Largely Increased Lithium Storage Ability of Mangnese Oxide through

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Largely Increased Lithium Storage Ability of Mangnese Oxide through Continuously Electronic Structure Modulation and Elevated Capacitive Contribution Shifei Huang, Da Tie, Miao Wang, Bo Wang, Peng Jia, Qingjie Wang, Guoliang Chang, Jiujun Zhang, and Yufeng Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04258 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Sustainable Chemistry & Engineering

Largely Increased Lithium Storage Ability of Mangnese Oxide through Continuously Electronic Structure Modulation and Elevated Capacitive Contribution Shifei Huang1, Da Tie1, Miao Wang1, Bo Wang1, Peng Jia1, Qingjie Wang2, Guoliang Chang3, Jiujun Zhang3, Yufeng Zhao1,2* 1Key

Laboratory of Applied Chemistry, State Key Laboratory of Metastable Materials Science and

Technology, Yanshan University, West Hebei Street No. 438, 2State

Qinhuangdao 066004, China;

Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources Co.

Ltd. Zunyi, Guizhou 3Institute

563003, China

for Sustainable Energy /College of Sciences, Shangda Road 99, Shanghai University,

Shanghai 200444, China; Corresponding email:[email protected]

Abstract Ultrathin MnO2 sheets assembled three dimentional flower microsphere grown on nitrogen doped graphene is synthesized through a hydrothermal treatment method. When tested as an anode in a lithium-ion battery, the obtained material exhibits a high discharge capacity of 993 mAh g-1 in the second cycle at 0.1 A g-1, which goes up to 2243 mAh g-1 gradually after 135 charge/discharge cycles. This phenomenon turns out to be related to the deep coupling between nitrogen doped graphene and MnO2 caused by morphology evolution of the composite upon cycling. And, kinetics analysis reveals the elevated capacitive contribution after cyclic reaction, indicating the ever enhanced phase interface charge storage mechanism associated with the morphology evolution. Otherwise, the first-principles calculations also indicate the

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electronic structure of MnO2 can be efficiently modulcated by coupling with conductive graphene substrate, through covalent C-Mn or N-Mn bond, thus the deep coupling between nitrogen doped graphene and MnO2 during the charge/discharge process will gradually promoting elevated charge mobility and charge storage ability. This work provides a novel insight from the atomic scale to understand the capacity rising obsession for transition metal oxides, both theoretically and experimentally. Keywords: MnO2, deep coupling, morphology evolution, kinetics analysis, lithium ion batteries Introduction TMO materials has attracted intensive attention for electrochemical energy storage technologies such as lithium ion batteries (LIBs). As abranch of TMOs, manganese oxides have been branded as very promising battery anode materials for LIBs owing to their high theoretical capacity, narrow voltage hysteresis, environmental friendly, inexpensive and abundant material supply.1-2 However, pristine α-MnO2 shows the typical feature of low intrinstic electrical conductivity.3 Generally, due to the existence of polarization, the theoretical capacity of pristine α-MnO2 is far from being reached and the rate performance is greatly restricted. On account of this, nano-sizing the material to shorten the ion transport pathway or coupling with electric conducting materials to improve the electronic conductivity, have become effective strategies to boost the electrochemical performance of manganese oxides.4-7


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Moreover, as far as the present state of the study is concerned, the capacity of the manganese oxides and carbon composites usually decreases during the initial cycles and gradually increases subsequently,4, 8-11 and the sample mophology can be varied during the cycling process.4, 12 The simliar phenomena were widely observed in other transiton metal oxides (TMOs),13-17 which attracted scientists to discover the possible reasons. Nevertheless, the deficiency of dynamic information or kinetics analysis during cycling, makes it difficult to identify the accurate origin of the capacity change. Herein, we constructed an α-MnO2 microsphere assembled with ultrathin nanosheets anchored on nitrogen doped graphene (MnO2@NG) through a hydrothermal treatment method. When tested as an anode in a lithium-ion battery (LIB). The as synthesized MnO2@NG exhibits a high discharge capacity of 993 mAh g-1 in the 2-nd cycle at 0.1 A g-1, which goes up to 2243 mAh g-1 gradually after 135 cycles. The morphology evolution of MnO2@NG upon cycling is visualized by TEM, indicating significant particle size refinement of both MnO2 and NG during the cycling. The constant morphology evolution during cycling process would result in further coupling between NG and MnO2, and a corresponding electronic structure engineering of both MnO2 and NG. We thus deduce that this constant electronic structure modulation should cause continuous capacity enhancement during long-term test. Kinetics analysis indicates the elevated capacitive contribution after cyclic reaction, which should be resulted from the ever enhanced phase interface charge storage mechanism associated with the morphology evolution. Otherwise, DFT calculations were also



