Defect and Surfactant Mediated Electrochemical Deposition of Lithium and Sodium

NON-TECHNICAL SUMMARY

The electric storage capacity of Li-ion battery technology is limited, leading to issues such as cell phones that cannot retain enough charge for a full day and electric vehicles that cannot travel for more than 300 miles before they need to be plugged in. Graphite, currently used as a lithium reservoir in Li-ion batteries, has limited storage capacity rate capabilities.

Lithium is stored inside the atomic structure of graphite, which minimizes the formation of lithium metal filaments, or dendrites, that can cause the battery to short-circuit. However, if charging is done too quickly, there is insufficient time for the lithium ions to be transported into graphite, resulting in lithium accumulation on the surface of the graphite, raising safety concerns.

In principle, if lithium and other alkali metals such as sodium could be directly electrodeposited onto the surface of a suitable substrate without the formation of dendrites, both the storage capacity and charging rate could improve significantly. The advantage of sodium is that it is much more earth abundant than lithium, which would significantly reduce costs.

By taking advantage of analogies between electrochemical and physical vapor deposition, this research is developing transformative electrodeposition methods to enforce the evolution of atomically flat surfaces that prevent dendrite formation. This knowledge is being used to guide models that inform the design of the next generation of alkali metal batteries.

The project involves a range of activities to broaden participation of underrepresented minorities in science and will introduce the next generation of materials scientists and engineers to the varied skills needed to maintain U.S. agility in battery technology. These efforts are contributing to and leveraging investments by the State of Arizona through its New Economy Initiative, which seeks to address local workforce needs and foster growth of industries through establishing the state-funded science and technology center on Advanced Materials, Processes and Energy Devices.

TECHNICAL SUMMARY

The electrochemical deposition of virtually all metals can result in the evolution of dendrites, where growth occurs at the tip of a protrusion involving tip-splitting phenomenon. Growth of a protrusion can also occur from its base, by an extrusion process at high enough homologous temperature, as a result of compressive stresses that evolve during electrodeposition.

This type of growth is called a whisker. The ambient temperature electrodeposition of both Li and Na often results in the development of dendrites and whiskers, which is the major issue in developing practical metal batteries, since this can lead to short circuiting of the batteries. The key issue this project addresses is the identification of electrochemical parameters that result in two-dimensional and atomically flat electrochemical growth of Li and Na without the development of dendrites or whiskers.

This project focuses on applying concepts derived from manipulating the kinetics of thin-film growth modes in physical vapor deposition to obtain atomically flat overlayers for film-substrate systems that conventionally result in three-dimensional Volmer-Webber growth. One of the approaches used is called Defect Mediated Growth and involves the use of potential pulsing routines and specific additives in the electrolyte referred to as mediator metals.

The other approach, called Surfactant Mediated Growth, involves an additive that floats on the surface of the depositing Li or Na and facilitates an interlayer exchange process promoting the development of flat electrodeposited films. Physical vapor deposition is used to identify the surfactants subsequently used in electrochemical deposition-dissolution cycling of Li and Na to obtain dendrite-free films.

Kinetic models of physical vapor deposition verify the role of the surfactants in the experiments and mechanism for formation of atomically flat overlayers. The surfactant-based approach also addresses pitting during the dissolution process and how preferential deposition inside a pit can be achieved without the occurrence of dendrites.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.