Eco-friendly polyvinyl alcohol/cellulose nanofiber–Li⁺ composite separator for high-performance lithium-ion batteries

Abstract

A novel eco-friendly polyvinyl alcohol/cellulose nanofiber–Li⁺ (PVA/CNF–Li) composite separator was prepared for lithium-ion batteries. In this membrane by a non-solvent induced phase separation (NIPS) wet-process, CNF–Li originating from wood pulp was successfully prepared and characterized by FT-IR and TEM. The composite separators showed excellent porosity of over 60%, and better ionic conductivity (∼1.1 mS cm⁻¹) as well as remarkable electrolyte uptake approaching 2.3. CNF–Li combined the excellent properties of both nanofibers and ion-conductive polymers such as CMC–Li. The introduction of CNF–Li in the separator increased the thermal dimensional stability and mechanical performance. Simultaneously, CNF–Li as a constituent in the membrane increased the contents of Li⁺, opening a way for Li⁺ transportation to improve batteries' Li⁺ diffusion efficiency and specific capacity. The battery with a 2 wt% CNF–Li separator retained 93% of the initial reversible capacity after 50 cycles, which was much higher than that of the commercial polypropylene (PP) separator with a value of 80%. The PVA/CNF–Li composite membrane produced via a relatively low cost and eco-friendly method can serve as a potential alternative of commercial PP separators applied in high-performance lithium-ion batteries.

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1. Introduction

Rechargeable power sources in a clean and smart energy environment with reliable electrochemical properties, robust safeties and from sustainable sources are greatly demanded by the tremendous progress of electric vehicles and energy storage systems.^1,2 Among different kinds of energy storage systems, lithium-ion batteries are surely an important trend, specifically in portable electronics.^3–7 As one of the major battery components, a separator acts as a barrier between the two electrodes, preserves the liquid electrolyte and provides microporous channels to allow ions to flow through, which is critical to batteries' safety and performance and should not be underestimated.^8–13

Nowadays, polyolefin polymers such as PE and PP are the most commercially available separators in the consumer market owing to their chemical stability and uniformity in pore size distribution. However, PE and PP separators have inferior thermal stability and are not well compatible with some electrolytes (due to hydrophobicity and low surface energies of the polyolefin), thus restricting their application in high performance batteries.

Despite such popularity, their inferior thermal stability and limited compatible with some electrolytes (due to hydrophobicity and low surface energies of the polyolefin) restricted the application in high performance batteries.^14–19 To overcome the drawbacks of polyolefin-based separators, several approaches have been explored, such as polymer coatings,²⁰ incorporation of inorganic nano-particles,^21–23 electro-spinning nonwoven separators.^24–27 However, separators fabrication usually involve toxic and contaminated solvents²⁸ and based on exhausted fossil oil that is stubbornly at odds with the achieve green, low-carbon and sustainable development, based on that, renewable polymers such as cellulose, PVA as an alternative to PP and PE^28–30 have attracted a lot of attention. Cellulose's application to energy storage systems^25,31,32 has been extensively investigated for its high dielectric constant, good chemical stability, good hydrophilicity, superior thermal stability^33–36 and natural abundance.²⁸ Many efforts have been made to develop high-performance separators from low cost, renewable cellulose.^25,28,37 It is also published that effective ion-conductive polymer like CMC–Li with groups of –COO–Li can increase the contents of freely moving lithium ions in lithium-ion batteries and shorten the diffusion pathway to the cathode particle surface.^49–51 Inspired by this, CNFs prepared by TEMPO-mediated system, almost all of the C6-primary hydroxyl groups exposed on the crystalline cellulose microfibrils are converted to –COO–Na, –COO–Li can get through ion-exchange method. Besides, cellulose nanofiber (CNF) is one of special forms cellulose presented big aspect ratio, high elastic modulus,⁴¹ low density, high crystallinity, small thermal expansion coefficient⁴² etc., attracting great attention in energy storage area as a reinforcing agent^43,44 and substrate.⁴⁵ Mechanically robust and thermal shrinkage of the separator are essential factor to evaluate safety characteristics and battery performance of the lithium-ion battery.^3,46

