be effectively reduced, and the stability of the material can be improved. For example, as seen in Fig. 7(c) and (d), when the materials are charged and discharged at a rate of 1 C, the voltage difference between the charge and discharge (4.25/3.95) of the LiMnPO4/C material is about 0.3 V, after Fe element doping, the voltage difference of LiMn0.875Fe0.125PO4/C material is reduced to 0.17 V. The decrease in polarization is due to the fact that Fe doping can improve the electrical conductivity of the material. The experimental results in this part are consistent with the results of Fe doping can reduce the band gap in the DFT calculation. As the charge-discharge rate increases, charging and discharging with a large current causes the degree of polarization to increase, and the voltage platform drops rapidly, causing the

Fig. 7. Experimental characteristics and charge-discharges curves. (a) XRD pattern and (b) SEM image of Fe-doped LiMnPO4/C. The charge and discharge capacity curves of (c) LiMnPO4/C and (d)LiMn0.875Fe0.125PO4/C.

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368

platform to gradually approach the slope of the curve. The capacity of LiMnPO4/C is 132 mAhg1 at 0.1C and 118 mAhg1 at 1C, while Fe-doped LiMnPO4/C is 150 mAhg1 at 0.1C and 136.5 mAhg1 at 1C. As the rate increases, the capacities of LiMnPO4/C and LiMn0.875Fe0.125PO4/C gradually decrease. The capacity of Fe doped-LiMnPO4/C at 1 C (136.5 mAhg1) is higher than the capacity at 0.1 C rate of LiMnPO4/C, while the capacity at 3 C is almost equal to the capacity at 1 C of LiMnPO4/C, indicating Fe doped can effectively improve the rate performance of LiMnPO4/C, which is consistent with the DFT calculation conclusion that Fe can promote the diffusion of lithium ions.

[4]

[5]

[6]

[7]

[8]

4. Conclusion In this work, DFT calculations were carried out to investigate the effects of different elements doping effects on the physical and electrochemical properties of LiMnPO4. The main findings are as follows: (1) Fe, Ni, V, Nb, Ti doping at the Mn site can reduce the band gap by introducing a transition metal TM (3d-t2g) or/and TM (3deg) impurity states in the band gap. Thereby improving the electronic conductivity of the material. (2) Fe, Mg, Ni, V, Nb, Ti binds surrounding O form a stronger bond than Mn, which can improve the stability of O in the crystal lattice. In addition, the doping causes the bond length of all MneO bonds of the material to be shorter than the undoped material, In addition, doping ions makes the difference in bond length between MeO gradually shrink, indicating that doping will moderate the John-Teller. (3) Among all the doped ions, only Fe and Nb doping can inhibit the O2 evolution reaction. The electronic structure calculation shows that the Fe3d orbital electrons below the Fermi surface preferentially oxidize over the O2p orbital electrons, inhibiting the O2 evolution. (4) In addition to Nb, other ion doping can reduce lithium ions diffusion barrier to promote the diffusion of lithium ions to different extents, and the effect of Fe doping is most significant. Based on the above conclusions, Fe seems to be the most ideal doping element, because Fe doping can simultaneously improve the electronic conductivity and the thermal stability of the material, while promoting the diffusion of lithium ions. Electrochemical tests show that Fe doping can improve electrochemistry behavior including specific capacity and rate performance.

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

Acknowledgements [23]

This work was supported by the National Key R&D Program of China (2017YFC0210406), the National Science Foundation of China (51874358, 51602352, 51772333, 51674300, 61533020), and the Fundamental Research Funds for the Central Universities of Central South University (2018zzts433). In addition, we also acknowledge the software support of the National Supercomputing Center in Shenzhen, China. References [1] J. Kim, A. Manthiram, A manganese oxyiodide cathode for rechargeable lithium batteries, Nature 390 (1997) 265e267. [2] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928e935. https://doi.org/10.1126/ science.1212741. [3] M.M. Thackeray, C. Wolverton, E.D. Isaacs, Electrical energy storage for transportation-approaching the limits of, and going beyond, lithium-ion

[24]

[25]

[26]

[27] [28]

[29]

[30]

