Mn2SiO4@C Cuboids as High

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Si-induced Synthesize MnO/Mn2SiO4@C Cuboid as High-Performance Anode for Lithium-Ion Batteries Hang Wei, Zhonghong Xia, and Dingguo Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13468 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Applied Materials & Interfaces

Si-induced Synthesize MnO/Mn2SiO4@C Cuboid as High-Performance Anode for Lithium-Ion Batteries Hang Wei a, b, Zhonghong Xia b and Dingguo Xia b* a

College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021,

P.R. China. b

College of Engineering, Peking University Key Lab of Theory and Technology for Advanced

Batteries Materials, Beijing 100871, P.R. China. College of Engineering, Peking University, Beijing 100871, P. R. China. KEYWORDS: MnO/Mn2SiO4@C, cuboids, induced, lithium-ion batteries, anodes

ABSTRACT: The exploration of anode material of lithium-ion batteries (LIBs) is still a great challenge because of its low electrical conductivity and poor durability. Transition metal oxide is proposed as a potential alternative, while the dimension and structure greatly affect its electrochemical properties. In this study, MnO/Mn2SiO4@C cuboid was prepared via polymerization-pyrolysis process. Larger MnCO3 precursor particles melted into monolithic carbon framework and formed smaller nanoparticles due to the inducing effect of Si element in phthalocyanino silicon (SiPc), thus the MnO/Mn2SiO4@C cuboids were obtained. Micron-scaled cuboid composite can lead to higher tap density and great electrical performance due to the lower interparticle resistance. Therefore, the as-prepared MnO/Mn2SiO4@C electrode exhibits a stable

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specific capacity of 585.9 and 423.9 mA h g-1 after 1000 discharge/charge cycles at 1 and 2 A g1

, respectively. Meanwhile, the excellent rate capacity of 246.3 mA h g-1 was achieved even at 30

A g-1. Additionally, this facile and economical strategy to improve electrode performance provides a commercially feasible way for the construction of high-performance lithium-ion batteries.

Introduction The performance of lithium ion batteries (LIBs), power density and energy density in particular, cannot meet the growing demand for portable device and electric vehicles1-4. Searching for novel electrode materials with high capability and cycling constancy is important for the growth of the next-generation LIBs. In contrast to the commercial graphite anode, transition metal oxides (TMOs) materials have garnered growing attention as potential high-capacity anode materials5-7. Among diverse transition metal oxides, MnO attracts much attention because of its low cost, low toxicity, high theoretical capacity and lower charge potential8-11. Nevertheless, the practical application of MnO anode material is still hampered by its poor capacity retention in long-term cycling and low rate capability resulting from the disadvantaged electrical conductivity and the collapsed structure of MnO material generated by drastic volume change during lithiation/delithiation process12-15. To overcome this drawback, well-designed nanomaterials have been considered to be promising approaches. It has been proved clearly that by reducing the diameter of MnO sample to the smaller size, the physical strain during the cycling processes can be mitigated, which means cracking and particle pulverization can be partially relieved16-19. Whereas there has been


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some published work relevant to the synthesis of low-dimensional MnO materials7, 14-20, it is still attractive to exploit novel synthetic technique that enable fine control over the product morphology, structure and further decrease of the particle size, all of which are closely related to the electrochemical performance of MnO electrodes. However, the crack and fracture of the composite is inevitable as a result of the volumetric expansion of the transition metal oxides material in the repeated changes. Therefore, if smaller transition metal oxide nanoparticles could be embedded into the unique conductive carbon matrix which can provide fast ion/electron transport, it will be a promising route toward the high-performance electrode materials. While it is obvious that this unique structure is attractive, the simple preparation process is still the challenge. Tremendous research was focused on the precise control of reaction conditions and the direct synthesis of ultrafine particles in various environment20-22, but smaller particles were prone to aggregation due to the large surface and the amount of final product was seriously affected by the complexity of experiments. Thus, it is of great significance that larger transition metal oxide particle precursor can melt into carbon matrix and become smaller nanopartilces at the same time. Herein, we put manganese oxide as an example, and synthesized a monolithic MnO/Mn2SiO4@C composite via a novel and facile polymerization-pyrolysis approach. Benefiting from the strong binding effect between Si and Mn, the smaller MnO/Mn2SiO4 was obtained and in-situ embedded in the amorphous carbon framework. Meaningfully, the resulting MnO/Mn2SiO4@C composite can be stable in the long-life charge/discharge process and exhibits excellent performances as an anode material for LIBs with reversible capacities as high as 585.9, 423.9 mA h g-1 at current densities of 1 and 2 A g-1 after 1000 cycles, respectively. Even at a high current density of 30 A g-1, the electrode can still show a stable capacity of over 246.3 mA h


