Mechanism Leading to Irreversible Capacity Loss in Li Ion Rechargeable Batteries

Abstract

The irreversible capacity loss that occurs during the first cycle in an Li ion battery was studied using Fourier transform infrared attenuated total reflectance, secondary ion mass spectrometer, x-ray photoelectron spectroscopy, and plasma spectrometer. The irreversible capacity loss was related to both the solvent decomposition and the reaction of Li with active sites in the bulk of the carbon electrode. Li remaining in the discharged electrode not only exists on the surface of the carbon but also in its bulk. The Li concentration on the surface of the carbon is higher than that in the bulk. The binding energy of Li remaining in the bulk of the discharged carbon electrode is higher by -2.5 eV than that of metallic lithium (52.5 eV) and lower by ~0.5 eV than that of Li remaining on the surface of the discharged electrode.Recently, lithium ion rechargeable batteries (LIBs) using carbon as an anode material have attracted a great deal of world wide attention. This is because, compared with lithium metal rechargeable batteries, they are safer, and they have a higher energy density as well as a higher voltage than nickel-metal hydride reehargeable batteries. In particular, it was reported that when disordered carbon is used as an anode material~ these materials store lithium with capacities surpassing the theoretical capacity of a graphite anode. 1-4 These results have further stimulated the development of LIBs. Recently, it has been commercialized in Japan. ~ Unfortunately, some problems continue to hinder their development. 6 One problem is that an irreversible capacity loss is observed during the first cycle because of side reactions, namely, Li charged into the carbon electrode cannot be discharged from the carbon electrode. It is well known that the lithium supply in LIB comes from the cathode when the cell is manufactured. To compensate for the loss of lithium which is irreversibly consumed, an excess of cathode material must be used. As a result, the energy density of the cell decreases and the cost of the cell increases.Reducing the irreversible capacity loss is important for LIB development. First, there is a need to understand how the lithium is lost. Studies on irreversible capacity loss have been reported. 7-1~ They suggest that the irreversible capacity loss is solely caused by the reduction of solvent (such as propylene carbonate, PC) on the surface of the carbon electrode to form Li~CQ. 7,10 Sleigh has conjectured that the loss is due not only to the incorporation of decomposed solvent, but also is related to the bulk carbon mater~al. However, no detailed description was given. 11The goal of this work is to investigate irreversible capacity loss and the properties of the Li remaining in the discharged carbon electrode by using electrochemical measurements and various analytical techniques such as Fourier transfo~zn infrared attenuated total reflectance (FTIR-ATR), secondary ion mass spectrometer (SIMS), x-ray photoelectron spectroscopy (XPS), and plasma spectrometer. ExperimentalRibbonlike carbon films...

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