Study of Performance Graphene Oxide Modification of LiFePO4/C Material for The Cathode of Li-Ion Batteries

The need for energy storage is increasing rapidly along with technological development. Lithium ion batteries are one of the energy storages that are in great demand due to their high specific capacity and energy density, discharge voltage of 3.4 volts, and environmental friendliness. LiFePO4 is a lithium-ion battery cathode material with a high specific capacity of 170 mAh/g and a discharge voltage of about 3.4 V, thermal stability, high energy density, environmentally friendly, and easy to obtain. However, it has low electrical conductivity and poor ion diffusion, which hinders energy storage. Carbon modification is a method that has the advantage of reducing particle size and preventing agglomeration in nanoparticles, so this method is widely researched to improve lithium ion diffusion coefficient and conductivity in lithium ion batteries. This study aims to describe the effect of GO modification on the characterization of LiFePO4/C-GO composite material and LiFePO4/C-GO composite material battery performance as a lithium ion battery cathode material. In this study, it can be seen that the addition of GO in LiFePO4 cathode material can improve battery performance. The LiFePO4/C-GO-10 cathode obtained the most effective results with the lowest Rct value of 95.21 Ω and the highest conductivity value of 14.3×10-6 S/cm indicating the best electron transport. The Rct value decreased with the addition of GO, and the conductivity value increased with the addition of GO.


Introduction
Along with the development of technology, electronic equipment especially portable equipment has also increased, such as cell phones, laptops, and digital cameras.In general, portable equipment relies on energy storage, namely batteries as an energy source.Batteries can produce electrical energy through electrochemical reactions of reduction and oxidation [1,2].The battery itself is divided into two types, namely primary batteries and secondary batteries.Primary batteries are batteries that cannot be recharged or disposable, while secondary batteries are rechargeable batteries that have a long lifetime.One type of secondary battery that is usually found in electronic equipment is a lithium ion battery.Lithium ion batteries have a high energy density, high standard potential value, and long lifetime.
Lithium ion batteries consist of several key components, including the cathode, anode, electrolyte, and separator.Among the various cathode materials used in lithium-ion batteries are LiCoO2, LiMnPO4, and LiFePO4.While LiCoO2 offers high electrical conductivity, it presents concerns due to its toxicity resulting from heavy metal content.Furthermore, in situations where lithium-ion batteries are subjected to improper use, LiCoO2 cathode material has the potential to explode [3].LiMnPO4 cathode material is environmentally friendly and affordable but has the disadvantage of poor performance.LiFePO4 cathode material has a high specific capacity of 170 mAh/g and a discharge voltage of about 3.4 V [4], thermal stability, high energy density, environmentally friendly, and easy to obtain [5,6].
As seen in Figure 1, LiFePO4 belongs exhibits the olivine structure.Within this structure, phosphorus oxide tetrahedron (PO4), lithium oxygen octahedron (LiO6), and ferrite octahedron (FeO6) are formed, with P, Li, and Fe serving as the central atoms.The PO4 tetrahedron possesses a robust covalent bond between P and O, ensuring the oxygen atom remains stable throughout the charging and discharging process without undergoing oxidation.This characteristic gives LiFePO4 a distinct advantage over other cathode materials regarding safety and cycle performance.
However, it has a low electrical conductivity of 10 -9 S/cm and poor ion diffusion, which hinders energy storage [8].Effective strategies to overcome these problems include carbon modification, ion doping, morphology optimization, and particle size reduction [9,10].Carbon modification is a method that has the advantage of reducing particle size and preventing agglomeration in nanoparticles [11,12], so this method is widely researched to improve lithium ion diffusion coefficient and conductivity in lithium ion batteries.
This study aims to synthesize LiFePO4/C composites modified with graphene oxide (GO) as a conductive agent.The inclusion of GO is beneficial for maintaining material stability and enhancing the diffusion coefficient of lithium ions.The graphitization level of graphene oxide facilitates electron transfer, thereby increasing the electrical conductivity of the composite material [4,13,14].To investigate the impact of GO modification on the electrochemical performance of the lithium ion battery cathode, GO was synthesized using Hummer's method and subsequently applied to the LiFePO4/C-GO composite.A variation in the volume of GO suspension (5 ml; 10 ml) from 1 mg/ml GO concentration was employed.

