INTRODUCTION

Li2FeSiO4 has been considered an attractive cathode material for rechargeable lithium batteries since it possesses high theoretical capacity (approximately 331 mAh·g−1), good thermal stability and cycling performance, nontoxicity, environmental friendliness, and is of low cost (Nytén et al., 2005; Guirish & Shao, 2015; Fujita et al., 2018). Many synthesis procedures have been proposed and used, such as the solid-state method, sol-gel technique, hydrothermal/solvothermal/supercritical fluid techniques, microwave method, spray pyrolysis/combustion/hydro-chemical techniques, polyol process and ionothermal techniques (Guirish & Shao, 2015; Ferrari et al., 2014). The mechanical activation reaction method is appropriate for large-scale production, but the impurities such as iron oxides, metallic iron, and lithium silicates, the larger particle size of the final product, and agglomerations could not be easily avoided. This has negative effects on the electrochemical performances of the products. The carbothermal reduction method is an efficient synthetic method to obtain metal or low-valent metallic oxide, and it has been applied to the synthesis of LiFePO4/C and Li2FeSiO4/C.

Most of the recent synthesis procedures are designed to optimize the intrinsic and extrinsic properties of Li2FeSiO4 by applying grain size reduction, morphology control, doping with some transition metal cations in Fe and Si sites, and conductive carbon coating, among others (Yi et al., 2017). Unfortunately, there remain difficulties in synthesizing a single-phase material. The potential and the low-cost advantage are not realized if expensive Fe2+ precursor compounds are used as the starting materials in the synthesis procedures. There are, however, some examples where Li2FeSiO4/C cathode material is successfully synthesized from Fe2O3 (Table 1).

In the present study, an easy and low-cost synthesis of Li2FeSiO4 was investigated using a Fe3+ precursor based on short-time preliminary milling of a reactant mixture in a planetary mill followed by calcination.

EXPERIMENTAL METHODS

Synthesis

Li2FeSiO4 was synthesized by ball milling-assisted solid-state reaction using FeC₂O₄∙2H₂O (iron II oxalate), 500 mesh commercial Fe₂O₃ (hematite), and synthesized hematite by a green route.

• ·Using Iron II oxalate as a precursor

  Dehydrated Iron II oxalate (Aldrich Chemistry, 99%), lithium silicate (GFS Chemicals,

  ≥99.5%), and typically 1.50 g of citric acid (C₆H₈O₇) with 15%w/w was ball milling using low energy for 36 hours with two different rpm´s (250 and 300) and hardened steel balls per gram of mixture 15:1 mass ratio. Solid-state reactions were carried out at 400°C for 2 hours and two different calcination temperatures (700°C and 750°C) for different calcination times in an inert Argon gas atmosphere.

  · Using Commercial Hematite 500 Mesh as a precursor

  500 mesh hematite (Applichem CAS1309-37-1) was ball milled at low energy (15:1 mass ratio) for 24 hours with 250 rpm, before mixing with other components. Then, a mixture of hematite, lithium silicate, and citric acid in stoichiometric amounts with 32% w/w was ground and mixed in an agate mortar. The mixture was calcinated at 400°C for 4 hours followed by calcination at 750°C for 10 hours, all in an Argon gas inert atmosphere. Also, a paste was formed by adding methanol to the reaction mixture and was ball milled for 24 hours using low energy conditions (250 rpm, ball/mass ratio 15:1) and then calcinated for 2 hours at 400°C, followed by 10 hours of calcination at 700°C/7050°C, all in an inert atmosphere of Argon gas.

  · Using Hematite obtained by a green route.

  The above procedure was repeated using hematite obtained by a green route (Freire et al., 2023). When heating the amorphous iron oxide precipitated from the methanolic Caesalpinia coriaria (Jacq.) Willd. fruits extract, hematite is obtained.

Characterization techniques

XRD measurements were performed in X-ray diffraction (PANalytical X'Pert powder diffractometer, Cu Ka radiation) in the range of 10° ≤ 2ϴ ≤ 80° at intervals of 0.02° in Bragg- Brentano geometry. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were recorded in the 4000-300 cm−1 range on an FTIR Frontier de Perkin Elmer spectrophotometer with ATR. A resolution of 2 cm−1 was used to obtain all the spectra. The room temperature Mössbauer spectra for all samples were collected in a spectrometer working at the standard transmission geometry by moving the vibrator with a triangular reference signal. A ⁵⁷Co/Rh source of 10 mC_i (925 MBq) of nominal activity was used. The spectrometer was regularly calibrated by collecting the RT Mössbauer spectrum of a standard

α-Fe foil. All spectra were fitted by using a Lorentzian or Voigt-based routine of the Recoil software (University of Ottawa, Canada).

