Coating of a Novel Lithium-Containing Hybrid Oligomer Additive on Nickel-Rich LiNi<sub>0.8</sub>Co<sub>0.1</sub>Mn<sub>0.1</sub>O<sub>2</sub> Cathode Materials for High-Stability and High-Safety Lithium-Ion Batteries
Yi–Shiuan Wu, Quoc‐Thai Pham, Chun‐Chen Yang, Chorng‐Shyan Chern, Lakshmipriya Musuvadhi Babulal, Manojkumar Seenivasan, Juliya Jeyakumar, Tadesu Hailu Mengesha, Tobias Placke, Gunther Brunklaus, Martin Winter, Bing−Joe Hwang
Abstract
In this study, we synthesized a Li-containing “BTJ-L” hybrid oligomer─obtained through polymerization of bismaleimide (BMI) with a polyether monoamine (i.e., Jeffamine-M1000, JA), trithiocyanuric acid (TCA), and LiOH─and coated it as an additive in various amounts (0.5–2 wt %) onto the surface of a Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode active material, forming BTJ-L@NCM811 electrodes for lithium-ion batteries (LIBs). Relative to CR2032 coin-type cells incorporating a pristine NCM811 electrode, the cells with the 1 wt % BTJ-L@NCM811 electrode demonstrated a slightly higher initial discharge capacity (173 mAh g–1 vs171 mAh g–1) and higher values of average Coulombic efficiency, CEavg (99.5% vs98.9%) and capacity retention, CR (86.1% vs72.9%) after 100 cycles at 1C. Electrochemical impedance spectroscopy revealed that the decrease in the charge transfer resistance (Rct: 46.7 Ω vs171.1 Ω) and the superior Li+ ion diffusivity (DLi+: ∼1.09 × 10–12 cm2 s–1 vs ∼1.61 × 10–13 cm2 s–1) of the cells incorporating the BTJ-L@NCM811 electrode after cycling at 1C could be attributed to the excellent wettability toward the electrolyte and the extra Li+ ions contributed by the hybrid BTJ-L oligomer additive. Therefore, the BTJ-L oligomer coating layer functioned much like an artificial cathode electrolyte interphase (CEI) layer, impairing the dissolution of transition metals (TMs) from the cathode materials into the carbonate-based electrolytes. Furthermore, in situ microcalorimetry manifested that the total exothermic heat generation (Qt) of the coin cells containing the 1 wt % BTJ-L@NCM811 electrode operating at 1C in isothermal modes (35 and 55 °C) during the charging process was dramatically lower (by ca. 45%) relative to that of the cells incorporating the pristine NCM811 electrode. On the basis of an ARC-HWS analysis, the delithiated pristine NCM811 electrode shows thermal reactivity with the electrolyte at a much earlier stage in comparison to the 1 wt % BTJ-L@NCM811 counterpart (843 min vs 1039 min) between 171 and 192 °C. Thus, Ni-rich NCM811 cathode materials coated with trace amounts (i.e., 1 wt %) of the BTJ211-L1 hybrid oligomer additives displayed both enhanced electrochemical performance and remarkably improved thermal stability. Accordingly, this Li-containing BTJ-L hybrid oligomer appears to be a great candidate material for coating high-Ni oxide cathode materials to enhance the safety and electrochemical performance of LIB cells.