Gel-type polymer separator with higher thermal stability and effective overcharge protection of 4.2 V for secondary lithium-ion batteries

Abstract

Overcharge protection by electroactive polymer composite separators is an alternative solution for the alleviation of safety concerns of rechargeable Li or Li-ion batteries. The use of gel-type electrolyte with less free organic solvent may benefit the safety performance as well as provide enhanced thermostability. Herein, a kind of gel-type polymer electrolyte separator, prepared by a facile electroactive conducting polymer solution dip-coating method, for effective overcharge protection of Li-ion batteries with enhanced thermal stability and electrochemical safety due to the introduction of heat-resistant polyfluorene end-capped with polysilsesquioxane, may be beneficial for solution of the Li-ion battery safety problem.

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Introduction

High-energy and/or high-power-density lithium-ion batteries with reliable electrochemical properties are urgently required for rapidly growing industrial and domestic fields such as smart electronics, electric vehicles, and grid-scale energy storage systems.^1–3 Development of advanced lithium-ion batteries with higher capacity and energy density, however, is temporarily stagnating with thorny issues of performance deterioration and safety problems concerned with complex physicochemical/electrochemical phenomena between major cell components including electrodes, electrolytes, separator membranes etc., in addition to the thermal/chemical instability at the electrode material–liquid electrolyte interface due to unwanted side reactions.^4–6 Among these safety hazards, overcharge has long been considered as a primary concern for it may lead to dangerous accidents such as thermal runaway, fire and explosion. By forming a reversible resistive shunt between the current collectors/electrodes, electroactive polymers may be utilized to prevent a cell from overcharging, which is self-activated by voltage and does not cause interference during normal cell charge and discharge.^7–9 Electroactive polymers with the ability to switch rapidly between conductive and insulating states by oxidation/reduction (e.g., doping/dedoping) as overcharge protection agents in lithium batteries have been recently exploited.¹⁰ Amongst these, polyfluorenes are an important class of photoactive and electroactive materials for processable aromatic polymers with unusual and widely tunable optical or electrical properties, e.g., thermochromism, and conducting base-doped polyelectrolytes as well-known p-type conductors.¹¹ The polyfluorenes are, up to now, the most effective polymers as electroactive separators that offer overcharge protection for high-voltage cathodes operating above 4 V with spinel structures.^12,13

Products which use secondary batteries have become diversified with more complex functions, and there might be the risk of instability when meltdown occurs rapidly due to the poor performance at high temperatures and high discharge rate, since the materials for the separators of commercialized lithium batteries are mainly composed of polyethylene (PE), polypropylene (PP) or PE/PP composites. As an alternative solution to this problem, the fluorinated polymer polyvinylidene fluoride (PVDF) with high mechanical and chemical stability and electrolyte uptake as well as enhanced thermal endurance has been chosen as a promising separator material.^14–19 And replacing the liquid electrolyte currently in use by a gel polymer electrolyte yields some more advantages including high energy density, structural stability, and low volatility, as well as improved cycling performance since there is not much free solvent to decompose at high potential or voltage.²⁰ So the combination of an advantageous polymer electrolyte with polysiloxane-modified electroactive polyfluorene may endow secondary Li-ion batteries with better safety properties based on the enhanced thermal stability and electrolyte trapping ability plus the buffering of the volume change of the polyfluorene as an optimized overcharge protection active material.

Here, by incorporating a reversible silicone-capped electroactive polyfluorene into highly porous PVDF membrane separators to form a polymer electrolyte, we demonstrate that an electroactive interpenetrating polymer network (IPN) composite with higher thermal stability than commercial separators and ultra-low threshold value of electroactive conducting polymers (ECPs) can provide reversible and stable overcharge protection at a higher voltage of 4.2–4.3 V.

Experimental

Materials and methods

Neutral poly[9,9-di-(2-ethylhexyl)fluorenyl-2,7-diyl] end-capped with polysilsesquioxane (PFO-PSQ), purchased from American Dye Source Inc., was used as the ECP close to the cathode. Neutral poly(3-butylthiophene-2,5-diyl) (P3BT), with a regiorandom conformation purchased from J&K Scientific Ltd, was utilized as a protective ECP close to the anode. Those polymers were used as received. LiFePO₄ particles and LiCoO₂ particles (99.8% metals basis, Aladdin Reagent, China) were used directly without further treatment as cathode materials.

