1. Introduction
Compared with traditional rechargeable secondary batteries, lithium-ion batteries, as a new type of rechargeable battery, offer numerous advantages, including a high voltage, long cycle life, and environmental friendliness. The primary cathode materials for lithium-ion batteries include lithium cobalt oxide (LiCoO
2), lithium nickel oxide (LiNiO
2), lithium manganese oxide (LiMn
2O
4), lithium manganese dioxide (LiMnO
2), and lithium iron phosphate (LiFePO
4). In 1997, A.K. Padhi, K.S. Nanjundaswamy, and J.B. Goodenough proposed olivine-type LiFePO
4 as a new cathode material [
1]. The crystal structure of LiFePO
4 is shown in
Figure 1. Olivine-type LiFePO
4, used as a cathode material in lithium-ion batteries, features a slightly distorted, hexagonal, close-packed arrangement of internal atoms. It belongs to the orthorhombic crystal system with a space group of Pnma [
2], in which phosphorus (P) atoms are bonded to four oxygen (O) atoms to form the PO
4 structure. Lithium (Li) and iron (Fe) atoms form octahedral structures, LiO
6 and FeO
6, respectively, with six surrounding O atoms. The magnetic properties of LiFePO
4 are primarily antiferromagnetic below the Neél temperature
TN, due to superexchange interactions between the internal Fe atoms through the Fe–O–P–O–Fe bond [
3].
LiFePO
4 offers several advantages, including inexpensive raw materials, a high theoretical capacity [
4,
5], high energy density, good stability, and non-toxicity [
6,
7]. However, its application as a cathode material is mainly limited by its low conductivity [
8,
9]. There are three primary methods to improve conductivity: reducing grain size, doping with other metal ions, and coating with conductive elements [
10,
11]. Guangcong Zeng et al. [
12] prepared both coated and uncoated LiFePO
4 materials and found that the sample that was coated with C-Nb
2CTx exhibited superior electrochemical properties. Similarly, Xiaohua Chen et al. [
13] prepared zinc oxide and carbon co-modified LiFePO
4 nanoparticles (LFP/C-ZnO) using an inorganic-based hydrothermal route and found this to significantly boost its performance. Meanwhile, Abdurrahman Yolun et al. [
14] prepared an Ru-substituted LiFePO
4 cathode material and demonstrated that it exhibits excellent electrochemical performance. There are many synthetic methods for producing olivine-type LiFePO
4, including the sol-gel method [
15,
16], hydrothermal method [
17,
18], co-precipitation method [
19,
20], high-temperature solid-state reaction method [
21,
22], microwave method [
23,
24], and thermal reduction method [
24,
25,
26]. The sol-gel method, in particular, is an important approach for preparing LiFePO
4; due to its homogeneous precursor, low heat treatment temperature, simplicity of equipment, and ease of control, this method is widely favored among researchers [
27].
Furthermore, Dan Li et al. [
28] prepared carbon-coated Li
1-3xLa
xFePO
4/C (
x = 0~0.025) materials with smaller particles and uniform morphology by combining solid state reaction and microwave heating. The results showed that the conductivity was improved after La-ions doping, thereby increasing the discharge capacity of the electrode material. Yung Da Cho et al. [
29] prepared La-doped LiFePO
4 materials, and the results showed that the structure remained unchanged after La-ion doping. However, the cyclic stability of the electrode materials’ capacity was effectively improved. Shaohua Luo et al. [
30] prepared Li
1-xLa
xFePO
4 (
x = 0.0025~0.01) through a two-step solid state reaction. They found that the microstructure and grain size of samples doped with La ions hardly changed, and, among the samples, Li
0.99La
0.01FePO
4 showed the most excellent electrochemical performance. The carbon source also has a significant influence on the performance of LiFePO
4 [
31]. Chaoqi Shen et al. [
32] prepared high-performance LiFePO
4/C composite using an optimized solid-state synthesis route. M. Swierczynski et al. [
33] found that lithium iron phosphate carbon (LiFePO
4/C) composite demonstrates excellent performance, with 8000 complete service cycles at 25 °C. Xingling Lei et al. [
34] prepared LiFePO
4 cathode material by introducing carbon and found that it exhibits good crystallinity and is of the olivine type, with a microscopic particle size of approximately 200 to 500 nm. Seo Hee Ju et al. [
35] observed that pure olivine can be prepared by adding carbon, and increasing the amount of nano carbon black will, to a certain extent, increase the particle size of LiFePO
4.
