Sodium-ion pouch cells with up to 1000 cycles are presented by using a NaNi1/3Fe1/3Mn1/3O2 cathode, a hard carbon anode, and a functional electrolyte. The functional electrolyte is composed of 1 M NaPF6 dissolved in a 1:1 (v/v) mixed solvent of propylene carbonate (PC) and ethyl methyl carbonate (EMC) with 3–4 wt% of two or three additives, including fluoroethylene carbonate (FEC), prop-1-ene-1,3-sultone (PST), and 1,3,2-Dioxathiolane-2,2-dioxide (DTD). It is shown that the capacity retentions of the cells increase to 84.4% and 92.2% after 1000 cycles for electrolytes containing FEC-PST bi-additive and FEC-PST-DTD tri-additive, respectively, as compared with that containing FEC single additive. Using X-ray photoelectron spectroscopy, inductively coupled plasma optical, and transmission electron microscopy, post-mortem analyses on the surface of the cycled electrodes indicate that PST and DTD are beneficial to the anode by forming an organic compound rich solid electrolyte interphase (SEI), and to the cathode by forming a dense and thick cathode electrolyte interphase (CEI) that consequently prevents transition metal ions from dissolving into electrolyte.