Design And Characterization Of Core-Shell Li-rich Manganese-Based Layered Cathode Materials

Lithium-ion batteries (LIBs) have attracted increased attention for energy storage development due to the immense demand from portable electronics, (hybrid) electric vehicles and stationary electrical energy storage in a grid. Although today’s commercial LIBs can deliver 150-200 Wh kg-1, it is difficult to improve its energy density limited by the theoretical capacities of cathode material and carbon-based anode. The cathode material is currently becoming a bottleneck in the performance improvement of LIBs. Li-rich manganese-based layered material (LR-NMC), with a high reversible capacities and energy densities (>200 mAh g-1 and 300 Wh kg-1), has been put forward as one of the most promising candidates. However, it still remains significant challenges because of large irreversible initial capacity loss (40-100 mAh g-1), short cycle life and poor rate performance. Based on the Li-rich Li-Mn-Ni-Co-O system, the objective of this thesis is to find a core-shell structure cathode material which is consists of LR-NMC core and fast lithium ion conduction shell. Therefore, the composite material can provide a good Li+ conducting property and inhibit the side reaction in the electrode/electrolyte interface.In consideration of spinel LiMn2O4 and Li-Mn-Ni-Co-O system are manganese-based cathode materials, and both have limitations of poor rate capability and performance degradation. Consequently, perovskite structure La-Sr-Mn-O (LSM) and Li-La-Ti-O (LLT) fast lithium ion conduction are employed to coating LiMnO4. Our focus is placed on the internal relationship between the surface modification and electrochemical properties as well as energy storage mechanism of the new system will be intensively discussed. The LSM and LLT surface coating have been proven to be effective in improving the electrochemical performance of LiMn2O4, especially the rate capability and cycling performance at elevated temperature. The significant improvement for rate and cycle performance could be attributed to high ionic conductivity and controllable Mn dissolution from LSM and LLT coated layer. The results herein support the idea that design fast ionic conductivity coated layer to form core-shell structure could be a feasible strategy for LR-NMC.LR-NMC are prepared successfully by co-precipitation of the corresponding metal salt solutions using NaOH and NH3H2O as precipitation and complexing agents. The effects of pH value, synthesis temperature, transition metal molar ratio on the morphology, microstructure and electrochemical performance of the prepared precursors and the I(003)/I(104) ratio of LR-NMC are investigated. It is found that the optimum prepared condition is control pH value at 11, 450 ℃ and 900℃ two steps sintering processing. Spherical Li1.2Mn0.54Ni0.13Co0.13O2 reveals a uniform distribution of particle size, layered structure, and solid solution characteristic. An initial discharge capacity of 247.9 mAh g-1 with a coulombic efficiency of 75.1% can be obtained at 0.1 C between 2.0 and 4.6 V. It exhibits a better electrochemical performance and delivers a discharge capacity of 236.2 mAh g-1 at the 1 C rate with capacity retention of 88.3%after 50 cycles.Fast lithium ion conduction of Li0.34La0.5iTiO3 (LLT), (La1/2Li1/2)o.95Sr0.05TiO3 (LST) and Al-doped Li7La3Zr2O12 (LAZ) were applied to surface modify Li1.2Mn0.54Ni0.13Co0.13O2 cathode material. Microstructure, morphology and electrochemical performance also the advantages of core-shell LM@LLT, LM@LST and LM@LAZ cathode materials are studied systematically. Charge-discharge tests show LM@LLT material exhibits improved electrochemical performance. The initial coulombic efficiency is enhanced to 78.3% with high initial discharge capacity of 231.3 mAh g-1 at 1 C rate after LLT coating. Meanwhile, it also shows higher stable cyclic performance above 217.4 mAh g-1 after 55 cycles at 1C rate, with excellent capacity retention of 96.4%. LM@LST has an obviously enhanced electrochemical performance compared with pristine sample. It shows discharge capacity of 267.6 mAh g-1 with a coulombic efficiency of 83.6% at 0.1 C rate. More than 95.3% of the capacity is retained after 55 cycles at the 0.1 C rate in a cut-off voltage range of 2.0-4.6 V. The LM cathode coated with the LAZ surface layer shows improved rate capability and cycle stability. Particularly, the 500℃ heat treatment LM@LAZ composite cathode material shows high discharge specific capacities (258.5 mAh g-1 at 0.1 C) and improved initial coulombic efficiency (80.7%). Moreover, it displays extremely high discharge capacity of 234.3 mAh g-1 and unique capacity retention of 94.1% after 100 cycles at 1 C rate. According to the analysis from cyclic voltammograms and electrochemical impedance spectroscopy, the improvement on the electrochemical performance is attributed to the coated Li+ superconductive layer can reduce side reactions of LM with the electrolyte, and thus form the cathode-electrolyte interface layer with enhanced Li+diffusion. As a result, protection of cathode material by Li0.34La0.51TiO3, (La1/2Li1/2)0.95Sr0.05TiO3 or Al-doped Li7La3Zr2O12 can address some of the capacity fading issues related to the LR-NMC at room temperature.