Ion Transport Properties in Surface Films Generated on LiNi0.5Mn1.5O4 Positive Electrode

Abstract

A high-voltage positive electrode can increase both energy density and output, and thus commercialization is under consideration. However, the mainly used organic electrolytes at present are electrochemically decomposed in the high-voltage region (>4.3 V vs. Li/Li+) to form a concomitant surface film on the positive electrodes, which cause to high resistance in the electrochemical cells. The interface of the electrode is an important key factor that determines not only the resistance itself, but also the reversibility and kinetics of the electrode. Therefore, it is important to understand the ion transport property of the surface film and to study a strategy of overcoming a series of resistances at interfaces based on these studies. At present, there has not been much studied on the physical properties of the positive electrode interface. On the other hand, a relatively large portion of the negative electrode interface is known about its characteristics. Therefore, in this study, it was tried to understand the characteristics of the surface film on positive electrodes, comparing with the properties for the negative electrodes. The surface film on negative electrodes, which is usually referred to solid electrolyte interphase (SEI): it secures excellent passivating ability within initial cycling and transport Li+ ion through its solid matrix to the active materials. Unlike the negative electrode surface film, however, the positive electrode interface has no excellent protecting ability and is rather permeable, so that lithium ions are not transferred through the solid phase, but delivered by the permeability of the liquid electrolyte. In addition, this permeability causes a continuous electrolyte side reaction when the positive electrodes are exposed to a high-voltage condition, because the electrolyte is easily accessible to the electrode surface due to passivating ability flaw of the surface film. It is one of the main deterioration for the high-voltage positive electrode. Second, it was examined the some of fundamental reason for charge transfer resistance at the surface film/electrodes interfaces, and studied how to overcome this resistance. Unlike the SEI on negative electrodes, a phenomenon, in which lithium ions are not densely accumulated on the surface of the high-voltage positive electrode interface was observed. The concentration of redox species at the electrode surface is one of the factors that could determine the rate of charge transfer reaction. It is thus examined that Li+ ion, the redox mediator in lithium-ion batteries, whether it can also influence charge transfer reaction into electrode. It was ascertained that the lithium ion concentration on the electrode surface affects the charge transfer resistance, and the charge transfer reaction was improved by organizing a high concentrated electrolyte based on phenomenon and the permeability. A high concentration electrolyte using a low viscosity solvent improves the charge transfer resistance and improves the lithium insertion rate so that more lithium ions can be delivered from the electrolyte to the electrode active material even at high C-rates. It was further studied to realize this advantage in a real full-cell. It is should be significantly considered to select the counter electrode, which must show fast delithiation property. Considering this point, after positive and negative electrodes, a high-power property is also realized in a full-cell.