MCC/TEOS-based PVDF Separator Membranes Characterizations
A PVDF-based membrane with the obtained MCC/TEOS was generated by two steps of membrane fabrication, including spin-coating and phase inversion processes. Under these processes, the MCC/TEOS-based PVDF separator membranes allowed the uniform distribution of these composites which were 0.3 mm thick. TEOS was suggested as an additive to enhance the hydrophilicity of PVDF-based membrane by conjugating at hydroxyl group alongside of linear structure of cellulose chain. The all details of MCC and MCC/TEOS characterizations are presented in the Supporting Information. Consequently, MCC/TEOS-based PVDF separator membranes were primarily confirmed by FTIR spectroscopy, which was compared to the pristine PVDF membrane, as illustrated in Fig. 2. Based on the chemical structure of PVDF, the pristine PVDF membrane exhibited the characteristic peak at 1400, 1175, and 875 cm− 1, corresponding to -CH2-in-planeblending, -CF2 stretching, and skeletal vibration of the C-C bond, respectively, [27]. The peak frequencies at 1070, 760, and 666 cm− 1, were assigned to the vibration of C-F bonding, which was under different mode including stretching, bonding, and wagging [28]. Compared with the spectra of all MCC/TEOS-based PVDF separator membranes, there were no significant change at the main characteristic absorption peak of to the pristine PVDF membrane, which could be implied that the MCC/TEOS-based PVDF separator membranes maintained the properties of PVDF.
The chemical compositions of MCC/TEOS-based PVDF separator membranes were also further confirmed by XPS measurement. Figure 3 reveals the respective wide scan, C 1s core-level, O 1s core-level, and F 1s core-level of MCC/TEOS-based PVDF separator membranes compared with pristine PVDF membrane. Considerably the pristine PVDF membrane (Fig. 3a), the peak of C 1s could be curve-fitted with four Gaussian peaks centered four peak components, with binding energies (BEs) at 290.4 eV for the CF2 species and 295.9 eV for the CH2 species are clearly obtained within all samples [29]. Moreover, in the spectrum of pristine PVDF membrane, the two more peaks centered at 284.6 and 285.8 eV are corresponded to C-H and C-atom in strong electron withdrawing environment (e.g., ester group, amide group or bonded with electron withdrawing atoms such as N or O, signed with COO), respectively [30]. These characteristic peaks might be attributed to the additive components which were added for commercial PVDF [31]. The peak component area ratio of [-CH2-]:[-CF2] at approximately 1:1 corresponded to the chemical structure of PVDF (-(CH2-CF2)n-) [32]. Moreover, the F 1s core electron spectrum exhibited the peak component with BEs at 686.4 eV which was assigned to CF2 of PVDF [33]. In contrast with the pristine PVDF membrane, five Gaussian peaks for CF2, C = O and C-O (adventitious carbon), CH2 and C-C exhibited on MCC/TEOS-based PVDF separator membranes. The notification of a distinctive O 1s signal in the wide scan spectra of 2 wt% and 3 wt% MCC/TEOS-based PVDF separator membranes, as shown in Fig. 3b,c. The spectra could be deconvoluted to three Gaussian peaks centered at 533.53, 532.44, and 531.49 eV for 2 wt% MCC/TEOS-based PVDF separator membrane and at 533.38, 532.37, and 531.50 eV for 3 wt% MCC/TEOS-based PVDF separator membrane, respectively. These characteristic peaks were assigned to the oxygen species associated with the adventitious carbon contamination, C-O, C-OH, and C = O, respectively [30]. These results indicated that the MCC/TEOS was presented on the surface of the MCC/TEOS-based PVDF separator membranes. As consideration, owing to the addition of MCC/TEOS contents, the C-C intensity area of MCC/TEOS-based PVDF separator membranes was decreased approximately at 5–7% while the pristine PVDF membrane presented at 15%. It could be implied that the MCC/TEOS might represent into PVDF structure.
