Sodium-ion batteries (SIBs) are considered as a promising alternative to lithium-ion batteries (LIBs), Since they are composed of naturally abundant elements, cheaper and larger scale battery modules can be assembled. However, due to their similar electrochemical mechanisms, their practical utilization is hindered by high reactivity of conventional carbonate electrolytes with electrode materials leading to the capacity fading over repeated charging cycles. To overcome this obstacle, localized high concentration electrolytes (LHCEs) are used as an effective approach to maintain localized solvation structure of active electrolyte salts by using hydrofluoroether as diluents. Earlier studies are mainly focusing on the interactions between electrolytes with anode materials. Little was known about the electrochemical reactions of LHCEs on SIB cathode. In this work, we examined the reactivities of carbonate electrolytes and LHCEs on NaNiO₂ surfaces. Ab initio molecular dynamics simulations showed that carbonate electrolytes involve more dehydrogenation reactions than LHCEs. The proton-transfer reactions between electrolyte molecules and cathode surface oxygens cause the formation of hydroxyl (-OH) groups, which leads to reduction of neighboring surface nickel atoms from Ni³⁺ to Ni²⁺. As cathode surface gets desodiated, more proton-transfer reactions were observed and Ni migrations from transition metal layer to Na layer were also identified. Consequently, it results in the oxygen loss of NaNiO₂ which accelerates the layered-rock salt phase transition of bulk cathodes.