Yulong Liu

Origin of the high oxygen reduction reaction of nitrogen and sulfur co-doped MOF-derived nanocarbon electrocatalysts

Developing an economical, highly active and durable material to replace the conventional, expensive noble metal electrocatalyst is an important milestone in the development of fuel cell technology. Nanocarbon materials are considered as promising catalysts toward the oxygen reduction reaction (ORR) in fuel cells, due to their reasonable balance between low-cost, long-life durability and high catalytic activity in alkaline media. In this work, we present the fabrication of N,S-co-doped nanocarbon derived from a metal-organic framework (MOF) precursor for use as an electrocatalyst towards ORR. High resolution transmission electron microscopy (HRTEM) mapping demonstrates the uniform distribution of N and S atoms into the nanocarbon skeleton. The nitrogen absorption–desorption isotherms indicate that the MOF-derived N,S-co-doped nanocarbon has a high specific surface area (2439.9 m2 g−1) and a porous structure. Importantly, the N,S-co-doped nanocarbon exhibits higher catalytic activity toward ORR, better long-term stability and methanol tolerance than commercial Pt/C catalyst. First-principles calculations demonstrate that the remarkable electrochemical properties of N,S-co-doped nanocarbon are mainly attributed to the synergistic effect from the N and S dopants. Moreover, for the first time, it is revealed that the N,S-coupled dopants in nanocarbon can create active sites with higher catalytic activity for ORR than the isolated N and S-dopants. This finding on the structure–performance relationship of the co-doped nanocarbon provides guidelines for the design of high performance electrocatalysts.

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.

Production of Lithium-Ion Cathode Material for Automotive Batteries Using Melting Casting Process

In the 1990s, LiFePO4 (LFP) was discovered as a cathode material for lithium ion batteries and was successfully used in the variety of devices such as power tools, E-bikes and grid accumulators. New challenges associated with use of lithium ion batteries for automotive applications demand higher performance and operating requirements, yet these requirements need to be achieved at affordable cost and without compromising vehicle safety. The advantages of LFP as a cathode material include thermal stability, limited environmental impact and potential of low cost as compared to the cathode chemistries containing cobalt. Currently, solid state and hydrothermal processes are used to synthesize LFP at the industrial scale. However, they require multiple, time-consuming steps and costly precursors. Recently, a melting-casting process to produce LFP cathode material was investigated. The motivation behind this new process is the great flexibility of raw materials including chemical makeup and particle size, and the use of lower cost, commodity chemicals, with the benefits of increased kinetics in the molten state and energy efficiencies leading to overall process cost savings. Also, if successful this process could represent a novel application of conventional casting. Melting lithium-, iron- and phosphorus-bearing precursors in near stoichiometric ratios and casting LFP material that forms around 1000 °C requires fewer processing steps and shorter reaction time. Initially, electric resistance furnaces were utilized to melt the precursors to synthesize LFP. In this investigation, induction furnace was utilized to significantly reduce the melting cycle time. Various precursors and process parameters were tested from small laboratory samples of less than 1 kg to pilot-scale casting of approximately 40 kg. Cast LFP samples were evaluated using SEM/EDX microscope, differential scanning calorimetry, thermal analysis, X-ray diffraction and battery assemblies in coin cells, and compared against commercial LFP product.

Stabilizing the Interface of NASICON Solid Electrolyte against Li Metal with Atomic Layer Deposition

Solid-state batteries have been considered as one of the most promising next-generation energy storage systems because of their high safety and energy density. Solid-state electrolytes are the key component of the solid-state battery, which exhibit high ionic conductivity, good chemical stability, and wide electrochemical windows. LATP [Li1.3Al0.3Ti1.7 (PO4)3] solid electrolyte has been widely investigated for its high ionic conductivity. Nevertheless, the chemical instability of LATP against Li metal has hindered its application in solid-state batteries. Here, we propose that atomic layer deposition (ALD) coating on LATP surfaces is able to stabilize the LATP/Li interface by reducing the side reactions. In comparison with bare LATP, the Al2O3-coated LATP by ALD exhibits a stable cycling behavior with smaller voltage hysteresis for 600 h, as well as small resistance. More importantly, on the basis of advanced characterizations such as high-resolution transmission electron spectroscope-electron energy loss spectroscopy, the lithium penetration into the LATP bulk and Ti4+ reduction are significantly limited. The results suggest that ALD is very effective in improving solid-state electrolyte/electrode interface stability.