High Cation Transference Number Polymer Electrolytes Improve Capacity Retention in High Energy Density Solid-State Batteries

Solid-state batteries are a promising alternative to conventional lithium-ion batteries, because of their potential to deliver improved safety, high energy, and power density. In a solid-state battery’s cathode, the active particles are surrounded by solid electrolytes to facilitate ion transport in and out of the cathode. The discharge capacity and cycle life from such an all-solid-state battery depends on the quality of the contact between the cathode particles and the solid electrolyte, the chemical and electrochemical stability as well as the transport properties of the solid electrolyte. To maximize capacity retention, all three aspects need to be optimized.

In this work, we analyze the capacity fade mechanisms of a series of polymer-based solid-state batteries, by investigating the quality of contact, thickness of the cathode electrolyte interphase (CEI), as well as the polymer electrolytes’ transport properties. Using a solvent-free infiltration method, we demonstrate that intimate contact between the polymer electrolyte and the cathode can be achieved. Cryogenic transmission electron microscopy (cryo-TEM) results indicate that the quality of contact does not change as cycling goes on. Cryo-TEM further reveals the presence of a very thin (a few nanometers) CEI layer. Comparing a high Li⁺ transference number (t₊ = 0.75) polymer electrolyte and a low t₊ one, the high t₊ polymer electrolyte shows dramatically improved capacity retention in thick cathode with loadings above 10 mg/cm² while the two electrolytes show comparable capacity retention in the thin cathode with loadings around 2 mg/cm². Continuum scale modeling shows that concentration gradient plays a important role in the state of charge of the cathode particles in the thick cathode near the current collector. The high t₊ polymer electrolyte can alleviate such concentration gradient and improve capacity retention.

Acknowledgements: This research was sponsored by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy for the Vehicle Technologies Office’s Advanced Battery Materials Research Program (Simon Thompson, Program Manager).