The use of solid-state electrolytes (SSE) promises electrochemical cells with high energy densities as they can enable high-voltage cathodes and lithium metal anodes while minimising the risk of dendrite formation and widening the thermal and electrochemical stability window [1]. Among different types of solid-state electrolytes that have been researched until now, cubic Li₇La₃Zr₂O₁₂ (LLZO)  has been shown to have good thermal, mechanical, and electrochemical stability [1]. Nonetheless, one of the main challenges of inorganic SSE is the poor interfacial contact with the electrodes, especially with the cathode active material (CAM). The rigidity of the SSE and CAM makes it necessary to co-sinter the SSE/CAM composite at high temperatures (>1000 ºC) to densify the catholyte to guarantee conformal contact between the solid components [2]. However, CAMs usually react at these temperatures, which leads to formation of poorly lithium-ion conducting intermediate secondary phases due to interdiffusion of atoms and chemical reactions between the SSE and CAM [3].

Recent research elucidating decomposition products resulting from processing of LLZO and various cathode materials has mainly focused on LiCoO₂ (LCO) given its high thermodynamical stability against LLZO [4-7]. However, the limited practical reversible capacity of LCO, high cost, and ethical issues concerning cobalt-rich cathode materials motivate the optimisation of other CAMs that are equally stable against LLZO [8]. Among them, LiNi_xCo_yMn_zO₂ (NMCs) are perceived as strong candidates to decrease the Co content while increasing the specific capacity and maintaining sufficient thermal stability for co-sintering with LLZO [8,9]. A few studies have recently attempted to probe the interfacial stability between LLZO/NMC, but the nature of decomposition products, reaction mechanism, and onset temperature are inconsistent between reports and poorly understood [8-13]. Moreover, these reports do not account for the formation of secondary phases containing dopant elements such as Al³⁺ in cubic LLZO.

To develop a fundamental understanding of the effect of sintering conditions on catholyte composition and reaction products, a comprehensive study of composite materials from microscale to atomic level is required [8]. Commonly used techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), or Raman spectroscopy can provide some information about the material’s local composition but cannot accurately detect any amorphous secondary phases formed in the nanometre scale that are buried at interfaces between SSE and CAM. Consequently, decomposition products can be easily overlooked, resulting in a wide range of reported results. On the other hand, magic angle spinning nuclear magnetic resonance (MAS-NMR) is an ideal technique to identify the decomposition products and probe their local structure even if they are present in small amounts and are highly disordered and amorphous in nature [14].

In this work, we present a detailed characterisation of the changes in the crystal structure and composition of Al-doped LLZO (Al-LLZO) and NMC811 composite as a function of temperature, sintering atmosphere, and conductive agent using ex-situ/variable-temperature XRD, thermogravimetric analysis/differential scanning calorimetry coupled with mass spectrometry (TGA/DSC-MS), and MAS-NMR. Ex-situ ²⁷Al MAS NMR spectroscopy provides evidence of the evolution of Al³⁺ coordination environment upon annealing, providing accurate and high-resolution atomic scale insights on elemental interdiffusion and reaction onset temperature between Al-LLZO and NMC811.