Crystallographic Habit Tuning of Li2MnSiO4 Nanoplates for High

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Crystallographic Habit Tuning of Li2MnSiO4 Nanoplates for High-Capacity Lithium Battery Cathodes Zhengping Ding, Yiming Feng, Datong Zhang, Ran Ji, Libao Chen, Douglas G. Ivey, and Weifeng Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17587 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

Crystallographic Habit Tuning of Li2MnSiO4 Nanoplates for High-Capacity Lithium Battery Cathodes Zhengping Ding,† Yiming Feng,†, Datong Zhang,† Ran Ji,† Libao Chen,† Douglas G. Ivey,‡ and Weifeng Wei*,† †

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan

410083, People’s Republic of China ‡

Department of Chemical & Materials Engineering, University of Alberta, Edmonton,

Alberta, Canada T6G 1H9

KEYWORDS: Lithium manganese orthosilicate cathodes, preferential growth, enhanced performance, first-principle calculations, lithium ion batteries ABSTRACT: Li2MnSiO4 has attracted significant attention as cathode material for lithium ion batteries due to its high theoretical capacity (330 mAhg-1 with two Li+ ions per formula unit), low cost and environmental friendly nature. However, its intrinsically poor Li diffusion, low electronic conductivity and structural instability preclude its use in practical applications. Herein, elongated, hexagonal prism shaped Li2MnSiO4 nanoplates with preferentially exposed {001} and {210} facets have been successfully synthesized via a solvothermal method. Density functional theory (DFT) calculations and experimental characterization reveal that the formation mechanism involves the decomposition of solid precursors to nanosheets, selfassembly into nanoplates and Ostwald ripening. Hydroxyl-containing solvents such as ethylene glycol (EG) and diethylene glycol (DEG) play a crucial role as capping agents to tune the preferential growth. Li2MnSiO4@C nanoplates demonstrate a near theoretical

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discharge capacity of 326.7 mAh g-1 at 0.05 C (1 C = 160 mAh g-1), superior rate capability and good cycling stability. The enhanced electrochemical performance is ascribed to the electrochemically active {001} and {210} exposed facets that provide short and fast Li+ diffusion pathways along [001] and [100] axes, a conformal carbon nanocoating and a nanoscaled plate-like structure that offers a large electrode/electrolyte contact interface for Li+ extraction/insertion processes.

1. INTRODUCTION Orthosilicate materials based on Li2FeSiO4 and Li2MnSiO4 have attracted extensive attention as potential alternatives to conventional positive electrode materials for lithium ion batteries (LIBs).

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The key feature of these orthosilicate materials is possible extraction of

two lithium ions due to a two-electron redox process, offering a much higher theoretical capacity (e.g., ~330 mAh g-1 for Li2MnSiO4) than commercially available LIB positive electrode materials.1, 6-9 Moreover, since the high oxidation state Mn4+ is more accessible than Fe4+, the second Li ion should be extracted more readily from Li2MnSiO4 than from Li2FeSiO4.10-12 Thus, in principle, Li2MnSiO4 could be a very promising high-energy cathode due to the high stability of Mn4+ ions and favorable discharge voltage at ~4V.1, 11 Nevertheless, the practical reversible capacity of Li2MnSiO4 is generally limited to about one Li ion extraction/insertion because of its very low electronic conductivity and poor Li+ diffusivity, as well as structure collapse induced by Jahn-Teller distortion.13-17 Several strategies have therefore been explored to mitigate the above-mentioned problems through coating with conductive materials18-21 and nanostructuring3, 22-24, but a full two Li ion capacity and stable cyclic performance are still unattainable. Recently, Rangappa et al. demonstrated ultrathin Li2MnSiO4 nanosheets (~3 nm thick) prepared by a supercritical fluid process with high discharge capacities of 220 mAh g-1 at room temperature and ∼340 mAh/g at 45 ± 5°C, respectively.22 This significant advancement indicates that the ultrathin nanosheets with high


