Polymer wiring of insulating electrode materials: An approach to improve energy density of lithium-ion batteries

The poor electronic conductivity of LiFePO₄ has been one of the major issues impeding it from achieving high power and energy density lithium-ion batteries. In this communication, a novel polymer-wiring concept was proposed to improve the conduction of the insulating electrode material. By using a polymer with tethered “swing” redox active molecules (S) attached on a polymer chain, as the standard redox potential of S matches closely the Fermi level of LiFePO₄, electronic communication between the redox molecule and LiFePO₄ is established. Upon charging, S is oxidized at the current collector to S⁺, which then delivers the charge (holes) to the LiFePO₄ particles by intermolecular hopping assisted by a “swing” – type motion of the shuttle molecule. And Li⁺ is extracted. Upon discharging, the above process is just reversed. Preliminary studies with redox polymer consisting of poly (4-vinylpyridine) and phenoxazine moiety tethered with a C₁₂ alkyl chain have shown promising result with carbon-free LiFePO₄, where effective electron exchange between the shuttle molecule and LiFePO₄ has been observed. In addition, as the redox polymer itself could act as binder, we anticipate that the polymer-wiring concept would provide a viable approach to conducting-additive and binder free electrode for high energy density batteries.