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g-1. To the best of our knowledge, this simple and novel process which makes larger particle precursor to smaller product has never been reported; more importantly, it can be promisingly expanded to synthesis various functional materials in catalysis, sensors, and energy storage. Experimental Sample preparation MnCO3 precursor was prepared from KMnO4 powder via a hydrothermal method23. 0.7 g KMnO4 were dispersed into glycol to form a homogeneous solution (H2O: glycol =3: 1 (volume ratio, a total of 40 mL)) and a purple solution was formed at room temperature. Then the solution was carried into a 50 ml Teflon-lined autoclave, which was sealed up and held at 180 oC for 24 h. After completion of the reaction, the MnCO3 sample was collected and washed thoroughly with distilled water, and followed by drying at 100 oC in vacuum oven. In a typical synthesis of MnO/Mn2SiO4@C composite, 0.008 mol MnCO3, 0.002 mol SiPc, 0.001 mol sodium dodecyl sulfate (SDS) and 0.003 mol pyrazine (pyz) were first put into 35 mL of N, N-dimethyl formamide (DMF) to obtain a homogeneous solution, which was then carried into a 50 mL Teflon-lined autoclave, and heated in an air-circulating oven at 160 °C for 4.5 h. After the temperature of the reactor was lowered to room temperature, the product was harvested by a vacuum rotary evaporation method and then dried in a vacuum oven at 80 °C for 12 h. Ultimately, the sample was calcined for 60 min at high temperature 700 °C under Ar atmosphere in a pipe furnace and the MnO/Mn2SiO4@C composite was acquired. In order to further explain the experimental results, MnO@C and MnO/Mn2SiO4@C products with different Si/Mn ratio were synthesized by different amount of Pc (Phthalocyanine) and SiPc with the same experimental method.


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Characterization X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu Kα irradiation (λ = 1.5406 Å). X-Ray photoelectron spectroscopy (XPS) measurements were carried out investigating the electronic state of all the products, which used Axis Ultra system with achromatic Al Kα X-ray source (1486.6 eV). The Raman spectroscopy was carried out on a Renishaw RM1000 Raman spectrometer using a confocal laser (λ=514.5nm). Thermogravimetric Analysis (TGA) was acquired with a TG/DTA6300 thermal analyzer at 5 oC min−1 under a stream of air. Nitrogen adsorption-desorption isotherm measurement was investigated by Brunauer– Emmett-Teller (BET) method which used the Micromeritics ASAP 2010 analyzer. The morphology and structure of the samples were obtained by Hitachi S-4800 scanning electron microscopy (SEM) coupled with a BRUKER QUANTAX EDS spectrometer. Transmission electron microscopy (TEM) and high resolution transmission electron micro- spectroscopes (HRTEM) were taken with a TECNAI-F20 microscope. Electrochemical measurements The working electrode consists of the MnO/Mn2SiO4@C nanocomposite, Super P carbon, and sodium alginate binder at a weight ratio of 7:2:1 in distilled water as a dispersant. Then the slurry was cast onto stainless steel foils. After being dried at 60 oC, the electrodes were then transferred to vacuum oven and dried at 120 oC for another 12 h, and the loading amount of active materials is about 2-3 mg cm-2.The 2032-type coin cells were prepared in an Ar atmosphere filled glovebox for the electrochemical measurements. Lithium metal was acted as the reference/counter electrode, and a Whatman glass fiber were used as separator. The electrolyte was using ethylene carbonate/dimethyl carbonate (volume ratio =1:1) solution with 1 M LiPF6 as


