Reaction of Li_1.3Al_0.3Ti_1.7(PO₄)₃ and LiNi_0.6Co_0.2Mn_0.2O₂ in Co-Sintered Composite Cathodes for Solid-State Batteries

Abstract

All solid-state batteries offer the possibility of increased safety at potentially higher energy densities compared to conventional lithium-ion batteries. In an all-ceramic oxide battery, the composite cathode consists of at least one ion-conducting solid electrolyte and an active material, which are typically densified by sintering. In this study, the reaction of the solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) and the active material LiNi0.6Co0.2Mn0.2O2 (NCM622) is investigated by cosintering at temperatures between 550 and 650 °C. The characterization of the composites and the reaction layer is performed by optical dilatometry, X-ray diffractometry, field emission scanning electron microscopy with energy dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, as well as scanning transmission electron microscopy (STEM). Even at low sintering temperatures, elemental diffusion occurs between the two phases, which leads to the formation of secondary phases and decomposition reactions of the active material and the solid electrolyte. As a result, the densification of the composite is prevented and ion-conducting paths between individual particles cannot be formed. Based on the experimental results, a mechanism of the reactions in cosintered LATP and NCM622 oxide composite cathodes is suggested.

Active Interphase Enables Stable Performance for an All‐Phosphate‐Based Composite Cathode in an All‐Solid‐State Battery

“…Literature showed that the elemental inter-diffusion in thermally unstable CAM/SSE combinations caused decompositions of CAM or SSE where the decomposed byproducts acted as insulating phases, strongly impeding the Li + transportation. [16,19,[21][22][23][24][25] Thus, thermal and chemical compatibilities of CAM/SSE at the processing temperature and (electro)chemical properties of the CAM/SSE interphase are the determining factors for the choice of CAM/SSE combinations.Phosphate-based NASICON-type Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) is attractive to be employed as the SSE for ASSBs due to its competitive ionic conductivities and good chemical stability in ambient atmosphere. [26][27][28] LATP is considered to have better thermal stability with phospho-olivine LiMPO 4 (M: Fe, Mn, Co, Ni) cathodes because the oxygen atoms are tightly bound to P with the strong covalent bonds in both LATP and LiMPO 4 .…”

“…Literature showed that the elemental inter-diffusion in thermally unstable CAM/SSE combinations caused decompositions of CAM or SSE where the decomposed byproducts acted as insulating phases, strongly impeding the Li + transportation. [16,19,[21][22][23][24][25] Thus, thermal and chemical compatibilities of CAM/SSE at the processing temperature and (electro)chemical properties of the CAM/SSE interphase are the determining factors for the choice of CAM/SSE combinations.…”

High-Temperature Thermal Reactivity and Interface Evolution of the NMC-LATP-Carbon Composite Cathode

“…The threshold temperature is much lower than that predicted in a recent study of NMC-LATP thermo-mechanical compatibility (<550 °C) as the impact of the electrically conductive additive was not considered. 23 Under air, the transformations involve a cation redistribution in the NMC cathode material, leading to Mn-depletion at the surface. This cation migration is further accompanied by the loss of lithium and oxygen from the lattice as temperature increases and finally results in a phase transition from layered to spinel-type compounds and/or rocksalt oxides (depending on the temperature) as determined from XPS, EDX, and XRD.…”

“…Oxide-based ceramics as LATP require temperature-assisted densification techniques, with temperature ranges typically around 1000 °C and above . However, at such temperatures, the cathode components are not stable and can react to form resistive byproducts as well as to the structural degradation of the active material, − which lead to the severe decay of the electrochemical performance. Ideally, the composite properties should remain unchanged, although chemical and structural modifications at the interfaces could be accepted, provided that Li-ion mobility is not obstructed. , …”

“…Noteworthy, our system includes electrically conductive additives as it cannot be eluded in cathode compositions. This is an essential aspect that has been often overlooked in previous works. ,, The threshold conditions at which the selected materials remain compatible have been determined in view of the unified effects of structural, chemical, and electrochemical stability of the components after heat treatments at different atmospheres and temperatures. From the combination of our observations, a degradation mechanism is proposed.…”