Ionic and Electronic Transport Properties of Lithium-Metal-Oxygen Systems

Abstract

Mixed conducting spinel (e. g. LixMn1.5Ni0.5O4 ) and layer structure materials (NCA, NMC) have been used as cathode for lithium ion batteries due to their high energy and power density. Ionic and electronic transport properties of these materials play an important role to help designing particle morphology and loading of active materials in order to get better electrochemical performances. Also these compounds can be ordered or disordered depending on the arrangement of Mn and Ni in the crystal structure. The charge-discharge behavior of ordered and disordered structures has been tested by several research groups. However still it is not clear which structural composition is suitable for a high performance battery. It is also not clearly understood if the rate performance of the material is limited by bulk transport properties or interfacial reactions. Here we report on the electronic and ionic conductivity and diffusivity of these materials which has been determined separately by using ion and electron blocking cell configurations as a function of lithium concentration on sintered dense pellets. For spinel, the disordered phase exhibits about fifteen time higher electronic conductivity than the ordered phase at room temperature in the lithiated state. The conductivities of the partially delithiated ordered phase measured at a given temperature increase monotonically with increasing delithiation at x = ~0.3 and beyond that the electronic conductivity is almost leveled up. In contrast, the electronic conductivity of disordered phase initially decreases with delithiation and comes down to the level of lithiated ordered phase. After that it exhibits almost the same electron conducting behavior as of ordered phase [1, 2]. The lithiated ordered and disordered phases exhibit the same order of magnitude of ionic conductivity and diffusivity. On the other hand electronic conductivity of layer structure materials gradually increase with increasing the state of charge whereas ionic diffusivity gradually decreases up to 50% state of charge and again increases on further charging [3, 4]. Chemical diffusion during electrochemical use is limited by lithium transport, but is fast enough over the entire state-of-charge range to allow charge/discharge of micron-scale particles at practical C-rates. References R. Amin and I. Belharouk, J. Power Sources, 348 (2017) 311–317.R. Amin and I. Belharouk, J. Power Sources, 348 (2017) 318–325.R. Amin,Y.-M. Chiang, J. Electrochem. Soc. , 163 (8), (2016) A1512-A1517.R. Amin, D. B Ravnsbæk, Y.-M. Chiang, J. Electrochem. Soc. 162 (7) (2015) A1163-A1169.