Understanding the Improved Kinetics and Cyclability of a Li2MnSiO4

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Understanding the Improved Kinetics and Cyclability of a Li2MnSiO4 Cathode with Calcium Substitution Yiming Feng,§,† Ran Ji,§,† Zhengping Ding,† Datong Zhang,† Chaoping Liang,† 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 T6G 1H9, Canada

‡

S Supporting Information *

ABSTRACT: Limited practical capacity and poor cyclability caused by sluggish kinetics and structural instability are essential aspects that constrain the potential application of Li2MnSiO4 cathode materials. Herein, Li2Mn1−xCaxSiO4/C nanoplates are synthesized using a diethylene-glycol-assisted solvothermal method, targeting to circumvent its drawbacks. Compared with the pristine material, the Ca-substituted material exhibits enhanced electrochemical kinetics and improved cycle life performance. In combination with experimental studies and first-principles calculations, we reveal that Ca incorporation enhances electronic conductivity and the Li-ion diffusion coefficient of the Ca-substituted material, and it improves the structural stability by reducing the lattice distortion. It also shrinks the crystal size and alleviates structure collapse to enhance cycling performance. It is demonstrated that Ca can alleviate the two detrimental factors and shed lights on the further searching for suitable dopants.

1. INTRODUCTION Recently, Li-ion batteries (LIBs) as energy storage devices have captured most of the market share of portable electronics and electric vehicles (EV).1−3 However, the current commercially cathode materials, such as layered lithium transition metal oxides, spinel Manganite and olivine-structural phosphate,4 restricts themselves to future application in EVs due to limited energy density. Silicate-based materials, Li2MSiO4 (M = Fe and Mn), characteristic of high reversible theoretical capacity, high safety, low cost, and abundant sources, have aroused extensive attention and may meet the requirement for high energy density cathode materials.5−14 In addition, Mn is more readily oxidized to the high chemical state of 4+ than Fe, so the second Li ion should be more accessible in Li 2 MnSiO 4 than from Li2FeSiO4.5−7,12,13 Thus, a theoretical capacity as high as 330 mAhg−1 and a reasonably high operating voltage, 4.0 V vs Li+/Li, make Li2MnSiO4 a promising alternative cathode material for next-generation LIBs.8,11−13 As a member of polyanion cathode materials, the crystal structure of Li2MnSiO4 can be viewed as a framework with distorted hexagonal packing of O anions with half of the tetrahedral sites occupied by Li, Mn, and Si.9 A variety of polymorphs, including © XXXX American Chemical Society

monoclinic Pn, P21/n and orthorhombic Pmn21 and Pmnb phases, form by adopting different patterns of occupied tetrahedral interstices in the lattice. Among these four space groups, the low-temperature orthorhombic Pmn21 polymorph, which has favorable thermodynamic stability and exhibits the lowest Li diffusion energy barrier, is an ideal candidate for high energy density cathodes. However, the structure of Pmn21 can change into other polymorphs either during the material preparation process, i.e., the synthesis methods and heat treatment temperatures, or during cycling. The change into other polymorphs often lead to worse overall performance.11,12 More importantly, the Pmn21 Li2MnSiO4 also comes with Mn-antisite defects, the Jahn−Teller distortion of Mn3+ ions, and structural destabilization (Li2MnSiO4 transforms into MnSiO4) during the charge− discharge process.8,11−13 On the other hand, the Pmn21 Li2MnSiO4 has a low intrinsic electronic conductivity, less than 10−14 S cm−1,12 which is 3 or 4 orders of magnitude lower than that of LiFePO4.14 This low conductivity results in sluggish kinetics and delays the access of the second Li extraction. Received: December 27, 2017

A

DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX

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In order to overcome these drawbacks, various experimental efforts including carbon nanocoating,15,16 heteroatom substitution,17,18 and nanostructurization19,20 have been applied. Heteroatom substitution distinguishes itself among those

Both drawbacks come from the intrinsic properties of Li2MnSiO4, which cause the unsatisfied electrochemical performance and limit the realistic application of Li2MnSiO4 cathode materials.

Figure 1. (a,(b) XRD patterns for LMS, LMS−Ca, LMS/C, and LMS/C−Ca. (c,d) Rietveld refined XRD patterns; the dark crosses, red line, blue line, and orange line represent the observed pattern, calculated pattern, difference curve, and Bragg positions, respectively.

