Combining composition graded positive and negative electrodes for higher performance Li-ion batteries

Homogeneous electrode structures used in Li-ion batteries (LIB) lead to inhomogeneous active material utilization and gradients of overpotential and Li-ion concentration at the cell-scale, which are detrimental for both capacity retention at high charge-discharge rates and for battery life-time. To account for these gradients, we demonstrate that heterogenous electrode structures with engineered gradients in material distribution can improve LIB C-rate and long-term cycling performance when compared with conventional uniform electrodes in LiFePO4 parallel to Li4Ti5O12 full-cell LIBs. An improvement in C-rate performance of > 120% and a capacity degradation rate reduced to <50% over uniform electrode cells was achieved at 1C, and graded cells showed a dramatically improved power-energy density balance. Graded electrodes had a relatively low cell polarization that became more marked as the C-rate increased. Cycled graded electrodes had reduced solid electrolyte interphase (SEI) formation when compared with uniform electrodes according to XPS surface analysis, which was consistent with their reduced charge transfer resistance measured by impedance spectroscopy. The origin of the improved performance arises from a more uniform overpotential distribution across the thickness of the graded heteroelectrodes.