Synthesis And Surface Modification For 5 V Spinel LiNi_0.5Mn_1.5O₄ Cathode Materials

Spinel LiNi_0.5Mn_1.5O₄?LNMO?is considered as one of the most promising high voltage cathode materials,because of its high operating voltage plateau?4.7 V?and theoretical specific capacity(147 mAh g^-1).However,many challenges and problems should be solved during the development of LNMO.First of all,the electrochemical performance of cathode is highly related to structure,particle size,size distribution and morphology.Therefore,how to design the available methods to synthesize pure phase and special morphology in order to improve transformation of lithium ion and electrons in LNMO cathode materials as well as be used in the full cell testing is one of the most important part in this thesis.Secondly,there are many interface reactions during charge/discharge process at the high voltage,such as the irreversible phase change at surface,dissolution of transition metals,Jahn-Teller effect,and oxidation of electrolyte.The application of surface coating is considered as an effective method for providing a stable interface between the active material and electrolyte.Numbers of coating materials have been employed for use on LNMO,including metal oxides,metal phosphates,and solid-state electrolyte.However,the traditional coating approaches have limited control over surface coverage,thickness and uniformity.Therefore,how to design advanced surface modification methods,which have ability to deposit highly conformal and uniform layers with well controlled thickness,as well as study the effect of coating layer to the structure and electrochemical performance of LNMO is another important part in the thesis.Finally,MnO₂,which is an important precursor of LNMO,is also be studied in this thesis,in order to research the performance in the aqueous electrolytes.The main contents and conclusions are shown as follows:1.The uniform double-shell LNMO hollow microspheres cathode materials are prepared via a facile molten salt and annealing method.Both the outer and inner shells are composed of nanoparticles,which allows easy penetration of the electrolyte into the whole microspheres and can buffer the large volume variations of electrode materials during the repeated Li ion insertion/extraction.The double-shell LNMO hollow microspheres cathode materials exhibit excellent performance.Otherwise,Li₄Ti₅O₁₂?LTO?spheres anode are also synthesized by a molten salt and template method and the formation mechanism of LTO sphere is also studied in this part.The as-prepared LTO has good cyclic stability.Meanwhile,the cycling stability of the full cell,which adopt double-shell LNMO hollow microspheres as a positive electrode and LTO spheres as a negative electrode,are also discussed in this part.2.Ultrathin and uniform AlPO₄ film is employed as a coating material for high voltage LNMO cathode,which is deposited by the atomic layer deposition?ALD?.The study demonstrated that 10 ALD-cycles of AlPO₄ coating on LNMO has the best long cycling performance.The AlPO₄ coating layer was shown to effectively suppress dissolution of Mn and maintains the integral structure of LNMO.The ultrathin ALD AlPO₄coating layer acts not only as a barrier layer to separate LNMO and electrolyte,but also transforms into a stable SEI layer during cycling,effectively preventing electrolyte decomposition and protecting LNMO structure during electrochemical reaction.Finally,thermal stability of LNMO cathode materials was investigated via DSC analysis,further demonstrating enhanced safety properties of LNMO with AlPO₄ coating.3.The amorphous Li₃PO₄ solid-state electrolyte film coating on the surface of LNMO cathode is prepared by the advanced ALD technique.10 ALD-cycles of Li₃PO₄ coating sample shows the highest initial discharge capacity of 122.6 mAh g^-1,stable cycling performance with the capacity retention of 78.4%,and excellent rate performance(5 C,59mAh g^-1).ALD Li₃PO₄ did not change the valence of Mn before charge/discharge cycling.After cycling,the valence of Mn still can keep the same status,which means that the Li₃PO₄coating layer as a stable interface protects LNMO electrode materials against TMs(Mn³⁺and Mn²⁺)dissolution as well as reliefs the side reaction during the interface between LNMO electrode and the electrolyte.Meanwhile,the Li₃PO₄ coating layer remains the same chemical states before and after cycling,indicating stability of ALD Li₃PO₄.More important,the uniform coating layer protects the surface structure of LNMO,avoiding the formation of inactive rock-salt phase on the surface,which improving initial discharge capacity,cycling and rate performances.4.The hybrid anatase TiO₂/amorphous Li₃PO₄ film?TLPO?is prepared by the advanced ALD technique,and then coating on the surface of LNMO cathode.the TLPO coated sample with LNMO powder substrate shows the best performance.The valence of Mn before and after charge/discharge cycling did not show obvious change,which means that the hybrid TLPO coating layer has ability to protect LNMO electrode materials against the side reaction with the electrolyte.TEM tests show the similar results as the synchrotron-based data.Hybrid TLPO coating layer maintains the same status before and after cycling,which indicates the high stability during charge/discharge process.More important,the hybrid TLPO coating layer can effectively suppress the formation of isolated SEI,proving that the ability of avoiding the side reaction.The hybrid TLPO coating layer can obvious improve the transformation of lithium ion and electrons during the charge/discharge process,therefore highly improve the discharge capacity,cycling and rate performances.5.A facile hydrothermal method to synthesize MnO₂ nanorods/graphene composite materials for supercapacitor is shown in this part.The main purpose of this study is to investigate the influence of ratio of MnO₂ nanorods/graphene composites for the electrochemical performance.It shows that the MnO₂-5/G nanocomposite has the best electrochemical performance.