This study examines the effects of a graphene nanosheet (GNS) conductive additive on the performance of a highly packed (2.5 g cm(-3)) lithium-ion battery cathode containing 92 wt% Li-1.1(Mn0.6Ni0.4)(0.9)O-2 microspheres (approximately 6 mu m in diameter). GNSs, approximately 2.0 nm thick and 0.5-1.0 mu m in width, are introduced into an electrode slurry in the form of a dispersion in N-Methyl-2-pyrrolidone. They are substantially smaller than the oxide particles; therefore, their presence exerts no adverse influence on the packing density of the electrode. A small quantity of the GNS additive (<= 200 ppm relative to the oxide mass) can significantly increase the overall electronic conductance and improve the conductance uniformity of the oxide electrode, leading to reduced polarization and enhanced specific capacity and rate performance. However, the GNS additive also promotes solid-electrolyte interphase formation, resulting in resistance buildup and capacity deterioration upon cycling. This study is the first to identify such an adverse effect caused by a graphene additive. The interplay between the positive and negative effects has led to an optimal GNS additive content of approximately 100 ppm, enhancing both the rate and cycle life performance.