J. Cent. South Univ. Technol. (2007)02-0186-05

DOI: 10.1007/s11771-007-0037-9                                                                                             

First-principle investigation on stability of Co-doped spinel λ-Mn_4-xCo_xO₈

HUANG Ke-long(黄可龙)¹, CHEN Chun-an(陈春安)¹, LIU Su-qin(刘素琴)¹,

LUO Qiong(罗 琼)¹, LIU Zhi-guo(刘志国)²

(1. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Department of Chemistry and Chemical Engineering, Hunan University of Technology, Zhuzhou 412007, China)    

Abstract: The mechanism of stability of Co-doped spinel λ-MnO₂ that is referred to as spinel Li_xMn₂O₄ (x=0) was studied by using the first-principle calculation method. The total energy and formation enthalpy can be decreased remarkably due to the Co substation, resulting in a more stable structure of λ-Mn_xCr_2-xO₄. The bond order and DOS analysis were given in detail to explain the nature of stability improvement. The calculated results show that as the content of Co dopant increases, the bond order of Mn—O becomes larger and the peak of density of states around Fermi level shifts toward lower energy. The charge density distribution illustrates that the Mn—O bonding is ionic and partially covalent, and the covalent Mn—O bonding becomes stronger with the increase of Co dopant content. The results confirm that the Co-doping will enhance the stability of λ-MnO₂ and hence improve the electrochemistry performance of Li_xMn₂O₄.

Key words: first-principles; stability; electrochemical performance; Co-doped λ-MnO₂                                      

1 Introduction

Despite the fact that spinel LiMn₂O₄ possesses a series of attractive characteristics such as high cell voltage, large durability under extreme temperature circumstance and low cost, there are still some open questions arising from its practical application in cathode of lithium ion battery^[1-3]. Some studies were reported that the structure of cathode Li_xMn₂O₄ becomes less stable when lithium is almost completely removed during charging and host material transforms to λ-MnO₂^[1,4]. These changes result in the decrease of charging- recharging circles. Therefore the formation of λ-MnO₂ during the charging of spinel LiMn₂O₄ cathode has a side effect on electrochemistry performance of LiMn₂O₄.

 In order to improve the performance of LiMn₂O₄ cathode, a great deal of experimental work has been conducted by doping a small amount of Co, Cr or Ni to substitute Mn^[5-8]. All kinds of models are constructed to account for the mechanism of the transition metal-doped spinel LiMn₂O₄ based on the first-principles, but most are focused on the correlation between the total energy and the intercalation voltage^[9-10]. Although some of them can give insight into understanding the electronic attributes of spinel LiMn₂O₄, a little information about the stability of the metal-doped Li_xMn₂O₄ is acquired especially at the time when x nearly reaches zero. In this study, the effect of Co on the stability of λ-MnO₂ was explored.

2 Computational methods

2.1 Method details

The CASTEP package, an implementation of the first principle plane-wave pseudopotential, was designed for simulating the material properties^[11]. The DFT that is highly successful for many classes of systems forms the basis for the CASTEP package^[12]. The GGA-PW91 functional is specified to describe the system because it generally produces a more accurate and reasonable result after different functionals, including LDA, GGA and meta-GGA, and was carefully evaluated in the system^[10].

The electronic wavefunctions, determined by 340 eV cut-off, were expanded to the extent that is sufficient for leading to the convergence with respect to the total energy and forces acting on the atom. The energy reaches the convergence with the accuracy of 2×10^-6 eV/atom and meanwhile the force is converged with the accuracy of 0.025 eV/nm. The Monkhorst-Pack k-points 5×5×6 were assigned to define the accuracy of the sampling^[13]. No observed Jahn-Teller effect is found in the doped λ-MnO₂ as Mn is expected to appear with formal valence of +4. Consequently, the Jahn-Teller effect is not taken  into account in this case^[13-14].

2.2 Geometric structure and calculation models

The spinel λ-MnO₂ belongs to space group Fd-3m.

It is experimentally observed that in spinel λ-MnO_2, Mn occupies Wyckoff site of 16d and 32e for oxygen atoms^[15]. The different sites are clearly identified by the label of O_I, O_II, O_III, O_Ⅳ and M₁, M₂, M₃, M₅, M₆, M₇, M₈(Fig.1).

