Ketjen black (KB) has a large specific surface area and is potentially more effective as the cathode conductive additive. However, large KB flakes often overlay together to form thick stacks. In order to overcome this problem, maleic anhydride (MA) was grafted onto the KB surface by a plasma-induced method. Such a MA-KB additive was found to have a more homogeneous distribution in the LiFePO4 cathode, which resulted in a reduction in the electrical resistivity and electrode polarization. As the consequence, greatly improved cell performance was achieved, including higher capacity, higher discharge voltage, higher rate capability, longer cycle lifetime, and a much flatter discharge curve. Detailed analysis on the discharge curves and the electrochemical impedance spectroscopy showed that although both the electrical resistivity and electrode polarization were reduced by the better distribution of the MA-KB additive, the reduction in electrode polarization was more important in cutting the capacity loss.