Composite Modified Lithium Manganate And Its Application In Aqueous Lithium Ion Batteries

The Aqueous lithium ion batteries has the advantages of high safety,low cost and environmental friendliness,and has received great attention.As a typical positive electrode material,lithium manganate(LiMn2O4,abbreviated as LMO)is favored by people for its own advantages.However,the long-term charge and discharge cycle of the material in the aqueous electrolyte causes the loss of manganese and the Jahn-Teller deformation of the electrode material itself,so that the cycle stability and rate performance of the sample can not meet the needs of people.In order to solve these problems,coating and doping methods are generally used to improve the performance of lithium manganate.By coating,the contact area of the LMO with the electrolyte can be reduced,and the occurrence of side reactions can be prevented.However,the common coating does not effectively improve the cycle stability of the material,and the introduction of the coating layer may cause the conductivity of the material to deteriorate,affecting the rate performance of the material.In this paper,graphene oxide(GO)and titanium dioxide(TiO2)are used as a matrix for the uniform dispersion of lithium manganate,and carbon nanotubes(CNT)or polypyrrole(PPy)are coated in situ,so as not only in lithium manganate and aqueous solution.A barrier is provided between the electrolytes to reduce the exposure of the aqueous phase electrolyte,effectively alleviating the loss of manganese,and the conductivity of the LMO electrode material is effectively improved by uniform distribution of CNTs or uniform coating of PPy.Furthermore,its cycle stability and rate performance in an aqueous lithium ion battery are improved.(1)The GO aerogel was used as the carrier,and the CNT was introduced in situ to encapsulate the LMO uniformly in the three-dimensional interconnection network formed by the two.The LMO,GO and CNT composite aerogel(abbreviated as GO/CNT@LMO)was prepared..The barrier of GO and the strong electrical conductivity of the CNTs allow the material to exhibit good electrochemical performance.For example,the material has good charge and discharge cycle stability and rate performance in 0.5 M LiNO3 aqueous solution,which is superior to LMO and GO@LMO electrodes.After 400 cycles,GO/CNT@LMO has a high discharge capacity of 88 mAh g’1 and a capacity retention rate of 80%.(2)A composite aerogel of LMO and TiO2 and CNT was prepared by in-situ introduction of CNT by sol-gel method(abbreviated as C-T-LMO-X,X is the mass of CNT(unit:mg),and the value is 30,50,70,100).The aerogel has a rigid-flexible structure,and the rigidity of the TiO2 aerogel can well fix the LMO particles and alleviate the loss of manganese caused by the Jahn-Teller effect during charging and discharging.CNTs have excellent electrical conductivity and can closely bond TiO2 and LMO.The network structure thus designed enables the prepared C-T-LMO-X(X=30,50,70,100)composite aerogel to exhibit perfect charge and discharge cycle performance and rate performance.Taking C-T-LMO-50 sample as an example,the reversible discharge specific capacity of C-T-LMO-50 sample is still 93.7 mAh g-1 after 200 charge and discharge cycles at a current density of 0.5 A g-1,which is much higher than that of bare.The LMO has a specific capacity of 55.8 mAh g-1;at a large current density of 3 and 7 A g-1,the average specific discharge capacity remains at 97.6 and 93.1 mAh g-1.(3)PPy@LMO-200 and PPy@LMO-600 composite materials with core-shell structure were prepared by in-situ coating of PPy on the outer layer of LMO.As a conductive coating material,PPy not only slows the loss of manganese in the electrolyte,but also increases the conductivity of the material.This core-shell composite exhibits particularly good lithium storage properties compared to LMO without PPy.For example,at a current density of 0.5 A g-1,after 200 cycles of charge and discharge,the reversible discharge specific capacities of PPy@LMO-200 and PPy@LMO-600 are stable at 97.1 and 102.2 mAh g-1,respectively,which is significantly higher than Naked LMO sample(55.1 mAh g-1);at a high current density of 7 A g-1,the average discharge specific capacity is as high as 108.2 and 102.8 mAh g-1,respectively,and returns to a small current density of 0.3 A g-1,the discharge capacity was restored to 102.9 and 110.4 mAh g-1,respectively.