Effect of salt concentration profiles on protrusion growth in lithium-polymer‑lithium cells

• Published in: Solid state ionics Vol. 358; p. 115517
• Main Authors: Frenck, Louise, Veeraraghavan, Vijay D., Maslyn, Jacqueline A., Müller, Alexander, Ho, Alec S., Loo, Whitney S., Minor, Andrew M., Balsara, Nitash P.
• Format: Journal Article
• Language: English
• Published: United States Elsevier B.V 15.12.2020; Elsevier
• Subjects: Concentrated Solution Theory; Electrochemical properties; ENERGY STORAGE; Lithium; Polymer electrolyte; Rechargeable batteries; Salt concentration; Salt Concentration Gradient; Polymer electrolyte; Electrochemical properties; Concentrated Solution Theory; Lithium; Salt concentration; Salt Concentration Gradient; Rechargeable batteries
• ISSN: 0167-2738; 1872-7689
• DOI: 10.1016/j.ssi.2020.115517

The formation of dendrites and other protrusions on lithium metal anodes is a subject of continued interest due to the potential to incorporate these anodes in next-generation rechargeable batteries with increased energy densities. Solid polymer electrolytes show improved stability against lithium metal compared to liquid carbonate electrolytes. We have studied the effect of salt concentration on the formation of protrusions formed on electrodeposited lithium through a rigid block copolymer electrolyte, polystyrene-block-poly(ethylene oxide) (PS-b-PEO or SEO), in a lithium‑lithium symmetric cell. The cell lifetime decreases by a factor of 100 when salt concentration is increased by a factor of 5. Our main objective is to understand the reason for this observation. We show that this decrease is not due to a salt-induced change of the morphology of the block-copolymer electrolyte, nor is it due to a salt-induced change of mechanical properties. We use an approach based on Newman's concentrated solution theory to fully characterize ion transport in the block-copolymer electrolyte, and report the conductivity, salt diffusion coefficient, cation transference number, and thermodynamic factor. Neither cell lifetime nor protrusion density in failed cells correlate with any of these electrochemical parameters. However, the electrochemical parameters can be used to predict salt concentration profiles in our symmetric cells. We posit that an important parameter in protrusion growth is the magnitude of the salt concentration gradient, ∆. We observe a direct correlation between ∆ and lithium protrusion growth. •The lifetime of lithium-lithium symmetric cells decreases by a factor of 100 when salt concentration is increased by a factor of 5.•The decrease of cell lifetime is not due to salt-induced morphological change nor salt-induced mechanical change.•Concentrated solution theory was used to fully characterize ion-transport in the block copolymer electrolyte.•Prediction of salt concentration profiles in the lithium-lithium symmetric cells were made.•We observe a direct correlation between the magnitude of the salt concentration gradient and lithium protrusion growth.