Fabrication of cellulose-based polymer electrolyte membrane with green solvent DMSO via N-TIPS technique

Solid polymer electrolyte membrane for Li-ion battery was fabricated via N-TIPS technique. To substitute commercially available polyolefin-based separator with more sustainable source, cellulose-based membrane was employed as polymer matrix. Green solvent dimethylsulfoxide (DMSO) was used to replace conventional solvents. Impedance measurement using electrochemical impedance spectroscopy was performed to determine ionic conductivity of the solid polymer electrolyte membrane. Room temperature ionic conductivity of the membrane in CR2032 battery cell was 1.1×10−4 mS/cm, indicating good performance and feasibility for Li-ion battery application. This result is hundred times higher compared to cellulose-based membrane fabricated via similar route using conventional N-methyl pyrrolidone (NMP) solvent (1.38×10−6 S/cm). Further dielectric properties measurement using temperature variation in an enclosed chamber showed that the polymer electrolyte membrane obeys Arrhenius behaviour.


Introduction
Since it was first commercialized in 1991 by Sony, lithium-ion batteries have emerged as one of the most widely commercialized energy storage systems because they can be used for portable devices, large-scale power sources, and electric vehicles, In general, battery consist of two electrodes, a separator membrane, and electrolyte.The charging process transfers lithium ions from the cathode to the anode, whereas the discharging process when the battery is in use transfers lithium ions from the anode to the cathode [1].
The separator and electrolyte pair plays an important role as it separates the two electrodes preventing short circuits while allowing efficient diffusion of lithium ions between them.Therefore, an ideal separator is composed of thin and porous electrical insulators and ionic conducting materials [2].The separator must also allow fast ion transfer between the electrodes, be easily wetted by liquid electrolytes, and be resistant to electrochemical interactions between the electrodes and electrolytes.The liquid electrolyte used in commercial batteries is lithium hexafluorophosphate (LiPF6) electrolyte salt dissolved in carbonate solvents [3].When heating occurs without oxygen inside battery cells at 1239 (2023) 012003 IOP Publishing doi:10.1088/1755-1315/1239/1/012003 2 >70°C, the liquid electrolyte decomposes and produces hydrocarbon gas.Electrolyte salts have even begun to decompose at temperatures >50°C and produce the toxic gases pentafluorophosphate PF5 and hydrogen fluoride HF.The Celgard© commercial separator begins to melt at 130°C so the electrode material degrades and produces oxygen [4].
The use of liquid electrolytes carries a serious safety risk associated with possible electrolyte leakage which can result in the flammable batteries and explosion.Solid electrolytes are being developed to avoid these undesirable events, in which a separator sheet that separates the two poles also functions as an electrolyte so that electrolytes in liquid form are no longer needed [5].
The Fenton, Wright, and Armand era in the 1970s, which paved the way for polymer electrolyte research by exploring polyethylene oxide (PEO), was continued in the following decade, along with the progress of introduction of electronic goods in everyday life.Secondary batteries have also been developed, and solid polymer electrolytes have begun to be considered as potential candidates for inclusion as lighter, cheaper battery components with performance equal to or superior to that of liquid electrolytes [6][7][8].
The polymer used as a solid polymer electrolyte membrane must have a porous morphological structure, because the pores in the membrane function as pathways for ions to move between chains [9].To perform this effectively, high porosity is required while maintaining good mechanical strength.Various fabrication methods have been explored to fabricate porous membranes, such as phase-separation techniques using non-solvent (NIPS), thermal (TIPS), and evaporation (EIPS) methods.Among these methods, NIPS has the potential to be developed as a continuous porous membrane fabrication method on a large scale [10][11][12].
There is also an urgent need to innovate battery separators based on renewable raw materials, considering that commercial battery separators are currently made from petroleum-based polymers.
Renewable biopolymers have the greatest potential for development into sustainable, non-toxic, and biodegradable battery separators [13].Cellulose is the most abundant biopolymer, followed by chitosan, lignin, alginate and starch.The potential for alginate from seaweed and starch from plantation products (corn, potatoes, cassava, etc.) is limited by the risk of competition with food production, making it difficult to realize large-scale biopolymer production.On the other hand, lignin and chitosan do not have the risk of competing with food production such as alginate and starch, but the production of lignin and chitosan from their raw materials (lignin from plants and chitosan from shrimp, insects and mushrooms) produces low yields.This makes it economically unfeasible to develop lignin and chitosan on a large scale for biopolymer applications [14].
Non-solvent Induced Phase Separation (NIPS) is a commonly employed technique for the fabrication of materials.The utilization of phase separation techniques is of significant importance in the development of microporous membranes.Among the various methods, non-solvent induced phase separation (NIPS) and thermally induced phase separation (TIPS) are particularly noteworthy because of their ability to facilitate pore formation without the need for pore-forming additives.The polymer solution was immersed in a coagulation bath via NIPS technique.The process of inducing phase separation in TIPS involves subjecting the polymer solution to thermal treatment involving both heating and cooling.The amalgamation of NIPS and TIPS (N-TIPS) has been regarded as a prospective approach for regulating the morphology and enhancing the performance, thereby achieving porous membranes with superior mechanical robustness.
The utilization of the phase separation technique enables the generation of membranes with properties.N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), and dioxane are among the most frequently used solvents in casting solutions.Nevertheless, these solvents present similar issues, specifically health, safety, and environmental concerns during their transportation, storage, and operation.Dimethylsulfoxide (DMSO) was used in this study to replace conventional, toxic solvents because it is safer for lithium-ion battery applications, non-toxic, and has a higher boiling point [15][16][17].This would promote the sustainable production of Li-ion battery polymer electrolyte membrane.