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salt. The electrochemical performance were carried out using a Neware battery tester with galvanostatic under different current densities of 200- 30000 mA g-1 over the voltage range of 0.01-2 V at room temperature. Results and discussion

Figure 1. Schematic illustration of the formation of the MnO/Mn2SiO4@C and MnO@C composite. The general preparation of MnO/Mn2SiO4@C composite is illustrated in Figure 1. Firstly, monodisperse MnCO3 nanoparticles with an average diameter of ~50 nm were prepared by a solvothermal method (Figure S1 in Supporting Information). Subsequently, the obtained MnCO3 was added into the Pc or SiPc precursor solution. After the polymerization process, MnO@C or Si-doped MnO@C composite could be easily synthesized by annealing the intermediate product at 700 oC for 60 min under the stream of Ar. The typical X-ray powder diffraction (XRD) patterns of the products sintered at 700 oC are shown in Figure 2a. The diffraction peaks could be easily ascribed to those from MnO (JCPDS no. 07-0230)7 and Mn2SiO4 (JCPDS no. 35-0748)24. The dominant phase of all the products is MnO, but the XRD patterns also demonstrate the


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progress of phase transformation under different Si/Mn molar ratios. The introduction of Si element resulted in the formation of Mn2SiO4 phase. Moreover, with the increasing of Si amount, the characteristic peaks related to Mn2SiO4 became more obvious, implying the formation of composite oxides.

Figure 2. (a) X-ray diffraction patterns of Si-doped MnO products obtained in different Si/Mn molar ratios, scanning electron microscopy images of Si-doped MnO products obtained in Si/Mn molar ratios of (b) 0, (c) 1/12, (d) 1/8 and (e, f) 1/4. SEM images from Figure 2 show the evolution of morphology of samples with various Si/Mn molar ratios. It could be clearly demonstrated that different Si /Mn molar ratio seriously affect the morphology of the resulting composites. Core-shell spherical MnO@C materials were observed in the absence of Si precursor, i.e. when only Pc was added as carbon source (Figure 2b), implying that the Pc was an excellent carbon source and did not affect the morphology of the MnCO3 precursor. However, with the increasing of the Si amount, the morphology of the MnO@C underwent transformation from nano-sized sphere to micro-sized cuboid. When the


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Si/Mn molar ratio is 1:12 (Figure 2c), part of spherical MnO@C composites were retained and cuboid products were also discovered, suggestive of the inducing impact of the Si element for MnO. When the Si/Mn molar ratio increased to 1:8 (Figure 2d), it can be found that most of the products changed to cuboid in morphology, but few spherical particles were decorated in the monolithic composites. In addition, the phase composition also changed to coexistence of MnO with Mn2SiO4. This is because the Si element acts as seed and serves as nucleation sites for the growth of MnO@C materials.

Figure 3. (a) SEM image, (b) SEM-EDS mapping, (c) TEM image and (d) cross-section TEM image of the MnO/Mn2SiO4@C composite. Interestingly, when the Si/Mn molar ratio changed to 1:4 (Figure 2e, 2f), all the spherical particles disappeared and the resultant monolithic composites were dispersed uniformly; the phase composition can be recognized as MnO/Mn2SiO4@C. Especially, as is shown in Figure 2f, the diameter of the monolithic composites is almost 3 µm. All the results above show that Si


