(6dd) Electrochemical Ion Insertion: Mechanisms and Applications in Energy Storage and Computing

Electrochemical ion insertion is a powerful method to tune the properties of materials. In such devices, an electronic current and voltage is used to move an ion in and out of the interstitial or layered sites of a crystal host [1]. By injecting electrons into the host using an external circuit, a positively charged ion like Li⁺or H⁺follows the electron from the ion reservoir into the host and vice versa, restoring electroneutrality. This changes the chemical stoichiometry of the host material, and allows us to conduct solid-state chemistry at room temperature by changing the current and voltage. This “materials switch” continuously and reversibly tune the stoichiometry of the host by electrically controlling the injected charge.

My research aims to use advanced characterization and microfabrication to further elucidate mechanisms of ion insertion as well as expand the range of ion insertion devices, with a focus on devices for energy applications. Two of my core interests today are batteries for energy storage and low-energy computing using artificial neural network. In the batteries space, I aim to understand the mechanisms of ion insertion through advanced characterization and precise quantification. In my doctoral work, I used synchrotron X-ray microscopy to observe the insertion and migration of lithium within individual particles in situ [2]. I correlated such nanoscale, time-resolved measurements with phase-field modeling accounting for both reaction and diffusion within solids [2-3]. I intend to further utilize advanced characterization, model microfabricated systems, and precise geometries to elucidate reaction, diffusion, and phase transformation processes in lithium insertion materials used for batteries. In particular, I want to treat individual battery particles as nanodevices in order to understand the intercalation mechanisms.

I also aim to create new, low-energy electronic devices that operate on the basis of electrochemical ion insertion. Here, electrochemical lithium insertion dynamically modifies the doping concentration, enabling such devices to be used as analog non-volatile memory artificial synapses. Inspired by the human brain, this post-Moore computing architecture can reduce the energy consumption of artificial neural networks by 2-3 orders of magnitude compared with an optimized silicon-based system, and is compatible with many of the deep learning algorithms used today. Future work here entails identifying and optimizing materials that improve the switching speed through engineering bulk diffusion and surface reaction, improving the endurance and non-volatility, and designing architectures for learning and training. My current work as a Truman Fellow at Sandia National Laboratories investigates the potential of TiO₂ for use as an artificial synapse.

Select publications:

[1] Li. Y. and Chueh, W. C. Electrochemical and chemical insertion for energy transformation and switching. Annual Reviews of Materials Research 48, 137-165 (2018).

[2] Lim. J*, Li. Y.* et al. Origin and hysteresis of lithium compositoinal spatiodynamics within battery primary particles. Science.353,566–571 (2016). Highlighted in DOE, BES Basic Research Needs 2017.

[3] Li, Y. et al.Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes. Nature Materials.13,1149–1156 (2014).

*Equal contribution author

Teaching Interests:

I have broad teaching interests within chemical engineering and materials science, including transport, reaction, thermodynamics, electrochemistry, renewable energy, crystal structures, and electronic properties. I will flavor my classes with an increased emphasis on applications towards energy technology, when appropriate. For example, in a new thermodynamics class that I designed with my PhD advisor, we structured thermodynamic concepts like entropy around important technologies like carbon capture and sequestration. Students engagements dramatically increased when they saw the connection between the core concepts and real-world applications. I will also aim to structure my classes around projects as much as possible, in the vein of my undergraduate, projected-based engineering curriculum at Olin College of Engineering. As the principal instructor for a batteries class at Stanford, students were tasked with proposing a new energy storage business, with representatives from local industry helping judge the novelty and feasibility of such new technologies. Students devised ideas including freeze-casting electrodes, batteries for space, and new battery charging algorithms. I was awarded the schoolwide Walter Gores Award at Stanford in 2016 for my teaching.