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.

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.

Spontaneous and field-induced crystallographic reorientation of metal electrodeposits at battery anodes

The propensity of metal anodes of contemporary interest (e.g., Li, Al, Na, and Zn) to form non-planar, dendritic morphologies during battery charging is a fundamental barrier to achievement of full reversibility. We experimentally investigate the origins of dendritic electrodeposition of Zn, Cu, and Li in a three-electrode electrochemical cell bounded at one end by a rotating disc electrode. We find that the classical picture of ion depletion–induced growth of dendrites is valid in dilute electrolytes but is essentially irrelevant in the concentrated (≥1 M) electrolytes typically used in rechargeable batteries. Using Zn as an example, we find that ion depletion at the mass transport limit may be overcome by spontaneous reorientation of Zn crystallites from orientations parallel to the electrode surface to dominantly homeotropic orientations, which appear to facilitate contact with cations outside the depletion layer. This chemotaxis-like process causes obvious texturing and increases the porosity of metal electrodeposits.