To investigate the morphology and porous structure of the MCC/TEOS-based PVDF separator membranes, FESEM analyses were conducted as shown in Fig. 4. The cross-sectional surface morphology of MCC/TEOS-based PVDF separator membranes exhibited rough fractured surfaces and connected pore formation due to phase separation between the agglomerations of MCC/TEOS-based PVDF composite system during phase separation process. The porous structure was formed as a function of immersion precipitation in the ternary phase diagram. The MCC/TEOS-based PVDF composite solution was cast as a thin film on a glass substrate and subsequently immersed in a non-solvent bath. Precipitation can occur because the good solvent in the matrix solution was exchanged for the non-solvent. The pore formation of MCC/TEOS-based PVDF separator membranes was obtained during non-solvent transfers into the matrix solution and the solvent in matrix solution transferring into the non-solvent system [34]. Compared with pristine PVDF membrane, with an increase in MCC/TEOS contents, owing to the certain boundaries between PVDF and MCC/TEOS, the porosity of MCC/TEOS-based PVDF separator membranes increased while decreasing pore size. In addition, EDS elemental analysis confirmed the presence of oxygen, which was related to the existence of uniformly distributed MCC/TEOS in the PVDF-based membrane.
As presented in Table 1, the porosity of MCC/TEOS-based PVDF separator membranes with different MCC/TEOS contents was compared to the pristine PVDF membrane. The porosities of MCC/TEOS-based PVDF separator membranes with 1, 2, and 3 wt% MCC/TEOS contents were approximately to be 86.2, 90.3, and 92.3%, respectively, which were significantly larger than those of the pristine PVDF membrane (82.5%). The blending of MCC/TEOS and PVDF resulted in dense solution and enhanced the trapping of the solvent, which was preferred for large porosity and high electrolyte uptake (Table 1). Contrary, the porosity of 4 wt% MCC/TEOS-based PVDF membrane was dramatically decreased to 71.8%. Due to the agglomeration of higher concentration of MCC/TEOS might be generated the specific bounds between MCC/TEOS and PVDF, resulting to limitation of membrane performance [35].
Table 1
Physical properties and electrochemical performance of the PVDF membrane with various MCC/TEOS contents
Membrane | Porosity (%) | Electrolyte uptake (%) | Ionic conductivity (mS/cm) |
Pristine PVDF membrane | 82.5 | 302.12 | 0.2534 |
1 wt% MCC/TEOS-based PVDF | 86.2 | 354.84 | 0.2673 |
2 wt% MCC/TEOS-based PVDF | 90.3 | 526.09 | 0.4368 |
3 wt% MCC/TEOS-based PVDF | 92.3 | 593.75 | 0.5144 |
4 wt% MCC/TEOS-based PVDF | 71.8 | 294.71 | 0.1840 |
Wettability and Liquid Electrolyte Uptake
The contact angle was measured to characterize the liquid wettability of the separator membrane. Figure 5 presents the contact angle of the pristine PVDF membrane and the MCC/TEOS-based PVDF separator membranes with 1, 2, 3, and 4 wt% MCC/TEOS contents were 78°, 68°, 66°, 63°, and 60°, respectively. For MCC/TEOS-based PVDF separator membranes, the contact angle decreases with increasing MCC/TEOS contents. These phenomena could be explained by the introduction of MCC-TEOS on the surface of MCC/TEOS-based PVDF separator membranes, which could increase the hydrophilic properties of the membrane and the pore volume and distribution, leading to enhanced liquid wettability of separator membranes. The smaller contact angle confirmed that adding MCC-TEOS was beneficial to improve the wettability of the biomembranes [36].
The assessment of electrolyte uptake capacities included quantitative testing. Effective separator membranes immediately wetted and absorbed significant quantities of liquid electrolyte to promote low cell resistance. As outlined in Table 1, upon saturation, the electrolyte uptake capacities of the pristine PVDF membrane, compared to MCC/TEOS-based PVDF separator membranes with 1, 2, 3, and 4 wt% MCC/TEOS contents, were 302.12%, 354.84%, 526.09%, 593.75%, and 294.71%, respectively. The diminished electrolyte uptake capacities observed in the pristine PVDF membrane and 1 wt% MCC/TEOS-based PVDF separator membrane were attributed to the lower porosity and smaller hydrophilic group on the surface of these separator membranes. Moreover, according to the smallest porosity and the agglomeration of MCC/TEOS contents into PVDF, the 4 wt% MCC/TEOS-based PVDF separator membrane presented the smallest electrolyte uptake capacities which was significantly corresponded to contact angle measurement. On the other hand, the PVDF membranes with 2 and 3 wt% MCC/TEOS contents exhibited significantly high porosity and electrolyte uptake were related with ionic conductivity measurement.
