Jurnal Kimia Sains dan Aplikasi Preparation of Zinc Oxide / Graphite Composite Using Solid - State Method as an Anode Material for Lithium - Ion Battery

Article history : Lithium - ion batteries using zinc oxide ( ZnO ) as anode material had a high theoretical capacity of about 987 mAh / g . Unfortunately , ZnO capacity can drop below 200 mAh / g after only a few cycles . For that reason , graphite was added in this study due to its stable theoretical capacity of around 348 - 374 mAh / g to maintain the stability of lithium - ion battery capacity . Zinc oxide / graphite ( ZnO / Graphite ) was prepared using a solid - state method , in which ZnO and graphite were mortared until homogeneous with the mass ratio of ( 2 : 1 ) , ( 1 : 1 ) , and ( 1 : 2 ) . The SEM images of all samples showed the agglomerate morphology between ZnO and graphite which affect the results of the battery performance test . The final result of the ZnO / Graphite anode can be considered a continuous anode material due to the stable cycle performance obtained in the range of 219.72 - 371.27 mAh / g with a decreased value of 40 % after 55 cycles .


Introduction
Lithiumion batteries are energy storage in great demand nowadays because of their outstanding performance, high power density, high energy, and long cycle life. Therefore, this is the key to the rapid development and improvement of portable electronic devices and their applications on a large scale [1]. The performance of lithiumion batteries is highly dependent on the active materials used in the anode and cathode of lithiumion batteries. Since the anode determines the lithiumion battery ' s output voltage range [2], it is essential to research and modify the active materials used in anodes for lithiumion batteries as they will eventually be employed in highpower applications such as electric vehicles.
In addition , metal oxides have prominent advantages compared to conventional carbon materials, such as high theoretical capacity, good safety performance, and wide availability [ 4 ]. Metal oxides that have been studied in previous studies were Fe 203 [ 5 , 6,7 ] and CuO [8, 9 ,10, 11].
The metal oxide that will be used is ZnO with a graphite composite since it has various advantages such as easy preparation, strong chemical stability, and inexpensive cost [12]. In addition, ZnO itself has a high theoretical capacity of 987 mAh / g [ 13 , 14 , 15 , 16].
However, the zinc oxide capacity can only drop below 200 mAh / g after a few cycles [ 17 ]. For this reason, graphite composites are used because it has a stable theoretical capacity in the range of 348 -374 mAh / g [18,19 ]. This is expected to maintain the stability of cycle performance of ZnO for lithiumion batteries.
The sample in this study was prepared using ZnO and graphite commercial, in which the mixing was conducted using a simple solidstate method. The solidstate The active materials that are in great demand for use as anodes in lithiumion batteries are metal oxides. This is because the metal oxide employed will experience a redox reaction during the chargedischarge process [ 3 ]. θ method is one of the most commonly used for lithiumion batteries because of its costeffectiveness and ease of synthesis [20]. Therefore, this present study aimed to synthesize ZnO / Graphite as the anode for lithiumion batteries using the solidstate method.

Materials
The active ingredients used as anodes for lithiumion batteries were zinc oxide (ZnO) (Loba Chemie, 99 %) and graphite (Graphite, 99 %) without further purification. The active ingredient used as the cathode for the lithiumion battery was technical NMC -811 (Nickel Manganese Cobalt -811), produced by the Centre of Excellence for Electrical Energy Storage Technology, Sebelas Maret University.

Sample Preparation of Zinc Oxide / Graphite (ZnO / Graphite)
This research consisted of three main stages of work procedures: material synthesis, battery assembly, and battery performance test, as shown in the flow chart in Figure 1. The material synthesis stage involved making ZnO / Graphite composites as the battery anode using the solidstate method. In the battery assembly stage, ZnO / Graphite that had passed sample characterization were assembled to become lithiumion batteries. The battery performance test was the stage to determine the performance of the lithiumion batteries that have been made.
hour to obtain a homogeneous sample. The resulting ZnO / Graphite were then characterized. Samples that pass the characterization stage will be used to prepare battery anodes.

. Battery Performance Test
Battery assembly testing must be done before performing a battery performance test. At the battery assembly stage, it started by dissolving ZnO / Graphite: AB (Acetylene Black): SBR (Styrene Butadiene Rubber): CMC (Carboxymethyl Cellulose) material in a ratio of 80:10: 7 :3 with distilled water. The ingredients were mixed using a stirrer to form a paste. The resulting paste was coated with a thin layer on top of Cu foil with a width of ± 5.5 cm and a thickness of ± 0.2 cm for each layer, with the final mass of the anode layer was around 4 g. The formed thin layer was then put in the oven until it dried.
The resulting thin anode layer was used to coat the battery components with a separatoranodeseparatorcathode arrangement in the battery winding process ( Figure 2). Furthermore, the batteries were arranged in a glovebox with an electrolyte solution. The type of electrolyte used was LiPFe. Electrolytes containing LiPFs usually show good conductivity and electrochemical stability and do not promote aluminum corrosion, a material commonly used as a positive electrode current collector [21].

