Characterization of Li+ and Na+ Insertion into Sodium Titanium Phosphate
Authors: Valdes-Espinosa Honorio , Oleson Karl , Hurt David , Hoo Shirley , Liu Yu , Madden Thomas 
Published: May 2020

Due to their high energy density, lithium-ion batteries based on organic electrolytes are ubiquitous in applications such as consumer electronics. However, high capital costs and flammability of the electrolyte have limited their use in residential or grid storage. An alternative involves replacing the organic solvent-based electrolytes with aqueous electrolytes and using electrochemical couples that operate within the water electrolysis stability window. Aqueous electrolytes have several advantages, such as ease of manufacturing, greater safety, and more cell design flexibility due to the higher conductivity compared to organic electrolytes 1.
In addition to aqueous Li-ion batteries, aqueous Na-ion batteries have attracted significant interest due to the increased availability of Na on earth and further cost reductions1,2. Furthermore, aqueous commercial batteries using electrolytes containing both Li+ and Na+ exist. For example, one such battery using a lithium manganese oxide cathode, a sodium titanium phosphate (NTP) anode, and a Li2SO4/Na2SO4 electrolyte has been commercialized by companies such as Aquion Energy and BenAn Energy2. Despite the advantages described previously, the operating voltage window of aqueous electrolytes is smaller than that of its organic counterparts. In addition, insertion behavior in aqueous electrolytes is more complex than what is observed in organic electrolyte systems3. This can be further complicated when using electrolytes with more than one cation4,5. One such example involves the insertion of Li+ and Na+ into NTP. In electrolytes where Na+ is the only cation, a single redox couple attributed to Na+ insertion is observed on CV experiments. Meanwhile, when both Li+ and Na+ are present, insertion of both cations occurs, with equilibration occurring after 10 CV cycles4. Despite this interesting behavior, little is known as to how the Li2SO4/Na2SO4 ratio affects the insertion behavior of NTP.
In this work, the effect of Li2SO4 and Na2SO4 concentration in the insertion behavior will be characterized using CV and galvanostatic cycling with ion monitoring. CV experiments will be used to characterize the relative extent of insertion for both Li+ and Na+ and the insertion potentials. Meanwhile, galvanostatic cycling will be used to assess the connection between the CV results and charge/discharge plateaus observed under different Li2SO4/Na2SO4 ratios. In addition, ion monitoring will be performed every 50 cycles at relevant conditions (bottom and top of charge and intermediate points) to measure concentration changes in the electrolyte concentrations. Furthermore, galvanostatic intermittent titration cycles will be incorporated into the cycling regime to characterize the effect of concentration in the open circuit voltage (OCV) and resistance, as well as exploring changes in these variables as a function of cycling.

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