Solid Electrolytes As Li-Ion Conducting Additive and Interfacial Stabilizer for High-Voltage Cathodes

Abstract

Li-ion batteries (LIBs) have been a great facilitator to enable portable devices and electric vehicles (EVs) because of their high energy and power densities compared with other energy-storage devices. The high energy of LIBs is attributed to the high operating voltages of oxide-based cathodes (e.g., ~ 4 Vvs.Li) and wide electrochemical stability windows of organic liquid electrolytes (e.g., 1 – 4 Vvs.Li). In pursuit of extending energy density and lowering cost of LIBs further, R&D efforts have been devoted to finding the chemistries that can increase the cell operating voltages.However, increasing the voltages beyond the highest occupied molecular orbital (HOMO) of the electrolytes (c.a., ~ 4 Vvs.Li) leads to unwanted parasitic reactions at cathode-electrolyte interphase (CEI). In addition to the electrolyte oxidations at CEI, the reaction byproducts such as transition metal ions, HF, and CO2 can migrate to anodes and degrade their solid-electrolyte interphase (SEI) layers. As a result, high-voltage LIBs suffer from rapid capacity fading, growth of cell impedance, and gas generation.To mitigate the issue, various approaches have been proposed such as artificial CEI layer on cathode, tuning chemical compositions of cathodes, coating graphite anodes, using electrolyte additives, and adopting functional binders for cathodes. However, there is no simple solution to resolve all the complex issues occurring in the high-voltage cells. Considering that such parasitic reactions originate from the CEI and propagate to the SEI of anodes, there is a dire need of multimodal strategies that can create synergistic effect on stabilizing the electrode-electrolyte interphases in cell-level.Our research team has been developing multi-strategy that can coherently improve the high-voltage stability of LIB cells. Among them, we will present our recent approach of using solid-electrolyte (SE) powders as Li-ion conducting additive and interfacial stabilizer for high-voltage cathodes. As the proof-of-concept, Li6.7La3Zr1.7Ta0.3O12 (LLZT) SE was blended with Ni-rich Li(Ni1-xMnx/2Cox/2)O2 (NMC) or LiNi0.5Mn1.5O4 (LNMO) cathodes by relying on conventional electrode fabrication processes in LIB cells. This simple process offers advantages over the conventional coating methods in terms of manufacturing friendliness, energy saving, and cost effectiveness.Among various SE materials, garnet-type LLZT shows great promises based on its wide electrochemical stability window, good mechanical properties, and reasonable Li-ion conductivity (10-3 – 10-4 S/cm). First, the LLZT blended cathode significantly improved specific capacity and its retention during cycling at high voltages: e.g., 4.5 Vvs.Li for Ni-rich NMC and 5.0 Vvs.Li for LNMO spinel. Compared with the SE-free cathodes (baseline samples), the SE blended cathodes consistently delivered improved specific capacities, capacity retentions, and Coulombic efficiencies in full-cells made with graphite anodes.The improvement mechanism of the LLZT blended cathodes can be explained by two folds. First, LLZT contacts with cathodes (e.g., NMC or LNMO) and improves Li-ion transport properties of CEI layer, as evidenced by electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analyses. Second, LLZT can scavenge moisture/proton in the liquid electrolytes and subsequently suppress the degradation of CEI and SEI layers during extended cycles, as evidenced by X-ray photoelectron spectroscopy (XPS). As a result, 5 wt% LLZO blended cathodes delivered a stable electrochemical performance even at the presence of 5000 ppm moisture (and thus HF) in an electrolyte. In contrast to the traditional surface coating methods, solid-electrolyte blending approach is cost-effective and manufacturing/environmentally friendly, and thereby can serve as a practical pathway for improving performances and stability of current battery cells for EV and small electronics.