Synthesis and Characterization of Network Type Single Ion Conductors Xiao-Guang Sun, Craig L. Reeder, and John B. Kerr* Lawrence Berkeley National Laboratory, MS 62-203, One Cyclotron Road, Berkeley, California 94720 Received November 10, 2003; Revised Manuscript Received January 9, 2004 ABSTRACT: New single ion conductors were synthesized by grafting the allyl group-containing lithium salt, lithium bis(allylmalonato)borate (LiBAMB), onto allyl group-containing comb-branch polyacrylate or polymethacrylate ethers by means of hydrosilylation. The highest ambient temperature conductivity of 3.5 × 10 -7 S cm -1 was obtained for a polyacrylate ether-based single ion conductor containing eight EO units in the side chain and five EO units in the cross-linking side chain, to which the anion was fixed with a salt concentration of EO/Li ) 20. For polyacrylate ether-based single ion conductors, an increase of chain length in both side chains and cross-linking anion chains favors an increase of ionic conductivity. The addition of 50 wt % EC/DMC (1/1, wt/wt) increased the ionic conductivity by more than 2 orders of magnitude due to both the increase in ionic mobility from the liquid phase and the increase in the concentration of free ions from the high dielectric constant of the solvent. The preliminary Li/Li cycling profiles of dry polyacrylate- and polymethacrylate ether-based single ion conductors are encouraging as almost no concentration polarization or relaxation was observed. The observed increase in cell potential with cycling is apparently due to an increase in the interfacial impedance associated with the SEI layer, and the cell failure is accompanied by the decomposition of the ester bond of the polyacrylate backbone. 1. Introduction Conventional solid polymer electrolytes are formed by the dissolution of salts in a suitable polymer host, and thus both cation and anion are mobile. 1 However, since lithium batteries only involve the intercalation/deinter- calation and plating/stripping of the lithium cations, practical cells using these conventional binary salt electrolytes will suffer from problems caused by con- centration gradients of the salt and cell polarization, which eventually results in battery failure. Therefore, in practical applications, polymer electrolytes with high Li + transference numbers are highly preferred. So far, there are two approaches of increasing Li + ion transfer- ence number in polymer electrolytes. One is to incor- porate electronically deficient moieties into the polymer chain, which can act as an anion-trapping site to limit the free movement of anions. 2,3 In this system, a Li + transference number as high as 0.7 has been realized. Another approach is to chemically attach the anions to the polymer backbones, so that Li + transference number of one, a single ion conductor, can be realized. 4-12 Most of the single ion conductors have been synthesized by fixing either alkyl sulfonate 4,5 or carboxylate 6,7 to the polymer backbone. However, the ion dissociation in polyether media is very limited, and as a result, the ambient temperature conductivities are very low, usu- ally in the range 10 -8 -10 -7 S cm -1 . To enhance the ionic dissociation and improve room temperature ionic con- ductivity, polyelectrolytes with different anions have been synthesized, 8-12 and the resulting conductivities have increased to the range 10 -7 -10 -5 S cm -1 . Among them, higher ionic conductivities are usually found for oligomers, low molecular weight polymers with poor mechanical properties, so they cannot be used for commercial devices where thin films are preferred. The synthesis of single ion conductors with both high ambi- ent conductivity and high mechanical strength presents a challenge. In this paper we report the synthesis of a new lithium salt, lithium bis(allylmalonato)borate (LiBAMB), and its incorporation into polymers to form single ion conduc- tors. Synthesis of this new salt is based on two consid- erations. One is that the new salt (LiBAMB) is similar to the recently studied lithium bis(oxalato)borate (Li- BOB) 13 in which the lithium cation is well dissociated from the anion. It is expected that the new salt should be well dissociated in the polymer media so that higher conductivities can be obtained. The other consideration is that LiBAMB has allyl groups on both sides, and after attachment to the polymer through tetramethyldisilox- ane by means of hydrosilylation, it not only acts as a lithium source but also acts as a cross-linking site to enhance the mechanical properties of the resulting single ion conductors. So far, examples of papers using lithium salts both as the lithium source and the cross- linker are rare. 8,10 In this paper the synthesis, charac- terization, and Li/Li cell performance of these new network single ion conductors are reported. 2. Experimental Section 2.1. Materials. Allyloxyethanol, 2-[2-chloroethoxy]ethanol, 3,4-dihydro-2H-pyran, acrylic acid, methacrylic acid, tetrabu- tylamonium hydrogen sulfate, triethylene glycol monomethyl ether, polyethylene glycol monomethyl ether acrylate (M n ) 454, n ) 8), polyethylene glycol monomethyl ether methacryl- ate (Mn ) 475, n ) 8), hydrochloric acid, diethyl malonate, trimethyl borate, allyl bromide, trichloromethylsilane, 1,2- dichloroethane, 1,1,3,3-tetramethyldisiloxane, sodium metal, lithium ribbon, toluene, benzene, anhydrous methanol, anhy- drous ethanol, anhydrous MgSO 4, NaOH pellet, and KOH pellet were all obtained from Sigma-Aldrich and were used directly without further purification. Tetrahydrafuran (B&J, distilled in glass) was obtained from VWR and was refluxed over CaH 2 for several days before use. AIBN was recrystallized from methanol twice. The platinum-divinyltetramethyldisi- loxane complex in vinylsilicon was obtained from Gelest, Inc. 2.2. Synthesis of Monomers (Scheme 1). 2-[2-(2-Chloro- ethoxy)ethoxy]tetrahydropyran was synthesized from the ad- dition of 2-(2-chloroethoxy)ethanol to 3,4-dihydro-2H-pyran with hydrochloric acid as catalyst. The crude addition product was dissolved in ether and neutralized with dilute NaOH * Corresponding author: e-mail jbkerr@lbl.gov; Tel 1-510-486- 6279; Fax 1-510-486-4995. 2219 Macromolecules 2004, 37, 2219-2227 10.1021/ma035690g CCC: $27.50 © 2004 American Chemical Society Published on Web 02/26/2004