Figure 2. Low-magnification TEM bright field images for (a) LMS, (b) LMS−Ca, (c) LMS/C, and (d) LMS/C−Ca. (e−h) High-resolution TEM images for LMS, LMS−Ca, LMS/C, and LMS/C−Ca. The insets in (e−h) show the reduced FFTs of the areas enclosed in the white squares. B

DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX

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reduced antisite defect and strengthened structural stability.29,30 Some basic questions remain unanswered, such as how Ca substitution brings about variations in the electronic structure and mitigates structural changes that occur upon charge/discharge at voltages greater than 4.5 V. In this work, we report for the first time on Li2MnSiO4 nanoplate composites substituted with a small amount of Ca cations synthesized through a facile solvothermal method. Calcium substitution results in enhancement of the electrochemical kinetics and improves cyclic performance. Experimental characterization and DFT calculations show that Ca substitution may reduce the lattice size and the number of Mn-antisite defects

methods, as it can modify the intrinsic properties of Li2MnSiO4 such as electronic conductivity, Li diffusivity, and structural stability. Thus far, various cations, such as Mg2+,21 Al3+,22 Cr3+,23 V3+,24 P5+,18 Fe2+,25,26 B3+,27 and Na+,28 have been incorporated into the Li2MnSiO4 lattice in order to improve the electrochemical performance. To the best of our knowledge, there is no report in the literature concerning the effects of Ca substitution on the structural stability and electrochemical properties of Li2MnSiO4. Thus far, it has been recognized that Ca substitution is beneficial for improving the reversible capacity and structural stability of phosphate and layer cathode materials, such as LiFePO4 and LiNi0.8Co0.2O2, because of the

Table 1. Refined Lattice Parameters and Volumes for LMS/C and LMS/C−Ca structure

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

V (Å3)

LMS/C LMS/C−Ca

6.3265 6.3219

5.4177 5.3812

4.9703 4.9790

90 90

90 90

90 90

170.36 169.38

Figure 3. XPS spectra for LMS/C and LMS/C−Ca. (a) Survey spectra; (b−e) high-resolution XPS spectra for Li 1s, Si 2p, Mn 2p, and Ca 2p. C

DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX

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values of 6.0 and 1.0 eV, respectively, were adopted for Mn ions in all cases, as the effective value of U−J was relevant for the calculations.38 The cutoff energies of 500 eV and the proper K-point meshes were tested to ensure a convergence criterion of 1 meV and the same volume density per unit cell for all compounds. A Monkhorst−Pack grid with 9 × 9 × 9 (for a unit cell of Li2MnSiO4 containing 16 atoms) meshes and 4 × 4 × 4 (for a 2 × 3 × 2 supercell containing 48 Li, 24 Mn, 24 Si, and 96 O atoms) meshes were employed in describing the irreducible Brillouin zone. The energy criteria were 0.03 eV Å−1 for ionic relaxations and 10−5 eV for the static calculations, respectively. The 4.17% Ca substitution was achieved by replacing one of the 24 Mn or Si atoms or two of the 48 Li atoms in the supercell with one or two Ca atoms, respectively.

