The revival of room-temperature sodium-ion batteries
Due to the abundant sodium (Na) reserves in the Earth’s crust and to the similar physicochemical properties of sodium and lithium, sodium-based electrochemical energy storage holds significant promise for large-scale energy storage and grid development. For example, high-temperature zero emission battery research activity cells based on Na/NiCl2 systems and high-temperature Na–S cells, which are successful commercial cases of stationary and mobile applications, have already demonstrated the potential of sodium-based rechargeable batteries. However, their high operating temperature of around 300 °C causes security issues and decreases the round-trip efficiency of sodium-ion batteries (SIBs). Room-temperature (RT) SIBs are therefore widely regarded as the most promising alternative technology to LIBs.
Over the history of batteries in the past 200 years, research on SIBs was fervently carried out side-by-side with LIB development. The electrochemical activity of TiS2 for lithium and its feasibility for energy storage was first put forward in the 1970s. Following this discovery, the capability of Na ions to be inserted into TiS+2 was realized in the early 1980s. With the discovery of graphite as a low-cost and moderate-capacity anode material for LIBs and the failure to intercalate sodium ions, rapid LIB development occurred in the 1990s, superseding the growth in sodium chemistry. Then, in 2000, the availability for sodium storage in hard carbon (HC), which would deliver an energy capacity similar to that of Li in graphite, rejuvenated research interest in SIBs.
A comparison of Sodium-ion battery and Lithium-ion battery
The revival of SIBs—coupled with the ever-increasing pressure from the lack of availability of lithium reserves and the corresponding escalation in cost—provides a complementary strategy to LIBs. SIBs have gained increasing research attention, combined with fundamental achievements in materials science, in the drive to satisfy the increasing penetration of renewable energy technologies. The cell components and the electrochemical reaction mechanisms of SIBs are basically identical to those of LIBs, except for the charge carrier, which is Na in one and Li in the other. The major reason for the rapid expansion in SIB materials chemistry is ascribed to the parallels in physicochemical properties between the two alkali metals.
First, the operating principles and cell construction of SIBs are similar to those of commercial LIBs, albeit with Na serving as the charge carrier. Four main components exists in a typical SIB: a cathode material (usually a Na-containing compound); an anode material (not necessarily containing Na); an electrolyte (in a liquid or solid state); and a separator. During the charge process, sodium ions are extracted from the cathodes, which are typically layered metal oxides and polyanionic compounds, and are then inserted into the anodes , while the current travels via an external circuit in the opposite direction. When discharging, Na leaves the anodes and returns into the cathodes in a process referred to as “the rocking-chair principle.” These similarities have enabled the preliminary understanding of and rapid growth in SIB technology。
Moreover, the larger ionic radius of Na brings its own advantages: increased flexibility of electrochemical positivity and decreased de-solvation energy in polar solvents. The greater gap in the ionic radius between Li and the transition metal ions usually leads to failure of the flexibility of material design. In contrast, a sodium-based system enables more flexible solid structures than a lithium-based system, and possesses enormous ionic conductivity. A typical example is β-Al2O3, for which Na intercalationhas the perfect size and high conductivity. More layered transition metal oxides with different M+x+ stacking manners can be easily realized in a sodium-based system. Similarly, the wide variety of crystal structures that are known for sodium ionic conductor (NaSICON) family is much more complicated than that of the lithium analogs. More importantly, a much higher ionic conductivity can be allowed in NaSICON compounds, which by far exceeds the ionic conductivity in lithium ionic conductor (LiSICON) compounds.
Last but not least, systematic investigations with different aprotic polar solvents have demonstrated that the larger ionic radius of Na causes a weaker desolvation energy. The smaller Li has a higher surface charge density around the core than Na when both possess the same valence. Li is therefore thermodynamically stabilized by sharing more electrons with the polar solvent molecules. That is, Li can be classified as a type of Lewis acid. As a result, a relatively high desolvation energy is needed for the highly polarized Li, leading to a relatively large transfer resistance being induced by the transport of Li from the liquid state (electrolyte) to the solid state (electrode). Since the desolvation energy is closely related to the transfer kinetics occurring at the liquid/solid interface, the relatively low desolvation energy is a significant advantage for designing high-power SIBs.