Research

Research motivation and thrust

Human development and improved standards of living have historically coincided with increases in energy consumption. As billions of humans enter the global middle class, the environmental consequences of fossil energy use place immense strains of the earth’s ecosystem. It is necessary to decouple development with carbon emissions through energy efficiency and renewable energy. Electrochemical systems play crucial roles towards achieving these goals.

The Li+ group aims to advance fundamental understanding and develop technologies in electrochemical materials and devices. We study the underlying physical principles behind such materials, devices, and systems, and exploit them into developing new technologies towards energy solutions. We specifically investigate transition metal oxides with point defects. Point defects, such as lithium interstitials and oxygen vacancies, act as electronic dopants and enable these materials to conduct both electrons and ions, or “mixed conduction.” Mixed conduction combined with electrochemistry enables the simultaneous addition or removal of electron and ions using current and voltage, allowing us to add or remove ~1022 cm-3 dopants dynamically under device operating conditions. This process is known as electrochemical ion insertion, and is crucial for a number of technologies including batteries, electrochromics, solid fuel cells, electro-catalysts, etc. The main thrusts of this group is to develop materials and devices for batteries and for in-memory, low-energy computing. 

We also aim to cross-pollinate and synergize the fields of electrochemistry and microelectronics. These fields are united by the crucial role of current and voltage in obtaining functionality. Key thrusts involve understanding electrochemical processes in the length, time, and operational environmental of microelectronics. We also seek to use the tools of microelectronics to better understand fundamental electrochemical processes.

Reference: Y. Li and W. C. Chueh, “Electrochemical and Chemical Insertion for Energy Transformation and Switching.” Annual Reviews for Materials Research. 48:137-165 (2018)

Mass transport and chemical diffusion in resistive redox memory

Led by Jingxian Li

Redox random-access memory (ReRAM) is a promising memory technology for information storage, including for extreme environments including high temperatures and high radiation. Unlike conventional electronic memory like Flash that stores information using electrons in energy traps in Si-based materials, ReRAM stores information using oxygen vacancy ions within transition metal oxides; these vacancies have very different transport properties than electrons. Our research aims to understand oxygen vacancies in ReRAM moves under concentration, electric field, and thermal driving forces. Understanding this oxygen vacancy migration in different materials is crucial towards understanding and improving resistive memories.

Schematic, switching profile, and SEM image of Redox random access memory (ReRAM) in our lab.

Analog resistive memory based on electrochemical redox

Led by Diana Kim

Machine learning and artificial intelligence require large data centers, consume extraordinary amounts of energy, will soon be major contributors to climate change. How can we make AI more energy efficient?

The energy intensity of of data-heavy processes like AI result from the need to move information from logic to processor elements. Neural networks can be 1000x more energy efficient by computing on a memory crossbar using physics (Ohm’s & Kirchoff’s Laws) to perform matrix multiplications, the most ubiquitous process.

Neuromorphic comptuting containing crossbar arrays of non-volatile memory elements enable orders of magnitude reduction in the energy consumption of data-intensive processes like machine learning.

For the past several decades, most research in the nonvolatile memory element have been focused on digital memory that stores (1) and (0). However, achieving memory with analog states have been much more challenging especially at the length scale of microelectronics. A substantial challenge is that there are not enough information storage elements (e.g., defects in a filament or electrons in a floating gate) to provide continuous analog states in nanoscale devices.

We believe that the atom is the highest density information storage element. With over 106 atoms in a small (30-nm)3 volume, we anticipate that the ability to dynamically control the number of atomic point defects in such materials can be used to achieve truly analog memory. Our initial exploration utilized bulk resistive memory, whereby analog information states are stored in the concentration of oxygen vacancies in the switching layer, and switched by electrochemically increasing or decreasing the oxygen vacancy concentration.

Our approach is to use the average behavior of all point defects in the bulk. With over 106 lattice sites in a small (30-nm)3 volume, the average behavior of all point defects is statistical. Harnessing this statistical behavior results in predictable and deterministic analogue switching.

 

Filamentary and bulk resistive memory. (a) Conventional filamentary memory devices store analogue information states in atomic-sized filaments. (b) Such devices switch stochastically due to the discrete nature of atoms in the filament and kinetic theory. (c) Bulk memory instead stores information using the point defect concentration, a continuous variable. (d) Bulk memory yields deterministic switching.

 

Past work:

Y. Li, E. J. Fuller, J. D. Sugar, S. Yoo, D. S. Ashby, C. H. Bennett, R. D. Horton, M. S. Bartsch, M. J. Marinella, W. D. Lu, A. A. Talin. “Filament-free bulk resistive memory enables deterministic analogue switching.” Advanced Materials, 2003984, 32 (2020)

E. J. Fuller, S. T. Keene, A. Melianas, Z. Wang, S. Agarwal, Y. Li, Y. Tuchman, C. D. James, M. J. Marinella, J. J. Yang, A. Salleo, A. A. Talin. ”Parallel programming of an ionic floating-gate memory array for scalable neuromorphic computing.” Science. 364, 570-574 (2019)

Electrochemical and microstructural heterogeneity in Li-ion battery particles

Led by Jinhong Min

Li-ion batteries are made from micron-sized, redox-active particles in porous electrodes. It is well established that Li-ion batteries are nonuniform and heterogeneous at different length scales. For examples, there exists substantial qualitative evidence that some particles charge faster than other particles, while other particles appear to degrade faster. It is unclear to what extent these particles are quantitative different from each other, or what the origins of such heterogeneity are.

To answer this question, we will quantify the electrochemical properties of individual battery particles. We will then correlate these properties with structural and compositional features. This research aims to determine how the composition and the microstructure of battery particles ultimiately affect their electrochemical properties. If we can obtain this, we can understand how to rationally engineer better battery materials.

Charge, discharge, and electrochemical impedance of a single micron-sized NMC-532 battery particle.

Past work:

Y. Li, H. Chen, K. Lim, H. D. Deng, J. Lim, D. Fraggedakis, P. M. Attia, S. C. Lee, N. Jin, J. Moskon, Z. Guan, W. E. Gent, J. Hong, Y. S. Yu, M. Gaberscek, M. S. Islam, M. Z. Bazant, W. C. Chueh. ”Fluid-enhanced Surface Diffusion Controls Intra-Particle Phase Transformations.” Nature Materials. 17, 915-922 (2018)

J. Lim,* Y. Li,* D. H. Alsem, H. So, S. C. Lee, P. Bai, D. A. Cogswell, X. Liu, N. Jin, Y. Yu, N. J. Salmon, D. A. Shapiro, M. Z. Bazant, T. Tyliszczak, W. C. Chueh. “Origin and hysteresis of lithium compositional spatiodynamics in battery primary particles.” Science, 353, 566-71 (2016)