Several researches have justly reported that CNF's mass production was successfully realized,^47–49 which solved the difficulties of sampling. CNF–Li getting from ion-exchange method may possess both the performances of CNFs and ion-conductive polymer like CMC–Li for same group –COO–Li is gotten through ion-exchange method, which possess both performances of CNF and CMC–Li for same group of –COO–Li. CNF–Li probably could do favor to the mechanical, thermal properties like CNF and simultaneously increase the contents of freely moving lithium ions in lithium-ion batteries and shorten the diffusion pathway to the cathode particle surface similar to CMC–Li.⁵⁰ Polyvinyl alcohol (PVA) with excellent thermal, mechanical, chemical stabilities and good film-forming property has also been used as battery components.^51–53 Herein, a new class of polyvinyl alcohol/cellulose nanofiber–Li⁺ (PVA/CNF–Li) composite lithium-ion battery separator is reported for the first time. Here the critical substrate PVA performs the crucial property, i.e., thermal shutdown ability. The characteristics of the composite separator are evaluated in terms of morphology, structure, thermal shrinkage, electrolyte wettability, ionic conductivity and the electrochemical performances. Another motivation of this work sheds light on an ideal substitute of synthetic polymer derived from fossil oil by renewable polymer as clean energy material owing to its versatile diversity and easy process ability.

2. Experimental

2.1. Materials

Poly(vinyl alcohol) (PVA) was supplied by Shanghai Yuanli Chemical Co. (Shanghai, China) with hydrolysis of 97% and molecular weight of 72 000 g mol⁻¹. Dried softwood breached eucalyptus pulp from South China University of Technology (SCUT), ethanol (100%), TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) (Aladdin Chemistry Co. Ltd), sodium hypochlorite, sodium bromide and lithium hydroxide (Sinopharm Chemical Reagent Beijing Co. Ltd), hydrochloric acid (36–38 wt%) (Sinopharm Chemical Reagent Beijing Co. Ltd). All solvents and chemicals were used as received.

2.2. Preparation for cellulose nanofiber–Li⁺ (CNF–Li⁺)

2.2.1. TEMPO-mediated oxidation. Cellulose fibers (15 g) was suspended in water (3000 mL) containing TEMPO (0.5 g) and sodium bromide (5 g). The oxidation reaction was started by adding 100 g NaClO solution. The pH of the reaction maintained at 10. The oxidized cellulose was then washed, sonicated and centrifuged to obtain cellulose nanofiber–Na (CNF–Na) with solid content of 1% according to the methodology reported by Isogai (Fig. 1).⁵⁴

Fig. 1 (a) The mechanism of preparation CNF–Li and (b–d) the fabrication method of separator.

2.2.2. Preparation for cellulose nanofiber–Li⁺ (CNF–Li⁺). To prepare the cellulose nanofiber (CNF–H) with protonated carboxyl groups, 1 M HCl was added to a 0.1% CNF–Na/water slurry to adjust the pH value to 2, and the slurry was stirred for 12 h. The CNF–H was washed with 0.50 M HCl and then with water, using filtration, until the pH of the CNF–H/water slurry was about 4. A certain amount of a 7 wt% aqueous solution of lithium hydroxide was then added to the aqueous CNF–H/water slurry with a rough estimation of 0.07 w/v% to keep the pH value to 10. The slurry was stirred at room temperature for 12 h or so. Then, the slurry was dialyzed for 5 days until its pH reached 7. Then 2 mg mL⁻¹ CNF–Li slurry/water was sonicated for 15 min at power of 300 W in an ice bath. Cellulose nanofibers dispersion was centrifuged at 10 000 rpm for 10 min to remove the unfibrilated cellulose. The cellulose nanofiber (CNF–Li) dispersion was stored at 4 °C with solid content of 1.64% before use.