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aspect ratios may possess distinct structural strength and exhibit enhanced kinetics because of the short Li diffusion path and large contact area with conductive material. In addition, considering the diffusion anisotropy of orthosilicates, it is of great importance to tailor the crystallographic habit planes of orthosilicate materials for enhanced kinetics. Theoretical computations reveal that for Li2MnSiO4 with the most stable orthorhombic Pmn21 structure, Li cations diffuse along two main migration pathways.25-26 Path A is a zigzag trajectory along the c axis and path B follows the a axis, with corresponding Li-Li migration energy barriers of 0.95 eV and 1.29 eV, respectively, which is indicative of higher Li mobility parallel to the c axis.26 Thus, if thin sheet- or plate-like Li2MnSiO4 materials with preferentially exposed (001) facets and minimum thicknesses can be obtained, the Li insertion/extraction kinetics would be substantially enhanced when compared with randomly oriented nanomaterials. In this regard, a facile solvothermal process has been developed to synthesize elongated, hexagonal prism Li2MnSiO4 nanoplate cathodes with tailored crystallographic facets, i.e., preferential growth of (001) and (210) nanoplates is achieved by adjusting the precursor and solvent (ethylene glycol (EG) and diethylene glycol (DEG)) content and solvothermal time. Combined with DFT calculations and experimental results, it is shown that hydroxylcontaining molecules act as a capping agent for manipulating crystal growth. The ultrathin nanoplates combine the benefits of enhanced Li diffusion across electrochemically active (001) and (210) planes and improved structural integrity, both of which ensure a two Li ion reversible capacity and outstanding rate capability.

2. EXPERIMENTAL SECTION 2.1. Materials synthesis. Experimental Details. Li2MnSiO4 nanoplates were synthesized via a solvothermal method. Typically, 50 mmol LiOHH2O and 12.5 mmol Si(OCOCH3)4 were dissolved in 30 mL EG to form solution A, while 12.5 mmol of MnCl24H2O was dissolved ACS Paragon Plus Environment

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in 30 mL EG as solution B. Solution B was then slowly dissolved into solution A and stirred for 30 min to form a uniform suspension. The suspension was then poured into a 100 mL Teflon-lined autoclave and heated to 230°C for 48 h. The Li2MnSiO4 nanoplates were filtered and washed with ethanol and deionized water for several times and dried at 100°C. For the synthesis of Li2MnSiO4 nanoplates using DEG as the solvent, the raw materials and procedures were unchanged except that EG solvent was replaced by DEG solvent. To improve the conductivity, 1 g of as-prepared Li2MnSiO4 nanoplates were mixed with 0.4 g sucrose in 40 mL ethanol; the dried precursor was then carbonized at 700°C for 5 h in an Ar atmosphere. 2.2. Materials characterization. X-ray diffraction (XRD) patterns were performed on a Bruker AXS D8 Advance X-ray diffractometer with Cu Kɑ radiation (λ= 1.54056 Å) at steps of 0.02°and a counting time of 2 s. Rietveld refinement was done using a GSAS+EXPGUI software. The morphology of the samples was characterized using a Nano SEM 230 field emission scanning electron microscope (FEI, USA) and a JEM-2100F field emission transmission electron microscope (JEOL, Japan). Thermogravimetric analysis was conducted under air flow with a temperature ramp up rate of 10°Cmin-1. X-ray photoelectron spectroscopy (XPS) spectra was collected by an X-ray photoelectron spectrometer (Thermo Scientific ESCALAB 250Xi, USA) and adjusted using the C 1s peak at 284.5 eV. Atomic Force Microscopy (AFM) measurements were carried out on NanoMan VS atomic force microscopy using tapping mode (Vecco, USA). Sample ethanol suspensions were dropped directly onto a substrate, which is consist of a thin layer of SiO2 on Si. After dried in a clean environment, the measurements were executed in air at ambient temperature and pressure.