carried out to reveal that the covalent coupling between MnO2 and NG can strongly influence the electronic structure of the individual component, and promote fast charge transfer at MnO2/NG interface. Therefore, downsizing the component thus enabling the deep coupling between NG and MnO2, could further modulate the electronic structure, which might be responsible for the enhanced electrochemical performance upon cycling. Experimental Materials preparation Synthesis of MnO2@NG: firstly, 0.25 g of PEO-PPO-PEO (P123, triblock copolymer) was dissolved in 1mL deionized (DI) water and 3 g ethanol mixed solution. Then 0.05 g of NG was added and uniformly dispersed ultrasonically for 2 hours, forming stable NG colloid. Then 3 mM of Mn(AC)2·2H2O was added and stirred for 1 hours. After that, 2 mM of KMnO4 dissolved in 20 ml DI H2O was slowly added to the above solution. The resulted mixture was then placed into a 100 mL Teflon-lined autoclave and heated at 160 oC for 2 hours. Finally, MnO2@NG was obtaind through centrifugation, washing with ethanol 3 times and freeze drying for 3 days, respectively. Results and discussion Characterization


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The XRD patterns of the MnO2@NG (Figure 1a) show two broad peaks at 2θ= 36.7o and 65.8o with relatively low intensity, corresponding to the not well crystallized α-MnO2.4,

18

The poor crystallization of MnO2 may be due to the low reaction

temperature (160 oC) during the hydrothermal process. In addition, the broad peaks at 23.4o and 43o attributable to the (002) and (101) reflection of the NG are observed. The Raman spectra of MnO2/NG composite are shown in Figure 1b. The characteristic peaks of D-band ( ～ 1355cm-1) and G-band ( ～ 1583cm-1) can be attributed to the NG reflects. And a Mn-O (Eg band) and Mn-O-Mn (Ag band) stretching vibration band at 569 and 633 cm−1 observed in the spectrum can be attributed to MnO2 reflects,19-21 which further indicate the hybrid structure of NG and MnO2.4 As contrast, the XRD and Raman characterization of pristine α-MnO2 are also provided (Figure S1a&b). The XPS survey spectrum of the MnO2@NG in Figure 1c shows the deconvoluted peaks of Mn (Mn3p (50.1 eV), Mn3s (84.1 eV), Mn2p3/2 (642.3 eV), Mn2p1/2 (654.0 eV), MnLM1 (855.1 eV), MnLM2 (902.1 eV)), O (O1s (530.1 eV), OKL1 (975.1 eV), OKL2 (995.1 eV)), C (C1s (285.1 eV)), N (N1s (399.1 eV)).22-23 No evidence of impurities was detected indicating the high purity of the materials. The C1s peak (Figure 1d) can be deconvoluted into five characteristic peaks at 284.7 eV (C-C/C=C), 284.8 eV (C-Mn), 285.6 eV (C-N), 286.0 eV (C-O), and 288.5 eV (O-C=O), respectively.24-26 The XPS analysis also clearly demonstrates the existence of plentiful oxygen functional groups on the surface of MnO2@NG. The O1s spectrum could be deconvoluted into five peaks at around 530.7 eV, 529.8 eV, 531.4 eV and 532.1 eV and 533.4 eV (Figure S1c), corresponding to C=O, Mn-O-Mn,



