The development of highly conductive and safe electrolytes for sodium-ion batteries is an emerging field beyond lithium battery technologies. In this work we have developed new ionogel electrolytes consisting of a binary mixture of an organic ionic plastic crystal, N-ethyl-N-methylpyrrolidiniumbis(fluorosulfonyl)imide (C2mpyrFSI), mixed with NaFSI supported on a mat of electrospun poly (vinylidene fluoride) nanofibers. The salt mixture near the eutectic composition (35 mol% NaFSI) was selected for further study after a detailed phase diagram analysis and ionogel electrolytes based on this were prepared. The ionic conductivity of the prepared ionogel composite reaches 2.6 × 10−3 S cm−1 at ambient temperature. This ionogel membrane possessed a relatively high Na-ion transference number of 0.44 at 50 °C and we demonstrate the performance of a Na metal full cell using a NaFePO4 cathode (1.75–4.0 V). The assembled cells show a good capacity retention with coulombic efficiency close to 100% at various C rates between C/20, C/10 and C/5, achieving 120 mAh g−1 at C/20. The long term charge/discharge performance is also demonstrated. Our study provides a feasible method to prepare highly conductive ionogel electrolytes for future Na-battery applications

Solvent-free solid polymer electrolytes (SPE) are gaining more attention to develop postlithium battery technologies due to the safety and performance benefits of solid-state batteries. In this work, we present a new SPE for a sodium metal battery based on high salt concentration polymer electrolyte membranes comprising mixed anions, polymerized ionic liquid (PIL), block copolymer (BCP) polystyrene-b-poly­(diallydimethylammonium)­bis­(trifluoromethanesulfonyl)­imide-b-polystyrene (PS-b-PDADMATFSI-b-PS) and NaFSI salt. The maximum salt concentration incorporated was up to 1:2 mol ratio (PIL block: NaFSI). The ionic conductivity was 10–3 S cm–1 at 70 °C for 1:2 composition, and the anion diffusion as measured by 19F NMR decreased. FTIR measurement indicates that the ion coordination in the polymer–salt mixtures changes with composition. The storage modulus as measured by dynamic mechanical analysis (DMA) was observed in the range 300 MPa at −40 °C to 35.8 MPa at 70 °C. The optimized electrolyte (1:2 mol ratio) membrane was investigated for its long-term stability against Na metal cycling with Na/Na symmetrical cells demonstrating stable Na plating/stripping behavior at 0.2 mA cm–2 at 70 °C. Finally, an Na|NaFePO4 cell cycled with a specific capacity of 118 mAh g–1 at C-rate C/20 at 70 °C and a good Coulombic efficiency (98%), showing the promising potential of these solvent-free triblock copolymer electrolytes in Na metal batteries.

Sodium ion batteries are widely considered to be a feasible, cost-effective, and sustainable energy storage alternative to Lithium, especially for large-scale energy storage applications. Next generation, safer electrolytes based on ionic liquid (IL) and organic ionic plastic crystals (OIPCs) have been demonstrated as electrochemically stable systems which show superior performance in both Li and Na applications. In particular, phosphonium‐based systems outperform most studied nitrogen‐based ILs and OIPCs. In this study triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide ([P1i444][FSI]) OIPC mixed with 20 mol% of NaFSI or NaTFSI were combined with an electrospun polyvinylidene fluoride (PVDF) support to create self-standing electrolyte membranes, and their thermal phase behaviour and ionic conductivity were investigated and compared with the bulk electrolytes. The ability of the solid-state composite electrolytes to support the cycling of sodium metal with good efficiency and without breakdown were examined in sodium metal symmetrical coin cells. The sodium transference number was determined to be 0.21. The electrochemical performance of Na/Na3V2(PO4)3 cells incorporating the composite electrolytes, including good cycling stability and rate capability, is also reported. Interestingly, the mixed anion systems appear to outperform the composite electrolyte containing only FSI anions, which may relate to electrolyte interactions with the PVDF fibres.

We discuss here the ion transport mechanism of a gel electrolyte comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solvated by two plastic crystalline solvents, one a solid (succinonitrile, abbreviated as SN) and another (a room temperature ionic liquid) (1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, (abbreviated as IL) confined inside a linear network of poly(methyl methacrylate) (PMMA). The concentration of the IL component (x) determines the physical properties of the unconfined electrolyte (i.e., SNIL-LiTFSI) and when confined inside the polymer network (GPE-x). The extent of disorder in the SNIL-LiTFSI and the GPE-x electrolytes is enhanced compared to both the bare SN-LiTFSI and IL-LiTFSI electrolytes. The enhanced disordering in the plastic phase alters both the local ion environment and viscosity. These changes strongly influence the ion mobility and nature of predominant charge carriers and thus the ion conduction mechanism in SNIL-LiTFSI and GPE-x. The proposed SNIL-LiTFSI and the GPE-x electrolytes show predominantly anion conduction (t ≈ 0.5); however, lithium transference number (t ≈ 0.2) is nearly an order higher than the IL-LiTFSI (t ≈ 0.02-0.06). The ionic conductivity of SNIL-LiTFSI is much higher (especially for x ≈ 0.1) compared to both SN-LiTFSI and IL-LiTFSI. The ionic conductivity of the GPE-x, though lower than the unconfined SNIL-LiTFSI electrolytes, is still very promising, displaying values of ∼10 Ω cm. The GPE-x displayed compliable mechanical properties, stable Li-electrode/electrolyte interface, low rate of Al corrosion, and stable cyclability over several tens of charge-discharge cycles when assembled in a separator-free Li-graphite cell. The promising electrochemical performance further justifies the simple strategy of employing mixed physical state plasticizers to tune the physical properties of polymer electrolytes requisite for application in rechargeable batteries.

There is still a sustained effort to explore and develop new electrode materials for Li‐ion rechargeable batteries. Presently, materials exploration not only focuses on the enhancement of Li‐ion battery performance, but also targets to make them cheaper and safer. This expectedly will make them market competitive against established battery chemistries. Graphitic carbon nitride (g‐C3N4), which has emerged as an important photon harvesting material, is demonstrated here as a potential efficient and cost effective alternative anode for Li‐ion cells. The g‐C3N4, synthesized here from pyrolysis of thiourea, possesses both graphitic and amorphous phases (T‐gCN). The intercalation of Li+ ions in the densely packed layered structure of T‐gCN (lithiated T‐gCN) results in an ionic conductivity ≈ 10−7 S/cm compared to the non‐lithiated T‐gCN which shows no ionic conductivity. The ionic transport takes place via both the amorphous and graphitic phases in T‐gCN. The T‐gCN when treated with an acidified dichromate solution disintegrates in to filaments, which on prolonged stirring self‐assemble into pillar‐like g‐C3N4 structures (T‐gCNP). The T‐gCNP exhibited higher crystallinity and an even higher ionic conductivity compared to the T‐gCN. The T‐gCN and T‐gCNP deliver modest specific capacities compared to battery grade graphite and other reported carbonaceous/non‐carbonaceous materials in the half‐cell operation. However, when coupled with cathodes such as LiFePO4 and LiMn2O4 in a full Li‐ion cell, the specific capacity obtained in the 1st discharge cycle for both LiFePO4 and LiMn2O4 are very close to their theoretical capacities. The cells display stable cycling and good current rate capability over widely varying current values. This is remarkable in spite of the fact that the samples are not completely crystalline with an average carbon content of approximately 31%. It is envisaged that a continuous network persists for electron transport for their participation in the reversible redox process.