Poly(ethylene oxide carbonates) solid polymer electrolytes for lithium batteries
Graphical abstract
Introduction
Solid electrolytes are expected to play an important role in the growing demand of energy consumption. New all-solid battery technologies based on lithium, lithium-sulfur or sodium chemistries are interesting alternatives which contribute to the requirements in this area [[1], [2], [3]]. However, battery charge/discharge time, safety, weight and cost are some of the critical features that an all-solid battery needs to address [4]. The use of polymer electrolytes is emerging due to the improvements in battery safety and flammability versus the current organic-liquid electrolytes. However, some limitations appear using polymer electrolytes [5] and the battery performance is affected by the slow ion movement, low lithium transference number, electrochemical window and the compatibility between the polymer electrolyte and the electrodes. Still, the possibility of tailoring the polymer chemical structure and morphology offers the possibility to overcome the limitations of solid polymer electrolytes (SPEs) [6].
Since the first report about ionic conductivity of salts in poly(ethylene oxide) (PEO) by Wright in 1973 [7], several generations of PEO SPEs for lithium batteries have been developed. However, low ionic conductivity and lithium transference number of PEO at room temperature, results in batteries usually operating at 70 °C. In the last decades, several attempts can be found in the literature in order to improve the PEO based SPEs properties; such as the incorporation of different types of salt [8], chemical functionalization [9], single-ion conducting polyelectrolytes [10,11], block copolymers [10,12], introduction of inorganic nanoparticles or nanofillers [13,14] or cross-linked PEO networks [15].
As an alternative to PEO, different families of polymers can be found as a SPE matrix in the literature [16,17]. Among them, aliphatic polycarbonates are becoming a popular family, due to the possibility of cycling the battery at room temperature [18,19]. Aliphatic polycarbonates' SPEs have shown excellent ionic conductivity values even at room temperature, good electrochemical stability and high lithium transference number. The improvements of the properties come from the plasticization effect of the lithium salt within the polycarbonate matrix which leads to very low glas transition values and high lithium mobility [20]. Moreover, the carbonate group shows a weak coordination with lithium ion as compared to the one of the ether groups which benefits the electrochemical properties [18,21]. Recently, two polycarbonates having 2–3 ethylene oxide links have been synthesized through polycondensation, showing promising properties, an ionic conductivities in the range of 10−5 S cm−1 at room temperature after addition of cellulose nanofibers [23]. In another example, Tominaga et al. reported the copolymerization between CO2 and different epoxies having ethylene oxide links in the final polymer [18]. In this particular case, the polycarbonates having few ethylene oxide units and carbonate showed an ionic conductivity in the range of 10−4 S cm−1 at 60 °C with high LiFSI content. Following this idea, in this work we further explored the combination of the two lithium friendly chemical groups, i.e., ethylene oxide and carbonate. We envision, on the one hand, that the ethylene oxide groups will be able to solvate different lithium salts through the chelating effect and, on the other hand, the lithium will be have the beneficial effect of the carbonate group. Thus, we synthesized a series of eight different poly(ethylene oxide)/carbonate polymers PEO-PC with different content of ethylene oxide and carbonate groups by polycondensation between poly(ethylene oxide) end-capped diol and dimetyl carbonate. In these polymers the amount of ethylene oxide units was varied from 2 to 45 units between each carbonate group. The objective is to find the optimal composition and to investigate the factors such as glass transition temperature, crystallinity and lithium salt content that affect the ionic conductivity of these SPEs. In order to understand the chemical structure and the dynamics of these systems, the SPEs were analyzed by FTIR and 7Li and 19F NMR. The study was complemented by 7Li and 19F diffusion coefficients measurements.
Section snippets
Materials
Dry dimethyl carbonate (99+ %) (DMC) and 4-dimethylaminopyridine (DMAP) (99%) were purchased from Across Organics. The diol diethylene glycol, hexaethylene glycol and poly(ethylene glycol) (Mn 1500 g mol−1) were supplied by Fisher Scientific and the diol triethylene glycol, tetraethylene glycol, poly(ethylene glycol) (Mn 600 g mol−1), poly(ethylene glycol) (Mn 1000 g mol−1), and poly(ethylene glycol) (Mn 2000 g mol−1) by Merck. All diols were dried by azeotropic distillation (60 °C) in toluene
Synthesis and characterization of poly(ethylene oxide carbonates) via melt-polycondensation (PEOX-PC)
Melt polycondensation is a versatile synthetic method for the synthesis of functional aliphatic polycarbonates. The synthesis involves the reaction between a diol molecule and dimethyl carbonate in bulk conditions at high temperature and vacuum. The synthetic route towards poly(ethylene oxide carbonates) polymers is depicted in Scheme 1. It shows the synthesis of eight different polymers, containing between 2 and 45 ethylene oxide units between each carbonate links. This synthetic approach
Conclusions
In this paper, we investigated the effect of the ethylene oxide and carbonate relative content for different poly(ethylene oxide carbonates) polymers synthesized by polycondensation. These PEO2-45-PC were formulated as solid polymer electrolytes (SPE) by adding different amounts of bis(trifluoromethane)sulfonimide lithium salt. The optimum SPEs showing the lowest glass transition temperature was the one based in the poly(ethylene oxide)/carbonate having a high amount (34) of ethylene oxide
Acknowledgements
We are grateful to the financial support of the European Research Council by Starting Grant Innovative Polymers for Energy Storage (iPes) 306250 and the Basque Government through ETORTEK Energigune 2013 and IT 999-16. Leire Meabe thanks Spanish Ministry of Education, Culture and Sport for the predoctoral FPU fellowship received to carry out this work. The authors would like to thank the European Commission for their financial support through the project SUSPOL-EJD 642671 and the Gobierno
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