Chemomechanical Failure of Solid Composite Cathodes Accelerated by High-Strain Anodes in All-Solid-State Batteries
Authors: Kang Junhee , Kim Beomsu , Yun Jonghyeok , Lee Jong-Won 
Published: February 2024

Lithium-ion batteries (LIBs) have played a major role in energy storage, particularly, for portable electronic devices. However, the energy density of the LIB has reached its limit, which is not high enough to fulfill an ever-growing demand for long-lasting batteries. More importantly, safety issues arising from flammable liquid electrolytes are considered critical problems to be addressed for large-scale batteries. All-solid-state batteries (ASSBs) with non-flammable solid electrolytes (SEs) have been emerging as a promising alternative to liquid-electrolyte-based LIBs. Among various types of SEs, sulfide-based materials, e.g., Li6PS5Cl (LPSCl), have gained much attention, owing to their high ionic conductivity at room temperature.

To develop ASSBs with high energy density, Li metal/alloys or Li-free Ag-C composites have been widely studied as anode materials; however, they undergo large volume changes during repeated charge-discharge cycling. In addition to the morphological and mechanical degradation of the anode itself, massive volume changes exert additional mechanical stresses on the cell components and interfaces between them, which can cause accelerated performance decay. While a few studies have been reported on the mechanical deformation of the SE layers induced by the volume changes of anodes, possible degradation of the composite cathodes has been largely overlooked.

Herein, we present a comparative study of ASSBs assembled with high-strain (Li-In) and zero-strain (Li4Ti5O12 (LTO)) anodes to understand the impact of anode volume changes on chemomechanical degradation behaviors of solid composite cathodes. An ASSB cell was composed of polycrystalline NCA-LPSCl composite cathode, LPSCl separator layer, and either Li-In or LTO anode. The Li-In-based cell suffered from severe capacity loss after 120 cycles, whereas the LTO-based cell showed a stable capacity retention over 200 cycles. Impedance decoupling via three-electrode set-up indicated rapid increases in interfacial and diffusion (Warburg) impedances of the composite cathode in the Li-In-based cell during cycling, as compared with those in the LTO-based cell.

In-depth chemical, electrochemical, and microstructural analyses revealed that the high-strain Li-In anode perturbs the structural integrity of the composite cathode and facilitates “dynamic” contacts between the cathode constituents upon repeated cycling. This leads to the enhanced parasitic interfacial reactions, as evidenced by the increased amount of resistive phases (e.g., P2S
x
, Li2S, and PO4
3-) in the cathode of the Li-In-based cell. The resulting chemically/electrochemically inhomogeneous interfaces between NCA and LPSCl cause localized concentrations of diffusion-induced stress within NCA, leading to the accelerated cracking of NCA aggregates. This study highlights the accelerated degradation phenomena of composite cathodes driven by high-strain anodes and provides insights into the design of ASSBs with long cycle lifetimes.