Mn-O-H, C-OH and C-O-C, respectively.27 There are four peaks in the high-resolution N1s spectrum (Figure 1e), which can be deconvoluted into pyridinic N (398.3 eV), pyrrolic N (398.9 eV), N-Mn (399.7 eV) and graphitic N (401.2 eV), respectively.26, 28-29 Figure 1f shows two characteristic peaks at ~642.3 and ~654.0 eV in Mn2p spectrum, ascribed to Mn 2p3/2 and Mn 2p1/2 respectively, with the spin-energy separation of 11.7 eV, indicating the presence of Mn4+ in MnO2, which is consistent with previous works.4, 30-34 The Mn 3/2p peak located at 640.8 and 644.1 eV can be assigned to Mn3+ and Mn2+ from MnO2 in the composites.27 The XPS analysis together with the DFT calculations further confirm the covalent interaction between MnO2 and NG through C-Mn and N-Mn in MnO2@NG composite. The covalent coupling between MnO2 and NG is expected to offer stable electrode integrity and fast electron and ion transport in energy storage applications. Field emission scanning electronic microscopic (FESEM) images (Figure 2a,b) demonstrate the well defined 3D microflower structure of MnO2 assembled on the NG nanosheets. The MnO2 microsflower is composed of numerous ultrathin nanosheets with sufficient void spaces between them, which is more obvious from the loose packed edges. The corresponding energy dispersive X-ray spectroscopy (EDX) mappings confirmed the uniform spatial distribution of Mn (Figure 2c) and O (Figure 2d) elements in the ultrathin MnO2 nanosheets. The magnified images in Figure 2e indicate that the lateral size of nanosheet is much larger than its thickness which results in its bending and curling (Figure 2a,b&e). High resolution transition electron microscopic (HRTEM) image (Figure 2f) indicates the thickness of the


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MnO2 sheet is only ~1.7-3 nm. The good contact between the edges of ultrathin MnO2 nanosheets and the NG sheets can not only ensure the fast and successive transportation of electrons in MnO2@NG, but also greatly reduce the solid-state transport pathway for Li ion diffusion. Figure 2g&h are the amplified HRTEM images of area 1 and 2 in Figure 2f, whereby the (211) plane of α -MnO2 corresponding to the lattice spacing of 0.24 nm is clearly revealed. The lack of large-range atomic ordering of MnO2 indicates its poor crystalatinity, and the amorphous structure is also observed in Figure 2g, h. The crystalized structure ensures good conductivity and the amorphous structure can also increase the active lithium storage sites of the material, this would enable the material with improved capacity and rate performance in charge storage applications. The corresponding selected-area electron diffraction (SAED) pattern confirmed the lower crystallization degree of the composite, and the dispersed diffraction rings can readily be indexed to the (211), (002) planes of the NG and (211), (002) planes of MnO2, respectively (Figure 2i). As contrast, the SEM, TEM and HRTEM images of pristine α-MnO2 are also provided (Figure S2). Electrochemical Performances The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) profiles were tested to verify the lithium storage performance of as prepared MnO2@NG (Figure 3). The CV curves test at 0.1 mV s-1 in the voltage range of 0.01-3.0 V vs. Li+/Li are shown in Figure 3a. In the 1st discharge cycle, the cathodic current starts from 0.8 V should be related to the SEI film formation and the decomposition of the



electrolyte, and the cathodic peak below 0.3 V is caused by the initial reduction of MnO2 to metallic Mn：

 - MnO 2  xLi   xe   Li x MnO2

(1)

Li x MnO 2  (4  x) Li   (4  x)e   Mn  2 Li2O (2) as well as the formation of Li2O and SEI layer.9,

35

(Mn4+ to Mn2+, Mn2+ to Mn0) In the 1st anodic process, the

anodic peaks at ~1.2 and ~2.1 V, which could be attributed to the two-step electrochemical oxidation of Mn. Otherwise, the nearly overlapped peaks on the subsequent CV curves indicates that the lithiation and de-lithiation reaction are highly reversible. As shown in Figure 3b, MnO2@NG exhibits a high discharge capacities of 993 mAh g-1 in the second cycle at 0.1 A g-1, and stabilizes at 2243 mAh g-1 after 135 cycles. The initial coulombic efficiency (CE) is calculated as ~60%. Figure 3c shows the galvanostatic charge-discharge voltage profiles of MnO2@NG at the 1st, 2nd, 10th, 50th, 80th, 100th and 135th cycles at 0.1 A g-1 within the potential range of 0.01-3.0 V(vs. Li+/Li ). In the discharge curves, a plateau at ~0.4-0.45 V is observed, which can be due to the reduction from high valence state of Mn to Mn0 during the lithiation process. The corresponding charge curves show no obvious platform area but a slope from ~1.0-1.3 V and ~2.0-2.3 V due to different oxidation stages. The phenomenon of increasing capacity is consistent with the other MnO2 and TMO based materials reported previously.4, 36-38 Moreover, the rate performance of MnO2@NG at 0.1-20 A g-1 was recorded (Figure 3d), high reversible specific capacities of 106 and 76 mAh g-1 can be reached even at 10.0 and 20.0 A g-1, and the capacity could be recovered to 1016 mAh g-1 when the current density are directly reduced from 20 to


