ИСТИНА

The enhancement of volumetric energy density of Li-ion batteries is critical to meet the requirements of electric vehicles and energy storage grids. The energy density of conventional LiFePO4 cathodes is limited by the amount of the added carbon conductor and inert PVdF binder which make up significant weight and volume fraction of the cathode. The performance of the electrodes can be improved by using conductive polymer binders combining adhesive properties with high electronic and ionic conductivity. We have developed composite binders based on poly(ethylene oxide) (PEO), polyphenylene oxide (PPO), or sulfonated poly(phenylene oxide) (sPPO) as ionically conducting binders and poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate) (PEDOT:PSS) polymer or carbon nanotubes (CNT) as electrically conducting components. Both PEDOT:PSS and CNT formed stable dispersions in PEO and sPPO solutions in “green” aqueous solvents but demonstrated poorer compatibility with PPO. The resulting composite binders have exhibited film-forming properties with electronic conductivity (1–40 S cm–1), Li+ ionic conductivity (10–5–10–4 S cm–1), and electrochemical stability over the potential range of 2.0–4.2 V vs. Li/Li+. The developed binders were used to fabricate LiFePO4-based cathode composites. The cathodes prepared with ionically conducting PEO and sPPO binders demonstrated lower charge transfer resistance comparing to conventional PVdF binder. It has been found that both PEDOT:PSS and CNT increased the macroscopic electronic conductivity of cathode blends, resulting in improved utilization of active material capacity. Even small amounts of PEDOT:PSS and CNT (2 wt.% and 0.5 wt.%, respectively) provided sufficient electronic conductivity of the cathode material (~10–2 S cm–1). Due to the low fraction of the electrically conductive components, the loading of the active material could be increased up to 95 wt.% resulting in enhanced energy density of the electrode. Carbon-free electrodes prepared with PEDOT:PSS and CNT also exhibited stronger adhesion to the aluminum current collector provided by sPPO and PEO co-binders. As a result, the cyclic stability of the developed cathodes was improved comparing to conventional binders. The PEO-based binders have revealed poor cyclic performance with conventional carbonate electrolytes due to PEO solubility of in the electrolyte solution. However, that issue has been resolved by using sulfolane-based electrolytes. The PPO and sPPO binders can be used in conventional carbonate electrolytes, showing higher stability and cyclability. The PPO binder revealed improved electrochemical stability comparing to sPPO binders. The rate capability of the cathodes is highly influenced by the cathode porosity. The cathode containing 2.5 wt.% of PEDOT:PSS, 2.5 wt.% of sPPO, and 95 wt.% of LiFePO4/C has demonstrated the optimal porosity of 43%. The packing density of the active material has been increased in the presence of sPPO binder, and the resulting cathodes have exhibited enhanced volumetric energy density at discharge rates 0.1C–50C In summary, the developed polymer binders can replace conventionally used carbon black and PVdF in the LiFePO4 cathode composition to enhance its specific capacity, cyclic performance and volumetric energy density. Acknowledgments: The authors acknowledge financial support from the Russian Science Foundation (project N 17-73-30006).