Enabling Anode-Free Sodium Metal Batteries with a Boron Cluster Electrolyte

The so-called anode-free cell configuration presents an advanced approach to increase energy density beyond intrinsic Na metal anode cells due to in-situ formation of the alkali metal film which is decoupled from initial anode weights. Simultaneously, this technology inherently overcomes the challenges and safety concerns associated with the processing of highly reactive materials during cell assemblies. The availability of sodium precursor supply chains and the natural high crustal abundance suggests that the anode-free sodium battery (AFSB) is a very attractive chemistry. However, longevity (long, stable cycling) issues associated with coulombic inefficiency, side reactions, and dendritic sodium nucleation represents a barrier to realizing this technology.

The highly reversible electrochemistry demanded of these systems is as intriguing from a fundamental perspective as they are practical. Currently, AFSB electrochemistry requires some novel chemical and/or architectural current collector modification to enhance the efficiency of metal electro-plating/stripping and sustain cycling over long periods of time. To this end, engineered sodiophilic (‘sodium-loving’) surfaces have proven a successful strategy to promote favorable metal nucleation and cycling. However, little variation in electrolyte chemistries beyond NaPF₆/glyme compositions have been adequately explored.

Here, we exploit the benefits of incorporating the [HCB₉H₉]^- carborane cluster as the anionic electrolyte component in AFSBs. We find the fluorine-free carboranyl electrolyte, with its inherent delocalized negative charge and non-ion pairing behavior, imparts substantial improvements over NaPF₆ electrolytes without the need to introduce engineered current collector surfaces. The stand-alone electrochemical and structural characterization of the electrolyte and its unique fundamental properties will be presented. This includes mechanistic studies on the sodium nucleation characteristics and surface passivation chemistry imparted by the anion and the ethereal solvent. Substantial performance improvements in a high cathode loading full cell are demonstrated that suggests electrolyte design is one of the most important facets in implementing AFSBs.