Abstract text (incl. figure legends and references)

Steady progress of battery performances attests the necessity of intense battery research. As promising high-voltage and high-capacity cathode material LiNi0.5Mn1.5O4 (LNMO) was investigated for Co-free next-generation Li-ion batteries. Our investigation focused on determining the structure property relationship of these materials at the microscopic and preferrably atomic level via different TEM methods to unravel the morphologic, crystallographic and chemical structure of carefully designed LNMO.

The main objectives of our research-project "H2O-LIMO" are related to the overcoming of the water sensitivity of Li transition metal oxides, especially LNMO, as Co-free cathode, which is possible only if a fundamental understanding of occurring processes is established:

-general effect of water exposure and the impact of tailored surface facets of LNMO;

-effect of adding inorganic acids (e.g., H3PO4), which have been proven [1] to stabilize the LNMO surface by forming a thin metal phosphate surface layer;

-understanding processes occurring at the LNMO/water interface.

First, the synthesis of single-crystalline LNMO particles with tailored surface facets has been carried out according to a combined precipitation/evaporation method [2]. Perfectly octahedral LNMO particles with solely {111} surface facets as well as truncated LNMO particles with a combination of {100} and {111} surface facets were obtained. Depending on the temperature and duration of the annealing, the preparation of either ordered (space group (sp): P4332) or disordered (sp: Fd-3m) LNMO is achieved. The mentioned cation ordering, the presence of Mn3+ in disordered LNMO as a result of oxygen vacancies, and that of solely Mn4+ in the ordered phase is expected. The Mn:Ni ratio, the vacancies as well as the initial oxidation state of the Mn cations is expected to play an essential role for the interaction with water. Similarly, the varying acidity of the different surface facets is expected to play a key role for the reaction with the aqueous environment.

The size morphology and crystallographic orientations were investigated by TEM bright- (fig.1a,c,d) and dark-field (fig. 1b) imaging, diffraction (fig.1e) and HRTEM (fig.1f) showing the expected octahedron faceted primary particles. Since the powder particles are disordered agglomerations of overlapping primary particles, it is difficult to find an isolated octahedron particle for the foreseen TEM analysis.Further, the effect of the (acidic) aqueous treatment on the electrochemistry of the new faceted synthesized particles has been studied by STEM-EDX. The atom concentration distributions especially at and around the particle surface (fig.2a,b) shows a change of the ratio Mn:Ni at the surface and an increased P concentration in this region. HRTEM and STEM-EDX were performed with a Thermofisher Talos 200X TEM at 200 kV and with an image Cs-corrected TEM FEI Titan 80-300 at 300kV.

The first results are promising a better understanding of the influence of the (acidic) aqueous treatment on the faceted synthesized LNMO particles and their electrochemical behaviour. The investigation will be extended to other material treatments (e.g. other acids).

[1] M. Künzel et al., ChemSusChem 11 (2018) 562

[2] M. Kuenzel et al., Materials Today 2020 (39) 127

Acknowledgement: This work was supported by DFG in the frame of the research project "H2O-LIMO" (Wechselwirkung von Wasser und wässrigen Säuren mit Lithiumübergangsmetalloxiden).