Improving Rate Capability and Coulombic Efficiency Via Crosslinking Sulfonated Chitosan Protective Coating on Natural Graphite Anode in Lithium Ion Batteries

Abstract

Graphite is the mostly used anode material for commercial lithium-ion batteries (LIBs) because of several advantages including low operating potential and low cost. However, the formation of thick solid-electrode interphase (SEI) passivating layer due to solvent and electrolyte decomposition at the graphite surface would be harmful to both Coulombic efficiency (CE) and cell performance at high C-rate. In this work, we successfully develop a multifunctional polymeric artificial SEI (A-SEI) protective layer with thickness in nanometers on active natural graphite (NG) particles in the anode to concurrently promote the rate capability and the CE of the lithium-ion batteries. The protective layer is composed of crosslinking sulfonated chitosan (X-SCS) containing sulfonated chitosan (SCS) with high degree of sulfonation as main polymer matrix and glutaraldehyde (GA) as the crosslinker. Different ratio of SCS and GA and a variety of curing conditions are applied to optimize the performance. The existence of sulfonic acid groups in X-SCS should allow lithium ions to be feasibly transferred between the electrolytes and the graphite as the pristine X-SCS membrane showed a good ion conductivity > 10-4 S cm-1 at room temperature upon only ~ 20 wt% liquid electrolyte uptake. X-SCS layer should also provide sufficient mechanical strength to protect graphite from exfoliation as a high than 1 GPa Young’s modulus was observed for the pristine X-SCS membrane. The complete coverage of X-SCS on NG was confirmed by testing the battery employing liquid electrolyte (LE) containing 40 wt% PC, which would cause serious exfoliation of NG and lead to unusual high discharge capacity and low CE in the first lithiation process. Normal discharge curves and a much improved CE are observed for X-SCS coated NG in addition to stable cycling performance up to 40 cycles, suggesting the coverage of X-SCS is complete and stable to effectively prevent the direct contact of LE with NG. X-SCS coated NG provided the corresponding half cell an average CE over 99.5% and capacity > 99.5% when operating at 0.3C and room temperature for 100 cycles. In contrary, the average CE for un-coated NG was only 98.7%. Using the charging (de-lithiation) capacity at 0.1 C as reference, an over 98% capacity retention was observed for X-SCS coated NG cell at 5 C while the capacity retention of the bare NG cell was only 90%.