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0.1 A g-1, indicating an outstanding rate and restorability performance. The discharge-charge cycling of MnO2@NG was performed at 5 A g-1 rate for 5000 cycles (Figure 3e). The electrodes suffer from capacity fading in the first several cycles, which can be attributed to incomplete redox reaction of MnO2. Subsequently, the capacity of the MnO2@NG electrode rises step by step from the 240th to ~974th cyle. The morphology evolution of MnO2 and NG is monitored by ex-situ TEM (Figure 3f-h), which demonstrates that the MnO2 microflowers are gradually unfolded to small pieces and uniformly dispersed on the NG sheets, and enable complete contact between the metal oxide and graphene conducting agent. Thus the covalent interaction between MnO2 NG can be further enhanced, meanwhile the nanovoids and interface surface would provide more active sites for charge storage. The subsequent capacity decrease from ~974th to ~5000th cycle should be due to the collapse of the internal structure of the composite (Figure 3i). And the crystal structure change of MnO2 and NG is also detected by ex-situ XRD (Figure S3) and HRTEM and SAED (Figure S4). The broad diffraction peak of the (002) crystal plane is shifted to ~21 o at different cycles, which indicates the expanded graphene layers because of the unremittingly intercalation-extraction process of li ion. The expanded graphene layers will provide enough space for lithium-ion storage and volume expansion.39-43 And as the charging-discharging went on, the peak of manganese oxide became inconspicuous may be due to the crystal transfer into amorphous forms or the influence of a large number of SEI components (Li2CO3: Li2O reacts with CO2 in the air) forming on the surface of the electrode materials (Figure S3). Meanwhile, as the number of cycles



increase, the crystal structure becomes more disordered (Figure S4). The amorphous process of the electrode material produces alarge number of defects and pores, which will provide more active sites for the storage of li ions. However, on the one hand, the materail will gradully pulverize due to continuous volume expansion, on the other han, the continuous growth of SEI will cause the consumption of li ions in the electrolyte, which will cause the attenuation of battery performance.44-45 Comparatively, the pristine MnO2 shows no morphology evolution during cyclic process (Figure S5), and discharging capacities of 303 and 92 mA h g-1 were achieved at rates of 0.1 and 0.5 A g-1. The lower capacity and the poorer cyclability of pristine MnO2 shows in Figure S6 further confirming the significant synergistic role of MnO2 and NG for the improvement of the eletrochemical performance of MnO2@NG. Kinetics analysis In order to further understand the energy storage behavior of MnO2@NG, the lithium ion storage kinetics is systematically studied. The EIS spectra (Figure 4a) suggest the gradual shrinkage of both the charge-transfer kinetic-controlled semicircles in the high frequency region, and SEI controlled semicircles in mid-frequency region, with the increase of charge and discharge cycles. This implies a great decrease of charge transfer resistance because of the intimate contact and enhanced interaction between MnO2 with NG upon cycling, which is coincidence DFT calculation results. Otherwise, the straight line of the mass transfer-controlled Warburg region in the low frequency region gradually becomes steeper, suggesting the promoted lithium insertion/extraction kinetics of MnO2@NG electrode. Thus the EIS analysis together ACS Paragon Plus Environment

