Nanofiber-reinforced polymer electrolytes toward room temperature solid-state lithium batteries
Graphical abstract
Introduction
The burgeoning application domains of electrochemical energy storage systems (e.g., portable electronic devices, electric vehicles, and stationary power sources) have incentivized a rapid development of rechargeable solid state batteries (SSBs) due to their higher energy density and better safety compared to the state-of-the-art lithium-ion batteries (LIBs) [[1], [2], [3], [4], [5], [6], [7]]. The importance of SSBs has been highlighted recently by the International Union of Pure and Applied Chemistry (IUPAC) as one of the top 10 emerging chemical innovations which will change our planet [8].
In contrast to conventional liquid electrolytes used in LIBs, solid electrolytes (SEs) possess inherently enhanced safety under abuse conditions (e.g., overcharge, cell crush, water submersion, etc …) due the absence of flammable liquid components, as well as better chemical and electrochemical compatibility with the ‘holy grail’ lithium metal (Li°) anode [9] and other metallic anodes [10], thereby favoring the stable and reliable operation of rechargeable batteries [4,[9], [10], [11]]. Currently, there are three families of SEs investigated as solid ionic conductors for SSBs, enlisting polymer electrolytes (PEs) [[15], [16], [17]], inorganic solid electrolytes (ISEs) [[4], [13], [14]]and composite/hybrid polymer electrolytes (CPEs/HPEs) [12], as shown in Fig. 1. Among these SEs, PEs containing lithium salts dissolved in electron-donating polymer matrices are the promising candidates to replace their liquid counterparts due to their better processability, flexibility and ease in structural design [17,18].
In general, PEs are classified into dry solid polymer electrolytes (SPEs, containing only salt and polymer matrix) and gel polymer electrolytes (GPEs, > 70 wt% liquid plasticizers) [17,19]. Poly(ethylene oxide) (PEO) is one of the most widely used polymer matrices for SPEs due to strong solvating power of ethylene oxide (EO) units, which facilitates the dissociation and dissolution of various lithium salts [20,21]. As the ionic transport occurs mainly in the amorphous phase, low ionic conductivity is obtained in PEO-based electrolytes due to the presence of crystalline phase at temperatures below the melting point of PEO (ca. 65 °C). As a consequence, PEO-based SSBs need to be operated at temperatures above >60 °C, which requires heating accessories for maintaining the temperature of the battery pack, thereby causing decreased overall energy density and efficiency [22,23]. Alternative polymer matrices such as polycarbonates (PC) [24,25] have been capturing attention towards RT operation of polymer-based SSBs; however, these polymers have poor compatibility against Li° electrode and reverse to their cyclic monomers, leading to the loss of mechanical properties upon cycling [26]. On the other hand, higher ionic conductivity and better interfacial contact are achieved with GPEs which are usually plasticized with flammable liquid solvents or safer ionic liquids. Nonetheless, GPEs have poorer mechanical properties and the reactivity of the added plasticizer with electrode materials can be lead intractable to reaction at electrodes and shorter life span [[27], [28], [29], [30], [31]].
Herein, we propose a nanofiber-reinforced polymer electrolyte (NRPE) comprising of poly(vinylidene fluoride) (PVDF) fibers along with high molecular weight but flowable polymer (FP) matrix containing polyether side moieties (so-called Jeffamine®) [32] and sulfonimide salts on the basis of the following considerations: 1) the liquid-like PE presents high mobility of Li+ ions and improved ionic conductivity due to the high amorphicity along with good adhesion properties and improved compatibility against Li° electrode [32]; 2) PVDF nanofibers improve the mechanical properties of PEs, leading to self-standing membranes with good electrochemical performance [[33], [34], [35]], which are essential for the scalable processing of large-format SSBs; and 3) the sulfonimide anions [i.e., bis(trifluoromethanesulfonyl)imide anion (TFSI−) and bis(fluorosulfonyl)imide anion (FSI−)] are chemically stable and structurally flexible, enabling the fast motion of ions and good thermal stability of the electrolytes. As shown in Fig. 1, the typical solvent casting technique using the Li salt/FP solution on top of the PVDF fibers yields self-standing membranes with good ductility (see Table S1 for electrolyte recipes and experimental section in Supplementary Information).
Section snippets
Synthesis of polymer matrix and PVDF nanofibers
Jeffamine M-2070-based flowable polymer matrix was prepared according to the procedures reported in our previous work [32]. The PVDF nanofibers were prepared by electrospinning as described by Howlett et al. [35].
Preparation of polymer electrolytes
For the NRPE electrolytes and PEO reference electrolytes (REs), the polymer matrix was dissolved in acetonitrile (ACN) and then a pre-determined amount of Li salt was added [‒CH2CH2O‒ (EO)/Li = 20]. Different methods were used for the membrane preparation. For the NRPEs, a certain
Results and discussion
Fig. 2 presents the morphological and thermal properties of as-prepared FSI-NRPE and TFSI-NRPE (the electrolyte compositions are summarized in Table S1). As seen in Fig. 2a, both electrolytes are soft and self-standing membranes with good ductility and the mechanical integrity of PVDF fibers is retained after the addition of Jeffamine-based flowable polymer electrolytes (FPEs). The mechanical properties of NRPE electrolytes are preliminary characterized by differential mechanical analysis and
Conclusions
In summary, the as-prepared NRPEs are self-standing, highly conductive (close to 10−4 S cm−1 at 30 °C), and stable against Li° electrode (<800 Ω cm2 at 30 °C). As a result, the corresponding Li° || LiFePO4 cells show decent performances even at RT (~0.7 mAh cm−2 at 30 °C). Further optimization on the electrolyte formation such as the use of SEI-forming additives and novel salts could be beneficial for improving the energy density of NRPE-based SSBs. Therefore, NRPEs combining both rigid PVDF
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by ELKARTEK-2016 from the Basque Government and Warsaw University of Technology Faculty of Chemistry (Poland). H.Z. thanks the Basque Government for the Berrikertu program (1-AFW-2017-2). We thank Fluolyte (Suzhou, China) for the generous supply of LiFSI, 3 M for providing the Al current collector, and Huntsman Corporation for Jeffamine poly(ether amines). We thank Neware (Shenzhen, China) for offering the battery cycler. X.W., P. H. and M. F. acknowledge the financial
References (39)
- et al.
Energy Environ. Sci.
(2014) - et al.
Joule
(2018) - et al.
J. Power Sources
(2018) - et al.
J. Power Sources
(2017) - et al.
Electrochim. Acta
(2011) - et al.
Electrochim. Acta
(2014) Solid State Ion.
(1994)- et al.
Electrochem. Commun.
(2015) - et al.
J. Power Sources
(2018) - et al.
J. Power Sources
(2010)