The Development of Organic Ionic Plastic Crystals As Solid State Electrolytes for Energy Storage

Organic ionic plastic crystals (OIPCs) are a class of solid-state electrolyte material quite unique in their mechanical and transport properties; they combine the advantages of fast ion conduction with good stability and non-volatility even above their melting point. Significant progress has been made recently in the development of OIPCs for electrochemical devices such as lithium batteries and dye-sensitised solar cells.[1] The negligible volatility of OIPCs renders them more suitable than molecular plastic crystals for long-term device use, while the high thermal and electrochemical stability of many OIPCs fulfils an essential requirement for solid state electrolytes for many device applications.

Understanding the ion transport behaviour of OIPCs, both in their pure and doped state (i.e. in the presence of a small amount of a lithium salt, to enable their use in lithium batteries), is crucial for their development as solid electrolytes.

Here we present our investigations into the ion transport mechanism in these materials using both experimental data (single crystal XRD, multinuclear solid-state NMR, DSC, ionic conductivity and SEM) and theoretical techniques (molecular dynamics and second moment-based solid static NMR line width simulations).[2][3]

We also discuss the physical properties and application of the OIPC triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide (P₁₄₄₄FSI) doped with 4 mol% LiFSI. The good conductivity of this material (0.26 mS cm^-1 at 22 °C) enables cycling of lithium cells (Li|LiFePO₄) at room temperature, achieving about 160 mA h g^-1 discharge capacity at 20 ^oC, which is the highest for OIPC based cells to date.[4]

References

[1] J. M. Pringle, Phys. Chem. Chem. Phys., 2013, 15, 1339

[2] L. Jin, K. M. Nairn, C. M. Forsyth, A. J. Seeber, D. R. MacFarlane, P. C. Howlett, M. Forsyth and J. M. Pringle, J. Am. Chem. Soc., 2012, 134, 9688-9697.

[3] L. Jin, S. de Leeuw, M. V. Koudriachova, J. M. Pringle, P. C. Howlett, F. Chen, M. Forsyth, Phys. Chem. Chem. Phys., 2013, 15, 19570-19574.

[4] L. Jin, P. C. Howlett, J. M. Pringle, J. Janikowski, M. Armand, D. R. MacFarlane and M. Forsyth, Energy and Environ. Sci., 2014, 7, 3352-3361.