Sodium-ion batteries (SIBs) is one of the more promising battery technologies that are starting to see commercialisation, were the low cost, abundance of necessary raw materials, higher safety and power and similar energy densities compared to lithium-ion batteries (LIBs) are often mentioned as their key advantages. SIBs, due to their great similarities with LIBs have seen a rapid development. One of the key differences, however, is the carbonaceous negative electrode, where SIBs, due to Na⁺ not forming stable intercalation compounds with graphite, the anode of choice for LIBs, are forces to use hard carbons. But, it was recently discovered by Jache et al.¹ that Na⁺ can form stable ternary graphite intercalation compounds through a solvent co-intercalation mechanism, thus enabling the use of graphite in SIBs. This sparked several follow-up studies exploring solvent co-intercalation as a way to use graphite in SIBs², but the question of how many solvent molecules are intercalated along with the cation, and thus the stoichiometry of the redox reaction, was never settled. Here, we present a novel electrochemical method to directly measure the number of solvents that are brought along by the cation into the graphite host structure.

The SIBs using the co-intercalation mechanism to enable the use of graphite has so far shown great kinetics, power densities and extreme cyclability, with great capacity retention over thousands of cycles². They are, however, plagued by a low specific capacity, a great volume expansion, and high average voltage in half-cells vs. Na⁺/Na, leading to poor energy density. But, the knowledge of how many glymes entering the graphite host gained from our measurements enabled us to rationally design new electrolytes, composed of low amounts of glymes mixed with another co-solvent, that both increases the energy density of the system, by lowering the average voltage while retaining the capacity, and reduces the volume expansion, while also reducing the cost of the electrolyte (Figure 1). The impact of the new electrolytes on the structure of graphite is studies using operando dilatometry and XRD, while the local structure in the electrolyte is investigated with ab initio methods – together giving a comprehensive view of the system.

Figure 1. Voltage profile of new electrolytes, showing a lowering of the average voltage a second plateau, and snap shot from ab initio molecular dynamics.

References:

1. B. Jache, P. Adelhelm, Angew. Chem. Int. Ed. 2014, 53, 10169-10173, DOI: 10.1002/anie.201403734
2. J. Park, Z-L. Xu, K. Kang, Front. Chem., 2020, 8:432, DOI: 10.3389/fchem.2020.00432