Table 1.
Summary of Li2FeSiO4/C cathode material is successfully synthesized from Fe2O3.
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RESULTS AND DISCUSSION

Li2FeSiO4 using FeC2O4·2H2O as precursor

The XRD pattern of powders obtained from mechanical milling is shown in Fig. 1.

Figure 1
Diffraction patterns of the precursors a) Li2SiO3 and b) FeC2O4·2H2O, and c) the mixture of milled powders using the oxalate precursor technique.
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The transformation of the precursors Li₂SiO₃ and FeC₂O₄·2H₂O is clearly seen, whose diffractograms are given in Fig. 1 (a) and (b). New weak and broad reflections are assigned to a mixture of anhydrous ferrous oxalate FeC₂O₄ (Brown & Bevan, 1966; Hermanek et al., 2007). Li₂SiO₃, and a phase designated as amorphous nanoparticles of F₂O₃ in correspondence with the Mössbauer data that is provided later. According to the X-ray, milling results in the amorphization and initial interaction of precursor materials, with the formation of an intermediate product of reduced particle size.

The transmission Mössbauer spectrum of the ball milled sample, exhibited in Fig. 2a, was fitted using a two-component model. The less intense doublet had a quadrupole splitting (QS) of 2.11 mm·s^-1and isomer shift (IS) of 1.23 mm·s^-1 (see Fig. 2a).

Figure 2.
ATR-FTIR spectrum of the milled powders using the oxalate precursor technique.
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This quadrupole splitting value was consistent with an electron configuration of high spin for the Fe (II) cation and the isomer shift value was in the range expected for Fe (II) in octahedral or quasi-octahedral environments (Drago, 1965). This doublet was assigned to anhydrous ferrous oxalate, in accordance with studies on the thermal decomposition of FeC₂O₄·2H₂O (Cari et al., 1975; Smrčka et al., 2016). The second doublet was fitted by using an IS of 0.36 mm·s-1 and QS of 0.76 mm·s-1, typical of high spin Fe³ ions in octahedral coordination, which could be assigned ions in octahedral coordination, which could be assigned to amorphous nanoparticles of Fe₂O₃ (Smrčka et al., 2016; Milivojevi et al., 2014; Machala et al., 2007). Thus, Mössbauer suggests an amorphous character for this phase of Fe₂O₃. If the crystal size is about 5-6 nm, XRD will not distinguish the nanocrystals of iron oxide (III), but Mössbauer will.

Figure 2 shows the infrared FTIR-ATR spectrum of the powders (oxalate technique) after the mechanical milling of the precursors. This profile is not related to those of the precursors. Several of the observed bands can be assigned to the oxalate anion since they coincide with well-known data on vibratory frequencies of spectroscopic studies of a large variety of metal oxalates (Dinnebier et al., 2003; Begun & Fleter, 1963). The peaks at 1651 cm-1 1330 cm-1

and 773 cm^-1 are interpreted as a result of vibrational modes of iron oxalate (II) (FeC₂O₄) in the solid state in which the oxalate anion is placed in a C2h symmetry site in the crystal structure. The peaks at 1651 cm^-1 and 1330 cm^-1 corresponded to asymmetric vibrations nas (CO₂) the peak at 773 cm^-1 was due to the asymmetric flex d (CO₂) (Dinnebier et al., 2003;). The rest of the spectrum of this spectrum could be associated with the presence of amorphous iron oxide (Raman et al., 1991) and Li2SiO₃ (Zhang et al, 2008; Cruz & Bulbulian, 2005; Yang et al., 2013). The signal at 955 cm^-1 may be related to vibrations O-Si-O, while the absorption at 505 cm^-1 can be due to a stretching band Fe-O and deformation Si-O-Li.

Figure 3 shows the evolution of the room temperature Mössbauer spectra of the milled powders heat-treated at 705 ° C for different times. The spectrum of the sample heated for two hours (see Figure 2b) consisted of a slightly asymmetric doublet and a sextet with broad lines. The doublet had hyperfine parameters (IS = 0.24 mm·s-1 and QS = 0.69 mm·s^-1) typical of the superparamagnetic Fe₂O₃. The broadened sextet with B = 47.9 T, IS= 0.28 mm·s^-1, and QS = -0.01 mm·s-1 resulted from the heat treatment, which induces more crystalline Fe₂O₃.

