Study On Modified And Electrochemical Properties Of Li[Li_0.23Ni_0.15Mn_0.62]O₂ Lithium-rich Cathode Material

In recent years,lithium-ion battery is widely used in portable electronic devices because of its high voltage,high capacity,no memory effect,such as notebooks,computers,mobile phones,cameras and so on.With the application of lithium-ion battery is more and more widely more,it is indispensable to explore new lithium-ion cathode materials which are low-cost,environmental and have excellent electrochemical performance.At present,the Li-rich layered cathode material received widespread attention due to high specific capacity（>200 mAh g-1）,wide voltage range,low cost and environment friendliness.However,these materials also have some drawbacks which can’t be ignored,such as high first irreversible capacity,unstable structure,and capacity decays faster along with cycle.In this paper,we mainly study and contrast the structure and electrochemical properties around before and after doping of Li-rich layered cathode material Li[Li_0.23Ni_0.15Mn_0.62]O₂.The main contents are as follows:The two samples,Li(Li_0.23Ni_0.15Mn_0.62)O₂ and Al-doped Li(Li_0.23Ni_0.15Mn_0.52Al_0.10)O₂ were prepared via sol-gel method by using citric acid as chelating agent.And the structure,morphology and electrochemical properties of the samples were studied.The results of XRD patterns shows that the XRD patterns of Al-doped sample is same as the un-doped sample.And no other impurity peaks are found except for Li₂MnO₃.Either sample possesses a layeredα-NaFeO₂ structure with a space group of Rm symmetry and the SEM results show that the two kinds of cathode materials have similar shapes and agglomeration phenomenon.The discharge capacity of Li[Li_0.23Ni_0.15Mn_0.62]O₂ is 65 mAh g^-11 in first cycle,and increases slowly to 200 mAh g^-11 after 50 cycles.The Al-doped sample has a specific capacity of 160 m Ah g^-11 in first cycle and later remains a relatively stable capacity(>180 mAh g^-1).Compared with Li[Li_0.23Ni_0.15Mn_0.62]O₂,Al-doped samples show a more stable capacity during the cycle process,indicating that the reversibility is better during lithium implantation/detachment process.Li[Li_0.23Ni_0.15Mn_0.52M_0.10]O₂（M=Mn,Fe,Ti,Mg）were successfully synthesized via sol-gel method by using citric acid as chelating agent.The results of[Li_0.23Ni_0.15Mn_0.52M_0.10]O₂（M=Mn,Fe,Ti,Mg）XRD patterns show that the spectra of the synthesized materials are basically the same,indicating that Fe,Ti and Mg are successfullydopedintothei[Li_0.23Ni_0.15Mn_0.62]O₂crystalstructure.The electrochemical test results shows that the charge/discharge capacities were814.8/69.8,617.3/70.3,114.2/72.5,850.4/44.9 m Ah g^-11 in sequence,and the corresponding coulombic efficiency were 8.6%,11.4%,63.5%and 5.3%.The electrochemical properties were tested at a current denisity of 20 mA g^-11 and 2.0 V and 4.8 V voltage range.After 50 cycles,the discharge capacities were 197.2,68.1,121.8,105.7 mAh g^-1,respectively,maintaining 282%,96.9%,168%,235%initial discharge capacity.Since Ti is electrochemically inactive in the material,the capacity is lower than Li[Li_0.23Ni_0.15Mn_0.62]O₂,but the irreversible capacity is improved significantly.The cyclic voltammogram show that the doing of Ti and Mg in Li[Li_0.23Ni_0.15Mn_0.62]O₂ only have an effect in stable crystal structure,they don’t have oxidation-reduction reaction.Impedance analysis results suggest that the key factor affecting the electrochemical performance is the charge transfer resistance of the lithium ion at the interface between the surface film and the active material.