Characteristic of Nanofiber PVA-Graphene Oxide (GO) as Lithium Battery Separator

Batteries have many uses, so a lot of research on batteries has been developed. The part of the battery that has not been studied much is the separator, which has a crucial role as one of the battery components. The separator is the main component in the lithium-ion battery, which functions to prevent short circuits, transport free ions, and isolate electricity. The separator must have adequate porosity, high conductivity, and good thermal stability. The purpose of this research is to analyze the characteristics of the nanofiber membrane, which will be applied as a separator in lithium batteries. The material that can meet the characteristics of the battery separator is PVA-GO nanofiber. Graphene oxide was synthesized using Hummer’s method, while PVA-GO nanofiber was synthesized by electrospinning. The characterization of the separator includes conductivity, impedance, and porosity tests. The GO variations given to PVA were 0.1, 0.2, 0.3 and 0.4 gr. The resulting fiber diameter ranges from 162-194 nm, with the smallest fiber diameter being 0.2 gr GO. Nanofiber with characteristics as a membrane for separators is PVA-GO 0.4 gram, with an electrical conductivity value of 5.91×10−4 S/cm and a porosity of 42%.


Introduction
Lithium batteries are devices that are in great demand, as devices that are easy to develop and are currently developing rapidly, apart from the characteristics of batteries that have high energy density and output power and have been widely used in small portable equipment, electric vehicles, medical, and microelectronics equipment [1].Lithium batteries have several components that significantly affect the battery's performance, including the electrodes (anode and cathode), electrolyte solution, and separator.Much research has been done on the anode and cathode of a battery, but not much has been done on the battery separator.The separator is an essential component of the battery.The separator is a component that separates the positive and negative electrodes of a battery, which is used to prevent short circuits while at the same time inhibiting free ion transport and isolating electricity [2].
The working principle of the lithium battery is divided into two parts: charging and discharging.Lithium ions flow from the anode to the cathode when the battery is in use, and electrons move from the cathode to the anode.Lithium ions flow from the cathode to the anode when the battery is charging, and electrons move from the anode to the cathode.As the lithium ions move from one electrode to the other, there is a constant flow of electrons.This flow provides energy to keep electronic devices running/functioning.This cycle can be repeated hundreds of times.Therefore, this type of lithium-ion battery can be recharged [3].The separator component is crucial to the battery, so a high-performance separator is needed.The separator's performance can be seen from its infinite electronic resistance and high ionic conductivity [5].In addition, the separator material must be chemically and electrochemically [6] for both electrodes and electrolytes to achieve stability over a long period [7].The electrical conductivity that qualifies as a separator ranges from 10 -7 -10 -3 S/cm [8].The electrical conductivity of the separator material is related to the membrane structure of the separator, namely porosity.Proper porosity in the separator will optimize sufficient electrolyte storage to achieve high ionic transfer [5].If the porosity is too high, the separator will be brittle and affect battery safety.Still, if the porosity is too small, the electrolyte that fills the separator is inadequate, resulting in reduced electrical conductivity and battery performance.Porosity for an ideal separator ranges from 40% -50% [9].The membrane used for the battery separator can be in the form of tiny fibers or nanofibers.Nanofiber has a porous structure with a large surface area to volume ratio, flexible surface, excellent electrolyte wettability which causes large electrolyte absorption, high electrical conductivity which accelerates ionic transport and superior mechanical properties so that nanofiber is suitable for use as a battery separator [10,11].Electrosipinning method, is a method that can be used as an alternative method in the manufacture of nanofiber.Electrospinning is a new and effective technological method in manufacturing polymer nanofibers [12,13].
Electrospinning is an attractive technique because of its cost-effectiveness and ability to produce continuous nanofibers with small diameters and large surface areas [14].In addition, electrospinning produces nanofibers with good mechanical properties, homogeneity, and high porosity ≥90% [15].
The membrane separator that has been developed is made from polymers with the types of polymers used are polyethylene (PE) and polypropylene (PP).However, the thermal characteristics of PE and PP are not good enough to limit the use and safety of batteries [2].Poor thermal stability will cause thermal shrinkage when heated abnormally [2].So a separator with good electrochemical and thermal stability is needed.PVA (Polyvinyl Alcohol) is a polymers with characteristics that match the characteristics of the separator membrane.PVA is a water-soluble polymer with excellent wettability, biodegradability, non-toxicity and mechanical properties [16,17].It is also known that the melting point of PVA reaches 200℃, indicating that PVA has good chemical and thermal stability [16,18], so PVA has great opportunities when used as a battery separator material that works at high temperatures.However, PVA is a non-conductive material, so nanofiller is needed to produce thermally and electrically conductive polymer composites.To turn PVA into a conductive material, one of the materials that can be a PVA filler is an allotrope of carbon, including graphite, graphene or graphene oxide.The characteristics of carbon allotropes have different properties and structures.The graphite structure is weakly bonded graphene layers, while graphene is a covalently bonded carbon atom.Graphene oxide (GO) almost resembles graphene, but there are defects due to the insertion of oxides in the form of oxygen or hydrogen into carbon bonds [19].Graphene oxide also has many hydrophilic groups on its surface [20], has a high surface area and good electrical conductivity [17].In addition, GO has the uniqueness of excellent dimensional, chemical, thermal stability and low permeability, low cost, surface functional and minimum thickness [21] Graphene Oxide can be obtained from graphite oxidation with Hummer's method and through Graphite Oxide peeling.The hydrogen bonds in GO result in a high proton conductivity when a polymer such as PVA is combined with GO.Functional groups such as carboxylic acids and intermolecular hydrogen bonds provide additional pathways for multiplying protons [5].
Research on PVA-GO nanofiber membranes produced by electrospinning performs excellently as a battery electrolyte.Adding GO to PVA facilitates ion transport resulting in high electrical conductivity and good cycle performance [22].Another study by Basha et al. [23] has made a PVA/PVP/GO electrolyte membrane with the addition of GO which has succeeded in increasing the conductivity up to 6.13 × 10 -4 S/cm.Another study from Ahmad et al. [24] made an electrolyte membrane (PVDF-HFP) with the addition of GO, which increased the mechanical and thermal properties and the conductivity of 4.23 × 10 -4 S/cm.The research that has been carried out is limited to the characteristics of electrical conductivity and is used as an electrolyte or cathode solution in batteries.Still, by looking at the electrical conductivity characteristics of PVA-GO and the character of PVA which has good thermal stability, PVA-GO can be used as a battery separator membrane lithium.

