Study On Surface Modification And Electrochemical Performance Of Nano Fe-Mn Based Li-rich Cathode Materials

Modern society is puzzled by the depletion of petroleum,global warming and environmental pollution. The development of electric vehicles(EVs) with low or zero emission, including plug-in hybrid electric vehicles and pure EVs should be the best choice for the low-carbon society. Lithium-ion batteries with high mass- and volumeenergy density as well as long cycle-life have been regarded as the most promising power sources for portable electronic devices and electric vehicles. With expansion of the applications of the lithium-ion batteries, high performance and low cost are urgently required. Fe-Mn based lithium-rich cathode(1-x) Li2 Mn O3·x Li Fe O2 are potential materials used in large lithium-ion batteries because they are capable of providing high specific capacity and high energy density, environmental benignity and naturally abundant in inexpensive Fe and Mn elements. However, the materials still suffer from large irreversible capacity lose at the first cycle, poor rate capability, and low cycling performance.As a coating material, Zr O2 with acid and basic sites can protect the active materials from the attack of HF, and retain the stablity of matrix structure even at high operation voltage. To modify the surface and enhance the cyclability of Fe-Mn based lithium-rich material Li1.26Fe0.22Mn0.52O2, Zr O2 is used to deposite onto the active material by facile diping method, and the structure, morphology, electrochemical performances are characterized by XRD, SEM, TEM, galvanostatic cycling and EIS, respectively. With modification, the discharge capacity and capacity retention are evidently improved. At 0.2C, after 50 cycles the capacity retentions of the pristine material and modified one are 137.5m Ah/g and 155.1 m Ah/g, with the retain rates of 70.62% and 78.4%(calculated with the 2nd cycle). EIS measurements after cycles indicate that Zr O2 modification depresses the increase of film resistance and charge transfer resistance, suggesting that the modification material improves the interface and facilitates capacity retention.As Ru O2 facilitates fast electron transfer and fast Li+ ion diffusion, the nanosized Fe-Mn Li-rich cathode material is coated by Ru O2 and chemically activated simultaneously via Ru Cl3 treatment. The surface basicity of Li-rich materials can chemically deposit Ru3+ to achieve the goal that Ru(OH)3(The K?sp of Ru(OH)3 is 10-36) covers the nano materials. The process provides extra H+, thus H+/Li+ exchange occurs with Li extraction from materials. After annealing, Ru(OH)3 transforms into Ru O2. ICP measurements show that the Li amounts in the treated samples decrease with increasing the amount of the Ru Cl3, XRD results reveal that the superstructure peaks of Li2 Mn O3 clearly decrease monotonously. It suggests that Li2 Mn O3 is chemical-delithium activated. The analysis of ICP、XPS and EDS elemental mapping indicate Ru O2 distributes uniformly on the surface of the material. The electrochemical test shows the first efficiency and rate capability are markedly improved. At 0.2C, the initial cycle efficiencies of the samples 2wt%-LMFO and 4wt%-LMFO are 83.0% and 100.9%, respectively, with discharge capacities of more than 220 m Ah/g. Additionally, 2wt%-LMFO exhibits best integrated rate and cyclic performances. The differential capacity(d Q/d V) analyses imply the existence of spinel structure.Owing to Ru O2 modified Fe-Mn materials with inferior cycle performance at low rate to the Zr O2 modified one, the Ru O2 and Zr O2 mix-functional modifications for the nanosized Fe-Mn materials are attempted. The spinel phase increases after the second modification and the particles size and the crystallization level decrease shown in XRD, SEM and TEM patterns. These are associated with the new spinel phase evolving at relatively-low treatment temperature. The dual-modified sample shows best rate capability. The initial discharge capacities reach 216 m Ah/g, 196.2m Ah/g, 185.5m Ah/g, 155.1m Ah/g at 0.2C, 1C, 2C,3C, and the increments compared to the pristine sample are 22.9m Ah/g, 68.6m Ah/g, 52.9m Ah/g,39.2m Ah/g, respectively. Noteworthily, the initial cycle efficiencies of the dual-modified sample at different rate exceed 100%. However, the cycle performance deteriorates. This may be due to the smaller particle size and weak crystallization.