Free energy for protonation reaction in lithium-ion battery cathode materials

Calculations are performed of free energies for proton-for-lithium-ion exchange reactions in lithium-ion battery cathode materials. First-principles calculations are employed for the solid phases and tabulated ionization potential and hydration energy data for aqueous ions. Layered structures, spinel LiMn₂O₄, and olivine LiFePO₄ are considered. Protonation is most favorable energetically in layered systems, such as Li₂ MnO₃ and LiCoO₂. Less favorable are ion-exchange in spinel LiMn₂O₄ and LiV₃O₈. Unfavorable is the substitution of protons for Li in olivine LiFePO₄, because of the large distortion of the Fe and P coordination polyhedra. The reaction free energy scales roughly linearly with the volume change in the reaction