Surface/Bulk Phase Modification Research of High-Capacity Lithium/Sodium-Rich Cathode Materials for Secondary Batteries

Author:Guo Li Chao

Supervisor:li jia jun


Degree Year:2017





With environmental issues becoming increasingly serious,renewable,green and clean energies are gradually replacing the traditional ones.High-performance chemical power sources are urgently demanded by grid storages and electric vehicles.Alkali-ion secondary batteries(AIBs),such as lithium-ion secondary batteries(LIBs)and sodium-ion secondary batteries(SIBs),are one of the most important chemical power sources and have attracted much attention.Cathode materials determine the upper limit of the energy density and power density of the AIBs.Therefore,the challenge in enhancing the overall properties of secondary batteries is to develop high-performance lithium/sodium-rich cathode materials.This study focuses on the phase modifications,introducing one or two kinds of modification phases at the surface region or within the bulk of the cathode materials.A well combined interface of the modification phase and the substrate phase was engineered,and multifunctional phase modification was achieved.The adsorption behavior,the phase transformation and other issues have also been proved by first-principle calculations.Firstly,to tackle with the issues of slow mass transfer kinetics and unstable surface structures,a modified structure with double phases on the surface Li-rich cathode was synthesized.The formation process and the impact mechanism of the double phases modification were discussed.The orientation relationships of hetero-phase interfaces were selectively analyzed.Secondly,to deal with the fact that Na-rich cathode materials were hard to synthesize and suffered from poor cycle life,the work studied the formation conditions of Na-rich phase,and the fading process of Na-rich cathode materials.The modification mechanism of the structural integrated doping phase was clarified.The research contents of this study include:1)Surface double phase network modified Li-rich cathode materials(MCNT-LR)were synthesized by a physical adsorption approach.The double phases referred to the multi-wall carbon nanotubes(MCNT)and the spinel phase.The effects of the MCNT contents on the network morphology,crystal structure and electrochemical property were discussed.The phase compositions and interfacial bonding at the modified surface region were selectively analyzed.The localized redox reactions were discovered between MCNT and Li-rich phase,leading to a“layered to spinel”phase transformation at the interface.The spinel phase had coherent or semi-coherent relationships with the substrate Li-rich phase.The double phase networks,which included the MCNT network and the spinel phase network,substantially increased the lithium and electron transfer kinetics,which efficiently enhanced the rate capability of Li-rich cathode materials.At the current rate of 5C,the most optimized MCNT-LR sample achieved a discharge capacity of about 150 mAh g-1,which was increased by 140 mAh g-1 as compared with the capacity of un-modified Li-rich cathode materials.2)In order to enhance the long cycle performance at high current rates,the surface double phase shell modified Li-rich cathode materials were synthesized via a chemical adsorption approach.Specifically,the double phase shells referred to the thiomolybdate ionic group outer shell and spinel phase inner shell.The influences of the ionic group on the morphology,phase composition and chemical bond were discussed.The structure character and the formation mechanism of double phase shells were clarified.With the discovery of electron transfer from the ionic group to the substrate phase,the chemical adsorption behavior of the ionic group was proved by both theoretical calculations and experimental measurements.Judging from electrochemistry tests,double phase shells not only accelerate the mass transfer kinetics,but also ensure the surface stability,hence improving the cycling performance of Li-rich cathode materials at high current rates.3)To solve the problem of the uncontrollable synthesis of high-quality Na-rich phase,Fe and Mn based Na-rich cathode materials(MF-PBA)were synthesized via the metal organic framework(MOF)self-assembly method.The study analyzed the effects of solvent compositions,NaCl additives,precipitation reaction temperatures,as well as the follow-up thermal treatment on the morphology,elementary compositions,phase structures,and electrochemical properties of MF-PBA materials.The crystal structures,X-ray diffraction patterns and voltage plateau profiles of MF-PBA in different crystal phases were simulated via first principle calculations;and these data were used to support the experiment analysis.The study found that water/ethane solvent and high content NaCl were the preconditions of synthesis of Na-rich cathode materials.The slow nucleation/growth rate and high Na+content were both the essential factors for the growth of high-quality Na-rich phase.In addition,lower precipitation reaction temperature and the thermal treatment can improve the cycle stability of MF-PBA to some extent.4)To clarify the failure mechanism and improve the cycle performance of MF-PBA,MOF self-assembly method was applied to synthesize the copper doped MF-PBA cathode materials(CMF)and un-doped MF-PBA(MF).The differences and similarities between CMF and MF samples were analyzed in the aspect of morphology,phase,valence bond,electrochemical property and reaction kinetics.The research found that with the condition of the low doping content,CMF sample maintained the same phase as MF.The doping phase was uniformly dispersed in the prime particles,and had a good structural integration with the substrate phase.In combination of the theoretical calculations and the electrochemical profiles of CMF vs MF,the failure process of MF-PBA were illustrated from the perspective of phase transformation.The doping effects on MF-PBA were analyzed in detail;and a reinforcing mechanism of the structural integrated doping phase was clarified.