Superior Performance Hybridized Supercapacitor / Lithium-Ion Battery Redox Electrodes.
(2013) Material Research Society (MRS) - Fall Meeting 2013 — Location: Boston (USA) (1.December.2013)
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- High specific energy, high power density, long cycle life, low cost and safer batteries are required for the advancement of electric vehicles. Current Li-ion batteries (LIBs) have highest energy density but they suffer from low power density. 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, electric double-layer supercapacitors store energy through accumulation of ions on the electrode surface, have very low energy storage capacity but very high power density. By combining lithium iron phosphate (LiFePO4), a high-energy density LIB material, with poly(2,2,6,6-tetramethyl-1-piperinidyloxy-4-yl methacrylate) (PTMA), a high-power density redox capacitor, we construct a high performance hybridized lithium battery 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 enhanced 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 an electrochemical paradox: generation and co-existence of higher redox potential species (PTMA) in the oxidized form with lower redox potential species (LiFePO4) in the reduced form. Being thermodynamically unstable, 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 [1]. [1] A. Vlad et al. in preparation.
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Vlad, A., Singh, N., Rolland, J., Melinte, S., Ajayan, P., & Gohy, J.-F. (2013). Superior Performance Hybridized Supercapacitor / Lithium-Ion Battery Redox Electrodes. Proceedings of the 2013 MRS Fall Meeting. Published. Material Research Society (MRS) - Fall Meeting 2013, Boston (USA). https://hdl.handle.net/2078.5/231878