Separators were prepared through the non-solvent induced phase separation (NIPS) wet-process method.⁴⁶ PVA powder was dissolved in deionized water at 90 °C using mechanical stirring in microwave oven with power of 200 W for about 25 minutes to make PVA aqueous solution (10 wt%). Before the film building, the solution should be cooled down to room temperature and remove entrapped air bubbles. Homogenous PVA solution was casted onto a glass plate at room temperature, using a casting knife with 150 μm thickness and 2.5 cm s⁻¹ velocity, and immediately immersed in a coagulation bath containing 100% ethanol at room temperature for 36 h. The separator was then removed from ethanol and dried at room temperature for 24 h before further characterization. 1 wt%, 2 wt% and 3 wt% cellulose nanofiber–Li of (CNF–Li)/polyvinyl alcohol (PVA) composite separator and pure PVA separator (later with PVA-1%, PVA-2%, PVA-3% and PVA in the paper) were prepared. Meanwhile, a commercialized PP separator was also investigated for comparison in this study.

3. Results

3.1. Characterization of cellulose nanofiber–Li⁺ (CNF–Li)

In the TEM image of CNF–Li sample (Fig. 2a), nanofibers are long fibers with, diameter of 4–8 nm and length of 800–1000 nm (aspect ratio ≥ 100) and they are homogeneously dispersed owing to TEMPO-mediated oxidation and the uniform distribution of Li⁺ on the surface. The FT-IR spectra of the initial product CNF–Na, the intermediate product CNF–H and the resultant CNF–Li are shown in Fig. 2b. –OH stretching at 3400–3300 cm⁻¹ can be found in all of these spectra and the CNF–H spectrum is considerably different from those of CNF–Na and CNF–Li, which demonstrated that the molecular structure of CNF–H is significantly different from the others. The peaks of CNF–H and CNF–Li spectra are very different at 1720, 1638 and 1590 cm⁻¹, respectively. It can be explained –C=O and –C–O bonds coupled with the same carbon atom are homogenized after the carboxylic acid protons replaced by Li ions. The bond length of homogenized carbon oxygen was between double bond –C=O and single –C–O because stretching vibration of two homogenized carbon oxygen was strong coupled, and there are two absorption bands at 1720 and 1638 cm⁻¹, which confirmed the formation of CNF–H. As an important intermediate product from CNF–Na to CNF–Li, CNF–H can help judge the resulting product CNF–Li. In addition to the characteristic peaks of the common structure of cellulose nanofibers either, there were more obvious difference between two kinds of carboxymethyl cellulose salt at 565 cm⁻¹, therefore CNF–Li was successfully synthesized.

Fig. 2 (a) TEM image of CNF–Li sample and (b) FT-IR spectra of CNF–Na, CNF–H, CNF–Li.

3.2. Morphology and mechanical properties of the composite separator

The surface and cross-section SEM images of the PVA, PVA-1% and PVA-2% separators are shown in Fig. 3a, b, c showed surface SEM images of PVA, PVA-1%, PVA-2% separately; Fig. 3d, e, f demonstrated cross-sectional SEM images of the PVA, PVA-1% and PVA-2% respectively. The surface of separator is highly homogeneous and smooth without obvious defects. Moreover, the separator possessed highly interconnected sponge-like porous structure, which was formed during the solvent/non-solvent exchange process in the coagulation bath.⁵³ Separators thickness was about 25 μm. The pore distribution was tested through mercury intrusion method. Fig. 3g, h, i presented the pore size distribution of the PVA, PVA-1% and PVA-2% respectively. The pore was very uniform and the average pore size were 0.17 μm, 0.32 μm, 0.26 μm respectively. The data of porosity was measured by immersing the separator into n-butanol for 2 h and then calculated according to an equation,^25,55 listed in Table 1. It is observed that the porosity of PVA-based composite separators' was above 60%, much higher than that of the PP separator (about 40%). In the NIPS process for separator fabrication, the PVA based separators with symmetric structure were formed because of the trade between thermodynamic enhancement and kinetic hindrance,^25,55 while the kinetic hindrance was dominated in this process.⁵³ The well defined three-dimensional porous separator based on PVA can be obtained by a simple phase separation process without pore forming additives. As we can see the data in the Table 1, the introduction of CNF–Li had no side-effects to the porosity of obtained separators. It can be speculated that higher porosity will contain more electrolyte and result in more effective ion-conducting during cycling.⁵³

Fig. 3 The SEM images (a–f) and pore size distribution (g–i) of the PVA based separators.

