Is Li/Graphite Half-Cell Suitable for Evaluating Lithiation Rate Capability of Graphite Electrode?

Li plating on graphite anode is known to be one of the bottlenecks for fast charge of Li-ion batteries and a Li/graphite half-cell has been often used to determine lithiation rate capability of the graphite electrode. In this work, a three-electrode Li/graphite cell with a graphite loading of 3.72 mg cm−2 is used to simultaneously record the cell's voltage and graphite's potential during the Li/graphite cell is lithiated at different current rates. It is surprisingly found that the polarization of a Li/graphite cell is dominated by the Li counter electrode, and that the lithiation capacity of graphite is centered in a narrow potential range between 0.068 V and 0.200 V vs Li/Li+. Impedance analysis reveals that the former is because the Li counter electrode has much larger charge-transfer resistance compared with the graphite electrode. As such, over-potential of the Li counter electrode can readily drive the potential of graphite to negative, leading to Li plating, even at moderate lithiation rates. The results of this work indicate that the lithiation rate capability of graphite determined from a Li/graphite half-cell is largely undervalued, and that an alternative technique is needed for accurate determination of the lithiation rate capability of graphite electrode.