and also suppress volume shrinkage during the extraction of Li ions from Li2MnSiO4. Both of these account for the improved cell electrochemical performance, making this approach a promising strategy for modifying layer exfoliation of Li2MnSiO4.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Nanoplated Li2Mn1−xCaxSiO4/C (x = 0 and 0.04) Composites. Li2Mn1−xCaxSiO4/C (x = 0 and 0.04) compounds were prepared utilizing a DEG-assisted sovolthermal method. LiOH·H2O (1.7127 g, 40 mmol) and tetraethyl orthosilicate (TEOS, 2276 μL, 10 mmol) were dissolved in DEG (40 mL) in turn. In addition, MnCl2·4H2O (1.9199 g, 9.6 mmol) was added to DEG (20 mmol) by stirring. CaCl2 (0.0462 g, 0.4 mmol) was then dissolved in the solution and sonicated for 1 h. After that, the two solutions were blended and further stirred for more than 10 min before being injected into a Teflon-lined stainless steel autoclave (100 mL) and being heated at 230 °C for 72 h. After cooling, the precipitate was centrifuged, washed with deionized water and ethanol several times, and dried at 80 °C overnight in vacuum. For carbon coating, the precursor material was mixed with pheno-formaldehyde (PF, 0.2 g, 20 wt %), dissolved in absolute ethyl alcohol (30 mL), and ultrasonicated for 40 min. The powder was obtained by evaporation at 80 °C and then treated at 120 °C for 24 h. Finally, the as-synthesized product was heated to 600 °C in a vacuum atmosphere, and a carbon-coated sample was obtained. The precursors and end-products were named as LMS, LMS−Ca, LMS/C, and LMS/C−Ca, respectively. 2.2. Materials Characterization. X-ray diffraction patterns (XRD) of the composites were obtained using a Rigaku K/Max02500 Diffractometer with Cu Kα radiation (λ = 1.54056 Å) at steps of 0.02° and a dwell time of 2 s. Rietveld refinements were conducted to analyze the lattice parameters and bond length of the as-prepared samples using GSAS+EXPGUI software.31 A field emission scanning electron microscope (FE-SEM, Nova NanoSEM 230, U.S.A.) and a field emission transmission electron microscope equipped with an image aberration corrector (TEM, FEI Titan G2 ETEM, U.S.A.) were employed to characterize the morphology and structural features of products. X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALAB 250Xi X-ray photoelectron spectrometer using monochromatic Al Kα X-rays (1489.6 eV). All XPS spectra were calibrated by using the C 1s peak with a binding energy of 284.8 eV. The carbon coating on the samples was evaluated by thermogravimetric analysis (TGA, SDTQ600) and differential scanning calorimetry (DSC, SDTQ600) under air flow at a heating rate of 10 °C/min between room temperature and 800 °C. 2.3. Electrochemical Measurements. Electrodes were prepared by mixing 80 wt % active material with 10 wt % polyvinylidene fluoride (PVDF) as a binder and 10 wt % acetylene black in N-methyl pyrrolidone. The thickness of the calendered electrodes without the Al current collector was about 20 μm, and the active material mass loading was estimated to be about 2.0 mg cm−2. A CR2016 type coin halfcell was assembled in an Ar-filled glovebox, with Li foil as the counter electrode, Celgard 2400 membrane as the separator, and 1 M LiPF6 dissolved in ethylene carbonate and dimethylcarbonate (1:1 w/w) as the electrolyte. The cells were charged and discharged in the voltage range of 1.5−4.6 V (1C = 166 mAhg−1) with a battery testing system (LANHE CT2001A, Wuhan LAND Electronics Co., P. R. China). Electrochemical impedance spectroscopy (EIS) was performed by utilizing an electrochemical workstation (PARSTAT 4000, Princeton Applied Research) in the frequency range from 100 kHz to 0.1 Hz with an amplitude of 5 mV. 2.4. Theoretical Calculations. In this work, first-principles calculations were performed utilizing the Vienna ab initio simulation package (VASP) based on density functional theory (DFT) with the project-augmented wave (PAW) method.32,33 The generalized gradient approximation (GGA) functional of Perdew−Burke− Ernzerhof34 was used to describe exchange and correlation interactions. To correct electronic delocalization and self-interaction errors of TM oxides, the Hamiltonian (GGA + U method) was applied to calculate the electrochemical properties accurately.35−38 The U and J

3. RESULTS AND DISCUSSION XRD patterns and full Rietveld refinement of the synthesized materials are depicted in Figure 1. As shown in Figure 1a,b, all the diffraction peaks can be indexed to the orthorhombic polymorph with the space group Pmn21 (ICSD: 16−1305), indicating that pure Li2MnSiO4 was obtained in the pristine, Casubstituted, and carbon-coated materials via the solvolthermal method. The broadened diffraction peaks in the XRD patterns indicate that the materials are composed of nanoscale grains. SEM secondary electron (SE) images, TEM images, and corresponding FFT patterns shown in Figures S1 and 2, respectively, depict the nanoplate-like morphology and crystalline nature of the materials. For the carbon-coated samples, a conformal ∼2 nm thick carbon layer is visible on the nanoplate surfaces (Figure 2g,h), and the carbon contents of both samples are estimated to be 20.2 and 20.6%, respectively, based on the thermal gravimetric analysis (Figure S2a,b). A closer examination of the XRD patterns (Figure 1a,b) shows that the first and second most intense diffraction peaks in all the materials are related to the (210) and (002) crystal planes, respectively, rather than (011) and (210) planes in the standard reference, which is indicative of the preferred crystallographic orientation within the materials.39,40 Rietveld refinement was performed to analyze differences in crystal structure based on the Pmn21 phase, and the results are presented in Figure 1c,d and Tables 1, S1, and S2. It is apparent that Ca substitution leads to shrinkage