ZnO-C2
In testing battery performance, the test equipment used was an eightchannel battery analyzer (BTS-5V6 A, China), which obtained the results of the battery chargedischarge capacity. The performance test is the formation test by running the chargedischarge performance at C / 20 and then continuing with the Crates to see the capacity ' s performance when discharge values were set at different stages. Crate testing was carried out with charge set at 0.5 C and discharge rate at 0.1C, 0.2C, lC, and 2C. The settings 0.1C and 0.2C aimed to determine the slow chargedischarge capacity, lC for the standard of chargedischarge, and 2C for the fast chargedischarge.
The EIS (Electrochemical Impedance Spectroscopy) (EZstat Pro Nuvant) test analyzed the electrochemical properties of lithiumion batteries with the frequency range of 0.01 Hz to10 kHz and the amplitude of 5 mV. The impedance data obtained would be plotted into Nyquist plot using software Origin 2018 version and fit in data using software ZsimDemo 3.2 to acquire the resistance value. Figure 3 shows the XRD results of ZnO / Graphite composites. XRD results from samples that had been stacked were compared with XRD ZnO pure (commercial) and graphite (commercial) results to obtain a data match. Based on the XRD results, the ZnO -Co. 5 revealed the two highest peaks at 2 = 26.8°and 36.59°w ith a crystal size of 18.27 nm. Then, the ZnO -Ci obtained the two highest peaks at 2 = 26.71°a nd 36.49°w ith a crystal size of 17.31 nm. The Zn0 -C2 produced the two highest peaks at 2 = 26.68°and 36.47°w ith a crystal size of 14.75 nm. diffraction peaks, all samples of ZnO / Graphite had similar data with JCPDS No. 36 -1451 , which showed that ZnO crystals had the first three highest peaks, indicating the diffraction planes of (100), (002), and (101). This peak pattern suggested that the observed ZnO phase had hexagonal Wurtzite [22]. Meanwhile, the results of the graphite analysis had the same data as JCPDS data No. 00 -012 -0212, which showed that the crystal system was also hexagonal. Furthermore, sample characterization was conducted using SEM to determine the surface morphology of ZnO / Graphite, and the results are shown in Figure 4 . Figure 4 shows the SEM images of ZnO / Graphite and pure graphite. Graphite morphology is in the form of coarse grains with an average diameter of 35.38 pm (Figure 4 (a)). The ZnO morphology in Figure 4 (b) has a fine structure, even though mainly ZnO is reported to have a spherical and less homogeneous morphology. Figure 4 (c-d) shows the morphology of the ZnO / Graphite composite that has been grounded with a mortar for an hour. It is evident that the ZnO particles adhered to the graphite surface and that the produced particles tended to agglomerate into one more significant piece. Based on Figure 4 (b-d), it can be said that the mortar process for ZnO and Graphite samples with their mass ratio (Table 1) still requires a longer time, causing uneven agglomeration and less homogeneous morphology. The more homogeneous the ZnO / Graphite composites, the more ZnO crystal growth can be increased, which is helpful for the chargedischarge process. The size of ZnO nanoparticles also has a different effect because the smaller particle size will also cause an increase in surface area, which is directly proportional to the number of reactions that occur so that the reaction results will be better [ 23 ]. Incorporating ZnO particles serves as a conductive band and network to allow ZnO particles to combine, significantly reducing the transfer resistance between these particles [ 24 ]. This highly affects the intercalation of Li + ions so that it can facilitate the transfer between Li + ions during the battery chargedischarge process.

. Crystal Structure and Morphological Analysis
The elemental composition of the samples was characterized by SEM-EDX, as shown in Table 2. Table 2  demonstrated the presence

