Chemical and thermal stability of electrospinned polyvinylidene fluoride/graphene composite membranes

The effect of graphene content, spinning flow rate, and distance on the chemical and thermal stability of electrospinned polyvinylidene fluoride (PVDF)/graphene composite membranes was systematically investigated. Long-duration soaking experiments by three typical solution media, diluted hydrochloric acid, dimethyl sulfoxide, and mixed-solution of lithium hexafluorophosphate/methyl ethyl carbonate, were used to evaluate the chemical stability of the selected samples. The relevant results of relative mass change rate all indicate that the chemical stability of PVDF composite materials increases with increasing graphene content as well as flow rate or decreasing distance. Moreover, there is a clear consistency between thermal stability and chemical stability. The results of the thermal shrinkage rate evidence that the thermal stability of the samples could be enhanced by increasing graphene content, flow rate, and decreasing spinning distance.


Introduction
The stability or reliability is one of the fundamental requirements for the sustained and safe application of materials and their components.Taking battery separators as an example, when the temperature of the battery increases due to prolonged use or abnormal conditions, the separator should continue to effectively isolate the positive and negative electrodes and stabilize ion channels [1] .Compared with traditional polypropylene and polyethylene-based membrane materials, polyvinylidene fluoride (PVDF) based materials have significant advantages in many aspects, such as good ion conductivity over a wide temperature range and being able to effectively block positive and negative electrodes.
On the one hand, from the molecular structure of PVDF, because the bond energy of carbonfluorine (-C-F-) is relatively strong, and every two fluorine atoms surround a carbon atom, the carbon atom is not easy to react with other atoms, which means that its chemical properties are generally stable.In addition, PVDF films often exhibit good mechanical properties and can be dissolved in many organic solvents, making it an ideal membrane material.Therefore, in recent years, developers have carried out a lot of research and development work on PVDF membranes [2][3][4][5] .It is worth mentioning that the vinylidene fluoride-hexafluoropropylene copolymer has become one of the preferred materials for polymer lithium-ion battery separators due to its excellent comprehensive performances including good electrical insulation, thermal stability, chemical resistance, and so on [5] .
On the other hand, polymer/inorganic composite membranes are increasingly receiving attention from researchers [6][7][8][9] .By complementing organic and inorganic materials, the safety and charging and discharging performance of batteries can be improved to a certain extent.These organic/inorganic composite membranes not only have the flexibility of organic materials and effective pore closure function but also have the function of low heat transfer rate of inorganic materials and difficulty in expanding the thermal runaway point inside the battery.For the typical PVDF/inorganic particle composite membranes, it is naturally worth exploring how much the addition of these inorganic materials affects the stability of integrated composite materials, compared to pure substances.In light of this, this paper focuses on the chemical stability and thermal stability of the composite materials based on the previous research on the preparation of membrane materials, in order to provide strong data support for the subsequent research on the application of battery separators.

Preparation of composite materials
The detailed preparation procedure of the PVDF/graphene composite materials was reported in the previous investigation [10] .The specific process parameters used for the selected seven samples involved in the current study are listed in Table 1.The samples can be divided into three groups for comparative analysis, based on the orthogonal experiment design method.

Stability characterization
The chemical stability of the received samples was evaluated by soaking experiments in three typical solution media solvents (diluted hydrochloric acid, dimethyl sulfoxide, and mixed-solution of lithium hexafluorophosphate/methyl ethyl carbonate) for different durations.The appearance and mass changes of the samples before and after solution soaking served as the most basic criteria for stability evaluation.The relative mass change rate of each sample was determined to evaluate its chemical stability in the designated solution medium.The thermal stability of the selected samples was first evaluated by the DSC analysis under the N 2 atmosphere (99.9%) with a flow rate of 20 ml/min.The current testing temperature range was set to 298~673 K and the heating rate was 10 K/min.According to the related results, the dimensional thermal stability of the samples was then further characterized by thermal shrinkage rate testing.Three groups of small square samples of the same size were placed in a drying oven respectively at 393 K, 423 K, and 453 K for 0.5 h.After that, the samples were taken out for multiple dimensional measurements and the thermal shrinkage rate was determined by the dimensional change before and after heat treatment.