Figure 4. Formation energies and optimized crystal structure (inset) for Ca substitution at Li, Mn, and Si sites. All tetrahedra point in the same direction along the c-axis and are linked by corner-sharing (blue: SiO4 tetrahedra, violet: MnO4 tetrahedra, orange: CaO4 tetrahedra, and yellow: Li atom). D

DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of the lattice parameters a, and b and unit cell volume, associated with slight expansion of the lattice parameters c. Also, the percentages of MnLi antisites for the LMS/C and LMS/C−Ca composites are 6.3 and 1.6% (Tables S1 and S2), respectively, indicating that Ca ions substituting in Li sites can reduce the number of Mn-antisite defects and facilitate Li-ion diffusion.41 The chemical state of the elements was evaluated using XPS, as shown in Figure 3. The survey spectra in Figure 3a depict five characteristic peaks corresponding to Li 1s, Mn 2p, Si 2p, O 1s, and C 1s in both samples and an extra Ca 2p peak in the Ca-substituted sample. The Mn 2p spectra (Figure 3d) show that Ca substitution affects the valence of Mn in Li2MnSiO4 by shifting the 2p peak to a lower binding energy, which suggests that the Mn−O bond was enlarged to some extent. The slight difference in oxidation state of Mn could possibly influence the redox plateaus and kinetic performance of the Ca-substituted

material. In addition, as shown in Figure 3b,c, with Ca substitution, the binding energies of Li 1s and Si 2p shift to higher values of 55.09 and 101.85 eV from the original 54.89 and 101.74 eV values, respectively (Table S3). Considering the larger ion radius of Ca2+, Ca substitution in Mn sites results in smaller Li−O and Si−O bond lengths or higher oxidation states for Li and Si, suggesting that a denser Li2MnSiO4 structure is obtained. Substitutional Ca ions act as an effective pillar during the charge−discharge process. Therefore, it is reasonable to deduce that there is a substantial change in the local environment of all MO4 tetrahedra (M = Li, Mn, and Si) induced by Ca substitution in Mn site. To get more insight into the local structure changes around the substitutional Ca ions, DFT simulations with a supercell of Ca-substituted Li2MnSiO4 were performed. Three distinct tetrahedral sites occupied by Ca ions in total, i.e., Li, Mn, and Si sites,

Figure 5. (a) Charge−discharge profiles and (b) corresponding dQ/dV vs voltage plots for LMS/C and LMS/C−Ca taken at C/20 between 1.5 and 4.6 V. (c) Cycle performance of LMS/C and LMS/C−Ca tested at C/10 for 50 cycles. (d) Cycle performance for LMS/C and LMS/C−Ca at different rates (C/10 → C/5 → C/2 → 1C → 2C → 5C → C/10). (e) Long-term high-rate cycling life at C/2 for 200 cycles of LMS/C and LMS/C−Ca. E

DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a,c) EIS spectra; (b,d) the relationship between Zre and ω−1/2 in the low-frequency region for LMS/C and LMS/C−Ca.

Table 2. EIS Parameters for LMS/C and LMS/C−Ca Cathodes before and after the First Cycle before the first cycle

after the first cycle

sample

Rct(Ω)

σw(Ωcm2s−1/2)

DLi+(cm2s−1)

Rct(Ω)

σw(Ωcm2s−1/2)

DLi+(cm2s−1)