2.2.
Preparation of Graphene Oxide Graphite powder and NaNO3 powder were dissolved with 97% H2SO4 under water freezing point conditions (0-5°C) with continuous stirring for 30 minutes.KMnO4 was added slowly, and the temperature was kept below 15°C for 30 minutes until the solution was purple.And it was continued stirring at room temperature for 3 hours until the solution was brown.In this process distilled water is added slowly and the temperature of the solution is maintained at 95-100 ° C in stirring for 3 hours.H2O2 was added slowly to remove permanganate compounds.At the final stage, the solution was washed with 1M HCl and distilled water until the pH was neutral and oven dried at 60°C for 6 hours.

2.3.
Preparation of LiFePO4/C-GO Composite LiFePO4 powder and carbon were milled for 1 hour.GO suspension was made with 0.05 grams of GO dissolved in 50 ml ethanol.The solution was then sonicated for 1 hour.Next, LiFePO4 and carbon mixture were added slowly and continued sonication for 30 minutes.The solution was then dried in an oven at 80°C for 3 hours.LiFePO4/C-GO composites were made with variations in GO suspension volume of 5 mL and 10 mL.

Fabrication of LiFePO4/C-GO Coin Cell
The LiFePO4/C-GO active material powder obtained was then made into cathode composite pellets by mixing it with PVDF and acetylene black dissolved in N,N-Dimethylacetamide (DMAc) solvent.Composite pellets were made with variations in the composition ratio of filler, activated carbon, and matrix, namely 85:10:5.The composite that was made is then coated on Al-foil that has been cleaned with acetone and determined the area using insulating paper with a very thin thickness as a pellet.After the coating process is complete, the coated sheet is dried at 80°C and after drying, it is stored in an oven at 50°C.The cathode sheet is then cut into circles with a diameter of 1.6 cm, the anode sheet comes from a disassembled lithium ion battery and is then cut into circles with a diameter of 1.6 cm, the separator uses polyethylene with a diameter of 1.9 cm, and the electrolyte uses 1 M LiFP6 solution.The coin cell is assembled and the coin cell is pressed using a coin cell pressing tool.

2.5.
Characterization of LiFePO4/C-GO The physical and electrochemical characteristics of the LiFePO4/C-GO material include XRD, Raman, SEM, IMAGEJ software, Gamry impedance test.XRD testing was performed to identify the phases formed from LiFePO4 and LiFePO4/C-GO cathode active materials.Raman Spectroscopy was performed to analyse the chemical structure and the irregularity and regularity in the sample as indicated by the I D /I G ratio.D band is associated with defects in the sample, and G band is associated with graphite structure in the sample [17].The microstructure of the LFP/C-GO composite was measured by Scanning Electron Microscope and Energy Despersive X-ray at 20 kV-30 kV magnification, ImageJ software was used to determine the average diameter of the material.EIS testing is carried out to observe the interaction of electrons and ions that move from cell components during electrochemical reactions so that the amount of electronic conductivity of the test material can be determined.The greater the conductivity value, the better the lithium ion battery cathode material.Equation ( 1) is the equation for calculating conductivity: Where, R = Resistivity of the material (ohm) ρ = Material type resistance (ohm.cm)d = Thickness of material (cm) A = cross-sectional area of material (cm 2 ) Because σ = 1/ρ, the equation ( 1) becomes With σ = conductivity (S/cm) [3].

X-Ray Diffraction Test
Based on the phase identification of the sample diffraction pattern using X'pert HighScore software, the phase formed is LiFePO4 with olivine structure indexed by space group Pnmb with orthorhombic crystal system.Figure 2 shows the XRD test results of LiFePO4 and LiFePO4/C-GO samples have almost the same peaks and do not show any carbon peaks.The XRD results are based on JCPDS database reference No. 00-040-1499 [4].

