Mesoporous poly(vinylidene fluoride-co-trifluoroethylene) membranes for lithium-ion battery separators
Graphical abstract
Introduction
An increasingly technological society resulting from the growth of electronic miniaturization and the appearance of new gadgets has led to a growing demand for smaller and lighter energy storage systems with improved safety, energy and power characteristics [1,2]. These energy storage systems are not just used for portable consumer devices but are becoming relevant in transportation, such as electric vehicles (EVs), leading to a society highly dependent on energy storage systems [3,4].
One of the most efficient technologies for energy storage are lithium-ion batteries, which are expected to become increasingly relevant for the next-generation rechargeable batteries [[5], [6], [7]].
Lithium-ion batteries present some advantages as power sources, as they are lighter, cheaper, show higher energy density (290 Wh.kg−1), less charge loss, no memory effect, prolonged service-life, and higher number of charge/discharge cycles, when compared to other battery technologies [8,9].
Lithium-ion batteries are fabricated from three key components: anode, cathode and a separator membrane, which is typically soaked by a electrolyte solution (liquid electrolyte where salts are dissolved in aqueous or organic solvents) [10]. The main functions of the separator membrane is to become the medium for ions transfer between the electrodes, prevent contact between the anode and the cathode, regulate cell kinetics and promote safety in the charge and discharge mechanisms [11]. The incorporation of the lithium solution into the separator membrane is usually achieved by uptake, i.e. the immersion of the polymer membrane directly into the lithium solution until the weight remains unchanged [12]. The properties of the separator membrane are dependent on the polymer membrane characteristics, including thickness, permeability, overall porosity, pore size and interconnectivity, wettability, electrolyte absorption and retention, chemical, thermal and mechanical stability [13].
Different processing techniques have been used to obtain porous membranes for battery applications, including template synthesis [14], dry and wet processes [15,16], electrospinning [17], preirradiation grafting [18] and solvent casting techniques with thermally induced phase separation (TIPS) [19,20] or non-solvent induced phase separation [21], among others. The most used host polymer types are poly(ethylene) (PE) [22], poly(propylene) (PP) [23], poly(ethylene oxide (PEO) [24,25], poly(acrylonitrile) (PAN) [24,26], poly(vinylidene fluoride) and its copolymers (PVDF-TrFE and PVDF-co-hexafluoropropene, PVDF-HFP) [[27], [28], [29], [30]]. In order to improve the thermal and mechanical properties of porous polypropylene separators (PP) for lithium-ion batteries, a SiO2 layer has been placed in both sides of the separator, these layers improving the capacity and performance of this separator when compared to pristine PP separator [31]. The effect of temperature on the performance for macroporous separators based on polyethylene and polypropylene were investigated and these separators presented performance degradation when submitted to an aging treatment [32]. Further, electrospun nanofiber separators of organic F-doped poly-m-phenyleneisophthalamide have been proposed for lithium-ion batteries. This separator is characterized by lower electronic conductivity and higher electrochemical stability window when compared to commercial polyethylene membranes [33]. A new multifunctional separator based on poly(ethylenealternate-maleic acid) dilithium salt into a poly(vinylidene fluoride-hexafluoropropylene) copolymer matrix has been developed showing improved battery performance due to Mn chelate ions [34]. PVDF and its copolymers belong to the fluorinated class and are characterized by exceptional properties for battery separator applications such as high polarity, excellent thermal and mechanical properties, wettability by organic solvents, being chemically inert and stable in cathodic environment and controllable porosity in binary and ternary systems [35,36].
The advantages of the co-polymer PVDF-TrFE for battery separator applications with respect to PVDF and other copolymers are that the porous PVDF-TrFE membranes exhibit low degree of crystallinity, high contents of polar phase (β-phase) and the possibility of controlling its porosity in binary systems at room temperature [37].
