Synthesis And Properties Of Bifunctional Layered LiMnO₂

Pre-lithiation is a research hotspot of lithium ion capacitors and high energy density lithium ion batteries.Layered Li Mn O₂has been widely studied as a cathode material due to its low cost,safety and high theoretical specific capacity.However,it has the disadvantages of low first coulombic efficiency and poor cycling performance,which just meets the needs of pre-lithiation agent.Ideally,its large irreversible capacity can not only be used to achieve the pre-lithiation of the anode,but also its stable residual reversible capacity can be used as an active substance to contribute capacity to the cuthod,so as to realize the dual functions of“pre-lithium agent”and“active substance”.In this paper,bifunctional layered Li Mn O₂materials were prepared by hydrothermal reaction,ball mill-high temperature solid phase method,spray pyrolysis-high temperature solid phase reaction/microwave heating reaction.The structure and properties of the materials were characterized by XRD,SEM,BET,ICP,XPS and CV,EIS,charge and discharge measurements,etc.The effects of preparation methods and conditions on the structure and properties of the materials were investigated.The layered Li Mn O₂material was prepared by hydrothermal method with Mn₂O₃and Li OH·H₂O as raw materials.The effects of hydrothermal reaction temperature,reaction time and Li/Mn ratio on orthogonal lamellar Li Mn O₂（o-Li Mn O₂）materials were studied.The results show that the crystallinity and phase purity can be improved by increasing the molar ratio of Li/Mn（≥4）and prolonging the reaction time.The material prepared by holding for 12 h at 180℃deliversan initial charge capacity of 159.1 m Ah/g at 2-4.2 V,an irreversible capacity of 78.1 m Ah/g,and a discharge capacity of 81.9 m Ah/g after 100 cycles at 0.4C,indicating that the residual capacity of the Li Mn O₂after pre-lithiation remains souble during cycling.o-Li Mn O₂material was prepared from commercial Mn₂O₃and Li₂O materials by ball milling-high temperature solid state reaction.The effects of temperature and product ball milling time were investigated.The results show that the first coulombic efficiency and cycling performances of the products were improved obviously after ball milling.After holding at 700℃for 10 h and then ball milling for 4 h,the initial charge capacity at 2-4.2 V was 172.2 m Ah/g,the discharge capacity was 134.7 m Ah/g,the coulombic efficiency was 78.3%,and the discharge capacity remained115.9 m Ah/g after 100 cycles at 0.4C.Compared with the material without ball milling,the contribution ratio of pseudocapacitance decreases from 83.6%to 60.8%.The spherical porous Mn₂O₃precursor was prepared by spray pyrolysis at 850℃with the concentration of 0.3 mol/L and the gas flow rate of 4 L/min.On this basis,o-Li Mn O₂materials were prepared by high temperature solid-phase reaction and microwave heating reaction respectively.The effects of Li/Mn and temperature on materials were explored.The optimal conditions of high temperature solid-phase reaction were obtained as follows:700℃for 10 h;The optimal condition of microwave heating method was 600℃for 5 min.The materials prepared by microwave method have the intial charge capacity of 188.7m Ah/g and irreversible capacity and 59.8 m Ah/g within 2-4.2 V,respectively,and the discharge capacity was 120.8 m Ah/g after 100cycles.The initial charging capacity and irreversible capacity within 3-4.5V were up to 238.9 m Ah/g and 145.1 m Ah/g,respectively.The pre-lithiation effect of o-Li Mn O₂material prepared under the optimized conditions of microwave heating was verified by full battery,ICP analysis and three electrodes.The results showed that the o-Li Mn O₂material has the potential to act as a“pre-lithiation agent”and“active substance”bi-functional material.