UC Berkeley

This study examines the feasibility of utilizing traditional flexographic printing technology for large-scale zinc-based battery manufacturing for grid energy storage applications. The design and development of functional flexographic inks is the main goal of this study. Printed battery electrochemical performance is also a focus area.

Long-life, energy dense, cost effective electrochemical energy storage systems for power grid applications have become a fast-emerging industry in recent decades. Grid energy storage is widely regarded as an important component of the smart grid, because of its potential role in complementing intermittent renewable energy sources. However, battery technologies have not improved much over the past few decades. Both new battery chemistries and fabrication processes are needed to significantly reduce battery cost and to allow for easy integration with renewables.

Printable batteries were designed based on fundamental electrochemical principles governing the battery performance, including thermodynamics, reaction kinetics as well as transport properties. With cost and application factors taken into account, practical battery system design criteria were also summarized with regard to battery geometry, chemistry and fabrication technology.

A survey of current main printing technologies was conducted. Based on the criteria developed for functional printing process design and selection, a comparison of the technologies was made and a roll-to-roll flexographic printing process for rechargeable zinc-based battery manufacturing was proposed. Based on the fundamental operating mechanism of flexography, key criteria for developing functional flexographic printing inks were established, including composite ink rheology (steady-state viscosity and yield stress), ink wettability as well as ink dispersing qualities. The ink viscosity significantly influences the ink transfer efficiency while the yield stress critically determines its structural integrity once transferred on flexible substrate. The ink wettability indicates the ink spreading properties and film uniformity while the ink dispersing quality affects the ink homogeneity from before printing through the printing process. A variety of MnO2 cathode inks were formulated and analyzed based on these criteria. A novel type of aqueous cathode ink based on PSBR polymeric binder showed excellent flexographic printability.

Extensive electrochemical characterizations with the flexographically printed PSBR-based composite MnO2 cathode were then conducted. Full cells consisting of dispenser-printed electrolytes and zinc foil anodes were assembled. The cyclic voltammetry method was used to study the reversible zinc intercalation through ionic liquid electrolyte into the aqueous-based cathode. Galvanostatic cycling showed that the cell capacity stabilized after about twenty cycles and the capacity varied significantly with discharge current density. Electrochemical impedance spectroscopy measurements revealed the interfacial resistance between the gel electrolyte and zinc foil, as well as the evolution of impedance components through cycling, for a full zinc- based cell system. Coin cells based on zinc/ionic liquid electrolyte/MnO2 chemistry were made in an inert argon environment and then characterized to study zinc-based chemistry performance in this controllable environment. The coin cells showed comparable behavior to batteries printed in the ambient environment. Printable PSBR-based nickel current collector inks have also been developed for an entirely printable zinc-based battery, to conveniently integrate with other electronics on non-conductive, flexible substrates.

An integrated energy-harvesting prototype was fabricated, which was consisted of dispenser- printed thermoelectric energy harvesting and electrochemical energy storage devices with a commercial voltage step-up converter. Parallel-connected thermoelectric devices with low internal resistances were designed, fabricated and characterized. The use of a commercially available DC-to-DC converter was explored to step-up a 27.1mV input voltage from a printed thermoelectric device to a regulated 2.34V output. The voltage step-up circuit efficiency reached as maximum of 32.4% during the battery charging process while the battery charging efficiency was approximately 67%. The prototype presented in this study demonstrates the feasibility of deploying a printable, cost-effective and perpetual power solution for practical wireless sensor network applications. This work paves the path for potential integration of printable photovoltaic cell, zinc-based battery as well as relevant electronics for grid energy storage applications.