Solid electrolytes (SEs) are expected to enable safe lithium-metal batteries with increased energy density compared to conventional lithium-ion batteries. Therefore, they are particularly interesting for the application in electric vehicles. However, lithium solid-state batteries face a number of challenges, hindering their widespread use to date. The exact challenges depend on the chemistry of the SEs. For example, the ionic conductivity of polymer electrolytes is typically insufficient, and their Young’s Modulus is too low to efficiently suppress lithium dendrite growth. In contrast, inorganic SEs are stiff and consequently difficult to process. To achieve dense electrolyte layers and limit lithium dendrite growth, they require high fabrication pressure. High pressure is also needed upon cycling to maintain good contact between the inorganic electrolyte and the electrodes. Furthermore, the inorganic electrolytes with the highest conductivity (sulfides) release toxic H2S gas when they come into contact with moisture.
In this work, sulfide electrolyte particles are functionalized with polymer coatings with the aim to reduce moisture sensitivity (H2S generation) and to enable lower fabrication and cell pressure compared to the pristine sulfide electrolyte while maintaining high conductivity and efficient suppression of lithium dendrite growth. The electrolytes are tailored via the choice of the polymers in a way that they exhibit high chemical and electrochemical stability towards both the cathode and the lithium anode. The effect of the polymer coatings is evaluated via pressure- and time-dependent impedance spectroscopy, linear sweep voltammetry, and the measurement of the H2S generation in an atmosphere with a defined humidity. The synergistic effects of the sulfide/polymer combination shall eventually lead to safe lithium-metal batteries with high energy density and long cycle life.
While one of the polymer coatings tested (FUNCY-SSB 1) leads to an acceleration of H2S formation and to a significant conductivity drop (compared to the pristine sulfide SE), the other functional-coated sulfide electrolyte (FUNCY-SSB 2) exhibits a reduced H2S-generation rate compared to both the FUNCY-SSB 1 SE and the pristine SE. Furthermore, the FUNCY-SSB 2 electrolyte has a high room temperature conductivity (similar to the pristine material) and the conductivity decrease in test cells due to pressure relaxation is lower (–0.01 mS cm-1 or –76%) compared to cells containing pristine SE (–0.38 mS cm-1 or –83%) in seven days. The electrochemical stability of the FUNCY-SSB 2 electrolyte at low potentials is also slightly improved compared to the pristine SE. However, the FUNCY-SSB 2 SE is unstable at high potentials. Further work will therefore include the screening of further coating materials to obtain a highly conductive composite with high stability towards moisture and high potentials to be used as catholyte. Additionally, the SE(s) will be validated in Li|Li and NMC|Li test cells.