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with the DFT calculations further confirms that the deep coupling between MnO2 and NG during cycling offers elevated charge transport ability. On the other hand, the discharge curves at the cycle number of 2nd, 88th and 135th cycles of MnO2@NG are illustrated in Figure 4b. It can be calculated that the discharge capacity corresponding to the platform area (II) is about 300 mAh g-1 under different cycles; while the capacity contribution up and below the platform potential shows a marked increase by the number of cycles. This should be attributed to that the continuous particle refining during the charge-discharge process results in more and more active surfaces and phase interfaces with plenty of holes, vacancies or free electrons.46 Therefore, the charge storage through surface adsorption (area I (which contains surface adsorption and the two-step electrochemical oxidation of manganese process)) or nanovoids filling (area III (which contains nanovoids filling and conversion reaction into Mn and Li2O phases process)) are intensively boosted with the morphology evolution.4-5. The capacitive contribution of the MnO2@NG is calculated by using the power-law formula (Equation S1)38, 47 through analyzing the CV curves obtained at different scan rates (Figure 4c & Figure S7a-b). The dynamic analysis results show that the b values for the cathodic (c)\anodic (a) peaks of the fresh and the 88th cells were calculated to be 0.68\0.78 and 0.74\0.87, respectively, indicating a favored capacitive kinetics of MnO2@NG for lithium ion storage. That is to say, MnO2@NG reveal a hybrid charge storage mechanism. The ratio of capacitive contribution can be obtained through the formula (Equation S2),48-50 the fresh and the 88th cells exhibits a 44%, 49%, 54%, 58%, 61% and 51%, 54%, 63%, 68%, 70% for capacitive



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contribution at the scan rate of 0.2 to 1 mV s−1, respectively (Figure 4d-e and Figure S7c-f). Thus, the capacity of the MnO2@NG is connected with the improved interfacial li storage. In other words, the MnO2/NG interface in the composite provides a more feasible Li+ ion transport channel and facilitates the pseudocapacitive behavior of Li+ in the MnO2@NG electrode. This feature is conduced to the rapid transfer of Li+, resulting in long cycle life and enhanced reversible capacity. DFT calculations The DFT calculations were performed using the CASTEP module to understand the Li

storage

behavior

of

MnO2/carbon

composite.51-54

The

electron

exchange-correlation interaction was setup using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) parameterization.55 The Monkhorst-Pack56 k-point sampling of a 3×3×1 grid to integrate over the Brillouin zone for the cell and a plane-wave cutoff energy of 550 eV were employed for the plane waves. The structures were optimized until the energy and the force were converged to 1.0×10-5 eV/atom and 0.03 eV/Å, respectively. A vacuum space as large as 15 Å was used along the c direction normal to the substrate to avoid periodic interactions. To promote the interaction between MnO2 and carbon, we use N-doped graphene in the model to make sure the formation of strong covalent bond between MnO2 and carbon, which is denoted as MnO2@NG. The molecular structural model of NG, MnO2 and MnO2@NG were constructed as shown in (Figure 5a and Figure S8). The bond length of the C-Mn is 2.237 Å and it is 1.954 Å for the N-Mn. These values are comparable to those in other Mn-containg componds, illustrating the strong


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interaction between MnO2 and NG sheet. The charge density difference between the MnO2@NG system with (Equation 3)57 was calculated (Figure 5b (front view) and Figure 5c (side view) ). ∆ρ = ρ[MnO2@NG] ―ρ[MnO2] ―ρ[NG]

(3)

Large degree of charge transfer between Mn and the adjacent N/C atoms can be observed, which means strong electronic interaction at the interface of MnO2/NG. This would effectively strengthen the affiliation between the MnO2 nanoparticles and the NG sheets. Actually, pristine α-MnO2 is an insulator with band gap of 0.7 eV.3 However, the MnO2 in MnO2@NG displays conduction characteristics with overlapped valance and conduction bands (Figure 5d). Thus, coupling MnO2 with NG enables electron transfer from the highly conductive NG to MnO2, leading to electron-enrichment on the MnO2 side, which can ensure uniform charge distribution in the electrode material. The localized electron accumulation leads to the down-shift of the valance and conduction bands of MnO2@NG due to the enhanced electron mobility,

thereby

improving

the

electrical

conductivity

properties.