367

batteries, Energy Environ. Sci. 5 (2012) 7854e7863. https://doi.org/10.1039/ c2ee21892e. A. Urban, D.H. Seo, G. Ceder, Computational understanding of Li-ion batteries, npj Comput. Mater. 2 (2016) 16002. https://doi.org/10.1038/npjcompumats. 2016.2. Y.S. Meng, M.E. Arroyo-de Dompablo, Recent advances in first principles computational research of cathode materials for lithium-ion batteries, Accounts Chem. Res. 46 (2012) 1171e1180, https://doi.org/10.1021/ar2002396. G. Yoon, D.H. Kim, I. Park, Using first principles calculations for the advancement of materials for rechargeable batteries, Adv. Funct. Mater. 27 (2017) 1702887. https://doi.org/10.1002/adfm.201702887. L. Wu, J.J. Lu, G. Wei, et al., Synthesis and electrochemical properties of xLiMn0.9Fe0.1PO4yLi3V2(PO4)3/C composite cathode materials for lithiumeion batteries, Electrochim. Acta 146 (2014) 288e294. https://doi.org/10.1016/j. electacta.2014.09.076. L. Wu, J. Zheng, L. Wang, et al., PPy-encapsulated SnS2 nanosheets stabilized by defects on a TiO2 support as a durable anode material for lithium-ion batteries, Angew. Chem. Int. Ed. 58 (2019) 811e815. https://doi.org/10. 1002/anie.201811784. A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positive-electrode materials for rechargeable lithium batteries, J. Electrochem. Soc. 144 (1997) 1188e1194. https://doi.org/10.1149/1.1837571. V. Aravindan, J. Gnanaraj, Y.S. Lee, LiMnPO4eA next generation cathode material for lithium-ion batteries, J. Mater. Chem. A 1 (2013) 3518e3539. https:// doi.org/10.1039/c2ta01393b. J.O. Herrera, H. Camacho-Montes, L.E. Fuentes, LiMnPO4: review on synthesis and electrochemical properties, J. Mater. Sci. Chem. Eng. 3 (2015) 54e64. https://doi.org/10.4236/msce.2015.35007. T. Shiratsuchi, S. Okada, T. Doi, Cathodic performance of LiMn1xMxPO4 (M¼ Ti, Mg and Zr) annealed in an inert atmosphere, Electrochim. Acta 54 (2009) 3145e3151. https://doi.org/10.1016/j.electacta.2008.11.069. C. Hu, H. Yi, H. Fang, Improving the electrochemical activity of LiMnPO4 via Mn-site co-substitution with Fe and Mg, Electrochem. Commun. 12 (2010) 1784e1787. https://doi.org/10.1016/j.elecom.2010.10.024. zen, et al., Improving the electrochemical activity of D. Wang, C. Ouyang, T. Dre LiMnPO4 via Mn-site substitution, J. Electrochem. Soc. 157 (2010) A225eA229. https://doi.org/10.1149/1.3271112. C.G. Son, H.M. Yang, G.W. Lee, Manipulation of adipic acid application on the electrochemical properties of LiFePO4 at high rate performance, J. Alloys Compd. 509 (2011) 1279e1284. https://doi.org/10.1016/j.jallcom.2010.10. 009. D. Wang, H. Buqa, M. Crouzet, High-performance, nano-structured LiMnPO4 synthesized via a polyol method, J. Power Sources 189 (2009) 624e628. https://doi.org/10.1016/j.jpowsour.2008.09.077. L. Liang, X. Sun, J. Zhang, et al., In situ synthesis of hierarchical core doubleshell Ti-doped LiMnPO4@ NaTi2 (PO4)3@ C/3D graphene cathode with highrate capability and long cycle life for lithium-ion batteries, Adv. Energy Mater. (2019) 1802847. https://doi.org/10.1002/aenm.201802847. C. Delacourt, The existence of a temperature-driven solid solution in LixFePO4 for 0  x 1, J. Nat. Mater. 4 (2005) 254e260. https://doi.org/10.1038/ nmat1335. S.W. Kim, J. Kim, H. Gwon, et al., Phase stability study of Li1x MnPO4 (0x1) cathode for Li rechargeable battery, J. Electrochem. Soc. 156 (2009) A635eA638. https://doi.org/10.1149/1.3138705. G. Chen, T.J. Richardson, Thermal instability of Olivine-type LiMnPO4 cathodes, J. Power Sources 195 (2010) 1221e1224. https://doi.org/10.1016/j. jpowsour.2009.08.046. S.P. Ong, A. Jain, G. Hautier, et al., Thermal stabilities of delithiated olivine MPO4 (M¼Fe,Mn) cathodes investigated using first principles calculations, Electrochem. Commun. 12 (2010) 427e430. https://doi.org/10.1016/j.elecom. 2010.01.010. J. Kim, K.Y. Park, I. Park, et al., Thermal stability of FeeMn binary olivine cathodes for Li rechargeable batteries, Mater. Chem. 22 (2012) 11964e11970. https://doi.org/10.1039/c2jm30733b. D.H. Snydacker, C. Wolverton, Oxygen evolution from olivine Mn1x MxPO4 (M¼ Fe, Ni, Al, Mg) delithiated cathode materials, Phys. Rev. B 95 (2017), 024102, https://doi.org/10.1103/PhysRevB.95.024102. S.J. Clark, M.D. Segall, C.J. Pickard, et al., First principles methods using CASTEP, Z. Krist. Cryst. Mater. 220 (2005) 567e570. https://doi.org/10.1524/zkri.220.5. 567.65075. K. Laasonen, R. Car, C. Lee, et al., Implementation of ultrasoft pseudopotentials in ab initio molecular dynamics, Phys. Rev. B 43 (1991) 6796. https://doi.org/ 10.1103/PhysRevB.43.6796. J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865e3868. https://doi.org/10.1103/ PhysRevLett.77.3865. J.P. Perdew, K. Burke, M. Ernzerhof, Perdew, burke, and ernzerhof reply, Phys. Rev. Lett. 80 (1998) 891. https://doi.org/10.1103/PhysRevLett.80.891. H. Chen, J.A. Dawson, J.H. Harding, Effects of cationic substitution on structural defects in layered cathode materials LiNiO2, J. Mater. Chem. A 2 (2014) 7988e7996. https://doi.org/10.1039/C4TA00637B. F. Zhou, M. Cococcioni, C.A. Marianetti, et al., First-principles prediction of redox potentials in transition-metal compounds with LDAþ U, Phys. Rev. B 70 (2004) 235121. https://doi.org/10.1103/PhysRevB.70.235121. G. Hautier, S.P. Ong, A. Jain, Accuracy of density functional theory in predicting

