Composite cathodes combining solid-state electrolytes (SSEs) and cathode active materials (CAMs) in All-Solid-State-Batteries (ASSBs) benefit from an extended interaction of the SSEs and the CAMs in a mixed ion- and electron-conductive SSE-CAM structure. Ideally, SSE and CAM should form an interconnected 3D-network with a low interfacial impedance improving charge transfer and transport and therefore minimizing loss in the battery [1,2]. Among SSEs, phosphate-based NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) combines competitive ionic conductivity, good chemical stability, and promising thermal stability with phosphate-based CAMs such as LiFePO4 (LFP), LiCoPO4 or LiMn0.5Fe0.5PO4 at 700-800 °C in inert atmosphere [3-5].

A main drawback of thick electrodes is their limited ionic diffusion so thus lowering their rate cyclability compared to conventional electrodes. Although previous research revealed that for additive-free single-phase electrodes an appropriate porosity is necessary to have sufficient electrochemical activity [6], a relatively compact structure in dual-phase all-ceramic cathodes is beneficial for shortening the diffusion pathways for both ions and electrons [3]. The present work was focused on the preparation for the first time of ultra-thick dual-phase LATP-LFP ceramic cathodes for ASSBs via a combined approach based on a facile high-pressure low-temperature (HPLT) technique followed by direct co-firing. Variable-loading (40-55 vol.% of solids) of commercially available LATP and carbon coated LFP powders (40:60 wt.%) were pre-mixed with a multicomponent binder comprised of PP, PW and SA (50:46:4 wt.%) in a Haake Rheomex at 180 °C and pressed at 50 MPa and 190 °C for 50 min in a hot plate press. A one-step debinding and sintering strategy, consisting of 1h-dwells at 190, 400 and 600 °C followed by a 2h-dwell at 650-850 °C in flowing Ar/H2 atmosphere, was applied to the pressed LATP-LFP disk-shaped samples of 800-1000 µm of nominal thickness and 16 mm of diameter. XRD studies confirmed that no secondary phases can be detected in samples sintered up to 700 °C, whereas a significant fraction of impurities such as Li2FeTi(PO4)3 and AlPO4 can be found in samples sintered at higher temperatures. Combined SEM/EDS investigations showed a homogeneous distribution of LATP and LFP phases in a relatively compact structure (porosity <20 vol.%) with no evidence of diffusion across the LATP-LFP interphase. The electrochemical performance of ceramic LATP-LFP disks was evaluated at 30 °C by electrochemical impedance spectroscopy using gold electrodes and chronopotentiometry in two-electrode coin cells (CR2032) with commercial separators/liquid electrolytes and Li-metal at C/50-C/2 in a potential range from 4.0 to 2.8 V. The processed composites with a high mass loading (120 mg/cm2) were stable and demonstrated competitive ionic and electronic conductivities, a high areal capacity (>20 mAh/cm2), promising cycling capabilities (167 mAh/g at C/10), and a high coulombic efficiency (99 %).
 
[1] Grey, C. P. et al. (2020). Nature communications, 11(1), 6279
[2] Chen, C. et al. (2021). Advanced Energy Materials, 11(13), 2003939
[3] Xu, Q. et al. (2022). Small, 18(21), 2200266
[4] Manthiram, A. (2020). Nature communications, 11(1), 1550
[5] Gellert, M. et al. (2018). Ionics, 24, 1001-1006
[6] Sotomayor, M. E. et al. (2018). Journal of Materials Chemistry A, 6(14), 5952-5961