Hanshuo Liu

Spatially resolved surface valence gradient and structural transformation of lithium transition metal oxides in lithium-ion batteries

Layered lithium transition metal oxides are one of the most important types of cathode materials in lithium-ion batteries (LIBs) that possess high capacity and relatively low cost. Nevertheless, these layered cathode materials suffer structural changes during electrochemical cycling that could adversely affect the battery performance. Clear explanations of the cathode degradation process and its initiation, however, are still under debate and not yet fully understood. We herein systematically investigate the chemical evolution and structural transformation of the LiNixMnyCo1-x-yO2 (NMC) cathode material in order to understand the battery performance deterioration driven by the cathode degradation upon cycling. Using high-resolution electron energy loss spectroscopy (HR-EELS) we clarify the role of transition metals in the charge compensation mechanism, particularly the controversial Ni2+ (active) and Co3+ (stable) ions, at different states-of-charge (SOC) under 4.6 V operation voltage. The cathode evolution is studied in detail from the first-charge to long-term cycling using complementary diagnostic tools. With the bulk sensitive 7Li nuclear magnetic resonance (NMR) measurements, we show that the local ordering of transition metal and Li layers (R3[combining macron]m structure) is well retained in the bulk material upon cycling. In complement to the bulk measurements, we locally probe the valence state distribution of cations and the surface structure of NMC particles using EELS and scanning transmission electron microscopy (STEM). The results reveal that the surface evolution of NMC is initiated in the first-charging step with a surface reduction layer formed at the particle surface. The NMC surface undergoes phase transformation from the layered structure to a poor electronic and ionic conducting transition-metal oxide rock-salt phase (R3[combining macron]m → Fm3[combining macron]m), accompanied by irreversible lithium and oxygen loss. In addition to the electrochemical cycling effect, electrolyte exposure also shows non-negligible influence on cathode surface degradation. These chemical and structural changes of the NMC cathode could contribute to the first-cycle coulombic inefficiency, restrict the charge transfer characteristics and ultimately impact the cell capacity.

Nanoscale Manipulation of Spinel Lithium Nickel Manganese Oxide Surface by Multisite Ti Occupation as High-Performance Cathode

A novel two-step surface modification method that includes atomic layer deposition (ALD) of TiO2 followed by post-annealing treatment on spinel LiNi0.5 Mn1.5 O4 (LNMO) cathode material is developed to optimize the performance. The performance improvement can be attributed to the formation of a TiMn2 O4 (TMO)-like spinel phase resulting from the reaction of TiO2 with the surface LNMO. The Ti incorporation into the tetrahedral sites helps to combat the impedance growth that stems from continuous irreversible structural transition. The TMO-like spinel phase also alleviates the electrolyte decomposition during electrochemical cycling. 25 ALD cycles of TiO2 growth are found to be the optimized parameter toward capacity, Coulombic efficiency, stability, and rate capability enhancement. A detailed understanding of this surface modification mechanism has been demonstrated. This work provides a new insight into the atomic-scale surface structural modification using ALD and post-treatment, which is of great importance for the future design of cathode materials.

Understanding Properties of Functional Materials with Atomic-Resolved Electron Energy Loss Spectroscopy

The recent improvements in electron microscopy instrumentation have lead to the adoption of electron energy loss spectroscopy (EELS) beyond the traditional fields of applications related to chemical analysis of materials and biological structures. EELS is increasingly attracting the attention of the solidstate physics and nano-optics communities due to the unparalleled spatial and energy resolution of this technique. This growing interest is supported by recent publications showing the realization of atomicresolved spectroscopy in complex solids and over ten years of research by several groups around the world probing surface-plasmon resonances. In this presentation, we focus on examples of applications of spatially resolved EELS for the study of energy-related materials, mainly Li-based layered compounds, and complex oxides with potential electronic applications, showing how atomic resolved measurements provide insight into the macroscopic properties of these materials.

Microscopy and Spectroscopy of Catalysts and Energy Storage Materials

Electron microscopy has always played an important role in the development and the understanding of new materials. In the last ten years there have been significant advancements in instrumentation, enabling improved studies of materials at the nanoscale. In the area of catalysts and energy storage materials, detailed microscopy of material structure, composition and bonding at the nanometer length scale are needed to optimize material properties and performance. Here we summarize recent examples of work related to the study of nano-alloy catalysts used in proton exchange membrane fuel cells and commercial Li ion battery materials, illustrating the crucial role of imaging and spectroscopy for the characterization of these materials.

Studying tomorrow's materials today: Insights with quantitative STEM, EELS

The development of aberration correctors for the scanning transmission electron microscope has revolutionized the field of electron microscopy and dramatically improved the analytical “toolkit” of materials scientists. In particular, when combined with electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM) makes it possible to detect compositional and spectroscopic changes at the atomic level that can be used to understand the structure, and ultimately the performance of materials. Here we present some examples of quantitative STEM and EELS as applied to the study of graphene-based materials, complex nanoparticles used in electrocatalysts for fuel cells, group IIInitride nanowires used for light emitting devices, and the defects generated in implanted Si.