. Impedance Test Results and Charge-Discharge
Battery Figure 5 shows the results of the Nyquist plot data tested by Electrochemical Impedance Spectroscopy (EIS).
EIS analysis was performed to investigate the electrodes ' charge transfer resistance and ion diffusion performance [ 25 ]. The data obtained from the EIS test were in the form of Nyquist plots and linear plots of 4 variations of anode samples. Wang et al. [26] said that the Nyquist plot is associated with the charge transfer impedance of the electrode, and the linear plot is associated with the Warburg impedance reflecting the diffusion of solidstate Li + to most of the active materials of the materials used. The Nyquist plot or semicircle radius directly demonstrates the magnitude of the charge transfer resistance [ 27 ]. Based on the EIS results ( Figure 5 and Table 3 ), the graphite plot in Figure 5 (a) had the most significant resistance, whereas the ZnO -Co. 5 plot in Figure 5 (c) had the least resistance. This can be seen from the semicircle that intersects the xaxis. Therefore, the smaller the semicircular plot, the smaller the resistance and the better or greater the conductivity of the battery because the conductivity value is inversely proportional to the resistance value and confirmed by the resistance value obtained from fitting the data through the equivalent circuit using a Nyquist plot (Table 3 ). Regarding the results of the morphological analysis of the four anode samples, all zinc oxide graphite composites had a large surface area due to the homogeneity of the mortared ZnO / Graphite. The specific structure of the large ZnO particles can increase the active ingredient ' s electrochemical reaction area and reduce the zinc electrode ' s resistance and polarization [ 27 ]. Zhang et al. [28] reported that the priority in determining the performance of battery cells also lies in the surface chemistry aspect, which produces good surface contact to ensure the intercalation and deintercalation processes run well during the battery chargedischarge process. Figure 6 displays graphs of the charge and discharge process for the four anode modifications performed with the first three cycles, which constitute the formation phase, to determine the precise lithiumion battery capacity. This is attributed to the fact that the battery capacity depends on the active material type and the electrochemical reaction rate during charging and discharging. The broader surface contact between the active materials will also increase the battery capacity. The amount of lithiumions that may be transported to the anode increases with the electric current generated during discharging [ 29 ].  [ 24 ] added that a full battery of ZnO / Graphite shows a capacity of around 280 -400 mAh / g. and is a superior speed capability. In Figure 6 , all ZnO / Graphite batteries have a specific discharge capacity stabilizing in the second cycle. This is because, during the first chargedischarge process, the lithiumion battery undergoes surface electrochemical interphase (SEI), which is electrolyte decomposition that produces a high irreversible capacity during the first discharge process and starts to stabilize in the second and subsequent cycles. The active material of SEI is derived from the positive electrode material and is responsible for the irreversible capacity, which suffers significant losses in the first cycle of most Liion batteries [ 30 ]. This has been demonstrated by the calculated Columbia efficiency (CE) values in Table 4 . CE is the ratio of discharge capacity value divided by the charge capacity value [ 31 ]. This suggests that ZnO can be considered a material that can improve the charge transfer efficiency and electron transfer rate of the electrodes, which will help enhance stability and good capacity during battery discharge. Figure 7 shows the performance value of the specific discharge capacity rate of the battery (the Crate). The Crate testing was carried out under three chargedischarge cycles, with charge setting at 0.5 C and discharge rate at 0.1, 0.2, 1, and 2C. ZnO -Ci anode and graphite have the most stable specific discharge capacity. As can be observed , the particular capacity value did not This result is in line with the research conducted by Wang et al. [ 33 ], which reported that a stable cycle was achieved after several cycles. According to the EIS results, the ZnO / Graphite anode also showed a lower charge transfer resistance than the commercial graphite electrode, meaning that the ZnO / Graphite anode can produce better electrode reaction kinetic characteristics (charge transfer and polarization). It also could be ascribed to better electron availability and possibly Li + . A better type of crystal is also a fast pathway for mass transport and electron transfer, increasing Li ' s storage capacity [ 34 ].
All variations of ZnO / Graphite anodes had sufficient specific capacity rate stability, which can be concluded that ZnO / Graphite may be considered a continuous anode material for lithiumion batteries. Apart from the high specific capacity and stability of ZnO, the active material ZnO contained in the anode can also act as a continuous anode material because it has a stable structure [ 29 ].
However, some considerations are still required in producing anode material from ZnO / Graphite, such as the homogeneity of the sample. The homogeneity of the samples must be taken into account since the shape of the sample has a significant influence on its chemical properties for the cycle performance of the chargedischarge process for lithiumion batteries.

. Conclusion
ZnO / Graphite composites were prepared using a significantly decrease even when it was configured with different Crates. Figure 7 shows that the higher the Crate, the smaller the specific capacity for discharging the battery. The specific discharge capacity decreases towards the next cycle, and the capacity decrease during the discharging and charging processes due to the volume expansion of the ZnO particles. The results showed that morphological control could overcome problems using ZnO material as an anode of lithiumion batteries, such as volume expansion and battery shutdown in just a few cycles [22], This phenomenon can also be attributed to the enlarged interplanar distance and reduced activation barrier for using Li + with the active materials [ 32 ]. Zhang et al. [28] also added that the chargedischarge capacity for the second cycle, both ZnO and graphene, showed good electrode stability and consistent cycle performance.