Methods
The material used to synthesize graphene oxide is graphite powder (Sigma Aldrich).The method used in manufacturing graphene oxide is the Hummer's method.Graphite was dissolved in H2SO4 and added with NaNO3, stirred at a temperature below 20˚C in an ice bath for 40 minutes.Then the solution was added with KMnO4, stirred at 95˚C for 60 minutes.300 ml of distilled water was added gradually and stirred until the solution was homogeneous for about 60 minutes.After a homogeneous solution, H2O2 was added and stirred for 30 minutes.The solution was precipitated for twelve hours, and the precipitate was filtered and then dried at 80˚C for 7 hours.After the drying process, the resulting powder is used to manufacture nanofiber by incorporating a solution of graphene oxide powder into a solution of Polyvinyl Alcohol with a percentage of 10%.Polyvinyl Alcohol solution was made from PVA powder dissolved in distilled water and stirred at 100˚C for 2 hours.Graphene oxide solution was prepared in several compositions 0.1; 0.2; 0.3 and 0.4 grams.
PVA-GO solution was synthesized into nanofibers by the electrospinning method, with a voltage of 20 kV, a distance of 15 cm from the spet to the collector drum, a flow rate of 1 ml/hour and a spinning time of 2-3 hours.Nanofiber produced from the spinning process was characterized using SEM -EDX, EIS and thermal stability to identify fiber morphology and size, porosity, electrical conductivity and thermal stability.

