Toward a Better Understanding and Optimization of the Electrochemical Activity of Na-Ion TiO2 Anatase Anodes Using Uniform Nanostructures and Ionic Liquid Electrolytes

TiO2 anatase has emerged as a promising anode for Na-ion batteries (SIBs). However, widespread use of this anode is severely limited by a series of factors that need to be identified and understood to further improve their electrochemical response. Here, we have taken benefit from the versatility of a self-assembly seeding-assisted method to obtain a variety of uniform high-surface-area undoped TiO2 anatase nanostructures. Electrodes built from these uniform nanostructures in combination with a safe ionic liquid electrolyte have allowed a systematic study on some of the factors that determine the electrochemical activity of Na-ion anatase anodes. Interestingly, the inherent low penetrability of the ionic liquid electrolyte has resulted in an unexpected asset to clarify large differences in Na+ uptake by different nanostructures. Basically, solid electrolyte interface (SEI) effects were maximized and therefore clearly separated from electrochemical reactions strictly associated with the anatase anode. Thus, for electrodes built from nanostructures that preserved their initial conformation after cycling, the first discharge showed Na+ uptakes well-beyond those of the Ti4+/Ti3+ redox couple. This large uptake has been associated with an apparent reversible reaction that operates below ca. 0.5-0.7 V and an irreversible mechanism that operates at lower voltages (ca. 0.3 V). However, for electrodes built from nanostructures that favored SEI formation, the irreversible reaction associated with the plateau at ca. 0.3 V was not observed during the first discharge. In accordance, the total Na+ uptake did not reach values beyond those of the corresponding Ti4+/Ti3+ redox couple. Irreversibility, in this case, is associated with SEI formation. Our results also establish the strong effect that size at different scale levels has in the electrochemical response of anatase anodes for SIBs (changes from ca. 6 to 11 nm in crystal sizes and from 50 to 80 in nanostructure sizes led to pronounced differences). This result emphasizes that any conclusions on mechanistic studies other than size effects must be done under strict control on size at various scales (size as a strict control variable at crystal level and nanostructure or in more general terms aggregate scale levels). Finally, we have found that at 30 and 60 degrees C the performance of the best of the electrodes, with the low-flammable and low-volatile ionic liquid electrolyte, is comparable to that of similar nanostructures immersed in their Li-ion electrolyte counterparts. This result is promising, as in stationary applications where SIBs could replace Li-ion batteries, large accumulation of storage components imposes more strict safety criteria. Basically, power criteria can be relaxed in response to more strict safety criteria.