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Enhancement in thermal stability and surface properties of LiFePO4/VFLG composite prepared via sol-gel route Amun Amria,*.
Yola Bertilsya Hendria.
Sunarnoa.
Yoyok Dwi Setyo Pambudib.
Mazhibayev Assylzhanc.
Kambarova Elmirac.
Khusnul Aind.
Khairulazhar Jumbrie.
Zhong Tao Jiangf.
ChunChen Yangg Department of Chemical Engineering.
University of Riau.
Pekanbaru 28293.
Indonesia Research Center for Nuclear Reactor Technology.
BRIN.
Tangerang Selatan 15314.
Indonesia Department of Chemistry.
Taraz University named after M.
Kh.
Dulaty.
Taraz 080000.
Kazakhstan Department of Biomedical Engineering.
Universitas Airlangga.
Surabaya 60115.
Indonesia Department of Fundamental and Applied Sciences.
Universiti Teknologi PETRONAS.
Seri Iskandar 32610.
Malaysia Surface Analysis and Materials Engineering Research Group.
Murdoch University.
WA 6150.
Australia Battery Research Center of Green Energy.
Ming Chi University of Technology.
Taishan.
New Taipei City 24301.
Taiwan Article history:
Received: 26 February 2025 / Received in revised form: 9 May 2025 / Accepted: 10 May 2025 Abstract Thermal and surface properties of LiFePO 4/very-few-layer graphene (LiFePO4/VFLG) composite manufactured through the sol-gel route have been researched for lithium-ion battery cathode application.
VFLG was acquired from a facile, cost-effective, and environmentally benign fluid dynamic shear exfoliation process.
The composites were characterized through thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), field-emission scanning electron microscopy (FESEM) interlinked with energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Braneur-Emmett-Teller (BET) The TGA-DSC results showed that the integration of VFLG could enhance the thermal stability of the composite by inhibiting oxygen diffusion on the LiFePO4 surface.
FESEM-EDX analysis, meanwhile, confirmed the homogeneously distributed VFLG in the composites.
TEM
results revealed that the average particle sizes of the composites decreased by about 21.
2% compared to the bare LiFePO 4.
TEM and HRTEM results confirmed an intimate contact between VFLG intimately and LiFePO4 particles via plane-to-point contact, contributing to the control and reduction of particle size.
Furthermore, physisorption via BET analysis revealed that incorporating VFLG provided a wider distribution of mesopores and increased pore diameter and pore volume by 128.
7% and 656.
3%, respectively, compared to sole LiFePO4.
These significant improvements were related to the flexibility and ability of a thin layer of VFLG to limit the growth of LiFePO 4 particles.
This approach offers a promising strategy to enhance the thermal stability and surface properties of lithium-ion battery cathodes.
Keywords: Sol-gel route.
LiFePO4/VFLG composite, thermal stability, surface properties, pore distribution Introduction Lithium iron phosphate (LiFePO.
, considering its long cycle life, high safety, and eco-friendliness, has emerged as a prospective cathode material for lithium-ion batteries (LIB.
Its lower electrical conductivity and thermal stability, however, still limit its widespread use, particularly in highperformance applications requiring these properties .
Various efforts have then been made to overcome these limitations by modifying its structure and composition .
the strategies investigated, the addition of carbon-based conductive materials has shown significant potential for the enhancement of these properties .
* Corresponding author.
Tel.
: ( .
751- 72497.
fax: ( .
751 - 72566 Email: taufiqihsan@eng.
https://doi.
org/10.
21924/cst.
Integrating graphene is considered among the most effective ways to elevate the efficiency of LiFePO4 .
Graphene, a carbon allotrope with exceptional electrical conductivity, a large surface area, and excellent thermal behavior .
, has become a popular additive to advance the electrochemical capabilities of LIB cathode materials.
The integration of graphene can address the conductivity challenges of LiFePO4 by creating a conductive network enabling to improve electron transport and overall battery efficiency .
The thermal stability of LiFePO4 can also be enhanced by adding graphene, which serves as a thermal distributor, to decrease the risk of overheating and thermal runaway, which are the critical issues in battery applications .
