In an open-access paper in the journal ChemSusChem, a team at Penn State reports using cold sintering to reprocess solid-state composite electrolytes—specifically Mg- and Sr-doped Li₇La₃Zr₂O₁₂ with polypropylene carbonate (PPC) and lithium perchlorate (LLZO–PPC–LiClO₄). The low sintering temperature allows co-sintering of ceramics, polymers and lithium salts, leading to re-densification of the composite structures with reprocessing.

Reprocessed LLZO–PPC–LiClO₄ exhibits densified microstructures with ionic conductivities exceeding 10⁻⁴ S/cm at room temperature after 5 recycling cycles. All-solid-state lithium batteries fabricated with reprocessed electrolytes exhibit a high discharge capacity of 168 mA h g⁻¹ at 0.1 C, and retention of performance at 0.2 C for over 100 cycles.

Life cycle assessment (LCA) suggests that the recycled electrolyte outperforms the pristine electrolyte process in all environmental impact categories, highlighting cold sintering is a promising technology for recycling electrolytes.

—Lan et al.

Schematic illustration of different stages during reprocessing. Lan et al.

The researchers note that mechanical stress is the key factor leading to failure of solid-state electrolytes, despite modest breakdown at interfaces. Because solid-state electrolytes are in contact with lithium metal, interphases with restricted electron conductivity form. This hinders electron transport and limits the propagation of side reactions; hence, the interfacial layer is usually localized.

Cracks generated during charging and discharging cycles may propagate through the bulk phase, resulting in battery failure. The growth of lithium dendrites and the accumulation of isolated dead lithium causes a volume fluctuation of the cell, which induces internal stress and the formation of cracks and fractures in the microstructures of solid-state electrolytes. These structural defects in turn block lithium-ion transport pathways, leading to a high total impedance or eventually battery short-circuits,, the researchers explain.

In spite of the presence of these localized defects and fragmented microstructures, the vast majority of solid-state electrolytes remain active after cycling. Thus, once localized interfaces are eliminated, it is plausible that spent solid-state electrolytes may be recycled without undergoing sophisticated processes.

… we directly reprocessed LLZO–PPC–LiClO₄ composite electrolytes using cold sintering, and demonstrate recyclability with modest loss in ionic conductivity. Cold sintering, a low-temperature sintering technology (< 300 °C), is a process that enables the densification of composites comprised of ceramics, polymers, and salts. Driven by pressure and heat, densification happens with the assistance of a transient liquid phase, which functions as a medium to aid in the dissolution and regulate the precipitation and nucleation processes.

—Lan et al.

Resources

• Yi-Chen Lan, Masoud Ghasemi, Shelby L. Hall, Ryan A. Fair, Christina Maranas, Rui Shi, Enrique Daniel Gomez (2024) “Cold Sintering Enables the Reprocessing of LLZO-based Composites” ChemSusChem doi: 10.1002/cssc.202301920