2.3. Electrochemical measurements. Electrochemical tests were performed using a coin cell assembled in a glove box filled with Ar gas. For the electrode, 80 wt% active materials, 10 wt% ACS Paragon Plus Environment

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polyvinylidene fluoride (PVDF) and 10 wt% acetylene black were mixed together in Nmethyl-2-pyrrolidone (NMP) to form a slurry. The slurry was coated onto Al foil and dried at 110°C in a vacuum oven. Then it was punched into plates (12 mm in diameter) as the cathode, and the typical cathode mass loading is ~1.5 mg cm-2. Lithium metal was used as the anode, Celgard 2500 membrane was served as the separator and 1 mol/L LiPF6 in ethylene carbonate and dimethylcarbonate (1:1 v/v) served as the electrolyte. The cells were galvanostatic charged-discharged on a Land CT2100A battery testing system (Wuhan, China) between 1.5 V and 4.7 V vs. Li+/Li at 30°C. Electrochemical impedance spectra (EIS) were obtained using a PARSTAT 4000 electrochemical workstation (Princeton Applied Reasearch) in the frequency range from 100 kHz to 0.1 Hz. 2.4. Theory calculation. First principles calculations were done using the Vienna ab initio simulation package (VASP) on the base of density functional theory (DFT).

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The electron-

ion interactions were defined through the projector-augmented wave (PAW). 28 Exchange and correlation items were described by the generalized gradient approximation (GGA) proposed by Perdew-Burke-Ernzerhof (PBE)

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. An energy cutoff of 500 eV and appropriate k-point

meshes were selected to guarantee that the total energies converged within 1 meV per formula unit of Li2MnSiO4. The GGA+U method was applied to accurately calculate the electrochemical properties and partially correct for electron overdelocalization (and selfinteraction errors). 30 Within this approach, an effective value Ueff = U - J of 5 eV was used. The lattice parameters of the bulk Li2MnSiO4 are a= 6.3704 Å, b= 5.4359 Å and c= 5.0274 Å, which is in accordance with the experimental results.

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The slab technique was

used to model the surfaces of Li2MnSiO4 in which a set of infinite layers divided by vacuum layers are repeated regularly along the surface. When creating a surface, the SiO4 tetrahedra were preserved due to the highly covalent Si-O bonds. The lattice parameters of the supercell (including slab and vacuum) were fixed and only atoms near the surface were allowed to relax until the forces were smaller than 0.03 eV/Å. The vacuum layers were set to be 12 Å thick ACS Paragon Plus Environment

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and the inner part of the slab was frozen at the bulk position. The surface free energies (Esurf) were calculated based on Equation (1):

=

− 2

(1)

where Ebulk is the total energy per formula unit of bulk Li2MnSiO4, Eslab is the total energy of the given supercell containing n formula units of Li2MnSiO4 and S is the base area of the supercell. The adsorption energies for EG and DEG molecules adsorbed on the (1×1) unit cell of (001), (010), (100) and (210) surfaces were calculated. The EG and DEG molecules were located on top of the slab and the dipole correction (VDW) was added in the calculations. A cutoff energy of 500 eV and k-point meshes of (5×4×1), (5×5×1), (4×3×4) and (5×2×1) were used for the calculation of the (001), (010), (100) and (210) surfaces, respectively. The adsorption energy was calculated according to Equation (2). = ( + ) −

(2)

where Eads is the adsorption energy, Esurf is the total relaxation energy of the slab model, Eadsorbate is the energy of the adsorbed molecule and Etotal is the energy of the system after adsorption of EG and DEG molecules.

3. RESULTS AND DISCUSSION Powder XRD and Rietveld refinement results for the as-prepared Li2MnSiO4 nanoplates are shown in Figure 1 and Table S1. All diffraction peaks are indexed to the orthorhombic Pmn21 space group with a = 6.2967(8) Å, b = 5.4000(1) Å and c = 4.9609(4) Å, suggesting that the pure Li2MnSiO4 phase has been prepared by the solvothermal method. 3 As shown in Figure 1a, the diffraction peaks exhibit some broadening, which indicates that the sample is nanostructured; the average grain size was calculated as ~17 nm based on the Rietveld


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refinement results (Table S1). Interestingly, the measured intensities of the (210) and (002) peaks differ from those of the standard pattern (Figure 1b), i.e., the intensity ratios for I(210)/I(010) and I(002)/I(010) are much higher than those for the reference data (3.30 and 1.73 vs. 1.455 and 1.291, respectively (Table S2)). Large differences in peak intensity ratios between the experimental and reference data indicate a possible preferred orientation along a particular facet or direction.32-34 The XRD data suggest that the as-prepared Li2MnSiO4 particles may have tailored crystallographic facets. This hypothesis is further substantiated by the following microscopy analysis.

