Synthesis And Electrochemical Performance Optimization Of Layered Manganese-based Lithum-rich Cathode Materials

Most of the cathode materials used in Li-ion batteries contain expensive Co,Ni etc.metal elements,which restricts the development and application of high energy density Li-ion batteries.The development of low cost and high energy density cathode materials as well as the improvement of the corresponding preparation technology have become key issues in this field.Therefore,the synthesis and electrochemical performance optimization of layered Mn-based Li-rich materials were studied.The main contents and conclusions are as follows:1.A series of Li-rich layered xLi2MnO3·(1-x)LiMnO2(x=0.91,0.78,0.67,0.54,0.42 and 0.32)nanorods with a diameter of 100-200 nm and length of 400-1000 nm were prepared through a pyrolysis reduction process of the in situ formed lithium stearate on m-Li2MnO3 nanorods.In those pure Mn-based Li-rich solid solutions synthesized,the m-Li2MnO3 portion can improve the layered structre stability due to its retardation effect of the layered-to-spinel transformation,while the m-LiMnO2 portion can enhance the reversible specific capacity but lower the cyclic stability.Among those solid solution materials,xLi2Mn03·(1-x)LiMnO2 nanorods with x=0.54 exhibits appropriate Li2MnO3/LiMnO2 contents to balance the Mn valence state and relieve the Janhn-Teller effect,and thus causing the best cyclic and rate performance with an initial discharge capacity of 250.9 mAh g-1 and a capacity retention rate of 105.3%at the 50th cycle.The present layered xLi2MnO3·(1-x)LiMnO2 solid solutions with adjustable composition is a new kind of pure Mn-based Li-rich cathode materials with low cost,high specific capacity and electrochemcical stability,and thus have enormous potential in the practical application.2.Nanoplate-,nanorod-and nanoparticle-like m-Li2MnO3 were prepared via solid-state reaction method,and then layered Li-rich cathode materials(xLi2MnO3·(1-x)LiMnO2)with similar compositions but different morphologies were synthesized by using pyrolysis reduction process of lithium stearate.In which,nanoplate-like pLM-RR has an average thickness of-35 nm and an average diameter of-200 nm;nanorod-like rLM-RR has an average diameter of?150 nm and length in the range of 400-1000 nm;and nanoparticle-like gLM-RR has an average diameter of?50 nm.Although the initial reversible discharge capacities of pLM-RR(223.7 mAh g-1)and rLM-RR(225.8 mAh g-1)are slightly lower than that of gLM-RR(228.8 mAh g-1),pLM-RR and rLM-RR exhibit much higher discharge capacities,capacity retention rates and rate performances than gLM-RR during the following 50 cycles' charge/discharge processes.The reason is that the 1D nanorod and 2D nanoplate structures can alleviate the intergranular stress during the electrochemical cycling,and effectively inhibit the structure collapse caused by the powdering of the material particles,and thus lead to rLM-RR and pLM-RR showing more excellent electrochemical cycling stability than gLM-RR.The present results provide a reference for the regulation of the morphology and electrochemical performance of cathode materials for Li-ion batteries.3.To further improve the electrochemical performance of the nanoplate-like pLM-RR(0.55Li2MnO3·0.45LiMnO2),amorphous V2O5 layer was coated on the nanoplate' surfaces through thermal decomposition method.The experimental results show that the V2O5 coating not only can accommodate those Li+ ions that cannot completely reinsert into the pLM-RR during the discharge process to improve the discharge capacity and Kulun efficiency,but also alleviate the electrolyte corrosion of the pure Mn-based Li-rich materials under high voltage to improve its cycling performance.Among which,3%V2O5 coating has a good coating effect and can promote the Li+ diffusion(the apparent diffusion coefficient is?20 times of pLM-RR),which can effectively improve the rate performance of the pure Mn-based Li-rich cathode materials.Under the current density of 150,300,600 and 1500 mA g-1,3%V2O5 coated pLM-RR shows discharge capacities of 275,256,233 and 174 mAh g-1,while the pLM-RR unde those current densities only exhibits capacities of 255,225,191 and 131 mAh g-1,respectively.The presnt facile and efficient method provides an important guidance to improve the electrochemical performance of this pure Mn-based Li-rich materials.4.To further improve the electrochemical properties of nanoplate-like pLM-RR(0.55Li2MnO3·0.45LiMnO2);and use those redundant Li+ ions that will release during the pyrolysis reduction process of m-Li2MnO3 by stearic acid(SA),FePO4 nanoparticles were loaded on the nanoplate surfaces through precipitation method,then turn the m-Li2MnO3 and FePO4 into Mn-based Li-rich material and LiFePO4 through the pyrolysis reduction process of SA,and get nanoplate-like pLM-RR/LiFePO4 composite material.It shows a discharge capacity of 200 and 126 mAh g-1 in 1C and 5C,and the capacity retention rate is 91%in the current density of 0.1 C for 100 cycles,and a better cycling stability than the nanoplate-like pLM-RR.The method is simple,resources saving,and the prepared material having relative good electrochemical performances,and thus has relative high practical application value.