Raman Spectroscopy Test
Figure 3 shows the Raman spectroscopy test results of carbon, GO, and LFP/C-GO samples.The Raman spectra of Carbon powder shows the D band at a wavelength of 1367.31cm -1 and the G band at a wavelength of 1599.60 cm -1 .The GO powder shows the D band at a wavelength of 1349.82 cm -1 and the G band at a wavelength of 1602.54 cm -1 .While the LFP/C-GO powder shows the D band at a wavelength of 1353.09cm -1 and the G band at a wavelength of 1597.04 cm -1 .The intensity ratio of the ID/IG peak is inversely related to the level of graphitization in the carbon material.A smaller ID/IG ratio, the higher the degree of order in the carbon material.The I D /I G peak intensity ratio is 0.74 for carbon, 1.03 for GO, and 0.76 for LFP/C-GO composite.This ratio shows that the GO produced has few defects, while carbon and LFP/C-GO composites have a higher degree of graphitization, which can increase the electronic conductivity of the material [13,18,19].In addition, the Raman spectrum of the LFP/C-GO composite shows peaks derived from the orthorhombic phase of LiFePO4 at wavelengths of 1085.94 cm -1 ; 715.11 cm -1 ; 608.36 cm -1 ; 273.06 cm -1 ; 149.49cm -1 .The Raman vibrations of Fe-O and  4 3− in LiFePO4 are represented by the peaks observed at wavelengths ranging from 500-100 cm -1 and 1120-520 cm -1 respectively [20].The peak at 1085.94 cm -1 indicates intermolecular stretching motion and antisymmetric stretching mode of  4 3− .The peak within the 1100 and 900 cm -1 range corresponds to the intermolecular stretching motion and the antisymmetric stretching mode of  4 3− ions [21].The peaks at 990, 1058, and 945 cm -1 correspond to anti-symmetric (ν3) and symmetric (ν1) stretching of the P-O bond [13].The peak at 715.15 cm -1 corresponds to the symmetrical bend (ν2) and anti-symmetrical bend (ν4) of the O-P-O angle [13].The peak at 608.36 cm -1 is associated with the combined antisymmetric stretching mode  4 3− .The peaks observed within the range of 700 and 550 cm -1 and 500-400 cm -1 are attributed to the combined antisymmetric stretching mode of

Scanning Electron Microscope Energy Dispersive X-Ray Test
Figure 4a.shows the surface morphology of spherical LiFePO4/C particles.The EDX spectrum of LiFePO4/C in Figure 4c.shows the Fe, P, O and C content with a weight percentage of 53.5%, 11.9%, 21.5%, 13.1% and atomic percentage of 25.4%, 10.2%, 35.6%, and 28.9%, respectively.The presence of C content proves that there is carbon in the LiFePO4/C composite.The surface morphology of LiFePO4/C-GO particles shows a spherical shape with GO layer as shown in Figure 4b.The EDX spectrum of LiFePO4/C-GO in Figure 4d.shows the content of Fe, P, O and C with weight percentage of 11.6%, 5.6%, 37.2%, 45.3% respectively and with atomic percentage of 3.2%, 2.8%, 35.8%, 58% respectively.The EDX spectrum in Figure 4d.shows a higher C content, thus proving the presence of GO in the LiFePO4/C-GO sample.For the LiFePO4/C-GO sample, it can be seen that the LiFePO4/C particles are in contact with GO and form a network to improve the electron and ion transport of the electrochemical performance of the material [16].

Electrochemical Analysis
EIS testing was carried out to detect the possibility of intercalation process of lithium ions of LFP cathode and LiPF6 electrolyte solution.The EIS test results are represented in the form of graphs consisting of semicircles and slopes.The Nyquist plot shows the relationship between real impedance (Zreal) and imaginary impedance (Zim) at frequencies of 0.1 to 100 kHz, where the x-axis represents the real impedance and the y-axis represents the imaginary impedance.Figure 5 shows the EIS graphs for the LFP, LFP/C-GO-5, and LFP/C-GO-10 samples.From the graph plotting results, the Resistive charge transfer (Rct) and   values are shown in Table 1.Based on the graph, the Rct values of the samples are 252.3Ω, 195.6 Ω, and 95.21 Ω for LFP, LFP/C-GO-5, and LFP/C-GO-10, respectively.The conductivity value is obtained using equation (2) and the LFP/C-GO-10 sample has the highest conductivity value.In contrast, the lowest conductivity is obtained by the LFP sample without the addition of carbon and GO.The higher the Rct value, the more difficult the electron transfer process, so the lower the Rct value, the better the electron transport process.

Figure 3 .
Figure 3.The Result of Characterization Raman Spectroscopy Carbon, GO, LFP/C-GO.

Table 2 .
Comparison of Rct values of LiFePO4/C cathode materials.