Na-Ion Batteries As a Pathway for Seawater Desalination
Authors: Smith Kyle Christopher 
Published: February 2020

Aqueous Na-ion intercalation compounds have shown promise for hybrid Na-ion batteries (NIBs) used in energy storage applications [Whitacre, Tevar, and Sharma, Electrochem. Comm., 12, 463-466 (2010)]. Because of their stability in aqueous electrolytes and their ability to store Na ions that are prevalent in natural saltwater, Na-ion intercalation compounds have also found use in seawater desalination [Pasta, Wessells, Cui and La Mantia, Nano Lett., 12, 839-843 (2012)], a process that could potentially provide fresh water access to many drought-stricken regions of the world. While these previous desalination devices utilized Na-ion intercalation electrodes paired with costly Ag/AgCl electrodes, recent work has predicted that NIBs using symmetric intercalation electrodes separated by a Cl-ion selective membrane can be used to remove seawater-level concentrations of NaCl [Smith and Dmello, J. Electrochem. Soc., 163, A530-A539 (2016)]. This method of desalination, referred to as Na-ion Desalination (NID), provides new opportunities for the application of NIB intercalation compounds.

Here, modeling results are presented that demonstrate the efficacy of Na-ion intercalation compounds to function in an NID cell. Energy consumption and the degree of salt removal are predicted using a two-dimensional porous-electrode model. Both Na0.44MnO2 and NaTi2(PO4)3 are investigated as intercalation compounds in these cells with Na0.44MnO2showing lower levels of energy consumption due to its fast intercalation kinetics. While conventional NIBs use porous separators to isolate electrodes, Cl-ion selective membranes are used in NID cells because cells using such membranes showed substantial drop from influent salinity (63%) compared to the same cells with porous separators (22%).

Conventional NIBs employ an asymmetric electrode arrangement to maximize cell voltage and, consequently, the energy density of an NIB. In contrast, NID devices can be designed either for desalination with minimal energy input. To minimize the energy input during desalination, a symmetric electrode arrangement can be used, as shown for the NaTi2(PO4)3-based cell in Fig. 1. Here, the predicted cell voltage (top left) and electrolyte salinity (bottom left) are shown under galvanostatic cycling conditions at 0.3C rate as aqueous NaCl electrolyte flows continuously through the cell’s porous electrodes from a reservoir with 700 mM salinity. Initially, the cell operates in a mode where cell voltage is negative when current is applied to the cell’s positive terminal and the cell outputs energy. But once the two-phase reaction plateau of Na
x
Ti2(PO4)3 is reached, cell voltage becomes positive and remains roughly constant until the terminal charging voltage is reached. During this process Na ions stored in Na
x
Ti2(PO4)3 in the positive electrode are released into the electrolyte, while Na ions from the negative electrolyte simultaneously intercalate into Na
x
Ti2(PO4)3 (Fig. 1, top right). Switching the direction of current application to discharge generates desalinated and concentrated solution in the positive and negative electrodes, respectively. The high charge capacity of Na-ion intercalation compounds used in NID cells could also be used to reduce the generation of undesirable brine water. When equal electrolyte flow velocities are employed in opposing electrodes, these cells are predicted to generate equal amounts of desalinated water and brine water. But, if flow velocity is reduced inside the concentrating electrode, a lesser amount of brine water can be produced, while maintaining the same rate of desalinated water production. For 700mM NaCl feedwater our simulations reveal that up to 80% of feedwater can be desalinated with approximately 60% salt removal, leaving only 20% feedwater as brine. For 70mM NaCl concentrations characteristic of brackish water even lower levels of brine are produced. These modeling studies provide a basis for experimental development of NID cells.

Figure 1