Effect of electrode charge balance on the energy storage performance of hybrid supercapacitor cells based on LiFePO4 as Li-ion battery electrode and activated carbon

Abstract

Hybrid supercapacitors using asymmetric, LiFePO4 (LFP) lithium intercalation and electric double layer activated carbon (AC) electrodes combining the high energy battery ability and high power supercapacitor ability in one device are reported. In AC/Li half-cell, AC electrode has 44.5 mAh g−1 capacity and operative voltage > 2 V (Li/Li+) to exhibit supercapacitor-like ion adsorption mechanism. The cyclic voltammetry (CV) and charge/discharge (CD) analysis on AC||LFP hybrid cells with AC/LFP mass ratio, 0.33, 1.2, 1.93, and 3.19, show the transformation from a redox reaction dominated like in battery to a system in which both faradaic redox and the physical electrostatic adsorption processes forming electric double layer are fully balanced in terms of stored charges on both electrodes. The capacity analysis by discharge at different rates shows the high capacity of 125 and 70 mAh g−1 with steep fall at high rates for AC/LFP mass ratios 0.33 and 1.2, respectively. With a specific capacity of ~ 40 mAh g−1, the hybrid cells with AC/LFP mass ratios 1.93 and 3.19 show significant capacity retention at high rates. Optimized by charge balance, the AC||LFP hybrid cell exhibits a high specific energy of 35 Wh kg−1, a high specific power of 1.69 kW kg−1, and a long cycling life.