Figure 1. (a) Powder XRD pattern for Li2MnSiO4 nanoplates, as well as Rietveld refinement of Pmn21. (b) Expanded diffraction areas between 14.5° and 18° and between 30° and 39°, respectively.

As shown in Figure 2a-2c, SEM and TEM images of the Li2MnSiO4 particles indicate that rod-shaped nanoplates with lengths of ~40 nm and widths of ~10 nm are obtained after processing. To better visualize the crystal structure and morphology of the nanoplates, the sample was further characterized by high resolution TEM (HRTEM). In Figure 2d, the dspacings of the lattice fringes are 0.544 and 0.275 nm, corresponding to the (010) and (210) planes, respectively, of the Pmn21 structure, indicating that the zone axis is along the [001] direction. This is also confirmed by the corresponding fast Fourier transform (FFT) pattern,


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which is indexed to the Pmn21 space group with a [001] zone axis (inset of Figure 2d). In addition, the angle A was measured as 120.4°, corresponding to the angle between (010) and (210) facets and the angle B was measured as 119.2°, which is representative of the angle between (210) and (-210) facets. Figure 3 shows an atomic force microscope (AFM) image and height profiles for the Li2MnSiO4 nanoplates. The heights measured between the surface of Li2MnSiO4 nanoplates and the SiO2 substrate are within the range of 5 to 20 nm. Hence, as shown in Figure 2e, the likely shape of the nanoplates is an elongated hexagonal prism with a long y axis (~35 nm) and short x and z axes (5-20 nm). The main exposed planes are {001}, {010} and {210} facets. As mentioned above, Li ions predominantly diffuse along the [001] and [100] directions in the orthorhombic Pmn21 structure (Figure 2f). Therefore, with preferentially exposed active surface planes, the elongated, hexagonal prism shaped Li2MnSiO4 nanoplates are expected to have superior Li+ transport kinetics, which is beneficial for high performance cathode materials in LIBs.


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Figure 2 ． (a) Representative SEM secondary electron (SE) micrograph, (b, c) low magnification TEM bright field (BF) images and (d) HRTEM image (inset: FFT diffraction pattern) of Li2MnSiO4 nanoplates. (e) Schematic illustration of an elongated, hexagonal prism shaped Li2MnSiO4 nanoplate. (f) Crystal structures and diffusion pathways of Li+ ions in Li2MnSiO4 with the orthorhombic Pmn21 structure.


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Figure 3. (a) Atomic force microscope (AFM) image and (b) corresponding height profiles (A-B, C-D and E-F) of Li2MnSiO4 nanoplates.

To understand the formation mechanism of elongated, hexagonal prism shaped Li2MnSiO4

nanoplates, the growth process was investigated by examining the products

collected at different intervals of reaction time and through DFT calculations. Based on the TEM image (Fig 4a) and XRD pattern (Figure 4g), it is clear that the product after 1 h reaction time is large amorphous sediments. As the reaction time is increased to 3h, a diffraction peak corresponding to the (210) plane of Li2MnSiO4 appears in the XRD pattern and the large sediments gradually decompose and form nanosheet-like products (Figure 4b). As the reaction time is further increased to 9 h, most of the nanosheets have self-assembled into nanocrystals (Figure 4c-4d). As shown in Figure 4e-4f, nanoplates with a higher degree of crystallinity and the elongated, hexagonal prism shape are formed when the reaction time reaches 12 h or even 48 h. The diffraction peaks, indexed to the orthorhombic Pmn21 structure, also become stronger and sharper with increasing reaction time (Figure 4g). On the basis of the morphological and structural analysis, a proposed formation mechanism of the Li2MnSiO4 nanoplates is depicted in Figure 4h. Large amorphous


