(Invited) Non-Conventional, Multimodal Strategies to Create Synergistic Effects on Electrode-Electrolyte Interphase Stabilities in Lithium-Ion Batteries
Authors: Kim Jung-Hyun 
Published: November 2022

Ni- or Mn-rich cathode materials have attracted great interest for electric vehicle (EV) applications due to their high gravimetric and volumetric energy densities and low- or cobalt-free compositions. For example, Ni-rich LiNi1-xCox/2Mnx/2O2 (NMC) layered cathodes can deliver > 200 mAh/g when they operate at above 4.3 Vvs.Li. Cobalt-free LiNi0.5Mn1.5O4 (LNMO) spinel operates at around 4.75 Vvs.Li and delivers an excellent rate capability due to three-dimensional Li-diffusion pathways in its lattice. However, due to lack of high-voltage stabilities of conventional, carbonate-based electrolytes, such high-voltage operation (> 4.3 Vvs.Li) led to unwanted electrolyte oxidation and simultaneous parasitic reactions occurring at CEI. Different types of cathodes have their own failure mechanisms which are often associated with side reactions occurring at solid-electrolyte interphase (SEI) on graphite anodes. This was possible because the reaction by-products from CEI could migrate and attack SEI layers on anodes.

To address the high-voltage related issues, various approaches have been proposed such as tuning chemical compositions of cathode active materials, surface modification of cathodes or anodes, and stabilizing the CEI or SEI using electrolyte additives. In literature, however, there was no single solution that can resolve all the complex issues occurring in the high-voltage battery cells. For example, coated layers on active materials are prone to fail during long-term cycling due to mechanical failure (e.g., crack and pulverization) of active materials. In addition, not only active materials but carbon conductors (e.g., acetylene black) inside a cathode leads to the electrolyte oxidation, prompted by its high electronic conductivity and large surface area. Considering that such parasitic reactions originate from CEI and propagate to anode SEI, there is a dire need of multimodal strategies that can create synergistic effect on stabilizing the electrode-electrolyte interphases in cell level.

In this presentation, I will introduce non-conventional strategies that can effectively enhance electrode-electrolyte interphase stabilities and improve full-cell (i.e., using graphite anodes) performances. First, formation of self-healing CEI on cathode active materials will be demonstrated. The interface of LNMO spinel oxide will be passivated by Ti-enriched shell via sacrificial transition metal dissolutions and suppress the side reactions occurring at CEI. Second, a strategy for passivating both cathode active materials and carbon black conductor by using functional binders will be demonstrated. Since the binder coating will be performed in-situ during a slurry preparation, no extra process will be required while implementing this approach to a cell manufacturing. Finally, solid-electrolyte implemented NMC or LNMO cathodes will be demonstrated as an effective approach to stabilize the high-voltage performance by offering good Li-ion conduction pathways at CEI layer.

Improvement mechanism of each approach will be also presented based on physical and electrochemical characterization data. Electrochemical impedance spectroscopy (EIS) revealed a suppression of CEI interfacial resistance during cycling, suggesting the enhanced stability of the multifunctional CEI. Various microscopy and spectroscopy data acquired from cycle-aged CEI and graphite SEI will be corelated to the cell performance data and identify the CEI property-performance relationship. These unique multifunction CEI strategies will be applicable to various cathode materials for the next-generation Li-ion batteries.

Figure 1