Extensive Sodium Metal Plating and Stripping in a Highly Concentrated Inorganic−Organic Ionic Liquid Electrolyte through Surface Pretreatment

Unveiling the Impact of the Cations and Anions in Ionic Liquid/Glyme Hybrid Electrolytes for Na-O2 Batteries

A series of hybrid electrolytes composed of diglyme and ionic liquids (ILs) have been investigated for Na-O₂ batteries, as a strategy to control the growth and purity of the discharge products during battery operation. The dependence of chemical composition of the ILs on the size, purity, and distribution of the discharge products has been evaluated using a wide range of experimental and spectroscopic techniques. The morphology and composition of the discharge products found in the Na-O₂ cells have a complex dependence on the physicochemical properties of the electrolyte as well as the speciation of the Na⁺ and superoxide radical anion. All of these factors control the nucleation and growth phenomena as well as electrolyte stability. Smaller discharge particle sizes and largely homogeneous (2.7 ± 0.5 μm) sodium superoxide (NaO₂) crystals with only 9% of side products were found in the hybrid electrolyte containing the pyrrolidinium IL with a linear alkyl chain. The long-term cyclability of Na-O₂ batteries with high Coulombic efficiency (>90%) was obtained for this electrolyte with fewer side products (20 cycles at 0.5 mA h cm^-2). In contrast, rapid failure was observed with the use of the phosphonium-based electrolyte, which strongly stabilizes the superoxide anion. A high discharge capacity (4.46 mA h cm^-2) was obtained for the hybrid electrolyte containing the pyrrolidinium-based IL bearing a linear alkyl chain with a slightly lower value (3.11 mA h cm^-2) being obtained when the hybrid electrolyte contained similar pyrrolidinium-based IL bearing an alkoxy chain.

SEI Formation on Sodium Metal Electrodes in Superconcentrated Ionic Liquid Electrolytes and the Effect of Additive Water

We have previously reported that water addition (∼1000 ppm) to an N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (C₃mpyrFSI) superconcentrated ionic liquid electrolyte (50 mol % NaFSI) promoted the formation of a favorable solid electrolyte interphase (SEI) and resulted in enhanced cycling stability. This study reports the characterization of Na-metal anode surfaces cycled with these electrolytes containing different water concentrations (up to 5000 ppm). Morphological and spectroscopic characterization showed that water addition greatly influences the formation of the SEI and that ∼1000 ppm of water promoted the formation of an active and more uniform deposit, with larger quantities of SEI species (S, O, F, and N) present. Water addition to the electrolyte system is also proposed to promote the formation of a new complex between the FSI anions, water molecules, and sodium cations as components of the SEI. For both dry and wet (∼1000 ppm) electrolytes, the SEIs were mainly composed of NaF, metal oxide (i.e., Na₂O), and the complex, suggested to be Na₂[SO₃-N-SO₂F]·nH₂O (n = 0-2). Postcycling SEM analysis of the Na-metal electrodes after extensive cycling (500 cycles, 1.0 mA·cm^-2, 1.0 mA·.cm^-2) was used to estimate the minimal average cycling efficiency (ACE), which was enhanced by water addition: up to ∼99% for the 1000 ppm cell compared to ∼98% for the dry cell. Two distinct deposit morphologies, a microporous and a compact layer deposit, were evident after extended cycling in the wet and dry electrolytes. The presence of both the microporous and compact layer deposits on Na-metal surfaces cycled with the wet electrolyte, along with the distinct chemistry and morphology of the SEI, all contributed to a more stable symmetric cell voltage profile and lower cell polarization. In contrast, a higher fraction of microporous deposits and the absence of compact layer formation in the dry electrolyte were associated with higher cell polarization potentials and the occurrence of dendrites.

High Coulombic Efficiency Na-O2 Batteries Enabled by a Bilayer Ionogel/Ionic Liquid

Sodium-oxygen (Na-O₂) cells are a promising high energy density storage technology with a theoretical specific energy of 1605 Wh kg^-1. However, this technology faces certain challenges in order to achieve both a high practical energy density as well as long-term cycling capability. In this Letter, a superior Coulombic cyclic efficiency, close to 100%, has been demonstrated by the use of a bilayer electrolyte composed of an ionogel and an ionic liquid electrolyte, reported herein for the first time. The presence of the ionogel plays a major role in the prevention of side reactions originating at the anode, providing a promising route to extend cell cycling, whereas the ionic liquid is essential to support high reaction rates at the cathode.

Anion amphiprotic ionic liquids as protic electrolyte matrices allowing sodium metal plating

Competition between hydrogen bonding and sodium coordination enables sodium metal plating from anion amphiprotic ionic liquids.

Synthesis and physicochemical characterization of room temperature ionic liquids and their application in sodium ion batteries

Sodium ion batteries (SIBs) based on IL electrolytes have attracted great attention, particularly in large-scale energy storage systems for renewable energy due to the abundance of sodium and the excellent safety resulting from the use of non-flammable ionic liquid (IL) electrolytes. In this article, a series of 15 functionalized room temperature ionic liquids (RTILs) suitable as electrolytes is presented. Special emphasis was laid on the purity of the synthesized RTILs and a consistent and uniform characterization of their physicochemical properties. Evaluation of the viscosity, conductivity, and thermal and electrochemical stabilities resulted in clear structure-property relationships, rendering the ether functionalized RTILs most promising for application in SIBs. Electrochemical investigations of the ether functionalized IL electrolytes in SIB half cells (Na0.6Mn0.9Co0.1O2 as cathode material) proved their compatibility with a SIB system. Stable cycling performance was achieved with the piperidinium based RTIL IL 6 outperforming the organic electrolyte by far with a retention of 81% after 350 cycles. These results show the suitability of RTILs to enhance the performance of SIB systems and serve as a basis for the design of high performance IL electrolytes.

Variations and applications of the oxygen reduction reaction in ionic liquids

Increasing energy demands call for new energy storage technologies with high energy density to meet current and future needs. Metal-air batteries are especially attractive due to their superior specific energy, which is as much as 8 times that of today's best Li-ion batteries. However, the practical values achieved to date are far from the theoretical one and require further research to enhance the battery performance. The electrolyte plays an important role in the performance of the battery whose properties can be tuned by varying the chemical composition and through the use of additives. That is the case of ionic liquids which offer a wide variety of anion-cation combinations to realise maximum performance. This feature article overviews recent developments in ionic liquids as electrolytes for both magnesium-air and sodium-air batteries.

Solid-State Lithium Conductors for Lithium Metal Batteries Based on Electrospun Nanofiber/Plastic Crystal Composites

Organic ionic plastic crystals (OIPCs) are a class of solid-state electrolytes with good thermal stability, non-flammability, non-volatility, and good electrochemical stability. When prepared in a composite with electrospun polyvinylidene fluoride (PVdF) nanofibers, a 1:1 mixture of the OIPC N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C₂ mpyr][FSI]) and lithium bis(fluorosulfonyl)imide (LiFSI) produced a free-standing, robust solid-state electrolyte. These high-concentration Li-containing electrolyte membranes had a transference number of 0.37(±0.02) and supported stable lithium symmetric-cell cycling at a current density of 0.13 mA cm^-2 . The effect of incorporating PVdF in the Li-containing plastic crystal was investigated for different ratios of PVdF and [Li][FSI]/[C₂ mpyr][FSI]. In addition, Li|LiNi_1/3 Co_1/3 Mn_1/3 O₂ cells were prepared and cycled at ambient temperature and displayed a good rate performance and stability.