368

[31] [32]

[33]

[34] [35]

[36]

[37]

[38]

[39]

[40]

[41] [42]

[43] [44]

H. Zhang et al. / Journal of Alloys and Compounds 793 (2019) 360e368 formation energies of ternary oxides from binary oxides and its implication on phase stability, Phys. Rev. B 85 (2012) 155208. https://doi.org/10.1103/ PhysRevB.85.155208. Z. Hu, H. Metiu, Choice of U for DFTþ U calculations for titanium oxides, J. Phys. Chem. C 115 (2011) 5841e5845, https://doi.org/10.1021/jp111350u. S. Lutfalla, V. Shapovalov, A.T. Bell, Calibration of the DFT/GGAþU method for determination of reduction energies for transition and rare earth metal oxides of Ti, V, Mo, and Ce, Chem. Theory Comput. 7 (2011) 2218e2223. https://doi. org/10.1021/ct200202g. L. Wang, T. Maxisch, G. Ceder, Oxidation energies of transition metal oxides within the GGAþ U framework, Phys. Rev. B 73 (2006) 195107. https://doi. org/10.1103/PhysRevB.73.195107. H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188. https://doi.org/10.1103/PhysRevB.13.5188.  nsson, A fast and robust algorithm for G. Henkelman, A. Arnaldsson, H. Jo Bader decomposition of charge density, Comput. Mater. Sci. 36 (2006) 354e360. https://doi.org/10.1016/j.commatsci.2005.04.010. K. Momma, F. Izumi, VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data, J. Appl. Crystallogr. 44 (2011) 1272e1276. https://doi.org/10.1107/S0021889811038970. A. Jena, B.R.K. Nanda, Engineering diffusivity and operating voltage in lithium iron phosphate through transition-metal doping, Phys. Rev. Appl. 7 (2017), 034007. https://doi.org/10.1103/PhysRevApplied.7.034007. Y. Gao, X. Wang, J. Ma, et al., Selecting substituent elements for Li-rich Mnbased cathode materials by density functional theory (DFT) calculations, Chem. Mater. 27 (2015) 3456e3461. https://doi.org/10.1021/acs.chemmater. 5b00875. J. Duan, Y. Cao, J. Jiang, Novel efficient synthesis of nanosized carbon coated LiMnPO4 composite for lithium ion batteries and its electrochemical performance, J. Power Sources 268 (2014) 146e152. https://doi.org/10.1016/j. jpowsour.2014.06.020. G. Rousse, J. Rodriguez-Carvajal, S. Patoux, et al., Magnetic structures of the triphylite LiFePO4 and of its delithiated form FePO4, Chem. Mater. 15 (2003) 4082e4090. https://doi.org/10.1021/cm0300462. F. Zhou, K. Kang, T. Maxisch, et al., The electronic structure and band gap of LiFePO4 and LiMnPO4, J. Solid State Commun. 132 (2004) 181e186. Z.X. Nie, C.Y. Ouyang, J.Z. Chen, et al., First principles study of JahneTeller effects in LixMnPO4, J. Solid State Commun. 150 (2010) 40e44. https://doi. org/10.1016/j.ssc.2009.10.010. R.G. Burns, Mineralogical Applications of Crystal Field Theory, vol. 5, Cambridge University Press, 1993. I.B. Bersuker, Electronic Structure and Properties of Transition Metal Compounds: Introduction to the Theory, John Wiley & Sons, 2010.

