Park Chung Bin, Sung Bong June
Department of Chemistry, Sogang University, Seoul 04107, Republic of Korea.
Phys Chem Chem Phys. 2022 Oct 27;24(41):25171-25181. doi: 10.1039/d2cp02689a.
Various dopant alkali ions have been introduced into organic ionic plastic crystals (OIPCs) in order to design solid electrolytes with the desired thermal stability and ionic conductivity. We performed extensive molecular dynamics simulations to investigate at the molecular level how dopant alkali ions affect the rotational and the translational diffusion of ions and the thermal stability of OIPCs. We introduced lithium (Li), sodium (Na), and potassium (K) ions as dopants into 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF]) OIPCs at the molecular level. We found that as smaller alkali ions are doped, larger domains of the crystals are disrupted. This makes it harder for OIPCs doped with smaller alkali ions to maintain their crystal structure such that the melting temperature of OIPCs decreases and phase transitions between rotator phases change. The size of dopant alkali ions also affects the rotational diffusion of matrix ions of [MMIM] and PF: the rotational diffusion of matrix ions near Li ions becomes more heterogeneous and facilitated than those near other kinds of alkali ions. We also find that alkali ions of different kinds diffuse translationally in OIPCs different transport mechanisms: while the Li ion undergoes continuous (anion-associated) diffusion through an amorphous region, the K ion hops between neighbor lattice sites. To investigate the effects of the relative size between matrix cations and dopant ions on translational diffusions, we also simulate OIPCs with longer alkyl chains such as 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF]) crystals. We find that as the size of imidazolium cations increases, the hopping diffusion of the K ion becomes suppressed and the K ion is more likely to diffuse through amorphous domains.
为了设计出具有所需热稳定性和离子电导率的固体电解质，人们已将各种掺杂碱金属离子引入有机离子塑性晶体（OIPC）中。我们进行了广泛的分子动力学模拟，以在分子水平上研究掺杂碱金属离子如何影响离子的旋转和平动扩散以及OIPC的热稳定性。我们在分子水平上将锂（Li）、钠（Na）和钾（K）离子作为掺杂剂引入1-甲基-3-甲基咪唑六氟磷酸盐（[MMIM][PF]）OIPC中。我们发现，随着较小的碱金属离子被掺杂，晶体的较大区域会被破坏。这使得掺杂较小碱金属离子的OIPC更难保持其晶体结构，从而导致OIPC的熔点降低，转子相之间的相变发生变化。掺杂碱金属离子的大小也会影响[MMIM]和PF基质离子的旋转扩散：Li离子附近的基质离子的旋转扩散比其他种类碱金属离子附近的基质离子的旋转扩散更加不均匀且更容易。我们还发现，不同种类的碱金属离子在OIPC中以不同的传输机制进行平动扩散：Li离子通过无定形区域进行连续（与阴离子相关）扩散，而K离子在相邻晶格位点之间跳跃。为了研究基质阳离子和掺杂离子之间的相对大小对平动扩散的影响，我们还模拟了具有更长烷基链的OIPC，如1-乙基-3-甲基咪唑六氟磷酸盐（[EMIM][PF]）和1-丁基-3-甲基咪唑六氟磷酸盐（[BMIM][PF]）晶体。我们发现，随着咪唑阳离子大小的增加，K离子的跳跃扩散受到抑制，K离子更有可能通过无定形区域扩散。