LMS/C LMS/C−Ca

143.40 82.97

45.47 44.31

3.53 × 10−14 3.72 × 10−14

413.30 185.90

116.92 68.80

5.34 × 10−15 1.54 × 10−14

material has better rate performance than the pristine material. To elucidate the enhanced kinetics of Ca-substituted material, EIS analysis was done according to the eqs 5 and 6 in the SI, and the results are shown in Figure 6 and Table 2. Compared with pristine LMS/C, the Ca-substituted material exhibits a lower charge transfer resistance and a higher Li diffusion coefficient before and after the first cycle, indicating that appropriate amounts of Ca substitution are beneficial in improving both electronic conduction and Li-ion migration. As depicted in Figure 5c and Table S5, the specific capacity of pristine LMS/C delivers a discharge capacity of 141.1 mAhg−1 and degrades rapidly to 57 mAhg−1 after 50 cycles with noticeable decay in discharge voltage (Figure S4a). In contrast, LMS/C−Ca exhibits a significantly improved performance with an increased first discharge capacity of 151.4 mAhg−1 and a discharge capacity of 115.7 mAhg−1 after 50 cycles (Figure 5c), which corresponds to effective suppression of the discharge voltage decay (Figure S4b). The long-term cyclic performance (Figure 5e) shows that LMS/C−Ca possesses a capacity retention rate of about 70%, which exceeds that of pristine

were optimized to determine the most energetically favorable doping site, as described in detail in the Supporting Information and Figure S3. As shown in Figure 4, Ca substitution at Mn sites is the most energetically stable, compared with substitution at Si and Li sites. The simulated local structure changes are consistent with the experimental XRD results. The variations in electronic structure, electrochemical kinetics, and cyclability induced by local structural distortion will be further discussed in the following sections. Galvanostatic charge−discharge measurements were performed to compare the electrochemical properties of pristine and Ca-substituted materials within the cutoff voltages of 1.5−4.6 V. As shown in Figure 5a, with Ca substitution, the charge plateaus slightly shift to lower voltages, and the discharge plateaus shift to higher voltages, which is further confirmed by the corresponding dQ/dV curves (Figure 5b). The Ca-substituted material exhibits an initial discharge capacity of 190.5 mAhg−1 (∼1.15 Li transferred per unit formula), which is higher than that of pristine LMS/C (180.8 mAhg−1). The rate capacity shown in Figure 5d also confirms that the Ca-substituted F

DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) Calculated voltage-composition curves compared with experimental voltage data; (b) corresponding lattice parameters; (c) and (d) relative changes in unit cell volume formation energies Li2yMn1−xCaxSiO4 (x = 0 and 0.04).

(x = 0 and 0.04) are plotted in Figure 7d. Obviously, the ΔE (y) value for delithiation from Li2Mn0.96Ca0.04SiO4 is substantially smaller than that from pristine Li2MnSiO4, indicating that Ca substitution is beneficial for attaining a lower voltage and removing more than one Li ion from the structure. To further confirm the effect of Ca substitution, ex situ XRD analysis was carried out on fully charged and discharged materials, as shown in Figure S5 and Table S7. It is apparent that, after charging to 4.6 V and discharged to 1.5 V, the volume and lattice parameter changes of the Pmn21 unit cell are substantially suppressed in LMS/C−Ca, suggesting that Ca substitution can improve the structural stability of Li2MnSiO4. Thus, Ca substitution can effectively enhance the electrochemical kinetics and inhibit capacity fading during the charge−discharge process.

LMS/C (50.3%), as the C rate is changed back to C/10 after 200 cycles at C/2. This suggests that the cycle performance and capacity retention of Li2MnSiO4 are significantly enhanced by Ca substitution. To understand the origins of enhanced kinetics and cyclability caused by Ca substitution, DFT calculations were performed to obtain average voltages (details in the Supporting Information), lattice parameters, volumes, energies, and volumes changes for Li2Mn1−xCaxSiO4 (x = 0 and 0.04) during charge−discharge processes, as presented in Figure 7 and Table S6. The average voltages are lower than what are projected by the dQ/dV curves when Ca is substituted into the Li2MnSiO4 crystal structure at Mn sites. The lowered voltages after Ca substitution can be attributed to weakened Mn−O bonds that may facilitate the oxidation of Mn2+ during the delithiation process (Figure 7a). The changes in the lattice constants and volume of Li2Mn0.96Ca0.04SiO4 during the Li intercalation/ deintercalation process are also smaller than those of Li2MnSiO4 (shown in Figure 7b,c and Tables S4 and S6). In particular, the volume change of the Ca-substituted composite (−1.10%) is less than half of the pristine composite (−2.76%) after the second Li+ extraction, suggesting that Ca substitution can effectively mitigate layer exfoliation and restrain the rapid capacity fading of Li2MnSiO4 during consecutive cycles. It is well established that the changes in lattice angle (γ) are significant, and the influence the cycling reversibility of Li2MnSiO4.42 Smaller changes to the γ angle were observed for the Casubstituted structure during the charge process (Table S4), indicating that improved cycling stability and capacity retention can be expected for Ca substitution. The calculated formation energy ΔE (y) values (using eq 4 in the SI) for Li2Mn1−xCaxSiO4

