Electrode thickness control: Precondition for quite different functions of graphene conductive additives in LiFePO4 electrode

Because of high electrical conductivity, graphene has been widely investigated as conductive additive in lithium-ion batteries. Whereas, it is found that graphene has quite different influences on the rate performance in diverse evaluation systems, such as commercial soft-packed cells and coin half-cells. It has been proved that the coin cells show better high-rate performance with the increase of graphene content, while it is of the opposite trend in commercial cells. In normal cases, the electrode thickness of coin cells is much smaller than that of commercial cells. Herein, it is found that the electrode thickness has a considerable effect on the high-rate performance of LiFePO4 electrode in which graphene is used as the conductive additive. Thicker electrode results in a longer Li-ion diffusion path, and thus the steric effect that graphene could hinder the Li-ion diffusion is amplified, inducing much higher polarization and poorer performance at high rate. Comparatively, this phenomenon is not obvious in thinner electrode. Thus, when more graphene is introduced in thick electrode, the power performance is greatly weakened as is observed in the commercial cells. This finding is of great importance for designing a high-performance commercial lithium-ion battery with graphene additives. (C) 2015 Elsevier Ltd. All rights reserved.