Hybrid supercapacitor-battery materials for fast electrochemical charge storage

Abstract

High energy and high power electrochemical energy storage devices rely on different fundamental working principles--bulk vs. surface ion diffusion and electron conduction. Meeting both characteristics within a single or a pair of materials has been under intense investigations yet, severely hindered by intrinsic materials limitations. Here, we provide a solution to this issue and present an approach to design high energy and high power battery electrodes by hybridizing a nitroxide-polymer redox supercapacitor (PTMA) with a Li-ion battery material (LiFePO4). The PTMA constituent dominates the hybrid battery charge process and postpones the LiFePO4 voltage rise by virtue of its ultra-fast electrochemical response and higher working potential. We detail on a unique sequential charging mechanism in the hybrid electrode: PTMA undergoes oxidation to form high-potential redox species, which subsequently relax and charge the LiFePO4 by an internal charge transfer process. A rate capability equivalent to full battery recharge in less than 5 minutes is demonstrated. As a result of hybrid's components synergy, enhanced power and energy density as well as superior cycling stability are obtained, otherwise difficult to achieve from separate constituents.

Figure 1. Hybridization principle and materials.

(a), Voltage vs. capacity profiles for typical capacitor and battery materials as well as for traditional hybridization. The stored charge (Q) is low given the low specific capacity of the capacitive component and the LIB plateau vanishes at high charge-discharge rates. (b), Voltage profile of the hybrid PTMA-LiFePO₄ electrode at a current density of 26 mAh/g_hybrid. Both plateaus are discernible in charge and discharge and correspond to distinct redox processes of PTMA and LiFePO₄. For constituents mass ratio of 1.6, the capacity contribution ratio of each component is ~1, corresponding to a specific capacity of ~126 mAh/g_hybrid.

Figure 2. Cyclic voltammetry of the hybrid electrode.

(a), Voltammograms recorded between 2.5 and 4.5 V vs. Li/Li⁺ at different scan rates. The two cathodic peaks are separated at low scan rates and overlap at higher scan rates. (b), Cathodic peaks position function of the scan rate for the hybrid electrode. Red arrow marks the onset of the overlapping region. No distinct LiFePO₄ redox peak is observed beyond. (c), Cathodic peak current function of the scan rate for the hybrid electrode. The slope of the log-log dependence is indicative of the charge transfer kinetics. The dashed curves in (b), and (c), are extrapolated from the voltammetry scan of the LiFePO₄ electrode (see SI).

Figure 3. Electrochemical properties of the hybrid electrode.

(a), Capacity retention function of C-rate and applied current density (symmetrical charge – discharge conditions) for LiFePO₄, PTMA and hybrid electrode. The green and black top axes correspond to PTMA and LiFePO₄ respectively. The rate of 1 C corresponds to 100, 170 and 130 mAh/g for PTMA, LiFePO4 and hybrid electrode respectively. (b), Capacity retention at a charge - discharge rate of 5 C for LiFePO₄, PTMA and hybrid electrodes. Inset: voltage profiles for the hybrid electrode at 1^st and 1,000^th cycle.

Figure 4. Charge storage mechanism in the hybrid electrode.

(a), Schematic representation of the fast charge - relaxation process. The green and black half-cylinders are indicative of the stored charge in PTMA and LiFePO₄ respectively. The relative position of the red disk indicates the polarization of the constituents in the hybrid electrode. Except during the charge, the polarization is actually the equilibrium OCP for each component. Fast charging leads to a thermodynamically unstable configuration, characterized by evanescent co-existence of high potential PTMA⁺ with low potential LiFePO₄ species forcing an internal charge transfer equilibration. The depicted situation is representative of 50% SOC, i.e., at equilibrium, only the LiFePO₄ component will be charged. (b), Relaxation curves after charging the hybrid electrode at different rates and to different SOC. (c), Voltage profile at different discharge rates (the curves are shifted for clarity). For every discharge, the charge rate was 45 C.

Figure 5. Fast recharge of the hybrid electrode.

(a), Voltage profile during charge (green) and relaxation (gray) sequences. Top panel: relaxation periods of 30 minutes; bottom panel: 30 and 10 seconds. The effective charge duration for each step is highlighted. The apparent charge time is 1 hour and 5 minutes respectively. (b), Corresponding discharge profiles (red curves, rate of 1 C) as well as the discharge profile for constant-current charge mode for an equivalent period of time, 1 hour and ~5 minutes respectively (grey curves, rate of 1 C).