4. CONCLUSION In summary, Ca ions have been successfully introduced into Li2MnSiO4 nanoplates via a diethylene-glycol-assisted solvolthermal method. We found that Ca substitution can significantly improve the structural stability and electrical conductivity, two inherent drawbacks of pristine Li2MnSiO4, and thus markedly enhance the electrochemical performance of the Li2MnSiO4. The Ca atom, acting as a pillar ion, helps to keep the structural integrity of Li2MnSiO4 by suppressing layer exfoliation during dilithiation/lithiation. In addition, Ca substitution can effectively reduce the volumes and Li/Mn disorder of the pristine LMS/C composites, leading to faster diffusion of Li+ and better rate properties. Furthermore, Ca substitution can lower the binding energy of Mn ions in Li2MnSiO4, which promotes the oxidation of Mn during Li extraction and allows the extraction of second Li+, as a result, a higher specific G

DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry capacity. Our findings demonstrate that the use of calcium substitution is anticipated to be an efficient way to improve the electronic conductivity and Li-ion diffusivity and enhance the long-lifetime cyclic performance of pristine Li2MnSiO4 and can serve as a guidance for future development of Li2MnSiO4 cathode materials.

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(6) Yang, J.; Zheng, J.; Kang, X.; Teng, G.; Hu, L.; Tan, R.; Wang, K.; Song, X.; Xu, M.; Mu, S.; Pan, F. Tuning structural stability and lithium-storage properties by d-orbital hybridization substitution in full tetrahedron Li2FeSiO4 nanocrystal. Nano Energy 2016, 20, 117−125. (7) Yang, J.; Kang, X.; He, D.; Zheng, A.; Pan, M.; Mu, S. Graphene activated 3D-hierarchical flower-like Li2FeSiO4 for high-performance lithium-ion batteries. J. Mater. Chem. A 2015, 3, 16567−16573. (8) Gummow, R. J.; He, Y. Recent progress in the development of Li2MnSiO4 cathode materials. J. Power Sources 2014, 253, 315−331. (9) 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. (10) Aravindan, V.; Karthikeyan, K.; Ravi, S.; Amaresh, S.; Kim, W. S.; Lee, Y. S. Adipic acid assisted sol-gel synthesis of Li2MnSiO4 nanoparticles with improved lithium storage properties. J. Mater. Chem. 2010, 20, 7340−7343. (11) 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, 21027. (12) Dominko, R. Li2MSiO4 (M = Fe and/or Mn) cathode materials. J. Power Sources 2008, 184, 462−468. (13) Rangappa, D.; Murukanahally, K. D.; Tomai, T.; Unemoto, A.; Honma, I. Ultrathin Nanosheets of Li2MSiO4 (M = Fe, Mn) as HighCapacity Li-Ion Battery Electrode. Nano Lett. 2012, 12, 1146−1151. (14) Chung, S. Y.; Bloking, J. T.; Chiang, Y. M. Electronically conductive phospho-olivines as lithium storage electrodes. Nat. Mater. 2002, 1, 123−128. (15) Sun, D.; Wang, H.; Ding, P.; Zhou, N.; Huang, X.; Tan, S.; Tang, Y. In-situ synthesis of carbon coated Li2MnSiO4 nanoparticles with high rate performance. J. Power Sources 2013, 242, 865−871. (16) Zhang, Q.; Zhuang, Q.; Xu, S.; Qiu, X.; Cui, Y.; Shi, Y.; Qiang, Y. Synthesis and characterization of pristine Li2MnSiO4 and Li2MnSiO4/C cathode materials for lithium ion batteries. Ionics 2012, 18, 487−494. (17) Dong, Y.; Zhang, W. L.; Wang, C. M.; Shi, T.; Chen, L. Synthesis of La-doped Li2MnSiO4 nano-particle with high-capacity