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Abstract
All solid-state lithium batteries (ASLBs) offer the potential for higher power and energy density than traditional lithium-ion batteries and could be inherently safer due to solid-state electrolytes (SSEs) replacing flammable organic liquid electrolytes.
Achieving the high energy density targets in ASLBs will require thick cathodes optimized for full utilization of active material. Composite cathodes in ASLBs are comprised of cathode active material (CAM), SSE, and potentially carbon additive. Well-engineered electrodes require properly balanced CAM content, cathode thickness, effective electronic conductivity, and effective ionic conductivity. Electrode microstructure (material distribution, particle size distribution, percolation pathways, void spaces), chemo-mechanics (volume changes during cycling), SSE decomposition, and interphase formation are all important design considerations in sulfide-based ASLB composite cathodes. The current distribution, i.e. the inhomogeneous rate of electrochemical reaction, plays a large role in composite cathode design.
This subject of this work is understanding particle contacts within sulfide-based composite cathodes. Contacts between CAM particles define the conduction pathways for electron (e-) transport. Contacts between SSE particles define the conduction pathways for lithium-ion (Li+) transport. Contact points between CAM and SSE define the locations where the electrochemical charge transfer reaction can occur. These types of ASLBs are cycled under high compression (~50 MPa) to maintain the quality of point contacts, and this presents the challenge that any ex situ materials characterization of the composite cathodes will not be representative of the in situ cathode under compression. Because the CAM and the anode both experience a volume change during battery cycling, the point contacts are expected to evolve during operation. In order to study these effects of the composite cathode microstructure on performance, we use operando and in situ electrochemical and materials characterization methods to determine transport and kinetic parameters within operating cells. Energy dispersive X-ray diffraction (EDXRD) is used in conjunction with electrochemical impedance spectroscopy (EIS) and transmission line modeling (TLM) to compare electrode microstructure modeling with actual current distributions within ASLBs. This allows effective values of tortuosity factor and contact area to be calculated. These values are a consequence of the cathode microstructure but can be known without detailed microscale knowledge of individual particles.
