This review summarizes the use of ionic liquids in Na secondary batteries and discusses their electrochemical performance with various electrode materials.

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Positive electrode materials with a wide operating temperature range and high energy and power densities are required for the construction of practical sodium secondary batteries. High performance is expected at elevated temperatures because electrode reactions and ion diffusion are enhanced under such conditions. In the present study, carbon‐coated Na superionic conductor‐type Na3V2(PO4)3 is prepared via a sol–gel method and investigated as a positive electrode material for sodium secondary batteries operating under low‐to‐intermediate temperature conditions. The Na3V2(PO4)3/C material utilizes fully the safety and large liquid‐phase temperature range of Na[bis(fluorosulfonyl)amide]–[1‐ethyl‐3‐methylimidazolium][bis(fluorosulfonyl)amide] ionic liquid electrolytes. Electrochemical testing of the Na3V2(PO4)3/C composite reveals superior cycling and rate performance at 253–363 K. Cycling tests at 1C (117 mA g−1) confirm capacity retention of 99% and Coulombic efficiency of over 99.9% after 300 cycles at 298 and 363 K. Even faster‐current cycling at 20C results in capacity retention of 89.2% after 5000 cycles at 363 K. The high rate capability at 363 K (50.1 mA h g−1 at 58500 mA g−1) leads to excellent power and energy densities that exceed those of NaCrO2 and Na2FeP2O7.

The limited cyclability and inferior Coulombic efficiency of graphite negative electrodes have been major impediments to their practical utilization in potassium-ion batteries (PIBs). Herein, for the first time, potassium difluorophosphate (KDFP) electrolyte additive is demonstrated as a viable solution to these bottlenecks by facilitating the formation of a stable and K +-conducting solid-electrolyte interphase (SEI) on graphite. The addition of 0.2 wt% KDFP to the electrolyte, results in significant improvements on the (de)potassiation kinetics, capacity retention (76.8% after 400 cycles with KDFP vs. 27.4% after 100 cycles without KDFP) as well as average Coulombic efficiency (~99.9 % during 400 cycles) of graphite electrode. Moreover, the KDFP-containing electrolyte also enables durable cycling of the K/K symmetric cell at higher efficiencies and lower interfacial resistance as opposed to the electrolyte without KDFP. X-ray diffraction and Raman spectroscopy analyses have confirmed the reversible formation of a phase-pure stage-1 potassium-graphite intercalation compound (KC 8) with the aid of KDFP. The enhanced electrochemical performance by the KDFP addition is discussed based on the analysis of the SEI layer on graphite and K metal electrodes by X-ray photoelectron spectroscopy.

Symmetric cell electrochemical impedance spectroscopy (SCEIS) is a powerful method to analyze electrode materials for secondary batteries. The EIS results are used to obtain information related to electrochemical processes such as charge-transfer resistance. In this study, SCEIS is employed to investigate the electrochemical performance of the Na2FeP2O7 positive electrode for sodium secondary batteries operating at temperatures ranging from room to intermediate temperatures using the ionic liquid (IL) electrolytes, Na[FSA]-[C2C1im][FSA] (ILFSA) (C2C1im = 1-ethyl-3methylimidazolium, FSA = bis(fluorosulfonyl)amide). The obtained SCEIS for Na metal, acetylene black, α-Al2O3, and V2O5 revealed that the resistance of the high-frequency region in the Nyquist plots is a combination of several factors (the Na[FSA] fraction, ionic conductivity of the electrolyte, and electronic conductivity of the composite electrode). The activation energies obtained by the Arrhenius plots for both the high-frequency and charge-transfer resistance of Na2FeP2O7/ILFSA/Na2FeP2O7 SCEIS showed that a significant decrease in the charge-transfer resistance contributes to the high rate performance in the intermediate temperature range.