[45] D. Choi, J. Xiao, Y.J. Choi, et al., Thermal stability and phase transformation of electrochemically charged/discharged LiMnPO4 cathode for Li-ion batteries, Energy Environ. Sci. 4 (2011) 4560e4566. https://doi.org/10.1039/ c1ee01501j. [46] J. Heyd, G.E. Scuseria, M. Ernzerhof, Hybrid functionals based on a screened Coulomb potential, J. Chem. Phys. 118 (2003) 8207e8215. https://doi.org/10. 1063/1.1564060. [47] M.D. Johannes, K. Hoang, J.L. Allen, et al., Hole polaron formation and migration in olivine phosphate materials, Phys. Rev. B 85 (2012) 115106. https://doi.org/10.1103/PhysRevB.85.115106. [49] V.I. Anisimov, J. Zaanen, O.K. Andersen, Band theory and Mott insulators: hubbard U instead of stoner I, Phys. Rev. B 44 (1991) 943e954. https://doi.org/ 10.1103/PhysRevB.44.943. [50] V.I. Anisimov, F. Aryasetiawan, A.I. Lichtenstein, First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDAþU method, J. Phys. Condens. Matter 9 (1997) 767. [51] S.L. Dudarev, G.A. Botton, S.Y. Savrasov, C.J. Humphreys, A.P. Sutton, Electronenergy-loss spectra and the structural stability of nickel oxide: an LSDAþU study, Phys. Rev. B 57 (1998) 1505e1509. https://doi.org/10.1103/PhysRevB. 57.1505. [52] L. Wang, T. Maxisch, G. Ceder, Oxidation energies of transition metal oxides within the GGAþU framework, Phys. Rev. B 73 (2006) 195107. https://doi.org/ 10.1103/PhysRevB.73.195107. [53] H. Lin, Y. Wen, C. Zhang, et al., A GGAþ U study of lithium diffusion in vanadium doped LiFePO4, Solid State Commun. 152 (2012) 999e1003. https:// doi.org/10.1016/j.ssc.2012.03.027. [54] C. Ouyang, S. Shi, Z. Wang, et al., First-principles study of Li ion diffusion in LiFePO4, Phys. Rev. B 69 (2004) 104303. https://doi.org/10.1103/PhysRevB.69. 104303. [55] D. Choi, D. Wang, I.T. Bae, et al., LiMnPO4 nanoplate grown via solid-state reaction in molten hydrocarbon for Li-ion battery cathode, Nano Lett. 10 (2010) 2799e2805. https://doi.org/10.1021/nl1007085. [56] J. Yang, J.S. Tse, Li ion diffusion mechanisms in LiFePO4: an ab initio molecular dynamics study, J. Phys. Chem. A 115 (2011) 13045e13049. https://doi.org/10. 1021/jp205057d. [57] M.S. Islam, D.J. Driscoll, C.A.J. Fisher, et al., Atomic-scale investigation of defects, dopants, and lithium transport in the LiFePO4 olivine-type battery material, Mater. Chem. 17 (2005) 5085e5092. https://doi.org/10.1021/ cm050999v. [58] C. Delacourt, Toward understanding of electrical limitations (electronic, ionic) in LiMPO4 (M¼Fe,Mn) electrode materials, J. Electrochem. Soc. 152 (2005) A913eA921. https://doi.org/10.1149/1.1884787.