Effect of Polymer Wrapping on Carbon Nanotubes in Li-O2 Battery Cathode
Authors: Fujigaya Tsuyohiko , Jang Il-Chan , Kanamori Ryota , Ishihara Tatsumi , Nakashima Naotoshi 
Published: February 2020

Nonaqueous lithium (Li)-oxygen (O2) batteries have recently attracted significant attentions as a promising electric generation system due to their highest theoretical specific energy (approximately 11680 W h kg-1) together with the discharge-charge cycle repeatability (i.e., rechargeability). For the ideal reaction of the nonaqueous Li- O2 battery, the lightest metal (Li) reacts with O2 (air) as 2Li + O2 → Li2O2 at the cathode. Since the reaction is reversible, the battery shows a rechargeablity.

The exceptionally high energy density of the Li-O2 batteries is mainly derived from the following two factors. First, the active cathode material (O2) in the batteries is abundantly stored in the surrounding environment, therefore, it is never exhausted. Second, the anode material (Li metal) has the highest specific capacity (3842 mAh/g), lowest atomic mass and low electronegativity (-3.04 V vs. SHE) among all solid electrode materials, which maximizes the energy density when it reacts with O2.

However, one of the critical problems in the system is that the nonhomogeneous deposition of solid Li2O2 upon discharging in the cathode due to the local reaction at the catalyst nanoparticles and/or the diffusion of the intermediate precipitates as an agglomerate. Since the Li2O2 is insoluble and an insulating material, such a nonhomogeneous deposition causes clogging of the void and prevents diffusion of the electrolyte and O2 as well as the electron conduction, which limits the capacity and increases the overpotential. What is worse, the formation of the large Li2O2 particles also limits the charging reaction (Li2O2 → 2Li + O2) mainly due to the poor electron conductivity of the Li2O2, which reduces the rechargeability.

In order to overcome these issues, developments of the novel cathode which can accommodate a substantial amount of Li2O2 were designed. Along this line, carbon nanotubes (CNTs) were extensively investigated in place of the typical electrode based on carbon black (CB) since the CNTs can form a continuous mesospace with a stiff network structure without using any binder materials, in which the CNTs were decollated with metal nanoparticles as the catalyst to lower the overpotential. It has been pointed out that the metal catalyst often induced the local deposition of the Li2O2 at the catalyst sites, and in addition, the graphitic surface causes the diffusion of the Li oxide intermediate, both of which end up clogging the electrode. Therefore, a novel strategy to homogeneously catalyse the discharge reaction on the CNTs without using neither the metal nanoparticles nor the graphitic surface is desired.

In this study, we proposed a polymer coating of CNTs as a novel electrode, in which a homogenous ultrathin polymer coating on the entire surface of the CNTs is expected to facilitate a homogeneous deposition of Li2O2 by the affinity of Li+ with the wrapped polymer. As the coating polymer, we chose poly[2,2’-(2,6-pyridine)-5,5’-bibenzimidazole] (PyPBI) since 1) since their polymer is known to homogeneously coat CNT surfaces with a thickness of ~1 nm, and 2) it is expected to promote the formation of Li2O2 through the coordination of Li+ to the PyPBI ligand, which leads to a homogeneous Li2O2 deposition. As the result, we obtained a much higher charging capacity and better rechargeability of the SWNT/PyPBI-cell compared to that of the SWNT-cell. Since the PyPBI coating system provides a non-destructive functionalization of the pristine SWNT’s surfaces, it is readily expected that the electrode possesses a high durability under highly oxidative conditions. The present strategy opens a new route for the development of a high performance electrode for the Li-O2 battery.