It is only slightly more than 50 years since the important paper of Whittingham and Huggins discussing the solid electrolyte Na-ion conductor beta-alumina was published [1]. This was a very instrumental paper for the research at Uppsala University and for our start as battery researchers. Again, Na-ion batteries and solid-state Na-ion conductors have become important topics to enable the realization of Na-ion batteries.
This presentation will discuss both Na-ion battery electrode materials and Na-ion solid electrolytes as well as interface chemistries in Na-ion battery full cells. While ceramic Na-ion conductors attract the largest attention in the global research community, our group is studying solid polymer electrolytes such as high-molecular-weight poly(trimethylene carbonate) (PTMC) but also completely new polyester-polycarbonate (PCL–PTMC) copolymers, both containing the NaFSI salt.
The new PCL–PTMC:NaFSI system has glass transition temperatures ranging from −64 to −11 °C which is increasing with increasing salt content, from 0 to 35wt%. The ionic conductivities at 25°C is ranging from 10−8 to 10−5 S cm−1. The optimal salt concentration is dependent on the level of crystallinity of the co-polymer, which is largely determined by the CL content. At 70 and 80mol% CL, the PCL–PTMC:NaFSI system is fully amorphous and exhibited high conductivities at lower salt concentrations. When the CL content increases to 100 mol%, high ionic conductivities is observed for high salt concentrations.
We are focusing of low cost Na-ion batteries based on abundant raw materials. In our case, it means iron-based cathodes vs. hard carbon anodes. Na2−xFe(Fe(CN)6) all-solid-state polymer electrolyte full cell was assembled to demonstrate the practical application of the material and cycled for more than 120 cycles at ∼22°C. Na2−xFe(Fe(CN)6 is generally called Prussian Blue. This compound has also attracted the attention of Prof. Huggins in his more recent work [2]. The properties of Prussian Blue compounds are that they are simple to prepare, that they can be made in many different crystalline structures and that they can show high rate performance in aqueous electrolyte systems. The capacity obtained is not that high and the capacity fading is also large in the aqueous electrolyte. Solid polymer electrolytes could therefore be one route for more stable and long-cycling Na-ion batteries with Prussian Blue cathodes.
Building on these developments we here present how, by a salt substitution from NaTFSI to NaFSI, it is possible to attain significant improvements in battery stability for solid-state Na-ion cells with PTMC-based electrolytes. A high capacity of >90 mAhg–2 of Prussian blue for an excess of 80 cycles is demonstrated at 60 °C, which is superior to other solid polymer electrolyte based Na-batteries demonstrated in scientific literature. We further show the effects of salt concentration on both conductivity and battery performance to reach the conclusion that a high conductivity is far from the only factor contributing to long-term battery cycling performance but also how interfaces to the electrode materials are stabilized.
The interfacial characteristics of these systems are also discussed in the presentation.
Despite the early and very comprehensive work done fifty years ago, there is still a need for new innovative solid electrolyte Na-ion conductors to reach the goal of sustainable and low-cost Na-ion batteries with high efficiency.
References:
[1] M.S. Whittingham and R.A. Huggins, Solid State Chemistry, National Bureau of Standards, October 18-21, 1971
[2] M. Pasta, C.D. Wessells, R.A. Huggins, Yi Cui, Nature Com. 3 (2012) 1149