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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP The separator in a conventional battery presents intriguing interfaces with electrolyte as well. It may further be desirable to conceptualize and advance interfaces or interphases that can switch between different states, chemical or physical, so as to block or facilitate the transport of specific energy carriers within a cell component on demand.62 For example, can we create responsive membranes that adapt their transport selectivity for ions vs. dissolved active materials when the active materials are present at a specific state of charge? Doing so could dramatically improve the efficiency of energy storage devices, particularly those implementing flow or conversion electrodes. A remarkable example of redox-switchable ion selectivity by a responsive polymer membrane was recently reported. It bears resemblance to how the transport of ions across biological cell membranes is regulated by transmembrane proteins (Figure 2.3.13).76 The ion-transporting character of transmembrane proteins is sensitive to the environment; any perturbations to that environment are typically met with an adaptive response. An analogous strategy was used to achieve adaptive ion transport in microporous polymer membranes while in a lithium–sulfur battery.81 Along the polymer backbone were placed redox-active switches that were activated in situ by the battery’s dissolved polysulfides as they entered the membrane’s pores. This transformation had little influence on the membrane’s ionic conductivity; however, the polysulfide- blocking ability of the membrane was enhanced. In turn, these membranes offered the cell improved capacity retention, energy efficiency, and cycle life by sequestering dissolved polysulfides in the cathode. a LiTFSI Li2Sn Adaptive Ion Transport b c 30 20 10 0 40 LiTFSI Reduction by Li2Sn Li2Sn Non-Selective Selective Adaptive 0 5 10 15 Time (h) LiTFSI Reduction by Li2Sn 7 Å Pores Size Sieving Li2Sn Supported Membrane Figure 2.3.13. Example of redox-switchable ion selectivity by a responsive polymer membrane. (a,b) The ion-transport selectivity of membranes cast from polymers of intrinsic microporosity (top right inset) can be enhanced to the benefit of Li–S battery cycle life when redox-switchable phenazine- containing monomers are activated in situ (inset at bottom) by reducing polysulfides that are endogenous to the cell. (c) This leads to a feedback loop whereby progressive reduction of the membrane by polysulfides only further restricts their access to the membrane’s pore voids, which slow the rate of polysulfide crossover in the cell relative to non-transformable size-selective membranes and conventional, non-selective Celgard separators. From Ref. 76. Designing and Controlling Synthesis and Fabrication of the Tailored SEI: It may also be possible to intentionally reconfigure an interphase through its controlled dissolution-precipitation, interface reconstruction, amorphous-crystalline phase transformations, or transitions between rigid and elastic states. For example, can we create an ionically conductive and electronically insulating interphase on metal anodes that is capable of self-repair indefinitely after interphase-disrupting dendrite-forming events? Doing so would resolve a long- standing challenge associated with the continuous consumption of interphase-forming agents added to the electrolyte, which are depleted rapidly and thus have diminishing influence on anode stability. Furthermore, can we synthesize an elastic and ionically conductive interphase that can accommodate extreme volume changes during cycling and, in turn, rigidifies or becomes ionically insulating when a dendrite emerges at the electrode surface? Doing so would enable a dual-responsive mechanical blocking ability while also starving the dendrite of its constituent ions and thus stunting its growth. Our ability to control the spatiotemporal aspects of these reconfiguring events will likewise be critical to achieving a specific type of adaptive behavior in response to an excursion or perturbation in the system. Self-repairing or self-rectifying interphases may be realized when coupled to metal anodes. They represent the most efficient use of mass and volume in an energy storage device as the penalty of a host material (e.g., graphite, Si, or Sn) is eliminated. Rather than rely on an intrinsic SEI for protection, methods for creating 50 PRIORITY RESEARCH DIRECTION – 3 Redox Switchable Unit Charge Blocking Concentration (mM)PDF Image | Next Generation Electrical Energy Storage
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