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C: Energy Conversion and Storage; Energy and Charge Transport 2
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Unveiling the Role of Oxygen Vacancy in LiMnO upon Delithiation Fei Li, Xiao-Hua Zhang, Jianyan Lin, Jiani Ma, Shoutao Zhang, and Guochun Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b07070 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019
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The Journal of Physical Chemistry
Unveiling the Role of Oxygen Vacancy in Li2MnO3 upon Delithiation Fei Li‡, Xiaohua Zhang‡, Jianyan Lin, Jiani Ma, Shoutao Zhang and Guochun Yang* Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China
Abstract: Li2MnO3 is one of the most promising electrode materials due to its high energy density. Oxygen vacancy (VO) inevitably appears in Li2MnO3 in the preparation and/or charging process. Thus far, the role of VO in redox chemistry or structural deterioration remains elusive or even controversial. Here, we study Li extraction energy, Mn migration dynamics, and structural evolution of Li2MnO3 with or without VO at the delithiation states of Li1.5MnO3, LiMnO3 and Li0.5MnO3 via first-principles calculations. With respect to the perfect crystal, VO, inducing charge compensation, enhances the electrochemical activity in the range of the considered Li content. On the other hand, VO facilitates Mn migration and causes the structural transformation at the delithiation states of LiMnO3 and Li0.5MnO3. This can be attributed to the destruction of the local structure of Mn ion induced by VO. Notably, we propose that V doping can effectively inhibit the structural phase transition, originating from the feature that V transfers more electron to oxygen and strengthens the chemical bonding of the local structure. Our research unveils the role of VO in structural transformation during delithiation, and provides an effective strategy for boosting its performance.
1. Introduction Electrode material with a high capacity is of great importance to the realization of high-energy-density lithium-ion batteries (LIBs).1–7 Li-excess layered Li2MnO3 has a theoretical capacity of 459 mAh/g,8 which is much higher than those of conventional electrode materials (e.g. 274 mAh/g for LiCoO2,9 170 mAh/g for LiFePO4,10 and 148 mAh/g for LiMn2O411). On the other hand, its operating voltage is high (~4.4 V).8 As a consequence, Li2MnO3 has become one of the most promising electrode materials in LIBs.12,13 Unfortunately, the battery made from Li2MnO3 suffers from structural fading and performance deterioration during cycling.14–17 Extensive efforts have been made to explore the origin and mechanism behind the undesirable performance of Li2MnO318–23 Some research results provide valuable clues for further optimizing the battery performance.13,24–26 In parallel, structural modification27–29 or elemental substitution26,30–33 have
been applied to Li2MnO3 to strengthen its structural stability, and improve its electrochemical 34–37 performance. Thus far, one of the most recognized mechanisms is that the oxidation of oxygen anion is responsible for the charge compensation in the Li extraction process.38,39 At the same time, oxygen evolution causes the breakage/formation of Mn-O bond, and the layered-to-spinel structural transition.23 This has been confirmed by the observed oxygen gas release and X-ray diffraction spectra.40,41 Oxygen vacancy (VO) usually presents in Li2MnO3 originating from imperfect crystal growth.42–44 On the other hand, delithiation is in favor of the formation of VO.45 Thus, understanding the effect of VO on the structural transition (i.e. from layered to spinel structure) is rather important. Unfortunately, this remains elusive or even controversial. In more detail, Lim et al. reported that VO hinders the structural transformation of Li2MnO3 in view of the increased kinetic barrier of Mn migration, and improves the 1
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reversible capacity of Li2MnO3.45 On the other hand, Chen’s group demonstrated that VO facilitates structural transformation of Li2MnO3 (i.e. Mn migration into Li layer) in the case of Li extraction nearest to Mn ion at the temperature of 3000 K through first-principles molecular dynamics simulations.23 Moreover, Okamoto found that the Li extraction energy gradually decreases and the charge compensation contributed by Mn increases as the ratio of VO increases.46 The presence of these differences can be attributed to the research perspective and condition. In fact, the Li extraction process in Li2MnO3 with VO (VO-Li2MnO3) is rather complex, accompanied by oxygen dimerization47–49 and the redox reaction of Mn and O ions.46 On the other hand, the role of VO in the charging process closely correlates with the content of delithiation. In this work, first-principles calculations are employed to explore the effect of VO on the Li extraction energy, the kinetic barrier of Mn migration, and the structural transformation as a function of Li content (e.g. Li1.5MnO3, LiMnO3 and Li0.5MnO3), associated with the oxygen dimerization. For comparison, Li2MnO3 is also involved. The involvement of VO is in favour of Li extraction, enhancing the electrochemical activity. On the other hand, the effect of VO on structural transformation and oxygen dimerization depends on the content of Li extraction. Our work not only resolves the dispute in the effect of VO on the phase transition of Li2MnO3, but also provides an opportunity for fully understanding the role of VO in Li2MnO3 upon charging. 2. Computational details The structural relaxations and total energy calculations are carried out in the framework of density functional theory (DFT)50,51 within the generalized gradient approximation (GGA)52 using the Vienna Ab Initio Simulation Package (VASP).53,54 Here, projector augmented wave (PAW)55 pseudopotentials are used to describe the electron-ion interaction, and the generalized gradient approximation (GGA) parameterized by Perdew and Wang (PW91) is employed for electron exchange-correlation
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functional.55,56 The valence configurations of Li, O, and Mn elements are 1s22s1, 2s22p4, and 3d64s1. Here, a 2 × 2 × 2 supercell containing 192 atoms is used, as shown in Figure 1. The cutoff energy of 450 eV and appropriate Monkhorst-Pack k-meshes57 with spacing of 2π × 0.05 Å-1 are adopted. The Hubbard U correction, with a U value of 5.0 eV for Mn,58 is applied to describe the strong correlation effect. Its effectiveness has been validated by several previous works.38,58,59 The energetically favorable site of VO is determined by calculating the defect formation energy of various O positions in Li2MnO3. Here, three delithiation states with or without VO (e.g. Li1.5MnO3, LiMnO3 and Li0.5MnO3) are fully considered. For Li1.5MnO3 and LiMnO3, we used the stable configurations built by Kristin A. Persson.60 For Li0.5MnO3, the stable structure is constructed after fully considering all possible combinations. Nudged elastic band (NEB) method is used to calculate the energy barriers of Mn ion migration.61–64 The occurrence of structural transformation (i.e. Mn ion migration into Li layer) in Li2MnO3 depends on the two important quantities: reaction enthalpy and reaction energy barrier upon the structural transformation. In general, a negative enthalpy and a small energy barrier (
