Characterization of Electrode-electrolyte interfaces of Lithium ion batteries with phosphorus-containing electrolyte additives

Abstract

In recent years, researches on highly efficient energy storage systems have been actively conducted due to an increase in energy demand. In particular, lithium ion batteries (LIBs) are attracted attention as the most promising power storage device because they have higher energy density, higher charge/discharge rates and longer life as compared with other types of batteries. However, the LIBs developed so far does not exhibit sufficient energy density to drive electric vehicles and to store renewable energy due to the active material of graphite, which has a low theoretical capacity of 372 mAh/g. To increase the energy density of a LIBs, silicon-based anode materials have been actively studied because of their high theoretical capacity, which shows about ten times that of a graphite anode of 4200 mAh/g. Despite the high energy density, silicon-based anode materials are not commercially available as anode materials due to volume changes of up to 300% during charging / discharging. This causes the electrolyte to decompose continuously on the electrode surface, consuming lithium ions during repeated charge and discharge cycles. As a result, a very thick solid electrolyte interface layer (SEI layer) is formed on the surface of the silicon anode material, and the electrode material loses its inherent electrochemical characteristics, thereby greatly shortening the performance and lifetime of the LIBs. As a method for solving such a problem, studies have been actively carried out to economically and effectively modify the surface of an electrode using a small amount of an additives. Various additives have been researched and developed in order to form stable solid electrolyte interface layer (SEI layer) which can prevent the continuous decomposition of the electrolyte. In this study, I investigated electrochemical properties of phosphorus-containing electrolyte additives and new electrolyte additives having single-ion conducting properties by using cycle test and electrochemical impedance spectroscopy (EIS). As phosphorus-containing electrolyte additives, lithium difluoro bis (oxalate) phosphate (LiDFOP), diphosphoryl chloride (DPC), diphosphoryl fluoride (DPF) and diphosphoryl chloride derivatives are investigated. The batteries with Phosphorus-containing electrolyte additives showed better capacity retention and lower the impedance compared to those of batteries with pristine electrolyte. Possible SEI layer formation mechanisms are studied by using X-ray Photoelectron spectroscopy (XPS) and IR spectroscopy. Additionally, new additive, Vinyl phosphonic acid dilithium salt (VPLi), having single-ion conducting properties was synthesized. The polymer formed on the electrode surface by decomposition of VPLi contains anions which suppress the transport of anion on the electrode-electrolyte interface. But the impedance of the cell with VPLi additive was relatively high which higher the polarization of the cell confirmed by symmetric cell test. The XRD experiment confirmed that the impedance was increased due to the formation of a large amount of Li2O. Even though the impedance of cell was relatively high, VPLi improved the cycle performance. So, I conclude that the phosphorus-containing additives, especially having single-ion conducting properties, are effective to improve the battery performance.