(Invited) Improvements in High Energy Density Silver-Zinc Button Cells for Wearable Devices

Abstract

The silver (Ag)-zinc (Zn) system has the highest theoretical specific (Wh/kg) and volumetric energy density (Wh/L) amongst all rechargeable batteries with the added benefit of safety and non-flammability compared to lithium-ion or lithium metal batteries, which makes it a good and ideal candidate for portable applications like hearing-aids. However, historically, the Ag-Zn battery has suffered from poor rechargeability and low energy utilization. At ZPower, we have tailored the chemistry and engineered a new Ag-Zn battery system, which is highly energy dense and rechargeable, thus making it possible to use it portable applications and giving more power to the consumer to allow them to contribute to a greener future. As aforementioned, enhancing the capacity of the active materials is paramount for high energy density, which we achieved by developing proprietary additives and methods. For the cathode, silver oxide (AgO) was used in our system. We developed novel proprietary conductive coatings for AgO, which allowed high capacity utilization and retention for many cycles. We conducted gassing tests, where we found the coatings considerably reduced oxygen generation. The role of these coatings was further studied through charge algorithm cycling, where we found that it reduced AgO cathode’s impedance compared to the control tests and maintained its active surface area. More importantly, we found that the morphology of the proprietary conductive coatings played a crucial role in efficiently utilizing the AgO cathode’s capacity. For the anode, Zn was used, where it has a theoretical specific capacity of 820 mAh/g; however, Zn has a long history of poor rechargeability due to problems like shape change (zinc redistribution), dendrite formation and passivation. In the past, previous researchers tried altering the Zn formulation to increase utilization; however, these attempts were not fruitful. We have engineered a new Zn anode that can access 94% of its theoretical capacity in a single primary discharge and 56% of its theoretical capacity rechargeably for over 300 cycles. We achieved this breakthrough by developing proprietary additives and methods that localize the Zn to the current collector and prevent dendrite formation. We will comment on the anode performance further in the talk. Finally, we will present impedance models and data of the ZPower Ag-Zn system measured with respect to state of charge, storage time and temperature, which is vital from a manufacturing standpoint to understand cell properties and make future cell and electrode improvements. We will comment on our future work of identifying impedance contribution from the cathode and anode components of the battery and using this data as a guideline for improving electrode composition and cell performance.