Li-rich Manganese Cathode Materials Synthesized From Nanostructural Precursor And Their Performances

Due to the high capacity,high operating voltage,long cycle life,wide working temperature,low self-discharge rate,safety,and environmentally friendly property,Li-ion battery has got a broad applications in the mobile portable electronics,driving power source and energy storage.Thus,it also becomes one of the current hot topics.In a Li-ion battery,cathode material,as one of the three major components,determines the specific capacity and cost of the whole battery.Therefore,it has attracted a great interest in looking for a new cathode material with high performance or enhancing the performances for the existing cathode materials,which shows a special theoretical and practical significance.Li-rich Manganese-based cathode materials are a series of solid solutions with a variable ratio of Li₂MnO₃ and LiMO₂（M=Ni,Co,Mn,etc.）.Compared to the commercialized cathode materials,Li-rich Manganese-based cathode materials possess an obvious advantage of a much higher mass-specific capacity（25% higher than common commercialized cathode materials）.However,Li-rich Manganese-based cathode materials have a lot of intrinsic defects.Firstly,the initial coulombic efficiency is about 70%,which is much less than 90% demanded by the practical application.Secondly,during the process of charge and discharge,Li-rich Manganese-based cathode materials show the serious structural degradation.The layered structure is gradually transformed into a spinel structure,which leads to the decrease in the specific capacities and voltage.Thirdly,Li-rich Manganese-based cathode materials present a low compact/tap density.Compared to the high-density ternary cathode materials,Li-rich Manganese-based cathode materials show a plain volume energy density,although they exhibit an obvious advantages of mass energy density.Forthly,compared to the three-dimensional Li-ion migration channel of spinel structure,Li-rich Manganese-based cathode materials just deliver a two-dimensional Li-ion migration channel,so their rate performances are not so satisfied as the demand on the application in electric cars,especially for the demand on the car power during the climbing stage.Lastly,other defects,such as the requirement on the high voltage electrolyte,side reactions during the process of charge and discharge.Generally,nanostructured materials possess a high mass surface area and surface energy,which is benefit to migrations and interphase reactions of Lithium ions.Combining the characteristics of Li-rich Manganese-based cathode materials with nanostructured materials,here we selected a nanostructured precursor as the research target material to synthesize the cathode materials in order to solve the problems faced by researchers of Li-rich Manganese-based cathode materials.On the basis of the related research works,this paper aims to improve the tap density,initial coulombic efficiency,discharge capacity and rate capability of Li-rich Manganese-based cathode materials by optimizing synthetic methods,and then explore the effects of the evolved structure and doping element on the properties of the cathode materials.1.In order to improve the tap density of Li-rich Manganese-based cathode materials,the micro spheres of the transition metal carbonates were synthesized by the solvo-thermal method and co-precipitation method.It was found that the precursor by the solvo-thermal method has a micro-nano structure.The micro spheres are composed by nanoplates.Li-rich Manganese-based cathode materials sintered from this micro-nano precursors show an improved tap density by ¹4% and increased energy density by more than 22% compared to the sample sintered from the co-precipitated synthesized precursor.At the same time,the discharge capacities and the rate performances of the cathode materials from solo-thermal method are slightly higher than those of the sample from the co-precipitated method.By analysing the structural difference of precursors,it was believed the enhanced tap density was related to the hierachical nano structure.The results bring us a method for designing the cathode materials with high tap density.2.The transition metal hydroxide with a thickness of 10 nm was successfully prepared by adding a bi-functional template in solution during the synthesis of precursors by the co-precipitation method.Li-rich Manganese-based cathode material,Li1.2Mn0.54Co0.13Ni0.13O2 was then prepared by sintering this nanoplate precursor and Li2CO3.The electrochemical results show that the initial coulomb efficiency of as-prepared cathode materials is up to 85%,while the efficiency of the cathode material from contrast precursor is only about 77%.Moreover,the cathode material from nanoplate precursor show an initial discharge capacity of 308 mAh/g at 0.1 C and a capacity of 224.8 mAh/g at 1 C（rate performance）,while the cathode material from contrast precursor exhibit an initial capacity of 245 and 187.6 mAh/g at the same conditions.Combining with the experimental results,structure of cathode material,Rietveld refine on XRD spectra and electron cloud from corresponding Fourier transformation,the reasons of the improved electrochemical performances are found: the crystal cell at a（or b）direction get enlarged about 0.05% which facilitates the Li+ migration from transition metal layer to lithium layer.That is,the enlarged crystal cell made a good electrochemical performances.The results enriched our knowledge in Li-ion migration in Li-rich layered materials.3.Based on the preparation of the nanoplate precursor,we optimized the synthetic parameters,and then discussed the mechanism of the bi-functional template.By designing a set of orthogonal experiments,we investigated the effects of the concentration of the reactant and template,and the ratio of two templates on the morphology of nanoplate precursor.Under the optimal conditions,the as-prepared Li-rich Manganese-based cathode material shows a discharge capacity of 323.8 mAh/g,and an initial coulombic efficiency of 81.3%.In addition,the cathode sample exhibits a good rate performance,where the discharge capacity is up to 195.8 mAh/g at 3 C.After 56 charge/discharge cycles,the capacity retention rate is 96%,which shows a good cycle performance.According to the experimental results,we proposed that the formation of the nanoplate precursor should be a self-assembly process under the help of the templates.4.Due to the same chemical valence,Sn⁴⁺ ions can replace Mn⁴⁺ ions in the crystal of Li-rich Manganese-based cathode material and then forms a Sn doped cathode material.Since the Sn-O bonds between Sn⁴⁺ and O^2-are stronger than the Mn-O bonds formed between Mn⁴⁺ and O^2-,the substitution of Mn by Sn may stabilize the structure of the cathode material.Nanoplate precursor shows a high reactivity during the sintering process,and the required distance for doping ions get short（migration from the surface to the doping position）.So,we prepared a series of Sn doped cathode materials,using the as-synthesized precursor nanoplates.By the experiments’ results of Sn doped cathode materials,we found that the cathode material with 1% Sn doped showed the best rate and cycle performance.Further inversitigation suggested that the Sn doping could benefit the migraton of Li ions and thus contribute the activation of Li-rich layered cathode materials.This paper explored the electrochemical performances of Li-rich cathode material synthesized by precursors with Nano-structure,and then obtained the following conclusion.As possessing a much larger special surface area and（a dimension,at least）limited thickness,nano structural precursors have a much higher reaction activity and a reduced diffusion distance for lithium ions during the solid reaction with lithium salt at high temperature.Therefore,Li-rich cathode materials prepared from the nano structural precursors exhibit high crystallinity,appropriate lattice parameters and,to a certain extent,the morphology of precursor.The former can reduce the interphase between crystals within cathode materials,and thus benefit the migration of Lithium ions,which finally improves the special capacities,rate and cycling performance of cathode materials.The later could afford us a route to enhance the tap density of cathode materials.