Synthesis And Modification Of LiMn₂O₄ And LiNiO₂

With the development and popularity of mobile electronic devices and new energy products,lithium-ion batteries have become an indispensable part of people’s lives.Lithium-ion battery is the best choice from powering electronic watches,mobile phones,laptops and other portable electronic devices,to electric vehicles and electric energy storage stations.However,the application of lithium-ion batteries is still facing many challenges,such as insufficient safety performance,insufficient energy density,etc.For a lithium-ion battery,the cathode material not only accounts for the most significant proportion of the entire cost,but also determines mainly its energy density,cycling life and safety.Therefore,improving the cathode material of a lithium-ion battery is crucial to its further development.In this thesis,two typical cathode materials were synthesized and investigated.The first one was spinel LiMn₂O₄ cathode material which has been commercialized for decades and the second one was layered LiNiO₂ cathode material which has great development prospect in the future.The main work was divided into three parts:（1）Nb₂O₅ was used to coat LiMn₂O₄ cathode material,which effectively improved the high temperature cycle performance and rate capability of the material;（2）Ni（OH）₂precursor was synthesized by the electrolysis method,and corresponding LiNiO₂cathode material with very low Li/Ni mixing degree and very high specific capacity was successfully synthesized with Li OH-Li NO₃ eutectic Li salt.Moreover,the degradation mechanism of LiNiO₂ cathode material during the cycling was further investigated in detail;（3）Nb₂O₅ was added in the electrolysis process to modify the LiNiO₂ cathode material,which effectively regulates the morphology of the primary particle of LiNiO₂ cathode material by coating on the surface of the material to protect the material from the erosion of the electrolyte.Therefore,Nb modified LiNiO₂ cathode material exhibited a good electrochemical performance.The first part of this thesis focuses on LiMn₂O₄ cathode material.Although LiMn₂O₄ cathode material has been widely recognized and applied in the market,but it has the problems of Mn³⁺ion disproportionation and associated Mn²⁺ion dissolution,which will become extremely serious under the induction of hydrofluoric acid at high temperatures.Therefore,it has poor electrochemical cycle performance at elevated temperatures,which is the primary factor limiting the further development of LiMn₂O₄cells for long service life applications.According to our previous experience with Nb₂O₅,it has a strong resistance to hydrofluoric acid.Therefore,this thesis selected Nb₂O₅ to coat the LiMn₂O₄ cathode material.The thesis explored the changes of the electrochemical properties of the LiMn₂O₄ cathode at different coating temperatures and amounts,and found that under low coating temperature and a small coating amount of Nb₂O₅,the high temperature cycling performance of LiMn₂O₄ cathode material was significantly improved,proving that Nb₂O₅ can effectively protect the electrode material from the erosion of electrolyte.However,under a larger coating amount of Nb₂O₅ and the high coating temperature,the high temperature cycling performance of LiMn₂O₄ cathode material was not improved effectively,but showed slightly decreased cycling performance.In the subsequent in-depth investigation of this thesis,it was determined that a small amount of Nb⁵⁺ions entered the lattice during the coating process.Due to the conservation of charge,the pentavalent Nb⁵⁺ions promoted the transfer of Mn ions to the low valence state,which encourged the disproportionation of Mn³⁺ions and the dissolution of Mn²⁺ions in LiMn₂O₄ cathode materials become more and more serious.As a result,the material’s high temperature electrochemical cycling performance was damaged,and the problem of Nb⁵⁺ions entrying will become more with the increase of coating temperature and coating amount.Therefore,Nb₂O₅ can effectively protect LiMn₂O₄ cathode material from hydrofluoric acid erosion,but it also needs to control the coating conditions to achieve the optimized material.Next,considering the limited energy density of LiMn₂O₄ itself,in order to further promote the market application prospects of lithium-ion batteries,the second part of this thesis mainly focuses on the synthesis of cobalt-free LiNiO₂ cathode materials with high energy density.The traditional Ni（OH）₂ precursor used in LiNiO₂ cathode materials is mainly synthesized by coprecipitation,which needs to regulate a series of parameters such as p H,ammonia concentration,feed rate ratio,etc.Moreover,the coprecipitation equipment requires high requirements,the operation is complex,the consistency of the material is poor,and the production cost is high.Therefore,this thesis used the electrolysis method to synthesize Ni（OH）₂ precursor,which only needs a constant current and voltage power supply,Ni sheet,electrolyte,and magnetic stirrer.The reaction rate can be simply controlled by current,and the whole production process is simple to operate and convenient.Considering the similarity of Li⁺and Ni²⁺ion radius,Li/Ni mixing problem is inevitable during the LiNiO₂ cathode material synthesis process.This thesis selected Li OH-Li NO₃ eutectic Li salt as the lithium source to reduce the pre-sintering temperature,LiNiO₂ cathode material with extremely low Li/Ni mixing was successfully synthesized,and the electrochemical test also showed that the synthesized material had a very high discharge capacity（235.2 m Ah/g）.In addiation,the reason for the low Coulomb efficiency of the LiNiO₂ cathode material in the initial cycle was revealed by the analysis of d Q/d V data of the initial and the second cycle.After that,this thesis also conducted a detailed failure analysis and exploration of the LiNiO₂ cathode materials after electrochemical cycling.Combined with structural refinement,electron microscopy and spectral analysis,this thesis found that the bulk phase structure of the LiNiO₂ cathode materials did not change much after cycling,while the structure and composition of materials’surface had changed significantly.It was determined that the degradation of LiNiO₂ cathode material in the electrochemical cycle mainly came from the change of the surface.Therefore,in the subsequent modification of LiNiO₂ cathode materials,the surface of the material was also mainly focused.After a thorough understanding of the synthesis of LiNiO₂ and the causes of degradation of cycling performance,and understanding gained from modification of the cathode material LiMn₂O₄ by Nb in the first part of this thesis The third part of this thesis mainly focuses on the modification of LiNiO₂ cathode materials,using Nb element to modify the LiNiO₂ cathode material.The introduction of Nb element was in the precursor preparation process,which can make Nb to be more effectively dispersed in the internal region of the whole LiNiO₂ cathode material.The electrochemical text results showed that the cycling and rate performance of LiNiO₂ cathode materials modified by Nb element were significantly improved.While exploring the mechanism analysis,this thesis found that the introduction of Nb element could make the LiNiO₂cathode material form a smaller primary particle size and a denser secondary particle structure,which could effectively resist the corrossion of the electrolyte during the electrochemical cycle.Advanced characterization further revealed that the introduced Nb element acted on the modification of LiNiO₂ cathode material in two forms:Part of Nb stayed at the grain boundary and surface of LiNiO₂ cathode material,which could effectively protect the surface of the material,and the other part of Nb entered the interior of the surface lattice of the material.Therefore,it also played a pillar effect in the electrochemical cycle and effectively alleviates the structural changes of LiNiO₂cathode materials.For the failure analysis of the Nb-modified LiNiO₂ cathode material after cycling,it was found that the surface structure of the modified LiNiO₂ cathode material did not change significantly,and the change of the surface composition was not as drastic as that of the original LiNiO₂ cathode material.In summary,using Nb element can effectively improve the high temperature performance of spinel LiMn₂O₄ cathode material,which is helpful for its further commercialization.Meanwhile,the synthesis and modification of LiNiO₂ cathode materials may help to accelerate its commercialization.In the research process,the exploration of Nb element and the application of electrolysis preparation of precursors also inspire the research work of other lithium-ion battery cathode materials.