| Lithium-rich manganese-based solid solution (LMSS) has developed into one ofthe most promising cathode materials for high-power lithium-ion batteries on accountof high specific capacity, low cost and light pollution to environment. However, itsrate capability is not satisfied up to now because the Li2MnO3in the LMSS is aninsulative component, which decreases the electronic conductivity of this material.Moreover, its coulombic efficiency in the first cycle is very low when charged to ahigh potential (~4.7V), due to elimination of Li2O. These above-mentioned problemsseriously limited the wide application of the LMSS in lithium-ion batteries. Toovercome these problems, we have carried out the following researches.Firstly, the LMSS material was prepared through a sol-gel method. Scanningelectron microscope (SEM) image shows the obtained LMSS is smooth in the surfaceand little in the particle size. X-ray diffraction (XRD) pattern indicates the LMSS hasa pure phase with good crystal structure. The LMSS material can deliver the specificcapacities of258.3,240.2,225.4,204.2and174.9mAh·g-1at the rates of0.1,0.2,0.5,1and2C, respectively, in the first cycle within the potential range2~4.8V. Thecorresponding coulombic efficiencies are74.1%,75.2%,77.1%,81.1%and79.3%while the capacity retentions are73.8%,69.9%,69.6%,68.1%and66.4%after50cycles. And we can see that the discharge capacity at the2C rate is67.7%of that atthe0.1C rate. The results suggest that this LMSS material is not satisfied in theelectrochemical properties, such as rate capability and cycling performance. Inaddition, the coulombic efficiency in the first cycle is low.Secondly, the LMSS material was surface-coated with AlF3/C hybrid layer toform the LMSAC sample. XRD measurement displays the crystal structure of theLMSAC does not change and SEM micrographs reveal its surface varies incomparison to its pristine state. BET measurement results suggest the surface-coatedsample possesses relatively lower specific surface area and less pore volume than thepristine one. Compared to the efficiency of75.2%for the pristine sample (at0.2Crate), it reaches89.0%for the LMSAC in the first cycle. The coulombic efficiency inthe first cycle are90.0%,89.0%,86.7%,88.0%and85.1%when charged anddischarged at the rates of0.1,0.2,0.5,1and2C, respectively. After50cycles, theLMSAC remain97.4%and96.8%of the initial capacities at0.1C and1C rates,respectively, indicating an enhanced cycling performance. The discharge capacity at2 C rate is210.1mAh·g-1, which is about75.6%of that at0.1C rate. The rate capabilityof the LMSAC is superior to the LMSS.Finally, the LMSS material was surface-coated with V2O5/C hybrid layer withvarious carbon contents to form the LMSVC samples. XRD patterns indicate that thecrystal structure of the LMSVC does not change after surface coating. From BETmeasurement, it can be seen that the specific surface area of the LMSVC is larger thanthat of the LMSS. The LMSVC sample possesses a porous structure. Among theLMSVC sample with different carbon contents, the LMSVC1, which contains5wt%carbon, exhibits the optimized electrochemical properties. It can deliver a specificcapacity of272.3mAh·g-1at the0.1C rate accompanying with a coulombic efficiencyof95.1%in the first cycle. The capacity remaining ratio at this rate is88.9%after50cycles. The initial discharge capacities of LMSS、LMSVC1、LMSVC2and LMSVC3are258.3,272.3,262.0and231.0mAh·g-1, respectively. The corresponding capacityretentions are73.8%,88.9%,82.1%,79.7%after50cycles. It is found that theLMSVC1sample has the highest retention as well as the best cycling performance.Electrochemical impedance spectra testified that the charge transfer resistance couldbe depressed by the V2O5/C hybrid layer during cycling. The results suggest thesurface-coating of V2O5/C hybrid layer could effectively decrease the irreversiblecapacity in the first discharge and improve the rate capability and cyclingperformance of the LMSS material to a certain extent. |
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