Dihexyl-Substituted Poly(3,4-Propylenedioxythiophene) as a Dual Ionic and Electronic Conductive Cathode Binder for Lithium-Ion Batteries

Abstract

The polymer binders used in most lithium-ion batteries (LIBs) serve only a structural role, but there are exciting opportunities to increase performance by using polymers with combined electronic and ionic conductivity. To this end, here we examine dihexyl-substituted poly­(3,4-propylenedioxythiophene) (PProDOT-Hx2) as an electrochemically stable π-conjugated polymer that becomes electrically conductive (up to 0.1 S cm–1) upon electrochemical doping in the potential range of 3.2 to 4.5 V (vs Li/Li+). Because this family of polymers is easy to functionalize, can be effectively fabricated into electrodes, and shows mixed electronic and ionic conductivity, PProDOT-Hx2 shows promise for replacing the insulating polyvinylidene fluoride (PVDF) commonly used in commercial LIBs. A combined experimental and theoretical study is presented here to establish the fundamental mixed ionic and electronic conductivity of PProDOT-Hx2. Electrochemical kinetics and electron spin resonance are first used to verify that the polymer can be readily electrochemically doped and is chemically stable in a potential range of interest for most cathode materials. A novel impedance method is then used to directly follow the evolution of both the electronic and ionic conductivity as a function of potential. Both values increase with electrochemical doping and stay high across the potential range of interest. A combination of optical ellipsometry and grazing incidence wide angle X-ray scattering is used to characterize both solvent swelling and structural changes that occur during electrochemical doping. These experimental results are used to calibrate molecular dynamics simulations, which show improved ionic conductivity upon solvent swelling. Simulations further attribute the improved ionic conductivity of PProDOT-Hx2 to its open morphology and the increased solvation is possible because of the oxygen-containing propylenedioxythiophene backbone. Finally, the performance of PProDOT-Hx2 as a conductive binder for the well-known cathode LiNi0.8Co0.15Al0.05O2 relative to PVDF is presented. PProDOT-Hx2-based cells display a fivefold increase in capacity at high rates of discharge compared to PVDF-based electrodes at high rates and also show improved long-term cycling stability. The increased rate capability and cycling stability demonstrate the benefits of using binders such as PProDOT-Hx2, which show good electronic and ionic conductivity, combined with electrochemical stability over the potential range for standard cathode operation.

Li+ and Oxidant Addition To Control Ionic and Electronic Conduction in Ionic Liquid-Functionalized Conjugated Polymers

“…6,13 Paradoxically, ionic aggregation can lead to higher ionic conductivity if aggregates percolate through the material. 12,[15][16][17] Ion transport in many polymeric systems can be characterized by transport through a transient network of solvation sites, [18][19][20][21] where the mobility of ions is, in part, connected to the density and connectivity of solvation sites. 18 Local ion transport in aggregated domains is postulated to be higher in some systems due to the close proximity of solvation sites.…”

“…1,2,53 Mixed conducting conjugated materials are commonly employed as protective coatings and binders for cathode materials in batteries due to their easy processability and facile ion and electron transport in the presence of liquid electrolyte. 18,[54][55][56][57][58] Increasing interest in solventfree battery construction, however, has created a demand for materials that can conduct both ions and electrons without the presence of solvent. 18,19,56,57 Conjugated polymers with ionic liquid-like side chains, in which an ionic liquid group is covalently tethered to the polymer backbone, have considerable promise in the field of solvent-free mixed conduction.…”

“…18,[54][55][56][57][58] Increasing interest in solventfree battery construction, however, has created a demand for materials that can conduct both ions and electrons without the presence of solvent. 18,19,56,57 Conjugated polymers with ionic liquid-like side chains, in which an ionic liquid group is covalently tethered to the polymer backbone, have considerable promise in the field of solvent-free mixed conduction. 59,60 Large, polarizable ionic side-chain moieties serve to weaken ionic associations and increase ion dynamics without the presence of solvent, while the conjugated backbone imparts electron conductivity.…”

“…A series of peaks starting with a first order peak at q = 0.247 Å -1 and higher order peaks at integer multiples indicate a lamellar side chain stacking structure, while a peak at q=1.674 Å -1 corresponds to π-π stacking. 18,19,[63][64][65] The addition of LiBF4 and NOBF4 to P3HT-IM introduces ionic and electronic carriers, respectively, without significantly perturbing the long-range ordering. To induce mobile electronic carriers and increase the electrical conductivity, an oxidant was added to P3HT-IM in solution state before casting.…”

“…Electrochemically doped conjugated polymers with electronic conductivities in the range of 10 -3 to 10 -1 S/cm have been shown to dramatically increase the performance of NCA electrodes when employed as battery binders. 18 As the oxidant concentration is increased beyond roxidant = 0.8, ! "!#$%&'(# decreases.…”