Table 1 Physical properties of separators

Separator	PP	PVA	PVA-1%	PVA-2%	PVA-3%
Thickness (μm)	25.0	23.0	23.0	25.0	25.0
Porosity (%)	41.0	64.7	65.7	66.9	67.1
Tensile strength (MPa)	12.0	13.0	15.0	18.0	6.5

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Lithium ion battery separator should be mechanically robust enough to withstand the high tension after casual collisions and prevent internal short-circuits caused by the rough electrode surface, debris and growth of lithium dendrite.^3,46 The stress–strain curves of separators were depicted in Fig. 4 and Table 1. It was clearly demonstrated that the import of the certain amount of CNF–Li had positive effects on the strength for the PVA based separators. The tensile strength of PVA, PVA-1%, PVA-2% was 13.0 MPa, 15.0 MPa and 18.0 MPa respectively, but that of PVA-3% decreased to 6.5 MPa, probably resulted from the serious agglomeration of CNF–Li under high viscosity because small particles were found by snake eyes on the surface of the wet membrane during processing. Also, the tensile strengths of PVA based separators in this study are a little better than that of commercial PP separator (12.0 MPa).²⁵ Studies^43,44 have found that CNF is good strengthening agent in composite materials. Besides, –COO⁻ on the CNF–Li hindered the aggregation of nanofibers. A large number of hydroxyl groups on both components formed hydrogen bonds in homogeneous solution, also contributing to establishing a well-mixed system. CNF–Li played a positive role in the mechanical performance of the composite separators.

Fig. 4 Stress–strain curve of separators.

3.3. Electrolyte wettability

The wettability of separators was evaluated by liquid electrolyte contact angle as shown in Fig. 5. Wetting test was carried out by dropping a drop of electrolyte onto the surface of each separator and measuring the contact angle as soon as liquid electrolyte touched surface.

Fig. 5 The liquid electrolyte contact angles of separators.

As illustrated in Fig. 5a, b, c, d presented liquid electrolyte contact angles of PP, PVA, PVA-1% and PVA-2% respectively, the contact angle of PVA based separators was only about 8.5 °C compared with 42.5 °C of the PP separator, which was much lower than that of the PP separator. While the amount of the CNF–Li in PVA based separators didn't affect wettability much because their contact angles were nearly the same. To further investigate the electrolyte performance of the separators, the electrolyte uptake was studied according to the electrolyte uptake test.^3,19,53 The electrolyte uptake of PP, PVA, PVA-1%, PVA-2% was 1.17, 2.22, 2.30 and 2.34 respectively. The difference in electrolyte wettability confirmed that the obtained PVA based separators was more electrolyte-philic, which led to better transportation of Li⁺ ions through the separator.¹⁹ The excellent electrolyte wettability of PVA based separator is mainly attributed to the following two reasons. One is the strong affinity of PVA and CNF–Li chains toward electrolyte solvent molecules.^29,53 The other is the capillary force due to the interconnected sponge-like porous structure in PVA based separator. Considering the superior electrolyte wettability, the PVA based separator is expected to endow the lithium-ion batteries with higher ionic conductivity and better discharge performance.¹¹