In this study, we systematically investigated the effects of different carbon sources and La doping on the structure and properties of LiFePO4. Five different carbon sources, including ethylene glycol (C2H6O2, analytical grade AR), polyethylene glycol 4000 (PEG4000; HO(CH2CH2O)nH, chemically pure CP), polyvinyl alcohol (PVA-124; [CH2CH(OH)]n, analytical grade AR), citric acid (C6H8O7·H2O, analytical grade AR), and glucose (C6H12O6·H2O, analytical grade AR), were used, and LiFePO4/C composite particles were synthesized using a one-step sol-gel method. Then, LixLayFePO4 (x = 0.9~1.0, y = 0~0.1) materials were prepared using the sol-gel method, which offers precise control, straightforward operation, and simple synthesis conditions. The magnetic properties of the samples with varying doping ratios, calcination temperatures, and calcination times were studied to select the optimal doping ratio, calcination temperature, and calcination time, thereby improving the properties of the samples for an improved application value.
3. Experimental Section
The LiFePO
4/C composites were prepared using the sol-gel method. This study focused on the effects of different carbon sources on the phase crystal structure, particle morphology, and magnetic properties of the LiFePO
4/C composites. The chemical reagents used for their preparation included various carbon sources (C
2H
6O
2, C
2H
6O
2, HO(CH
2CH
2O)
nH, [CH
2CH(OH)]
n, C
6H
8O
7·H
2O, and C
6H
12O
6·H
2O), a phosphorus source (NH
4H
2PO
4), an iron source (Fe(NO
3)
3·9H
2O), and a lithium source (LiOH·H
2O). All of the reagents were purchased from Xilong Science Co., Ltd. The molar ratio of the carbon sources, phosphorus source, iron source, and lithium source was 2:1:1:1. HO(CH
2CH
2O)
nH and [CH
2CH(OH)]
n were weighed to match the mass of C
2H
6O
2. The dissolution sequence of the chemical reagents for the preparation of the LiFePO
4/C is shown in
Figure 18. Preparation process of LiFePO
4/C was showed in
Figure 19. Li
xLa
yFePO
4 (
x = 0.9~1.0,
y = 0~0.1) materials were prepared by the sol-gel method with citric acid as a complexing agent to investigate the impact of different La
2+ ratio, calcination temperature, and calcination time. The flow chart of the process is shown in
Figure 20. La(NO
3)
3·6H
2O was purchased from Tianjin Guangfu Fine Chemical Research Institute.
