In this Work Package (WP2), Bedimensional and Politecnico di Torino designed and manufactured graphene-based reference electrodes printed on battery separators, i.e., Celgard 2500. These reference electrodes, named level-2 sensors, enable: 1) the monitoring of the potentials of the cell electrodes distinctively under both static and dynamic operation; 2) operando half-cell impedance measurements through artifact-free electrochemical impedance spectroscopy (EIS) measurements.
The reference electrodes were printed from slurries containing: 1) lithium iron phosphate (LFP) or lithium titanate (LTO) as the active materials that undergo two-phase reactions upon intercalation or deintercalation with Li, resulting in stable and constant equilibrium potential; 2) polyvinylidene difluoride (PVDF) or carboxyl methyl cellulose (CMC) as binders that can be dissolved in solvents (N-Methyl-2-pyrrolidone and water, respectively) compatible with polypropylene separators; 3) and carbon black (CB) and wet-jet milling (WJM)-produced single-/few-layer graphene (SLG/FLG) as the conductive materials. The content of the various materials was optimized to balance electrical, mechanical, and thermal performances, while adjusting the rheological properties of the slurries to be processed by high-throughput printing techniques, i.e., screen and stencil printing. Printed reference electrodes exhibit an electrical resistance as low as 0.06 Ω×cm. This property enabled the modelling of the reference electrodes with equivalent electrical circuits, proving their suitability for EIS measurements at frequencies as high as 100 kHz, as needed for the evaluation of battery components.
The reference electrodes were incorporated into pouch cell, evaluating both three- and four-electrode configurations using the EC:DEC 1:1 LiPF6 1 M as electrolyte and NMC622 and graphite as anode and cathode, respectively. The functionalities of the printed reference electrodes were then evaluated in 5 cm2 pouch cells through extensive electrochemical characterizations, including galvanostatic charge-discharge (GCD) cycling and EIS measurements. Meanwhile, no interference of the reference electrodes with battery cell performances has been demonstrated compared to baseline cells without reference electrodes.
The results of WP2 activities were used for the realization of 1 Ah multi-layered pouch cell prototypes by battery manufacturers participating to SENSIBAT, i.e., VAR and ABEE, who also optimized the protocols to incorporate printed reference electrodes into practical cell configurations. These protocols included the methods to connect Al tab to printed reference electrodes through ultrasonic welding, as well as the positioning of the reference electrode-coated separators within multi-layered pouch cells.
The data obtained in reference electrode-incorporating pouch cells have been used to develop advanced battery management systems (BMSs) for Li-ion batteries with enhanced safety. Preliminary cost analysis concluded that the addition of printed reference electrodes doesn’t increase the overall cost of the cells considerably, making them suitable for market deployment and offering the opportunity to further drive down the cell cost by increasing their useful cycle life.