Synthesis And Optimization Of Mn-based Layered Oxides As Cathode Materials For Lithium Secondary Batteries

The new and green energy technologies has been pursued because of the massive consumption of the traditional fossil fuel and the accompanying environmental pollution. With high capacity and power, the electrochemical secondary batteries have been considered as highly efficient energy conversion and storage devices. However, lack of high-performance cathode materials for batteries has always been a bottleneck for the development and application of advanced secondary batteries, especially in the field of electric vehicles. Thus, many researchers have focused on the development of high-performance cathodes, In this thesis, with the purpose of exploiting high energy and high power cathode materials, we selected the high-capacity layered lithium-rich cathode as the research object after reviewing and analyzing the latest development of cathode materials for lithium ion batteries. The synthesis, modification, structural and morphology design of layered transition oxides have been systematically investigated and also applied into lithium or sodium ion batteries for their electrochemical performance. The main research achievements are listed below:1. Li-rich materials Li1.2Mn0.54Co0.13Ni0.13O2 were synthesized at 700,800,900 and 1000? for 20h by sol-gel method, respectively. The electrochemical performance tests indicates that 800? is the optimum calcination temperature, and the samples shows the best electrochemical performance under this temperature. Meanwhile, we prepared single Li2MnO3 and Li[Mn1/3Co1/3Ni1/3]O2 material under the same conditions, which is the single phase in Li-rich materials. We also prepared the mixture by ball-milling Li2MnO3 and Li[Mn1/3Co1/3Ni1/3]O2 with mole ratio of 1:1. The structures and the electrochemical performance of all the samples were characterized and tested. Lastly, we selected the sample synthesized at 800? as the study objective which was further observed by HRTEM and FFT characterization. The results have demonstrated that the Li-rich materials was rather a solid solution of Li2MnO3 and Li[Mn1/3Co1/3Ni1/3]O2 at the nanometer scale than the simple mixture of them. As we know, structure determines the performance, this work plays an important role in understanding the complex structure and electrochemical properties of Li-rich materials.2. Lithium-rich layered oxide Li[Li0.2Mn0.54Co0.13Ni0.13]O2 coated with V2O5 layers (labeled as LMNCO@V2O5) has been synthesized and its electrochemical properties as cathode material for lithium ion batteries have been measured and compared with pristine Li[Li0.2Mn0.54Co0.13Ni0.13]O2 (labeled as LMNCO) and LMNCO-V2O5 composite.As a lithium ions insertion host materials, both the V2O5 in the LMNCO@V2O5 and LMNCO-V2O5 can reduce the irreversible capacity losses and improve the Coulombic efficiencies of the cathode in the first charge-discharge cycle. However, for improving the cycling stabilities and the high-rate capabilities of the LMNCO, the effects of the V2O5 coating layers in the LMNCO@V2O5 are far beyond the effects of the V2O5 nanoparticles in the LMNCO-V2O5. When charged-discharged galvanostatically at 25 mA g-1 between 2.0 and 4.8 V (vs. Li+/Li), the LMNCO@V2O5 with 3 wt.% V2O5 exhibts a discharge capacity of 279.5 mAh g-1 in the first cycle and maintains a discharge capacity of 269.1 mAh g-1 after 50 cycles, with capacity retention of 96.3%. In contrast, the discharge capacity of the pristine LMNCO changes from 251.2 mAh g-1 in the initial cycle to 202.2 mAh g"1 in the 50th cycle, with no obvious improvement in the capacity retention. At high rate of 1250 mA g-1, the discharge capacity the LMNCO@V2O5 can reach 113.6 mAh g-1,which is much higher than the capacities that the pristine LMNCO and the LMNCO-V2O5 can reach. Different effects of V2O5 are due to their different roles in the cathode materials. While the V2O5 coating layer in the LMNCO@V2O5 can reduce the charge transfer resistance at the electrode-electrolyte interfaces and improve the transportation of lithium ions among the LMNCO particles, the V2O5 nanoparticles in the LMNCO-V2O5 can only work as a Li+ ions insertion host material.3. Spinel phase LiMn2O4 was successfully embedded into monoclinic phase layered-structured Li2MnO3 nanorods and these spinel-layered integrate structured nanorods showed both high capacities and superior high-rate capabilities as cathode material for lithium-ion batteries (LIBs). The pristine Li2MnO3 nanorods were synthesized by a simple rheological phase method using ?