T. Nishida1), K. Nishikawa1), H. Matsushima1), T. Wakatsuki1), T. Homma2) , M. Rosso3) and Y. Fukunaka4)
1)Dept. of Energy Sci. & Tech., Kyoto University
2)Dept. of Applied Chemistry, Waseda University
3)Ecole Polytecnique, Paris
4)Nanotechnology Research Center, Waseda University
Development of advanced battery is the key for future space engineering field. Long life cycling must be maintained as well as the safety issue. Moreover, battery life must be predicted to reduce the number of space debri. Such technology achievements in space engineering field should be fed-back to the terrestrial applications including large scale energy storage devices. Generally speaking, battery reversibility is frequently governed by electrode surface flatness. Metal electrodeposition may generate irregular deposits with various morphologies. Coupling phenomena between the morphological or microstructural variations and ionic mass transfer rate must be fully understood to tailor the interface to create the unique physical properties as well as the ultra-high energy storage capacity. It is also indispensable for us to prolong the charging/discharging operation of secondary battery after longer term utilization under various environments. For liquid electrolyte, a precise study of these morphologies may be complicated by convective motion caused by gravitational force of buoyancy. Now the coupling phenomena are discussed in the case of Li in organic solvent PC or ionic liqid and Ag (or Cu) in AgNO3 (CuSO4) aqueous solution because they have been employed as negative electrode in space application as well as commercial use.
The dendrite growth behavior of Li metal galvanostatically electrodeposited on Ni substrate in LiClO4-PC electrolyte was in-situ observed by a laser scanning confocal microscope. A Li dendrite precursor is stochastically evolved on Ni substrate probably through SEI. The initiation period of dendrite precursor becomes shorter with increasing current density and decresing LiClO4 concentration. Once it has been initiated, the ionic mass transfer rate starts to control the growth process of dendrite arm length, exceeding over the surface chemistry determining step.
In order to excule the complexities due to SEI formation during the dendrite formation process, more simplified coupling behavior was studied in aqueous solution.
3M AgNO3aqueous solution was contained in a quasi-two dimensional electrolytic cell. Holographic interferometry technique was employed to monitor the ionic mass transfer rate. The thickness of electrolyte layer sandwiched by two slides glasses was measured to be 55 to 70 micrometer.
The concentration profiles were formed around dendrite tips of silver electrodeposited at extremely high current densities. A uniform diffusion layer was formed along the cathode surface at the start of electrolysis. Electrochemical nucleation and growth phenomena on a substrate is expected under a considerably high overpotential. Following a certain incubation period, several dendrite tips began to protrude into the diffusion layer. The growth rates of these dendrites were related to concentration gradients around the tips, diffusivity, and transfer number of Ag+ ion. Giant-Macro step flow accompanied with dendritic growth of electrodeposited Ag in AgNO3 solution was dynamically observed in the image of Laser Confocal Scanning Microscope.
These experiments must be engaged in microgravity facility which provides an ideal environment to built the physical model on electrolyte/electrode interfacial coupling phenomena without natural convection.