Study On Surface Coating And Application Of Spinel Mn-Based Cathode Materials For Lithium-Ion Batteries

In order to reduce the pollution of automobile exhaust and relieve environmental pressure,the development of new energy electric vehicles has drawn wide attention.Due to the advantages of good safety,high operating voltage,environment friendly,mature production technology,and better flexibility for design,lithium ion batteries（LIBs）have been the preferred battery for energy supply of new energy vehicles.The increasing use and promotion of LIBs lead to higher requirements,especially energy density,safety and cost.To increase the capacity and voltage of cathode materials is an effective method to increase the energy density of LIBs.Spinel Mn-based cathode materials(LiMn₂O₄ and LiNi_0.5Mn_1.5O₄)of LIBs have the advantages of high voltage,good safety,low cost,rich resources and superior columbic efficiency.Even LiMn₂O₄ has a market price only 1/4 or 1/5 of the ternary cathode materials,which is the cheapest commercial cathode material and one of the most potential cathode materials for LIBs.However,several critical problems involve its poor stability with electrolytes and its capacity degradation upon extended electrochemical cycling at elevated temperatures which limits its further practical applications.Partial substitution and surface coatings of metal oxides have been demonstrated as efficient approaches to solve the issues.Surface coating materials,particularly metal oxides,show significant effect on cycling performances.But these oxides with low ion/electron transferring capability have a negative effect on specific capacity and rate performance for the bulk materials.This work is intended to improve the comprehensive electrochemical performances of Mn-based cathode materials for LIBs by designing novel coating structure of Al₂O₃ oxide,coating fast ion conductor with superior ion conductivity,and coating spinel material with the same crystal stricter to bulk with superior electron/ion conductivity.These strategies not only keep cycling stability but reduce capacity fading and interface impedance,which leads to increased rate performance by designing novel coating structure of Al₂O₃,fast ion conductor La_0.7Sr_0.3MnO₃ and active anode material NiFe₂O₄.By a simple sol-gel route,a novel kind of 3D hierarchical Al₂O₃ nanosheets were designed and fast ion conductor La_0.7Sr_0.3MnO₃ layer was prepared as coating materials on the surface of LiMn₂O₄ particles,inverse spinel NiFe₂O₄ layer were prepared on LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ respectively.In addition,based on the conclusion obtained from the results in half cell using Li metal as conuter electrode,the modified LiMn₂O₄cathode materials by Ni Fe₂O₄ and LiMn₂O₄,and LiNi_0.5Mn_1.5O₄ was tested in full battery using graphite as anode material to explore the effect in practical application.The main research contents are as follow:（1）Three-dimension hierarchical Al₂O₃ nanosheets wrapped LiMn₂O₄ cathode materials.A three dimensional Al₂O₃ nanosheets layer was uniformly coated on LiMn₂O₄ particles,which was synthesized by a facile sol-gel approach with pseudo-boehmite as raw material and ammonia as precipitant.The irregular particle shaped AlOOH precursor change into nanosheets through stripping and curling by heat-treatment.Compared to the tranditional layered Al₂O₃ shell,the prepared flower-like Al₂O₃ nanosheets with high specific area largely sequesters acidic species produced by side reaction between electrode and electrolyte.The inner coating layer wrapping spinel LiMn₂O₄ effectively inhibits the dissolution of Mn by suppressing directive contact with electrolyte to enhance cycling stability,resulting in a superior cycling performaces.The rate performance is improved because of the better electrolyte storage of the assembled hierarchical architecture of the 3D coating layer affording unimpeded Li⁺diffusion from electrode to electrolyte.The electrochemical results reveal the as-prepared coated Li Mn₂O₄ sample with a coating amount of 1wt%exhibits superior cycle stability under room temperature even at elevated temperature.At room temperature,the initial specific discharge capacity is 128.5mAh·g^-1 and the coulombic efficiency is 96.2%at 0.1 C,the specific discharge capacity is 90.1mAh·g^-1 at 10 C,and retains 89.8%of the initial capacity after 800 cycles at 1 C rate.When cycling at 55°C,the composite shows 93.6%capacity retention after 500 cycles.