A new device concept, Na-Ion Desalination (NID), is introduced to desalinate NaCl from water using Na-ion battery (NIB) intercalants. A two-dimensional porous-electrode model is used to predict the performance of NID cells operating at sea- and brackish-water salinity-levels by simulating electrolyte transport, membrane polarization, and simultaneous electrochemistry. Anion-selective membranes are used to isolate electrodes in NID cells because simulations showed substantial drop from influent salinity (63%) compared to porous separators used in conventional NIBs (22%). Symmetric NIB intercalants and energy recovery were used to minimize energy consumption. A Na0.44MnO2-based NID-cell with 0.5 mm-thick electrodes desalinated 700 mM influent by 63% while consuming only 50% more energy (0.74 kWh/m(3)) than thermodynamic minimum when cycled at C/2 rate. The high volumetric charge-capacity of NIB intercalants enabled 59-64% drop in influent salinity with water-recovery levels up to 80% and 95% for 700 mM and 70 mM influent, respectively. The present predictions of NID performance are optimistic (side reactions, intercalant decomposition, membrane leakage, and competing cation-intercalation are neglected), providing mechanistic insights into NID operation that will guide NID development in the future.(C) The Author(s) 2016 Published by ECS. All rights reserved.