Conducting‐Polymer/Iron‐Redox‐ Couple Composite Cathodes for Lithium Secondary Batteries

Abstract

Conducting polymers, such as polyacetylene, polyaniline, and polypyrrole (PPy), have been studied as electrode materials for rechargeable batteries because they are electrochemically active and permit penetration of the electrolyte into the polymer mass. These polymers can be charged and discharged by a redox reaction involving lithium ions or counter anions of the electrolyte.[ /3dn + 1 pair of a first-row transition element. The operating voltage of the lithium-ion rechargeable battery should then reflect the energy level of the redox couple over the capacity range of the attached couple, which is expected to flatten the charge-discharge curves over this capacity range and add this capacity to that of the polymer.As an example of the chemical bonding of a redox couple to the polymer, we have covalently anchored ferrocene groups to the PPy backbone. Previous work on ferrocene tethered to PPy was mostly focused on membrane materials. [3] In this study, [(ferrocenyl) amidopropyl]pyrrole is used in conjunction with pure pyrrole to make the copolymer chains (Fig. 1).To examine the physical binding of redox couples to the polymer, we have incorporated the oxide LiFePO 4 into PPy; this oxide exhibits a flat V OC value of 3.4 V versus Li + /Li 0 and has a theoretical specific capacity of 170 mA h g -1 .[4] Although LiFePO 4 is inexpensive, thermally safe, and environmentally friendly, it is plagued by a low conductivity because of a subtle first-order phase change that distinguishes it from FePO 4 .[4]This phase change is responsible for the desirable flat V OC behavior, but has made it difficult to realize the theoretical capacity, particularly at high charge-discharge rates. Previous work has shown that the conductivity problems can be somewhat alleviated by a partial surface coating of carbon on the LiFePO 4 particles. [5] By physically incorporating fine carbon-coated LiFePO 4 particles into PPy, we have fabricated a cathode that is sufficiently porous to allow good penetration of the electrolyte to the surfaces of the oxide particles; the conducting polymer eliminates the need for carbon-black and polymer binder additives while providing additional capacity. In these proofof-concept experiments, no attempts have been made to optimize the polymer/oxide ratio of the composite cathodes. PPy/ferrocene polymer films with a uniform thickness have been electrodeposited onto a stainless-steel mesh (Fig. 2a). These films show an uneven surface morphology, which is typical of nucleation events, causing uneven current distribution. COMMUNICATION 848

Olivine-type cathode for rechargeable batteries: Role of chelating agents

“…Unusual chargedischarge behavior and the increase in specific capacity upon cycling, was observed for PVP (from 64 to 75 mAh/g) and for mixed chelating agents (from 69 to 100 mAh/g) cathodes upon successive cycling. A similar trend of charge-discharge behavior was also observed in the literature for PPy/LiFePO 4 composite [24] using non-aqueous solvents as an electrolyte. It is reported in the literature that this unique behavior is observed due to the change in volume of trapped polymer layer for an initial oxidation and then the structure is stabilized [24].…”

“…A similar trend of charge-discharge behavior was also observed in the literature for PPy/LiFePO 4 composite [24] using non-aqueous solvents as an electrolyte. It is reported in the literature that this unique behavior is observed due to the change in volume of trapped polymer layer for an initial oxidation and then the structure is stabilized [24]. Also, Sasaki et al [25] reported that the trapped organic between electrode-electrolyte interface increase the lithium ion diffusion rate with better capacity retention.…”

Nanostructured conductive polymers for advanced energy storage

“…44 To overcome this problem, redox-active components are doped into a polymeric matrix or linked to polymer chains where they can act as counterion dopants to enhance the electronic conductivity and function as redox-active sites to enhance the capacity. Goodenough et al 45 covalently anchored ferrocene groups to the PPy backbone, thus increasing the specific capacity and rate capability, as well as lowering the overpotential at high discharge rates (Fig. 7c and d).…”

A chemistry and material perspective on lithium redox flow batteries towards high-density electrical energy storage

“…Park and co-workers proposed to use chemically and/or physically attached ferrocene/ferrocenium redox couple to polypyrrole, which is electrochemically active as Li-ion battery cathode materials, to promote its capacity and alleviate the ohmic/concentration polarization. 114 Astruc and co-workers demonstrated dendritic molecular electrochromic batteries based on redox-robust metallocenes. 115 One of the most appealing characteristics of ferrocene as active material for Li-redox flow battery is the prompt redox kinetics.…”