Among vacancy defects, V O and V Li have much lower formation energies (thus exist in higher concentrations) than V Mn and V Ni under all conditions (see Figure ). Therefore, oxygen and lithium vacancies are discussed here. For lithium vacancies, while V - Li (removing a Li + ion) generates little disturbance to the system, V 0 Li (removing a Li atom) reveals the delithiation mechanism. The average Madelung site potential on Ni is -24.55 V, whereas the Ni 3+ site has the most negative potential, with -27.38 V.