These

investigations directly evidence that, the electronic structures of MnO2 can be significantly modified by coupling with NG.5 Furthermore, the projected density of states (DOS) show increased electronic states near the Fermi level in MnO2@NG (Figure 5e), suggesting that more charge carriers can be introduced by the N-doped graphene framework, thereby enhancing the conductivity of the basal MnO2 layer. The continuous distributions of the p-orbits of N and C and their overlap with that of oxygen near Fermi level indicates the covalent interactions between the NG and



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MnO2, which can promote enhanced charge transfer in the MnO2/NG interface of the MnO2@NG hybrid structure.5 This covalent interaction would be reinforced by further downsizing the NG and MnO2 during long cycle test, thus improve the electron transfer ability and the overall electrochemical performance. Conclusions In summary, this study demonstrates an efficient way to construct a high performance MnO2 based electrode by deep coupling with electric conductive carbon substrate, and elucidate the capacity rising problem from a new perspective based on the chemical interaction

for

metal

oxides/carboneous

sbustrate,

both

theoretically

and

experimentally. The as synthesized MnO2@NG exhibits a high discharge capacities of 993 mAh g-1 in the second cycle at 0.1 A g-1, and goes up to

2243 mAh g-1 gradually

through 135 cycles. Kinetic analysis indicate the increased capacitive contribution after cyclic reaction, which should be attributed to the newly formed phase/interface during cycling. Meanwhile, DFT calculations uncovers the origin of this extremely high performance should be related to the ever boosted covalent interactions between MnO2 and NG through C-Mn and N-Mn bond. These results will show new light to understand the electrochemistry storage mechanism of transition metal oxides (TMOs), guiding new electrode structure design of nano TMOs based electrodes materails as anodes for next generation LIBs.

AUTHOR INFORMATION


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Corresponding Authors *Yufeng

Zhao: Corresponding email:[email protected] (Y. Zhao);

Notes The authors declare no competing financial interest. Acknowledgements We thank the financial supports from the National Natural Sciecne Foundation of China (51774251), Hebei Natural Science Foundation for Distinguished Young Scholars (B2017203313, B2015203096), Hundred Excellent Innovative Talents Support Program in Hebei Province (SLRC2017057), State key Project of Research and Development of China (2016YFA0200102), NSFC-RGC Joint Research Scheme (51361165201), Scientific Research Foundation for the Returned Overseas Chinese Scholars (CG2014003002) and the open funding from State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, 2017-KF-14). ASSOCIATED CONTENT Supporting Information Supporting information includes the detailed description of material characterization and electrochemical measurements, the XRD, Raman and XPS spectra, the SEM, TEM, HRTEM and SAED images of pristine α-MnO2 and MnO2@NG, the Ex-situ



XRD, HRTEM and corresponding SAED iamges patterns of MnO2@NG, the electrochemical performance of pristine α-MnO2 tested against lithium, the Kinetic analysis and the atomic structural model of NG and MnO2, respectively. Reference (1) Sun, Y.; Hu, X.; Luo, W.; Xia, F.; Huang, Y. Reconstruction of conformal nanoscale MnO on graphene as a high‐capacity and long‐life anode material for lithium ion batteries. Adv. Funct. Mater. 2013, 23 (19), 2436-2444. DOI 10.1002/adfm.201202623. (2) Li, L.; Raji, A. R.; Tour, J. M. Graphene-Wrapped MnO2-Graphene Nanoribbons as Anode Materials for High-Performance Lithium Ion Batteries. Adv. Mater. 2013, 25 (43), 6298-6302，DOI 10.1002/adma.201302915. (3) Ling, C.; Mizuno, F. Capture Lithium in α-MnO2: Insights from First Principles. Chem. Mater. 2012, 24 (20), 3943-3951, DOI 10.1021/cm302347j. (4) Jiang, Y.; Jiang, Z. J.; Chen, B.; Jiang, Z.; Cheng, S.; Rong, H.; Huang, J.; Liu, M. Morphology and crystal phase evolution induced performance enhancement of MnO2 grown on reduced graphene oxide for lithium ion batteries. J. Mater. Chem. A 2016, 4 (7), 2643-2650, DOI 10.1039/C5TA10614A. (5) Chu, Y.; Guo, L.; Xi, B.; Feng, Z.; Wu, F.; Lin, Y.; Liu, J.; Sun, D.; Feng, J.; Qian, Y. Embedding MnO@Mn3O4 Nanoparticles in an N-Doped-Carbon Framework Derived from Mn‐Organic Clusters for Efficient Lithium Storage. Adv. Mater. 2018, 30 (6), 1704244, DOI 10.1002/adma.201704244. (6) Wu, Z. S.; Ren, W.; Xu, L.; Li, F.; Cheng, H. M. Doped graphene sheets as anode materials with superhigh rate and large capacity for lithium ion batteries. ACS nano 2011, 5 (7), 5463-5471, DOI 10.1021/nn2006249. (7) Qie, L.; Chen, W. M.; Wang, Z. H.; Shao, Q. G.; Li, X.; Yuan, L. X.; Hu, X. L.; Zhang, W. X.; Huang, Y. H. Nitrogen ‐ Doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv. Mater. 2012, 24 (15), 2047, DOI 10.1002/adma.201104634. (8) Zang, J.; Ye, J.; Qian, H.; Lin, Y.; Zhang, X.; Zheng, M.; Dong, Q. Hollow carbon sphere with open pore encapsulated MnO2 nanosheets as high-performance anode materials for lithium ion batteries. Electrochim. Acta 2017, 260 (10), 783-788, DOI


