Synchrotron X-Ray Studies of the Solid Electrolyte Interface on Cycled Lithium-ion Battery Electrodes

摘要

Lithium-ion batteries are promising candidates for hybrid electric vehicles (HEV). Batteries that exceed the requirements for HEV, have been designed and built. These batteries are based on thin LiNi-0.80Co-0.15Al-0.05O-2 cathodes, thin carbonaceous anodes, and a 1 M LiPF6 electrolyte with an ethylene carbonate (EC): ethyl methyl carbonate (EMC) (1:1) solvent. A major technical barrier is that the batteries lose their high power capability during cycling or prolonged storage, particularly at elevated temperatures. Electrochemical impedance spectroscopy indicates that the power fade is mostly due to an increase in cathode impedance in cycled and abused cells. X-ray diffraction and hard X-ray absorption spectroscopy clearly shows that the bulk of the cathode material is largely intact. It is conceivable that the power fade is related to electrolyte and cathode decomposition products that are formed on the surface of the cathode. The decomposition products form the solid electrolyte interface (SEI) and might affect ionic motion by pore plugging, or could contribute to resistive electrical paths to parts of the cathode structure. Although the presence of a SEI on the cathode has long been suspected, confirmation of such a layer has proved elusive. In order to study the nature of the SEI layer it is important to use structural techniques with some surface sensitivity. We have explored the use of soft X-ray absorption spectroscopy to study the nature of the SEI layers in lithium-ion battery cathodes.