LETTERS nature materials | VOL 3 | JANUARY 2004 | www.nature.com/naturematerials 29 T he future of lithium metal batteries as a widespread, safe and reliable form of high-energy-density rechargeable battery depends on a significant advancement in the electrolyte material used in these devices. Molecular solvent-based electrolytes have been superceded by polymer electrolytes in some prototype devices 1 , primarily in a drive to overcome leakage and flammability problems, but these often exhibit low ionic conductivity and prohibitively poor lithium-ion transport 2–4 . To overcome this, it is necessary to encourage dissociation of the lithium ion from the anionic polymer backbone, ideally without the introduction of competing, mobile ionic species. Here we demonstrate the effect of zwitterionic compounds, where the cationic and anionic charges are immobilized on the same molecule, as extremely effective lithium ion ‘dissociation enhancers’.The zwitterion produces electrolyte materials with conductivities up to seven times larger than the pure polyelectrolyte gels,a phenomenon that appears to be common to a number of different copolymer and solvent systems. We have previously demonstrated the efficacy of ionic liquids— liquids composed entirely of cations and anions—in enhancing the conductivity of lithium copolymer polyelectrolyte systems 5,6 . However,the mobility of the ionic liquid ions in an electric field can be prohibitive in achieving the high lithium ion mass transport rate required for high-rate operation, for example, in a lithium battery. Hence, zwitterions such as those shown in Fig. 1, which are structurally similar to the successful imidazolium-based ionic liquids, but with covalently bonded cations and anions, were identified as potential dissociating agents, without the associated problems of ion mobility. The zwitterion 1-butylimidazolium-3-(n-butanesulphonate) (Fig. 1) was synthesized by procedures similar to those by Yoshizawa et al. 7 , who reported the use of structurally analogous zwitterionic compounds as dissociation-enhancing solvents for lithium salts such as lithium bis(trifluoromethanesulphonyl)amide. They also reported the effect of addition of a neutral polymer to these systems 8 , observing enhanced conductivities (up to 10 –6 S cm –1 ) and a lithium ion transport number of 0.57. However, the use of lithium salts in these systems means that the problems of competing ion migration remains. Use of a lithium polymer, as reported here, in combination with the zwitterions as a dissociation enhancer, results in a single-ion conducting system, with only the lithium ions contributing to the conductivity of the system, and hence allows the high lithium ion transport number that is required for electrolytes for lithium ion secondary-battery applications. The zwitterion is a white powder at room temperature, of melting point 152 °C, with an inherently low conductivity (<10 –7 S cm –1 at 70 °C). The solid-state structure has been investigated by X-ray crystallography 9 . The applicability of this material as a lithium ion dissociator in a range of lithium polyelectrolyte systems was first examined using a random copolymer of 10 wt% lithium 2-acrylamido-2-methyl-1- propanesulphonic acid (AMPSLi) and 90 wt% N, N′-dimethylacryl amide (DMAA). The copolymer is a transparent solid material, P(AMPSLi-c-DMAA), at room temperature. Clear, flexible polyelectrolyte gels were prepared by mixing this copolymer with propylene carbonate (PC), with or without zwitterion, and stirring at 60 °C for 24 hours. The zwitterion becomes insoluble in this system at concentrations of 10 wt% and above, and so was used at a weight ratio of copolyme/zwitterions/solvent of 1:1:9. Figure 2a shows the startling effect of addition of 9 wt% zwitterion on this system, with the conductivity more than tripling (5.6 × 10 –5 S cm –1 compared with 1.6 × 10 –5 S cm –1 at 30 °C). The applicability of the zwitterion effect to other polyelectrolyte systems was investigated using a lithium methyl acrylate copolymer system, P(MALi-c-DMAA), composed of 10 mol% of lithium methacrylate (MALi) and 90 mol% of N,N′-dimethylacryl amide (DMAA). In this system polyethyleneglycol (PEG) was used as a solvent,and again the polyelectrolyte formed was a clear elastomeric gel at room temperature. As before, addition of 9 wt% zwitterion to this system results in an increase in conductivity (3.87 × 10 –5 compared with 1.98 × 10 –5 S cm –1 at 70 °C),shown in Fig. 2b. Thus, the zwitterion effect is not a unique feature of one system, but is equally applicable to other copolymers and, importantly, to different solvent systems. Figure 1 The zwitterion 1-butylimidazolium-3-(n-butanesulphonate). The zwitterion effect in high-conductivity polyelectrolyte materials CHURAT TIYAPIBOONCHAIYA 1 , JENNIFER M. PRINGLE 1 , JIAZENG SUN 2 , NOLENE BYRNE 2 , PATRICK C. HOWLETT 1 , DOUGLAS R. MACFARLANE* 1 AND MARIA FORSYTH 2 1 School of Chemistry and 2 School of Physics and Materials Engineering, Monash University, Wellington Road, Clayton, Victoria 3800, Australia *e-mail: Douglas.MacFARLANE@sci.monash.edu.au Published online: 21 December 2003; doi:10.1038/nmat1044 SO 3 – N N + ©2004 Nature Publishing Group