Ionic and electronic conductivities of atomic layer deposition thin film coated lithium ion battery cathode particles

Abstract

It is imperative to ascertain the ionic and electronic components of the total conductivity of an electrochemically active material. A blocking technique, called the “Hebb–Wagner method”, is normally used to explain the two components (ionic and electronic) of a mixed conductor, in combination with the complex ac impedance method and dc polarization measurements. CeO₂ atomic layer deposition (ALD)-coated and uncoated, LiMn₂O₄ (LMO) and LiMn_1.5Ni_0.5O₄ (LMNO) powders were pressed into pellets and then painted with silver to act as a blocking electrode. The electronic conductivities were derived from the currents obtained using the dc chronoamperometry mode. The ionic conductivities were calculated based on results of the electronic conductivities and the mixed conductivities obtained using the ac impedance method. The results showed that the ionic conductivities of the LMO and LMNO particles coated with CeO₂ thin films were twice as much as those of the uncoated LMO and LMNO particles. Also, LMO particles coated with insulating materials, such as alumina and zirconia ALD films, were tested and compared. No significant effects of the substrates on the ionic conductivities of the coated and uncoated samples were noticed, although the electronic conductivities of the LMO samples were found to be higher than those of the LMNO samples. Indeed, the ionic conductivity of the CeO₂ films and the optimal film thickness achieved by ALD helped overcome the trade-off between long cycle-life and the reduced initial capacity fade of the LMO when used as a cathode in lithium ion batteries.