Fig.1 Primitive cell of M₄O₈ (M represents metal atom (Co or Mn))

A primitive cell of λ-MnO₂ containing four Mn atoms and eight O atoms was employed in the study, and then Mn atoms in cell were substituted by Co atoms to model stoichiometric Mn_4-xCo_xO₈ (x=0, 1, 2, 3, 4). During the calculations, the unit cell of Mn_4-xCo_xO₈ was maintained as the cubic symmetry of host. The structural parameters of Mn_4-xCo_xO₈ are optimized by full relaxation of the cell and the calculated results are presented in Table 1.

Table 1 Characteristic property of various Co-dopant primitive M₄O₈ cells

3 Results and discussion

3.1 Total energy and formation enthalpy

The total energy is calculated to obtain the formation enthalpy. The formation enthalpy is given by the difference in total energy between Mn_xCo_4-xO₈ and the sum of oxides Mn₄O₈ and Co₄O₈, i.e.

        (1)

where E(Mn_xCo_4-xO₈)，E(Mn₄O₈) and E(Co₄O₈) represent the total energy of per formula unit of Mn_xCo_4-xO₈, Mn₄O₈ and Co₄O₈, respectively.

According to Table 1, both the total energy and formation enthalpy suggest that Co substitution can improve stability of the structure. It has a consistent declining trend of the total energy; however, the formation enthalpy may well reflect the nature of the structural stability and it is also in good agreement with the experimental analysis^[16].

The energy change is mostly due to the electron transfer that causes the interaction force among the ions when Mn atoms are replaced.
3.2 Bond length and volume

The average bond length of Mn—O tends to decrease from in λ-Mn₄O₈ to in λ-MnCo₃O₈ as more Co is substituted for Mn (Table 2). By comparison, the average bond length of Co—O is shorter than Mn—O in all primitive cells. It can be observed that bond length of Co—O remains almost unchanged no matter what the Co content is, being fixed at about 0.191 4 nm. The contraction of   Mn—O can certainly help to form a more λ-Mn_4-xCo_xO₈ state. The experiment and X-ray absorption spectroscopic studies have born out such a conclusion^[4,17]. Although the Mn—O or Co—O bond length is somewhat smaller than the counterpart of experimental value, such a trend of bond length decrease from λ-Mn₄O₈ to MnCo₃O₈ is so consistent that the overall errors can be ignored^[18].

Table 2 Total energy and formation enthalpy of structure M₄O₈ with varying Co dopant

There is also a decline in volume from λ-Mn₄O₈ to MnCo₃O₈ in Table 2. The average bond length of Mn—O is constricted to a slight extent of about 0.26%. The volume contractions as well as the decrease of Mn—O bond are desirable factors for achieving a stable λ-  Mn_4-xCo_x O₈ structure in the light of classic chemistry theory. The shorter Mn—O helps to further bind the molecule together in view of atoms and molecules theory, and it follows necessarily that the Co- doped structure contacts more closely^[18]. This has been also confirmed by the experimental result^[7].

3.3 Ionicity and bond order

According to the Mulliken population(Table 1), the charges distributed to O and Mn in λ-Mn₄O₈ are -0.39 and 0.79 respectively; whereas the formal charges of O and Mn in λ-Mn₄O₈ correspond to -2 and +4. The fact that Mn—O bond has not only characteristic of ionicity but also partial covalence can be inferred. As illustrated in Table 1, the Mulliken charge of Mn varies from 0.79 to 0.83 as Mn is gradually replaced with Co and simultaneously the average charge build-up that encompasses O atom appears with a remarkable increase from -0.39 to -0.49. In the light of the electrostatic equilibrium theory, the more charge accumulation there is around atoms, and the more attractive force of the electron density will exert on the molecule^[18]. Therefore, it will give an explanation about why the Co-dopant is capable of enhancing the stability of the structure.

As observed in Table 1, the average bond order for Mn—O goes up with Co substitution content increasing. The approximate increment of bond order from λ-Mn₄O₈ to MnCo₃O₈ corresponds to 0.06, which is just responsible for the enhancement of the structure. Though the average bond order of Co—O ranks lower than that of Mn—O in magnitude, it still plays a key role in improving the stability of the material by considering its contribution to lift Mn—O bond order up.

