Structure And Morphology Control For The Electrochemical Properties Of Li₂MnO₃ Based Oxide Cathode Material

Due to the depletion of fossil energy and the deterioration of the ecological environment,the development of clean energy has become more and more critical for sustainable development in the modern society.Lithium-ion battery,the typical representative of clean energy,can satisfy the diversified application and functional requirements.Currently,most of the commercial cathode materials suffer low capacity,which is the key issue to improve the energy density and power density.Thus,the development and application of advanced batteries are seriously limited.This paper focus on the layered lithium rich manganese based materials.The morphology and orientation are controlled,and the relationship between morphologies,particle size,interface reaction and electrochemical properties is systematically studied.The main achievements are as follows:The control of particle size and morphology is studyed and the thermodynamic regions for synthesizing Li₂MnO₃ are obtained via hydrohermal process.Dissolution-precipitation mechanism of the hydrothermal reaction is proved.KMnO₄,which is introduced as the second oxidant,can provide an oxidizing reaction environment and result in a continuous change of the particle sizes from 30 to10nm.Smaller particles size is beneficial to the capacity delivery and structural stability upon cycling.Monocrystalline nanowire bundles and polycrystalline nanowires are synthesized by different precursors.The varied crystalline orientation and stacking faults of small crystalline domians endow polycrystalline nanowires with higher electronic and ionic conductivity,leading to higher electrochemical activity.Ni²⁺,Co³⁺and PO₄^3-ions are doped for stabilizing the layered structure and improving the cycling and rate performance.By the hydrothermal coprecipitation,lithium rich materials can be obtained,in which H⁺and Ni²⁺ions may locate in Li layers.Unevenly distributed Ni²⁺ions cause the generation of spinel phase,which gives rise to poor cycle performance and lower capacity.Furthermore,the morphology of the hydrothermal product can not be maintained after the calcination process.Thus,the rate and cycling performance are very poor.By co-precipitation reaction,PO₄^3-ions are introduced into the lattice,which can increase the layered spacing,broaden the lithium-ion migration channels and enhance the migration of charge carriers,leading to high discharge capacity and rate capability.The development experience of lithium rich cathode material plays a critical role in the development of advanced lithium ion battery.In order to investigate the interface reaction and the thermodynamic stability of Li₂MnO₃ in the electrolyte,Li₂MnO₃ nanoparticles are aged at a constant temperature and cycled at a constant current.Surface species of Li₂MnO₃ powders are analyzed and the results reveal that:surface structure of the cathode material reacts with the electrolyte in both the aging process and dynamic cycling process;electrolyte decomposition and continuous side reaction occur,leading to the formation of solid electrolyte interface film?SEI?;SEI film contains inorganic species?Li_xPF_y,Li₂CO₃,LiF,etc?and organic lithium salt?RCH₂CO₂Li and ROLi?,which are mainly originated from the decomposition of the electrolyte and the reaction with the surface structure of the electrode material;the composition of SEI film is changing in both the static aging and cycling process;thus,active Li⁺ions are consumed and the impendence is raised,which may impede the capacity delivery.The research of the interface reaction provides a theory foundation for the modification of lithium ion battery cathodes and promotes the development of lithium ion batteries with high energy density and safety.As surface species and structure are varied during the charging process,we imitate the formation of SEI film during the initial cycle.Part of the Li and O atoms are chemically extracted from Li_1.86MnO_2.93.93 samples and Li_1.16MnO_2.58.58 particles are obtained.XPS photoelectron spectroscopy is used to analyze the effect of Li₂O-removal on the SEI composition.SEI film is mainly composed of Li_x PO_yF_z,Li_xPF_y,Li₂CO₃,RCH₂CO₂Li,ROLi and LiF.During the charging process,lattice O^2-ions overflow from the crystalline structure and produce strong oxidizing O^2-/O^-/O₂,which may cause the decomposition of the SEI film.The chemical extraction of Li₂O can reduce the formation of O^2-/O^-/O₂species and inhibit the decomposition of SEI film.Thus,the relative content of Li_xPO_yF_z and Li₂CO₃decreases significantly;the decreased thickness and varied composition of SEI film give rise to the impedance drop.This study provides a scientific guide for the research of lithium rich battery,and promotes the development of high voltage lithium ion battery.Through solid phase diffusion,the composites are obtained from the mixture of Li₂MnO₃ and LiMn_1/3Ni_1/3Co_1/3O₂ particles.Li₂TiO₃ buffer layer is introduced by a sol-gel method.High charging voltage can break the barrier and accelerate the migration of Mn,Ni and Co elements into Li layers;the reduction peaks of Mn,Ni and Co shift to the low voltage upon cyling;high upper limit voltage inspires voltage drop seriously.The introduction of Li₂TiO₃ buffer layer can modify the composited region and stablize Li₂MnO₃ phase and LiMn_1/3Ni_1/3Co_1/3O₂.Ti⁴⁺ion possesses large radius and chemical stability properties,which can inhibit the migration of Ni,Co and Mn ions in the lithium rich materials.Thus,the voltage decay can be alleviated.This basic research can accelerate the development and commercial application of the layered lithium rich batteries.