Electrochemical Energy Storage

PI: Leon Shaw (MMAE)

Li-ion batteries (LIBs) have revolutionized portable electronic devices in the past two decades because of their high output voltage, high specific energy, long cycle life, and no memory effect. However, further improvements in their properties, such as shortening charge time from hours to minutes and extending battery usage time before re-charge from one day to multiple days, can enable the broad market penetration of electric vehicles and open up new applications. One of the areas investigated by our group is hierarchical design and synthesis of novel anodes with high specific capacity, high power, and long charge/discharge cycle life for next-generation LIBs and Na-ion batteries. For LIB applications, we have been focusing on Si anodes with combined features of nanoscale Si building blocks, a conductive shell, and engineered void space (denoted as Si@void@C micro-reactors) to achieve breakthroughs in high specific capacity, high power, and long cycle life simultaneously for next-generation LIBs. For Na-ion batteries, our focus is on phosphorus (P) anodes with engineered nanostructures. Significant progress has been made in both anodes, particularly for Si anodes which can be charged to the full capacity in three to six minutes at the current density of 8 A/g with 1,000 cycle stability and still exhibit specific capacities two to three times that of the graphite anodes that require approximately four hours to be fully charged with approximately 330 mAh/g capacity.

PI: Sohail Murad (ChBE)

The lack of economical and efficient energy storage devices is one of the major hurdles to the widespread utilization of renewable solar and wind energy. The redox flow battery (RFB) is an attractive option because of its excellent safety, high capacity, high efficiency, modularity, and small environmental footprint; however, in its current development state, it is not commercially viable largely because of inefficiencies in the ion exchange membrane (IEM), which is a key factor determining its cost-effectiveness, energy efficiency, and battery lifetime. Research and development efforts on IEMs for RFBs have largely focused on polymer-based materials. These materials have fundamental deficiencies, associated with their polymeric nature, related to ion crossover and chemical instability in high concentration electrolyte solutions of RFBs; therefore, alternative IEMs fabricated from new materials are required. The goal of this project is to explore nanoporous zeolite thin films as a new class of highly efficient and durable IEMs for RFBs. A key objective is to understand the mechanisms of proton conduction and field-driven ion transport in the zeolite membranes. The research will primarily focus on the siliceous MFI-type zeolite membranes for two model RFB systems including the Fe/Cr RFB and the all-vanadium RFB. The specific objectives include: synthesizing MFI zeolite membranes with different thickness, orientation, and framework composition and investigating the effects of these structural and chemical properties on the membrane performance in RFBs; experimentally studying the transport properties for proton and relevant metal ions with and without applied electric fields; and performing molecular simulations of the electrical-field-driven and chemical-potential-gradient-driven ion transport processes.

PIs: Károly Németh (PHYS), Leon Shaw (MMAE)

2D materials, such as functionalized (chemically surface modified) graphene or hexagonal boron nitride (h-BN) provide an excellent platform for simultaneous achievement of high power and high energy densities. This has been demonstrated in the literature using graphene oxide (GO) cathode materials in combination with Li and Na anodes. Unfortunately, GO is an explosive material, and its industrial production is both hazardous and difficult on the large scale. This motivated us to replace graphene oxide with much safer and stable FBN-s. Our theoretical efforts explore the possible functionalization for stable FBN cathodes, while our experimental efforts aim to synthesize and test selected candidate FBN-s. Furthermore, FBN-s may also be 3D crystals, such as Li3BN2. We have demonstrated by experiments and spectroscopic measurements that Li3BN2 is an intercalation cathode material whereby two Li-ions per formula unit can be extracted and intercalated, while there is only a negligible volume change of the crystal. This transition-metal-free Li-ion intercalation cathode can offer a specific capacity of near 900 mAh/g based on the use of -2, -2.5, and -3 charged nitrogen ions. This experimentally demonstrated specific capacity of near 900 mAh/g is the greatest ever reported for intercalation cathodes. Further investigations are needed to fully explore the field of FBN-s and related materials.

PI: Carlo Segre (PHYS)

Sn-based anode materials have high initial capacities but fade rapidly due to loss of electrical connectivity resulting from the conversion to metallic Sn and subsequent high volume expansion upon alloying with Li. We have been studying the degradation mechanisms of Sn-based materials using in-situ x-ray absorption spectroscopy for the past six years. As part of this long-term study, we have developed the first detailed structural model of Sn-Li alloys enabling the quantitative measurement of the degree of lithiation using extended X-ray absorption fine structure (EXAFS). We have found that nearly all Sn-based materials show increased cyclability when prepared as nano-composites with graphite. Most recently, we have published a study on an Sn4P3/graphite composite anode material with superior capacity and cycling performance (651 mAh g-1 after 100 cycles). Detailed EXAFS modeling and detailed analysis of local environment changes are correlated to the cell capacity and reveal the mechanism of the lithiation/delithiation process. In the first two lithiation/delithiation cycles crystalline, Sn4P3 is fully converted to an amorphous SnPx phase, which in further cycles participates in reversible conversion and alloying reactions. The superior reversibility of this material is attributed to the highly dispersed SnPx in the graphite matrix, which provides enhanced electrical conductivity and prevents aggregation of Sn clusters during the lithiation/delithiation process. The gradual capacity fading in long-term cycling is attributed to the observed increase in the size and the amount of metallic Sn clusters in the delithiated state, correlated to the reduced recovery of the SnPx phase. We have applied these results to other Sn-based materials, which also show significant improvement in cyclability.

PI: Leon Shaw (MMAE)

Na-ion batteries can play a critical role in grid-scale electric energy storage for widespread integration of renewable energy, making clean energy affordable to Americans and the technology greener and more energy-efficient. A critical issue for grid-scale electric energy storage is the long charge/discharge cycle life of the storage device. This project is aimed at addressing this issue by investigating how mechanical activation induced by high-energy ball milling at room temperature alters structural defects in NaCrO2 crystals and how the structural defects in NaCrO2 improve the electrochemical cycle stability of NaCrO2. Through seamless integration of experiments, DFT modeling, and MD simulation, this joint project between Shaw’s and Chen’s groups is developing mechanistic understandings at the atomic level and using the newly created knowledge to guide rational design and synthesis of NaCrO2 with the controlled structural defects and desirable chemical dopants for superior capacity retention, high round trip energy efficiency, and long cycle life for Na-ion batteries.