Stabilizing Lithium Metal Anode by Ion Depletion-Induced Phase Transformation in Polymer Electrolytes

Ion depletion in liquid electrolytes is widely accepted to promote dendrite growth in metal anodes due to enhanced local electrical field and magnified concentration fluctuation at the electrode/electrolyte interface. Here we report unexpected opposite behaviors in solid polymer electrolytes, showing that ion depletion leads to uniform lithium deposition. Such stabilization originates from ion depletion-induced phase transformation, which forms a new PEO-rich but salt/plasticizer-poor phase at the lithium/electrolyte interface, as unveiled by stimulated Raman scattering microscopy. This new phase leads a significantly higher Young’s modulus (~2-3 GPa) than the bulk polymer electrolyte (< 10 MPa), which effectively suppresses dendrite growth. Further battery tests show that LiFePO₄/PEO/Li cells with such ion depletion-induced phase transformations can be reversibly cycled for 200 times, while cells without such transformation fail within only ten cycles, demonstrating the effectiveness of this strategy to stabilize the lithium anode.