In recent years, the demand for lithium ion batteries (LIBs) significantly increases due to the wide range of application, from consumer electronics to automotive industry. The improvement of the LIB’s energy density is one of the key factors to the mass market penetration of electric mobility. Currently, LIBs are applied in hybrid (HEV), plug-in hybrid (PHEV) and fully electric (EV) vehicles. Particularly, layered lithium Ni‑Co‑Mn oxides LiNi_xCo_yMn_zO₂ (NCM, x + y + z = 1) cathode materials are the most promising candidates for LIBs due to their high capacity, lower cost and enhanced stability compared with LiCoO₂.^[1]

Although the state-of-the-art NCM cathode material is NCM-523, the trend is going towards a higher energy density. Recently, the focus of research is on the cathode materials with high nickel content. For example NCM-811 with a Ni content of 80% can deliver a discharge capacity of 200 mAh/g at 4.3 V. At the same time, the high Ni content causes some disadvantages, such as an unstable electrode structure and rapid capacity fading.

One of the challenges is the synthesis and stabilization of Ni-rich materials. In order to avoid cation mixing, an excess of lithium must be ensured. Consequently, the unreacted lithium on the surface of the active material can react with water and CO₂ from the air. As a result of the contamination, LiOH and L₂CO₃ are formed on the surface of the active material. These residues can form an insulating layer of LiF in contact with the electrolyte, which leads an increased internal resistance of the electrode. Furthermore, the highly reactive Ni⁴⁺ ions in the delithiated state of the electrode of Ni-rich cathode materials can oxidize the electrolyte and result in active material loss.^[2]

In this contribution, we investigate the effect of surface modification of NCM-811 via an inorganic coating on the electrochemical performance. The inorganic coating acts as protective layer on the surface of the active material, preventing the direct contact of the cathode material with the electrolyte. Moreover, it suppresses the side reactions on the particle surface. The effect of the coating on the structural and electrochemical properties was investigated in NCM-811/graphite full cells. The cells with coated electrodes were significantly improved with respect to their long-term cycling performance compared to the uncoated electrodes (Figure 1).

[1] S.-K. Jung, H. Gwon, J. Hong, K.-Y. Park, D.-H. Seo, H. Kim, J. Hyun, W. Yang and K. Kang, Advanced Energy Materials, 2014, 4, 1300787.

[2] W. Liu, P. Oh, X. Liu, M.-J. Lee, W. Cho, S. Chae, Y. Kim and J. Cho, Angewandte Chemie International Edition, 2015, 54, 4440-4457.

Figure 1: Cycling performance of uncoated and coated NCM-811/graphite LIB full cells at room temperature using a voltage window of 2.5 – 4.3 V. Three formation cycles at 0.1C and following cycling at 0.5C rate.