Generating Oxygen Vacancies in MnO Hexagonal Sheets for Ultralong

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Generating Oxygen Vacancies in MnO Hexagonal Sheets for Ultralong Life Lithium Storage with High Capacity Yihui Zou,† Wei Zhang,† Ning Chen,‡ Shuai Chen,§ Wenjia Xu,† Rongsheng Cai,∥ Christopher L. Brown,⊥ Dongjiang Yang,*,†,# and Xiangdong Yao*,#,⊗ Downloaded via UNIV OF EDINBURGH on January 29, 2019 at 08:43:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

State Key Laboratory of Bio-fibers and Eco-textiles, Collaborative Innovation Center of Marine Biobased Fibers and Ecological Textiles, Institute of Marine Bio-based Materials, Qingdao University, Qingdao 266071, P.R. China ‡ Canadian Light Source, Saskatoon S7N 0X4, Canada § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, 27 Taoyuan South Road, Taiyuan 030001, P.R. China ∥ Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, U.K. ⊥ Environmental Futures Research Institute, #Queensland Micro- and Nanotechnology Centre (QMNC), Griffith University, Nathan Campus, 170 Kessels Road, Brisbane, Queensland 4111, Australia ⊗ Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Department of Chemistry, Jilin University, Changchun 130023, P.R. China S Supporting Information *

ABSTRACT: The polar surface of (001) wurtzite-structured MnO possesses substantial electrostatic instabilities that facilitate a wurtzite to graphene-like sheet transformation during the lithiation/delithiation process when used in battery technologies. This transformation results in cycle instability and loss of cell efficiency. In this work, we synthesized MnO hexagonal sheets (HSs) possessing abundant oxygen vacancy defects (MnO-Vo HSs) by pyrolyzing and reducing MnCO3 HSs under an atmosphere of Ar/H2. The oxygen vacancies (Vos) were generated in the reduction process and have been characterized using a range of techniques: X-ray absorption fine structure, electron-spin resonance, X-ray absorption near edge structure, Artemis modeling, and R space Feff modeling. The data arising from these analyses inform us that the introduction of one Vo defect within each O atom layer can reduce the charge density by 3.2 × 10−19 C, balancing the internal nonzero dipole moment and rendering the wurtzite structure more stable, so inhibiting the change to a graphene-like structure. Density function theory calculations demonstrate that the incorporation of Vos sites significantly improves the charge accumulation around Li atoms and increases Li+ adsorption energies (−2.720 eV). When used as an anode material for lithium ion batteries, the MnO-Vo HSs exhibit high specific capacity (1228.3 mAh g−1 at 0.1 A g−1) and excellent cell cycling stabilities (∼88.1% capacity retention after 1000 continuous charge/discharge cycles at 1.0 A g−1). KEYWORDS: MnO-Vo, hexagonal sheets, oxygen vacancy, lithium ion batteries, DFT

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from its small intrinsic conductivity and internal structural reorganizations during the lithiation/delithiation process.6,12 Recently, tremendous efforts have been made in an attempt to circumvent these drawbacks, and so far, two strategies have shown potential. The first of these is the assembly of hybrid

he utilization of readily available and low-cost transition-metal oxides (TMOs) possessing high theoretical capacities as anode-electrode materials in lithium-ion batteries (LIBs) has attracted a great deal of attention of late.1−5 Among them, MnO exhibits high theoretical energy storage of 756 mAh g−1, high density (5.43 g cm−3), weak voltage hysteresis (2 h of hydrogenation presumably arise due to a plateau in defect number for all samples with 2 h or more under reducing conditions. The SEM and TEM images of MnO-Vo HSs and MnO HSs after 500 charge and discharge cycles are shown in Figures S9 and S10, respectively. It is interesting to note that while the MnO-Vo HS sample still exhibits a HS morphology, the structure of the MnO HS sample has been destroyed. Obviously, the presence of Vos at the electrode/ electrolyte interface facilitates and stabilizes lattice changes during Li+ deintercalation using modified surface thermodynamics that preserve the integrity of the electrode surface morphology. The rate performance was further investigated at different cycling rates (Figure 4d) in which the current density was from 0.1 to 2.0 A g−1 in a stepwise manner before returning to 0.1 A g−1. Pleasingly, the MnO-Vo HSs system exhibited superior rate characteristics compared to MnO HSs. For example, the defected MnO-Vo HSs-2h assembly delivered stable capacities of 1228.3, 1174.8, 979.4, 812.3, and 607.3 mA h g−1 from 0.1 to 2.0 A g−1. On cycling back to 0.1 A g−1, the specific capacity recovered to 1293.6 mA h g−1. In contrast, the control MnO HSs system exhibited poor rate capabilities, namely 464.9, 355.2, 266.8, 172.6, and 93.8 mA h g−1 from 0.1 to 2.0 A g−1. Accordingly, we propose that the superior cycle and rate performance of the MnO-Vo HSs are highly related to the incorporation of Vos, which both improves the conductivity and facilitates the Li+ insertion−desertion pathways into the various layers within MnO. The stability of the MnO-Vo HSs-2h system over 1000 cycles at 1.0 A g−1 is displayed in Figure 5a with the reversible capacity remaining at 825.2 mAh g−1. While the initial CE is around

