DFT Study of Surface and Interface Issues in High-Voltage Cathodes for Li-Ion Batteries

Abstract

The development of stable materials for cathodes in lithium-ion batteries (LiBs) is a major bottleneck for the commercialization of high-energy density rechargeable batteries. Lithium cobalt oxide (LiCoO2) is one of the most heavily used cathodes for LiBs. LiCoO2 exhibits fair electrical conductivity and Li+ mobility with an operational voltage of 4 eV. Moreover, the theoretical capacity of LiCoO2 is 274 mAh/g corresponding to a full Li+ extraction, producing CoO2. However, its practical capacity is only 130–150 mAh/g, indicating that only half of the Li atoms can be used during the “rocking-chair” de/intercalation process. Moreover, because of safety concerns (e.g. thermal stability and interaction with organic electrolytes with high volatility and flammability) its usage in high power and high energy density batteries is abandoned. Indeed, at high temperatures or high cut-off voltages, LiCoO2 decomposes and desorbs oxygen. The evolved oxygen reacts exothermically with the flammable electrolyte. This oxygen evolution jeopardizes the safety of the cell due to the magnitude of this highly exothermic reaction. In our previous work, we found that the (104) and (012) surfaces are more prone to O2 release. Ab-initio molecular dynamics (AIMD) results show that the under-coordinated oxygen atoms from the delithiated structures can combine and eventually evolve as O2. Thus, the intrinsic surface properties of cathodes and the interfacial interactions between cathode and electrolyte need to be considered to enhance the cathode stability. The surface engineering and design of the electrode material have been proposed as a promising path to achieving safe and high-voltage cathodes for LiBs. In this work, firstly we report the stability, interfacial chemistry, oxidation-reduction behavior and surface thermodynamics of the major surfaces of LiCoO2 based on the calculations obtained with the density functional theory (DFT) approach. Further, we present the results achieved from DFT-based calculations and experimental techniques (e.g. electrochemical cycling, in-situ heating transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman characterizations) on the surface coating approach to enhance the structural stability of LiCoO2 by providing a physical barrier for oxygen evolution. Our experimental work shows that the coated sample could exhibit more than 75% capacity retention after 40 cycles while the pristine sample failed after 20 cycles with the same condition. Our computations suggest that the interaction between the physical barrier and the electrode’s surface leads to an interfacial chemical bonding that prevents O2 evolution. Overall, we expect that this combined approach will allow improving the overall performance of the battery by addressing the multifaceted problems that play a role in the safety problems of cathodes.