Specific implementation steps of preparation of LixLayFePO4 are:
Step 1: Calculate the raw materials of LixLayFePO4 (x = 1.0, y = 0; x = 0.98, y = 0.02; x = 0.96, y = 0.04; x = 0.94, y = 0.06; x = 0.92, y = 0.08; x = 0.90, y = 0.10) according to the stoichiometric ratio, and then weigh the raw materials into small beakers, numbered as A1–A6, according to the calculated amount;
Step 2: Add about 100 mL of deionized water to the A1 to A6 small beakers and stir continuously to obtain a red wine clear liquid of the A1 to A6 solutions after dissolving entirely;
Step 3: Transfer the aqueous solutions A1 to A6 to the fume hood, then add ammonia into the aqueous solution until the pH value is 9 while stirring continuously, and subsequently obtain about 100 to 120 mL of burgundy clear Sol precursor solution B1 to B6;
Step 4: Continuously stir the precursor solutions B1 to B6 in a water bath at 80 °C for about 3 h, and stop when a wet gel is formed;
Step 5: Dry the wet gel at 120 °C for 12 h in an air-drying oven after the wet gel is aged at 80 °C for 12 h;
Step 6: Grind the dry gel with an agate mortar before putting 5 mL of dry gel powder or smaller pieces of dry gel into a lidded porcelain crucible;
Step 7: Introduce nitrogen for about 40 min to remove the air in the quartz tube after transferring the dry gel powder from the porcelain crucible to the tube furnace, and then calcine and cool to room temperature in a nitrogen atmosphere;
Step 8: Put the loose-shaped samples from the porcelain crucible to the agate mortar, then finely grind it again with the agate mortar to obtain the final samples.
The various analytical techniques (TG-DTA, XRD, SEM, Mössbauer, VSM) were used to determine the following features: the impact of different doping amounts of La2+ ion, calcination temperature, and calcination time on the structure, functional groups, chemical bonding, particle shape and size, magnetic performance, and hyperfine interaction of samples
4. Conclusions
In this study, five different carbon sources, including ethylene glycol (C2H6O2, analytical grade AR), polyethylene glycol 4000 (PEG4000; HO(CH2CH2O)nH, chemically pure CP), polyvinyl alcohol (PVA-124; [CH2CH(OH)]n, analytical grade AR), citric acid (C6H8O7·H2O, analytical grade AR), and glucose (C6H12O6·H2O, analytical grade AR), were used, and LiFePO4/C composites were synthesized using a one-step sol-gel method. The crystal phase structure, functional groups, chemical bonds, microscopic surface morphology, and magnetic properties of the LiFePO₄/C composites were analyzed using XRD, FTIR, SEM, and VSM. The results indicate that the LiFePO₄/C composites that were prepared with these five carbon sources exhibit the complete standard peaks characteristic of pure LiFePO₄. The introduction of different carbon sources results in shifts in certain infrared characteristic peaks. These five carbon sources significantly affect the microstructure and magnetic properties of the composites. The sample using ethylene glycol as the carbon source forms a porous network structure, which is conducive to improving electronic conductivity. However, the sample using glucose as the carbon source exhibits distinct particles with good dispersion, resembling olivine crystal morphology. The sample using PVA-124 as the carbon source shows the highest relative Ms, measured at 2.01 emu/g, whereas the sample using ethylene glycol as the carbon source has the highest coercivity, recorded at 170.67 Oe. In short, among the five samples, the LiFePO₄/C composite that was prepared using ethylene glycol as the carbon source shows better electromagnetic properties. All of the synthesized LiFePO₄ samples doped with La have an olivine structure. However, the presence of LaPO4 impurities from side reactions indicates that the La ions were only partially doped into the lattice. The IR analysis indicates that all samples show characteristic infrared absorption peaks; the magnetic analysis suggests that the observed weak ferromagnetism may be due to the presence of weak ferromagnetic impurities; the Mössbauer spectroscopy indicates that Fe2+ compounds are the main components in the samples, accompanied by some Fe3⁺ compounds, indicating the coexistence of Fe3⁺/Fe2⁺ valence states. The Mössbauer spectra of LixLayFePO4 (x = 1.00, y = 0; x = 0.96, y = 0.04; x = 0.92, y = 0.08) after being calcined at 700 °C for 10 h indicate that all samples contain Doublet(1) and Doublet(2) peaks, dominated by Fe2+ compounds. The proportions of Fe2+ are 85.5% (x = 1.0, y = 0), 89.9% (x = 0.96, y = 0.04), and 96.0% (x = 0.92, y = 0.08). The maximum IS and QS of Doublet(1) for the three samples are 1.224 mm/s and 2.956 mm/s, respectively.