Article pubs.acs.org/cm
High Ion Conducting Nanohybrid Solid Polymer Electrolytes via Single-Ion Conducting Mesoporous Organosilica in Poly(ethylene oxide) Youngdo Kim,† Suk Jin Kwon,† Hye-kyeong Jang,† Byung Mun Jung,† Sang Bok Lee,*,† and U Hyeok Choi*,‡ †
Functional Composites Department, Korea Institute of Materials Science, Changwon 51508, Korea Department of Polymer Engineering, Pukyong National University, Busan 48547, Korea
‡
S Supporting Information *
ABSTRACT: A novel mesoporous silica-based single-ion conductor for lithium-ion batteries was prepared via two-step selective functionalization of designated silica precursors into the inner pore wall of mesoporous silica. 2-[(Trifluoromethanesulfonylimido)-N-4sulfonylphenyl]ethyl (TFSISPE) group was first incorporated as a silica precursor having an anionic weak-binding imide group, and a dense brush of oligo-poly(ethylene glycol) (oligo-PEG) moieties, solvating Li+, was cografted to produce functionalized mesoporous silica (FMSTFSISPE) nanoparticles. FMS-TFSISPE showed a 2D hexagonal nanopore structure and a regular spherical shape with an average diameter of 50 nm. Poly(ethylene oxide) (PEO) was used to form a dispersion of the mesoporous silica nanoparticles into the polymer matrix. This new polymer−mesoporous silica nanohybrid solid electrolyte with the sole mobile Li ions (FMS-TFSISPEPEO) exhibits attractive electrical, mechanical, and electrochemical properties. The ionic conductivity and storage modulus both increase simultaneously upon addition of FMS-TFSISPE nanoparticles. A 30 wt % amount of FMS-TFSISPE nanoparticles leads to the highest ionic conductivity (σDC ∼ 10−3 S/cm at 25 °C) and storage modulus (G′ ∼ 104 Pa at 30 °C) with a high lithiumion transference number (tLi+ ∼ 0.9). Compared to conventional nonporous silica nanoparticles-incorporated PEO matrix (SiO2TFSISPE-PEO), FMS-TFSISPE-PEO exhibits 2 orders of magnitude higher ionic conductivity with lower activation energy, suggesting that the facile transportation of lithium ions is achieved through the continuous weak-binding and solvating nanopore channel of the mesoporous silica retaining a high surface area and pore volume.
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INTRODUCTION Lithium-ion battery (LIB) is defined as a family of rechargeable battery types where an electrolyte enables lithium-ion transport from a negative electrode to a positive electrode during discharge and back when charging.1 Since the pioneering work of Sony Inc. in 1990, the continuous development of design, performance, and application of LIB over recent years has reflected the gradual maturation of its use in daily lives, and it has attracted a broad attention from the commercialization of power tools, hybrid electric vehicles, and portable devices.2 The number of works3−7 and reviews8,9 have been greatly increasing attention on the topic of LIBs, which directly shows the rapid development of current fields and trends. In LIBs, one of the major components is an electrolyte that is required to retain high performance and long working cycle. The ultimate goal of the electrolyte is to provide an ideal ionic conduction over a wide range of temperatures and maintain high chemical stability and compatibility with electrode materials.10 LIB based on liquid electrolytes, which contain organic solvents with lithium salts, ensure that the high performance with respect to ionic conductivity is achieved due to high energy density and excellent rate capability.11,12 © 2017 American Chemical Society
However, the liquid electrolytes promote undesirable Li dendrite formation at the interface between electrolyte and electrode, thereby resulting in low stability and safety issues (such as inflammability, fire, and explosion), and hence limit the use on a large scale.13−17 Therefore, finding and preparing safer and reliable electrolytes is a crucial task to attain improvement in the performance and energy density of LIB. The problems of the nonaqueous liquid electrolytes could be mitigated by replacing them with solid polymer electrolytes (SPEs), which possess good mechanical strength, effectively preventing dendrite penetration and allowing for no leakage of electrolyte, for flame resistance, and for flexible geometry.14 Wright et al. first reported the ionic conductivity of poly(ethylene oxide) (PEO) complexes with alkali metal salts.18 This suggestion resulted in extensive study of the SPEs, which are promising candidates for fabricating lighter, thinner, and safer LIBs.19−28 SPEs in which various lithium salts are dissolved in a solvating aprotic polymer matrix have Received: March 2, 2017 Revised: April 12, 2017 Published: April 14, 2017 4401
DOI: 10.1021/acs.chemmater.7b00879 Chem. Mater. 2017, 29, 4401−4410
Article
Chemistry of Materials
binding with Li+ into the mesoporous silica to develop a singleion conductor. Second, although it has been recognized that ethylene oxide units, having a high donor number of Li ion (solvating Li+) and high chain flexibility (lowering glass transition temperature), play an essential role in the PEObased SPEs,50 the inherent PEO crystallinity significantly disturbs long-range ion transport. The several routes to suppress the formation of crystalline portions have been proposed, e.g., the inclusion of plasticizer51−54 or nanosized inorganic fillers.55−57 Of these ways, we embed the single-ion conducting mesoporous silica nanoparticles cografted with oligo-PEG strands into the PEO matrix, preventing PEO chains from being crystallized, thereby allowing simultaneously boosts of the ionic conductivity and mechanical strength.
attracted considerable research interest and show significantly improved characteristics, such as flexibility, multifunctionality, mechanical property, and thermal/chemical stability in contrast to liquid electrolytes.29,30 However, the SPEs display a relatively low room temperature ionic conductivity (less than 10−5 S/cm), due to a high degree of polymer crystallinity that limits the ion transport, as well as a low Li-ion transference number (tLi+ < 0.5), owing to the fact that the larger anions move faster and contribute more to the ionic conductivity than the small lithium cations.31−35 In order to overcome the low transference number, single-ion conductors that have anions covalently bonded to polymers are adopted to have unity transference number and the absence of anion concentration polarization.36−39 Recently, Colby et al. synthesized polysiloxane single-ion conductors containing two side groups: polar cyclic carbonates and weak-binding tetraphenyl borate anions with lithium counterions.36 Although the introduction of borate anions can lower the Li+ activation energy, their ionic conductivities were only 10−7 S/cm at 25 °C, due to extensive ion aggregation. Armand’s group has reported a single-ion conducting BAB triblock copolymer, polystyrene-b-PEO-bpolystyrene, wherein an anionic imide group (SO2NSO2CF3−) was covalently bonded to the polystyrene blocks. The anionic structure, greatly delocalizing negative charge, afforded weak interaction with Li+.37 Although the block copolymer achieved a high tLi+ value of 0.85 due to the suppression of long-range movement of anion, its ionic conductivity at room temperature was less than 10−6 S/cm. Balsara and his co-workers have focused on the relationship between ionic conductivity and morphology of this class of block copolymers.38,39 They discovered that at room temperature the lithium ions are trapped in the form of ionic clusters, resulting in a very low conductivity (
