Tailoring poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) membrane microstructure for lithium-ion battery separator applications

Novel battery separators based on poly(vinylidene fluoride-co-trifluoroethylene-chlorofluoroethylene)- P(VDF-TrFE-CFE)- were produced by different processing techniques (non-solvent and thermally induced phase separation, salt leaching and electrospinning), in order to evaluate their effect on separator morphology, degree of porosity and pore size, electrochemical parameters and battery cycling behavior. It has been demonstrated that the different processing techniques have a significant influence on the morphology and mechanical properties of membranes. The degree of porosity varies between 23 % and 66 %, for membranes obtained by salt leaching and thermally induced phase separation, respectively. The membranes present a high ionic conductivity value ranging between 1.8 mS.cm-1 for the electrospun membrane and 0.20 mS.cm-1 for the membrane processed by thermally induced phase separator. The lithium transference number value for all membranes is above 0.20, the highest value of 0.55 being obtained for samples prepared by salt leaching and thermally induced phase separation. For all membranes, battery capacity values have been obtained at different C-rates with excellent reversibility. P(VDF-TrFE-CFE) samples present an excellent battery performance at 1C-rate after 100 cycles with 74 mAh.g-1 and excellent coulombic efficiency, for membrane processed by the salt leaching technique. This work demonstrates that P(VDF-TrFE-CFE) terpolymer can be used as a porous membrane in lithium-ion battery separator application, the membrane processing technique allowing to tailor its morphology and, consequently, battery performance.

Ternary composites of poly(vinylidene fluoride-co-hexafluoropropylene) with silver nanowires and titanium dioxide nanoparticles as separator membranes for lithium-ion batteries

In the realm of polymer composites, there is growing interest in the use of more than one filler for achieving multifunctional properties. In this work, a composite separator membrane has been developed for lithium-ion battery application, by incorporating conductive silver nanowires (AgNWs) and titanium dioxide (TiO2) nanoparticles into a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer matrix. The composite membranes were manufactured by solvent casting and thermally induced phase separation, with total filler content varying up to 10 wt%. The ternary composites composites present improved mechanical characteristics, ionic conductivity and lithium transfer number compared to the neat polymer matrix. On the other hand, the filler type and content within the composite has little bearing on the morphology, polymer phase, or thermal stability. Once applied as a separator in lithium-ion batteries, the highest discharge capacity value was obtained for the 5 wt% AgNWs/5 wt% TiO2/PVDF-HFP membrane at different C-rates, benefiting from the synergetic effect from both fillers. This work demonstrates that higher battery performance can be achieved for next-generation lithium-ion batteries by using separator membranes based on ternary composites.

A Thermal-Ball-Valve Structure Separator for Highly Safe Lithium-Ion Batteries

The separator located between the positive and negative electrodes not only provides a lithium-ion transmission channel but also prevents short circuits for direct contact of electrodes. The inferior dimension thermostability of commercial polyolefin separators intensifies the thermal runaway of batteries under abuse such as short circuits, overcharge, and so on. a polyvinylidene fluoride/polyether imide (PVDF/PEI) separator with high thermal stability in which the high thermostable PEI microspheres are evenly dispersed in the PVDF film matrix and also located in the micro holes of the PVDF film is developed. They not only function as strong skeleton that enables the rare shrink of the separator at 200 °C avoiding short circuit but also act as ball valve that blocks the lithium ion transmission channel at 150 °C interrupting the further heat aggregation. Thus, the LiNi0.6Co0.2Mn0.2O2/Li batteries exhibit high cycle stability of 96.5% capacity retention after 100 cycles at 0.2C and 80°C. Further, the LiNi0.6Co0.2Mn0.2O2/graphite pouch cells are constructed and deliver good safety performance without smoke release and catching fire after the nail penetration test.

Smart and Multifunctional Materials Based on Electroactive Poly(vinylidene fluoride): Recent Advances and Opportunities in Sensors, Actuators, Energy, Environmental, and Biomedical Applications

From scientific and technological points of view, poly(vinylidene fluoride), PVDF, is one of the most exciting polymers due to its overall physicochemical characteristics. This polymer can crystalize into five crystalline phases and can be processed in the form of films, fibers, membranes, and specific microstructures, being the physical properties controllable over a wide range through appropriate chemical modifications. Moreover, PVDF-based materials are characterized by excellent chemical, mechanical, thermal, and radiation resistance, and for their outstanding electroactive properties, including high dielectric, piezoelectric, pyroelectric, and ferroelectric response, being the best among polymer systems and thus noteworthy for an increasing number of technologies. This review summarizes and critically discusses the latest advances in PVDF and its copolymers, composites, and blends, including their main characteristics and processability, together with their tailorability and implementation in areas including sensors, actuators, energy harvesting and storage devices, environmental membranes, microfluidic, tissue engineering, and antimicrobial applications. The main conclusions, challenges and future trends concerning materials and application areas are also presented.

Composite Membrane Containing Titania Nanofibers for Battery Separators Used in Lithium-Ion Batteries

In order to improve the electrochemical performance of lithium-ion batteries, a new kind of composite membrane made using inorganic nanofibers has been developed via electrospinning and the solvent-nonsolvent exchange process. The resultant membranes present free-standing and flexible properties and have a continuous network structure of inorganic nanofibers within polymer coatings. Results show that polymer-coated inorganic nanofiber membranes have better wettability and thermal stability than those of a commercial membrane separator. The presence of inorganic nanofibers in the polymer matrix enhances the electrochemical properties of battery separators. This results in lower interfacial resistance and higher ionic conductivity, leading to the good discharge capacity and cycling performance of battery cells assembled using polymer-coated inorganic nanofiber membranes. This provides a promising solution via which to improve conventional battery separators for the high performance of lithium-ion batteries.

