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Sodium ion batteries (SIBs) have resurfaced into the spotlight, given the supply chain uncertainties and the soaring demand for lithium-ion batteries (LIBs). Although, even now, their lower energy density may stall their commercialization in the portable sector, they are considered prime candidates for large scale electrochemical energy storage applications. Accordingly, advancing, establishing, and maintaining the safety of SIBs is crucial to prevent catastrophic thermal runaways and colossal financial losses to garner the trust of concerned authorities. Electrolytes play a pivotal role in the safety of batteries. Considering the above, this paper presents a comprehensive review of the progress in safe electrolytes for SIBs. It explains the various approaches employed to enhance the safety of high-risk based electrolytes and the electrochemical performance of intrinsically safe electrolytes. Moreover, a state-of-the-art review of the assembled cells/half cells employing different classes of electrolytes is also presented. Particular attention has been devoted to specifying the techniques and results, if available, of thermal stability and safety tests besides highlighting the electrochemical characteristics and performance, such as the cell capacity and cyclability, and electrolyte ionic conductivity and electrochemical stability window (ESW) of the electrolyte. Finally, challenges and future research directions have been summarized and recommended. This review concludes that solid state electrolytes with high conductivity are among the practical and safe electrolytes for SIBs.

The binary metal oxides show advantages in energy storage devices. Specifically, nickel cobaltite (NiCo2O4) materials showed promising pseudocapacitive properties, high electrical conductivity and large surface area by virtue of their effective porous structure. NiCo2O4nanosheets were hydrothermally grown in this work over flexible tapes of Aluminum (Al) and Copper (Cu). A nanosheets structure obtained of NiCo2O4as confirmed by SEM and AFM images. The measured thickness by 3D profilometer of NiCo2O4nanosheets based Al framework found to be 4.3 µm compared to 8.4 µm thick of film based-Cu framework. Asymmetric supercapacitor prepared from graphite and NiCo2O4electrodes separated by filter paper. Acidic aqueous electrolyte of H2SO4and basic aqueous electrolyte of KOH were employed to verify the cyclic activity and electrochemical reaction of asymmetric prepared supercapacitor devices. The basic KOH electrolyte shows a high stability and better charge transfer/ionic diffusion compared to the acidic H2SO4electrolyte in particular for NiCo2O4film-based Cu framework. The energy density and power density values were 0.9 W h kg−1and 66.45 W kg−1, respectively. The highest specific capacity (in F.g−1) = 10.09 coincides with NiCo2O4/Cu supercapacitor in the basic KOH electrolyte. The charge storage in the supercapacitor system of NiCo2O4and graphite can be ascribed in the form of Faradic charge transfer and capacitive non-faradic double layer, respectively.

A nanocomposite (nanoclay/conductive polymer) was prepared by a template-free method for supercapacitor applications. Halloysite Nanotubes (HNTs) were used as a nanoclay material, and synthesized polyaniline (PANI) by ammonium persulfate-initiated polymerization was used as a conductive polymer. The HNTs/PANI nanocomposite was deposited on an indium tin oxide (ITO) coated glass to obtain a working electrode by an electrochemical deposition method. Morphological and structural examination of the HNTs/PANI nanocomposite proved that the PANI nanoparticles were attached to the HNTs surface. The structural analysis demonstrated that the PANI and HNTs crystal size significantly decreased in the HNTs/PANI as a result of the composition of PANI molecules with HNTs structure. The main characteristic bonds and functional groups of HNT, PANI, and HNTs/PANI nanocomposites were determined by FT-IR analysis. Using cyclic voltammetry (CV), the electrochemical performance of the HNTs//PANI nanocomposite electrode was investigated. The specific capacitance values were reported via the CV curves as 264, 230.22, 175.2, and 154.8 F/g at scan rates of 50, 100, 200, and 300 mV/s respectively. The results showed that the specific capacitance at the slow scan rates revealed higher maximum specific capacitance values.