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element played a very important role in terms of inducing growth of MnO materials, and the introduction of Si element induced a more compact structure for the MnO@C composites. In order to further reveal the composition of monolithic MnO/Mn2SiO4@C composites, elemental distribution was characterized by line-scan SEM as well as energy dispersed spectroscopy (EDS) mapping, which is presented in Figure 3a and 3b. Across the whole portion of the micro-sized composite, Mn, Si, O and C elements were uniformly distributed on the whole cuboid, which confirms the uniform structure of the composite and the formation of Mn2SiO4. A typical transmission electron microscopy (TEM) image of the MnO/Mn2SiO4@C product is displayed in Figure 3c, in which a solid cuboid structure can be observed. And from a crosssection TEM image (Figure 3d and S2 in Supporting Information), it clearly indicates the existence of composite structure. MnO nanoparticles and Mn2SiO4 components were homogeneously dispersed in the amorphous carbon, and due to the close affinity between the MnO and Si, Mn2SiO4 was dispersed around the MnO particles. In addition, the lattice planes of the MnO can be determined clearly in the corss-section TEM image, which further demonstrated the concomitant existence of MnO. As we know, the carbon network and uniformly distributed structure may greatly improve the electrochemical properties of MnO/Mn2SiO4@C cuboid25. N2 adsorption-desorption isotherm and the pore size distribution are shown in Figure 4a and 4b. The Brunauer-Emmett-Teller (BET) surface area of MnO/Mn2SiO4@C composite is measured to be just about 16.1 m2 g-1. The pore size distribution (Figure 4b) indicates MnO/Mn2SiO4@C cuboid has more micropores with diameter of less than 10 nm, which should be beneficial to the infiltration of electrolyte and will be conductive to the high rate-capability26. The carbon content of the MnO/Mn2SiO4@C cuboid is estimated by TGA method. In Figure S3 (Supporting Information), through the total weight loss of MnO/Mn2SiO4@C composite between


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100 and 800 oC, the carbon content is calculated to be about 55 wt %. This is a high carbon amount due to the certain Si/Mn molar ratio from the precursors compared to other transition metal oxide materials, but considering the excellent conductive network for the host MnO/Mn2SiO4 composite and high concentration of active sites of amorphous carbon, the merits of MnO/Mn2SiO4@C cuboid still make it a good candidate as an anode material for LIBs. Especially, Figure S4 in the Supporting Information shows the TEM image of MnO@C material composed of the same carbon amount. It can be observed that the MnO@C material has nanosized diameter, and its BET surface area is measured to be about 90.5 m2·g-1, which is 5.5 times higher than MnO/Mn2SiO4@C cuboid. It is well known that the predicament of higher surface area and lower tap density caused when using nanosized materials as electrode is a serious problem for LIBs27. As such, with the inducing effect between Si and MnO, this problem can partially be resolved. Due to the micro-sized MnO/Mn2SiO4@C cuboid, their tap density is obviously higher than prime MnO@C nanosized particles packed at random.


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Figure 4. (a) Nitrogen adsorptionedesorption isotherms, (b) pore size distribution, (c) Raman spectra, (d) Mn 2p, (e) N 1s and (f) Si 2p XPS spectras of MnO/Mn2SiO4@C composite. Raman spectroscopy was provided to further affirm the existence of MnO and amorphous carbon (Figure 4c). The peak at 644.6 cm-1 is assigned to MnO28, and the peaks observed at 1338.6 and 1574.3 cm-1 can be assigned to the D band and G band of low-graphitized carbon according to the literatures27-29. D band is ascribed to the existence of defects and disorders in the sample which may be induced from the in-situ N doping of PcSi, and the G band represents the C-C stretching of typical graphite. Interestingly, the disordered carbon containing much defects and disordered structure not only possesses more Li storage sites than typical graphite, but also facilitates the diffusion of lithium ions. Thus, the higher intensity ratio ID/IG and the superior compatibility of the MnO/Mn2SiO4@C composite enables its excellent electrochemical performance. Actually, the Raman spectra of the Mn2SiO4 has a characteristic set of two intense lines near 839 cm-1 and 812 cm-1, which are assigned to the Si-O asymmetric stretching band and Si-O symmetric stretching band, respectively30. However, in Figure 4(c), the characteristic peaks of Mn2SiO4 cannot clearly observed. This is because that the amount of Mn2SiO4 in the MnO/Mn2SiO4@C composite is just only 15%, and the small Mn2SiO4 nanoparticles is uniformly coated in the composite cuboid. Although such a small content and the special material structure result in the difficulty in observing the characteristic peaks of Mn2SiO4 in Raman spectra, the XRD pattern and the XPS results can still prove the presence of the Mn2SiO4. In order to further demonstrate the surface composition of MnO/Mn2SiO4@C composition, XPS tests were carried out. Figure 4d presents the Mn 2p3/2 spectrum of MnO/Mn2SiO4@C samples, the spectrum can be deconvoluted into two signals (642.0 eV and 640.9 eV), which are ascribed to Mn2SiO4 and MnO24, 31, respectively. This is in line with the characteristic peaks of both