Results and discussion
Figure 1 shows the relative mass change rate of the seven samples in the mixed solution of lithium hexafluorophosphate/methyl ethyl carbonate.As the soaking time increases, the magnitude of mass change varies in different degrees.During the soaking period, some samples exhibit an increase in mass change, which is likely to be related to the small residual lithium hexafluorophosphate particles, according to the fact the concentration of Li ions in the solution has indeed changed.From the perspective of graphene content, the final absolute value of relative mass change rates from small to large are 1%, 0.5%, and 0.2%, indicating that the chemical stability of the sample increases with the increase of graphene content.In addition, a similar analysis indicates that the chemical stability of PVDF composite materials increases with increasing flow rate or decreasing distance.The relative mass change rate of the selected seven samples soaked in the solution of dimethyl sulfoxide for 24 h is summarized in Table 2. Compared to the mass changes of the sample soaked in the mixed solution of lithium hexafluorophosphate/methyl ethyl carbonate, the larger values of relative mass change rate indicate the chemical stability of the samples in dimethyl sulfoxide is relatively poor.These changes in such composite materials are consistent with pure PVDF substances.However, as a composite material, there is a greater margin to improve chemical stability by adjusting the corresponding process parameters to change the microstructure.The relative mass change rate exhibits varying degrees of change for the samples prepared by different conditions.In particular, the effect of graphene is more obvious than that of spinning distance or flow rate.The relative mass change rates of the selected seven samples soaked in the solution of hydrochloric acid for 60 h and 120 h are summarized in Table 3. From the comparison of these data, the hydrochloric-acid resistance of the PVDF composite material increases to varying degrees after more graphene is added, and with the increase of graphene content, the acid resistance of the sample is also enhanced.The differential thermal analysis curves of different samples are shown in Figure 2. We can see that the DTA curves of these seven different samples all have only one obvious endothermic peak ranging from 413 K to 461 K. Further analysis shows that the peak temperature of the PVDF/graphene composite materials is approximately around 443 K, which is slightly higher than that of the corresponding pure PVDF samples (433±3K).The intuitive results of the thermal shrinkage experiment indicate that all samples undergo significant changes in shape and size at 453 K and the samples become hard and black.But the samples are relatively stable at 423 K as well as 393 K and there is no significant change in the appearance and size visible to the naked eye.The quantitative results on thermal shrinkage rate are summarized in Table 4. Taking samples #1, #2, and #3 as an example, under the same conditions of spinning distance and flow rate, whether it is 393 K or 423 K, the thermal shrinkage rate of the sample decreases with the increase of graphene content.The more the graphene content is, the smaller the thermal shrinkage rate is, which means the better the thermal stability of the sample is.In addition, it can also be seen that the thermal shrinkage rate decreases as the spinning flow rate increases or the spinning distance decreases.

Conclusion
In this paper, the chemical and thermal stability of PVDF/graphene composite membranes prepared by the electrostatic spinning method were characterized.Three typical solution media, diluted hydrochloric acid, dimethyl sulfoxide, and mixed solution of lithium hexafluorophosphate/methyl ethyl carbonate, were used to investigate the chemical stability of the selected seven samples.The effect of graphene content, spinning flow rate, and distance on the samples were quantitatively compared by relative mass change rate.The relevant results all indicate that the chemical stability of PVDF composite materials increases with increasing graphene content as well as flow rate or decreasing distance.Moreover, there is an evident consistency between thermal stability and chemical stability.The results of the thermal shrinkage rate suggest that the thermal stability of the samples could be enhanced by increasing graphene content, flow rate, and decreasing spinning distance.

Figure 1 .
Figure 1.Relative mass change rate of PVDF/graphene composite materials in the mixed-solution of lithium hexafluorophosphate/methyl ethyl carbonate.

Table 1 .
The process parameters of seven PVDF/graphene composite materials.

Table 2 .
Relative mass change rate of the PVDF/graphene composite materials in dimethyl sulfoxide.

Table 3 .
Relative mass change rate of the PVDF/graphene composite materials before and after hydrochloric acid immersion.

Table 4 .
The thermal shrinkage rate of seven PVDF/graphene composite materials.