Doping effect of manganese on the structural and electrochemical properties of Li2FeSiO4 cathode materials for rechargeable Li-ion batteries

Journal Pre-proof Doping effect of manganese on the structural and electrochemical properties of Li2FeSiO4 cathode materials for rechargeable Li-ion batteries Narinthorn Wiriya, Patcharapohn Chantrasuwan, Songyoot Kaewmala, Jeffrey Nash, Sutham Srilomsak, Nonglak Meethong, Wanwisa Limphirat PII:

S0969-806X(19)31447-1

DOI:

https://doi.org/10.1016/j.radphyschem.2020.108753

Reference:

RPC 108753

To appear in:

Radiation Physics and Chemistry

Received Date: 12 November 2019 Revised Date:

22 January 2020

Accepted Date: 1 February 2020

Please cite this article as: Wiriya, N., Chantrasuwan, P., Kaewmala, S., Nash, J., Srilomsak, S., Meethong, N., Limphirat, W., Doping effect of manganese on the structural and electrochemical properties of Li2FeSiO4 cathode materials for rechargeable Li-ion batteries, Radiation Physics and Chemistry (2020), doi: https://doi.org/10.1016/j.radphyschem.2020.108753. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Author statements N. W., S. K. and P. C., W. L. collected and preformed the analysis of the presented data. S. S. verified the experimental results. N.W., W. L., J. N. and N. M. wrote the paper. N. M. and W. L. supervised the project and contributed as co-corresponding authors. All authors discussed the results and contributed to the final manuscript.

Doping effect of manganese on the structural and electrochemical properties of Li2FeSiO4 cathode materials for rechargeable Li-ion batteries Narinthorn Wiriya1, Patcharapohn Chantrasuwan1, Songyoot Kaewmala1, Jeffrey Nash2, Sutham Srilomsak1,3, Nonglak Meethong1,3,* and Wanwisa Limphirat4,* 1

Materials Science and Nanotechnology Program, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand 2 The Graduate School, Udon Thani Rajabhat University, Udon Thani 41000, Thailand 3 Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), Research Network of NANOTEC- KKU(RNN), Khon Kaen University, Khon Kaen 40002, Thailand 4 Synchrotron Light Research Institute, Nakhon Ratchasima 30000, Thailand *Corresponding author: E-mail: [email protected], [email protected], Postal address: Synchrotron Light Research Institute, 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand

Highlights ●

Li2Fe1-xMnxSiO4/C has been synthesized by a sol-gel method.

●

Electrochemical impedance spectra (EIS) analysis shows that Mn doping reduces charge transfer resistance (Rct) and increases Li-ion diffusion by a factor of 2.65.

●

The Fe-O bond length in the structure is longer with Mn doping.

●

The study is important to understand the local structural effects on electrochemical performance of the Li2Fe1-xMnxSiO4/C material.

Abstract Li2MSiO4, with two Li ions per molecule operates on both of the M2+/M3+ and M3+/M4+ redox couples resulting in a higher theoretical capacity that is >300 mAh.g-1. In this work, synthesis of a Li2Fe1-xMnxSiO4/C (LFMS) composite was done using a sol-gel method. XRD patterns can be indexed in the monoclinic phase with P21/n space group. Li2Fe0.8Mn0.2SiO4/C provides higher discharge potentials and capacities, hence higher energy densities than Li2FeSiO4 of about 60%

at 0.1C. (655 Wh.kg-1 vs. 408 Wh.kg-1, respectively). X-ray absorption spectroscopy (XAS) shows that the Fe-O bond length increases by Mn doping in the structure. EIS measurements show that Li-ion diffusion coefficients improved from 8.3 x 10-16 cm2.s-1 to 2.1 x 10-15 cm2.s-1 by Mn doping. The increased Fe-O bond length is correlated with improved lithium ion diffusion and its effect on electrochemical behavior. Keywords: Polyoxyanion cathodes, Li-ion batteries, X-ray absorption spectroscopy (XAS)