3.4. Thermal stability

Thermal shrinkage of the separator was another essential factor to evaluate safety characteristics and battery performance of the lithium-ion battery.^3,46 Fig. 6a showed photographic images of separators before and after exposure to 160 °C for 1 h. It can be seen that the PVA based separators exhibits superior thermal stability with almost no dimensional change after the heat treatment, as compared to PP separator showing thermal shrinkage of about 51%. The excellent thermal resistance of PVA material was also reported because its meltdown peak was about 275–280 °C, much higher than the common operating temperature of batteries.²⁶ The thermal shrinkage of the separators was further compared by measuring their dimensional changes subjected to heat treatment every 20 °C from 120 °C to 240 °C for 1.0 h. As shown in Fig. 6b, the difference in the thermal shrinkage between PVA based separator and PP separator becomes more pronounced as the temperature was increased. Since PP's melting temperature was about 200 °C, the PP separator continued to shrink with increasing temperature of dimensional shrinkage ratios of about 81% and 99%, respectively, when the temperature was raised to 160 °C and 200 °C. By comparison, the dimensional change of PVA based separator was very small over a wide range of temperature. For example, the shrinkage ratios of PVA separator are only 1% and 3% when temperature was raised to 160 °C and 200 °C, respectively. Besides, the more CNF–Li added, the smaller shrinkage we got. For example, the shrinkage of PVA-2%, PVA-1% and PVA at 240 °C was 62%, 64%, 70%, respectively. The results might be attributed to the separator materials and the preparation methods. The melting temperature of PVA was about 270 °C and the cellulose nanofibers' good thermal resistance was also reported⁴⁷, both of which contributed to good dimensional stability. Simultaneously, the separator from wet phase separation method possessed better thermal stability compared to that from stretching process. PP separator was more likely to lose dimensional stability than PVA base separator, so it was easier for PP separator to lose dimensional stability than PVA based separator via the NIPS wet-process⁵³ In addition, the introduction of CNF–Li improved the thermal stability of PVA separator to some extent for nanofibers' small thermal expansion coefficient⁴¹ and metal-ion exchange.⁴⁵ Good dimensional stability of CNF–Li/PVA composite membrane could effectively prevent internal electrical short circuit at elevated temperature when the battery is at high charged/discharged rates.

Fig. 6 (a) Photographic images for PP, PVA, PVA-1%, PVA-2% before (a–d) and after (e–h) exposure to 160 °C for 1.0 h, respectively. (b) Thermal shrinkage properties of separators.

3.5. Electrochemical performance of separators

Electrochemical stability of the electrolytes within the operation voltage of a battery system is important for practical battery applications. The electrochemical stability window of the separators was evaluated by linear sweep voltammetry as shown in Fig. 7a. The membranes were sandwiched between one lithium foil sheet and one stainless steel sheet for the measurement. When the voltage was higher than 4.3 V, a considerable increase in current flow was found for the PP separator, while, PVA, PVA-1% and PVA-2% showed higher decomposition voltage of 4.7 V, 5 V, 5.2 V. Besides, a distinct improvement was found with increasing amount of CNF–Li, further proving the inertness of PVA/CNF–Li separator. This result demonstrated the PVA based separator was stable enough to endure the operating voltage of the battery system.

Fig. 7 (a) Linear sweep voltammetry of the separators. (b) Nyquist plots of separators in symmetric lithium coin cells.

Based on the comparisons of electrolyte wettability and thermal stability between PVA based separators and PP separator, the electrochemical impedance spectra of the separators were illustrated as Nyquist plots of separators in symmetric lithium coin cells in Fig. 7b. Impedance spectroscopy was used to characterize the ionic conductance for separators impregnated with the liquid electrolytes and sandwiched between two lithium foils for the measurement. The impedance spectra depicted a smaller semicircle for PVA based separators than PP separator. The semicircle in the impedance spectra usually related to the interfacial resistance of a separator was reported, lower interfacial resistance usually indicated better interfacial compatibility with lithium electrodes^46,56 At the same time, Fig. 7b showed the larger amount of CNF–Li in composite separator led to better interfacial compatibility.

In addition, the stainless steel/separator/stainless steel cell's AC spectra were typically inclined straight line for stainless steel blocking electrodes. The line inclined towards the Z′ axis demonstrated the electrode/electrolyte double layer capacitance behavior.⁴⁷ Thus, the bulk resistance of the separator can be acquired from the high-frequency intercept of the Nyquist plot on the Z′ axis. The bulk resistances of PVA, PVA-1% and PVA-2% separator were about 3.4 Ω, slightly lower than the 4.4 Ω value of PP separator. PVA based separators exhibits better ionic conductivity with 1.1 mS cm⁻¹ compared to 0.55 mS cm⁻¹ for PP separator at room temperature.

