High-Capacity and Self-Stabilized Manganese Carbonate

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High-Capacity and Self-Stabilized Manganese Carbonate Microspheres as Anode Material for Lithium-Ion Batteries Liang Xiao, Shiyao Wang, Yafei Wang, Wen Meng, Bohua Deng, Deyu Qu, Zhi-Zhong Xie, and Jinping Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09022 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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ACS Applied Materials & Interfaces

High-Capacity and Self-Stabilized Manganese Carbonate Microspheres as Anode Material for Lithium-Ion Batteries Liang Xiao†, Shiyao Wang†, Yafei Wang†, Wen Meng†, Bohua Deng†, Deyu Qu†, Zhizhong Xie†, Jinping Liu*,†,‡ †

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, Hubei 430070, China

‡

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Manganese carbonate (MnCO3) is an attractive anode material with high capacity based on conversion reaction for lithium-ion batteries (LIBs), but its application is mainly hindered by poor cycling performance. Building nano-/porous structures and nanocomposites has been demonstrated as an effective strategy to buffer the volume changes and maintain the electrode integrity for long-term cycling. It is widely believed that micro-sized MnCO3 is not suitable for using as anode material for LIBs because of its poor conductivity and the absence of nanostructure. Herein, different from previous reports, spherical MnCO3 with the mean diameters of 6.9 µm (MnCO3-B), 4.0 µm (MnCO3-M) and 2.6 µm (MnCO3-S) were prepared via controllable precipitation and utilized as anode materials for LIBs. It is interesting that the asprepared MnCO3 microspheres demonstrate both high capacity and excellent cycling performance comparable to their reported nanosized counterparts. MnCO3-B, MnCO3-M and MnCO3-S deliver reversible specific capacities of 487.3, 573.9 and 656.8 mAh g-1 after 100 cycles, respectively. All the MnCO3 microspheres show capacity retention more than 90% after the initial stage. The advantages of MnCO3 microspheres were investigated via constant-current charge/discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results indicate that there should be substantial structure transformation from micro-sized particle to self-stabilized nanostructured matrix for MnCO3 at the initial charge/discharge stage. The evolution of EIS during charge/discharge clearly indicates the formation and stabilization of the nanostructured matrix. The self-stabilized porous matrix maintains the electrode structure to deliver excellent cycling performance, and contributes extra capacity beyond conversion reaction.

KEYWORDS: manganese carbonate; microspheres; self-stabilization; anode material; lithiumion batteries


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1. INTRODUCTION Lithium-ion batteries (LIBs) are promising power sources for electrical vehicles (EVs), however their limited practical energy densities of ~200-300 Wh kg-1 cannot fulfill the long-range cruise of EVs at the present time.1-2 Therefore, the research and development of novel electrode materials with higher capacities than those of commercial available electrode materials are of great importance. In recent years, transition metal oxysalts including carbonates and oxalates have emerged as potential anode materials with reversible capacities approaching to 1000 mAh g-1 based on so-called conversion reaction mechanism.3-6 Taking metal carbonates as an example, the total reaction could be expressed as the following equation:7-12 MCO3 + 2Li↔M + Li2CO3 (M = Co, Fe or Mn) (1) Among these carbonates, manganese carbonate (MnCO3) has attracted the most attention because of its facile preparation, nontoxicity, low cost and abundant resources,12-20 however, several drawbacks hinder the application of metal carbonates as anode material for LIBs. One is the low conductivity of MnCO3, the others are volume change, particle pulverization and consequently the collapse of electrode structure during repeated cycles, which result from the nature of conversion reaction accompanied with phase transformation. Building nano-/porous structures and nanocomposites have been demonstrated as effective methods to enhance the structure integrity, the conductivity and consequently the electrochemical performance of MnCO3 electrodes for both LIBs12,