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manganese oxides. Meanwhile, two signals at 398.2 and 400.2 eV (as shown in the Figure 4e) presents the high-resolution of N 1s spectrum, which are identified as graphitic nitrogen and conjugated nitrogen, respectively32, 33. What’s more, graphitic nitrogen as a kind of pyridine-type nitrogen results in most of the increase of reversible capacity, thus the higher graphitic nitrogen percentage may boost electrochemical performance. Interestingly, Figure 4f shows the Si 2p spectrum of the MnO/Mn2SiO4@C composition, the peak at 101.8 eV can be assigned to the Si-O34, which indicates the existence of Mn2SiO4. Based on characterization demonstrated above, it can be confirmed that this unique designed cuboid is dramatically different from MnO composites reported in the past. Such rationally design has multi-characteristics: (1) the tight integration between MnO/Mn2SiO4 and amorphous carbon permits the MnO/Mn2SiO4 to maintain the particle structure in the lithiation/delithiation process, and the amorphous carbon network can prevent particle cracking; (2) the carbon network are also used as electronic transmission channel and a mechanical skeleton which makes all particles are electrochemically active; (3) amorphous carbon completely warps the whole MnO/Mn2SiO4 particle, thereby restricting SEI formation considerably; (4) MnO, Mn2SiO4 and amorphous carbon are all electrochemical active, which constitute a high-capacity electrode; (5) higher tap density demonstrates higher energy density than other nano-sized MnO electrode. The

electrochemical

reactions

during

the

lithiation/delithiation

process

of

the

MnO/Mn2SiO4@C nanocomposites were obtained by CV profiles and the galvanostatic discharge/charge voltage plots. Figure 5a shows the typical CV profiles of the MnO/Mn2SiO4@C sample for the initial three cycles at a rate of 0.1 mV s-1 in the voltage range of 0.01–3 V vs. Li+/Li. In the first cycle, the broad peak at about 1.0 V can be ascribed to an irreversible reaction related to the formation of SEI layer35,

36

. The cathodic peak at 0.5 V corresponds to the


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reduction conversion reaction between lithium and Mn2+ to form Mn0 and Li2O as shown in the Equation (1-3)37-39: MnO + 2Li+ + 2e- → Mn + Li2O

(1)

Mn2SiO4 + 4Li+ + 4e- → 2Mn + Li4SiO4

(first discharge)

(2)

2Mn + Li4SiO4 4Li+ + 4e- + MnxSiOy

(Subsequent charge/discharge)

(3)

Figure 5. (a) CV curves of the MnO/Mn2SiO4@C nanocomposites for initial five cycles, (b) cycling performance of MnO/Mn2SiO4@C and bare MnO electrodes at 0.5 A g-1, and the corresponded Coulombic efficiency of the MnO/Mn2SiO4@C nanocomposites, (c) Rate performances and (d) long-term cyclic performance of MnO/Mn2SiO4@C composite. Additionally, the broad peaks at 0.7-1 V is related to the reaction of Li ions with functional groups on the amorphous carbon surface and partially reversible SEI formation, which is similar to many carbon materials40-42. And the widened peaks with some weak fluctuations present


