Hybrid Pseudocapacitor - Lithium-Ion Battery Redox Electrodes for Power Leveling Applications

Li-ion batteries (LIBs) have highest energy density but they suffer from low power density[1]. Energy is stored in LIBs by virtue of reversible Coulombic reactions occurring at both electrodes involving slow charge transfer in the bulk electrode materials and limited diffusion of ions from one electrode to the other. On the other extreme, supercapacitors store energy through accumulation of ions on the electrode surface, have very low energy storage capacity but very high power density[2]. 

By combining poly(2,2,6,6-tetramethyl-1-piperinidyloxy-4-yl methacrylate) (PTMA), a high-power density redox pseudocapacitor with lithium iron phosphate (LiFePO₄), a high-energy density LIB material, we construct a high performance hybrid electrode. The voltammetry response of the hybrid battery electrode contains two pairs of reversible redox couples at low scan rates. At high scan rates, the two oxidation peaks converge suggestive of electrochemical hybridization. The polarization in oxidation is limited by PTMA, avoiding voltage abuse on LiFePO₄ component. The hybrid electrode shows excellent capacity retention, 17.4% capacity loss after 1,500 cycles at 5C charge/discharge rate, mimicking the PTMA electrode behavior rather than that of LiFePO₄. Electrochemical impedance spectroscopy reveals improved charge transfer after cycling, consistent with an activation mechanism. The influence of the hybrid electrode configuration and composition on the battery performance is also detailed.

Kinetically controlled fast hybrid electrode charging leads to a thermodynamically unstable: generation and co-existence of higher redox potential species (PTMA) in the oxidized form with lower redox potential species (LiFePO₄) in the reduced form (Fig. 1). This configuration forces an internal charge transfer process that equilibrates the redox state of the hybridized species, leading primarily to charging of LiFePO₄. This translates into a highly relevant technological fact: whenever the electrode needs to be recharged, the rapid response of PTMA ensures the fast recharge. As such, >90% state-of-charge in the hybrid battery is reached within a five minutes time window of current pulse and relaxation sequences.

The contribution of components to the total stored charge is proportional to the amount of each component, whereas the electrode configuration and composition control the power and energy delivery performances. The power and energy density of the hybrid electrode can be precisely balanced by the respective amount of constituents, to fulfill the targeted application requirements. Coupling PTMA with a higher voltage cathode material results in enhanced power delivery characteristics. The hybrid electrode is found suitable to complement the variability of renewable energy sources (solar conversion – storage units), as well as potentially useful for load-leveling in micro-grids.

Following the same rationale, a hybrid supercapacitor – pseudocapacitor was built and tested and found to provide high specific energy while not compromising the specific power. Further design of multicomponent electrodes will be also detailed[3].

[1] Armand M, Tarascon JM (2008) Nature 451:652–657.

[2] Simon P, Gogotsi Y (2008) Nat Mater 7:845–854.

[3] Vlad A, et al., submitted.