Study On The Properties Of PAN-based Polymer Electrolytes For Lithium-ion Batteries

Polymer Lithium-ion batteries retain the inherent superiority of their counterparts with liquid electrolyte, that is high energy density and long cycling life; meantime, They contain the virtue of the structural properties of polymer, easier packing properties and high electrochemical and chemical stability. It is a revolution that polymer electrolytes replace conditional liquid electrolytes!It is interesting to find out if there are interactions between the components and how the components interact with each other. These questions are important because they are relevant to the transport mechanism of lithium ions in the solid electrolytes, which have not been fully understood to date, and the synthesis of gel electrolytes with better properties. In this paper, a novel PAN-based polymer electrolyte has been studied by Raman, IR and AC impedance spectroscopy. Based on our experimental facts, the ion conductivity mechanism of different polymer electrolytes has been put forward, at the same time; the role and the function of polymer have been re-evaluated.Detailed work as follows:The interactions between propylene carbonate (PC) and bis-(trifluoromethanesuphone)imide (LiTFSI) have been studied by Fourier transform (FT) Raman and infrared (IR) spectroscopy. It is found that the coordination of the Li"1" ions with the solvent PC and the influence of PC upon the structure of the TFSF anion lead to the formation of solvent-shared ion pairs in this liquid electrolyte.When PC,PAN and LiTFSI are mixed to prepare for Polymer electrolyte, the polymer can not be dissolved in PC before all the PC molecules are associated with the salt. As long as the polymer is dissolved in PC, the microstructure related to PC does not vary with the salt content in the polymer electrolyte while the interaction between the Li+ ion and the polymer is striking.The microstructure evolution of polymer electrolyte composed of PC, PAN and LiTFSI has been characterized by Raman spectroscopy and the conductivity of the electrolytes have been evaluate by AC impedance spectroscopy. It is found that the conductivity of the electrolyte depends strongly on the association of the lithium ions with the electron donor of the polymer and a transition from "Salt-in-Polymer" to "Polymer-in-Salt" electrolyte has been observed when the salt content is very high (PC:PAN:LiTFSI=1:1:5).Based on these facts, the distinct effects of the polymer are proposed on the transportation of ionic species in conventional "Salt-in-Polymer" electrolyte and in the "Polymer-in-Salt" electrolyte.