Cathode becomes the most explorable part of battery to increase the energy storage ability. Especially to store electric energy as much as possible. It is the oxidizing electrode that acquires electrons and is reduced during the electrochemical reaction (a electron transfer between two substances-one solid and the other a liquid). There are three cathodes option in lithium ion battery technology. Layered LiMO2 (M = Mn, Co, and Ni), Spinel LiMn2O4, and Olivine LiFePO4 (figure 1). Of course, each of these three cathodes have their advantages and disadvantages.
Figure 1. Crystal structures of LiMO2 (M = Mn, Co, and Ni), Spinel LiMn2O4, and Olivine LiFePO4 (ACS Central Science, 2017).
On the capacity aspect, the layered structure has a highest practical capacity (currently up to ~180 Ah/kg) among the three. But, it suffers form structural and chemical instabilities during the cycling depending on the chemical composition and state of charge. The LiMn2O4 spinel cathode with a three-dimensional structure and lithium ion diffusion provides good structural stability without phase transformation and high rate capability. However, this model suffers from a limited practical capacity (<120 Ah/kg). On the other hand, Olivine LiFePO4 offers good thermal stability and safety without oxygen release because of covalently bonded PO4. But, it suffers from limited practical capacity (<160 Ah/kg), lower operating voltage of ~3.4 V, and poor electronic and lithium ion conductivity. Scholars speaking, the limited conductivity have to be overcome by reducing the particle to nano size and coating with conductive carbon that will decrease low volumetric energy density. This volumetric factor is influenced by the crsytallographic density of structures (Layered > Spinel > Olivine). Therefore, among the three models, the layered oxides are the ones that can provide the highest energy density.
|Date||:||11 May 2021|