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around 1.25 V for the anodic scanning demonstrate the delithiation processes from Mn0 to Mn2+. Moreover, after activation of the first cycle, not only the cathodic plots but also the anodic curves are becoming similar to each other, showing the improvement of reaction kinetics and the weak polarization in the MnO/Mn2SiO4@C structure. Figure 5b displays the charge/discharge performance and the corresponding Coulombic efficiency of MnO/Mn2SiO4@C, pure MnO and graphite at a current density of 0.5 A g-1. The MnO/Mn2SiO4@C composite had an initial Coulombic efficiency at 68%. The irreversible capacity in the initial cycle was owing to the formation of a SEI film and the irreversible storage of Li on the MnO/Mn2SiO4@C composite. The lower initial Coulombic efficiency may lead the unusable of the electrode in the real LIBs, so other ways such as using prelithiated anode should be needed in the real LIBs. And after 200 cycles, a reversible capacity of 638.9 mAh g−1 with a high Coulombic efficiency about 99.5 % was measured, which is obtained on account of the total quality of MnO/Mn2SiO4@C composite and 1.7 times higher than the theoretical capacity of graphite (372 mA h g-1). In particular, pure MnO nanoparticles were prepared by direct calcination of MnCO3 in H2/Ar (5%), which was also measured in LIBs as comparison samples. When the pure MnO nanoparticles were used as electrode materials, the initial specific capacity achieved 783 mA h g−1 at a current density of 0.5 A g-1, but then a fast capacity fade could be clearly discovered as the cycle proceeding. This arises as a consequence of the expansion process and lower conductivity of MnO during the cycling processes, which results in the disintegrating and lower activity of the active materials of the electrodes. In addition, the MnO@C nanocomposite were also presents better cycling performance than pure MnO (Fingure S5 in the Supporting Information). The initial capacity of the MnO@C composite is around 1015 mAh g-1 at a 0.5 A g-1 and drops to 607 mAh g-1 at the second cycle, which shows a lower initial


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Coulombic efficiency. Then the capacity of MnO@C grows incessantly with the increasing cycles. The specific capacity of the MnO@C composite reach to 811.6 mAh g-1 after 200 discharge/charge cycles. The phenomenon which battery capacity increase gradually with the growing cycles is also frequently discovered from other transition metal oxides anodes, which is harmful for the cycling of the real LIBs. That is because the increasing capacity and lower Coulombic efficiency leads to the instable electrode structure and constant consumption of the electrolyte9. The galvanostatic voltage/capacity curves of MnO/Mn2SiO4@C was provided in the Figure S6 (Supporting Information), the capacity–voltage profiles of the MnO/Mn2SiO4@C composite electrode at a current density of 0.5 A g-1 between 0.01 and 2 V were consistent with CV curves. During the initial cycle, an irreversible capacity emerged, which indicated the formation of the SEI layer. In the subsequent cycles, one voltage plateaus could be observed at around 0.5 V during the discharge processes, also matching with the front CV results. Moreover, the galvanostatic charge/discharge profiles clearly illustrated that the charge and discharge capacities at the 200th cycle were close to those at the 50th cycle, which indicated the MnO/Mn2SiO4@C electrode obtained a high reversible capacity. The rate capability of MnO/Mn2SiO4@C was measured at the same current densities from 0.2 to 30 A g-1 (Figure 5c). At the current density of 0.2 A g-1, MnO/Mn2SiO4@C composite obtained a high average capacity of 671.2 mA h g-1. The capacities decreased gradually as current density increases. The specific discharge capacities were 540.1, 536.7, and 498.9 mA h g-1 at current rates of 2, 3, and 4 A g-1, respectively, corresponding to a truly exceptional rate performance. Impressively, even reaching a very high current density of 30 A g-1, a capacity of 246.3 mA h g-1 could still be obtained. After an ultrafast cycling at rate of 30 A g-1, the current density of 0.2 A g-1 was recovered, the average capacity