14, 19-20

and supercapacitors.21 For instance, graphene was

usually used as conductive matrix and volume buffer to prepare various MnCO3/graphene composites, and enhanced performance of the composites have been demonstrated.14, 19-20 As to the micro/nano structure investigation, morphologies of MnCO3 including nanocube,7,

21-22


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nanosphere,21 submicron peanut,15 porous sphere,17 microdumbbells,18 flower-to-petal,19 ellipsoid23 and twinborn sphere23 were prepared to study the micro/nano structure effects on the performances. All these studies can be divided into two categories: one is to construct MnCO3 particles with porous structures15, 17, 23 or hierarchical structures;18-19, 23 the other is to prepare submicron or nanosized MnCO3 particles, namely, downsizing the dimensions of MnCO3 materials.7, 15, 22 Although micro-sized MnCO3 with spherical morphology has the advantage of high tap density, it is barely utilized as potential anode material for LIBs considering its poor ionic and electronic conductivity and the absence of nanostructure. In the present work, however, we firstly attempt to utilize the bulk MnCO3 spheres with micron-sizes as LIB anode. Simple solution precipitation method was employed to obtain MnCO3 microspheres and rationally control the size, since it is a well-understood method widely used for the growth of MnCO3 21, 2425

. It is interesting that the as-prepared MnCO3 microspheres demonstrate both high capacity and

excellent cycling performance comparable to the reported nanosized or hierarchically porous counterparts. The reversible capacities of our MnCO3 microspheres are found to be higher than its theoretical value of 467 mAh g-1 based on the reaction in equation (1). Actually, the capacities ranging from 600 to over 900 mAh g-1 have been reported in lots of studies for MnCO3 anode in LIBs12-20. It implies that extra capacity is available beyond the equation (1) for MnCO3. Extra capacity is a common feature of conversion reaction-based anode materials including transition metal oxides26-28 and oxysalts.3-6 Tirado group22, 29 and Kang et al.17 ascribed the extra capacity of MnCO3 beyond equation (1) to the nonfaradic contribution, namely interfacial capacitance. Zhao et al. ascribed the extra capacity to the pseudocapacitance occurring at the interface


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between lithiated and delithiated phases.19 Zhou et al. declared that MnLix alloy formed below 0.35 V is the origin of the extra capacity.14 Zhong et al.20 and Gao et al.16 suggested that the reoxidation of Mn2+ to higher valence results in capacity elevation for MnCO3. Therefore, the reaction mechanism of MnCO3 during cycling is still ambiguous and even controversial so far, and needs to be studied further. In the present work, to further understand the lithium storage details of MnCO3 microspheres, electrochemical impedance spectroscopy (EIS) of MnCO3 electrodes at different states of charge/discharge and after different cycles was systematically monitored and analyzed for the first time. In combination with the investigations of voltage profiles and cyclic voltammetry (CV) curves, the EIS results clearly suggest that the excellent performance of MnCO3 microspheres originates from the structure modification from micro-sized particle to self-stabilized nanostructured matrix during initial cycling stage. In addition, the particle-size effect on the performance of the as-prepared MnCO3 microspheres and the relating mechanism are studied as well. Our work presents an alternative way to develop high-performance metal oxysalts for LIB application.

2. EXPERIMENTAL SECTION The spherical MnCO3 samples were prepared by controllable precipitation with manganese sulfate (MnSO4, AR) and ammonium hydrogen carbonate (NH4HCO3, AR) according to our previous report with modification.25 Firstly, 7 mL ethanol (AR) and 70 mL MnSO4 aqueous solution (0.0143 mol·L-1) were mixed thoroughly via stirring. Then, 70 mL NH4HCO3 aqueous solution (0.143 mol·L-1) was added into the abovementioned MnSO4-ethanol solution with controllable dropping rate under vigorous stirring. After the addition of all the NH4HCO3