3.4 Total and partial electron density of state

For a typical oxide with octahedrally coordinated transition-metal, d orbitals are not equivalent on the scale of energy due to the crystal field splitting effect^[1]

(Fig.2). The two orbitals are of higher energy and lie above the Fermi level extending over from 0 to 3.5 eV in the DOS curve (Fig.3) and they are the main component parts of antibonding. The two e_g* can be easily distinguished from the band structure (Fig.2). The bonding region is mainly composed of the O-p orbital, which is generally intermixed with metal-s, metal-p orbitals and metal-d(Fig.2). The range from -19.5 eV to -1.5 eV is regarded as the bonding region. And the extent from -1.5 eV to 0 has a distinctive mark of non-bonding. The structural stability can essentially be determined by the bonding population. The following parts will elaborate on such correlations between the bonding population and structure.

Fig.2 Schematic of band structure expected for oxides with octahedrally coordinated transition-metal cations

Fig.3 DOS and PDOS of primitive cell(Dashed line indicates Fermi level)

The first region involved in bonding part in DOS resides is between -19.5 eV and -17.5 eV and its major part is constituted by metal-s orbital with a very slight metal-d orbital(Figs.2 and 3). It is inevitable that the metal-s and metal-d should only lead to the formation of σ overlap according to the orbital theory^[18]. The difficulty is that we can’t decide the intensity of σ overlap because the patterns in DOS along this region keep almost the same. Therefore, we have to examine them carefully by comparing the respective orbital integrals in Table 3. It is evidently seen that the amount of electrons around Mn-s orbital becomes larger with Co dopant increasing and the net gains of electrons on Mn-s amount to 0.04 e from λ-Mn₄O₈ to MnCo₃O₈. The difference is likely due to the varying of Co substitution in the material. It is believed that the build-up of electrons along this bonding region contributes to strengthened σ overlap and leads to a more stable structure.

Table 3 Integral of different orbitals indicating total amount of electrons below Fermi level

Another region extending from -6.5 eV to -1.5 eV in DOS is a bonding region as well, which consists predominantly of O-p orbital hybridized partially with metal-d orbital(Fig.3). Table 3 shows that the electrons concentrated on the O-p orbital are about 4.51 e in the λ-Mn₄O₈, and it becomes larger when Mn is continuously replaced with Co, arriving at 4.59 e in the MnCo₃O₄. Similarly, the charge transfer to O atoms is of great benefit to strongly bind the molecule together.

The DOS peaks around the Fermi level, covering the energy range from -1.5 eV to 0 eV, are mainly made up of the metal-3d orbitals but still partially mixed with the O-p orbital(Fig.3). The bond along this region is characterized as a non-bonding type^[1]. It is apparently seen from Fig.3 the DOS peak through the Fermi level shifts down as Mn is replaced. Such a shift, which is probably caused by the lower Co-d states in energy, implies that part of the electrons in the anti-bonding region is transferred to the non-bonding one. As a result, the repulsive force exerted by anti-bonding region will be expected to be weaker, which is easily balanced by the force of attraction from bonding region.

4 Conclusions

1) The mechanism of stability of Co-doped Li_xMn₂O₄ (x=0) was investigated by the first-principle plane-wave pseudopotential method. The calculated results show that Co doping plays an important role in modifying the total energy, bond population and electronic structures of λ-MnO₂ and results in a more stable structure of Li_x Mn₂O₄ cathode at the end of its charging.

2) The total energy can be reduced considerably and the formation enthalpy is decreased, which indicates that the stability is strengthened due to the Co substitution.

3) Bond length and volume tend to decrease with the content of Co dopant increasing; whereas the bond order of Mn-O increases.

4) The density of states of Mn_{1- x}Co_xO₂ reveals that more electrons are transferred in the two binding region and the antibonding region eclipses due to the peak of states around Fermi level shift toward the lower energy. 

5) Co doping enhances the stability of λ-MnO₂ host materials.

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Foundation item: Project(20376086) supported by National Natural Science Foundation of China

Received date: 2006-05-21; Accepted date: 2006-07-27

Corresponding author: HUANG Ke-long, Professor; Tel: +86-731-8879850; E-mail: klhuang@mail.csu.edu.cn

 (Edited by YANG Bing)