New ALD ZnO Precursors and Processes for Protective Coatings on Lithium-Ion Battery Cathodes

Abstract

The ever-increasing demand for Li-ion batteries commands their performance to increase while at the same time size and mass to decrease. One promising cathode material to achieve this is the Ni-rich LiNi0.8Mn0.1Co0.1, (NMC811), a high-capacity layered oxide cathode material. However, NMC811 suffers from capacity fading during electrochemical cycling of the cells. This phenomenon is connected to the dissolution of the active transition metal ions in the cathodes, phase changes and to side reactions happening at the interface of the cathode and the electrolyte or with formed HF but remains poorly understood.[1] To mitigate these problems, many different coating processes have been employed. In the realm of atomic layer deposition (ALD), the predominant coating material is the very stable insulating Al2O3 prepared from trimethyl aluminum and H2O. However, ZnO is an alternative transition metal oxide coating that shows promise since it is an amphoteric conductor. Recently, amorphous ZnO coatings prepared by sol-gel methods were shown to significantly boost the capacity retention of NMC cathodes.[2] Taking inspiration from a recent study that compared different ALD Al2O3 precursors and their ability to precisely tune the protective coating, we set out to investigate the suitability of ZnO ALD films as protective coatings and the influence of different precursors used for that purpose.[3] We selected four Zn precursors for cathode surface treatments in this study with different coordination motifs, metal-to-ligand bonds and reactivities, (Fig. 1). The ZnO ALD processes for these precursors were established by in-situ spectroscopic ellipsometry, and the resulting films were subjected to compositional analysis by XPS and RBS/NRA proving high purity ZnO. Further, UV-VIS measurements were conducted to investigate the band gaps of the materials which showed interesting precursor dependent differences. To analyze the growth behavior of the different precursors, in-situ FTIR studies were conducted hinting at distinct decomposition/deposition behavior of each precursor, as shown by the FTIR difference spectra for ZnEt(HMDS) in Fig. 2. The in-situFTIR studies also revealed different nucleation behaviors for the ALD ZnO precursors on the NMC811 cathode surface. Next, we coated NMC811 powder with 1 and 5 ZnO ALD cycles and performed XPS to verify successful ZnO ALD, analyze the surface species, and probe for potential oxidation state changes caused by the applied precursors. Whereas the observed changes in the oxidation states were small, a reproducible reduction was seen for the Ni, Co, and Mn transition metals in the cathode material, see exemplarily the ratio of Co 2+ and 3+ species in Fig. 3. Coin cell cycling tests of cathodes fabricated from the ALD ZnO-coated NMC811 powders prepared using the different Zn precursors are ongoing. These results will be especially noteworthy and showcase how the choice of ALD precursor influences the properties of a battery protective coating and allows fine tuning of the cathode surface properties.[1] Z. Ruff, C. Xu, C. P. Grey, J. Electrochem. Soc. 2021, 168, 060518.[2] Y. Li, X. Li, J. Hu, W. Liu, H. Maleki Kheimeh Sari, D. Li, Q. Sun, L. Kou, Z. Tian, L. Shao, C. Zhang, J. Zhang, X. Sun, Energy Environ. Mater. 2020, 3, 522–528.[3] D. Kang, R. E. Warburton, A. U. Mane, J. Greeley, J. W. Elam, Applied Surface Science 2022, 599, 153329. Figure 1