Abstract
Na batteries are seen as a feasible alternative technology to lithium ion batteries due to the greater abundance of sodium and potentially similar electrochemical behavior. In this work, mixed phase electrolyte materials based on solid‐state compositions of a trimethylisobutylphosphonium (P111i4) bis(trifluromethanesulphonyl)amide (NTf2) organic ionic plastic crystal (OIPC) and high concentration of NaNTf2 that support safe, sodium metal electrochemistry are demonstrated. A Na symmetric cell can be cycled efficiently, even in the solid state (at 50 °C and 60 °C), for a 25 mol% (P111i4NTf2)–75 mol% NaNTf2 composition at 0.1 mA cm−2 for 100 cycles. Thus, these mixed phase materials can be potentially used in Na‐based devices under moderate temperature conditions. It is also investigated that the phase behavior, conductivity, and electrochemical properties of mixtures of NaNTf2 with this OIPC. It is observed that these mixtures have complex phase behavior. For high compositions of the Na salt, the materials are solid at room temperature and retain a soft solid consistency even at 50 °C with remarkably high conductivity, approaching that of the pure ionic liquid at 50 °C, i.e., 10−3–10−2 S cm−1.
Highlights
Organic ionic plastic crystals are a class of materials that have recently been shown to be good candidates as solid state electrolytes for electrochemical devices including lithium batteries,[1,2,3,4,5,6,7,8,9] solar cells,[10,11,12] and fuel cells.[13,14,15]
The pure P111i4NTf2 presents as phase I over a wide range of temperatures from -48 ̊C to 40 ̊C, this relatively high remaining melting entropy for the pure organic ionic plastic crystal (OIPC) is higher than the Timmermans' criterion for molecular plastic crystals,[16] the degree of plasticity and ion dynamics in this system may not be as high as those OIPC materials such as P1444FSI,[9] P1224PF6,[32] and P1224SCN,[36] with final entropy of melting significantly less than 20 Jmol‐ 1K‐1
A two−step increase in conductivity is observed in 20, 25 and 50 mol%; the first conductivity step is attributed to the solid−solid phase transition as we have just discussed and the second is related to the peritectic transition. 25 and 50 mol% exhibit a higher ionic conductivity at temperatures lower than 36 C compared to the other measured compositions
Summary
Organic ionic plastic crystals are a class of materials that have recently been shown to be good candidates as solid state electrolytes for electrochemical devices including lithium batteries,[1,2,3,4,5,6,7,8,9] solar cells,[10,11,12] and fuel cells.[13,14,15] The term ‘plastic’ reflects the high degree of mobility, and deformability under low applied stress, that is observed in these materials in one or more of the submelting phases. Plastic crystals have one or more solid-solid phase transitions before the final melt with the final entropy of melting (∆Sf) being less than 20kJmol-1 as defined by Timmermans’ criterion.[16] The presence of more disorder in the higher temperature OIPC phases, results in the creation of vacancies and extended defects that lead to the diffusional motions that result in the plasticity. These diffusive motions lead to significant ionic conductivity which make these materials excellent candidates as safe electrolytes for electrochemical devices. A combination of differential scanning calorimetry (DSC), conductivity, scanning electron microscope (SEM), Nuclear magnetic resonance spectroscopy (NMR) and electrochemical methods are employed to investigate the phase behaviour, transport and electrochemistry in these new materials
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