Establishing in-situ heating experiments of monolithic LiNiO₂ particles in O₂ atmosphere

Abstract text (incl. figure legends and references)

The state-of-the-art cathode materials for lithium-ion batteries are the layered transition metal oxides Li(Ni_1-x-yCo_xMn_y)O₂ (NCM) and Li(Ni_1-x-yCo_xAl_y)O₂ (NCA). They deliver specific capacities of more than 170 mAh g-1 [1]. Manufacturers and researchers constantly try to reduce the cost as well as increase the capacity of batteries. Increasing the Ni content to 80% or higher fulfills on the one hand these goals but leads on the other hand to a reduced cycling stability of the battery, as the polycrystalline particles undergo an anisotropic volume change during (de-)lithiation [2]. By using monolithic crystals instead of polycrystalline materials, these drawbacks may be mitigated [3]. Furthermore, an annealing step at elevated temperatures can optimize the surface morphology and structure of the particles leading to an improved electrochemical performance [4]. The annealing process is executed in an oxygen atmosphere to prevent oxygen loss from the grains [5].

This study aims at gaining a thorough understanding of the influence of the annealing process on the structure of Ni-rich layered transition metal oxides. For this purpose, the model system LiNiO2 (LNO) is investigated during annealing in an oxygen atmosphere in in-situ high angle annular darkfield (HAADF) scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) experiments.

The monolithic LNO particles agglomerate to secondary particles with a diameter of approximately 4 μm. To prepare an electron transparent sample, the focused ion beam (FIB) cutting method is employed. For apposite placement of the particle on the SiN window of the heating chip, a Si lamella with a cut-out hole, in which the particle is placed before thinning, is used as support. The chip with the particle is then mounted in the enclosed gas cell TEM holder, which is connected to an oxygen bottle. Between the bottle and the holder, a flow valve controls the flow of oxygen, whereas a pressure valve downstream of the holder regulates the pressure to 1 bar in the holder tip.

We report on our experiments to heat a secondary LNO particle in an oxygen atmosphere. A heating ramp of 1 °C / s was employed up to a temperature of 700 °C. Starting at around 450 °C, structural changes of the particle begin to arise. These changes firstly appear at the grain boundaries of the monolithic crystals and later proceed to the bulk of the grains. Eventually, the whole secondary particle is severely disintegrated, as the HAADF STEM image in figure 1 shows. We propose that this disintegration occurs due to oxygen loss from the particles as EELS data in figure 2 obtained from the same region before and after heating implies.

The first experimental data has shown that our work-flow from the elaborate sample preparation to in-situ heating and the particles" investigation works fundamentally. In future experiments we will refine our set-up and parameters to try to suppress oxygen loss and disintegration of the LNO particles during heating.

Figure 1: HAADF STEM images of LNO secondary particle a) before and b) after in-situ heating to 700 °C.

Figure 2: STEM EELS signal of oxygen before (blue) and after (red) in-situ heating to 700 °C.

[1] de Biasi, L. et al. Adv. Mater. 31 (2019)

[2] Kurzhals, P. et al. J. Electrochem. Soc. 168 (2021)

[3] Langdon, J. et al. Energy Stor. Mater. 37 (2021)

[4] Huang, B. et al. Solid State Ion. 345 (2020)

[5] Karki, K. et al. ACS Appl. Mater. Interfaces 8 (2016)