The addition of graphene in the LiFePO4 cathode has also been reported to significantly impact the morphology of the material, leading to the generation of numerous mesopores that This open access article is distributed under a Creative Commons Attribution (CC-BY) 4.
0 license Amri et al.
/ Communications in Science and Technology 10.
68Ae74 then create a unique 3D conductive network structure .
This unique structure later on results in a better rate and cyclic efficiency of the cathode due to its high conductivity and abundant mesopores, which accelerate the Li ion transport .
An excessive graphene addition, nonetheless, can reduce the total energy density and performance.
as a consequence, a careful control in adding the graphene amount is highly critical .
Furthermore, the structure and number of graphene layers also determine the morphology of LiFePO4 .
Both single-layer and multilayer graphene can improve the morphology and electrochemical capabilities of LiFePO4.
The excessive number of layers, however, can become a significant barrier to lithium-ion movement in which it can increase the ion transport path length, and ultimately reduce ionic conductivity .
The thinner the graphene layers, the more efficient the conductive network formed within the cathode material .
Typically.
FLG .
ew-layer graphen.
consists of 4-8 graphene layers .
, .
, while VFLG .
ery-few-layer graphen.
is primarily composed of 1-3 graphene layers .
Chemical vapor deposition (CVD) that produces singlelayer graphene can deliver superior LIB performance .
Nevertheless, the transfer process, its homogeneity within the cathode composite, and the high processing costs of CVDderived graphene still limit its commercial feasibility .
Incorporating graphene oxide (GO) or reduced graphene oxide .
GO) to LiFePO4 cathodes shows good dispersion and homogeneity, as well as excellent specific capacity .
, but defects in rGO can reduce charge carrier mobility and limit high-temperature stability .
Furthermore, the prolonged GO preparation process involving less environmentally friendly chemicals has reduced interest in the use of this Wang J.
et al.
reported that few-layer graphene (FLG), obtained through a liquid-phase exfoliation method, could significantly enhance the electrochemical capabilities of LiFePO4 cathodes, even at relatively low loadings .
wt%) .
However, the preparation of a cavitation jet system to form graphene flakes and powder .
two-step proces.
is cumbersome and involves the use of acetone, which is relatively less environmentally friendly .
Based on the above discussion, the use of thinner graphene can lead to a more efficient conductive network, thereby optimizing LiFePO4 cathode performance.
The addition of very few-layers graphene (VFLG) to LiFePO4 cathodes has attracted significant attention as a promising approach to enhance the cathode performance in relation to the minimal number of graphene layers, the facile and low-cost production process, and its environmentally friendly nature .
In our previous study, we successfully enhanced the electrochemical capabilities of LiFePO4 cathodes after the addition of VFLG using the sol-gel method in which it resulted in improvements in the lattice parameter of LiFePO4, an improvement in specific discharge capacity reaching 58.
Cyclic voltammetry and electrochemical impedance spectroscopy exhibited that LFP/VFLG displayed minimal internal resistance, excellent electrochemical reaction reversibility, an elevated Li diffusion coefficient, and minimal polarization .
This study explored the outcome of adding VFLG on the thermal behavior and morphology of LiFePO4 The direct integration of low-cost very few-layer graphene (VFLG) .
btained through a straightforward and environmentally friendly liquid shear exfoliation proces.
into LiFePO4 lithium-ion cathodes, offers a novel strategy to elevate the performance of LiFePO4 lithium-ion cathodes.
This method reduces both production costs and environmental impact.
The investigation involved thermal analysis using TGA-DSC, morphological analysis via FESEM-EDX and TEM-HRTEM, as well as physisorption analysis through BET testing.
The results indicated improvements in thermal properties and morphology after the addition of VFLG to LiFePO4 in view of the flexibility and thin-layer structure of VFLG, which restricted particle growth within LiFePO4.
In the context of LiFePO4 as a cathode for lithium-ion batteries, the addition of VFLG provides more conductive sites, optimizing the electrochemical capabilities of LiFePO4 as a LIB cathode.
Materials and Methods Preparation of VFLG VFLG was synthesized from natural graphite using facile two-step shear exfoliation in an aqueous solution .
