Superior Electrochemical Performance and Rechargeability of Zinc Anodes Via Additive Manufacturing

Abstract

Current roll-to-roll slurry-cast battery electrode manufacturing methods create serious restrictions for electrochemical energy storage, including: poor conductivity due to the presence of electrochemically inactive and electronically insulating binders, low energy density due to the need for foil current collectors, and limited form factors for electrode size/shape and cell design. Here, we report the benefits of advanced manufacturing methods for zinc anode rechargeability and performance.Zinc-based batteries pose an attractive alternative to lithium-ion (Li-ion) systems due to their excellent gravimetric and volumetric capacity, the increased safety and ionic conductivity of aqueous electrolytes, and the relative abundance and low price of zinc.Zinc metal is used as an anode in commercial alkaline batteries (i.e., Zn-MnO2) and other primary systems whose specific energies are comparable to (or higher than) Li-ion, such as Zn-Ag, Zn-Ni, and Zn-air, but which lack sufficient rechargeability to compete with Li-ion. Commercial zinc anodes comprise either zinc foil, which has high electronic conductivity but very low surface area, or zinc powders (i.e., zinc-binder powder composites or pastes), which have high surface area but poor inter-particle conductivity and long-range electrode connectivity; unfortunately, both of these form-factors significantly hinder zinc anode performance.One groundbreaking approach to overcome these problems utilized a monolithic 3D porous zinc “sponge” electrode architecture to produce high surface area and improved electronic conductivity [1]. These electrodes achieved ~ 90% zinc utilization when discharged in a primary Zn–air cell, and could be electrochemically cycled without macroscale dendrite formation in symmetric test cells (i.e., pure Zn sponge electrode vs. sponge electrode with Zn core/ZnO shell). Furthermore, nickel–zinc sponge alkaline cells achieved >100 high-rate cycles to 40% DODZn at lithium-ion-commensurate specific energy, and tens of thousands of cycles mimicking start-stop microhybrid vehicle duty cycles. The authors attributed this exceptional performance to architectural aspects of the electrodes, namely the interconnected porous voids and zinc domains. However, the authors acknowledged that the 3D porous “sponge” architecture could be improved through advanced manufacturing methods, such as laser powder bed fabrication (LPBF), which creates direct metallic connections between individual metal particles, without the need for solvents or non-conductive binders/surfactants. Additive manufacturing also provides exquisite control over electrode form factor, with high precision and resolution. Although porosity poses problems for the structural integrity of additively manufactured parts, the granular nature of sintered zinc powder has several potential benefits for electrochemical performance, including:(i) the direct, metallic connections between zinc particles are not degraded during charge/discharge, which preserves metallic conductivity throughout extended cycling;(ii) maximizing the total surface area of electrode in contact with the electrolyte.Another principal advantage of LPBF additive manufacturing is the ability to tune the density by adjusting the laser input to control the degree of melting. However, the main challenge for optimizing these additively fabricated samples is to create electrodes that are thin and porous enough to achieve complete zinc discharge and maximize the electrified electrode-electrolyte interfacial contact area without compromising the electrodes’ dimensional stability and integrity during extended charge/discharge cycling.Here, we present an electrochemical analysis of binder-free, pure zinc samples fabricated additively from zinc metal powder using LPBF. The manufacturability of thin-walled metallic deposits was demonstrated over a wide range of operating conditions, and the structures were characterized in terms of their thickness (200-500 µm), density (ρ=4-6.7 g/cm3) and roughness (Ra =10-15 µm).The electrodes provided a useful starting point for an evaluation of their electrochemical performance as anodes, and demonstrated facile electrochemical reversibility and extended cyclability, even without special electrolytes or electrode additives to improve electrochemical performance. The samples displayed stable and consistent cyclic voltammetry (CV) cycling at scan rates between 0.01 V s-1 and 1 V s-1, and completed multiple cycles at 50 mV/s without significant loss of capacity. Further, zinc-zinc symmetric cells containing additively manufactured anode and cathode displayed low overpotential (i.e., approximately 25 - 35 mV) and survived more than 2000 continuous hours of galvanostatic charge/discharge testing at 10 mAh cm-2. Full-cell galvanostatic charge/discharge testing is currently underway.This investigation provides a foundational starting point for further investigations of the benefits of additive manufacturing as a tool; for optimizing the architecture and structural features of zinc anodes. The results of our preliminary study portend well for our future efforts, which will focus on optimizing the thickness, density/porosity, geometric precision, and particle connectivity of additively manufactured zinc electrodes. REFERENCE [1] Parker JF, Chervin CN, Nelson ES, Rolison DR, LongJW.Wiring Zinc in Three Dimensions Re-Writes Battery Performance-Dendrite-Free Cycling. Energy Environ. Sci. 2014, 7, 1-3.