First-principles modeling of thermal stability and morphology control of cathode materials in Li-ion batteries

摘要

We compute the energy of a large number of oxidation reactions of 3d transition metal oxides using the generalized gradient approximation (GGA) to density functional theory and GGA+ U method. Two substantial contributions to the error in GGA oxidation energies are identified. The first contribution originates from the overbinding of GGA in the O₂ molecule and is only present when the oxidant is O₂. The second error occurs in all oxidation reactions and is related to the correlation error in 3d orbitals in GGA. The constant error in the oxidation energy from the O₂ binding error can be corrected by fitting the formation enthalpy of simple non-transition metal oxides. Removal of the 02 binding error makes it possible to address the correlation effects in 3d transition metal oxides with the GGA+U method. Building on the previous success of obtaining accurate oxidation energies from first-principles calculations, we present a new method for predicting the thermodynamics of thermal degradation of charged cathode materials for rechargeable Li batteries and demonstrate it on three cathode materials, LixNiO₂, LixCoO₂, and LiMn2O₂. The calculated decomposition heat for the three systems is in good agreement with experiments. The electrolyte can act as a sink for the oxygen released from the cathode. Although oxygen release from the cathode is generally endothermic, its combustion with the electrolyte leads to a highly exothermic reaction, which is the main source of safety problems with lithium batteries. This thesis also studies surface properties and morphology control of olivine structure LiMPO₄ (M=Fe, Mn). The calculated surface energies and surface redox potentials are very anisotropic. With the calculated surface energies, we provide the thermodynamic equilibrium shape of a LiMPO₄ crystal under vacuum. We furthermore establish an ab initio approach to study surface adsorption and Li dissolution in aqueous solutions. We demonstrate for LiFePO₄ that ab initio calculations can be used effectively to investigate the crystal shape dependency on practical solution parameters, such as electric potential E and solution pH. Our first-principles work is helpful in finding a synthesis condition that favors the production of platelet shape LiFePO₄ with large area of reaction active (010) surface.