Polymer composite separators were prepared by impregnating porous PVDF membranes (pore size 0.45 μm; Shanghai Xinya Co. Ltd) with a 2 wt% solution of PFO-PSQ or P3BT in chloroform. The as-prepared composite membranes were then immersed in and pulled from 1 M LiPF₆ EC/DMC (ethylene carbonate and dimethyl carbonate, v/v = 1 : 1) electrolyte as gel-type separators. Also, the typical commercial Celgard 2500 microporous membrane was modified with the same above-mentioned processes for comparison. All cells were assembled in an argon atmosphere glove box with an oxygen and water content less than 1 ppm. The polymer loading was 0.1–0.2 mg per square centimeter over a geometric area. The polymer-coated electrode was assembled as a CR2032 coin cell with lithium metal as counter and reference electrodes. In addition, samples for determination of the capacity of such electroactive polymers were prepared by dissolving PFO-PSQ in chloroform and casting the solution onto a porous artificial graphite current collector (90% graphite and 10% PVDF binder).

Characterization

The surface morphology was investigated by using scanning electron microscopy (Hitachi S-4800) and EDX spectra recorded with a suitably equipped Bruker QUANTAX 400 spectrometer. Surface chemical components and states of the composite membrane were studied using X-ray photoelectron spectroscopy (XPS) with a Thermo Scientific ESCALAB 250 spectrometer with monochromatic Al Kα radiation (10 mA, 15 kV, 1486.6 eV) under high vacuum (2 × 10⁻⁹ mbar). Thermogravimetric analysis (TGA 1, Mettler Toledo) was performed in nitrogen or air atmosphere at a heating rate of 20 °C min⁻¹. Electrochemical experiments were carried out with a VSP ultimate electrochemical workstation (BioLogic Science Instruments, France) for CV and PEIS processes, and a LANHE CT2001A battery testing system (Wuhan Land Electronic Co., China) for cycling characteristics.

Results and discussion

Scheme 1 illustrates the fabrication process of the gel-type electroactive composite membrane as separator and the assembly into a cell for potential overcharge protection application. The porous PVDF membrane as a flexible matrix was dip-coated using an electroactive polymer solution, i.e., PFO-PSQ or P3BT, to form an IPN structure with an interconnected electronic switch. The detailed preparation process was described in the Experimental section.

Scheme 1 (a) Structural formula of poly[9,9-di-(2-ethylhexyl)fluorenyl-2,7-diyl] end-capped with polysilsesquioxane cage structure and structural formula of poly(3-butylthiophene-2,5-diyl), abbreviated as PFO-PSQ and P3BT, respectively. (b) Schematic of the preparation process for the gel-type electroactive composite separators and the assembly into a cell for overcharge protection application.

The gel polymer PVDF membrane was chosen for supporting the high voltage polymer PFO-PSQ due to the enhanced interfacial adhesion between them and the higher thermal stability as well as the safety based on gel-type batteries. This will allow for a uniform distribution of electroactive polymer on the internal surfaces of the skeleton-like substrate and produce a highly porous composite membrane that promotes better utilization of the electroactive polymer and high ion conductivity in the separator. P3BT, which has an onset oxidation potential of 3.2 V vs. Li/Li⁺, was impregnated into a microporous PVDF membrane and placed next to the anode to prevent lithium dendrite penetration.¹⁰ An overcharge-protected cell, with PFO-PSQ impregnated into a PVDF membrane as the high voltage gel-type composite separator and P3BT into the same PVDF membrane as the low voltage gel-type composite separator, was assembled and tested at room temperature. Fig. 1 shows the SEM and elemental mapping images of the composite membranes consisting of PFO-PSQ/PVDF and Fig. 2 shows those of the counterpart consisting of P3BT/PVDF against lithium metal anode. The polymer deposited from CHCl₃ solution formed a uniform deposit, producing a composite membrane containing 1 wt% of PFO-PSQ or 2 wt% of P3BT. The average pore size of the composite membrane is 0.45 μm, with a thickness of ca. 40 μm. The rough and porous nature of the composite membrane with interconnected channels is beneficial for electrolyte adsorption and wettability, and higher conductivity is easily obtained by soaking the film into standard electrolyte solutions.^21,22 The sulfur and fluorine SEM-EDX mapping of the P3BT/PVDF composite membrane shows the uniform distribution of the related electroactive polymer inside the porous matrix intimately, i.e., the characteristic elements such as C, F, Si and S typically included in the PVDF matrix and the ECPs PFO-PSQ and P3BT. The preparation of composite membranes with varying degrees of nanolayer-level mixing between the electroactive and supporting polymers for gel-type electrolyte is possible through adjustment of the dip-coating conditions.

Fig. 1 SEM and elemental mapping images of the PFO-PSQ/PVDF membrane. (a) SEM image, and (b–f) elemental mapping images of the integrated elements and C, F, O and Si, respectively, in top views; and (g and h–l) those in cross-sectional views of the composite membrane.