The solution is derived from an affordable domestic dishwashing liquid comprising 18.
9% sodium lauryl sulfate (SLS) as an active and stabilizing compound.
In the first step, the solution is exfoliated through a turbulence-assisted shear exfoliation (TASE) process in a rotating blade mixer, and after 24 hours, the liquid is separated from the residue.
In the second step, the resulting liquid was fed into the highly effective L5M highshear mixer for 120 min to complete the exfoliation process.
This process in the end produced the graphene solutions.
Preparation of LiFePO4 and LiFePO4/VFLG composites The synthesis of LiFePO4 was carried out using lithium dihydrogen phosphate (LiH2PO.
(Sigma-Aldrich.
St Louis.
MO) and iron .
citrate (C6H5FeO.
(Sigma-Aldrich.
Ou 99%) as the base materials.
The solutions of LiH2PO4 and C6H5FeO7, both in equimolar proportions, were combined and heated to 60EE while stirring for 10 minutes with a heated magnetic stirrer.
The solution was dehydrated at 70EE for 24 hours to produce a dense xerogel, which was later grinded in a mortar for around 20 min and calcined under an Argon atmosphere at 700EE for 10 hours.
To prepare LiFePO4/VFLG composites, 8% wt VFLG liquid was added, and all the preparation methods followed the same thorough route .
Characterizations LiFePO4 LiFePO4/VFLG characterized through thermogravimetry (TGA) analysis, differential scanning calorimetry (DSC) analysis, fieldemission scanning electron microscopy (FESEM) interlinked with Energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high-resolution (HRTEM), physisorption analysis.
Here.
TGA and DSC analysis were performed by means of a TGA/DSC simultaneous thermal analyzer (TGA/DSC 3 High-temperature furnace.
Mettler Toled.
FESEM analysis was performed using FEI Nova NanoSem 230 (FEI.
Hillsboro.
Oregon.
USA) equipment Amri et al.
/ Communications in Science and Technology 10.
68Ae74 interlinked with EDX (EDX Max 20.
Oxford Instrument.
Oxford.
UK).
Furthermore.
TEM was performed using a JEOL JEM 1400 Instrument (JEOL.
Peabody.
MA.
USA) with a 120 kV voltage applied for acceleration, while HRTEM was processed using the JEOL JEM-2100F Field Emission TEM (JEOL JEM-2100F.
JEOL Ltd.
Tokyo.
Japa.
, with a 200 kV accelerating voltage applied.
For the physisorption analysis, the specific surface areas of LiFePO4 products were assessed using the the multi-point Braneur-Emmett-Teller (BET) technique.
while the Barrett-Joyner-Halenda (BJH) pore size distribution was determined from the desorption segment of the isotherm with Quantachrome Instrument (Version 3.
Results and Discussion TGA-DSC analysis Fig.
1 and Fig.
2 present the diagram of the formation of LiFePO4/VFLG composites and the TGA-DSC curves of LiFePO4 and LiFePO4/VFLG composites at a 10 K/min heating rate under airflow, respectively.
As observed, the TGA-DSC curves for LiFePO4 and LiFePO4/VFLG composites had an almost similar trend.
The TGA curve of LiFePO4 and LiFePO4/VFLG composites (Fig.
) showed a significant mass loss in the temperature range around 400EE-550EE with the mass loss of LiFePO4 and LiFePO4/VFLG composites by 6.
8 % and 6.
5 %, respectively.
This mass loss was related to an oxidation reaction of carbon and LiFePO4 occurred at high temperatures, as seen in equation .
, as reported by Belharouak et al.
LiFePO4/VFLG composites.
The exothermic peaks of LiFePO4 and LiFePO4/VFLG composites were observed at 416.
43EE, respectively.
The slight shift of the exothermic peak to a higher temperature in LiFePO4/VFLG composites indicated that the oxygen diffusion onto the LiFePO4 surface was slightly inhibited by the VFLG layer, leading the oxidation temperature to slightly increase .
15% increas.
Nevertheless, as reported by Wei et al.
this layer will not retard lithium-ion diffusion in the charge/discharge process in LIB application .
The inhibition of oxygen diffusion at the LiFePO4 surface may occur in view of several possibilities.
