Voltage, stability and diffusion barrier differences between sodium-ion and lithium-ion intercalation materials

Abstract

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 AMO 2 and AMS 2 structures, the olivine and maricite AMPO 4 structures, and the NA-SICON A 3 V 2 (PO 4 ) 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, ∆V Na-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 ∆V Na-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 AMPO 4 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.

Role of Na+ Interstitials and Dopants in Enhancing the Na+ Conductivity of the Cubic Na3PS4 Superionic Conductor

“…In this article, we performed a first-principles investigation of the phase stability and Na + ionic transport in undoped and M 4+ -doped (M = Si, Ge, and Sn) c-Na 3 PS 4 . We demonstrate that pristine c-Na 3 PS 4 is in fact an extremely poor ionic conductor, but with the introduction of Na + interstitials, a reasonably high conductivity can be achieved.…”

Computational studies of solid-state alkali conduction in rechargeable alkali-ion batteries

“…For example, in layered structures, such as the AMO 2 cathode and graphite anode, 45,135 it is clear that the interlayer spacing has a substantial effect on the alkali migration barriers, whereas the relationship between alkali ion radii and bottleneck size seems to have a larger role in rigid close-packed anion frameworks (e.g., olivine LiMPO 4 ). 11 In addition to migration barriers, the concentration of carriers is another factor contributing to overall ionic conductivity. In this respect, experimental composition scanning is generally still a more rapid mean of determining optimal dopant concentrations because of the inherent difficulty in investigating small concentration changes using computational methods.…”

“…62 However, the maricite structure is uninteresting for battery applications given the lack of viable alkali diffusion pathways in the structure. Ong et al 11 previously performed a comparison of the alkali migration barriers in the olivine and layered structures using NEB calculations (Figure 7). They found that the Na migration barrier is slightly higher than the Li migration barrier in the olivine structure.…”

“…Indeed, even the reasons for the different conductivities exhibited by different alkali ions (e.g., Li + and Na + ) are not well understood. Certainly, the significantly larger size of Na + (102 pm) compared with Li + (76 pm) could be expected to mean lower migration barriers for Li + in bottle-neckconstrained crystals, such as the anion-close-packed olivine, but there is both experimental 27 and theoretical 11 evidence of more facile Na + transport in the more open-layered and NAtrium SuperIonic CONductor (NASICON)-type structures. Furthermore, despite their chemical similarity, Na-ion chemistry can exhibit different crystal structures from Li-ion chemistry.…”

“…With one of the highest energy densities of known energy storage technologies, Li-ion batteries are finding increasingly large-scale applications in electrified transportation and grid storage. In recent years, there has also been a resurgence of interest in rechargeable sodium-ion (Na-ion) batteries, [6][7][8][9][10][11][12] which function on the same basic principle but with Na + instead of Li + because of concerns regarding the abundance and cost of lithium. Figure 1 shows a representation of a rechargeable alkali-ion battery.…”