Effect of LiFePO4 Cathode Thickness on Lithium Battery Performance Ariska Rinda Adityarini 1, a, Eka Yoga Ramadhan 1, b, Endah Retno Dyartanti 3, c and Agus Purwanto 4,d Departement of Chemical Engineering, Sebelas Maret University, Indonesia * aguspurw@gmail.com, Keywords: LiFePO 4, Lithium battery, X-Ray Diffraction Abstract: Lithium ion battery is composed of three main parts, i.e. cathode, anode and electrolyte. In this work, we investigated the effect of LiFePO4 cathode composite s thickness on performances of lithium battery. LiFePO4 cathode was prepared in a slurry that consisted of lithium iron phosphate (LiFePO4) powder as active material, acetylene black as conductive additive, poly(vinylidene fluoride) (PVDF) as binder, and N-methyl-2-pyrrolidone (NMP) as solvent. The slurry was then deposited on the aluminum substrate using doctor blade method in different thickness. The cathode layers were deposited with the thickness of 150, 200, 250 & 300 µm. The structure characterization of the material was analyzed with XRD, while the material s morphology was analyzed with Scanning Electron Microscope (SEM). Performances of lithium ion battery with LiFePO4 cathode were evaluated using charge-discharge cycle test. It is found that battery made of cathode layer with 250 µm thickness shows the best performances. Introduction Battery is the most common source of energy used by people. The type of battery, which is being developed, is secondary lithium ion battery. Lithium ion battery composed of three main parts, i.e. cathode, anode and electrolyte. For cathode material, lithium compounds that were well developed are lithium cobalt oxide (LiCoO2) [1,2,3], lithium manganese oxide (LiMn2O4) [4,5,6], lithium ferri phospate (LiFePO4) [7,8,9], etc. As reported in 1997, LiFePO4 could be used as positive electrode of secondary lithium battery [10]. LiFePO4 had many advantages, for example, it has cheap materials, good cycle, stability, and it is also environmentally friendly [11,12]. Since the publication of the report in 1997, there have been many works that study how to enhance the quality of LiFePO4 material for secondary lithium battery cathode. Different polymer as binders affects LiFePO4 cathode s electrochemical performance [13]. Currently, poly (vinylidene fluoride) (PVDF) is the most used binder in commercial secondary lithium battery because it has great electrochemical stability [14]. Another study found that zinc-doping improved the performance of LiFePO4 including discharge capacity and rate capability [15]. Polythiopene coating could also increase reversible capacity and enhance the cycling ability of LiFePO4 electrodes [16]. Jin et. al. reported that LiFePO4 with multiwalled carbon nanotubes structure not only enhanced lithium-ion diffusion coefficient and electronic conductivity but also decreased the size of crystallite and charge transfer resistance of LiFePO4 composite[17]. Xiao et. al [18] demonstrated that an additional layer of acetylene black film on the surface of an active material could be applied to make a novel sandwich-like three-layer electrodes. LiFePO4/C in the three-layer electrodes shows a much better rate capability. The aforementioned publications focused on the half-cell of Lithium battery. Meanwhile, this study would report the effect of LiFePO4 cathode thickness on performances of cylindrical type-18650 lithium battery which is usually produced industrially.
Experimental The materials used in this research are lithium ferri-phosphate (LiFePO4), polyvinylidene fluoride (PVDF), and acetylene black (AB). The solvent was N-methyl-2-pyrollidone (NMP) (MTI corp., USA). LiFePO4 powder, PVDF and AB were weighed with the mass ratio 8.6:1:0.4 and placed inside the oven in a 100 o C temperature for 5 minutes. LiFePO4 and AB were milled using ball mill for 5 minutes, while PVDF and NMP were mixed inside a vacuum mixer for 60 minutes. Both were then mixed in the vacuum mixer until became homogenous. The slurry was coated on an aluminium sheet using doctor blade method in varied thickness. The cathode film was placed inside the vacuum oven with 120 o C temperature to dry it. The dried cathode film was pressed using hot rolling press machine at 100 o C. The cathode film was made with four thickness variations (150, 200, 250 & 300 µm ).The structure characterization from the sample was analyzed with XRD, while the material s morphology was analyzed with Scanning Electron Microscope (SEM). The fabrication of lithium battery was done by rolling LiFePO4 cathode film, separator, and Carbon-based anode film using Winding Machine. LiPF6 as electrolyte was filled into the cell case inside the argon-filled glove box. Electrochemical performances of secondary lithium battery were analyzed using the eight channel battery analyzer (MTI corp, USA). Results and Discussion Before conducting a detail investigation, the as-received LiFePO4 was characterized using XRD technique. The XRD Pattern of LiFePO4 is shown in Figure 1.The XRD pattern matched very well with the LiFePO4 reference indicated by the purity of their crystal phase [19]. The crystal structure is orthorhombic. Intensity 2θ ( ) Fig.1 XRD Patterns of LiFePO4 To determine the particle morphology of LiFePO4, scanning electron spectroscopy (SEM) characterization was conducted. The SEM image of LiFePO4 particles is shown in figure 2. The SEM observation shows that LiFePO4 powder has irregular shape. The particles size is varied from
100 to 600 nm. Using the as-received materials, a 18650 type battery lithium was fabricated to evaluate the effect cathode thickness on the battery performance. Fig.2 SEM Image of LiFePO4 Powder Surface (a) (b) (c) (d) Fig. 3 The Charging-Discharge curve of battery with cathode composite in (a) 150 µm, (b) 200 µm, (c) 250 µm (d) 300 µm
The electrochemical performances of the prepared battery were analyzed using Eight Channel Battery Analyzer. The thickness variation of cathode film caused the difference of electrochemical performances. This was reflecyed on the overview of charge/discharge capacities versus voltage that were done in 10 cycles as shown in Figure 3. Fig.3 (a, b, c, d) shows the effect of cathode film thickness variation on charge/discharge capacity for first ten cyclic. X axis shows battery capacity (mah), meanwhile Y axis shows battery voltage (V). Battery capacity reduction / degradation level was shown by the gap between the curves. Smaller gap between curves means lower degradation level. By observing cathode whose thickness ranges from 150 to 250 µm, it could be stated that the thicker the film, the smaller the degradation level. The minimal degradation is shown by cathode which has thickness of 250 µm. Cathode thickness of 300 µm shows the worst degradation level. This happened because the film was broken at that thickness. Fig. 4 The initial discharge capacity of secondary lithium battery in a variation of thickness Discharge capacity at the first cycle from fig. 3 was shown in fig.4. The first discharge capacity of the lithium ion battery cathode thickness of 150, 200, 250, 300 µm is respectively 104.89, 116.15, 116.67, 58.18 mah/g. The highest capacity was obtained from battery constructed from the 250 µm cathode thickness. The capacity of battery constructed from 300 µm cathode composite s thickness decreased because the layer was broken in that thickness. Conclusion The effect of cathode thickness on the electrochemical performance of the cylindrical type lithium battery was investigated. The battery was made from LiFePO4 material. The XRD characterization showed that the material is pure LiFePO4 without any crystal impurities. Using SEM analysis, this study found that LiFePO4 powder had size ranging from100nm to 600 nm. The electrochemical performance of the prepared battery indicated that battery having 250 µm thickness of cathode produced the highest discharge capacity of 116,67 mah g -1. Acknowledgments This work was financially supported by RISPRO-LPDP (contract no PRJ-906/LPDP/2013) References [1] C.H. Chen, Fabrication of LiCoO2, Thin Film Cathodes for Rechargeable Lithium. Solid State Ionics, 80 (1995) 1-4.
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