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precursors were formed based on Reaction (1). 4LiOH + MnCl2 + Si(OC2H5)4 = Li2MnSiO4 + 4C2H5OH + 2LiCl

(1)

With increasing reaction time, the precursors decompose to generate nanosheets under solvothermal conditions. After aging for longer times, the metastable nanosheets tend to recrystallize into nanoparticles by the Ostwald ripening process. EG or DEG, used as the solvent in the experiment, can easily adsorb on the surface of particular Li2MnSiO4 crystallographic planes. As a result, specific facets are stabilized and grow into plate-shaped nanocrystals with preferred facets.

Figure 4. TEM BF images of precipitates obtained at different reaction times: (a) 1 h, (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h and (f) 48 h. (g) XRD patterns for precipitates obtained at different reaction times. (h) Schematic illustration of the formation process for elongated, hexagonal prism shaped Li2MnSiO4 nanoplates.


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Table 1. Adsorption energies of (001), (010), (100) and (210) surfaces for Li2MnSiO4 adsorbing ethylene glycol (EG)

Surfaces Number of oxygen atoms E (surf) / eV

(001)

(010)

(100)

(210)

4

2

2

3

-433.1193

-329.0321

-292.9944

-561.4525

E (adsorbate) / eV

-53.1109

E (total) / eV

-486.4180

-382.4841

-345.7988

-617.0326

E(ads)* / eV

0.1878

0.3411

-0.3065

2.4692

* E(ads) = E(surf) + E(adsorbate) – E(total) In the solvothermal synthesis process, surface energy and adsorption energy are the crucial factors to control the formation of the nanostructures with specific morphologies. Therefore, experimental results and first-principles calculations were combined to study the surface energy and adsorption energy of crystal planes adsorbed by hydroxyl-containing molecules. The surface energies of (001), (010), (100) and (210) facets were calculated using the vacuum model and the results are listed in Table 1. As shown in Table 1, the surface energy of (210) facets is -561.4525 eV, which is lower than that of (001), (010) and (100) facets, indicating that (210) facets are more stable. Thus, (210) facets appear first during synthesis. The second most stable surface is the (001) facets appear with increasing reaction time, correlate with the appearance of the (002) peaks in the XRD results. Solvents with hydroxyl groups can easily adsorb on a specific crystallographic facet, adjust their surface energy and, in turn, affect the crystal shape.

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As EG molecules are introduced, chemical

absorption occurs on Li2MnSiO4 surfaces by hydrogen bonds between the hydroxyl groups and the oxygen atoms of the different crystal facets. The adsorption energies of EG molecules on (001), (010), (100) and (210) facets were calculated and are shown in Figure 5 and Table 1. The adsorption energies of EG molecules on (001), (010), (100) and (210) facets are 0.187825, 0.341152, -0.3065 and 2.4692 eV, respectively (Table 1), demonstrating that EG molecules ACS Paragon Plus Environment

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are adsorbed on (001), (010) and (210) facets and desorbed on (100) facets. When EG solvent is replaced with DEG solvent during the solvothermal process, a similar trend is observed for the adsorption energies of DEG molecules on different crystallographic facets (Table S3). In addition, similar Li2MnSiO4 nanoplates with preferentially exposed {001} facets can be obtained through the DEG-assisted solvothermal process (Figure S1). Consequently, elongated, hexagonal prism shaped Li2MnSiO4 nanoplates with preferentially {001}, {010} and {210} facets exposed are successfully synthesized through this solvothermal reaction using EG and DEG as capping agents.


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Figure 5. DFT calculations of the surface adsorption energy of EG molecules on different Li2FeSiO4 surfaces. (a) (001) plane, (b) (010) plane, (c) (100) plane and (d) (210) plane.