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solution, the obtained suspension was maintained under vigorous stirring for 3 hours. All the operations were carried out at room temperature. MnCO3 samples were separated from the reaction mixture by centrifugation, and then washed several times with deionized water and ethanol to remove impurities. Finally, all the MnCO3 samples were dried at 50°C in a vacuum oven for 3 hours before using. By adjusting the dropping rate of precipitator, spherical MnCO3 with different sizes were prepared. The morphologies of as-prepared MnCO3 samples were examined with Zeiss ULTRA PLUS43-13 field emission scanning electron microscope (FE-SEM). The powder XRD patterns of prepared MnCO3 samples were obtained on Rigaku D/MAX-RB (Cu Kα radiation) with scan rate of 5o/min. X-ray photoelectron spectra (XPS) were recorded on a VG Multilab2000X X-ray photoelectron spectrometer with an Al Kα excitation source, where binding energies were calibrated by referencing the C 1s peak (284.6 eV) to reduce the sample charge effect. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images were taken with a JEM 2100F electron microscope operating at 200 kV. The MnCO3 electrodes were prepared by laminating the mixed slurry composed of 80 wt.% of MnCO3 sample, 10 wt.% of acetylene black and 10 wt.% of polyvinylidenedifluoride (PVDF). The mass loading of MnCO3 electrode was about 5 mg cm-2. 2032-type coin-cell were assembled in argon filled glove box with a MnCO3 electrode as anode, a lithium foil as counter electrode, a Celgard separator and 1 M LiPF6 in a 1:1 ethyl carbonate (EC): dimethyl carbonate (DMC) solvent as electrolyte. The cycling performance of MnCO3 samples was tested with constant current charge-discharge on NEWARE BTS battery tester at room temperature and all potentials refer to Li+/Li. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests of both fresh and cycled coin-cells were carried out on Autolab PGSTAT 302N


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electrochemical workstation. The discharged MnCO3 electrode was obtained from disassembled cell, washed three times with anhydrous ethyl methyl carbonate (EMC), and dried in the antechamber of the glove box under vacuum. The morphology was examined by Hitachi S-4800 FE-SEM.

3. RESULTS AND DISCUSSION Figure 1 shows the FE-SEM images and particle size distributions of as-prepared MnCO3 samples. To facilitate the following discussions, the spherical MnCO3 samples with the mean diameters of 6.9 µm, 4.0 µm and 2.6 µm are denoted as MnCO3-B (Figure 1A, 1B and 1C), MnCO3-M (Figure 1D, 1E and 1F) and MnCO3-S (Figure 1G, 1H and 1I), respectively. Although the SEM images show several twinned particles due to the agglomeration effect and insufficient stirring in some parts of the reaction system, most particles are dispersed well. The particle size distributions of all the three samples are narrow. In our experiment, with vigorous stirring, the slower dropping rate of precipitator results in bigger mean particle size of the spheres, indicating a nucleation-growth mechanism for MnCO3 preparation. As a result, the uniform spherical MnCO3 with different diameters were successfully prepared via a controllable precipitation route.


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Frequency / %


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Figure 1. The FE-SEM images and the particle size distributions of micro-sized MnCO3 spheres with different diameters, (A,B,C) MnCO3-B, (D,E,F) MnCO3-M, (G,H,I) MnCO3-S.

The XRD patterns with the indexed peaks in Figure 2A reveal that all the three samples are calcite-type rhombohedral structure (JCPDS no. 86-0173, space group R-3c) without impurities. The crystal structure of MnCO3 is shown in Figure 2B, consisting of MnO6 octahedra with CO3 equilateral triangles arranged in the same plane perpendicular to the c-axis. There are full of cube's corner points emerging on the surfaces of all the MnCO3 samples (Figure 1B, 1E and 1H), since calcite-type compounds or minerals prefer cubic morphologies. In our experiments, vigorous stirring is the dominated factor determining the formation of spherical


