What Affects More the NMC811 Cathode Powders: Lithium Excess, Washing or Aging?
Authors: Winkowska-Struzik Magdalena , Buchberger Dominika , Uhrynowski Witold , Struzik Michał , Czerwinski Andrzej 
Published: February 2024

Lithium Nickel Manganese Cobalt Oxides with R-3m space group have gained significant attention in recent years due to their high energy density, high power capability, and long cycle life. There are several synthesis methods to obtain NMC active powders, like solid-state or sol-gel, but co-precipitation synthesis is a widely used method for preparing NMC cathode materials for lithium-ion batteries, offering advantages in terms of control over particle size, morphology, and homogeneity (Xie et al., 2016).

The main difficulty of synthesizing Ni-rich materials is the minimization of the degree of Li/Ni cation mixing, which refers to the interchange of the nickel and lithium ions in the 3a/3b sites of the crystal lattice. Perfect NMC materials have a hexagonal layered structure of α-NaFeO2-type, where Li and transition metal atoms form alternating layers between oxygen layers. To minimize phenomenon of cation mixing an excess of lithium source prior to sintering is required (Arai et al., 1995).

However, unreacted lithium remain on the surface of NMC after the synthesis in the form of Li2O and it turns into LiOH and Li2CO3 as impurities if stored in ambient conditions. Those residual lithium compounds (RLC) on NMC811 surface can have significant impacts on the cathode's morphology, crystal structure, surface chemistry and, thus, electrochemical performance. Despite it was previously shown that Ni-rich materials have poor charge-discharge capacity retention, related to the electrolyte oxidization at the positive electrode surface (Jung et al., 2017) and/or structural changes of the material, such as large c-axis shrinkage, at high potentials (Teichert et al., 2020), many efforts are made to synthesize Ni-rich NMC for battery applications.

Our research explores the impact of lithium excess on the physicochemical properties of NMC811. We compared self-made fresh samples with commercially available NMC811 in terms of structural parameters, morphology and electrochemical performance at high C-rates before and after lithium surplus was washed. Lithium removing procedure was applied to mitigate issues with RLC during battery operation, such as electrolyte degradation and gassing (Hartmann et al., 2022). However, over-washing can change physical properties of NMC811, including surface reconstruction as a consequence of the leaching of active transition metal ions and lithium from the cathode. Therefore, it is important to carefully optimize the washing procedures to balance the removal of RLC with the preservation of the cathode's electrochemical performance and stability.

Here we want to point the effectiveness of washing procedure on NMC811 morphology, structural stability and electrochemistry. Additionally, our studies addressed the effects of calendar aging of active powders, i.e. prolonged shelf-storage in an ambient conditions for washed and unwashed samples, especially an influence of LiOH and Li2CO3 on structural parameters, electrodes preparation and electrochemical behavior. We want to open the discussion about preferable strategy on storage of Ni-rich NMC

The work was supported by the University of Warsaw through Action I.3.4. “The circular economy – energy storage”: "New electrode materials for next generation Li-ion batteries" (project no. PSP 501-D112-20-0001340) within the “Excellence Initiative – Research University (2020-2026)” programme of the Ministry of Science and Higher Education, Poland.

References

Arai, H., Okada, S., Ohtsuka, H., Ichimura, M., Yamaki, J., 1995. Solid State Ion 80, 261–269.

Hartmann, L., Ching, C.H., Kipfer, T., Koch, M., Gasteiger, H.A., 2022. J Electrochem Soc 169, 070516.

Jung, R., Metzger, M., Maglia, F., Stinner, C., Gasteiger, H.A., 2017. J Phys Chem Lett 8, 4820–4825.

Teichert, P., Eshetu, G.G., Jahnke, H., Figgemeier, E., 2020. Batteries 6, 8.

Xie, H., Hu, G., Du, K., Peng, Z., Cao, Y., 2016. J Alloys Compd 666, 84–87.