75.0%, it increases and settles close to 100% after only 11 cycles, suggesting the development of highly reversible Li+ insertion/ extraction kinetics. The performance of the systems described in this work is far superior to that reported for MnO/C electrodes so far.14−17,33−38 In order to probe the effects that defects have on the masstransfer process within MnO HSs electrodes, the EIS measurement was performed from 100 kHz to 0.01 Hz. This is illustrated in Figure 5b, where a semicircular plot occupies the highfrequency domain being consistent with charge-transfer resistance.40 Of note is that MnO-Vo HSs-2h has a smaller diameter semicircular profile than that of MnO HSs, indicating the fast charge transfer process resulting from the Vos. Information on the apparent Li+ ion diffusion coefficient (DLi+) can be obtained according to the equation as follows:41 DLi + = R2T 2/2A2 n 4F 4C 2σ 2

In addition, Z′ ∝σω−1/2, in which σ is the Warbug factor and is shown in Figure 5c. Accordingly, the DLi+ values of MnO HSs and MnO-Vo HSs-2h were calculated to be 2.29 × 10−14 and 3.24 × 10−13 cm2 s−1, respectively, indicating that the Vos are favorable for improving the Li+ ion diffusion by a factor of about 10. To model the effect of Vos on Li+ kinetics, DFT was used to simulate the adsorption sites for Li+ on a MnO surface and calculated the corresponding adsorption energies based on the atomic structure model of MnO and MnO-Vo with a single layer height (Figure 6a,b) (see the computational details in the Supporting Information). These simulations suggest that the introduction of each oxygen defect can reduce the quantity of electric charge by −3.2 × 10−19 C at each O atom. This change in G

DOI: 10.1021/acsnano.8b08608 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano

Preparation of the MnO-Vo HSs. MnO-Vo HSs were prepared by a two-step procedure. Step one: The MnCO3 HS precursor was synthesized by the following process: SA (0.4 g) was dissolved in an aqueous solution of Li2CO3 and KMnO4 (total of 70 mL of 0.06 mol L−1 0.02 mol L−1 respectively) with vigorous magnetic stirring. Once dissolved, the resulting solution was poured into a sealed 100 mL Teflon lined cup with heat treatment at 140 °C for 14 h and then cooled to room temperature. The resulting precipitates were obtained via centrifugation, which was also purified with deionized water and ethanol and dried at 60 °C for 8 h to obtain MnCO3 HSs. Step 2: The products were calcined at 550 °C under an Ar/H2 (0.95/0.5) atmosphere for 1, 2, 3, and 5 h, respectively. The final products were denoted as MnO-Vo HSs-1h, MnO-Vo HSs-2h, MnO-Vo HSs-3h, and MnO-Vo HSs-5h, respectively. For comparison, the MnO HSs were prepared by calcinations under Ar atmosphere at 550 °C. Characterization. The XRD, TEM, TEM, HRTEM, STEM, XPS, Raman, BET, TGA, ESR, and XANES were performed to characterize the as-prepared materials. The test details can be found in our previous work.18,19 LIB Measurements. The process of the assembling and testing MnO-Vo HSs and MnO HSs as anode material for LIBs can be found in our previous work.6 The loading amount of active material was ∼0.7 mg cm−2.

