My research focuses on improving the energy density, cycle life, and safety of lithium-ion batteries.
One of my group’s specialties is performing advanced diagnostics on electrode materials like olivine-type phosphates, high-capacity “layered-layered” lithium nickel-manganese-cobalt oxides, and high-voltage Ni/Mn spinels. The crystal structures, compositions, microstructures, and surface characteristics of these materials separately and interdependently affect performance and stability. Empirically optimizing these parameters is, therefore, expensive and impractically time-consuming. We synthesize single crystals of the electrode materials with well-controlled physical properties and use them as the platform to perform fundamental studies on performance-limiting physical properties, phase-transition mechanisms, interfacial behavior, and transport phenomena. By establishing the relationships among properties, electrochemical performance, and long-term stability, electrode materials with improved energy density and stability can be rationally designed and developed.
Our single-crystal syntheses are based on hydrothermal, solvothermal, and molten-salt techniques. Crystals with a range of physical properties are prepared by controlling the reaction precursors and synthesis conditions that affect the crystal nucleation and growth process.
The diagnostic work relies heavily upon the use of advanced instrumentations for electron, vibrational, ultrafast laser, and synchrotron-based microscopies, spectroscopies and spectromicroscopies. Some techniques that we frequently use include the following: X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), Fourier transform infrared (FTIR) and Raman spectroscopy/microscopy, secondary and transmission electron microscopy (SEM and TEM), transmission and scanning transmission X-ray microscopy (TXM and STXM), electron energy loss spectroscopy (EELS), selected area electron diffraction (SAED), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC).
Another area of our research is developing reliable overcharge protection for lithium-ion batteries. For PHEV and EV-type batteries that use hundreds or thousands of individual cells in series and parallel stacks, overcharge is a serious issue that poses safety risks and reduces battery life. Our approach protects cells by forming a reversible resistive shunt between the current collectors during overcharging. It is self-activated by voltage and is capable of high-rate and low-temperature protection.