Study On The Synthesis And Modification Of LiNi_0.8Co_0.15Al_0.05O₂ As High-Performance Lithium-Ion Battery Ternary Cathode Material

Nickel-rich ternary cathode materials are considered to be the idea choice for high-performance lithium ion batteries due to their high capacity and high working voltage.However,large-scale applciatons of these materials have been hindered by their shortcomings includeing poor rate capability,cycling stability,and thermal stability.In order to overcome these problems,in this thesis,we selected lithium nickel cobalt aluminum oxide(Li Ni_0.8Co_0.15Al_0.05O₂,NCA)as the research object to study our proposed modification approach,namely full concentration-gradient structure design and further titanium（Ti）doping and surface coatng,resulting in the final optimized NCA cathode material with significantly enhanced rate capability,cycling stability,and thermal stability over its traditional NCA counterpart.Meanwhile,by comprehensively investigating the physical properties of all materials tested,including partical size distributions,tap densities,morphologies（SEM,TEM）,structural phases（XRD）,and compositions（ICP,XPS,EPMA）,we systematically studied the mechanisms behind the improvement of the overall properties of our modified NCA cathode material.The relevant research and achievemnts are briefly described as follows:First of all,it is known that the composition of the precursor of traditional NCA,i.e.,Ni_0.8Co_0.15Al_0.05（OH）_2.05,is Ni:Co:Al=0.8:0.15:0.05 where the distributions of all elements from the core to the surface are constant.From that,in our first concentration-gradient design,the core of the precursor is designed as Ni:Al=0.95:0.05（content of Co is 0）and the surface as Ni:Co:Al=0.55:0.4:0.05,that is,from the core to the surface,the content of Al remains unchanged while that of Ni gradually decreases and Co increases.Oppositely,in our second concentration-gradient design,the core of the precursor is Ni:Co=0.85:0.15（content of Al is 0）and the surface is Ni:Co:Al=0.75:0.15:0.1,that is,from the core to the surface,the content of Co remains unchanged while that of Ni gradually decreases and Al increases.Using the conventional co-precipitation method,the traditional Ni_0.8Co_0.15Al_0.05（OH）₂ precursor as well as the proposed Co-concentration-gradient Co CGNi_0.8Co_0.15Al_0.05（OH）_2.05（Co CGNCA）precursor and the Al-concentration-gradient Al CGNi_0.8Co_0.15Al_0.05（OH）_2.05（Al CGNCA）precursor have been successfully synthesized,all showing uniform particle sizes（about 10μm）and well-defined morphologies.Further,higher contents of Co on the surface of Co CGNCA and Al on Al CGNCA,resepectively,over the traditional Ni_0.8Co_0.15Al_0.05（OH）_2.05 precursor,were verified.Subsequently,these precursors were separately mixed with Li OH,followed by a high-temperature solid phase calcination to produce traditional non-concentration gradient Li Ni_0.8Co_0.15Al_0.05O₂（LNCAO）,Co-concentration-gradient Co CGLi Ni_0.8Co_0.15Al_0.05O₂（LCo CGNCAO）,and Al-concentration-gradient Al CGLi Ni_0.8Co_0.15Al_0.05O₂（LAl CGNCAO）cathode materials,respectively.By processing the three cathode materials above into electrodes and testing cells,the electrochemical measurements showed that LNCAO has an initial discharge capacity of190.18 m Ah g^-1 corresponding to a Coulombic efficiency（CE）of 90.75%,but its capacity retention at 3C（vs.0.1C）(1C=200 m A g^-1)is 42.58%only and the capacity retention after charge/discharge at 1C for 100 cycles is as low as 22.42%.Upon modification,LCo CGNCAO has an initial discharge capacity of 193.85 m Ah·g^-1corresponding to a CE of 91.09%,along with an enhanced capacity retention of 50.40%at 3C and an enhanced capacity retention of 32.56%after charge/discharge at 1C for 100cycles.Further,LAl CGNCAO possesses an initial discharge capacity of 192.10 m Ah g^-1corresponding to a CE of 91.24%,along with its capacity retention at 3C further enhanced to 57.47%and the capacity retention after charge/discharge at 1C for 100 cycles further enhanced to 46.28%.The improved rate capability of concentration-gradient cathode materials can be attributed to their restrained cation mixing and enlarged lattice spacing,while the improved cycling stability to their restrained cation mixing and reduced surface-impurity content（suppressing the corrosion of the electrode by the electrolyte）.Moreover,compared to LCo CGNCAO,the further improvements of these physical properties of LAl CGNCAO are responsible for its further enhanced rate capability and cycling stability.At the same time,the thermal stability tests in the range of 150-350℃indicated that LNCAO has a high total heat release up to 4356 J·g^-1,much higher than those of LCo CGNCAO(3739 J g^-1)and LAl CGNCAO(2164 J g^-1).The improved thermal stability of concentration-gradient cathode materials can be attributed to their reduced surface-content of oxidative Ni⁴⁺（restraining the oxidation between the delithiated NCA and the electrolyte）,where the Al surface-coating of LAl CGNCAO can further suppress this kind of oxidation as well as the release of lattice oxygen so as to further enhance its thermal stability.Finally,Ti doping and surface coating were performed on the best concentration-gradient cathode material optimized above,i.e.,LAl CGNCAO,to further improve the overall properties for our proposed modified cathode material.Specifically,through a facile hydrolysis reaction,a Ti compound was coated onto the precursor of LAl CGNCAO,i.e.,Al CGNCA,followed by Li OH-mixing and high-temperature solid phase calcination of the resultant new precursor to produce Ti-doped/coated LAl CGNCAO,i.e.,Ti-Al CGLi Ni_0.8Co_0.15Al_0.05O₂（Ti-LAl CGNCAO）.Through optimization,the best Ti modification amount was determined to be 2 mol%,resulting in a material of 2%Ti-LAl CGNCAO.This final optimized cathode material has an initial discharge capacity of175.42 m Ah g^-1 corresponding to a CE of 84.59%,along with its capacity retention at 3C further enhanced to 66.58%,the capacity retention after charge/discharge at 1C for 100cycles further enhanced to 70.04%,and the total heat release furher reduced to 1540 J g^-1,clearly showing the superior overall performances over its LAl CGNCAO counterpart.In this material,the enhanced Li⁺-layer structural stability,enlarged lattice spacing,and facilitated Li⁺diffusion enabled by Ti doping should be responsible for its improved rate capability and cycling stability,while the suppressed corrosion of the electrode by the electrolyte,restrained dissolution of the electrode,and alleviated release of lattice oxygen enabled by Ti surface coating for the enhanced thermal stability.In summary,using the Ni-rich NCA ternary cathode material as the research object,we have comprehensively and systematically investigated our modification approach through concentration-gradient structure design and further Ti doping and surface coatng.The final optimized Ti-doped/coated concentration-gradient NCA cathode material possesses stabilized structure,increased cation mixing,enlarged lattice spacing,facilitated Li⁺diffusion,reduced surface-impurity content,reduced surface-content of oxidative Ni⁴⁺as well as the suppressed corrosion of the electrode by the electrolyte,restrained dissolution of the electrode,and alleviated release of lattice oxygen,thereby exhibiting significantly enhanced overall perormanes in rate capability,cycling stability,and thermal stability over its trdaditonal NCA counterpart.Therefore,our research of the present work about the modification and optimization of lithium-ion battery cathode materials would stand for a remarkable significance for practical applications and also an important value for academic researches.