Voltage, Stability and Diffusion Barrier Differences between Sodium-ion and Lithium-ion Intercalation Materials

To evaluate the potential of Na-ion batteries, we contrast in this work the difference between Na-ion and Li-ion based intercalation chemistries in terms of three key battery properties – voltage, phase stability and diffusion barriers. The compounds investigated comprise the layered AMO2 and AMS2 structures, the olivine and maricite AMPO4 structures, and the NASICON A3V2(PO4)3 structures. The calculated Na voltages for the compounds investigated are 0.18-0.57 V lower than that of the corresponding Li voltages, in agreement with previous experimental data. We believe the observed lower voltages for Na compounds are predominantly a cathodic effect related to the much smaller energy gain from inserting Na into the host structure compared to inserting Li. We also found a relatively strong dependence of battery properties with structural features. In general, the difference between the Na and Li voltage of the same compound, ∆VNa-Li, is less negative for the maricite structures preferred by Na, and ∗To whom correspondence should be addressed 1 more negative for the olivine structures preferred by Li. The layered compounds have the most negative ∆VNa-Li. In terms of phase stability, we found that open structures, such as the layered and NASICON structures that are better able to accommodate the larger Na+ ion generally have both Na and Li versions of the same compound. For the close-packed AMPO4 structures, our results show that Na generally prefers the maricite structure, while Li prefers the olivine structure, in agreement with previous experimental work. We also found surprising evidence that the barriers for Na+ migration can potentially be lower than that for Li+ migration in the layered structures. Overall, our findings indicate that Na-ion systems can be competitive with Li-ion systems. Introduction Rechargeable lithium-ion (Li-ion) batteries1–4 have become a mainstay of the digital age with extensive applications in portable electronics. With Li-ion battery technology poised to move into larger scale applications such as plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs), much research has targeted the development and optimization of lithium-ion batteries, in particular, the development of cathodes with higher energy and power densities. The typical cathode in a Li-ion battery is an intercalation compound, which as the name implies, store Li+ ions by inserting them into their crystal structure in a topotactic manner. Current cathodes are typically lithium transition-metal oxides or chalcogenides, which contain interstitial sites that can be occupied by Li+. The insertion of each Li+ is accompanied by the concomitant reduction of a transition metal ion to accommodate the compensating electron. The earliest commercial intercalation cathode can be traced back to the work of Whittingham,2 who first demonstrated electrochemical activity in layered LiTiS2 in the 1970s. However, that material had too low a voltage to be commercially useful and was superseded by layered LiCoO2 in the 1980s.3 Though LiCoO2 and its substituted variants currently dominate the world market in lithium batteries, other promising cathode materials, such as the spinel LiMn2O4 5,6 and the olivine LiMPO4 materials,4 have emerged and are increasingly finding adoption, especially in applications