Microporous membranes based on PVDF-TrFE as battery separators have been prepared by thermal induced phase separation (TIPS) [30] and composite membranes have been developed with lithium salts [29] and fillers such as zeolite (NaY) [38], barium titanate (BaTiO3) [39], carbon nanotube (CNT) [40], montmorillonite (MMT) [41]. It has been reported that the membrane structure of PVDF-TrFE prepared by TIPS strongly depends on the solvent evaporation temperature on the polymer/solvent ratio. The porosity of membranes, ranging from 70 to 80%, determines the electrolyte solution uptake, being larger (up to 600%) for the samples with larger porosity. The electrolyte solution improves the ionic conductivity of the microporous membranes with the ionic conductivity decreasing with increasing degree of porosity of the membranes [28,30]. For PVDF-TrFE composites, the optimized filler contents leading to membranes with better electrochemical performance have been shown to be 4 wt% of MMT, 16 wt% of NaY, 16 wt% of BaTiO3 with 500 nm average size and 0.1 wt% of MWCNT. Further, filler type deeply affects membrane separator performance in lithium-ion batteries [42]. Recently, it has been demonstrated that the best fluoropolymer-based separator membrane with degree of porosity larger than 50% is PVDF-TrFE due to the higher β-phase content that facilitates faster lithium ion migration due to its higher polarity and electromechanical response with respect to other polymer phases [43]. On the other hand, typically obtained pore size obtained in PVDF-TrFE ranges from 2.5 to 4 μm, whereas the ideal pore size is below 1 μm in order to avoid the trend for particle penetration within the separator and the formation of dendrites during excessive loading [44]. In order to improve those issues, the goal of the present work is to prepare porous membranes of PVDF-TrFE with pore size below 1 μm with different degrees of porosity through a simple, reproducible and scalable method. For a given average pore size, the effect of degrees of porosity (ranging from 30% to 70%) on the lithium ion battery cycling performance was evaluated. Based on the experimental results and theoretical simulations, the ideal degree of porosity should be above 50% [45].
The porosity within PVDF-TrFE was obtained by using ZnO nanoparticles with sizes between 90 and 210 nm. Moreover, the ZnO nanoparticles are low-cost, non-toxic and simple to remove from the polymer in acidic solution. Further, the fabrication method of this separator allows a suitable control of the degree of porosity and pore size and can be easily up-scaled for industrial production. Moreover, impressive electrochemical performance of the Li/C-LiFePO4 half-cells, make such mesoporous separator membranes a promising alternative to commercial LIBs separators.
Section snippets
Materials
Poly(vinylidene fluoride-trifluorethylene) (PVDF-TrFE) (70/30) and poly(vinylidene fluoride) (PVDF, Solef 5130) were purchased from Solvay.
Zinc oxide nanoparticles (ZnO, average particle size (APS): 90–210 nm) were acquired from Iolitec. C-LiFePO4 (LFP) and carbon black (Super P-C45) were supplied by Phostech Lithium and Timcal Graphite & Carbon, respectively.
The solvents N,N′-dimethylpropyleneurea (DMPU), N,N-dimethylformamide (DMF), the conventional electrolyte 1 M LiPF6 in ethylene
ZnO removal
ZnO particles were used as sacrificial additives for the preparation of the PVDF-TrFE membranes after considering their advantages: low cost, non-toxicity and easy removal by acidic solution, whereas PVDF-TrFE shows excellent chemical stability against acid solutions [51,52]. After the membranes are formed, the ZnO particles are removed from the PVDF-TrFE membrane to give rise to a porous membrane structure and improve electrolyte uptake, ionic conductivity and electrochemical performance. In
Conclusions
Mesoporous membranes of poly(vinylidene fluoride-co-trifluoroethylene) with controlled porosity were prepared through ZnO particle removal from the polymer matrix. The membranes were prepared with different ZnO initial contents between 10 wt.% and 70 wt.%. The ZnO removal from the PVDF-TrFE membranes is fast and efficient and the ZnO removal originated the porosity of the PVDF-TrFE matrix. The morphology, mechanical properties, uptake value and ionic conductivity value of the membranes depend
Acknowledgments
The authors thank the FCT (Fundação para a Ciência e Tecnologia) for financial support under the framework of Strategic Funding grants UID/FIS/04650/2013, UID/EEA/04436/2013 and UID/QUI/0686/2016; and projects no. POCI-01-0145-FEDER-028157, PTDC/FIS-MAC/28157/2017 and POCI-01-0145-FEDER-028237. The authors also thank the FCT for financial support under grant SFRH/BPD/112547/2015 (C.M.C.). J.N.P. wish to thank the financial support of the project Centro-01-0145-FEDER-000017 - EMaDeS - Energy,
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Equal contribution.