We report the structural and electrochemical characteristics of lithium (Li)-ion battery (LIB) electrolyte solutions using an ethylene sulfite (ES) solvent that is used as an electrolyte additive for LIBs. From dilute to highly concentrated ES solutions with lithium bis(fluorosulfonyl)amide (LiFSA), the formation of Li-ion complexes was investigated using a combined Raman and infrared spectroscopy study with the aid of density functional theory (DFT) calculations to quantitatively determine their solvation and ion-pair structures depending on the Li salt concentration (cLi). The results reveal that, in the dilute solutions (<1.0 mol dm-3), Li-ions are fully solvated with ES molecules to form a tetrahedral-like [Li(ES)4]+ complex; however, with the increasing cLi (up to 2.5 mol dm-3), the Li-ion complex changes in structure to form contact ion-pairs coordinated with both ES and FSA anions. It also reveals that further increasing cLi to approximately 3.0 mol dm-3 leads to the ionic aggregate formation, i.e., multiple Li-ion complexes linked via several FSA anions. LiFSA/ES electrolyte solutions exhibited a reversible Li-ion insertion/deinsertion reaction into/from the graphite anode irrespective of cLi. This is due to the high-grade ES-derived passivation films on the electrode as a result of the preferential reductive decomposition of the ES molecules trapped within the Li-ion coordination sphere. According to the charge-discharge test, the concentrated LiFSA/ES solutions exhibited the high C-rate performance, which is superior to the concentrated electrolyte solutions using conventional organic solvents.