1. Introduction Recently, polyoxyanion cathodes of orthosilicates, Li2MSiO4 (where M= Mn2+, Fe2+, Co2+) have attracted significant research attention. The advantages of such materials are their structural stability with extensive cycling, safety, low cost, high thermal stability due to strong Si-O bonding, and environmental friendliness (Dominko et al., 2006; Dominko, 2008; Deng et al., 2011; Zhu et al., 2014; Sasaki et al., 2015; Zhu et al., 2015;). Li2MSiO4, with two Li ions per molecule, operates on both of the M2+/M3+ and M3+/M4+ redox couples, resulting in a higher theoretical capacity that is >300 mAh.g-1 (Masquelier and Croguennec, 2013; Ding et al., 2017; Zhang et al., 2018).However, its discharge capacity is lower than for a material with one Li-ion per molecule because of poor electrical conductivity (on the order of 10-12-10-16 S cm-1 for Li2FeSiO4 and Li2MnSiO4) (; Gong & Yang, 2011; Bai et al., 2012 Masquelier and Croguennec, 2013; Yang et al., 2013; Yi et al., 2016). Improvements in the discharge capacity have been made by decreasing the particle sizes of the materials to reduce the electron transfer distances and by carbon coating (Fu et al. 2013; Jiang et al., 2014; Singh and Mitra, 2014; Zhu et al., 2015; Kumar et al., 2018). Li2FeSiO4 and Li2MnSiO4 have attracted significant attention because iron and silicon are among the most abundant, non-toxic and safe cathode materials available for high energy applications. The silicates also show polymorphism. Three commonly reported polymorphs of silicate (Li2MSiO4) are Pmn21, Pmnb (orthorhombic) and P21/n (monoclinic) (Armstrong et al., 2010; Sirisopanaporn et al., 2011). Li2MnSiO4 commonly provides higher discharge potential than Li2FeSiO4, but its cycle stability is extremely poor compared to that of Li2FeSiO4 because of the Jahn-Teller distortion effect involving Mn3+ in the crystal structure (Li et al. 2007; Bhaskar et al 2012; Liu et al. 2013; Devaraj et al., 2013; Song et al., 2015, Babbar et al., 2017; Cheng et al., 2017). These Li2FeSiO4 and Li2MnSiO4 materials have been improved to

yield high capacity and increased lifetime by incorporating carbon, graphene (Singh & Mitra, 2014; Gao et al., 2014; Zhu et al., 2014; Zhu et al., 2015) and doping with transition metals such as Mg, Cd, Cr, Co, Zn, Cu, Ni and Mn (Zhang et al., 2010 a,b; Deng et al., 2011; Shao et al., 2013; Zhang et al., 2014; Zhang et al., 2015; Qu et al., 2016). Mn doping is apparently beneficial for improving the rate capability and increasing specific capacity because Mn2+ has an atomic radius that is similar to Fe2+. Additionally, it can decrease charge-transfer resistance and increase Li+ diffusion (Qu et al., 2016; Yi et al., 2016). Guo et al., (2010), Gong and Yang (2011) and Chen et al., (2013) synthesized Li2Fe1-xMnxSiO4 with a high capacity due to the more facile oxidation of Fe2+ to Fe4+ than that of Mn2+ to Mn4+, but this material provided poor cycle stability. Yi et al. (2016) improved electrochemical performance by Mn doping into Li2FeSiO4. They found that Mn doped at a 5% level exhibits good rate capability. However, the increased capacity can also be explained through the decreased charge transfer resistance and increased Liion diffusion measured by electrochemical impedance spectroscopy (EIS), although the local structures of Mn doped materials have not been investigated. In this research study, Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C materials were prepared via a sol-gel method to provide increased capacity. The electrochemical behavior and local structure of Li2FeSiO4 and Li2Fe0.8Mn0.2SiO4 were studied using X-ray diffraction, galvanostatic cycle testing and X-ray absorption spectroscopy (XAS) techniques. The current study fills this gap in our knowledge of the local structure of such materials and relates this information to their electrochemical behavior.