As result from the electrolyte uptake curves (Fig. 6a), 2 wt% and 3 wt% MCC/TEOS-based PVDF separator membranes exhibited significantly higher rates of electrolyte absorption compared to the pristine PVDF membrane. Within a 10-minute, PVDF membranes with 2 wt% and 3 wt% MCC/TEOS separator membranes almost reach their saturation points in terms of electrolyte absorption. Consequently, the total electrolyte absorption rates are 533.75% and 597.71% for the PVDF membranes with 2 wt% and 3 wt% MCC/TEOS contents, respectively, which was considerably higher than that of the pristine PVDF membrane (310.92%). Additionally, in the electrolyte retention tests (Fig. 6b), the 3 wt% MCC/TEOS-based PVDF separator membrane demonstrates the highest electrolyte retention rate of 87.5% after 100 min at room temperature. This value was notably greater than that of the pristine PVDF membrane (60.6%). The reduced electrolyte uptake was attributed to the relatively lower porosity resulting from the obtained structure. In addition, the 3 wt% MCC/TEOS-based PVDF separator membrane effectively captured the lithium electrolyte components, enhancing the electrolyte retention rate. Under the synergistic effect of MCC and TEOS, the 1–3 wt% MCC/TEOS-based PVDF separator membranes exhibited outstanding dimensional stability with electrolyte dropping, while the pristine PVDF membrane and the 4 wt% MCC/TEOS-based PVDF separator membrane exhibited
instability, as can be seen in Fig. 6c. These results could be implied that the 3 wt% MCC/TEOS-based PVDF separator membrane exhibited outstanding affinity for electrolytes, which could provide excellent electrochemical performance for the assembled cells.
Ionic Conductivity
Ion conductivity serves as a crucial parameter for assessing the appropriateness of a separator in lithium-ion batteries (LIBs), exerting a significant influence on battery performance. Figure 7 shows the point where the Nyquist curve intersects with the Z′ axis revealing the bulk resistance of the separator. The bulk resistance of the pristine PVDF membrane, in comparison to those with 1, 2, 3, and 4 wt% MCC/TEOS contents were 184 Ω, 191 Ω, 72 Ω, 67 Ω, and 218 Ω, respectively. In the instance of the 3 wt% MCC/TEOS-based PVDF separator membrane exhibited significantly lower bulk resistances, measuring about 67 Ω. Considering the separator thickness and bulk resistance, the ionic conductivity value of the 3 wt% MCC/TEOS-based PVDF separator membrane was determined to be 0.5144 mS/cm. The heightened ionic conductivity observed in this separator could be attributed to its rich porous structure and effective electrolyte uptake, facilitating pore channels for lithium-ion transportation between the electrodes. Conversely, 4 wt% MCC/TEOS-based PVDF separator membrane exhibited the largest bulk resistances reaching approximately 218 Ω. Consequently, the resultant calculated ionic conductivity was significantly low, measuring about 0.1840 mS/cm due to an excessively high amount of MCC/TEOS filler. This high filler content posed an issue by potentially blocking the pores within the structure, thereby impeding the efficient transport of Li ions. This exceptionally low ionic conductivity rendered the separator unsuitable for utilizing in LIBs.
Thermal and Dimensional Stability Evaluation
Figure 8a presents the thermal decomposition of cellulose and PVDF based composite. It exhibited in a similar composition. Based on the guidance of temperature elevation, it can be classified into 3 temperature ranges. From room temperature to 300°C, 3–5% of weight loss was observed. It was typically involved to evaporator of humidity onto surface It can be suggested that composite was easily to be observed by water molecule, in contrast, the pristine PVDF membrane could be decomposing at 450°C, the weight loss of MCC/TEOS-based PVDF separator membrane could be distinctly divided into two stages. In the first stage was the loss amount of MCC/TEOS. The reduction of amount was different depending on the amount of MCC/TEOS. In the second was the loss of amount of PVDF. The initial decomposition temperature of PVDF-MCC/TEOS membrane was 325°C indicating the decomposition of MCC/TEOS, the final decomposition temperature was 480°C. These could be confirmed the exist of MCC in the composite separator membranes [37].
