Surface Structure And Morphology Of Spinel Cathode Materials For Lithium-ion Batteries

The application of Lithium ion batteries has been moving from portable power source to electric vehicles（EV）and grid storage,which needs high safety and specific power.Considering the cathode and anode materials,their chemical potential and capacity are the limiting factors to the specific energy for a Li-ion battery and the diffusion coefficient of Li⁺in the materials limits the kinetic performance.This thesis focuses on the spinel materials with three-dimensional Li⁺transport channel,including LiMn₂O₄ with 4.1 V and LiNi_0.5Mn_1.5O₄ with 4.7 V voltage platform vs.Li⁺/Li.Research results of their surface atomic structure,components and morphology,cycling performance have been obtained,which could provide a fundamental understanding about the reconstruction of surface atomic layer in the operation process.The surface evolution of LiMn₂O₄ spinel cathode without/with Al₂O₃ surface modification at the atomic-scale was compared.Al₂O₃ modified LiMn₂O₄ was synthesized by wet chemistry.The presence of Al³⁺rich surface layered structure Li（Al_x,Mn_y）O₂ within a f ew nanometers was confirmed by XPS,SIMS and Aberration-corrected scanning transmission electron microscopy STEM/EDS in the surface-modified LiMn₂O₄ sample,viewed along the[110]_spinel,[211]_spinelpinel crystallographic direction.Layered structure Li（Al_x,Mn_y）O₂ could be reserved,which was observed at the direction of[100]_layered when it was charged to 4.3V.This layered structure Li（Al_x,Mn_y）O₂ contribute significantly to the improved high-temperature and high-voltage cycling performances of LiMn₂O₄,in addition to the common results that Al₂O₃ act as the scavenger of HF.LiNi_0.5Mn_1.5O₄ was prepared by a co-precipitation method using NiSO₄,MnSO₄and NaOH,NH₄OH as the starting materials,due to their low cost and superior dispersion effects.Small amount of residual SO₄^2-could be present on the surface of the LiNi_0.5Mn_1.5O₄ particles and further it could adjust the surface energy of plane,change its growth speed and control the ultimate morphology of particles.A surface-adsorption/desorption controlled crystal growth for spinel LiNi_0.5Mn_1.5O₄ cathode was demonstrated by an ingenious reversible experiment and also confirmed by DFT calculation method for the first time.By employing STEM/EELS method combined with the FIB,which could lift the sample to a 100 nm sheet along the[110]_spinelpinel direction,we revealed that the presence of oxygen-deficient and Ni-rich areas is related to the rock-salt phase on the surface layer of truncated octahedral grain.Thus we concluded that the atomic structure and composition of the surface region are critical to its morphology and electrochemical performance.It is found that some Ta⁵⁺ions can be doped into the 16d sites to substitute the Ni²⁺ions in the surface layer and more Ta⁵⁺directly form the LiTaO₃ phase at the outermost layer.Mix（C₂H₅O）₅Ta directly with Ni_0.25Mn_{0.75（）OH2} and Li₂CO₃ and heat to 900℃are favorable to the homogeneous Ta⁵⁺doping into crystal lattice of LiNi_0.5Mn_1.5O₄.On the contrary,thermal treating of（C₂H₅O）₅Ta coated LiNi_0.5Mn_1.5O₄ at 500℃could only get LiTaO₃ coating.The electrochemical performance of Ta⁵⁺doped Li Ni_0.5Mn_1.5O₄ cathode is improved whereas the LiTaO₃coated LiNi_0.5Mn_1.5O₄ shows a deteriorative performance both at the room or elevated temperature.The underlying mechanism is revealed,the bulk and surface regions become different when Ta⁵⁺ions diffuse into the different depth.Lattice structure could be more stable when part of Ni²⁺is replaced by Ta⁵⁺,whereas the extraction of Li⁺in surface layer of LiNi_0.5Mn_1.5O₄ to form LiTaO₃ will distort the lattice structure of LiNi_0.5Mn_1.5O₄.This will further affect the different surface side reactions,which is affirmed by the analysis of XRD,SEM,EIS and XPS etc.