Tailoring Surface of Ni-Rich LiNi_1-xCo_x/2Mn_x/2O₂ by Using Lithium-Ion Conducting Solid-Electrolytes

Ni-rich LiNi_1-xCo_x/2Mn_x/2O₂ (NMC) layered cathode materials have attracted great interest for electric vehicle (EV) applications due to their high gravimetric and volumetric energy densities. However, the Ni-rich NMC cathodes still experience a series of degradation issues, especially when their charging voltage exceeds 4.3 V_vs.Li. Therefore, current cell operating conditions limits their usage, which corresponds to about 60 – 70 % of their theoretical capacities. Current understanding on the degradation mechanism of Ni-rich NMC at high voltages (e.g., &gt; 4.3 V_vs.Li) includes; (1) Ni²⁺ migration to vacant Li⁺ layer, leading to an irreversible phase transformation from layered structure to rock-salt structure; (2) oxidative decomposition of electrolytes and unwanted parasitic reactions at cathode/electrolyte interfaces (CEI), increasing the internal impedance and consumption of Li-ions; (3) large volumetric changes and subsequent mechanical stresses inducing surface cracking and particle pulverization during extended cycling.<br/>One of the most prominent approaches to improve the high-voltage stability is coating the surface of the Ni-rich NMC with inorganic materials such as Al₂O₃, ZrO₂, and Li₃PO_4. However, the commonly used coating techniques involve extra processing steps and/or unwantedly increase CEI impedance due to sluggish Li-ion transportation properties of the coated materials. Therefore, in this work, we will present solid-electrolyte (SE) implemented composite NMC cathodes prepared without any extra process, by relying on conventional electrode fabrication processes in Li-ion battery cells. This simple process offers advantages over the conventional coating methods in terms of manufacturing friendliness, energy saving, and cost effectiveness.<br/>Among various solid electrolytes, Li_6.7La₃Zr_1.7Ta_0.3O₁₂ (LLZT) with garnet structure showed great promises based on its wide electrochemical stability window, good mechanical properties, and reasonable Li-ion conductivity (10^-3 – 10^-4 S/cm). Based on these promising properties, we examined the effect of LLZT on the NMC + LLZT. First, the NMC + LLZT composite cathode significantly improved specific capacity and its retention during cycling at high voltages (e.g., 4.5 V_vs.Li). Compared with the baseline data (i.e., simple NMC cathode), the composite cathode showed 22% improved specific capacity and 11% improvement cycle life after 200 cycles at C/3-rate in full-cells (i.e., using graphite anode). Microscopy of the NMC + LLZT cathode revealed (i) a dispersion of LLZT nanoparticles on the surface of the NMC and (ii) LLZT microparticles interconnecting the NMC particles within the electrode microstructures. The X-ray photoelectron spectroscopy (XPS) data showed that LLZT-NMC cathode had thinner CEI than that of bare NMC, suggesting that LLZT coating could passivate the NMC and reduces parasitic reactions occurring at high voltages. Meanwhile, the high Li-ion conductivity of LLZT can provide facile lithium-ion conduction pathway via the NMC-LLZT-NMC network inside cathode microstructures, as evidenced by lower cell impedance from electrochemical impedance spectroscopy (EIS) combined with distributed time relaxations (DRT) techniques. As a result, the LLZT-NMC also deliver improved rate capabilities: e.g., ~ 5 times improved capacity at 5C-rate. The results suggest that tailoring the surfaces of Ni-rich NMC by using SE materials will be a promising approach to stabilize CEI of any types of high-voltage cathodes for next-generation EVs.