To improve the electronic conductivity, the synthesized Li2MnSiO4 nanoplates were modified with a carbon nanocoating using sucrose as the C source. The C content in the Li2MnSiO4@C nanoplates is about 14.34 wt% based on thermogravimetric analysis (TGA) (Figure S2). The Raman spectrum for Li2MnSiO4@C nanoplates shows two bands at 1349


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cm-1 and 1587 cm-1, representative of the D band and the G band, respectively (Figure S3). The intensity ratio of these two bands (ID/IG) is 0.776, indicative of a partially graphitized carbon layer and enhanced electronic conductivity. XPS was employed to investigate the element’s chemical states in Li2MnSiO4@C nanoplates. The survey spectrum in Figure S4a shows the characteristic Li 1s, Mn 2p, Si 2p, O 1s and C 1s peaks. Deconvolution of the high resolution C 1s spectrum results in three peaks centered at 284.8, 286.2 and 290.0 eV, which is representative of sp2-hybridized graphite-like C-C bonding, C-O bonding and O-C=O bonding (Figure S4b).

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The amount of C-C sp2 is 66.1%, indicating a high degree of

graphitized carbon, which is in line with the Raman result. The Mn 2p3/2 binding energy of 641.78 eV is consistent with that of Mn2+ (Figure S4c) and the satellite peaks observed at ~647.5 eV only arise from Mn2+, both of which confirm that the Mn valence is 2+.

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The

binding energy for Si 2p centered at 101.69 eV (Figure S4d) is in agreement with that of Si4+, indicating the presence of the orthosilicate [SiO4] structure.

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Figure S5 shows an XRD

pattern of the Li2MnSiO4@C nanoplates. All peaks can be well indexed according to the Pmn21 structure and the peak intensities for the (210), (020) and (002) reflections still deviate from those of the standard, indicating that the preferentially exposed facets were maintained after carbon coating. Moreover, as shown in Figure S6, the nanoplate morphology is retained in the Li2MnSiO4@C composite with a nanoscale carbon layer (~2-5 nm thickness). The FFT pattern for the nanoparticle is indexed as the Pmn21 structure along the [001] zone axis, suggesting an exposed (001) plane perpendicular to the [001] direction (Figure S6d). The electrochemical performance of Li2MnSiO4@C nanoplates was investigated by galvanostatic charge-discharge method at 30 °C. Figure 6a shows the initial charge-discharge curves (3 cycles) at 0.05 C in the potential range from 1.5 to 4.7 V (1 C = 160 mAhg-1). The initial discharge specific capacity of Li2MnSiO4@C nanoplates is 326.7 mAhg-1, corresponding to 1.98 Li ions stored per Li2MnSiO4 formula. The charge specific capacity (367.6 mAhg-1) is slightly higher than the theoretical capacity of Li2MnSiO4 (333 mAhg-1), ACS Paragon Plus Environment

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which can be ascribed to electrolyte decomposition at high voltages above 4.5 V.

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The

derivative plots (dQ/dV versus voltage) for the charge-discharge curves of Li2MnSiO4@C nanoplates are shown in Figure 6b. There are anodic peaks at 4.12 V and 4.38 V in the initial charge process, corresponding to the stepwise oxidation from Mn2+ to Mn4+. 3 During the first discharge process, cathodic peaks at 3.96 and 2.91 V are detectable, representative of reduction from Mn4+ to Mn3+ and Mn3+ to Mn2+. During the 2nd cycle, the anodic peaks at 4.12 and 4.38 V disappear and a new anodic peak at 3.20 V shows up, which can be ascribed to structural rearrangement and phase transformation of crystalline Li2MnSiO4 into an amorphous phase during the first charge process.

