https://www.nature.com/articles/s41467-026-78099-3
The closed loop of “Synthesis-Performance-Confirmation/Prediction” plays a key role in guiding energy materials synthesis for optimized performance. The synthesized materials enter this loop for further optimization, especially for AI training. Behind this, a core issue is whether the synthesized material is truly the material that we expected, for example from the element composition, crystal structure, phase, etc. During energy conversion reaction, material reconstruction has been intensively studied in electrochemistry due to the interaction of the material’s surface with the surrounding solution. Typical reconstruction can occur under oxidizing potentials such as during the oxygen evolution process, where metal oxides, sulfides, selenides, nitrides and phosphides can be readily transformed into more thermodynamically stable metal oxides or (oxo)hydroxides. Before the reaction occurs, the synthesized materials will often undergo a series of processing. Taking battery and catalyst electrode coating preparation as an example, the synthesized materials with conductive agents need to be dispersed in solvent. This process also calls forth the same issue of whether the materials in the coating are the same as the synthesized materials, and in other words, whether the synthesized materials are stable in solvent. To clarify this issue, advanced characterization is of utmost importance. Thus, in this work, we take the synthesis of CoAl2O4 and its leaching phenomenon in alkaline medium as an example to demonstrate the issue, raise our concern, and give our advice. This work also emphasizes that careful characterization is needed for synthesized multi-metal oxides in general, as the results are often used to construct the structure-performance relationship and for density functional theory calculations.
The CoAl2O4 in this work exists in the form of powders, which were obtained by annealing the Co-Al gels from nitrates, citric acid and urea. The crystallized CoAl2O4 has a cubic spinel structure, and the crystal structure can be determined by X-ray diffraction patterns. Reports have indicated that 400 °C is enough to crystallize the material. Increasing annealing temperatures will increase the particle sizes and change metal element occupation. We compared the CoAl2O4 powders annealed at 500, 700 and 900°C. All the synthesized powders demonstrate a cubic spinel structure and have an Al/Co atomic ratio of almost 2. However, these powders demonstrate different phenomenon in KOH solution in that CoAl2O4 synthesized at 900°C are stable in concentrated KOH solution, while Al leaching can be observed for CoAl2O4 synthesized at low temperatures. From the conventional X-ray diffraction characterization, we can still observe a pure cubic spinel phase at the leached CoAl2O4 after long-term soaking in concentrated KOH. It motivates us to explore the surface of the oxides. Further TEM characterization indicates that Al and Co distribution is homogeneous for CoAl2O4 synthesized at 900°C, while an Al excess surface exists for CoAl2O4 synthesized at low temperatures. Such Al excess surface will be partially leached but still maintain an Al rich surface. In contrast, impurity phases can be observed at the oxides that were soaked for short time or in less concentrated alkaline media. It also implies that this material is unstable under some specific conditions.
From our case, it can be assumed that the Co3O4 crystallizes and the crystallized Al-excess CoAl oxides envelop the Co3O4 at low temperatures, and with increasing the temperature Co and Al mutually diffuses to form a homogeneous CoAl2O4. The phenomenon above raises a crucial issue of whether the metal elements are distributed homogeneously in multi-metal oxides, how to confirm if the expected material is obtained, and whether it is rational to correlate the material and performance. When comparing the performance of the energy materials, the enhanced properties are commonly correlated to the composition in the studied coordination environment or crystal structure, supported by empirical assumptions and theoretical calculations. Extensive synthesis and analysis for the mixed oxides are based on the single-phase solid solution model, in which the metal cations are distributed homogeneously. The enhanced properties are commonly correlated to the composition in the studied coordination environment or crystal structure, supported by empirical assumptions and theoretical calculations. Therefore, the phase purity and homogeneous distribution of cations within the synthesized particles are the prerequisites for correlating the performance, structure and composition.
In oxide synthesis, heat treatments are crucial for the target phase formation. The relay-like phase formation in this work also applies to other metal oxide systems, such as the reported battery materials LiNi0.8Co0.2O2 and P2-Na0.67CoO2, and high-entropy/entropy-stabilized Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O during increasing heat treatment temperatures. We note that the required heat treatment temperature strongly relies on the used chemicals and preparation methods. For example, the combustion method used in this work involves an exothermic redox reaction, and thus a relatively low temperature is enough compared with the solid-state reaction. Though increasing heat treatment temperature is important for obtaining a single phase for some oxides, it does not mean high-temperature treatment is beneficial to other oxides. The different redox process is also carefully considered, for example, the decomposition of LiCoO2 and ZnCo2O4 at high temperatures.
However, when the synthesized material consists of a crystalline phase together with an amorphous phase, the amorphous component is often difficult to identify using the conventional X-ray diffraction analysis. Furthermore, in this study, the oxides synthesized at low temperatures are nanoscale in size and possess similar crystal structures, while their elemental distributions vary continuously. Distinguishing such individual phases only from X-ray diffraction patterns becomes highly challenging. Therefore, direct characterization of crystal structures and elemental distributions is indispensable for elucidating phase formation mechanisms. In particular, high resolution analysis of particle interiors and interfaces is required to identify phase boundaries and local compositional variations. Although conventional solid-solution phase diagrams provide valuable guidance regarding composition, heat-treatment conditions, and phase stability during materials synthesis, the emerging field of entropy-stabilized and high-entropy oxides offers additional opportunities for tailoring phase evolution and controlling the formation of intermediate phases or desired single-phase structures.