2. Experiments 2.1 Materials and synthesis Li2Fe1-xMnxSiO4/C (LFMS) composites (x = 0.0 and 0.2) were synthesized using a sol-gel method. Stoichiometric amounts of poly(ethylene glycol)-block-poly (propylene glycol)-blockpoly (ethylene glycol) (P123) (99%, Sigma-Aldrich) were used as carbon sources. Lithium acetate dihydrate (LiC2H5O2⋅2H2O), (99.9%, Sigma-Aldrich), iron nitrate nonahydrate (Fe(NO3)3⋅9H2O)

(98%,

Sigma-Aldrich),

manganese

acetate

tetrahydrate

((CH3COO)2Mn·4H2O) (99%, Sigma-Aldrich) and tetraethyl orthosilicate (TEOS) (99%, SigmaAldrich) were used to produce a Li:(Fe or Mn):Si molar ratio of 2:1:1. Typically, 2 g of P123 was dissolved in 20 ml ethanol and 2 mmol TEOS was added to the solution with mixing for

30 minutes. Then, iron nitrate nonahydrate was added to solution and stirred for a further 30 minutes. Next, lithium acetate dihydrate was added to the solution with continuous stirring. After 3 hours of stirring, the resulting dark red solution was heated to 80 oC and held overnight at this temperature, leading to its complete evaporation. Then, the dried mixture was calcined at 350 oC for 4 hours followed by an additional heat treatment at 650 oC for 12 hours under an Ar atmosphere. Finally, the resulting black product was ground in a mortar and pestle. 2.2 Measurements The X-ray diffraction (XRD) pattern of this material was measured for crystal structure analysis. The XRD data were achieved over 2θ angles in the range 10o to 70o with 0.02o increments using Kα radiation at room temperature. The particle size and morphology of the samples were evaluated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In XAS experiments, the Fe and Mn K-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements were carried out in the transmission mode using ionization chambers as the detectors at the X-ray absorption Beamline (BL-5.2) of the of the Synchrotron Light Research Institute, Thailand. The electron energy was 1.2 GeV with a beam current of 150–80 mA. A germanium (220) double crystal monochromator used for energy selection with a precision of ± 0.2 eV in the energy range of 7100 - 7200 eV and 6530-6590 eV for Fe and Mn K-edge energy, respectively. The photon energy was calibrated using Fe foil (at 7112 eV) and Mn mesh (at 6539 eV). After obtaining the raw data from XAS measurements, pre-edge subtraction, post-edge subtraction, background subtraction, and normalization was performed according to the standard analysis technique using Demeter version 0.9.26 (Ravel and Newville, 2005). The k2-weighted spectra were Fourier transformed within the limits of 3
Celgard 2500 membrane served as a separator. Charge and discharge profile measurements were made in the range of 1.5-4.8 V using a multichannel tester (BTS8-MA, MTI) at 30 oC. 3. Results and discussion

Fig. 1. Comparison of the XRD patterns of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C X-ray diffraction (XRD) pattern analysis of the crystal structure of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C are shown in Fig. 1. The XRD patterns of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C can be indexed to the monoclinic phase P21/n space group, which is consistent with the structural analysis of Kamon-in et al. (2014). The diffraction peaks shift in the 2θ range of 32°-37° corresponding to an increased Mn content. This is consistent with Dominko et al. (2010), who studied a similar material. The lattice parameters in the current study were calculated and the results were a = 8.2501 Å, b = 5.0182 Å, c = 8.2362 Å and β= 98.94º for Li2FeSiO4/C and a = 8.2564 Å, b = 5.0237 Å, c = 8.2421 Å and β= 98.99º for Li2Fe0.8Mn0.2SiO4/C. These parameters slightly increased with Mn content because of its larger ionic radius (0.80 Å) than that of Fe2+ (0.76 Å) and are consistent with Yi et al. (2016). In addition, the XRD peaks of Li2FeSiO4/C are much broader compared to Li2Fe0.8Mn0.2SiO4/C indicating a smaller crystallite size. Normally, the crystallite size can be calculated using the Debye-Scherrer equation. However, direct observations using TEM providing more accurate results as shown below.

The size and morphology of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C materials were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as shown in Fig 2. TEM and SEM images indicate that the nanoparticles of both samples present similar morphology with round shape. The size distribution is quite narrow with an average of 20−25 nm for Li2FeSiO4/C. However, the particle size distribution of Li2Fe0.8Mn0.2SiO4/C is slightly wider with an average of 20-40 nm. The particles size as well as particle size distribution increased with Mn doping in Li2FeSiO4/C.