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morphologies for as-prepared MnCO3. Without vigorous stirring, only cubic MnCO3 particulates with the sizes of several hundred nanometers can be attained.28 The chemical compositions of asprepared MnCO3 were further investigated with XPS. Due to the identical preparation route except dropping rates for all the samples, MnCO3-S was selected as a typical sample for XPS analysis. The survey spectrum in Figure 2C shows the presence of Mn, O and C with no observable impurity. The Mn 2p spectrum in Figure 2D shows two major peaks with binding energies of 640.8 and 652.9 eV with a separation of 12.1 eV, which are assigned to Mn 2p3/2 and Mn 2p1/2, respectively, and are consistent with Mn in the +2 state.14, 17 The carbonate group CO32is confirmed by the C 1s peak at a binding energy of 289.0 eV (Figure 2E) and the O 1s peak at 531.0 eV (Figure 2F).14, 17 The C 1s peak at 284.6 eV corresponds to the hydrocarbons adsorbed on all specimens, which is usually used as binding energy reference.30 Figure 3 presents the TEM images and HR-TEM image of MnCO3-S sample. Figure 3A confirms the spherical morphology of MnCO3-S with the mean diameter of about 2.6 µm, and both Figure 3A and 3B clearly reveal the cube's corner points emerging on the surface of spherical MnCO3 particles, consistent with the observation in SEM. Moreover, the HR-TEM image reveals the crystalline feature of the as-prepared MnCO3. The well-defined fringes can be seen in Figure 3C with the d-spacing of 0.29 nm, which corresponds to the (104) plane of MnCO3. It might suggest that the spherical MnCO3 particles are composed of close-packed cubic grains. In summary, all the XRD, XPS and TEM analyses indicate that pure-phase micron-sized MnCO3 spheres are prepared successfully.


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(B)

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Figure 2. (A) The XRD patterns of MnCO3-B, MnCO3-M and MnCO3-S, (B) crystal structure of MnCO3, the XPS spectra of MnCO3-S: (C) a survey spectrum, (D) Mn 2p, (E) C 1s and (F) O 1s.


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Figure 3. TEM images (A, B) and HRTEM image (C) of MnCO3-S. Figure 4 compares the cycling performances of spherical MnCO3 with different diameters. It is clearly shown that MnCO3 samples with smaller particle sizes deliver higher reversible capacities. Specifically, MnCO3-B, MnCO3-M and MnCO3-S deliver specific charge capacities of 766.72, 884.18 and 1115.48 mAh g-1 in the first cycle, respectively. The coulombic efficiencies of all the three samples are extremely low and less than 40% in the first cycle, which will be discussed later. The capacity degradations of the three samples have identical feature, which is slowly decaying in the initial 20 cycles and becoming relatively stable in the following cycles. Simultaneously, the coulombic efficiencies gradually increase to near 100% within initial 20 cycles. After 100 cycles, MnCO3-B, MnCO3-M and MnCO3-S deliver reversible specific capacities of 487.3, 573.9 and 656.8 mAh g-1, respectively. Herein, the reversible capacities of as-prepared MnCO3 exceed the theoretical capacity of 466 mAh g-1 based on equation (1), even


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after 100 cycles, which is consistent with the previous reports.14, 16-17, 19-20, 22, 29 The capacity degradation of MnCO3 during cycling was generally ascribed to its structure collapse and poor conductivity. Several approaches have been utilized in the literature to stabilize the electrode integrity and maintain the conductivity of electrode, including fabricating nanostructured MnCO3 and MnCO3 nanocomposites with nanostructured carbon materials.14, 17, 19-20 As to the microsized MnCO3 in the present study, high reversible capacity and excellent cycling performance after the initial stage (initial 20 cycles) are clearly demonstrated. Table 1 shows the comparison of the electrochemical performances of our MnCO3 microspheres with other previous manganese carbonate anodes. It can be seen that the performances of MnCO3 microspheres are at the same level with some nanostructured counterparts. These results indicate that the spherical morphology of micro-sized MnCO3 still possesses structural advantages regarding cycling stability.

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Figure 4. Cycling performances of MnCO3-B, MnCO3-M and MnCO3-S at 100 mA g-1 from 0.05 V to 3.00 V.