Fig. 2 SEM and elemental mapping images of the P3BT/PVDF membrane. (a) SEM image, and (b–d) elemental mapping images of the integrated elements and F and S, respectively, in top views; and (e and f–h) those in cross-sectional views of the composite membrane.

XPS was carried out to further examine the composition and the chemical state of the PFO-PSQ/PVDF electroactive composite membrane, as presented in Fig. 3, and the corresponding P3BT/PVDF composite membrane, as shown in Fig. 4. The XPS survey spectrum of PFO-PSQ/PVDF shows that the binding energies of 286.3, 532.3, 688.3 eV correspond to C 1s, O 1s, F 1s, respectively. The C 1s core-level spectrum consists of three main component peaks, at 291.6 eV attributable to the CF₂ species and at 286.7 eV and 284.8 eV attributable to the CH₂ and C–C species, respectively, in the high-resolution XPS spectrum of C 1s (Fig. 3b).^23,24 However, no peaks corresponding to silicon in PFO-PSQ were detected, because its content is too low, below the detection limit of XPS. The S 2p spectrum of P3BT/PVDF (Fig. 4d) verifies the existence of sulfur in the sample, which indicated the successful loading of P3BT onto PVDF and only one S species on the surface. The binding energies positioned at 163.9 and 164.9 eV are ascribed to S 2p_3/2 and S 2p_1/2, respectively, in the expected area ratio of ca. 2 : 1, which matched well with the literature value.²⁵ Therefore, both EDX and XPS results confirmed the electroactive polymers (PFO-PSQ or P3BT) combined well with the PVDF membrane. Incidentally, a very small peak appeared at 532–533 eV, which may be attributable to the contamination of the polymer prior to loading it into the spectrometer.

Fig. 3 XPS spectra of the PFO-PSQ/PVDF electroactive composite membrane. (a) Survey scan, (b) C 1s, (c) F 1s and (d) O 1s spectra.

Fig. 4 XPS spectra of the P3BT/PVDF electroactive composite membrane. (a) Survey scan, (b) C 1s, (c) F 1s and (d) S 2p spectra.

To investigate the thermal stability of PFO-PSQ/PVDF composite membrane, TGA was conducted, and the results are shown in Fig. 5. The PVDF microporous membranes as separators show an enhanced thermal resistance compared to the traditional PP or PE porous membranes, with the temperatures of onset of decomposition about 160 °C higher in air atmosphere and 60 °C higher in nitrogen atmosphere (as indicated in Fig. S1 in ESI†). Since the electroactive polymer PFO-PSQ was also thermally stable in nitrogen and air atmospheres up to 408 and 388 °C with a weight loss less than 5% (Fig. S2 in ESI†), respectively, its combination with PVDF separator with higher thermal stability than commercial PP separators will endow it with a better thermal resistance for ​safety-critical applications. For the as-prepared PVDF composite membranes, the irreversible decomposition started in nitrogen and air atmospheres at ca. 465 and 430 °C with a weight loss less than 5%, respectively, which is about 100 °C higher than that of commercial Celgard separators, i.e. PP microporous membrane. Significantly, the electroactive composite membrane of PVDF exhibits an enhanced thermal stability.

Fig. 5 TGA curves of the as-prepared electroactive composite membranes by combining PFO-PSQ with PP or PVDF matrices in (a) nitrogen atmosphere and (b) air atmosphere. The temperature of maximum decomposition is 433 °C and 494 °C for the PP-based and PVDF-based electroactive composite membranes in nitrogen atmosphere, and 388 °C and 490 °C in air atmosphere, respectively.

