Meng Jiang

From Grey Lab Page

My PhD research has been focused on the electrochemical and structural study on cathode materials for Li ion batteries. Efforts have been made on two aspects. First, synthesize materials and understand the relation between synthesis condition and structure as well as electrochemical performance. Second, study the structural change and working mechanism during electrochemical process by combining different techniques such as solid state Nuclear Magnetic Resonance (NMR), X-ray Diffraction (XRD), Transmission Electron microscopy (TEM), and X-ray Absorption Spectroscopy (XAS).

Layered Li[Li_1/3-2x/3Ni_xMn_2/3-x/3]O₂ are a series of solid solution compounds which are promising cathode materials for Li ion batteries. They have layered structure with Li layers (Figure 1), transition metal layers and oxygen layers. During electrochemical process, the Li ions can go in and out of the layers, which is so-called intercalation/deintercalation reaction (Figure 2). By changing the synthesis condition, different stacking faults were found by both high resolution XRD and ⁶Li NMR. Overcapacity and irreversible process in the electrochemical process are another two issues for those Li excess compounds where x is smaller than 1/2. A plateau was also observed at high voltage. Both long range and short range ordering changes were found in charged and discharged samples after high voltage process from NMR and XRD. The morphology of the crystals after cycling was different from pristine, and the evolution of oxidation states of transition metals was also observed from XAS.

The second project is to understand the mechanism of the conversion reaction during electrochemistry process. The general term for conversion reaction is: MX + Li<a>&#8596</a>LiX + M⁰.Here, M is 3d transition metal, X could be O, F, or S. During discharge, Li goes into cathode material, then MX compound is decomposed to nano sized LiX and M metal. During charge, Li is removed from cathode material, the reaction is reversed, and MX and Li metal is formed again (Figure 3). Differing from intercalation reaction like in LiCoO₂ and LiNiMnO₂, conversion reaction evolves redox couple M⁺ and M⁰ which could possibly give us more theoretical capacity. Therefore, the conversion reaction is a new direction to look for high capacity cathode materials. CuS, CuF₂, and FeF₃ were chosen as model compounds in this project. Different working mechanisms were carefully studied by ⁶³Cu, ^6,7Li and ¹⁹F NMR and high-resolution XRD. The different working mechanisms are understood for these three compounds.

A new research direction for cathode materials is organic compounds. Since the transition metal resources are limited, lots of efforts have been made to look for 'renewable' materials. A collaborative project with Prof. Jean-Marie Tarascon in France was carried out to study the structural change during cycling of some organic compounds which can accommodate Li ions in their structure. By doing ¹³C NMR, we can probe the local environment of the compound and study how the structure change during electrochemistry.

Figure 1. The structure of layered Li[Li_1/3-2x/3Ni_xMn_2/3-x/3]O₂. The pink ball is lithium, the red ball is oxygen, and the blue ball is transition metal.

Figure 2. A pictorial representation of the intercalation reaction.

Figure 3. A pictorial representation of the conversion reaction.