Table 1. Comparison of the electrochemical performances of our MnCO3 microspheres with other previous manganese carbonate anodes. Electrode materials MnCO3-S MnCO3-M MnCO3-B MnCO3 submicron particles CNT anchored MnCO3 nanoparticles MnCO3 particles with average size between 10 and 20 µm MnCO3@rGO Composite Submicron peanut-like MnCO3 Graphene-wrapped mesoporous MnCO3 Nanostructured porous MnCO3 spheres with the diameter of 500nm Hierarchical architectured MnCO3 microdumbbells MnCO3 spindle–GO composites MnCO3/large-area graphene composites MnCO3 ellipsoids composed of nanoscale primary particles MnCO3 twinborn spheres composed of nanoscale primary particles Mn0.9Co0.1CO3 submicron particles

Capacity / mAh g-1 656.8 573.9 487.3 500 618

Current density Cycles References / mA g-1 100 100 This work 100 100 This work 100 100 This work 58 25 (4,7,22) 100 100 (12)

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Table 2. Specific capacities and coulombic efficiencies in the initial two cycles for spherical MnCO3 samples. The 1st cycle Samples

Specific capacity / mAh g-1 charge

discharge

MnCO3-B

766.72

2066.9

MnCO3-M

884.18

MnCO3-S

1115.48

The 2nd cycle

coulombic efficiency

Specific capacity / mAh g-1

coulombic efficiency

charge

discharge

37.08%

632.2

913.94

69.17%

2326.9

38.00%

750.7

1041.7

72.06%

2936.12

38.00%

913.08

1271.26

71.82%

To illuminate the advantages of spherical morphology for micro-sized MnCO3, the voltage profiles and CV curves of as-prepared MnCO3 in the first and the second cycle were comparatively investigated in Figure 5. Accordingly, the specific capacities and coulombic efficiencies in the initial two cycles for spherical MnCO3 samples are summarized in Table 2. The cathodic scanning in CV tests for both the 1st and the 2nd cycle could be divided to three continuous regions marked as I, II and III in Figure 5A and 5B according to the current responses. Region I from 1.75 to 0.8 V is the onset of cathodic current, which is usually ascribed to the non-reactive lithium storage or double layer capacitance depending on the structural parameters (e.g. porosity) of the active substance and the operating voltage range19. Region II from 0.8 to 0.3 V corresponds to the sole cathodic peak in the CV curves, which is believed to originate from the bulk conversion reaction. The cathodic current peak located at about 0.5 V relates to the reduction of Mn2+ to Mn and the formation of Li2CO3. The broad anodic peaks from 0.5 to 1.75 V in CV are assigned to the oxidation of Mn and decomposition of Li2CO3 for MnCO3 recovery. Finally, Region III below 0.3 V corresponds to the sharply increasing cathodic current and is generally recognized as the origin of extra capacity beyond the theoretical value


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from conversion reaction. However, the reaction mechanism of region III is not clear and even controversial, which could be interfacial capacitance,17, 24-25 pseudo-capacitance19 or even the formation of MnLix alloy.14 As to the details of voltage profiles in Figure 5C and 5D, there are significant differences in region II and III for the first and the second cycles. The first discharge in region II is composed of a slow voltage decay centered at 0.5 V and a long plateau at 0.3 V distinguished by a voltage valley. However, the long plateau at 0.3 V disappears in the second discharge and a new plateau at 0.5 V emerges. In addition, the region III greatly shrinks in the second cycle as compared to the first discharge. As can be seen in Table 2, the coulombic efficiency sharply increases from 38% in the first cycle to 70% in the second cycle. These observations imply that there are substantial structure modifications for micro-sized MnCO3 during initial cycles.


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Figure 5. CV curves of spherical MnCO3 with different sizes for (A) the first cycle and (B) the second cycle at 0.1 mV s-1. Voltage profiles of spherical MnCO3 with different sizes for (C) the first cycle and (D) the second cycle at 100 mA g-1 from 0.05 to 3.00 V.


