(280a) Conformal Protective Coating of Cathode Materials By Oxidative Chemical Vapor Deposition for Enhancing Lithium-Ion Batteries.

Lithium-ion batteries (LIBs) are relatively energy dense, which make them suitable in energy-demanding applications such as electric vehicles and grid storage. To further increase energy density and cycle life, highly active cathode materials are being pursued, such as layered lithium transition metal oxides for the battery cathode. Despite their relatively high capacity, layered lithium transition metal oxides (e.g. nickel-cobalt-manganese or NCM) undergo crystal and interfacial degradation when subject to aggressive electrochemical and thermal forces, leading to a rapid drop in performance and poor cycle life. These problems are more severe under more stressful battery operating conditions, such as high voltage and high temperature cycling, which can lead to parasitic side reactions on the cathode surface. The failure mechanisms include interfacial degradation, electronic and ionic isolation, undesired cathode-electrolyte reactions, and transition metal dissolution that lead to impedance increase and capacity loss. Thus, the interfacial challenges of cathode materials need to be addressed to achieve longer cycle life. A proposed solution to overcome these challenges is to use a thin film deposition process to protect the electrode surface, which is a direct means to increase the longevity of cathode materials [1,2].

In this work, we illustrate the use of oxidative chemical vapor deposition (oCVD) as a thin film polymer deposition technique that allows a conformal protective barrier of a conducting polymer to grow on the surface of the LIB NCM cathode particles. The oCVD process, as a solvent-free technique, overcomes many obstacles posed by liquid-based processing methods, such as non-conformal and non-uniform coatings due to poor wettability, and damage to substrates from liquid solvents. oCVD is a versatile method that can synthesize electrically conductive poly(3,4-ethylenedioxythiophene) (PEDOT) in a solventless environment by enabling the oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) monomer using vanadium oxytrichloride (VOCl₃) oxidant in the vapor phase. The oCVD process enables ultra-conformal and highly uniform PEDOT coating on topologically complex structures. This approach makes oCVD a promising candidate for coating PEDOT as a protective layer on NCM cathode materials to enhance the structural stability, capacity retention, and cycle life of LIBs.

Method and Results

This work studies the encapsulation of NCM111 cathode materials with PEDOT coatings using a novel rotary oCVD reactor system that allows the coating of dry cathode particles. The goal is to determine the effect of PEDOT on enhancing LIB performance. The EDOT monomer and VOCl₃ oxidant are metered into the oCVD reactor at a flow rate of 2 and 1 sccm (standard cubic centimeters per minute), respectively, to achieve a 2:1 monomer to oxidant flow ratio. The reagents were fed from source jars that were heated to achieve sufficient vapor pressure. The reaction flask is charged with dry NCM cathode particles (1–2 g; 1–10 μm particle diameter) and is continuously rotated at 140 rpm to achieve constant particle agitation and surface exposure to the reagents. The flask temperature is maintained by a temperature-controlled oil bath that was varied from 90 to 130 °C. The deposition time was varied from 60 to 100 minutes to achieve different film thicknesses. The pressure was maintained by a capacitance manometer and butterfly valve, and was kept constant at 300 mTorr for all the trials.

The electrical conductivity of the PEDOT-coated cathode material on an aluminum current collector was measured with a four-point probe. The film conductivity was found to strongly correlate with temperature and deposition time. At a deposition temperature and time of 90 ºC and 80 min, respectively, the electrical conductivity reached a maximum of 600 S/cm. Further increasing the deposition time results in a decrease in conductivity, which is attributed to an increase in the density of film defects, impurities such as unreacted reagents, and polymer chain disorder. With the deposition time fixed at 80 min, the effect of the deposition temperature on the electrical conductivity was studied. By increasing the temperature to 130 ºC, the conductivity increased dramatically to 5000 S/cm. This is attributed to both a slower film deposition rate and more favorable polymer crystalline orientation. The higher flask temperature lowers the rate of monomer adsorption, which decreases the deposition rate and hence produces a more uniform coating with fewer impurities and film defects. The resulting thinner film also minimizes the increase in impedance caused by the increased diffusion distance of lithium ions across the PEDOT film. In addition, at higher temperatures, the crystalline orientation of the PEDOT film is known to transition from an edge-on orientation to a face-on orientation of the aromatic rings, which enhances the conducting pathway and hence electrical conductivity.

After optimizing the PEDOT deposition conditions to 80 min and 130 ºC, the coated NCM particles were assembled into LIB half cells with a Li metal reference anode and liquid electrolyte in an argon glove box. Uncoated NCM particles were also assembled into coin cells as a reference comparison. The cells were formation-cycled at C/10 (20 mA) at room temperature for 5 cycles. The PEDOT coated cells achieve an initial capacity of 220-240 mAh, while the uncoated cells achieve an initial capacity of 180-190 mAh. The higher initial capacity of the PEDOT coated cells is attributed to both the protective layer against electrolytic attack and the PEDOT film acting as active material that can store Li ions. After formation cycling, the coated cells have a starting capacity of 210 mAh, and the uncoated cells have a starting capacity of 160 mAh. The cells were subsequently cycled at 1C (200 mA) between 3 and 4.6 V at room temperature. The coated cells achieve 91% capacity retention after 100 cycles, while the uncoated cells have only 54% capacity retention. Furthermore, additional coin cells were assembled to study the effect of the PEDOT thickness on cycling performance. The thinner PEDOT films provide insufficient protection from the electrolyte, and the thicker films significantly increase the Li ion transport distance, both resulting in lower performance compared to the optimized conditions.

Implications and Future Work

This work shows that oCVD PEDOT can be used to fabricate conductive and protective coatings on NCM cathode particles to protect them from degradation in lithium-ion batteries. This technique can be expanded to other cathode materials that are especially vulnerable to electrolytic attack and suffer from low conductivity. PEDOT coated cathodes have been found to enhance both the capacity retention and cycle life of lithium ion batteries. More broadly, the fabrication of coated cathodes makes lithium ion batteries more practical for high storage and high power applications such as electric vehicles and grid storage, where high-capacity retention and cycle life are simultaneously sought after. Additional work is currently underway to not only coat NCM powders but also coat the assembled cathode structure to encapsulate not only the active cathode particles but also the electrically insulating binder, which should lower series resistance, improve mechanical stability and further enhance cycle life. Furthermore, the coated cells will be tested at more stressful conditions, such as elevated temperatures (>40 ºC) and higher currents (>2C), to determine whether the PEDOT coating allows the cells to endure harsher charge-discharge conditions. By optimizing the oCVD processing conditions, batteries with higher capacity and longer cycle life can be manufactured to meet increasing energy storage demands.

References

1.Im, Sung Gap, and Karen K. Gleason. "Systematic control of the electrical conductivity of poly (3, 4-ethylenedioxythiophene) via oxidative chemical vapor deposition." Macromolecules 40.18 (2007): 6552-6556.

2.Xu, Gui-Liang, et al. "Building ultraconformal protective layers on both secondary and primary particles of layered lithium transition metal oxide cathodes." Nature Energy 4.6 (2019): 484-494.