Effect of Additives on the Interfacial Degradation Phenomena of LiNi0.5Mn1.5O4 Thin-Film Electrodes

Abstract

LiNi0.5Mn1.5O4 shows promise as a positive electrode material for lithium-ion batteries. However, because the material has a higher operating potential than conventional cathodes, interfacial side reactions are accelerated during cycling, which degrades the outmost surface of the material. Although it has been reported that some additives offer effective protection against degradation, their protection mechanism has not been clarified in detail. Here, the effect of additives on the surface states of LiNi0.5Mn1.5O4 was investigated using thin film model electrodes. It was found that surface film was not formed on LiNi0.5Mn1.5O4 in additive-free electrolyte solution even after cycling at 55 °C, and severe dissolution of transition metal ions continuously occurred, leading to rapid capacity fading. Addition of ethylene glycol bis(propionitrile) ether (EGBE) effectively suppressed the capacity fading at 55 °C. Analysis with redox reaction of ferrocene on the electrodes, X-ray photoelectron spectroscopy, and scanning electron microscopy indicated that surface film hardly formed in EGBE-containing solution, but the dissolution was effectively suppressed. Because a nitrile group tends to adsorb on positive active material at high potential, it was concluded that the adsorption layer of EGBE impeded side reactions at the interface, resulting in improved cycleability of LiNi0.5Mn1.5O4.