Metal organic frameworks to transform the cyclability of metal-sulfur batteries

Metal sulfur batteries hold much promise as batteries due to their high potential energy density that are up to 5 times more energy dense than established "rocking chair" metal-ion batteries. However, metal-sulfur batteries, such as Li/S materials are compromised by their cyclability, as capacity fades quickly due to polysulfide species that attack the metal anode and then react with the sulfurous cathode, leading to metal sulfide species that poison the battery. Separators (typically glass fiber) are unable to eliminate the migration of species from cathode to anode; however we propose an important dual strategy modification to mitigate this. In this work, we will seek to coat separators with a metal organic framework (MOF) to precisely control the flow of species (S42- - S82-) and additionally use the MOF internal surface (specifically, undercoordinated metal sites) to trap labile polymeric sulphide species (especially S2- - S32-). We believe this novel dual-pronged attack provides a highly achievable mechanism for dramatically improving the lifetime of metal-sulfur batteries which is closely aligned to key EPSRC challenges in electrochemical sciences and computational and theoretical chemistry.

We will use a combination of theoretical approaches, to guide laboratory work with the expectation of a feedback loop between theory and experiment. We will focus on Na/K-S and Na/K-Se batteries and the polysulfide species during battery cycling, where using simulation, we will identify promising combinations of MOF films on glass fibre separators using established forcefields such as MOF-UFF or QuickFF. We already have extensive experience of mining and screening MOF databases3 and this will be used to identify MOFs that have narrow pores, that will hinder the migration of polymeric Sn2-(especically S42- - S82-) , NaxSy or KxSy species through the coated separator. The student will perform experiments to assess the distribution of oligomeric Sn2- in a MOF-free cell disperse MOFs in solution and then coat the separator and then vacuum filter to generate the coated separator. The coated separator will then be placed in the cell and cycling experiments will be undertaken to assess the performance of the material, feeding back to the predictions from simulation. In the latter stages of the work, we will seek to identify MOFs with undercoordinated sites to trap polysulphide species using screening and density functional theory (DFT). Once a ranked list of candidate has been identified, the coated separator will be assessed for its effectiveness and longevity in situ.

Overall the aim is build a protype metal sulfur battery with an energy density that surpasses current typical metal-ion batteries with lifetime that greatly exceeds current metal sulfur batteries which typically last for just a few cycles. In principle, this project could have a dramatic effect on the battery field and help to get away from relatively scarce metals that will be exhausted in a few decades.