Results and Discussion
The resulting PVA and PVA-GO solutions were tested by FTIR, as shown in Figure 3, to identify the functional groups of the two solutions.The functional group formed is a functional group of PVA characterized by a peak at 3285 cm -1 which is a hydroxyl group due to stretching of PVA by H2O.Meanwhile, other functional groups that appear are the C-H stretch and the C-O stretch at wave numbers 2915 and 1088 cm -1 [25].In the PVA-GO solution, apart from the functional groups present in the PVA solution, there is also the C=O stretch functional group at wave number 1651 cm -1 , which is the carboxyl and carboxylic groups of graphene oxide [26].Another characteristic of the separator that must be considered is the porosity of the membrane.The porosity of an ideal battery separator is around 40-50%, where ion transfer can still pass through the membrane.Porosity testing was carried out by immersing PVA and PVA-GO nanofibers in an n-butanol solution.This test is conducted to determine the electrolyte affinity's ability on the membrane [9].The porosity value is obtained from Equation (1).
Wwet is the mass after immersion, Wdry is the mass before immersion, ρb is the density of butanol, which is 0.81 gram/cm 3 , and V is the volume of the membrane.
The results of the porosity test on the membrane obtained values as in Table 2.The addition of graphene oxide will increase the porosity of the membrane.Still, the greater graphene oxide will also increase the fiber diameter size, affecting the pore size.So, it is necessary to optimize the addition of graphene oxide so that the porosity characteristics can meet the porosity standards on the membrane.The greater the porosity, the higher the electrolyte absorption as a battery separator [18].Vice versa, the smaller the porosity of a membrane, the lower the electrolyte absorption as a battery separator.Porosity that is too high will make the separator brittle so it affects battery safety.Still, if the porosity is too small, the electrolyte filled in the separator is inadequate, resulting in reduced ionic conductivity and battery performance [5].So proper porosity is provided for the separator in storing sufficient electrolytes to achieve high ionic transfer [5].The porosity for the ideal separator is 40-50% [9].In Table 2, it can be seen that the porosity of the membrane that meets the criteria is PVA-graphene oxide 0.2 and 0.4 gr.

Membrane
Before heating After heating PVA PVA-graphene oxide-0.2PVA-graphene oxide-0.4The thermal resistance of the separator also greatly affects the battery's performance.The thermal stability test was carried out by providing thermal treatment to the membrane, and it showed that the 0.2 composition of the membrane had a higher thermal resistance at a temperature of 200˚C, had not undergone any dimensional changes and did not experience significant shrinkage.The interaction between the PVA chains and graphene oxide sheets increases the thermal stability of PVA-Graphene oxide caused by the strong intermolecular hydrogen bonds with PVA molecules (hydroxyl groups) and graphene oxide sheets [27].
The resulting membrane was also subjected to electrical characterization as one of the membrane criteria for battery separators.The test performed was Electrochemical Impedance Spectroscopy (EIS), which produced a Nyquist chart.The Nyquist plot in Figure 6 shows the test results on PVA and PVA-GO membranes in several variations.The graph shows that for GO variations of 0.1, 0.3, and 0.4 gr, the resulting semicircle curve is very small compared to the curves of PVA and PVA-0.3GO.Calculating the conductivity value with equation 2 is obtained as in Table 3.
where σ is the ionic conductivity, d is the thickness of the membrane (cm), Rct is the interfacial resistance (kΩ), and A is the surface area of the membrane (cm 2 ) [8].The addition of GO to PVA will increase the number of ions in the PVA-GO membrane.Thus, the electrical conductivity of the membrane is expected to be higher.It can be seen from the table above that the conductivity is higher with the increase in the amount of GO added to the PVA.However, when 0.3 grams of GO is added, there is a very significant decrease in conductivity.This is associated with the morphology of the membrane, which has a smaller porosity and results in transport being hampered.Ions on the separator membrane.The porosity value needs to be considered in making membrane separators.Porosity is crucial in the performance of the separator and the battery.In addition, the tendency of GO aggregation at high concentrations or the dominance of the barrier effect can also increase impedance so that the ionic conductivity decreases [28].Increasing the amount of GO does not always increase conductivity, but an optimal composition will maintain conductivity and stability [29].Based on the ionic conductivity obtained, PVA-GO nanofiber meets the requirements as a separator because the conductivity value is between (10 -7 -10 -3 S/cm) [8].

Conclusion
The PVA-GO membrane can be used as a lithium battery separator, with characteristics that meet the criteria of a membrane separator, including an electrical conductivity of 5.91 × 10 -4 S/cm, porosity ranging from 40-50%, and thermal stability reaching a temperature of 200˚C.

Figure 2 .
Figure 2. Schematic of the manufacture of PVA-GO nanofiber separator.