The rapid research and technological improvement of sodium-ion batteries (NIBs) have made them the most practical complement to lithium-ion batteries (LIBs). Furthermore, high sodium ion diffusion kinetics provide several fast-charging electrode materials that are attractive for high-power applications. Reducing phase-to-phase and charge-transfer resistance through innovative electrolyte design, while not reducing lifetime, is critical to ensuring such applications. Here, we report the role of low-viscosity ester-based cosolvents in enhancing the conductivity of sodium-ion-based electrolytes. Our new electrolyte formulation has shown excellent power capacity, charging 18650 cells to 84% state-of-charge (SOC) in about 10 minutes. Furthermore, it improves low-temperature cycling performance but slightly degrades high-temperature performance compared with our co-solvent-free electrolyte. We believe that the guidelines adopted here will pave the way to find a better compromise between ultra-fast charging and high-temperature applications for optimal performance.
Lithium-ion batteries (LIBs) have played a pivotal role in the electrification of transportation and enabling smarter grids in recent years [1], [2], [3], [4]. Improvements in manufacturing techniques and supply chain establishment have reduced the cost of Li-ion batteries from over $1000/kWh to around $200/kWh [5]. However, progress in manufacturing is reaching its limit [5,6], and the sudden surge in the price of Li-based precursors [7,8] calls into question the pure reliance on Li-based batteries for electrochemical energy storage. Furthermore, the growing demand for low-cost batteries raises concerns about lithium resource availability and geopolitical independence [6,9]. To this end, sodium-ion batteries (NIBs) are emerging as attractive supplemental energy storage devices due to the low cost and population neutrality of the sodium precursor [9,10].

Additionally, Fast Charging (FC) is the new holy grail of acetate charge battery technology advancement. In 2018, the U.S. Department of Energy allocated more than $19 million to enable ultra-fast charging (XFC), with the goal of charging electric vehicles in 15 minutes or less by 2028 [11]. In this context, Na-ion batteries may become more important because some Na-ion electrodes and electrolytes have been reported to exhibit faster Na-ion diffusion kinetics than Li-ion batteries [12]. In particular, cathodes such as Na3V2(PO4)2F3 (NVPF) [13] and Prussian blue analogue (PBA) [14] are capable of intercalating (de)sodiumizing at rates up to 10C (full capacity within 6 min). On the other hand, hard carbon (HC), the most commonly used Na-ion negative electrode, exhibits indirectly better rate performance than its Li counterpart graphite due to its slightly higher redox potential [15]. The higher redox potential of hard carbons allows Na insertion at a faster rate without causing competing Na plating processes.