Selective removal of nanopores by triphenylphosphine treatment on the natural graphite anode

Abstract

Artificial graphite (AG) and natural graphite (NG) have different physical properties, which should have significant effects on their unique electrochemical characteristics: NG is known to have superior specific capacity and low cost, whereas AG shows superior cyclability and low-swelling characteristics. Herein, we present a straightforward strategy using triphenylphosphine to improve NG, enabling its physical properties to resemble those of AG and thereby improving its cyclability and swelling characteristics. The volume of a-few-nanometer-sized pores of NG is significantly reduced by triphenylphosphine treatment, which is similar to that of the AG. As a result, triphenylphosphine-treated NG (TPP) anode shows improved cyclability (83.6% after 300 cycles) compared to that of NG, confirming the correlation between nanopore volume and cyclability. Furthermore, the expansion of single-layer pouch cell prepared with TPP, as monitored by in situ thickness measurement, is suppressed to 9% after 55 cycles, which is superior to the pristine NG showing 14% expansion after the same number of cycles. Electrochemical impedance analyses of symmetric cells reveal that the changes of ionic and solid-electrolyte interface (SEI) resistance are accompanied by the reduction of nanopores, confirming that the formation of decreased SEI layer is a crucial factor for realizing less-swelling and highly durable anodes based on the cost-effective NG materials.