Garnet-Based Composite Cathodes for Polymer-Ceramic Solid-State Li Batteries

Abstract

Ceramic solid-state Li batteries (SSLBs) with high capacity composite cathodes have a huge potential to outperform conventional Li-ion batteries [1]. Among the most promising ceramic electrolytes is garnet-based Li7La3Zr2O12 (LLZ), which has a wide electrochemical window, stability vs metallic Li, and good Li-ion conductivity at room temperature [1]. However, so far, the electrochemical cycling stability of all ceramic solid-state SSLBs is low [1]. The volume changes in the cathode active materials (CAMs) during cycling can cause cracks at the rigid CAM/ceramic electrolyte interface if the density of the mixed cathode is low. To mitigate the detrimental effects of crack formation, polymer electrolyte can be added to fill the cracks could and ensure a good contact between CAM and ceramic electrolyte. Such a concept is called polymer-ceramic SSLB [2].LLZ-based polymer ceramic SSLBs have already shown good electrochemical properties. Commonly the cells are constructed based on a LLZ separator with a dense and porous side that are manufactured by tape-casting [2, 3]. The tape-casting process is not straight forward as it requires the preparation of a slurry with organic solvents. Furthermore, the grain morphology has a major impact on the final LLZ microstructure. Afterwards, the tape-casted LLZ separator is conventionally sintered and grain growth and loss of volatile elements, such as Li, occurs. The sintered LLZ separator is then infiltrated by CAM solution [3]. After drying the CAM solution the polymer electrolyte is added.In a powder-based process, co-sintering the LLZ and CAM simplifies the manufacturing. Another advantage is that the porous LLZ framework is stabilized by the CAM and allows to reduce the LLZ amount in the porous framework in order to achieve higher CAM loadings. However, the high sintering temperature and long dwell time for the sintering of garnet-based LLZ electrolyte lead to reactions with cathode active materials (CAMs) and hamper the manufacturing of composite cathodesA reduction in sintering temperature and dwell time can be achieved by advanced sintering techniques, such as Field-Assisted-Sintering-Technique/Spark Plasma Sintering (FAST/SPS). FAST/SPS applies rapid Joule heating and mechanical pressure. This shortens the required processing times due to faster heating and an increased sintering rate and reduces the sintering temperatures. By that less grain growth is observed, and the loss of volatile elements is reduced.Recently, we showed, that FAST/SPS can co-sinter LLZ and LiCoO2 (LCO) and allows the preparation of an LLZ/LCO composite cathode in one step. The LLZ/LCO phases are clean and provide a low interface impedance [4]. Another advantage is that the thickness of the composite cathode is easily scalable. However, the obtained density was around 95 % and only closed porosity was observed, that prevents the polymer infiltration.In this work, the FAST/SPS process is adjusted to manufacture phase pure porous LLZ/LCO-composite cathodes in one sintering step. The porosity is easily controllable, without the requirement of pore former by the applied pressure during FAST/SPS. Infiltration of polymer electrolyte into the pores within the composite cathode leads to significantly lowered interfacial impedances. The porous composite cathode can be assembled into a polymer ceramic SSLB by attaching it to an anodic half-cell (separator and anode) to demonstrate the functionality of the presented concept.