(Invited) AC Impedance Studies on Electrode/Electrolyte Interface in Lithium-Ion Batteries

Abstract

Lithium-ion batteries have been extensively used for portable devices and electric vehicles. For the latter use of lithium-ion batteries, rapid charge is required in addition to long cycle lives, high safety, etc. We have focused on the rate capabilities of lithium-ion batteries by considering the lithium-ion transfer at electrode/electrolyte interface and also ion transport inside composite electrodes. Lithium-ion transfer at electrode/electrolyte interface was studied by using various model electrodes. Highly oriented pyrolytic graphite (HOPG) was used as a model electrode of graphite negative electrode. Since the basal plane of HOPG has limited reaction sites, we can observe large lithium-ion (charge) transfer resistances by Ac impedance spectroscopy. Then, it is quite easy to evaluate the activation energies for lithium-ion transfer at HOPG electrode. The values are ranging from 50-60 kJ/mol. For the positive electrodes, we used thin films prepared by pulsed laser deposition (PLD). We fabricated LiCoO 2 and LiMn 2 O 4 thin film electrodes. By using the thin films, the lithium-ion transfer resistances become also clear. Again, the activation energies were obtained and the values are almost the same with those of graphite. Based on the activation energies for lithium-ion transfer at negative and positive electrodes, it becomes quite clear that the rate capabilities of lithium-ion batteries at the lower temperature region are quite dependent on the lithium-ion transfer resistances. We then focused on the ion transfer resistances inside the composite negative and positive electrodes. We used the four-probe cell to evaluate the ion transport resistances inside composite electrodes by Ac impedance spectroscopy. Consequently, the ion transport inside composite electrodes was found to be very slow for the high density electrodes. By the above results, I will talk about the rate determining steps of lithium-ion batteries at the conference. Acknowledgment This work was partially supported by CREST, JST.