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Figure 6. (A) Voltage profiles of intermittent discharge and charge at 100 mA g-1 for MnCO3-S at the 3rd cycle, (B) Schematic diagram of reaction mechanism and equivalent circuit, (C) Nyquist plots during discharge, and (D) Nyquist plots during charge.

Discussions on the microstructure evolution of as-prepared MnCO3 spheres during cycling actually need more characterizations in the future studies, herein we focus on the EIS investigation to analyze the reversibility of the MnCO3 spheres during repeated cycles. Cycling performance in Figure 4 shows that MnCO3-S has the best performance among all the three samples, and the coulombic efficiencies of all the samples increase to near 90% after the 3rd cycle. Therefore, the impedance changes of MnCO3-S during the 3rd cycle were firstly studied to illuminate the reversibility of the MnCO3 spheres. Figure 6A shows the voltage profiles of


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intermittent discharge and charge for MnCO3-S at 100 mA g-1 for the 3rd cycle, from which various states of charge (SOCs) marked with number (1)-(9) were gotten. Figure 6C and 6D display the Nyquist plot changes with SOCs for the discharge and charge processes, respectively. The reversible changes of the Nyquist plots during the 3rd cycle indicate a good reversibility for MnCO3 spheres. An equivalent circuit is suggested in Figure 6B to fit the EIS. Rs is assigned to the ohmic resistance of the coin cells, and W is the warburg resistance related to the diffusion of lithium ions. Three series resistances (R1, R2 and R3) coupled with constant phase elements (CPE1, CPE2 and CPE3) are used to fit the EIS as well. The fitting parameters of testing points (1)-(9) are listed in Table 3. During the discharge process, R1 suddenly emerges at point (4) with the voltage of 0.35 V, and increases to its maximum at the end of discharge. During the charge process, R1 quickly decreases at point (6) and disappears above 0.5 V. R2 and R3 show similar trends during charge and discharge as listed in Table 3, which are gradually increasing during discharge and decreasing back to the original state during charge. Table 3. Fitting parameters of electrochemical impedance spectroscopy at different SOCs for MnCO3-S during the 3rd cycle. Testing Testing Rs / Ω R1 / Ω R2 / Ω R3 / Ω Rs / Ω R1 / Ω R2 / Ω R3 / Ω points points (1) 7.229 4.941 1.553 (9) 7.232 4.232 1.272 (2) 7.159 5.285 1.557 (8) 7.17 4.194 1.58 (3) 7.054 5.924 2.479 (7) 7.111 9.489 1.004 (4) 6.886 27.95 8.575 4.847 (6) 7.179 4.931 10.26 1.688 (5) 6.714 34.36 10.34 8.595 (5) 6.714 34.36 10.34 8.595

The round-trip changes of R1, R2 and R3 indicate the reversible transformation of structure and component for MnCO3 electrode except for the bulk conversion reaction. The reversible formation and decomposition of conducting-type polymeric film catalyzed by cobalt nanograins in alkyl carbonate solution was demonstrated by Laruelle et al., which provided extra capacity


