Understanding surface chemistry of Li-ion battery (LIB) cathode materials is crucial for improving their performance. Decomposition of electrolyte and dissolution of transition metal species are interfacial processes both playing key roles in the formation of solid electrolyte interphase (SEI) and contributing to LIB degradation. Here, we focus on spinel Li_xNi_0.5Mn_1.5O_4-δ as a promising high-voltage cathode material for LIB. By employing a combination of static DFT and enhanced ab initio molecular dynamics (AIMD) approaches we investigate adsorption/decomposition of electrolyte species (e.g., dimethylcarbonate) and kinetics/mechanism of Mn and Ni dissolution into the electrolyte solution. Specifically, we analyze the roles of surface disproportionation reactions and transition metal oxidation states, lithiation/delithiation and formation of oxygen vacancies to identify factors controlling adsorption and dissolution processes.