Sulfolane-Based Flame-Retardant Electrolyte for High-Voltage Sodium-Ion Batteries

Abstract

Sodium-ion batteries hold great promise as next-generation energy storage systems. However, the high instability of the electrode/electrolyte interphase during cycling has seriously hindered the development of SIBs. In particular, an unstable cathode–electrolyte interphase (CEI) leads to successive electrolyte side reactions, transition metal leaching and rapid capacity decay, which tends to be exacerbated under high-voltage conditions. Therefore, constructing dense and stable CEIs are crucial for high-performance SIBs. This work reports localized high-concentration electrolyte by incorporating a highly oxidation-resistant sulfolane solvent with non-solvent diluent 1H, 1H, 5H-octafluoropentyl-1, 1, 2, 2-tetrafluoroethyl ether, which exhibited excellent oxidative stability and was able to form thin, dense and homogeneous CEI. The excellent CEI enabled the O3-type layered oxide cathode NaNi1/3Mn1/3Fe1/3O2 (NaNMF) to achieve stable cycling, with a capacity retention of 79.48% after 300 cycles at 1 C and 81.15% after 400 cycles at 2 C with a high charging voltage of 4.2 V. In addition, its nonflammable nature enhances the safety of SIBs. This work provides a viable pathway for the application of sulfolane-based electrolytes on SIBs and the design of next-generation high-voltage electrolytes.

1. Introduction

Sodium-ion batteries (SIBs) have become a promising alternative energy source for emerging energy storage devices due to the abundant content of sodium in the earth’s crust (23,000 ppm sodium versus 17 ppm lithium) and low cost (e.g., the cost of Na2CO3 is about 25–30 times lower than that of Li2CO3) [1–3]. Since lithium and sodium are both alkali metals and have similar chemical and electrochemical properties, both academia and industry acknowledge the importance of SIBs and actively seek to use existing lithium-ion battery technology to industrialize SIBs [4, 5].

However, compared with lithium ions (0.76 Å), the larger ionic radius of sodium ions (1.02 Å) will cause various structural evolutions during the Na+ insertion/extraction process, leading to problems such as deterioration of the host crystal structure and poor cycling stability [6]. At the same time, the larger atomic weight of Na (23 g mol−1) compared to Li (6.9 g mol−1), along with the higher standard electrochemical potential of Na (2.71 V for Na+/Na vs. 3.04 V for Li+/Li), poses challenges to SIBs in achieving comparable energy density to lithium-ion batteries [7]. Layered oxide cathode materials have been widely studied due to their high energy density and facile large-scale preparation [8–10]. Compared with most P2-NaxTMO2 cathodes (x varies between 0.6 and 0.8), the initial structure of O3-type NaxTMO2 (x is usually 1) is very similar to LiCoO2, and it can provide more capacity in the same voltage range [11–13]. Despite the high capacity, these materials exhibit complex phase transformations during the Na+ deintercalation process, leading to rapid capacity fading [14, 15]. Simultaneously, the cathode undergoes surface remodeling or reduction of the transition metal oxidation state in contact with the electrolyte, which indicates a charge transfer between the positive electrode and the electrolyte, a phenomenon that is more intense at high voltage [16–18]. An unstable cathode–electrolyte interphase (CEI) can lead to continuous electrolyte decomposition and transition metal dissolution during cycling [19, 20]. Therefore, it is important to implement strategies such as optimizing the electrolyte and