Surface science for improved lithium metal batteries
Keywords: Engineering.
Chemical Engineering.

Surfaces and interfaces abound in simple and complex devices alike, influencing phenomena like kinetics, transport, and thermodynamics. As such, the insights derived from the chemistry and physics of surfaces can be translated into practical engineering strategies for enabling better devices. This insight-engineering relationship has been demonstrated in numerous devices such as solar cells and transistors. Similarly, the plethora of surface-mediated processes in batteries positions them to benefit from the power of surface science. This dissertation was focused on the use of a surface modification technique called atomic layer deposition (ALD) for unraveling crucial phenomena that improve the performance of lithium metal batteries. Lithium metal batteries have immense energy storage capability owing to the high specific capacity and low redox potential of Li (3860 mAh/g and -3.04 v standard hydrogen electrode, respectively). However, the deployment of lithium metal batteries has historically been limited by the high reactivity of lithium. To address the high reactivity of lithium, two classes of strategies are often employed: control of the surface area of lithium to reduce its extent of reaction within batteries, and the in-operando design of interfacial films that isolate the reactive surface of lithium from other battery components. Though these strategies have been effective, they are often carried out using methods that obfuscate the physical origin of their success, making it challenging to identify which measures to take for commercial lithium metal batteries. To design lithium metal batteries with long term stability, it is essential to understand the physical principles underlying the control of lithium reactivity. In this dissertation, we introduce a new battery architecture that elucidates the physical principles for controlling lithium reactivity and provides strategies for design improvement. In the battery architecture, thin films grown using ALD are situated beneath lithium metal, providing insights into the control of lithium surface area and the chemistry of interfacial films formed atop Li. We find that films such as TiO2 featuring a low Li interfacial energy facilitate the growth of lithium particles with low surface area, resulting in up to four-fold improvement in battery cycling performance. We show that films with high electrical resistance like Al2O3, rather surprisingly, restrict the nucleation of lithium to pinhole sites, then facilitate the lateral growth of low surface area lithium particles which improve battery performance almost ten-fold. Additionally, we demonstrate that a polar film such as Al2O3 modifies the local distribution of reactive species near electrically active substrates like lithium, thereby altering the chemistry of interfacial films that form on the surface of lithium. In addition to the new battery architecture introduced in this dissertation, we have demonstrated other interfacial science approaches for controlling and understanding the reactivity of lithium. We have utilized molecular layer deposition to grow a new aluminum-methacrylate hybrid film with properties that could prove useful in stabilizing the interface of lithium metal. Through electrochemistry, we have also shown that current and voltage are parameters that can control the structure and chemistry of the interfacial film formed atop lithium metal, resulting in up to a 7% improvement in efficiency during battery cycling. Additionally, we have employed data science techniques to inform the discovery of battery electrolytes that react minimally with lithium, resulting in Coulombic efficiencies as high as 99.7%.