Study On Preparation And Lithium Storage Performance Of Manganese-based Cathode Materials

Lithium-ion batteries have been considered as an extensive energy storage for the smart phones and portable notebook computers because of small volume and long cycle life.However,with the rapid development of the electric vehicles（EVs）and hybrid electric vehicles（HEVs）,people put forward the higher requirements on the performance of lithium ion batteries,especially the energy density.Due to the specific capacity of the graphite anode materials is much higher than that of the normal cathode materials,so the cathodes have direct influence on the energy density of lithium-ion batteries.At present,in the field of the commercial cathodes such as LiCoO2,Ni-Co-Mn ternary composites,LiFePO₄ and LiMn2O₄,the spinel LiMn2O₄ has been payed closeattention by the advantages of the low cost,easy preparation,no pollution and three-dimensional lithium-ion channel.Meanwhile,there are still existing some problems.For example,LiMn2O₄ occurs Jahn-Teller effect during the charging-discharging process.Therefore,it leads to the lattice distortion of the cubic structure,which hindering the transport for lithium-ions,and resulting in the poor performance.In a general way,it is executable to coat with oxide layer or non-oxide layer on the outer surface of the spinel cathodes,which could reduce the irreversible dissolution of Mn2+ and improve the cycle performance.In addition,it is also performable to adulterate the metal ions,such as Ni,Co,Cr,Ti,etc.into the manganese-based crystal lattice LiMn2-xMxO₄（0≤x≤1）,which could change the overall valence of Mn,suppress the Jahn-Teller effect,and extend the cycle life.In this work,it is significant to improve the cycle stability of the manganese-based cathode materials by doping Ti and coating the conductive polymers and metal oxides on the base of the spinel manganese-based materials.The following are the main contents:（1）The monomer of acrylamide is the source of the soft template agent,and the polyacrylamide（PAM）soft template is formed by the in-situ polymerization.The LiMn_2-xTi_xO₄（x=0,0.25,0.5,0.75,1）precursors doping with different amounts of Ti is prepared by the PAM soft template method.The best experimental condition of the electrochemical property is filtrated via optimizing the doped amount of Ti.As a result,the initial discharge capacity of the sample LiMn_2-xTi_xO₄（x=0.75）is 286.1 mA h g^-1.After 100 cycles,the reversible capacity can still be maintained at 122.9 mA h g^-1,exhibiting a good cycle stability.（2）LiNi_0.5Mn_1.5O₄ is used as the raw material.The polyaniline（hereinafter referred to as PANI）is coated onto the surface of the LNMO particles by the simple chemical oxidation in-situ polymerization method,and the high-voltage cathode LNMO-PANI composite is successfully prepared for lithium ion batteries.The mass,thickness,and uniformity of the PANI layer are comprehensively investigated,as well as the effect on the electrochemical properties.As a result,when the mass of the aniline involved in the polymerization reaction is 5%,and the mass of PANI coated on the surface of the LNMO particles is about 1%.The thickness is about 20 nm.The initial discharge specific capacity of the LNMO-1%PANI composite is 123.8 mA h g^-1 at room temperature.After 200 cycles,the reversible capacity is still 123.4 mA h g^-1.And the capacity retention rate is as high as 99.7%.Meanwhile,the LNMO-1%PANI composite also showes a good rate capability and excellent high temperature performances.（3）LiNi_0.5Mn_1.5O₄ is also used as the raw material.A mixture of 80wt% active materials,10wt% conductive agent and 10wt% polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone solvent was ground into slurry,and the LNMO sheet is made from the slurry.Then it is the appropriate time to coat nano-Al₂O₃ on the surface of the LNMO sheet.The influence of crystal structure and electrochemical property for the coating layer of the nano-Al₂O₃ on the LNMO cathode is systematically investigated,especially improving the interfacial side reaction during the process of electrochemical cycle.As a result,the initial discharge capacity of the LNMO-Al₂O₃ composite electrode is 123.7 mA h g^-1 at room temperature.After 200 cycles,the reversible capacity still remains 129.1 mA h g^-1.Simultaneously,the LNMO-Al₂O₃ composite electrode also reveals an excellent high temperature performances and lower polarization resistance and interface resistance.