Sulfide Solid Electrolyte Viscoplasticity and Interface Contact Mechanics
Sulfide solid-state cells require continuous external stack pressure between 5 and 10 MPa to overcome viscoplastic voiding and maintain intimate interface contact.
Thiophosphate
Meaning
Phosphorus and sulfur based compounds constitute the chemical group used for solid state electrolytes in advanced lithium batteries. Sulfur based crystalline or glassy materials provide a pathway for lithium ions to move through a solid medium. Within the family of solid electrolytes, a thiophosphate offers some of the highest ionic conductivities reported for non liquid systems.
Safety and energy density both improve.
Ionic Conductivity
Lithium ions hop between sulfur sites in the lattice with very little resistance. The ionic conductivity of thiophosphate is often comparable to that of liquid electrolytes at room temperature. This high mobility is due to the large and polarizable nature of the sulfur atoms.
Faster charging becomes possible in a solid state format.
Interface Resistance
Stability against the lithium metal anode is a primary goal for any new electrolyte candidate. The interface resistance of thiophosphate varies depending on the specific ratio of elements like germanium or tin. Some variants form a stable layer that prevents the growth of dendrites.
This protects the cell from internal shorts.
Handling Requirement
Exposure to moisture leads to the generation of hydrogen sulfide gas and the degradation of the ionic path. Because of the handling requirement of thiophosphate, all manufacturing steps must occur in a high purity argon environment. Processing costs are higher than for traditional oxides.
Hermetic sealing is mandatory for the final product. Surface moisture can destroy the electrolyte structure within seconds of exposure. Specialized gloveboxes and moisture sensors are used throughout the assembly line to prevent any contact with air.