In commercial lithium ion batteries, layered transition metal oxides, such as LiCoO₂, are widely used. Despite a high theoretical capacity (274 mA h g⁻¹), batteries produced with LiCoO₂ cathodes are more reactive and have poor thermal stability which makes them susceptible to thermal runaway in cases of abuse, such as high temperature operation (>130 °C) or overcharging beyond 4.2 V versus Li/Li⁺. At elevated temperatures, the oxygen generated by LiCoO₂ decomposition can then react with the organic electrolyte, which is a safety concern due to the magnitude of such highly exothermic reaction which may spread to adjacent cells or ignite nearby combustible material. And the likely undetected overcharging will irreversibly damage batteries or lead to a battery explosion.^26–28 The measures for suppression of the amount of organic electrolyte, promotion of heat resistance of the separator, and introduction of internal protection systems are of importance and advantageous. So here the gel-type PVDF composite separators with enhanced thermostability and synergetic electroactive polymer are adopted for optimized battery overcharge protection. For example, the discharge capacities of unprotected and protected cells with LiCoO₂ as cathode are shown in Fig. 6. For the unprotected cell, it can be seen that LiCoO₂ as cathode active material quickly lost capacity in a few cycles due to overcharging to 4.5 V if the charging voltage was raised from 4.2 V to 4.5 V (Fig. 6a and b), while the protected cell with the as-prepared functional composite separator maintained its discharge capacity of 125 mA h g⁻¹ for over 50 cycles. Cyclic voltammetry study results of PFO-PSQ cast onto a porous graphite current collector shown in Fig. 6c indicate the electroactive polymer has a single redox couple between 2.0 and 4.5 V vs. Li/Li⁺, with the onset of oxidation occurring at about 4.0 V. The oxidation and reduction peaks are due to the intercalation and deintercalation of PF₆⁻ by the polymer. Because of the strong dependence of electronic conductivity on state of charge, a well-defined reduction peak is obtained only from the thin, porous polymer film.¹⁰ The reversible capacity is about 35 mA h g⁻¹ of polymer when charged to 4.4 V, while it altered slightly when charged between 4.3 and 4.5 V, and 4.4 V is an optimized upper voltage limit which contributes to the quite stable cycling profile (Fig. S3a in ESI†). Fig. 6d shows the charge/discharge behavior of LiCoO₂–Li cell with PFO-PSQ/P3BT-impregnated PVDF separator charged to about twice the theoretical capacity (274 mA h g⁻¹). This cell held a constant potential of 4.2 V, and did not reach the voltage limit of 4.5 V. The discharge capacity of the protected cell was 125 mA h g⁻¹, slightly lower than that of the unprotected cell. This may be attributed to the electroactive polymer self-discharge to the insulating state during the relaxation process, and the more porous separator has greater internal resistance. The steady-state potential at 4.2 V and discharge capacity of 125 mA h g⁻¹ were maintained for the first 50 cycles when the cell was cycled at 0.5C and overcharged by 50% of the initial discharge capacity (Fig. 6e). The protected cell was able to maintain the capacity during long-term overcharging, lasting at least 500 h, i.e. over 80 cycles, although the holding overcharge voltage increased to 5.0 V gradually (Fig. 6f). The electroactive polymer PEO-PSQ as a cathode material for Li-ion batteries also evidenced a good cycling performance with a discharge capacity of 36 mA h g⁻¹ and coulombic efficiency of 97% after 200 cycles (Fig. S3b in ESI†). The introduction of PEO-PSQ raised the overcharge protection voltage from 3.6 V to higher than 4.2 V with much wider application, compared to our related and previously reported polytriphenylamine-based materials (see Fig. S4 in ESI†).^29–31 The results clearly demonstrate that the reversible and efficient overcharge protection for lithium-ion batteries supported by the electroactive polymers was achieved.

Fig. 6 (a) Galvanostatic charge–discharge cycling profiles and (b) specific capacities as a function of the cycle time or number for the unprotected LiCoO₂ half-cell overcharging to 4.5 V compared with normal charging to 4.2 V. (c) Cyclic voltammograms of PFO-PSQ in 1.0 M LiPF₆ in 1 : 1 EC : DMC with a sweep rate of 1 mV s⁻¹, and Li foil as counter and reference electrodes. (d) Galvanostatic charge/discharge features of overcharge-protected LiCoO₂–Li cell with PFO-PSQ/P3BT-impregnated PVDF complex separator charged to twice the theoretical capacity (274 mA h g⁻¹) at 0.5C with 100% overcharge. (e) Specific capacities as a function of the cycle number and (f) the degrading cycling performance (after 80 cycles) of a LiCoO₂ half-cell overcharge-protected by the as-prepared PFO-PSQ/PVDF and P3BT/PVDF gel-type composite synergic separator (rate 0.5C, 50% overcharge).

Conclusions

In summary, reversible electroactive silicone-capped polyfluorene (PFO-PSQ) was uniformly impregnated into a gel polymer PVDF membrane through solution dip-coating successfully, which has better thermal stability, enhanced safety and higher power density for full batteries compared to commercial membranes. Reversible and efficient overcharge protection for lithium-ion batteries at a higher voltage of 4.2–4.3 V was also achieved, preserving the discharge capacity for over 50 cycles. It may provide a general and beneficial reference for the Li-ion battery safety solution.

Acknowledgements

We appreciate the financial support from the Science Foundation of Sichuan Province (2014JY0202, 2016JQ0025), the R&D Foundation of China Academy of Engineering Physics (2014B0302036) and National Natural Science Foundation of China (21401177, 51403193 and 21501160), the “1000plan” from the Chinese Government, the Collaborative Innovation Foundation of Sichuan University (XTCS2014009).

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Footnote

† Electronic supplementary information (ESI) available: TGA and electrochemical characterizations of related materials. See DOI: 10.1039/c6ra11638h

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