Toward Maximized Volumetric Energy Density Using Graphite Via Polymer Coating with High Degree of Electrolyte Impregnation

Abstract

Development of electric vehicles and portable electronics triggers a research boom for the advanced lithium ion batteries (LIBs) with high energy density. For these reasons, High specific capacity anodes including Sn, Si and Ge with the specific capacity over 1500 mAh g−1 has been highlighted as alternative anodes to achieve the high energy LIBs. Although the huge volume change considered as the main reason of short cycle life upon repeated charging-dischrage process has been recently handled through the nano-engineering, its utilization as a commercial anodes has been still hindered when considering the low volumetric energy density affected by the several factors (low coulombic efficiency, high electrode swelling ratio, high working voltage and low cycling performance). On the other hand, conventional graphite exhibits the low working voltage and high cycling stability, and its high true density of 2.25 g cc−1 and low expansion ratio of 8% during cycling could lead to the high theoretical volumetric energy density of over 770 Wh L−1. However, density of the currently used electrode using conventional graphite anodes is 1.4 ~ 1.6 g cc−1 bringing about volumetric energy density of 550 Wh L−1. In case of the high electrode density of over 1.9 g cc−1, the poor electrolyte impregnation causes the unreacted active material with electrolyte upon cycling, which leads to the unbalance of N/P ratio (much higher areal capacity of cathode than those of anode) resulting in the Li metal plating. Therefore, Promoting the electrolyte penetration to electrode which is comprised of the highly densified active materials is key factors to meet the high energy density lithium ion batteries by using conventional graphite anodes. In this study, we synthesized cyanoethyl polyvinyl alcohol (PVA-CN) having feature of high degree of impregnation and coated on conventional graphite surface. Because of the PVA-CN with the higher permittivity which is one of critical factors determining the impregnation of electrolyte than these of other polymer such as SBR and PVDF, PVA-CN coated graphite composite demonstrated high cycling performance under the high electrode density of 1.9 g cc−1 without any Li plating, leading to the high volumetric energy density of over 650 Wh L−1.