First, at high temperatures, graphene is able to form strong covalent bonds with oxygen atoms on the LiFePO4 surface.
These bonds then reduce oxygen mobility on the surface, thereby inhibiting oxygen diffusion.
A study showed that the strong CAeO bonds on the surface of graphene made it more difficult for oxygen to move freely .
The second possibility is that oxygen molecules adsorbed on the graphene surface may trigger competition with other molecules, such as hydrogen or nitrogen, which can also be adsorbed on the same surface.
This competition can then reduce the amount of oxygen interacting with the LiFePO4 surface .
Thirdly, graphene may cover the active sites on the LiFePO4 surface, commonly used for reactions with oxygen.
By covering these sites, graphene reduces the amount of oxygen that can diffuse and react on the LiFePO4 surface .
CAeLiFePO4 A O2Ic nO2Ic Ie 1/3 Li3Fe2(PO.
3 1/6 Fe2O3 nO2Ic .
Fig.
TGA curves of LiFePO4 and LiFePO4/VFLG, .
DSC curves of LiFePO4 and LiFePO4/VFLG FESEM-EDX analysis Fig.
The schematic of the formation of LiFePO4/VFLG composites Fig.
shows that the LiFePO4/VFLG composite exhibited a mass loss lower than that of pure LiFePO4, suggesting that the incorporation of graphene could improve the thermal stability of the LiFePO4/VFLG composite.
Graphene can shield the underlying substance from damage by reason of intense heat and distribute the thermal energy evenly to mitigate structural degradation .
This suggests that graphene can become a thermal barrier that is able to improve material resistance to degradation at high temperatures.
Fig.
illustrates the DSC curve of LiFePO4 and Fig.
3 reveals the FESEM images of LiFePO4 and LiFePO4/VFLG composites.
Fig.
, .
shows that the LiFePO4/VFLG composites provided an homogeneous distribution of LiFePO4 particles, likely due to the even dispersion of graphene in the LiFePO4 precursor suspension .
, which prevents the aggregation of LiFePO4 particles .
These findings are consistent with the report by Tian et al.
revealing that LiFePO4 modified with graphene of high electrical conductivity displayed a uniform particle A close view showed the appearance of mesoporous structures in LiFePO4 and LiFePO4/VFLG composites, as shown in Fig.
, .
It is worth noting that the porous structure in LiFePO4 and LiFePO4/VFLG composites is determined by the evolved gases .
ater vapor (H2O), carbon monoxide (CO), and carbon dioxide (CO.
) during the dehydration process, as well as the decomposition of iron .
citrate and lithium dihydrogen phosphate .
Fig.
shows the presence of graphene covering the Amri et al.
/ Communications in Science and Technology 10.
68Ae74 LiFePO4 surface.
As observed, graphene intimately contacted with LiFePO4 particles by plane-to-point contact as graphene sheets have the outstanding flexibility and thin and high surface area .
Graphene sheets act as a conductive additive and support the LiFePO4 structure, connecting separated or isolated LiFePO4 particles to form a more effective conducting network .
The conducting network of graphene provides a fast pathway for electron migration during the charge/discharge process, improving electron conductivity in LIB applications .
sheets were evenly distributed on the surface of the LiFePO4 particles to form LiFePO4/VFLG composites.
The interactions between LiFePO4 and graphene in LiFePO4/VFLG composites have been studied by Wang et al.
It has been reported that there is an interfacial binding between LiFePO4 and graphene in a parallel orientation .
The interfacial binding energies (Ebi.
per unit area of LiFePO4/VFLG was reported as 20.
meV/yI2 .
, where this value was within the Van der Walls binding energy interval .
-21 meV/yI.
, indicating that the Van der Walls bond has a significant contribution in the interaction between LiFePO4 and graphene .
Fig.
FESEM image of .
LiFePO4 at scale bar of 100 m, .
LiFePO4/VFLG at scale bar of 100 m, .
LiFePO4 at scale bar of 2 m, .
LiFePO4/VFLG at scale bar of 2 m Fig.
4 shows the EDX mapping of LiFePO4/VFLG composites in which Fe.
