Electron/ion-conductive Polymer Binders for Lithium-ion Batteries: Poly(phenanthrenequinone) and Poly(acrylic acid) Lithium Salt

Abstract

Among the components comprising a lithium-ion battery (LIB), the binder is a compound that binds mechanically the active material, the conductive additive, and the current collector, by which electrochemical reactions are well occurred in the electrode. As a conventional binder for LIBs, polyvinylidene fluoride (PVDF) has been widely used due to its superior chemical and electrochemical stability. According to that the researches regarding the active materials for higher capacity and higher power have been attracted attention, researches on the functional binders that can support the performances of them have been also conducted. In this study, electronic or ionic conductive polymer binders for high capacity silicon (Si) negative electrode and high power LiFePO4 positive electrode are characterized electrochemically. Si has been attracted much attention as an advanced negative active material due to its higher theoretical capacity (3579 mA h g−1) than conventional graphite. However, because Si experiences huge volume changes during charge-discharge, adhesive polymers such as carboxymethyl cellulose (CMC) and poly(acrylic acid) (PAA) have been considered to be used as alternate of PVDF binder having poor adhesion strength. In addition, nano-sizing of Si particles is a one of good strategy for achieving good cycle performance because absolute volume change of nano-sized Si is smaller than that of bigger Si particles. But, due to its high surface area, there is a problem about particle dispersion in slurry mixing process and high amount of conductive additive and polymer binder should be loaded to ensure the electrical network of the electrode. This brings lowered energy density of the electrode, especially volumetric energy density. To this end, in the first subject of the first part (chapter 2.1), a new conductive polymer binder 3,6-poly(phenanthrenequinone) (PPQ) based on 9,10-phenanthrenequinone is developed and the function as a conductive additive is tested. The PPQ conductive binder becomes an electronic conductor at the first lithiation reaction (n-type doping) and maintains the conductive nature in the reaction voltage range of Si. Nano-sized- Si electrodes prepared without the conventional conductive additive shows superior rate capability compared to the electrode prepared with the non-conductive polymer binder. The internal resistance, measured using intermittent titration technique (GITT), of the electrode prepared with the PPQ conductive polymer binder is smaller than the electrode with non-conductive binder in both lithiation and de-lithiation periods. This is due to the developed electron pathways between the nano-Si particles or between the Si particle and the copper current collector by the PPQ conductive binder that is uniformly dispersed within the electrode. In other words, the PPQ binder plays a role of a conductive additive in the electrode. Due to this feature of the PPQ conductive binder, the loading of the PPQ binder can be minimized down to 10 wt.% with reasonable cycle performance. Although nano-sized Si powder has been researched for a promising negative active material, it has some problems such as low dispersion in slurry mixing and low tap density due to its high surface area and inter-particle repulsion. As a solution of this, using of micrometer-sized Si particles can be considered. In the second subject of the first part (chapter 2.2), for an application of submicrometer-sized Si powder prepared by ball-milling, an adhesive polymer, poly(acrylic acid) (PAA), is blended to the electrode with the PPQ conductive polymer binder. While the adhesion strength of the electrode prepared with only the PPQ binder is marginal as 0.06 N cm−1, the electrode prepared to which 20 wt. % PAA to the total electrode weight was added showed 30 times high adhesion strength (~2 N cm−1). This implies that the PPQ and the PAA binder play roles of a conductive additive and a material providing mechanical integrity of the electrode by its high adhesive property, respectively. In the second part (chapter 3), the effect of a Li+ ion conductive polymer binder on the fast discharge of LiFePO4 positive electrode was studied. Due to its stable structure, flat discharge voltage profile, and high theoretical capacity (170 mA h g−1), LiFePO4 has been used for a positive active material for the batteries of power tools or electric vehicles. In order to decrease polarization during charge and discharge at high current rate, nano-sizing of the particles and surface carbon coating have been tried. But, the polarization can be also occurred due to Li+ ion transfer in the interface between the active material and the electrolyte. To confirm the effect of a Li+ ion conductive binder, poly(acrylic acid) lithium salt (LiPAA), electrochemical properties of the LiFePO4 electrodes with the binders is examined. Discharge capacities of the LiFePO4 electrode prepared with the LiPAA binder at high current rates are higher than the control, the electrode prepared with poly(acrylic acid) (PAA) binder. In addition, polarizations observed in each discharge voltage plateaus of the electrode with the LiPAA binder are smaller than the electrode with the PAA binder. The smaller polarization to Li+ ion transfer is observed in the LiPAA film compared with the PAA film, which was measured with a galvanostatic polarization test of a Li metal

polymer film

Li metal symmetrical cell. Also, resistance in the charge transfer in the electrode with the LiPAA binder is smaller than the electrode with the PAA binder, which was measured with an electrochemical impedance test. These imply that Li+ ion transfer in the interface between the active material and the electrolyte is assisted by the Li+ ion conductive binder because of higher Li+ affinity of carboxylate anion compared with carboxylic acid, so improved discharge rate capability property can be obtained.