Silk fibroin and sericin polymer blends for sustainable battery separators

Natural polymers are a promising alternative for reducing the environmental impact of batteries. For this reason, it is still necessary to study their behavior and implement its use in these devices, especially in separator membranes. This work reports on new separator membranes based on silk fibroin (SF) and silk sericin (SS) prepared by salt leaching method. The effect of the different SS relative content on the physiochemical properties of the membranes and on the electrochemical performance of the corresponding batteries with lithium iron phosphate (LFP) as cathodes has been reported. It is observed that the increasing of SS content leads to a decrease of the overall crystallinity of the membranes. All SF/SS membranes presented a well-defined porosity above 75% with a uniform distribution of interconnected micropores. The electrolyte uptake and the ionic conductivity are dependent on the relative SS content. The addition of 10 wt% of SS into SF membranes, induce a high ionic conductivity of 4.09 mS.cm^-1 and high lithium transference number (0.52), due to the improvement of the Li⁺ ions conduction paths within the blended structure. Charge/discharge tests performed in Lithium/C-LFP half-cells reveal a discharge capacity of 85 mAh.g^-1 at 2C after 100 cycles for batteries with a SF/SS separator, containing a 10 wt% of SS, which suggests a stabilizing effect of Sericin on discharge capacity. Further, a 50% and 35% of capacity of retention and capacity fade, respectively, is observed. The presented SF/SS membrane show high electrochemical stability, being suitable for implementation in a next generation of sustainable battery systems. This could allow the SS valorization considering that 150,000 tons of SS are abandoned each year, reducing the contamination of environmental effluents.

The 'use-by date' for lithium-ion battery components

Lithium-ion battery (LIB) manufacturing is based around the slurry tape casting of electrodes followed by the assembly of the dried electrodes into cells with a separator and electrolyte. Many aspects of the manufacturing process can affect the performance of a lithium-ion cell. One of the least understood aspects in academia is the effect of degradation of the materials during the manufacturing processes or the 'shelf-life' of the materials and components. Here, we discuss some of the time limitations and degradation issues observed during the manufacturing and testing of the components from an industrially sourced LiNi_0.6Mn_0.2Co_0.2O₂ (NMC-622)//graphite cell, and the affect that the component storage has, upon both the performance and the properties of the materials and cells. The materials are stored either in a dry room, vacuum oven or in a laboratory environment and the effect of the atmosphere upon the degradation components of the electrodes and electrolyte is characterized by analytical surface techniques and electrochemical analysis. We note that all storage affects the electrochemical performance, even storage in a vacuum oven or dry room. We propose that the electrodes and electrolytes should be used immediately after manufacture; however, we propose alternative methods for storage in case this is not possible. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Recent Development in Separators for High-Temperature Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are promising energy storage devices for integrating renewable resources and high power applications, owing to their high energy density, light weight, high flexibility, slow self-discharge rate, high rate charging capability, and long battery life. LIBs work efficiently at ambient temperatures, however, at high-temperatures, they cause serious issues due to the thermal fluctuation inside batteries during operation. The separator is a key component of batteries and is crucial for the sustainability of LIBs at high-temperatures. The high thermal stability with minimum thermal shrinkage and robust mechanical strength are the prime requirements along with high porosity, ionic conductivity, and electrolyte uptake for highly efficient high-temperature LIBs. This Review deals with the recent studies and developments in separator technologies for high-temperature LIBs with respect to their structural layered formation. The recent progress in monolayer and multilayer separators along with the developed preparation methodologies is discussed in detail. Future challenges and directions toward the advancement in separator technology are also discussed for achieving remarkable performance of separators in a high-temperature environment.

Effects of Island-Coated PVdF-HFP Composite Separator on the Performance of Commercial Lithium-ion Batteries

The widespread industrialization of high-energy density commercial lithium-ion batteries has long been challenged by issues of safety and efficiency stemming from uncontrollable lithium dendritic growths. Here, an island-coated composite separator has been fabricated using a pre-swelling process with water-based dispersions to address the issue of dendrite growth. The pre-swelling of the polymer particle surface balances the contradiction between the high crystallinity and electrolyte compatibility showing high electrolyte wettability and electrolyte uptake ability. Furthermore, the point-to-point surface structure can balance the high interfacial adhesion of electrodes and anti-deformation ability well, which is beneficial for preventing ripple-shaped and pot-shaped deformation, smoothing the solid particle morphology of the electrode and achieving a steady interfacial structure for lithium diffusion in cells. This new strategy constructs a non-continuous novel structure, achieving greatly improved dendrite growth suppressing and cell interface stabilization. This paper has opened up a new method for the development of low cost, simple process and easy industry of the lithium-ion pouch cell with improved quality and efficiency.

Fluorinated Polymers as Smart Materials for Advanced Biomedical Applications

Fluorinated polymers constitute a unique class of materials that exhibit a combination of suitable properties for a wide range of applications, which mainly arise from their outstanding chemical resistance, thermal stability, low friction coefficients and electrical properties. Furthermore, those presenting stimuli-responsive properties have found widespread industrial and commercial applications, based on their ability to change in a controlled fashion one or more of their physicochemical properties, in response to single or multiple external stimuli such as light, temperature, electrical and magnetic fields, pH and/or biological signals. In particular, some fluorinated polymers have been intensively investigated and applied due to their piezoelectric, pyroelectric and ferroelectric properties in biomedical applications including controlled drug delivery systems, tissue engineering, microfluidic and artificial muscle actuators, among others. This review summarizes the main characteristics, microstructures and biomedical applications of electroactive fluorinated polymers.