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Figure Captions Figure 1. XRD, Raman and

XPS characterization of MnO2@NG.

(a) The

XRD, Raman (b) curve of MnO2@NG, respectively, (c) XPS survery spectra of MnO2@NG, (d-f) deconvoluted XPS spectra of C1s (d), N1s (e) and Mn2p (f), respectively. Figure 2. Morphological characterization of MnO2@NG. (a-d) The SEM and corresponding mapping images of MnO2@NG, (e-f) TEM and HRTEM images of MnO2@NG,

(g, h) the enlarged images of (f) (reagion 1(g) and 2 (h), respectively),

(i) the SAED images of MnO2@NG. Figure 3. Electrochemical performance and morphology evolution of MnO2@NG. (a) CV curves of MnO2@NG at a scan rate of 0.1 mV s−1 for the first to five cycles in LIBs, (b, c) cycling performance and GCD profils of MnO2@NG at a current density of 0.1 A g−1 in LIBs，respectively, (d) rate performance of MnO2@NG at different rates gradually increasing from 0.1 A g-1 to 20 A g-1 and then back to 0.1 A g−1 in LIBs, (e) cycling performance of MnO2@NG at a current density of 5 A g−1 in LIBs,



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(f-i) TEM images of the MnO2@NG at 240 th (f), 600 th (g), 974 th (h), 5000 th (i) with the current density of 5 A g-1, respectively. Figure 4. Kinetics analysis of the lithium-storage behavior for the MnO2@NG electrode, (a-b) EIS profils and discharge curves at the cycle number of 2nd, 88th and 135th cycles of MnO2@NG with the current density of 0.1 A g−1 in LIBs, (c) log(i) versus log(v) plots at different cathodic/anodic peaks (fresh cells and the cycled electrode (after 88 cycles at 0.1 A g−1 (denoted as 88th cells)) at various scan rates of 0.2-1.0 mV s-1), (d-e) Normalized contribution ratio of capacitive (black) (which means

the

the

percentages

of

capacitive-controlled

capacities)

and

diffusion-controlled (red) (which means the the percentages of diffusion-controlled capacities) capacities of the fresh and 88th cells at a scan rate of 0.2-1.0 mV s-1. Figure 5. First-principles calculations of the lithium-storage behavior for the MnO2@NG electrode. (a) The optimized geometry of MnO2@NG and (b-c) are the corresponding charge density difference geometry for front and side view, respectively (isosurfaces level = 0.0028e bohr-3; yellow and light blue areas represent positive and negative charge differences). (d) Electronic band structure of MnO2@NG (Horizontal dash line represents the Fermi level). (e) Calculated partial density of states of MnO2@NG (Dash line represents the Fermi level).


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Figure 1



Figure 2


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Figure 3


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Figure 4




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Figure 5



For Table of Contents Use Only


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This work reported a new perspective of the origin of the ultrahigh capacity upon cycling of MnO2@NG.




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Largely Increased Lithium Storage Ability of Mangnese Oxide through

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