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with pseudo-capacitive behavior to the conversion reaction of CoO.31 The reaction mechanism of micro-sized MnCO3 with spherical morphology can thus be suggested herein (Figure 6B) referring to the reversible polymer-veil for CoO in Laruelle et al.’s work to explain the EIS. Micro-sized MnCO3 with spherical morphology should have a substantial structure modification in the first discharge from micro-sized MnCO3 to Mn nanograins embedded into Li2CO3 matrix surrounded by an inorganic SEI layer with an electrochemically active polymer-veil on the outer layer.31 In the other words, as usual, micro-sized MnCO3 is pulverized into nanostructured matrix, namely porous structure, during the first discharge. The extra capacity of MnCO3 comes from both the interfacial capacitance of the porous structure formed in initial cycles and the pseudo-capacitance related to the reversible formation and decomposition of the polymeric film. Moreover, the coating of SEI with polymer outer layer ensures a relatively stable matrix to maintain the electrochemical activity and the microstructure of single MnCO3 particles. This assumption is supported by the SEM images of discharged MnCO3 electrode in Figure S1 (Supporting Information). Since the matrix spontaneously forms during the electrochemical reactions and becomes more stable after several cycles, it is recognized as a self-stabilized matrix in this discussion. Based on the above discussion, R1, R2 and R3 in the equivalent circuit in Figure 6B are assigned to the resistances of polymer layer, SEI layer and charge transfer, respectively. The round-trip changes of R1 support the reversible formation and decomposition of the polymer layer. The changes of R2 and R3 indicate that the SEI layer is still not stable in the 3rd cycle corresponding to the coulombic efficiency below 100%. The voltage profiles in Figure 5 support the abovementioned reaction mechanism. It is reasonable that the long plateau at 0.3 V disappears and a new plateau at 0.5 V emerges in region II of the second discharge as compared to the first discharge, since the structure modification of


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micro-sized MnCO3 decreases the electrode polarization, i.e. newly formed porous matrix facilitate the reaction kinetics. The shrink of region III in the second cycle (Figure 5D) indicates that not all the polymer and the SEI layer are reversible, which is the reason why the coulombic efficiency sharply increases from 38% in the first cycle to 70% in the second cycle. Low coulombic efficiency of around 60~70% in the first cycle is a common behavior for the electrode materials based on conversion reaction involving in structure and phase modification, but the coulombic efficiencies of micro-sized MnCO3 in the present work is extremely low, even lower than that of submicro- and nanosized MnCO3. The irreversible capacity from SEI formation might be negligible if the surface area of pristine micro-sized particles is maintained during the electrochemical reactions. In this regard, the low coulombic efficiencies of MnCO3 microspheres support their substantial structure modification to nanostructured matrix in the first discharge. The large irreversible capacity of as-prepared MnCO3 can be ascribed to irreversible reactions with the electrolyte during the first discharge until the formation of porous matrix of Mn nanoparticles and Li2CO3 protected by SEI layer with polymer outer layer.7,

31

The sharply

increased coulombic efficiencies in the second cycle imply that the porous matrix coated by SEI layer with polymer out layer gradually becomes stable and reversible. Overall, the MnCO3 samples with smaller sizes deliver higher capacities as listed in Table 2. For spherical MnCO3 with smaller sizes, it is easy to transfer to porous matrix because of shorter solid path. The coulombic efficiencies seem independent of the particle sizes for the as-prepared microspherical MnCO3. This might be ascribed to the identical reactivity of micro-sized spherical samples.


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(A) 40

(D)

MnCO3-B

3.0

MnCO3-B

st

After 1 discharge nd After 2 discharge th After 20 discharge th After 50 discharge Fitting

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2 th 10 th 20 th 50 th 100

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Figure 7. The Nyquist plots after different cycles for (A) MnCO3-B, (B) MnCO3-M and (C) MnCO3-S and voltage profiles during cycling for (D) MnCO3-B, (E) MnCO3-M and (F) MnCO3S.


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Table 4. Fitting parameters of EIS at full-discharge state for MnCO3-B, MnCO3-M and MnCO3S after different cycles. Samples MnCO3-B

