Milling effect on the local structure, site occupation, and site migration in aluminum substituted lithium manganese oxides
Introduction
Lithium manganese spinel, LiMn2O4, is one of promising candidates for electrode materials of lithium rechargeable batteries. It has a high energy capacity per unit weight, is low cost and non-toxicity, hence it is environmentally friendly. It is expected as a replaceable electrode material for a commercial material such as LiCoO2. In this spinel compound and substituted spinel systems, however, some problems, for instance, relatively low capacity (~120 mA h/g) and fading upon cycling need to be improved for application to commercial electrode materials [[1], [2], [3], [4], [5], [6], [7]]. The crystal structure of LiMn2O4, belongs to a cubic system with space group , in which Li+, Mn3+/4+, and O2− ions are located in the tetrahedral 8a, octahedral 16d, and 32e sites, respectively, as shown in Fig. 1. A Li+ ion migrates between the 8a sites via the 16c interstice, which is located in the unoccupied octahedral position neighboring tetrahedral Li 8a one [[8], [9], [10], [11]].
Electrical conductivity studies have revealed that LiMn2O4 is a small polaron semiconductor because of unpaired eg electrons on Mn3+ sites, which are trapped in local lattice relaxation sites [[12], [13], [14], [15], [16]]. Electrical conductivity is 1.9 × 10−5 S/cm at room temperature and an activation energy is 0.16 eV from analyzing electron hopping mechanism between Mn3+ and Mn4+ ions [15]. Chemical diffusion coefficient of Li in LiMn2O4 is represented as D = 2.5 × 10−5exp(−0.26 / kBT), and a main contribution to the electrical conduction in LiMn2O4 is not a Li+ ion conductivity, but a small polaron conductivity [16].
The Jahn-Teller distortion arising from a Mn3+ (t2g3eg1) state is closely related to capacity fade of a stoichiometric LiMn2O4, because it directly disturbs the Li+ ionic conduction pathway. Therefore, substitution effect for the typical Jahn-Teller active Mn3+ ion has been widely studied to suppress the Jahn-Teller distortion in LiMn2O4 [[17], [18], [19], [20]]. Substitution for the Mn3+ ion effectively degrades the Jahn-Teller interaction and reinforces the chemical bonding between transition-metals and oxygen ions [21, 22]. On the other hand, it is also reported that such a doping often causes a degradation of the initial discharge capacity [23, 24]. Therefore, it is important to understand changes in local structure and Li+ ion migration in the impurity doped LiMn2O4 system.
NMR technique is a powerful tool to probe local structure around observed nucleus and electronic state of ions. So far, NMR studies on local structure have been performed; a 6Li NMR peak position in LixMn2O4 sensitively shifts with a decrease in Li content, and a new peak, which is assigned to Li+ ion occupying the 8a site in a Mn4+ rich phase, appears at around 950 ppm for x < 0.3 [25]. A single resonance peak is dominant in a normal cubic phase in LiMn2O4 prepared at 850 °C, while several peaks associated with Mn4+ are observed in the NMR measurement and no evidence for the Jahn-Teller distortion is observed in a sample containing considerable disorder prepared at relatively low temperature [26]. Doped Al ions occupy not only a six-fold coordinated Mn site, but also a four-fold coordinated Mn site in the Al doped LiMn2O4. Analysis of 27Al NMR spectrum has revealed that a ratio of distribution for six-fold and four-fold Al is 85%:15% [19]. In addition, some insights in the Li ion dynamics have been probed with regard to spinel systems; a 2D 7Li-NMR measurement reveals the Li+ ion exchange between the 8a and the 16c site in LiMn2O4 with an activation energy of 0.5 eV [27]. In cubic spinel Li4Ti5O12, ultraslow Li diffusion, which is dependent on the Li transport length scale, is observed in the spin–lattice relaxation measurement [28].
The ball-milling is extensively utilized to induce the disorder such as defects and dislocation in solids by a confliction energy between small hard balls and particles of sample materials. The structural disorder induced in crystal structure would affect a stability of crystal structure and bonding to surrounding ions. Indeed, they would cause changes in an activation energy and conducting behaviors in some dielectric lithium oxides [29, 30]. It is anticipated that such a structural change stimulates the Li+ ion motion and leads to an enhancement of electrical conduction in the Al doped LiMn2O4.
In this paper, we discuss milling effect on structural change, site migration, and electrical conductive behavior in LiMn2−xAlxO4 (x = 0, 0.05) based on results of 7Li and 27Al MAS NMR and electrical resistivity measurements.
Section snippets
Experimental
Al substituted LiMn2O4 was prepared by solid state reaction method. The mixture of powdered Li2CO3, MnO2, and Al(OH)3 weighted in a molar ratio of Li:Mn:Al = 1:2 − x:x was mechanically mixed for 40 min and was sintered at 750 °C for 72 h after sintering at 350 °C for 4 h in air. Finally, we obtained polycrystalline samples of LiMn2O4 (abbreviated as LMO) and LiMn1.95Al0.05O4 (abbreviated as LMA5). The obtained samples were milled with ethanol on a Fritsch planetary ball milling machine P-6 for
Crystal structure and valence of Mn
The reflection peaks in XRD patterns of the milled LMO and LMA5 became broader with an increase in milling time and were mainly identified as a cubic spinel structure with the space group of as shown in Fig. 2(a) and (b). The split peaks at around 64° as shown in insets of Fig. 2(a) and (b), consisting of reflection peaks from the cubic spinel phase and tetragonal phases (represented respectively by an asterisk and an open triangle), were observed in LMA5 as well as LMO [31]. The lattice
Discussion
The Al substitution and milling locally caused the lattice distortion, changes of the electronic state of Mn, and disturbance of conduction pathway. They are likely to affect the electrical conducting behaviors.
As shown previously, the chemical diffusion coefficient of Li+ ion in spinel LiMn2O4 is D = 2.5 × 10−5 exp (−0.26/kBT) cm2/s [16] and the conduction process for Li+ ion hopping contributes to 15% [36]. Electrical conductivity obtained is about 10−6 S/cm and is smaller by one order of
Summary
The milling process caused local structural disorder in LMO and LMA5 as observed in changes of the lattice parameter and the activation energy. The analysis of electrical resistivity on the basis of the simple Debye model indicated that the hopping time and the activation energy of charge carrier increased with an increase in milling time. The increase of the Mn4+ species caused by milling resulted in the decrease of Peff and degradation of antiferromagnetic correlation in LMA5. Such a valence
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research (C) (No.21560699) from the Japan Society for the Promotion of Science. This work also was partially performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”, Japan.
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