An Alternative Binder System for Improving the Fast-Charging Capability of Water-based Lithium-Ion Battery Graphite Anodes

Fast to ultra-fast charging times for battery electric vehicles are becoming of more and more interest, especially for car owners without own charging capabilities at home. From a technical point of view, it is important to offer short charging times without negatively affecting the lifetime and aging of the used lithium-ion battery (LIB).^1–3 High charging currents can lead to high overpotentials, which cause the anode potential to drop locally below the lithium (Li)⁺/Li⁰ equilibrium potential, resulting in Li deposition. Li deposition can lead not only to performance decay, but also to heat generation and safety issues.^3–7 To counteract high overpotentials during charging, one method is to accelerate the kinetically limited processes on the anode side, as this electrode is considered the main limiting factor when applying high charging currents.^{8, 9} Not only the choice of active material has a decisive influence on the fast-charging capability, but also the choice of passive materials such as binders.¹⁰ The conventionally used binder system for water-based graphite anodes, a combination of CMC and SBR, has good rheological as well as mechanical properties.^10–12 However, the Li-ion mobility of the binder system is limited, which leads to problems especially at high charging currents.¹³

This work presents an alternative binder system in comparison to the conventional binder system (CMC and SBR) for aqueous graphite anodes in LIBs. The used alternative binders were selected based on their chemical and electrochemical properties, taking into account, among other things the intrinsic Li-ion mobility. The various alternative binders were combined systematically based on their processability, rheological and electrochemical properties. The binder system developed in this process showed an improvement in fast-charging capability in research pouch cells (2.55 mAh cm^-2, NMC622 cathode) at room temperature, as well as at 45 °C. The improvement in the fast-charging capability could be attributed to an improved Li-ion mobility in the porous structure of the electrode, but also to a modified microstructure of the electrode. Overall, the fast-charging capability at room temperature at a charging current of 4C (10 mA cm^-2) in the CC-step was improved by over 150 % compared to the reference binder system. The alternative binder system also shows improved cycle stability with a SOH of 82.6 % after 1500 cycles, while the reference binder system only has a SOH of 79.3 %.

Acknowledgement

We gratefully acknowledge the financial support of the Federal Ministry for Economic Affairs and Energy (BMWK) as part of the project structur.e (03ETE018E).

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