Preparation And Properties Of High-rate Li-rich Mn-based Oxide Cathodes For Lithium-ion Batteries

Recently,research and investment around the world in the development of sustainable energies?solar and wind energy etc.?grow at an unprecedented high speed,due to the more and more concerns about the climate change and energy crisis.To store the sustainable energy for large-scale use in devices as well as electric vehicles,Li-ion batteries are demanded with higher energy and power,which thus leads to a requirement for cathodes with higher capacity and rate.As a result,Li-rich Mn-based layered oxides?LLOs?are one of the most promising candidates due to the highest capacity of any cathodes used at present.However,these materials have fatal drawbacks,such as poor rate capability,low initial Coulombic efficiency and serious voltage degradation.To optimize their rate capability,considerable methods have been developed to improve the conductivity of LLOs,such as doping and coating.Although the rate capability of LLOs has been improved by doping or coating,it is still difficult to meet the requirements of electric vehicles etc.on energy and power density of Lithium-ion batteries.Herein,this thesis will continue to optimize the rate capavbility of LLOs by improving ionic and electronic conductivity.To enhance the ionic conductivity of LLOs,the following methods have been adopted:shortening the paths of Li-ions transportation in active materials,improving the distributive uniformity of elements of active materials,and introducing the three-dimensional Li-ion channels into the interior of active materials.As for electronic conductivity of LLOs,it has been improved by the in-situ modification of rGO.The main work of this thesis is summarized as following:1.Three nanoplates with different exposed facets have been prepared by using diglycol-,CTAB-or NMP-assisted co-precipitated-precursor methods,respectively.The particle size,crystalline structure and TM valence state of the three nanoplates are almost the same,and the exposed facet is their main distinction.Their exposed facets are?001?,?102?and?101?,respectively.The cycling stability of?101?nanoplate is the worst,while?001?nanoplate has the best cycling property.The rate capabilitie of the three nanoplates decrease in the order:?101?>?102?>?001?.These can be attributed to:?1?Large-area inactive exposed facet can mitigate stucture transformation and electrochemical corrosion of LLOs,leading to slow voltage decay and capacity fading;?2?Large-area of active surface can shorten the paths of Li-ions transportation in LLO cathodes,resulting in an improved rate capability.In addition,by the comparative analysis of the nanoplates,we find:?1?TM ions in LLO cathodes migrate toward the active facets,and Ni and Co ions hop more preferentially than Mn ions during the cycling;?2?Both stucture transformation and electrochemical corrosion often occur on active surfaces;And?3?Co and Ni are easier to react with electrolyte than Mn.2.We have prepared some kinds of LLO cathodes by combining the solvothermal process and molten salt method,and researched the effect of molten salt method on the rate capability of LLO.In this thesis,Li₂CO₃ and NaCl-KCl are used as molten salt,respectively.Electrochemical measurements indicate the rate capability of the sample prepared by NaCl-KCl molten salt method is better than that synthetized by Li₂CO₃ molten salt method.It is because the content of surface Li for the former is more close to stoichiometric ratio than that for the latter.Due to the overmuch surface Li,Li₂MO₃ component will be enriched on the surface of LLO,leading to a difficult?de?insertion of Li from LiMO₂ component and an unideal rate capability.In a word,the main function of molten salt method is to improve the migration rate of ions during the heat treatment process,which will decrease the content of surface Li and enhance the uniformity of Li₂MO₃ and LiMO₂ components.Thus,the rate capability of LLO cathodes can be optimized.3.The rGO and spinel phase are introduced into LLO for the first time by an in-situ technique.This cathode is composed of LiMO₂ component,Li₂MnO₃ stucture,spinel phase,and a small amount of rGO?¹.08 wt%of carbon?.Thus,we haved name it as LS@rGO.In fact,LS@rGO is a novel cathode material of Li-ion batteries,in which LLO is the main part and spinel phase as well as a small amount of rGO is introduced by an in situ method.The discharge capacity of LS@rGO can still reach145 mA h g^-1 at an ultrahigh charge-discharge rate of 60 C(12 A g^-1).Such an excellent rate of LS@rGO should be close related with the in situ introduction of spinel phase and rGO.This is because the spinel phase can assist the transport of Li⁺within electrode materials,and the rGO component can greatly improve the electronic conductivity of electrode materials.Herein,the overall result is LS@rGO cathode can charge-discharge at an ultrahigh rate.Through unremitting efforts,we have finally designed a method to synthesize Li-rich Mn-based oxide cathode materials with high-capacity and high-rate.It will significantly improve the energy and power density of Li-ion batteries,which can promote the large-scale applications of electric vehicle and smart grid etc.,resulting in an alleviation of climate change and energy crisis.