Fig. 2. SEM (a,b) and TEM (c,d) images for Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C, respectively.

Fig. 3. Charge/discharge profiles at various C-rates of (a) Li2FeSiO4/C, (b) Li2Fe0.8Mn0.2SiO4/C and (c) rate capabilities of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C, and (d) discharge profiles at 0.1C of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C.

The charge and discharge profiles of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C were measured with a cut-off voltage range of 1.5-4.8 V at 30 oC. Fig. 3a and 3b shows specific capacities for Li2FeSiO4/C of around 160, 150 and 115 mAh.g-1at 0.1, 0.2 and 1C, respectively and for Li2Fe0.8Mn0.2SiO4/C of around 253, 195 and 159mAh.g-1 at 0.1, 0.2 and 1C, respectively. From the first charge process, the curve of Li2FeSiO4/C in Fig. 3a show two potential plateaus. The first plateau, at 3.2 V, is related to Fe2+ conversion to Fe+3. The other, at 4.2 V, represents a Fe3+ to Fe4+ redox couple (Zhang et al., 2015). The potential corresponding to the first plateau of Li2Fe0.8Mn0.2SiO4/C shows dramatically different behavior and must arise from the addition of

Mn. The potential was much greater than 3.2 V due to the higher potential of Mn. In subsequent cycles the charge profile shows the two plateaus. For the Li2FeSiO4/C, the first is related to Fe2+/Fe3+ transition and the second resulted from a Fe3+/Fe4+ transition. For the Li2Fe0.8Mn0.2SiO4/C, the first is related to a combination of Fe2+/Fe3+ and Mn2+/Mn3+ transitions, while the second resulted from a combination of Fe3+/Fe4+ and Mn3+/Mn4+ transitions. This is why we see that the potential is higher with Mn doping. Fig. 3c shows the rate capability of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C. It can be seen that the capacities of Li2Fe0.8Mn0.2SiO4/C are higher than that of Li2FeSiO4 at each rate. Li2Fe0.8Mn0.2SiO4/C exhibits a diminished capacity in subsequent cycles because of the Jahn-Teller distortion effect involving Mn3+ in the crystal structure. The structural distortion can be elucidated using the XAS technique. This can be done by examining the bond length changes during the charge and discharge processes. Mullaliu et al. (2019) used XAS to study how the Mn local environment of a manganese hexacyanoferrate material affected its electrochemical performance. The Jahn-Teller distortion in this octahedral complex material (Mn occupies the octahedral sites) occurs along the ± z-axis. Therefore, its expansion/contraction in ± z-axis can be clearly observed. The results showed that manganese hexacyanoferrate exhibited substantial Jahn–Teller distortion resulting in a 10% reduction of Mn-N bond distances along the ± z-axis. However, unlike other Mn‐active compounds, this material can be reversibly cycled during Mn2+/Mn3+transition. In the Li2MSiO4 structure, Mn occupies tetrahedral sites. Therefore, the Jahn-Teller distortion occurs in these sites. The bond distances change in this structure due to a combination of Jahn Teller distortion and structural phase transformation. The Jahn Teller distortion affects the centrosymmetry around Mn sites along all bonding directions and become less visible than in octahedral complexes where only the ±z axis is affected. Jahn-Teller distortion has previously been used to explain the poor cycling stability of Mn containing Li2FeSiO4 materials (Li et al. 2007; Bhaskar et al. 2012; Liu et al. 2013; Devaraj et al., 2013; Song et al., 2015, Babbar et al., 2017). In addition, phase transformation during charging from monoclinic to orthorhombic phases results in bond length changes as shown by Dominko et al. (2010). They used in situ XAS to show that during lithium extraction from Li2Fe0.8Mn0.2SiO4 to LiFe0.8Mn0.2SiO4, the Fe-O bond lengths were reduced by about 6% while the Mn-O bond lengths were reduced by about 3.6%. When the current density returns to 0.1C, the discharge capacity recovers to 159 mAh.g-1 of Li2FeSiO4, it can still retain structural integrity even at a high rate and Li2Fe0.8Mn0.2SiO4/C

exhibits relatively lower reversible capacity. The discharge specific capacities of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C shown in Fig. 3d were 160 and 253 mAh.g-1, respectively, corresponding lithium extraction of 1 and 1.58 atoms. The voltage plateaus increase from approximately 2.85 V to 3.4 V due to Mn doping. The energy densities of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C are 408 and 655 Wh.kg-1, respectively.

