Lithium-ion batteries are considered to be one of the most important technologies to enable a successful transition to renewable energies. Ni-rich layer oxides, with at least 80% nickel, are particularly interesting as the cathode active material (CAM) for electric vehicle applications due to their high specific capacity of around 200 mAh/g and high average potential (approx. 3.7 V vs. Li).[1] However, these materials suffer from degradation phenomena like the phase transition from a layered to a rock-salt type structure, degradation of electrolytes at the cathode electrolyte interface, loss of oxygen from the crystal structure, dissolution of transition metals and their subsequent deposition on the anode side, and particle cracking induced by anisotropic crystallographic deformation.[2] Particle cracking can be mitigated by using single-crystal CAMs instead of polycrystalline materials. These micron-sized particles are synthesized with one grain boundary, which leads to more robust particles with higher mechanical stability.[3] Moreover, the surface modification of the CAM with a coating was shown to be a feasible approach to mitigate material degradation during cycling. Coatings can reduce degradation in several ways, e.g., acting as a physical barrier between electrolyte and active material, enhancing electronic or ionic conductivity or improving structural stability.[4]
In this study, the surface of single-crystalline NCM811 particles was modified by applying a phosphonic acid-based coating via a wet-chemical approach. The modified particles are characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and thermogravimetric analysis with coupled mass spectrometry (TG-MS) to explore the binding of the coating materials and to analyse the surface morphology and the amount of coating. The results are compared to a pristine NCM811 reference. Finally, the rate capability and capacity retention of cells containing the modified CAMs is investigated and the effect of the coating on the electrochemical performance is evaluated.
[1] J. Kim, H. Lee, H. Cha, M. Yoon, M. Park, J. Cho, Advanced Energy Materials 2018, 8.
[2] S. S. Zhang, Energy Storage Materials 2020, 24, 247.
[3] M. J. Lüther, S.-K. Jiang, M. A. Lange, J. Buchmann, A. Gómez Martín, R. Schmuch, T. Placke, B. J. Hwang, M. Winter, J. Kasnatscheew, Small Structures 2024.
[4] U. Nisar, N. Muralidharan, R. Essehli, R. Amin, I. Belharouak, Energy Storage Materials 2021, 38, 309.