P, and O elements were uniformly In addition.
C element appeared to be well distributed in all parts, indicating a homogeneous graphene Table 1 presents the percentages of iron (F.
, phosphorus (P), oxygen (O), and carbon (C) contained in LiFePO4 and LiFePO4/VFLG composites.
The EDX results confirmed that graphene has been successfully incorporated into LiFePO4, as proven by increased carbon composition.
Moreover, both samples showed a Fe:P:O ratio close to 1:1:4, corresponding to the structure of LiFePO4.
Fig.
Energy spectrum of the elements in the sample, .
EDX mapping of carbon, oxygen, phosphorus, and iron elements, respectively.
Table 1.
Percentage of elements in LiFePO4 and LiFePO4/VFLG composites LiFePO4 Elements LiFePO4/VFLG Weight Atomic Weight percentages percentages percentages (%wt.
(%wt.
Atomic .
TEM and HRTEM analysis The presence of graphene was further identified using a TEM analysis.
Fig.
5 displays the TEM images of LiFePO4 and LiFePO4/VFLG composites.
It was found that the graphene Fig.
TEM images of LiFePO4 and LiFePO4/VFLG The mean particle sizes of LiFePO4 and LiFePO4/VFLG composites were 95.
98 nm and 75.
60 nm, respectively.
These results indicated that incorporating graphene reduced the particle size of LiFePO4.
This can be explained by graphene acting as a separator, impeding the growth and agglomeration of LiFePO4 particles .
Generally, smaller particle size will provide a sufficient contact area for LiFePO4/electrolyte and shorten the Li diffusion pathway, leading to the enhancement of the electrochemical capabilities of the LIB cathode .
Amri et al.
/ Communications in Science and Technology 10.
68Ae74 Fig.
6 illustrates HRTEM image of LiFePO4 and LiFePO4/VFLG composites.
It is observable that a thin layer of amorphous carbon coated both LiFePO4 and LiFePO4/VFLG composites with a thickness of about 1-2 nm, which originated from the citrate decomposition during the synthesis reaction (Fig.
The inserts in Fig.
showed a corresponding Fast Fourier-Transform (FFT) pattern, displaying a single crystal pattern with sharp diffraction points.
Furthermore.
Fig.
show the Fast inverse Fourier-Transform (Inverse-FFT) and line profile for inverse-FFT.
Based on the line profile of inverse-FFT, d spacing values of LiFePO4 and LiFePO4/VFLG composites were 0.
39 nm and 0.
391 nm, respectively, which corresponded to the orthorhombic .
Furthermore, the presence of graphene in LiFePO4 is seen in Fig.
, where graphene layers coated and connected LiFePO4 particles to form an effective conducting network .
phenomenon of particle agglomeration .
According to Kuo et al.
, graphene sheets can function as a robust scaffold to prevent particle agglomeration.
It has been reported that a large surface area can enhance the electrode/electrolyte interface .
roviding more active sites for electrochemical reaction.
and enrich the diffusion pathway for Li ions and electrons, improving the specific capacity and performance of the LIB battery .
Fig.
Nitrogen adsorption/desorption isotherm curve, and .
Pore distribution curve of LiFePO4 and LiFePO4/G Fig.
HRTEM image of LiFePO4, .
Inverse FFT of LiFePO4, .
FFT of LiFePO4, .
HRTEM image of LiFePO4/VFLG composites, .
InverseFFT of LiFePO4/VFLG composites, .
FFT of LiFePO4/VFLG composites BET analysis To investigate the surface characteristics of LiFePO4 and LiFePO4/VFLG composites, a BET analysis was conducted.
Fig.
presents the isotherm plot for nitrogen adsorption/desorption of LiFePO4 and LiFePO4/VFLG It can be observed that the two LiFePO4 and LiFePO4/VFLG composites featured a typical isotherm of type IV, indicating the mesoporous characteristics (>2 nm and < 50 n.
, in agreement with FESEM results (Fig.
The surface areas of LiFePO4 and LiFePO4/VFLG composites were measured by multi-point BET and listed in Table 2.