Fig. 4 Electrochemical impedance spectra (EIS) ofLi2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C. Electrical conductivity can be determined from the electrochemical impedance spectra (EIS), as shown in Fig. 4 for Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C. The spectra of the samples exhibit semicircular trend above the Z’-axis. At high frequencies the first intercept is approximately equal to the electrolyte solution resistance between the electrode and the current collector in the cell (Re). At medium frequencies, the second intercept is approximately equal to the charge transfer resistance (Rct). The upward sloping line at low frequencies represents impedance related to the diffusion of lithium ions within the electrode, known as the Warburg (Zw) resistance. Lithium ion diffusion (DLi+) can be calculated using the following equation (Kumar et al., 2018 and Guo et al., 2010):

=

2

where R is the universal gas constant, T is temperature (Ko), A is the area of the electrode, n is the number of electrons transferred during oxidation, F is the Faraday constant, C is the concentration of lithium ions, and σ is the Warburg resistance, which is related to Zʹ following: Zʹ=Re+Rct+σω-1/2. In Table 1, it can be seen that the materials exhibit a reduced charge transfer resistance (Rct) from 95.8 Ω to 65.2 Ω with Mn doping, indicating a lower resistance for lithium ion transfer within the active material. Therefore, Mn doping increases the lithium ion diffusion of Li2FeSiO4 by a factor of 2.65. This is consistent with a similar material reported by Yi et al. (2016). Table 1 Electrochemical impedance parameters of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C Sample

Re (Ω)

Rct (Ω)

DLi+ (cm2 S-1)

Li2FeSiO4/C

1.85

95.8

8.31x10-16

Li2Fe0.8Mn0.2SiO4/C

1.50

65.2

2.10x10-15

Fig. 5 shows that Fe and Mn K-edge XANES spectra of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C have absorption edges at ∼7120 eV and ∼6538 eV for Fe and Mn, respectively. These are similar to the spectra reported by Zhang et al. (2015). The pre-edge (feature A) occurs due to the transition of electrons from a 1s state to an unoccupied 3d state. The weak shoulder (feature B) results from the transition of electrons from a 1s state to an unoccupied 4p state with a shakedown process, followed by a ligand to metal charge transfer (Yoon et al. 2002; Farges, 2005). These features are used to elucidate two things. First, their position is used to indicate the oxidation state. Second, their intensity is used to indicate the loss of centrosymmetry. As shown in the inset of Fig. 5c, the position of the pre-edge features look similar to the standard materials, FeO and Fe2O3, with the respective oxidation states of +2 and +3. This indicates that the Fe ion in Li2FeSiO4 has an oxidation state similar to that of FeO, i.e., +2. In the same way, the Mn K-edge XANES spectra of Li2Fe0.8Mn0.2SiO4/C indicates that the Mn ion has an oxidation state similar to that of MnO, +2, as shown in Fig. 5d. These results are consistent with those of Dominko et al. (2010) and Zhang et al. (2015).

The intensity of the pre-edge features of Li2Fe0.8Mn0.2SiO4/C is greater than Li2FeSiO4/C indicating different degrees of centrosymmetry distortion around Fe after Mn doping. A higher pre-edge indicates a larger deviation from centrosymmetry (Hsu et al., 2010). Therefore, the effects of the Mn doping caused distortion in the structure of the materials.

Fig. 5 XANES spectra of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C, (a) Fe K-edge, (b) Mn K-edge, c) comparison of Fe K-edge energy with that of standard materials and d) comparison of Mn K-edge energy with that of standard materials.