（2）Fast ion conductor La_0.7Sr_0.3MnO₃-coated LiMn₂O₄ cathode material.La_0.7Sr_0.3MnO₃ has the advantages of higher ion transferring capability and decrease impedance contributed by coating layer.This work synthesized La_0.7Sr_0.3MnO₃ shell coating on surface of LiMn₂O₄particles by sol-gel.Based on good results obtained in half cell,the modified cathode material was also tested in 502030 typed soft-packed full batteries.The results show that La_0.7Sr_0.3MnO₃with a thin film of about 10 nm does not change the structure and morphology of LiMn₂O₄ bulk.The initial discharge capacity achieves 106 mAh·g^-1 at 0.1 C and 25 ^oC.At high rate,the discharge capacity of 89.6 mAh·g^-1 at 10 C;the capacity left 88.2 mAh·g^-1 after 800 cycles at 1C and the capacity retention is 83.6%.At elevated temperature,the discharge capacity remains92.9 mAh·g^-1 after 500 cycles and the retention is 91.8%.The La_0.7Sr_0.3MnO₃ coating suppresses the dissolution of Mn and reduces the impedances,enhancing the kinetics of lithium ion diffusion through the surface layer and the charge transfer reaction.As a result,the coating strategy improves the electrochemical performances,especially for rate and high temperature performances,of LiMn₂O₄ cathode material in the designed 502030 full battery.（3）Inverse spinel NiFe₂O₄-coated LiMn₂O₄ cathode material.The uniform inverse spinel NiFe₂O₄ thin film with a thickness of 10-11 nm was coated on the surface of spinel LiMn₂O₄ by a simple sol-gel method.The effect of coating contents on the electrochemical performances is studied and the best content was optimized.The electrochemical performances are tested in half cell with Li metal as counter electrode and full battery with graphite as anode material.The results reveal that the uniform inverse spinel NiFe₂O₄ widely used as active material in electrochemical catalysis,supercapacitors and anode materials for LIBs decrease the resistance of coating material and interface resistance between shell and core due to the similar crytal structure beside protective effect.The as-prepared NiFe₂O₄-coated LiMn₂O₄ composite with a coating amount of 1%shows capacity retention of 90.7%after 1000 cycles at 1 C and 25°C.At an elevated temperature of 55 ^oC,the initial discharge capacity at 0.1 C is 130.8 mAh·g^-1 and90.1 mAh·g^-1 at 10 C and the capacity retention is 84.5%after 400 cycles at 1C.While the full battery of modified LiMn₂O₄‖graphite shows capacity retention of 89.1%after 500 cycles.The results suggest the NiFe₂O₄ modified LiMn₂O₄ exhibits better cycling and rate performances due to superior electrochemical activity,thermal and structure stability.（4）Inverse spinel NiFe₂O₄-coated LiMn₂O₄ cathode material.The spinel LiNi_0.5Mn_1.5O₄exhibits great promise as a potential high-energy positive electrode for lithium-ion batteries due to higher working potential of 4.7 V（vs.Li）and energy density,abundant resources,good safty and environmental friendly.In view of the significant effect of NiFe₂O₄ on improvement of performances for LiMn₂O₄,the inverse spinel material was also introduced as surface coating material to satify the interface properties between LiNi_0.5Mn_1.5O₄ and electroyte via the same sol-gel route.Similarly,the different bulk cathode material did not change the layer structure.The coating layer is uniform and compacts on the surface of Li Ni_0.5Mn_1.5O₄ with a thickness of15 nm with the amount of 1%.The electrochemical test results show,the initial discharge capacity is 133 mAh·g^-1 and Coulombic efficiency is 97%at 0.1 C and 25 ^oC.The specific discharge capacity remains 116.6 mAh·g^-1 with a retention 91.8%at 1 C after 900 cycles.At the high rate,the specific discharge capacity achieves 124 mAh·g^-1 at 10 C.So the inverse spinel NiFe₂O₄ coating can also shows positive effect on the performances of LiNi_0.5Mn_1.5O₄.