-MnO2 nanowires as precursors; the spinel-layered integrate structured nanorods were fabricated by a facile partial reduction reaction using stearic acid as the reductant. Both structural characterizations and electrochemical properties of the integrate-stuctured nanorods verified that LiMmO4 nanodomains were embedded inside the pristine Li2MnO3 nanorods. When used as cathode material for lithium-ion batteries, the spinel-integrate structured Li2MnO3 nanorods (SL-Li2MnO3) showed much better performances than the pristine layered-structured Li2MnO3 nanorods (L-Li2MnO3). When charge-discharged at 20 mA g-1 in a voltage window of 2.0-4.8 V, the SL-Li2MnO3 showed discharge capacities of 272.3 and 228.4 mAh g-1 in the first and the 60th cycle, respectively, with capacity retention of 83.8%. The SL-Li2MnO3 also showed superior high-rate performances. When cycled at the rates of 1C,2C,5C and 10C rate (1C= 200 mA g-1) for hundreds of cycles, the discharge capacities of the SL-Li2MnO3 can still reach 218.9,200.5,147.1 and 123.9 mAh g-1, respectively. The superior performances of the SL-Li2MnO3 were ascribed to the spinel-layered integrated stuctures. With both large capacities and superior high-rate performances, these spinel-layered integrate structured materials should be one of the good cathode materials for the next-generation high-power LIBs.4. Hierarchical lithium-rich nanowires assembled with 5-20 nm nanocrystals were successfully synthesized by molten-salt method, in which NaCl served as a molten salt flux and a-MnO2 nanotubes were introduced as manganese source and templates. The results from parallel experiments have demonstrated that NaCl flux and one-dimensional a-MnO2 nanotubes were essential for building the final Li rich nanowires. When tested as cathode material for lithium ion batteries, the Li rich nanowires shows impressive electrochemical performance, such as higher discharging capacity, superior rate performance and stable cycling ability. Without any surface modification, the Li-rich nanowires can offer an initial charge and discharge capacity of 375.1 and 304.5 mAh·g-1, respectively, with a high coulombic efficiency of 81.2% at a current density of 25 mA g-1. When charge-discharged at 0.5C (1C= 250 mA g-1) in a voltage window of 2.0-4.8 V, the Li-rich nanowires still can maintain at a discharge capacity of 253.3mAhg-1 after 100 cycles with a high capacity retention of 90.3%. Even cycled at 30C rate for 1000 cycles, the discharge capacity of Li-rich nanowires can still reach 65 mAh·g-1 with no obvious capacity decay. These outstanding merits should ascribe to the one dimensional nanowire morphology which is favorable for electron transportation and the nanocrystals which can greatly shorten the distance for lithium ions and electrons transmission. Importantly, the cycled electrode materials have essentially maintained the one dimensional morphology and the good crystallinity, which is responsible for the respectable cycling performance.5. P2-Na0.5[Mn0.65Co0.2Ni0.15]O2 nanoplates with highly exposed (010) crystal planes were synthesized through a molten salt method. Meanwhile, the P2-Na0.5[Mn0.65Co0.2Ni0.15]O2 microflakes were also prepared by a sol-gel method for comparison. The crystal structural analysis indicated that both of them have an ideal layered structure with P2 phase. When tested as cathodes for sodium ion batteries, the nanoplates exhibits much higher rate performance and capacity than its counterpart microflakes. When charge-dicharged at 0.1C (14 mA g-1), the nanoplates showed discharge capacities of 169.4 and 136.3 mAh g'1 in the first and 50th cycle, respectively, with a capacity retention of 80.5%. While for microflakes, the discharge capacities in the 1st and 50th cycle is 156.1 and 107.1 mAh g-1, respectively, with a capacity retention of 68.6%. When cycled at the rates of 1C,2C,5C,10C and 20C rate (1C= 140 mA g-1) for hundreds of cycles, the discharge capacities of the nanoplates can still reach 110.5, 100.4,76.5,37.4 and 18.1 mAh g-1, respectively. The superior electrochemical performance for Na0.5[Mn0.65Co0.2Ni0.15]O2 nanoplates is mainly due to the active (010) planes which can be very favorable for fast sodium ion de/intercalation in the materials, thus highly improve the electrochemical performance.