Mixed spinel materials such as LiM_0.5Mn_1.5O₄ (M=Ni, Fe, Co) have emerged as a promising class of high-voltage cathodes for Li-ion batteries (LIB). These materials, however, suffer from transition metal dissolution that leads to the loss of active electrode species causing capacity fading and the formation of passivating solid electrolyte interphase (SEI). Herein, we employ a combination of static DFT, ab initio molecular dynamics and accelerated free energy sampling simulations to investigate adsorption/decomposition of electrolyte species, the mechanism and kinetics of Mn and Ni dissolution into the electrolyte. We especially analyze how dissolution pathways and rates are affected by oxidation states of transition metals on the surface, lithiation/delithiation state and the presence of surface protons and oxygen vacancies. This mechanistic study enables a better understanding of LIB interfacial chemistry and factors controlling dissolution process of redox active metals from spinel-type cathodes.