DSC was employed to investigate the thermal stability of the pristine PVDF membrane and its composite MCC/TEOS-based PVDF separator membranes, as shown in Fig. 8b. The pristine PVDF membrane exhibited endothermic peaks centered at 157.73°C, corresponding to the melting temperature of PVDF. In addition, the 1, 2, 3, and 4 wt% MCC/TEOS-based PVDF separator membranes exhibited endothermic peaks centered at 157.69°C, 157.04°C, 157.23°C, and 157.10°C, respectively. These results could be implied that the addition of MCC/TEOS might not affect to melting point temperature of PVDF, suggesting PVDF composite membrane with MCC/TEOS was suitable for LIBs due to melting temperature higher than PE commercial separator membrane at 135°C [38].
Moreover, the assessment of thermal shrinkage involved subjecting the membranes to a range of temperatures (100 to 150°C) for 30 minutes in an oven (Fig. 8c). The pristine PVDF membrane exhibited notable shrinkage beyond 140°C, potentially resulting in physical contact between the cathode and the anode at elevated temperatures. As shown in Fig. 8d, the pristine PVDF membrane displayed significant deformation due to shrinkage resulting from its lower shutdown temperature. In contrast, the 1, 2, 3, and 4 wt% MCC/TEOS-based PVDF separator membranes exhibited negligible dimensional changes owing to the heightened thermal stability of the pristine PVDF membrane. This stability, maintained even after thermal treatment at 150°C, can be attributed to the effectiveness of MCC/TEOS as a nucleation agent. The presence of MCC/TEOS significantly enhances thermal stability and interaction within the pristine PVDF membrane. Consequently, the 1, 2, 3, and 4 wt% MCC/TEOS-based PVDF separator membranes demonstrated superior thermal and dimensional stability, making them well-suited for use as separator membranes compared to their pristine PVDF membrane.
Electrical Properties of Separator Membranes
According to above mention results, the PVDF membranes with 2 wt% and 3 wt% MCC/TEOS contents exhibited superior ion conductivity due to their higher porosity, better electrolyte uptake, and greater hydrophilicity. Subsequently, the electrochemical performance of pristine PVDF membrane compared to PVDF membrane with 2 wt% and 3 wt% MCC/TEOS contents were demonstrated (Fig. 9). The initial charge/discharge curves of the MCMB-anode/NCM-cathode with different separators membranes at 0.2C are plotted in Fig. 9a. All the cells using the PVDF membrane with 2 wt% and 3 wt% MCC/TEOS contents exhibited clearly steady charge/discharge curves which were 88 mAh/g and 98 mAh/g, respectively, while the pristine PVDF membrane presented the specific capacity only 43 mAh/g. The significant improvement in the specific capacity of PVDF composite membrane with 3 wt% MCC/TEOS content was attributed to the synergetic impact on the lager electrolyte affinity of hydrophilic structures and the connected porosity that could facilitate the immigration of liquid electrolyte and Li+ transportations. It could be suggested that the addition of the optimized MCC/TEOS could promote the pathway for Li+ immigration and reduced the charge transfer resistance, resulting in improvement of battery performance. Considering the high concentration of 4 wt% MCC/TEOS-based PVDF separator membrane, as shown in Fig. 9b, because of its higher content of MCC/TEOS, which restricted the electrochemical performance of LIB, the cell showed an immediate voltage drop, probably from an internal short circuit. It could be implied that the excess fillers might block inside the pore, obstructing the electrolyte penetration.
Figure 9c presents the discharge capacities of the batteries with the pristine PVDF membrane compared to the PVDF membrane with 2 wt% and 3 wt% MCC/TEOS content at different current densities ranging from 0.2C to 2C. Typically, the discharge capacities of the batteries significantly decline the electrode resistance and provide great electrochemical reaction kinetics, resulting in reduced discharge potential drop during high-rate cycling. The result shows that through the addition of MCC/TEOS, the MCC/TEOS-based PVDF separator membranes exhibited higher discharge capacities than those of PVDF membrane. At a fast rate of 2C, the specific rate capacity of the battery using 3 wt% MCC/TEOS-based PVDF separator membrane was as high as 56mAh/g along with a stable retention time. It could be suggested that the addition of MCC/TEOS could make the MCC/TEOS-based PVDF separator membranes had better electrical performance than those without MCC/TEOS weight contents.