Self-assembled Janus-like multi-ionic lithium salts form nano-structured solid polymer electrolytes with high ionic conductivity and Li + ion transference number† Parameswara Rao Chinnam and Stephanie L. Wunder * A solid polymer electrolyte with high ambient temperature conductivity, 4  10 4 S cm 1 , and transference number, t Li + ¼ 0.6, is formed from blends of polyethylene oxide (PEO) and a multi-ionic polyoctahedral silsesquioxane lithium salt, POSS-phenyl 7 (BF 3 Li) 3 , with Janus-like properties. A two-phase morphology is proposed in which the hydrophobic phenyl groups cluster and crystallize, and the three –BF 3  form an anionic pocket, with the Li + ions solvated by the PEO phase. The high ionic conductivity results from interfacial migration of Li + ions loosely bonded to three –BF 3  anions and the ether oxygens of PEO. Physical crosslinks formed between PEO/Li + chains and the POSS clusters account for the solid structure of the amorphous PEO matrix. The solid polymer electrolyte has an electrochemical stability window of 4.6 V and excellent interfacial stability with lithium metal. Introduction There is strong demand for the development of safe, high capacity electrical energy storage devices such as lithium/ lithium ion batteries for use in electric vehicles and for storage of energy generated by wind, solar and other uctuating sour- ces. This has motivated the development of solid polymer electrolytes (SPEs) that are compatible with lithium metal, and thus could utilize its high specic capacity (3860 mA h g 1 ). Solid polymer electrolytes, of which the most investigated has been polyethylene oxide (PEO), are exible compared with inorganic solid electrolytes, and do not suffer from safety issues such as leakage, shorts due to dendrite formation, and explo- sions due to volatile solvents in the liquid electrolytes currently used in lithium ion batteries; further, they have longer cycle life due to the slower migration of degradation products to reactive centers in the electrodes in solid compared with liquid elec- trolytes. SPEs are critical components on the anode side in cathode ow batteries. 1,2 However, SPEs have lower ambient temperature ionic conductivities, s,(s < 10 5 S cm 1 ) than either liquid or gel electrolytes. 3 Single ion conductors (SICs), in which the anion is immobile, have even lower RT ionic conductivities (<10 6 S cm 1 ), but have lithium ion transference numbers, t Li + , the fraction of the charge carried by Li + , that approach 1, so that in principle all of the conductivity, although low, originates from the migration of the electroactive lithium species and minimizes polarization effects. 4,5 By contrast, lithium ion transference numbers for PEO electrolytes with mono-ionic lithium salts (LiX), where X is the anion, are typi- cally t Li + ¼ 0.2–0.3. 68 Previous attempts to improve conductivity (s), interfacial and transport properties of PEO have included the addition of plasticizers, 9 which yield materials with poor mechanical properties, and nanoparticle llers such ceramic ZrO 2 , SiO 2 , 10 Al 2 O 3 , 11,12 chitin 8 and polyphosphazine, 13 for which compre- hensive evaluation showed minimal improvement in conduc- tivity. 14 Conduction in PEO based electrolytes occurs predominantly in the amorphous phase, 15 but amorphous PEO, even with added salt or llers, is a (very) viscous liquid. Thus, preparation of SPEs/SICs from PEO has consisted of cross- linking polyethylene glycol, 16 or engineering a two phase morphology in which there is both a structural and a conductive phase, 17–23 either through block copolymers, or polymers with pendant oligomeric polyethylene glycols. Here we propose an alternative approach to engineering solid polymer electrolytes with good mechanical stability as well as high ionic conductivities, which is based on dissolution of a polyoctahedral silsesquioxane (POSS) multi-ionic salt, POSS- phenyl 7 (BF 3 Li) 3 (Fig. 1), with a high density of anion sites and Janus-like properties, in a polar matrix. Bi-functional, Janus St¨ ober SiO 2 , in which one side of the 150 nm nanoparticles were hydrophobically modied, have previously been investigated in “soggy sand” electrolytes (inorganic particles wet by liquid electrolyte). 24 The synthesis and characterization of POSS-phe- nyl 7 (BF 3 Li) 3 has been previously described. 25 The three Si–O– BF 3  groups form an anionic “pocket” on one side and the phenyl groups form a hydrophobic domain on the opposite side of the POSS-phenyl 7 (BF 3 Li) 3 . The motivation for the new approach came from previous studies of POSS-phenyl 7 (BF 3 Li) 3 Department of Chemistry, Temple University, Philadelphia, PA 19122, USA. E-mail: slwunder@temple.edu † Electronic supplementary information (ESI): EXAFS spectra, VTF and Arrhenius plots. See DOI 10.1039/c2ta00085g Cite this: J. Mater. Chem. A, 2013, 1, 1731 Received 18th July 2012 Accepted 16th November 2012 DOI: 10.1039/c2ta00085g www.rsc.org/MaterialsA This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1, 1731–1739 | 1731 Journal of Materials Chemistry A PAPER Published on 19 November 2012. Downloaded by Temple University on 11/11/2014 14:46:22. View Article Online View Journal | View Issue