Nitrile cellulose and sucrose-based additives for impedance reduction in Li and Li-ion cells

摘要

Reduced interfacial resistance was achieved in Li cells with new additives. These additives include Nitrile Cellulose (CECN), Nitrile Sucrose (SUCN) and lithium salts of nitrile Sucrose (LiSUCN). While CECN is white solid room temperature SUCN is viscous liquid at room temperature. These are stable to air and water. CECN and SUCN have respectively 3 and 8 nitrile groups per molecule. The salt Li{sub}4SUCN was formed by adding 1.08 g of SUCN to a 100 ml round bottom 2-necked flask in a glove box. To this was added 50 ml of Tetrahydrofuran (THF) at room temperature and the mixture was stirred well. The flask was tightly sealed and taken out of the glove box, cooled to 0°C in an ice bath and 2.8 ml of n-butyl lithium was injected through a septum. The solution was stirred for 2 hrs and the ice bath was removed and again stirred overnight. The solvent was pumped off leaving behind a bright yellow deposit. The flask was taken into the glove box and the yellow powder collected. The composition of the yellow powder was confirmed to be Li{sub}4SUCN. The salt Li{sub}4SUCN is very stable to air and water. A similar procedure was employed for the syntheses of other lithium salts and the weights of the chemicals were adjusted appropriately. These salts range in composition from LiSUCN to Li{sub}8SUCN. The physical properties of the salts change from off-white and sticky powder to deep yellow and hard solid as the lithium content increases. Polyvinylidene difluoride (PVDF) based gelled electrolyte thin films containing a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with and without additives were prepared. A doctor-blade technique was used to prepare these films. Electrochemical measurements of the PVDF gelled electrolyte films were made by sandwiching the film between two oxide free lithium foils and of the liquid electrolyte in "Tee" cells.