MnCO3-M

MnCO3-S

Cycle number 1 2 20 50 1 2 20 50 1 2 20 50

Rs / Ω 7.263 7.536 8.505 8.576 7.507 7.77 8.493 8.518 6.836 7.548 8.137 8.256

R1 / Ω 33.68 29.07 22.01 23.1 36.19 31.11 22.03 22.67 37.89 34.84 23.39 24.63

R2 / Ω 3.336 4.098 15.83 18.51 2.305 4.173 13.55 16.38 2.15 3.166 12.47 14.45

R3 / Ω 3.718 4.773 7.193 7.577 3.621 4.573 7.015 7.177 3.526 4.442 6.314 6.758

EIS at fully discharge state and voltage profiles during cycling were studied for the three MnCO3 samples in Figure 7, and the fitting parameters based on the equivalent circuit shown in Figure 6B are listed in Table 4. Generally, R1 increases in the order of MnCO3-B, MnCO3-M and MnCO3-S since spherical MnCO3 with smaller sizes is easier to transfer to porous matrix with shorter reaction path. R2 and R3 decrease in the order of MnCO3-B, MnCO3-M and MnCO3-S, indicating more stable SEI and smaller charge transfer resistance for smaller MnCO3 spheres. As to the cycling effect, Rs slightly increases until 50 cycles due to the plenty of electrolyte to sustain the reversible formation and decomposition of the polymer layer in coin cell. R1 gradually decreases during the initial 20 cycles and becomes stable after that, and consequently the coulombic efficiency gradually increases in the initial 20 cycles and becomes 100% after that. These trends are consistent with the capacity degradation of MnCO3 spheres, which is gradually decreasing during the initial 20 cycles and becomes stable after the initial stage. It is strongly demonstrated that the reversibility of polymer layer is gradually established and is


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related to the extra capacity of MnCO3 spheres. The evolution of R2 and R3 demonstrates a gradually formed stable SEI layer, which are gradually increasing during the initial 20 cycles and become relatively stable after that. As shown in Figure 7D, 7E and 7F, the voltage profiles only show slight variations from the 20th to the 100th cycle. In addition, the discharge capacities in the Region I from 1.75 to 0.8 V are stable and identical for all the three samples after 20 cycles. This result means that stable porous matrix forms after the initial cycles, since Region I relates to the non-reactive lithium storage or the double layer capacitance depending on the porosity. Based on all the above results, the high capacity and excellent cycling stability of micro-sized MnCO3 with spherical morphology are reasonable considering the self-stabilized porous matrix coated by SEI with polymer outer layer formed after the initial 20 cycles. It is believed that the electrochemical performance of our MnCO3 microspheres can be further improved with delicate design of the micro-/nanostructure and component (hybridization, etc.). 32-37

4. CONCLUSIONS In conclusion, spherical MnCO3 with the mean diameters of 6.9 µm (MnCO3-B), 4.0 µm (MnCO3-M) and 2.6 µm (MnCO3-S) were successfully prepared via controllable precipitation and were used as anode materials for LIBs for the first time. After 100 cycles, MnCO3-B, MnCO3-M and MnCO3-S deliver reversible specific capacities of 487.3, 573.9 and 656.8 mAh·g1

, respectively, which are at the same level with the nanostructured counterparts. The possible

advantage of micro-sized MnCO3 spheres is suggested as their structure transformation from micro-sized particle to self-stabilized nanostructured matrix during cycling. The matrix could be made of manganese nanograins dispersed into a Li2CO3 medium together with the coating of solid electrolyte interface (SEI) with polymer outer layer according to the previous reports.


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Consequently, the self-stabilized porous matrix maintains the electrode structure to deliver excellent cycling performance, and the reversible formation and decomposition of the polymeric film contribute extra capacity to MnCO3 spheres beyond conversion reaction. The fundamental finding of this work provides new thoughts for enhancing the performance of oxysalts as anode material for LIBs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key Research Program of China (No. 2016YFA0202602), the National Natural Science Foundation of China (Nos. 21673169, 11474226, 51672205), the Science Fund for Distinguished Young Scholars of Hubei Province (Grant No. 2013CFA023), the Youth Chenguang Project of Science and Technology of Wuhan City (Grant No. 2014070404010206), the Research Start-Up Fund from Wuhan University of Technology, and the Fundamental Research Funds for the Central Universities (Nos. WUT: 2015-IB-001, WUT: 2016-IB-005 and WUT: 2016IVA083)

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