Operando Visualization and Interfacial Engineering of Li Metal Anodes

Abstract

The grand challenge of climate change has created an enormous need for superior battery technologies that deliver higher energy/power densities at low cost without sacrificing safety and cycle life. Such batteries would enable widespread vehicle electrification, increased use of renewable energy sources, and dramatic improvements for myriad other energy storage applications. One of the most promising approaches to offer a step-increase in energy density, lithium (Li) metal anodes, boast 10x higher specific capacity than graphite anodes. Unfortunately, implementation has been limited by capacity fade and safety concerns that stem from interfacial instability and morphology evolution during cycling.

The primary goal of this thesis is to better understand the interfaces and interphases between Li metal electrodes and electrolytes, and thus enable enhanced performance through interfacial engineering. The work has two thrusts: 1) characterization of liquid and solid-state electrolyte (SSE) interfaces with Li metal to observe, correlate, and understand coupled electro-chemo-mechanical phenomena; and 2) development of atomic layer deposition (ALD) films for SSEs, interfacial coatings, and interlayers.

In thrust one, operando video microscopy is developed to characterize the dynamic changes occurring at Li metal interfaces and correlate them with their electrochemical signatures. This technique is used to develop a comprehensive model of reaction pathways on Li metal electrodes. This model explains how transitions between these reaction pathways, driven by spatially varying kinetics and morphology evolution on the electrode surface, result in distinct electrochemical signatures.

To better understand the Li/electrolyte interface in lithium metal solid-state battery (LMSSB)s, the correlation between surface chemistry, wettability, and interfacial impedance of LLZO SSEs is explored with x-ray photoelectron spectroscopy (XPS) and sessile drop tests. This demonstrates the coupled behavior at Li/SSE interfaces and that control of surface chemistry can enable higher rate capability. Operando video microscopy is adapted to SSE systems. Four distinct morphologies of Li penetration are identified in SSEs, and studied under a range of plating and stripping conditions. The voltage signatures of Li penetration in SSEs are compared with those of liquid electrolytes, to better understand the reaction pathways at the Li/SSE interface. The rate of propagation of Li penetration is quantified as a function of applied current to gain insight into the coupled electro-chemo-mechanical behavior of the system. Void formation in the Li electrode at the Li/SSE interface is observed during deep discharge, demonstrating the importance of morphology evolution during both plating and stripping.

In thrust two, ALD of Al2O3 is used to improve the homogeneity of Li flux across the electrode/electrolyte interface. Cycle life and deep discharge performance are doubled. Subsequently, ALD processes are developed for two SSEs, Al-doped LLZO and glassy Li3BO3-Li2CO3 (LBCO). Challenges with low ionic conductivity and post-annealing of the LLZO films are overcome with the LBCO films, which do not require crystallization to obtain high ionic conductivities. ALD LBCO films demonstrated to have approximately 6x higher ionic conductivity (2.2*10-6 S cm-1) than other reported ALD films. The films also have good electrochemical stability at both high and low potentials, and are incorporated into a Li metal battery with high Coulombic efficiency and good cycle life.

In summary, this thesis furthered the understanding and performance of Li metal anodes through the development and use of novel methods of characterization and means of interfacial modification. The implications of this work could aid in the development of next-generation Li metal batteries.