Methods
The initial step in the production of separators via N-TIPS technique involves the dissolution of cellulose acetate in DMSO.This process was performed on a hot plate maintained at 40°C until the solution became clear and homogeneous.The solution was permitted to undergo a period of settling to ensure the complete dissipation of gas bubbles.The homogeneous solution was uniformly dispensed onto a glass plate using a pipette and subsequently evaporated.The solution was cast onto a glass plate and then placed on a hotplate set to 70°C to facilitate the partial evaporation of the solvent.A meshcovered lid was then placed over the glass plate.The glass plate coated with the membrane was immersed in a coagulation bath containing distilled water for 20 min at ambient temperature.The membrane formed was carefully removed from the glass plate.A similar experiment was performed using NMP as the solvent.
Subsequently, the membrane was submerged in a liquid electrolyte solution containing 0.67 M LiClO4 for 2 h.The dielectric properties of the polymer electrolyte membrane were examined through the utilization of Electrochemical Impedance Spectroscopy (EIS).The configuration of the CR2032 battery coin cell involved the placement of a stainless steel (SS) plate on either side of the immersed membrane, resulting in an SS/membrane/SS configuration.The battery cell was subjected to measurement using a Metrohm Autolab potentiostat with a frequency range of 0.1 Hz to 1 MHz.To investigate the dielectric properties of the solid polymer electrolyte, a temperature variation experiment was conducted over a range of 30-60°C.

Results and discussion
EIS is a widely employed experimental technique for determining the ionic conductivity of electrochemical system.The results obtained from the EIS measurement are presented as a Cole-Cole plot, where the horizontal axis represents the real impedance (Z') and the vertical axis represents the imaginary impedance (Z") [18].Figure 1  The conductivity σ of a given material can be calculated using the formula σ = t/(Rs×A), where t is the thickness of the polymer electrolyte membrane, Rs is the bulk resistivity, and A is the electrolyte surface contact area.The Cole-Cole plot is characterized by semicircles in the high-to mediumfrequency range and a linear spike in the low-frequency range.The resistance, R, observed at the intermediate frequency is typically associated with the charge transfer process of lithium ions at the interface of the material, specifically between the anode and electrolyte.Conversely, the Warburg region observed at low frequencies is linked to the diffusion coefficient of the lithium ions within the material.The minor intercept is equivalent to the value of the bulk resistivity Rs.As shown in Figure 1, the Rs value of the plot decreases with increasing temperature.A lower resistance would result in more flexible motion of the polymer segments, creating more free volume and, therefore, higher ionic conductivity.The calculation results are listed in Table 1 Figure 2 shows the Arrhenius plots of CA-NMP and CA-DMSO polymer electrolyte complexes.It is noteworthy that there is a positive correlation between temperature and conductivity within the temperature range of 303-333 K.The regression values indicated a high degree of linearity, with values approaching unity, suggesting that all data points were aligned along a straight line.It can be deduced that the conductivity is facilitated thermally, suggesting that upon exposure of the sample to heat, a greater number of ions are generated, which indicates that increasing the temperature aids in the dissociation of more ions.The Arrhenius equation provides information regarding the correlation between temperature and conductivity, as shown below: where σ0 is a pre-exponential constant, Ea is the activation energy and k is the Boltzmann constant.It can be seen that CA-DMSO possesses higher ionic conductivity, as confirmed by the Cole-Cole plot and calculation results in Table 1.It also showed that the activation energy decreased for CA-DMSO complex, which resulted in higher ionic conductivity

Conclusion
A cellulose acetate membrane was fabricated via N-TIPS technique.The greener solvent DMSO was used to replace the conventional NMP solvent, with aquadest as the non-solvent for both experiments.CA-NMP and CA-DMSO membranes were activated using LiClO4 electrolyte salt which was then assembled into a CR2032 battery coin cell in SS/membrane/SS configuration.EIS measurements was performed in 0.1 Hz -1 MHz frequency range, for various temperature (30 -60°C).For both systems, the ionic conductivity increased with increasing temperature, resulting from the higher chain mobility and increased free volume upon heating.It was also found that the temperature dependent ionic conductivity behavior of both CA-NMP and CA-DMSO complexes followed the Arrhenius rule, with the CA-DMSO polymer electrolyte system possessing higher ionic conductivity by an order of magnitude compared to CA-NMP.These results demonstrate the potential application of green solvent DMSO in cellulose-based polymer electrolyte for Li-ion batteries.

Figure 1 .
depicts Cole-Cole plots of the CA-NMP and CA-DMSO solid polymer electrolyte systems.Cole-Cole plots of (a) CA-NMP, and (b) CA-DMSO polymer electrolyte complex

Figure 2 .
Figure 2. Arrhenius plot of CA-NMP and CA-DMSO polymer electrolyte complex

Table 1 .
. Room temperature ionic conductivity of CA-NMP and CA-DMSO polymer electrolyte