Stabilizing lithium plating in polymer electrolytes by concentration-polarization-induced phase transformation

It is widely accepted that concentration polarization in liquid electrolytes promotes whisker growth during metal deposition, and therefore, high salt concentration is favored. Here, we report unexpected opposite behaviors in solid polymer electrolytes: concentration polarization can induce phase transformation in a polyethylene oxide (PEO) electrolyte, forming a new PEO-rich but salt-/plasticizer-poor phase at the lithium/electrolyte interface, as unveiled by stimulated Raman scattering microscopy. The new phase has a significantly higher Young’s modulus (∼1–3 GPa) than a bulk polymer electrolyte (<1 MPa). We hereby propose a design rule for PEO electrolytes: their compositions should be near the boundary between single-phase and two-phase regions in the phase diagram so that the applied current can induce the formation of a mechanically rigid PEO-rich phase to suppress lithium whiskers. LiFePO4/PEO/Li cells with concentration-polarization-induced phase transformation can be reversibly cycled 100 times, while cells without such transformation fail within 10 cycles, demonstrating the effectiveness of this strategy.