X-Ray Microtomography Studies of Lithium Batteries with Polymer Electrolytes

Lithium metal anodes have the potential to enable rechargeable batteries with specific energy that is significantly higher than that of current lithium-ion batteries.  It is believed that a prominent failure mode in these batteries is due to growth of dendrites on the anode.  We demonstrate that the use of block copolymer electrolytes is an effective approach for slowing down dendrite growth. Efforts to solve the lithium dendrite problem have focused on preventing the growth of protrusions from the anode surface. We use synchrotron hard X-ray microtomography to directly probe the morphological changes that occur in batteries with lithium metal anodes.  Studies on symmetric lithium-polymer-lithium cells show that during the early stage of dendrite development, the bulk of the dendritic structure lies within the electrode, underneath the polymer-electrode interface. Furthermore, we observed crystalline impurities at the base of the subsurface structures. The portion of the dendrite protruding into the electrolyte increases upon cycling until it spans the electrolyte thickness causing a short circuit. Contrary to conventional wisdom, it appears that preventing dendrite formation in polymer electrolytes depends on inhibiting the formation of subsurface structures in the lithium electrode.  In contrast, studies on full cells with a lithium metal anode and a conventional lithium cathode show an entirely different failure mechanism.