Mixed-conducting properties of annealed polyacrylonitrile activated by n-doping of conjugated domains

Abstract

Critical limiting factors in next generation electrode materials for rechargeable batteries include short lifetimes, poor reaction reversibility, and safety concerns. Many of these challenges are caused by detrimental interactions at the interfaces between electrode materials and the electrolyte. Thermally annealed polyacrylonitrile has recently shown empirical success in mitigating such detrimental interactions when used in conjunction with alloy anode materials, though the mechanisms by which it does so are not well understood. This is a common problem in the battery community: an additive or a coating improves certain battery characteristics, but without a deeper understanding of how or why, design rules to further motivate the design of new chemistries can't be developed. Herein, we systematically investigate the effect of heating parameters on the properties of annealed polyacrylonitrile to identify the structural basis for such beneficial properties. We find that sufficiently long annealing times and control over temperature result in the formation of conjugated imine domains. When sufficiently large, the conjugated domains can be electrochemically reduced in a Li-ion half-cell battery, effectively n-doping the polymeric matrix and allowing it to become a mixed-conductor, with the ability to conduct both the Li-ions and electrons needed for reversible lithiation of an interdispersed alloy active material like antimony. Not only do those relationships inform design principles for annealed polyacrylonitrile containing electrodes, but they also identify new strategies in the development of mixed-conducting materials for use in next generation battery electrodes.