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No deviation of the anodic and

cathodic peaks is observed during the 2nd and 3rd cycles, suggesting good reversibility for the redox reactions during the charging-discharging process. Figure 6c shows the discharge profiles of Li2MnSiO4@C nanoplates at increasing current densities ranging from 0.05 C to 10 C. With increasing C rates, the discharge plateau shifts to a lower potential as a result of polarization induced by kinetic limitations. The discharge capacities for Li2MnSiO4@C nanoplates are 326.7, 242.2, 215, 172.2, 143.3, 116.4, 91.8 and 68.4 mAhg-1 at rates of 0.05 C, 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C and 10 C, respectively. Figure 6d shows the rate cycling performance of Li2MnSiO4@C nanoplates. When the rate is returned to 0.1 C after 40 cycles, the discharge capacity for Li2MnSiO4@C nanoplates can recover to 183.5 mAhg-1, corresponding to a capacity retention of 75.8%, which is indicative of good cycling stability. In addition, as shown in Figure 6e, the first discharge capacity of Li2MnSiO4@C nanoplates is 143.3 mAhg-1 at 1 C and a capacity retention of 50.45% is achieved after 100 cycles. The continuous capacity fading is mainly caused by the Jahn-Teller distortion of Mn3+ and the Mn dissolution in electrolyte, resulting in collapse of the crystal structure and amorphization of the materials.

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The capacities for Li2MnSiO4@C

nanoplates obtained in this work exceed those of the Li2MnSiO4 cathodes reported in most previous studies (Table S4). ACS Paragon Plus Environment

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Figure 6 Electrochemical performance of synthesized elongated, hexagonal prism shaped Li2MnSiO4@C nanoplates. (a) Charge-discharge curves at a rate of 0.05 C for the initial 3 cycles. (b) Differential capacity versus potential plots during the initial 3 cycles. (c) Discharge curves at different rates from 0.05 C to 10 C. (d) Rate cyclic performance at different C rates. (e) Cycling performance at a rate of 1 C. (1 C = 160 mAhg-1.). (f) Discharge curves of 1, 10, 20, 40, 60, 80, and 100 cycles at 1 C rate. The enhanced electrochemical performance can be attributed to the unique crystallographic habit-tuned Li2MnSiO4@C nanoplates. Firstly, the elongated, hexagonal prism shaped nanoplates with {001} and {210} exposed facets provide short and fast Li+ diffusion pathways along the [001] and [100] directions. In addition, the nanoscale plate-like structure offers a large electrode/electrolyte contact interface for Li+ intercalation and deintercalation reactions, leading to enhanced rate capability. As shown in Figure S7, the calculated Li ion diffusion coefficient is 1.004×10-13 cm2s-1, which is much higher than the reported Li ion diffusion coefficient (10-14~10-18 cm2s-1).

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Secondly, the

electronic conductivity is improved by the conformal C nanocoating. As shown in Figure S7, the C-coated Li2MnSiO4@C nanoplates demonstrate a much lower charge transfer resistance


