Rate Limitations in Composite Solid-State Battery Electrodes: Revealing Heterogeneity with Operando Microscopy

Abstract

Solid-state batteries (SSBs) show promise for improving energy density, cycle life, and safety. However, when active material particles are mixed with a solid electrolyte phase, the rate capability of the resulting composite electrode is often limited. As a consequence, tradeoffs between energy and power density arise, especially in thick electrodes. Herein, we fabricate graphite/Li 6 PS 5 Cl composite electrodes with varying active material fraction and thickness as model systems to probe the mechanisms that limit rate capability in composite SSB electrodes. Using operando optical microscopy, spatial variations in the local state-of-charge of graphite that arise as a result of current focusing are directly observed. Pairing these results with simulations, we identify the electrode properties that limit rate performance, including the electrostatic potential drop within the tortuous solid electrolyte phase and solid-state diffusion within the graphite domains. The results highlight the critical role of microstructure in designing composite SSB cathodes and anodes.

The Coupling between Voltage Profiles and Mechanical Deformations in LAGP Solid Electrolyte During Li Plating and Stripping

“…However, the magnitude of the strains in the neighboring areas increased with increasing current density. This demonstrates the impact of the early non-uniform deformations on the spatial distribution of Li plating and stripping on the solid electrolyteelectrode interface 10,21 .…”

Challenges and Opportunities for Fast Charging of Solid-State Lithium Metal Batteries

“…The morphological stability of the anode and electrochemical performance of the SSB depend on the kinetics of transport and reaction, and hence the thermal environment in the vicinity of different solid–solid interfaces within the system. Temperature-dependent mechanisms such as vacancy diffusion and viscoplasticity (creep) of lithium play a pivotal role in preserving the anode morphology during electrochemical operation. − Additionally, further enhancement in ionic transport within the solid electrolyte has also been considered beneficial toward achieving homogenized reaction distributions at the anode interface and improved ionic percolation within the cathode microstructure. , …”

“…With an increase in active material loading to achieve higher energy densities, ion percolation limitations within the cathode architecture become more critical. In this context, recent studies have reported a decrease in rate performance at higher active material loading (∼80 wt%). ,, This has been attributed to a higher electrostatic potential (IR) drop within the composite electrode structure, which results in spatial heterogeneity in state-of-charge throughout the electrode thickness . Therefore, energy and power density trade-offs exist in composite cathodes in SSBs, analogous to Li-ion batteries.…”

“…84−86 Additionally, further enhancement in ionic transport within the solid electrolyte has also been considered beneficial toward achieving homogenized reaction distributions at the anode interface 87 and improved ionic percolation within the cathode microstructure. 20,88 Operating the SSB at elevated temperatures that can thermally activate the kinetics of mechanisms such as ionic transport within the electrolyte/cathode, vacancy transport within lithium, and lithium creep behavior theoretically offers significant advantages in terms of interface stability, reduced internal resistance, and improved cell performance. However, low-temperature operation of the SSB would result in reduced kinetics of such transport mechanisms, potentially having a deleterious effect on interface behavior and performance.…”

“…In this context, recent studies have reported a decrease in rate performance at higher active material loading (∼80 wt%). 88,124,125 This has been attributed to a higher electrostatic potential (IR) drop within the composite electrode structure, which results in spatial heterogeneity in state-of-charge throughout the electrode thickness. 88 Therefore, energy and power density trade-offs exist in composite cathodes in SSBs, analogous to Li-ion batteries.…”