Fig. 6 Fourier transformation of the Fe K-edge EXAFS spectra for (a) Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C and (b) Mn K-edge EXAFS spectra for Li2Fe0.8Mn0.2SiO4/C and c) the expansion of Fe-O bond distances with Mn doping The Fourier transformation of EXAFS spectra of Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C are shown in Fig. 6. The spectra show significantly varying intensities indicating different local structures. The main peak with the highest intensity corresponds to the Fe-O bonds. The Fe EXAFS results, shown in Table 2, indicate that Fe is located in a distorted tetrahedron. It has two equidistant oxygen atoms, each at a distance of 1.84 Å from Fe, one oxygen atom is 1.87 Å and another oxygen atom is 2.00 Å from Fe. In Li2Fe0.8Mn0.2SiO4/C, there are two equidistant oxygen atoms 1.87 Å from Fe, with one oxygen atom 1.90 Å and another oxygen atom 2.04 Å from Fe. The average Fe-O distance increases with Mn doping. The Mn EXAFS results indicate

that Mn is also located in a distorted tetrahedron. It has two equidistant oxygen atoms, each at a distance of 2.02 Å from Fe, one oxygen atom is 2.05 Å and another oxygen atom is 2.19 Å. The bond length of Mn-O is larger than Fe-O. Figure 6c shows that Fe-O bond lengths increase by about 1-2% in all directions with Mn doping. The increase Fe-O bond distances upon Mn doping and the longer bond length of Mn-O occur due to substitution of Mn2+ with its larger ionic radius (0.80 Å) than Fe2+ (0.76 Å). Table 2. Parameters of the nearest coordination shells around Fe atoms in Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C. The coordination number (N0), distance (R) and Debye-Waller factors (σ2). Li2FeSiO4/C

Fe-O

Li2Fe0.8Mn0.2SiO4/C

Fe-O

Mn-O

N0

R[Å]

σ2[Å2]

2

1.84(1)

0.00232±0.0006

1

1.87(1)

0.00232±0.0006

1

2.00(1)

0.00232±0.0006

N0

R[Å]

σ2 [Å2]

2

1.87(1)

0.00633±0.0030

1

1.90(1)

0.00633±0.0030

1

2.04(1)

0.00633±0.0030

2

2.02(1)

0.00714±0.0015

1

2.05(1)

0.00714±0.0015

1

2.19(1)

0.00714±0.0015

In summary, the results indicate that the crystal structure did not change with Mn doping, only the lattice parameters varied. Additionally, Mn doping shows improved specific capacities and energy densities of the materials. The charge transfer resistance is reduced and Li ion diffusion increased by a factor of 2.65 with Mn doping. Fe-O bond lengths also increased as shown in Table 2. The Fe-O bond length increased significantly enough to affect the electrochemical behavior. It can change the crystal lattice, which facilitates lithium ion diffusion during the charge/discharge processes.

4. Conclusions

Li2FeSiO4/C and Li2Fe0.8Mn0.2SiO4/C were successfully prepared via a sol-gel method. XRD results reveal crystal lattice expansion with increasing Mn content. Mn doping leads to higher discharge potentials and capacities, hence higher energy densities. EIS measurements show that Li diffusion improved by a factor of 2.65. XAS results confirm that Fe-O bond lengths increased with Mn doping. This increased Fe-O bond length is correlated with improved lithium ion diffusion and its effect on electrochemical behavior. Acknowledgments This study was supported by the Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), Khon Kaen University and the National Nanotechnology Center (NANOTECH), NSTDA, Ministry of Higher Education, Science, Research and Innovation, Thailand, through its program of Research Network NANOTEC (RNN). NM and NW acknowledged partial supported by Research and Technology Transfer Affairs (KKUS60_004). WL acknowledged partial supported by the Synchrotron Light Research Institute, Thailand. References Armstrong, A. R., Lyness, C., Ménétrier, M., Bruce, P. G., 2010. Structural Polymorphism in Li2CoSiO4 Intercalation Electrodes: A combined diffraction and NMR study. Chem. Mater. 22(5), 1892-1900. Babbar, P., Tiwari, B., Purohit, B., Ivanishchev, A., Churikov, A., Dixit, A., 2017. Charge/discharge characteristics of Jahn–Teller distorted nanostructured orthorhombic and monoclinic Li2MnSiO4 cathode materials. RSC Adv. 7, 22990-22997. Bai, J., Gong, Z., Lv, D.,

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Declaration of interests The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: