Study of Preparation and Modification of Layered Lithium Nickel Cobalt Aluminium Cathode Material

Author:He Xiao Shu

Supervisor:du chun yu


Degree Year:2019





Along with the emergence and development of newly sprouted things including electric power tool,electric vehicles(EVs)and back-up power devices,it is increasingly urgent to develope lithium-ion batteries(LIBs)with high-energy density,high-power density and long cycle life.Cathode materials are the most critical factor to determine the performance and cost for LIBs,which has become the research focus in this field.Layered nickel-rich LiNi0.8Co0.15Al0.05O2(NCA)is a preferred cathode material for LIBs,owing to its low cost and superior electrochemical performance.However,large capacity fading of the material cycled at elevated temperature(≥50℃)restricts its large-scale applications,because the Ni4+of the delithiated NCA material can react with the electrolyte,and the side reactions occur more seriously as the temperature increases,.Surface coating can effectively address this issue to avoid the direct contact between bared NCA material and electrolyte.Metal oxides,fluorides and phosphates,as surface coating materials,are chosen by researchers at early,however,most of them are ionically and/or electronically insulating materials,and prone to increase the polarization of the battery,thus decreasing the capacity and rate capability of the cathode material.In this thesis,high-performance NCA cathode material was prepared by co-precipatition method,and ionic and/or electronic conductors were employed as coating material on NCA cathode,which significantly enhanced the cycle performance,especially at elevated temperature.Moreover,the capacities and rate capabilities were notably improved as well.Spherical Ni0.8Co0.15Al0.05(OH)2 precursor was prepared by co-precipatition method,and the influences of solid-state-reaction conditions on the crystal structure and electrochemical performance of the prepared NCA cathode material were investigated.XRD results show that the calcination time and temperature determine the structural integrity and Li/Ni disorder degree of the prepared NCA materials.Combined with the electrochemical performance,it can be obtained that the cycle performance of NCA cathode material improves with the decrease of Li/Ni disorder degree,and the superior rate capability depends on both the high value of lattice parameter c and low Li/Ni disorder degree of the NCA material.Fast Li-ion conductor Li2TiO3 was employed to coat on NCA cathode material,and the coated material was denoted as LTO-NCA.The influences of nanoparticles-coating method and synchronous lithiation method,on the crystal structure,superficial lithium residues,and electrochemical performance of the prepared LTO-NCA material were investigated.It can be obtained by analyzing the results of XRD/Rietveld,XPS,SEM-EDS,and TEM that the synchronous lithiation method can cause titanium doping into the NCA bulk structure,thus increasing the Li/Ni disorder ratio and superficial lithium residues.Therefore,the LTO-NCA material prepared by synchronous lithiation method shows inferior electrochemical performance compared to the pristine NCA.The identical characterizations were performed on LTO-NCA material prepared by nanoparticles-coating method,and the results indicate that this coating method cannot change bulk structure,thus the rate capability and cycle performance of this coated material were significantly enhanced by Li2TiO3 coating,especially for the cycle performance at elevated temperature.Electronically conductive Sb-doped SnO2(ATO)was employed to coat on NCA cathode material.The influences of coating content on the crystal structure and electrochemical performance of NCA materials were investigated.The results show that ATO coating can enhance the electronic conductivity,and cannot change the NCA bulk structure.The coated material with 0.5 wt%ATO shows the best rate capability and cycle performance.The capacity at 5 C-rate of this coated material was 10 mAh·g-1 higher than that of the pristine NCA,and its capacity retention was 20.81%higher than that of the pristine NCA cycled at 1 C-rate and 60℃for 200 cycles.The characterizations of the cycled cathodes show that the ATO coating can restrain the SEI film growth on the electrode surface and the structural degradation of bulk NCA material.Furthermore,the ATO&Li2TiO3 composite coated NCA material was prepared to simultaneously improve the ionic and electronic conductivity of NCA material surface.The results show that this composite coating makes NCA material much greater enhancements in capacity and rate capability.The capacity was 167 mAh·g-1 at 5 C-rate.The graphene nanodots(GNDs)and reduced graphene oxide(RGO)were employed to modify on NCA cathode material.The influences of the two modification materials on ionic and electronic conductivity,as well as the electrochemical performance for NCA material were investigated.The results indicate that the electronic conductivity of NCA material by GNDs and RGO modification are better than that of ATO coating,however,RGO modified NCA material has inferior capacity,owing to the large Li-ion transport barriers,which can be effectively overcome by the GNDs modification.The ionic conductivity of 0.5 wt%GNDs modified NCA material was close to that of the pristine NCA,and its electronic conductivity was 5 times of the pristine NCA.Therefore,the rate capability was significantly enhanced and better than those of ATO and ATO&Li2TiO3coated materials.Especially,the capacity of GNDs modified NCA material was increased by 22 mAh·g-1 at 5 C-rate.