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(63.7 Ω) than the bare Li2MnSiO4 nanoplates (231.3 Ω), implying higher electro-activity for Li storage and better rate performance. Thirdly, the unique nanoplate-like structure could provide sufficient reaction space for volume changes (27% for Li2MnSiO4 with two Li-ion extraction)39 and buffer the structural strain during the Li+ intercalation and deintercalation processes, resulting in a good cycle life. 4. CONCLUSIONS In summary, crystallographic habit tuning of Li2MnSiO4 nanoplates with preferentially exposed {001} and {210} facets has been achieved via a solvothermal process. On the basis of theoretical calculations and experimental characterization, a possible formation mechanism has been put forward and reveals the effect of hydroxyl-containing solvents as a capping agent to tune preferential growth. The Li2MnSiO4@C nanoplates demonstrate a near theoretical discharge capacity of 326.7 mAh g-1 at 0.05 C, with superior rate capability and good cycling performance. The improved electrochemical performance is attributed to the electrochemically active {001} and {210} exposed facets that provide short and fast Li+ diffusion pathways along the [001] and [100] directions. In addition, the nanosized plates offer a large electrode/electrolyte contact interface for Li+ extraction/insertion processes. ASSOCIATED CONTENT Supporting Information. This Supporting Information is available free of charge on the ACS Publication website. XRD Rietveld refinement, TEM and HRTEM images, TG curve, Raman spectrum, XPS spectra and EIS plots of the sample, List of electrochemical properties of Li2MnSiO4 based cathodes. AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] Author Contributions Z. D. and Y. F. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to acknowledge financial support from the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (51304248), the Innovation Program of Central South University (2016CXS003), the State Key Laboratory of Powder Metallurgy at Central South University and the Hunan Shenghua Technology Co., Ltd. REFERENCES (1) Cheng, Q.; He, W.; Zhang, X.; Li, M.; Wang, L., Modification of Li2MnSiO4 Cathode Materials for Lithium-Ion Batteries: a Review. J. Mater. Chem. A 2017, 5, 10772-10797. (2) Girish, H. N.; Shao, G. Q., Advances in High-Capacity Li2MSiO4 (M = Mn, Fe, Co, Ni, …) Cathode Materials for Lithium-Ion Batteries. RSC Adv. 2015, 5, 98666-98686. (3) Muraliganth, T.; Stroukoff, K. R.; Manthiram, A., Microwave-Solvothermal Synthesis of Nanostructured Li2MSiO4/C (M = Mn and Fe) Cathodes for Lithium-Ion Batteries. Chem. Mater. 2010, 22, 5754-5761. (4) Islam, M. S.; Dominko, R.; Masquelier, C.; Sirisopanaporn, C.; Armstrong, A. R.; Bruce, P. G., Silicate Cathodes for Lithium Batteries: Alternatives to Phosphates? J. Mater. Chem. 2011, 21, 9811-9818. (5) Ferrari, S.; Capsoni, D.; Casino, S.; Destro, M.; Gerbaldi, C.; Bini, M., Electrochemistry of Orthosilicate-based Lithium Battery Cathodes: a Perspective. Phys. Chem. Chem. Phys. 2014, 16, 10353-10366.


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Figure 1. (a) Powder XRD pattern for Li2MnSiO4 nanoplates, as well as Rietveld refinement of Pmn21. (b) Expanded diffraction areas between 14.5° and 18° and between 30° and 39°, respectively. 59x26mm (300 x 300 DPI)


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Figure 2． (a) Representative SEM secondary electron (SE) micrograph, (b, c) low magnification TEM bright field (BF) images and (d) HRTEM image (inset: FFT diffraction pattern) of Li2MnSiO4 nanoplates. (e) Schematic illustration of an elongated, hexagonal prism shaped Li2MnSiO4 nanoplate. (f) Crystal structures and diffusion pathways of Li+ ions in Li2MnSiO4 with the orthorhombic Pmn21 structure. 199x253mm (300 x 300 DPI)



Figure 3. (a) Atomic force microscope (AFM) image and (b) corresponding height profiles (A-B, C-D and E-F) of Li2MnSiO4 nanoplates. 80x40mm (300 x 300 DPI)


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Figure 4. TEM BF images of precipitates obtained at different reaction times: (a) 1 h, (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h and (f) 48 h. (g) XRD patterns for precipitates obtained at different reaction times. (h) Schematic illustration of the formation process for elongated, hexagonal prism shaped Li¬2MnSiO4 nanoplates. 119x90mm (300 x 300 DPI)



Figure 5. DFT calculations of the surface adsorption energy of EG molecules on different Li2FeSiO4 surfaces. (a) (001) plane, (b) (010) plane, (c) (100) plane and (d) (210) plane. 174x218mm (300 x 300 DPI)


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Figure 6. Electrochemical performance of synthesized elongated, hexagonal prism shaped Li2MnSiO4@C nanoplates. (a) Charge-discharge curves at a rate of 0.05 C for the initial 3 cycles. (b) Differential capacity versus potential plots during the initial 3 cycles. (c) Discharge curves at different rates from 0.05 C to 10 C. (d) Rate cyclic performance at different C rates. (e) Cycling performance at a rate of 1 C. (1 C = 160 mAhg1.). (f) Discharge curves of 1, 10, 20, 40, 60, 80, and 100 cycles at 1 C rate. 82x43mm (300 x 300 DPI)


Crystallographic Habit Tuning of Li2MnSiO4 Nanoplates for High

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