Practicality of separators in battery cell was evaluated by using a lithium metal anode and a LiCoO₂ cathode impregnated with the liquid electrolytes of 1 M LiPF₆ in the EC : DEC : EMC = 1 : 1 : 1 solvents charged over a voltage range of 2.8–4.2 V at a constant current density of 0.2C (0.42 mA cm⁻²) and discharged at different current densities ranging from 0.2C (0.42 mA cm⁻²) to 4.0C (8.4 mA cm⁻²).

Cycling stability of the cells with separators at room temperature under 0.2C was presented in Fig. 8a. The cells showed typical charge–discharge curve with initial discharge capacity of ∼145 mA h g⁻¹, and all of the cells demonstrated capacity fading to some extent with the current rate increasing, which were likely caused by the structural instability of LiCoO₂ and side reactions during charge/discharge.^46,57 After 50 cycles, the capacity retention ratio of the PVA based cells for PVA-2%, PVA-1% and PVA were about 93%, 90% and 86%, respectively, higher than that of the PP cell (80%). Typically, PVA based cells with larger content of CNF–Li demonstrated higher discharge C-rate capacities which led to better long-term stability and columbic efficiency in comparison to PP cells.

Fig. 8 (a) Cycling stability of the cells with separators (b) discharge rate performance for the high mass loading LiCoO₂ cathode.

Fig. 8b showed the PVA based cells also demonstrated better performance than PP cells at higher charge/discharge current densities. Capacity drop were observed with increasing C-rate in all of the cells, resulting from the electrical polarization caused by the increase in the serial resistance with increasing discharge C-rate.²⁹ The data demonstrated that the PVA based cells exhibited much better C-rate capability. For example, the PVA based cells kept specific capacity of 91.8, 82.3 and 73.8 mA h g⁻¹ at 4.0C, respectively, corresponding to 63%, 57% and 51% of those at 0.2C; while that of the PP cell was only 56.7 mA h g⁻¹, about 39% of that at 0.2C.

The improved cycle performance of the PVA based separators may be attributed to the following reasons. On one hand, PVA based separators with remarkable electrochemical stability and highly developed microporous structure as well as strong affinity between separator and electrolyte, which could result in more facile ion transport and better interface stability during cycling.^29,53 On the other hand, the incorporation of CNF–Li increased the amount of Li⁺ and opened a new way for Li⁺ transportation simultaneously which led to more facile ion transport and less dendrite formation.^38–40 Moreover, the larger amount of CNF–Li with better mechanical properties led to better resistance to lithium penetration.

Microporous structure, the addition of CNF–Li in PVA based separators were sufficiently well-tuned to afford high ionic conductivity, good electrolyte wettability and better mechanical properties, which consequently contributes to superior discharge performance for LIBs.

4. Conclusion

CNF–Li mannered with big respect ratio was successfully prepared which possessed both admirable properties of cellulose nanofibers and ion-conductive polymer like CMC–Li with groups of –COO–Li. PVA/CNF–Li based membrane for the potential lithium-ion battery separators fabricated by the NIPS wet-process presented excellent porosity, ionic conductivity and remarkable electrolyte wettability. The introduction of CNF–Li improved thermal stability, mechanical performance compared with the commercial PP separator. Besides, CNF–Li as a constituent increased the contents of Li⁺ and opened a way for Li⁺ transportation, improving the diffusion efficiency and specific capacity. The electrochemical characterization further confirmed the lithium-ion battery assembled with the PVA/CNF–Li composite separator exhibited preferable comprehensive performances, such as the cycle ability, C-rate capability and thermal resistance, compared with the PP separator. In summary, this novel and environment-friendly separator can function as a high performance separator, resulting in a good candidate for high performance LIBs.

Acknowledgements

This work was supported by the Municipal science and technology projects of Beijing [grant numbers Z141103004414111].

Notes and references

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