via polyol-assisted hydrothermal method. Electrochim. Acta 2015, 166, 40−46. (18) Zhang, S.; Deng, C.; Gao, H.; Meng, F. L.; Zhang, M. Li2+xMn1−xPxSi1−xO4/C as novel cathode materials for lithium ion batteries. Electrochim. Acta 2013, 107, 406−412. (19) Hwang, C.; Kim, T.; Shim, J.; Kwak, K.; Ok, K. M.; Lee, K. K. Fast ultrasound-assisted synthesis of Li2MnSiO4 nanoparticles for a lithium-ion battery. J. Power Sources 2015, 294, 522−529. ́ (20) Swiętosławski, M.; Molenda, M.; Furczoń, K.; Dziembaj, R. Nanocomposite C/Li2MnSiO4 cathode material for lithium ion batteries. J. Power Sources 2013, 244, 510−514. (21) Gummow, R. J.; Sharma, N.; Peterson, V. K.; He, Y. Synthesis, structure, and electrochemical performance of magnesium-substituted lithium manganese orthosilicate cathode materials for lithium-ion batteries. J. Power Sources 2012, 197, 231−237. (22) Zhang, M.; Zhao, S.; Chen, Q.; Yan, G. Li2+xMnSi1‑xAlxO4/C nanoparticles for high capacity lithium-ion battery cathode applications. RSC Adv. 2014, 4, 30876−30880. (23) Zhang, S.; Lin, Z.; Ji, L.; Li, Y.; Xu, G.; Xue, L.; Li, S.; Lu, Y.; Toprakci, O.; Zhang, X. Cr-doped Li2MnSiO4/carbon composite nanofibers as high-energy cathodes for Li-ion batteries. J. Mater. Chem. 2012, 22, 14661−14666. (24) Hwang, C.; Kim, T.; Noh, Y.; Cha, W.; Shim, J.; Kwak, K.; Ok, K. M.; Lee, K. K. Synthesis, characterization, and electrochemical performance of V-doped Li2MnSiO4/C composites for Li-ion battery. Mater. Lett. 2016, 164, 270−273. (25) Guo, H.; Cao, X.; Li, X.; Li, L.; Li, X.; Wang, Z.; Peng, W.; Li, Q. Optimum synthesis of Li2Fe1−xMnxSiO4/C cathode for lithium ion batteries. Electrochim. Acta 2010, 55, 8036−8042. (26) Bini, M.; Ferrari, S.; Capsoni, D.; Spreafico, C.; Tealdi, C.; Mustarelli, P. Insight into cation disorder of Li2Fe0.5Mn0.5SiO4. J. Solid State Chem. 2013, 200, 70−75.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03257. SEM images, TGA curves, DFT calculations details and results, charge−discharge profiles and data, ex situ XRD patterns, Rietveld refinement data for Li2Mn1−xCaxSiO4 (x = 0 and 0.04) cathodes, XPS spectra analysis data, and EIS analysis details(PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.-F.W.) ORCID

Weifeng Wei: 0000-0002-3088-6549 Author Contributions §

Y.-M.F. and R.J. contributed equally to this work. Y.-M.F. synthesized the materials, performed the electrochemical tests, the XRD characterization, Rietveld refinement analysis, and first-principles calculations; R.J. performed the electron microscopy imaging, diffraction and spectroscopy characterizations and analyses. Z.-P.D. and D.-T.Z. performed the XPS analysis; L.-B.C., D.G.I., and W.-F.W. supervised the research. C.-P. L., D. G. I, and W.-F. W. cowrote the manuscript. All authors contributed to the discussion and provided comments on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (51304248, 11504162), the Program for New Century Excellent Talents in University (NCET-11-0525), the Doctoral Fund of Ministry of Education of China (20130162110002), the Program for Shenghua Overseas Talents from Central South University, and the State Key Laboratory of Powder Metallurgy at Central South University, Changsha, China.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b03257 Inorg. Chem. XXXX, XXX, XXX−XXX