Carbon Nanotube Composite Electrodes Toward All Solid Alkali Metal Ion Batteries

Abstract

Owing to the superior energy density, rechargeable Li-ion batteries (LIBs) have been used as a major power source of portable electric devices such as cell phones and lap-top computers. In addition to small-scale applications, LIBs are also installed in recently developed large scale systems such as electric vehicles (EVs), renewable energy storage systems, and smart grids. In commercially available LIBs, rare metal oxides, such as LiCoO2 and LiMnO4 are used for the cathode material. However, the capacity of LIB is not large enough for EV with long driving range. Furthermore, rare metals used in the present LIB such as Co, Ni and Li raise the cost of LIB. Therefore, we have to explore new electrode materials having high ion adsorption capacity and available at low cost so as to develop long driving range EV. Phosphorus is a candidate for such an electrode materials, because phosphorus is one of the ubiquitous elements and can adsorb much alkali metal ions. However, most of the polymorphs of phosphorus except for black phosphorus, which is prepared by high pressure and high temperature treatment, are electric insulators. We also take large volume change of phosphorus electrode with ion adsorption into account. Single-walled carbon nanotube (SWCNT) has good electric conductivity and huge hollow core space in which some molecules can be encapsulated. Therefore, if we input phosphorus molecules into the hollow core of SWCNT, the materials would work as a good electrode. In fact, it was found by our preliminary experiments that phosphorus encapsulated in SWCNT (P@SWCNT) can adsorb and desorb alkali metal ions such as Li and Na [1]. However, since the experiments were performed under limited conditions, we should perform more sophisticated experiments to examine the electrode properties of P@SWCNT more precisely. In this study, we discuss how the electrode property of P@SWCNT changes with different kinds of electrolytes. In the previous work, we used mixture of Ethylene Carbonate and Diethyl Carbonate (EC/DEC) as an electrolyte. In the present study, we prepared two different kinds of electrolytes: Propylene Carbonate (PC) and polyethyleneoxide based composite polymer electrolyte (PEO). The former has wide liquid temperature range and shows very good low temperature performance than EC/DEC solvent and high conductivity thus we can try to evaluate electrode material at low temperature. The latter is known for one of the solid electrolyte and show from 10-3 to 10-4 S/cm at over 60 degrees Celsius. In the symposium, we’ll discuss the electrode properties of P@SWCNT with different kinds of electrolyte and also talk about the possibility of all solid state battery using P@SWCNT.