Figure 3.
Mössbauer spectra at room temperature powders milled (oxalate precursor) and thermally treated at 705 °C in Ar atmosphere for a) 0 h, b) 2 hours, c) 6 hours, d) 10 hours, and e) 20 hours
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The thermal treatment at 705°C for periods longer than 6 hours results in samples whose Mössbauer spectra have a central doublet, clearly of high spin Fe2+ in the tetrahedral coordination. The similarity in the position of the spectral lines of the doublet suggests similar local environments for the Fe. However, the values of the quadrupole splitting are significantly different between the samples. According to (Mali et al., 2011), the values of the quadrupole splitting can be strongly correlated with the degree of distortion of the FeO₄ tetrahedra in the orthosilicates. In the sample heated for 6 hours, it was fitted with two quadrupole sites with QS values of 1.88 mm·s^-1 and 2.19 mm·s^-1. The doublet in the spectra of samples heated for 10 hours, has a QS ~ 2.42 mm·s^-1, which coincided with those of the monoclinic structure P21/n of the lithium iron orthosilicate, Li2FeSiO4 (Jugović et al., 2014; Lv et al., 2011; Sirisopanaporn et al., 2010; Dominko, 2008). is noteworthy that by carefully controlling the experimental conditions, the purity of the ball-milled synthesized orthosilicate Li₂FeSiO₄ can be improved, A sample obtained using 300 RPM during milling, exhibits a Mössbauer spectrum consisting of two sets of Fe²⁺+ and Fe³⁺ doublets as depicted in Figure 4, a prominent one belonging to the lithium iron orthosilicate doublet (CS = 0.947(1) mm/s and QS = 2.400(2) mm/s) and a very small one (CS = 0.29(5) mm/s and QS = 0.88(9) mm/s) which is attributed to delithiated lithium iron orthosilicate Li2- xFeSiO₄ (Kyu Lee et al., 2013; Nytén et al., 2006).

Figure 4.
Mössbauer spectrum at room temperature of a lithium iron orthosilicate obtained using 300 rpm during ball milling.
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The delithiated orthosilicate Li2-xFeSiO4 is in very small quantities, so is difficult to clearly observe it in the XRD. It is interesting to point out that neither Fe⁰ impurities nor other iron oxides (lithiated magnetite and lithiated wustite) were observed in the spectrum of some samples heated for 20 hours, which in turn exhibited a smaller contribution to two magnetic sextets that may be attributed to non-stoichiometric magnetite,Fe_3-xO₄.

The milling causes the dehydration of ferrous oxalate and simultaneously, an atmosphere is generated that leads to the formation of amorphous nanoparticles of Fe₂O₃, like what happens during the thermal decomposition of oxalate. It is interesting to observe that from the pure divalent state of Fe²⁺ in FeC₂O₄, it converts to the trivalent state Fe³⁺ in mechanical milling, even in an argon atmosphere. The transformations can be described by the equations.

FeC2O4·2H2O ® FeC2O4 + 2 H2O (1)

2 FeC2O4 ® Fe2O3 + CO2 + 3 CO (2)

2 FeC2O4·2H2O ® Fe2O3 + 2 H2O + 3 CO2 + CO + 2H2 (3)

The formation of other oxides is induced during the first part of the heat treatmentat at 400

°C.

FeC2O4 ® FeO + CO + CO2 (4)

3 FeC2O4 ® Fe3O4 + 2 CO2 + 4 CO (5)

Fe3O4 + CO ® 3 FeO + CO2 (6)

Frecuently, the formation of metallic iron is observed after heat treatment.

Fe2O3 + 3 C ® 2 Fe + 3 CO (7)

FeO + CO ® Fe + CO2 (8)

4FeO ® Fe3O4 + Fe (9)

Li2FeSiO4 + C ® Fe + CO + Li2SO3 (10)

In the above discussion, it should be emphasized that the amorphous nanoparticles Fe₂O₃ react with Li₂SiO₃ after the intermediate step at 410 °C, so that the product of the solid- state reaction at temperatures around 705 °C is the orthosilicate Li₂FeSiO₄, the monoclinic polymorph (space group: P21/n).