surface dipole compensates the internal nonzero dipole moment, electrostatically stabilizing the wurtzite structure (Figure 6b). To model the adsorption behavior of Li+ on the MnO and MnO-Vo surfaces, we proposed three potential adsorption sites for Li+ on the MnO surface namely, (1) top sites (Figure 6c,d), (2) bridge site-1 (Figure 6e,f), and (3) bridge site2 sites (Figure 6g,h).42 The adsorption energy is defined as Ead = EMnOLi − EMnO − ELi, and the results from our calculations are shown in Table S2. The calculated adsorption energies are −2.267 eV for MnO and −2.720 eV for MnO-Vos on top sites, −1.597 eV for MnO and −2.337 eV for MnO-Vos on bridge site1, and −1.576 eV for MnO and −1.599 eV for MnO-Vos on bridge site-2, respectively. The data shows that the absorption energies of the MnO-Vo system are all larger than those of undefected MnO for all the three adsorption sites, supporting the hypothesis that Vos are favorable for Li+ adsorption on MnO. In addition, it is the “top site” that is the most preferable adsorption point for both MnO and MnO-Vo, while bridge site2 makes the minimum contribution to the adsorption process. To further compare and contrast the electronic structures of MnO both with, or without, Vos, charge density differences of Li+ adsorption on three different adsorption sites of MnO and defected MnO have been calculated (Figure 6c−h). Charge accumulation (yellow) and charge depletion (cyan) reveals that charge is transferred from the O atom to adsorbed Li+. Of note is that the charge accumulation region around Li+ on the surface of MnO is smaller than that of the defected MnO-Vos system for both “top site” and “bridge site-1”, indicating that Li+ adsorption is easier for MnO-Vos than MnO. The calculated adsorption energies also suggest that the contribution to the adsorption process by “bridge site-2” is insignificant when compared to other sites (calculated values of −1.576 eV for MnO and −1.599 eV for MnO-Vos). On the basis of these data, it can be proposed that the presence of oxygen defects (Vos) on the surface of MnO play a pivotal role in electronic structure tuning and accelerating Li+ transport processes.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b08608. Additional, TEM, SEM, XRD, TGA, BET, and electrochemical characterizations and additional tables (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: x.yao@griffith.edu.au. ORCID

Xiangdong Yao: 0000-0002-1235-5090 Notes

The authors declare no competing financial interest.

CONCLUSIONS Reduction and pyrolysis of MnCO3 hexagonal sheets facilitates the introduction of electrostatically stabilizing oxygen defect sites (Vos) into MnO, forming MnO-Vo HSs hexagonal sheets. This material displays excellent ion-transport rate performance and cell-cycling stability in LIBs. These enhancements can be attributed to the existence of Vos, which enhance electric structure and positively compensate for destabilizing internal nonzero dipole moments in the nondefected material. This study represents a successful application of Vos beyond MnO and could widen the application potential of cheap, abundant Mn-based oxide anode materials in renewable energy storage and conversion. Furthermore, ease of assembly of these materials opens up approaches to enhance existing materials that may have applications in more efficient and durable anode materials in battery and other technologies.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 51503109, 21501105, and 51473081), Natural Science in Shandong Province (ZR2018BEM002), Taishan Scholars Program, Outstanding Youth of Natural Science in Shandong Province (JQ201713), ARC Discovery Project (No. 130104759), and the Key Research and Development Program of Shandong Province (No. 2017GSF18128). REFERENCES (1) Dong, S.; Chen, X.; Gu, L.; Zhou, X. H.; Li, L. F.; Liu, Z. H.; Han, P. X.; Xu, H. X.; Yao, J. H.; Wang, H. B.; Zhang, X. Y.; Shang, C. Q.; Cui, G. L.; Chen, L. Q. One Dimensional MnO2/Titanium Nitride Nanotube Coaxial Arrays for High Performance Electrochemical Capacitive Energy Storage. Energy Environ. Sci. 2011, 4, 3502−3508. (2) Latham, A. H.; Wilson, M. J.; Schiffer, P.; Williams, M. E. TEMInduced Structural Evolution in Amorphous Fe Oxide Nanoparticles. J. Am. Chem. Soc. 2006, 128, 12632−12633. (3) Lv, C.; Yang, X.; Umar, A.; Xia, Y.; Jia, Y. A.; Shang, L.; Zhang, T.; Yang, D. Architecture-Controlled Synthesis of MxOy (M= Ni, Fe, Cu) Microfibres from Seaweed Biomass for High-Performance Lithium Ion Battery Anodes. J. Mater. Chem. A 2015, 3, 22708−22715. (4) Tabassum, H.; Zou, R.; Mahmood, A.; Liang, Z.; Wang, Q.; Zhang, H.; Gao, S.; Qu, C.; Guo, W.; Guo, S. A Universal Strategy for

EXPERIMENTAL SECTION Chemicals. Sodium alginate (SA) was provided by the Shanghai Aladdin Bio-Chem. Technology Co. (China). Lithium carbonate (Li2CO3) was from Sinopharm Chemical Reagent Co., Ltd. Mineral chameleon (KMnO4) was purchased from Sanhe Chemical Reagent Co., Ltd. Deionized (DI) water (18.2 MΩ) was used as a solvent in the synthesis. All of the reagents in the experiments were analytical grade and used without further purification. H

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DOI: 10.1021/acsnano.8b08608 ACS Nano XXXX, XXX, XXX−XXX