Percolation threshold of graphenenanosheets as conductive additives in Li4Ti5O12 anodes of Li-ion batteries

Abstract

Graphene nanosheets (GNSs) have been considered as potential conductive additives for electrodes in Li-ion batteries to replace the existing carbon black (CB). Graphene has exceptionally high aspect ratio and excellent electrical conductivity, enabling the formation of extensive conductive networks at a much lower content than CB. This paper reports the beneficial effects of GNSs with a low percolation threshold on electrochemical performance of Li₄Ti₅O₁₂ (LTO) anodes. The experimental results show that the GNSs with a diameter of 46 μm and a thickness of 4.5 nm have a percolation threshold of 1.8 wt%. The prediction based on the interparticle distance concept gives a percolation threshold of 0.54 wt% for GNSs, which is almost an order of magnitude lower than that for CB particles. The substantially low percolation along with a high electrical conductivity of GNSs explains why the LTO anodes containing only 5 wt% GNSs deliver a much better rate capability than those with 15 wt% CB. However, a higher GNS content of 10 wt% results in re-stacking GNSs, deteriorating the diffusion of Li ions through the thickness of GNSs. The parametric study indicates that the percolation threshold of GNSs is inversely proportional to the aspect ratio of GNSs.