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of MnO/Mn2SiO4@C composite could be achieved

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840 mA h g-1, which indicated the

outstanding electrochemical reversibility of this electrode. As we know, this high rate performance is outstanding compared to other MnO reported. Moreover, it is important to mention that even after 1000 charge−discharge cycles under 1, 2 and 10 A g-1, the reversible capacity of the MnO/Mn2SiO4@C electrode was 585.9, 423.9 and 372 mA h g-1, respectively (Figure 5d). The great high-rate behavior and cycling constancy of the MnO/Mn2SiO4@C composite are ascribed to the novel highly stable core-shell cuboid structure, resulting from the pores allowing the fast diffusion of Li+ and electron transfer between amorphous carbon matrixes and MnO/Mn2SiO4. EIS of different cycles are delivered shown in Figure S7 (Supporting Information). It is evidently discovered that the Nyquist plots at different cycles have analogous shapes, indicating the high structural integrity of MnO/Mn2SiO4@C cuboid. Meanwhile, the charge-transfer resistance gradually decreases with increasing cycle numbers, which may be due to the composite structure and its activation process43. The ex-situ XPS spectrum after 30 charged/discharged cycles were also measured as shown in Figure S8 (Supporting Information). It can be noted that binding energy signals of 642.2 eV and 640.8 eV in the Mn 2p3/2 spectra respectively represents Mn2SiO4 and MnO in the MnO/Mn2SiO4@C composite after cycling. And the values of 102.1 eV in Si 2p spectra reveals the existence of the Si-O bond. Especially, there is no apparent chemical state change in the XPS spectrum compared with the original data before cycling, indicating the stability of the composite electrode structure. In addition, the microstructural properties of the MnO/Mn2SiO4@C cuboid nanocomposite electrodes were also tested by using TEM to confirm the favorable cuboid structure (Figure 6). From the TEM images, after 80 cycles, the MnO/Mn2SiO4@C structure were almost retained, and no crack or


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fracture were observed. Not only does carbon matrix here provide tight binding for accommodation of volume expansion, the Si-induced MnO/Mn2SiO4@C cuboid also easily maintains their structural integrity. This morphological invariance corroborates the excellent cycling stability.

Figure 6. TEM images of the MnO/Mn2SiO4@ composite after 80 charge/discharge cycles at 0.5 A g-1. Conclusions In summary, the MnO/Mn2SiO4@C cuboid was synthesized by Si-induced mechanism in the simple polymerization-pyrolysis process, and the Mn/Si molar ratio of the reactant is the critical point in the cuboid preparation. The uniquely designed MnO/Mn2SiO4@C nanocomposite makes a higher electrochemical performance and better charge transfer kinetics properties than graphite due to the uniform distribution of MnO/Mn2SiO4 nanoparticles and the unique core-shell cuboid structure. Considering the MnO/Mn2SiO4@C cuboid composite with novel structure displayed remarkable electrochemical performance with stable specific capacity of 585.9 mA h g-1 at 1 A


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g-1 after 1000 cycles and 246.3 mA h g-1 at 30 A g-1, it could be regarded as a better anode material for advanced LIBs. Moreover, due to its simplicity and micro-sized morphology leading higher tap density, this method may be expanded to other transition metal oxide nanomaterials for improving their lithium-storage properties.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the New Energy Project for Electric Vehicle of the National Key Research and Development Program (2016YFB0100200) and the National Natural Science Foundation of China (51671004) and Program of Higher-level Talents of Inner Mongolia University (21300-5165155). REFERENCES (1)

Crabtree, G., Perspective: The Energy-Storage Revolution. Nature 2015, 526, S92.


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Mn2SiO4@C Cuboids as High

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