The measured BET surface areas of LiFePO4 and LiFePO4/VFLG composites were 36,42 m2/g and 43,39 m2 g, or increased by 1% after the addition of graphene, indicating that the presence of graphene increased the surface area of LiFePO4/VFLG composites effectively.
This is probably due to the high specific surface area of graphene .
0 mA/.
and the ability to bridge separated or isolated LiFePOCE particles, forming a conducting network that can effectively reduce the Fig.
presents the BJH pore size distribution curves of LiFePO4 and LiFePO4/VFLG composites.
It was found that the incorporation of graphene provided a wide distribution of mesopores .
nm Ae 30 n.
Du et al.
explained that the presence of mesopores in the LiFePO4 structure is beneficial to reduce the volume and structure changes and to maintain electrode stability during the electrochemical cycles in LIB Furthermore.
Table 2 lists the pore diameter and pore volume of LiFePO4 and LiFePO4/VFLG composites.
seen in Table 2, the pore diameter and pore volume of LiFePO4 increased by 128.
7% and 656.
3%, respectively, after the addition of graphene.
The significant increase in pore diameter and volume was likely related to graphene, whose flexible structure and high surface area assist in creating more void space within the composite.
This is reflected in the research by Weng et al.
, showing that the addition of graphene can increase the specific surface area and pore volume of the LiFePO4/graphene composite .
The bridging of graphene nanosheets and the creation of interconnected conduction networks through cross-linking between adjacent crystallites promote the formation of mesoporous structures, which ultimately increases pore diameter and volume .
As shown in the previous TEM analysis section, graphene can limit the growth of LiFePO4 particles, keeping these particles small and well-dispersed.
This results in a growth of the effective surface area and creates more space between particles, thereby increasing the pore volume of the composite.
Research indicated that LiFePO4/graphene composites with welldispersed graphene exhibited an improved electrode performance in LIB applications due to better particle distribution and increased pore size .
Table 2.
Values of pore diameter, pore volume, and surface area of LiFePO 4 and LiFePO4/Graphene composites Pore Diameter, nm (BJH adsorptio.
Pore Volume, cc/g (BJH adsorptio.
Surface area, m2/g (Multi-point BET) LiFePO4 LiFePO4/G Samples Amri et al.
/ Communications in Science and Technology 10.
68Ae74 Conclusion LiFePO4/VFLG composites were successfully generated through the sol-gel route.
TGA-DSC curves suggested that the integration of graphene strengthen the thermal stability of the LiFePO4/VFLG composite by inhibiting oxygen diffusion on the LiFePO4 surface.
This was confirmed by the lower mass loss of LiFePO4/VFLG compared to LiFePO4 in the TGA curve, as well as the shift of the exothermic peak towards higher temperatures .
n increase of 0.
15%) in the DSC curve.
FESEM-EDX analysis confirmed the presence of graphene, which was homogeneously distributed in the composites.
Meanwhile.
TEM results revealed that the average particle sizes of LiFePO4 and LiFePO4/VFLG composites were 95.
nm and 75.
60 nm, respectively.
TEM and HRTEM results furthermore confirmed that graphene intimately contacted LiFePO4 particles via plane-to-point contact, contributing to the control and reduction of particle size.
Physisorption analysis revealed the surface areas of LiFePO4 and LiFePO4/VFLG composites at 36.
42 mA/g and 43.
39 mA/g, respectively, representing an increase of 19.
1% after the addition of graphene.
The incorporation of graphene provided a wider distribution of mesopores .
nm Ae 30 n.
compared to bare LiFePO4.
The pore diameter and pore volume of LiFePO4 increased by 128.
7% and 656.
3%, respectively after the addition of VFLG.
These significant increases were related to the flexibility and high surface area properties of graphene, as well as its ability to limit the growth of LiFePO4 particles.
These excellent physicochemical properties of LiFePO4/VFLG composites provided a more conducive site for advancing the electrochemical capabilities of LiFePO4 cathode-based LIB.
Acknowledgements This work was supported by Badan Riset dan Inovasi Nasional (BRIN) and Lembaga Pengelola Dana Pendidikan (LPDP) Republic of Indonesia via the RIIM research grant (Contract number: 7672/UN19.
3/AL.
04/2.
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