Li₂FeSiO₄ using commercial Fe₂O₃ as precursor

The spectrum of the product obtained without milling is shown in Figure 5 (a). Three contributions is clearly observed. One doublet indicative of iron ions in a trivalent oxidized state (CS1=0.59 ± 0.02 mm/s, QS1 = 0.64 ± 0.02 mm/s, the second doublet with Mössbauer parameters CS2=0.83 ± 0.02 mm/s, QS2 = 0.76 ± 0.02 mm/s), and a third one, belonging to a

divalent oxidized state contributing with 24% to the spectral area, with CS = 0.959(4) mm/s and QS = 2.449(9) mm/s. The last doublet evidently is due to the formation of the lithium iron orthosilicate phase through a solid-state reaction. When ball milling is used in the synthesis procedure,

When the reaction mixture is ball-milled at 250 rpm, the Mössbauer spectrum typically consists of two doublets (Figure (5b)). One doublet indicative of iron ions in a trivalent oxidized state (CS1=0.265(5) mm/s, QS1 = 0.73(8)mm/s, while the majority phase exhibiting a doublet with Mössbauer parameters CS2=0.958(4), QS2 = 2.431(7) is attributed to the divalent oxidized phase of lithium iron orthosilicate, Li₂FeSiO₄ (Jugović et al., 2014; Lv et al., 2011;Sirisopanaporn et al., 2010; Dominko. 2008). This result demonstrates that by introducing milling in the synthesis procedure leads to the desired formation of Li₂FeSiO₄. As in the case of samples obtained using oxalate precursors, the small area contribution may come from delithiated orthosilicate Li₂-xFeSiO₄. In this last spectrum, traces of a magnetic components are observed. A sufficient electric conductivity is expected, with electrochemical performance comparable with Li₂FeSiO₄ prepared from Fe²⁺ precursor.

Figure 5.
Mössbauer spectrum at room temperature of a lithium iron orthosilicate obtained using hematite as a precursor of (a) unmailed sample, (b) ball-milled sample (250 rpm) prepared with commercial Fe2O3, and (c) ball-milled sample (250 rpm) prepared with green route Fe2O3.
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One should carefully observe the experimental synthesis conditions. This is illustrated in Figure 6, which shows the room-temperature Mössbauer spectrum of a sample obtained using a reduced amount of citric acid, 1.14 g instead of 1.50 g. Because of the reducing power diminishes, large concentrations of magnetite are obtained. Decreasing the grinding time (e.g., 10 h) also causes the formation of magnetite, although in a much smaller quantity.

Figure 6.
Mössbauer spectrum at room temperature of a sample obtained using commercial Fe2O3 as a precursor, a ball-milling procedure (250 rpm), and only 1.14 g of citric acid.
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Li₂FeSiO₄ using green route obtained Fe2O3 as the precursor.

As expected, the products obtained using hematite synthesized by a green route are quite similar to those using commercial hematite. The Mössbauer spectrum of a ball milled at 250 rpm sample also exhibits two doublets. The doublet corresponding to the lithium iron orthosilicate phase, the majority phase, has hyperfine parameters CS = 0.959(2) mm/s; QS = 2.439(4) mm/s. The subspectrum of the delithiaded orthosilicate, Li₂-xFeSiO₄, has parameters CS = 0.216(5) mm/s; QS = 0.589(8) mm/s, but in this case, it appears in a greater proportion due to the smaller particle size. Delithiation occurs on the surface of the particle, so the surface/bulk ratio is greater in these samples. The spectrum also shows the contribution of some magnetite in the sample.

CONCLUSION

Ball mill-assisted solid-state carbothermal synthesis of Li₂FeSiO₄ using Fe₂O₃ in an inert atmosphere is feasible. A homogeneous, crystalline Li2FeSiO4 is formed after brief annealing at temperatures of 700°C - 750°C, but preliminary milling is required to improve the amounts of orthosilicate formed,

Obtaining impurities such as Fe_xO, Fe₃O₄, and delithiated Li_2−xFeSiO₄ continues to be a problem. Improving the synthesis procedure is required. For example, controlling the amount of citric acid is necessary to ensure the reduction of Fe³⁺ to Fe²⁺. The added carbon, in the form of citric acid, acts as a reducing and covering agent, resulting in the formation of fine particulate Li₂FeSiO₄.

The carbothermal synthesis of Li₂FeSiO₄ using low-cost and readily available Fe₂O₃ and a previous low-energy mechanical alloying process is a convenient and ecologically clean method with a reduced material cost.

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