Structure Design Of Layered Lithiun-Rich Cathodes For Advanced Lithium Ion Batteries

Energy has a crucial role in to keep running modern social economic.The development of clean energy is being urgently needed due to the depletion of conventional resource.The energy storage system and conversion devices are important part in the energy utilization chain.As highly efficient one among them,lithium-ion batteries,have attracted much attention since their first commercialization.However,the development of advanced lithium-ion batteries,which are encountering ever-growing demand for further increases in energy density,is limited by lack of high-performance cathode materials.In this thesis,the layered lithium-rich manganese-rich cathode is selected as the objective due to their superior capacity,by reviewing and contrasting the existing cathode materials to demands of applications.We focus on addressing the problems,including poor rate capability,modest cycling performance,and voltage drop,of these layered lithium-rich cathode materials,and present rational structure design and synthetic strategy.The main obtained achievements and progress are listed as follows:Li_1.2Ni_0.2Mn_0.6O₂ was selected as a pristine material in the following study by comparing the preformances of a series of as-prepared lithium-rich cathode materials.xLi₂MnO₃·?1–x?LiNi_0.5Mn_0.5O₂?x=0.1⁰.8?,were successfully synthesized by a sol-gel method.The crystal structure,morphology and electrochemical performance of the as-synthesized materials respectively were investigated to reveal the role of Li₂MnO₃content.The results have shown that the materials with higher Li₂MnO₃ content yielded high initial discharge capacity but poor cycle stability,while the materials with lower Li₂MnO₃ content showed lower discharge capacity but better cycle stability,and spinel impurity phase was found as well.Based on the data we got,the optimal electrochemical properties were obtained when x=0.5 in x Li₂MnO₃·?1–x?LiNi_0.5Mn_0.5O₂.As a result,the layered lithium-rich cathode material with x=0.5 was selected as a pristine material in the following study.Different methods were compared here to achieve the hierarchical structure design,by which,we synthesized layered lithium-rich materials with exposed{010}planes.The hydrothermal method,carbonate-based coprecipitation method and hydroxide-based co-precipitation method were employed to synthesize layered lithium-rich cathodes with hierarchical structure.The results showed that the morphology of precursors could be easily tailored using hydrothermal method,but the morphology could not be maintained during the high-temperature treatment;the kinds of morphologies created by carbonate-based coprecipitation method is limited;hydroxide-based co-precipitation method could produce precursors with different morphology,and the morphology could be maintained during the high-temperature treatment after mixing with lithium salt.Thereby,we seleted hydroxide-based co-precipitation method for further structure design,based on which,We have developed a simple approach for synthesizing hierarchical quasi-sphere,whose surface is constructed with{010}planes,self-assembled from oriented Li_1.2Ni_0.2Mn_0.6O₂nanoplates.Inspired by this unique hierarchical structure that combines with the advantages of hierarchical architecture and electrochemical active{010}planes,both efficient ion and electron transport have been satisfied to afford a fast Li⁺transport kinetics.This material as a cathode manifested both superior rate behavior and excellent capacity retention.This material yields high maximal discharge capacities of 230.8mAh g^-1,216.5mAh g^-1 and188.2 mAh g^-1 at 1C,2C and 5C rates,respectively.This high capacity is taking the lead in China and the world as well.For the first time,we successfully combine with the advantages of hierarchical architecture and{010}planes of layered structure,affording both efficient ion and electron transport by the pure-phase material itself.We regard this as a novel solution to the well-known challenge in many fields of science and technology that synthesis of materials with high percentage of exposed high-energy facets.In addition,we present a self-templated synthetic route here that can achieve well-crystallized structures as well as preserve the micro/nano architecture at high temperature.Microstructure design was applied to address the intrinsic issues of layered lithium-rich cathode materials.We propose a minimally invasive structural treatment for Li₂MnO₃-based materials.This treatment can imitate the extraction of lithium ions,as well as the phase transitions occur in the electrochemically process,based on which,the host structure can be self-repaired by locally implantation of transition metals into the Li slabs instead of the formation of spinel-like structure.This self-repaired structure configuration has been confirmed to be responsible for the overall improvement of the electrochemical performances,including high capacity with excellent cycling ability,outstanding rate capability,and suppressive voltage decay.The refined material yields high maximal discharge capacities of 257.6 mAh g^-1,234.3 mAh g^-1,219.4 mAh g^-1 and 195.7 mAh g^-1at 1C,2C,5C and 10C rate,respectively.This high capacity is taking the lead in China and the world as well.In addition,the Li₂O acceptor employed here has shown the ability to leach Li⁺from the bulk phase,and subsequently react with it to form high Li⁺conductivity Li₃PO₄ surface amorphous films.This in-situ surface coating method of Li⁺conductive films,shows better intimate integration with the host material rather than the conventional methods.These results have confirmed the validity of the minimally invasive structural treatment to address the issues of lithium-rich cathodes;and more importantly,this study provides a general technique for designing other novel structure configurations,which can be applied to various intercalation materials that suffer from structural degradations during their applications.