Study On Synthesis And Modification Of Li₂MoO₃ Cathode Materials For Lithium Ion Batteries

Layered oxides Li₂MoO₃ meets the reguirements to an ideal replacement of Li₂MnO₃ in constructing Li₂MoO₃-based Li-rich cathode materials with its unique advantages.The Mo⁴⁺/Mo⁶⁺redox couple in Li₂MoO₃ can exchange two electrons and supply a higher theoretical capacity up to 339 mAh/g.The synthesis and modification of layered Li₂MoO₃ cathode material were systematically studied in this paper.Research progress and results achieved are as follows:Li2MoO3 cathode materials were synthesized by solid state reaction with excess5%Li displayed an initial discharge capacity of 180.6 mAh/g at a current density of5mA/g during 1.5⁴.5V,which had stable median discharge voltage（².2V）.We conducted a series of studies on modification of Li₂MoO₃ to improve its low capacity retention rate（56.8%）coupled with poor rate perpormance caused by its poor electronic conductivity and obvious polarization.First of all,XRD test showed that the formation of solid solution（1-x）Li₂MoO₃·xLiCoO₂ caused a regular change of lattice parameters with increasing of the value of x,indicated that Co entered the material lattice position.It is beyond the solid solubility of material when x is 0.2 with the presence of some impurity.SEM displayed the introduction of Co can effectively reduce the average grain size of materials.When the doping amount of Co is 0.1 calcined at 750°C for 48h under reducing atmosphere,the sample was obtained with partical size of approximately around 300 nm and a initial discharge capacity of 219.7 mAh/g at 5 mA/g,which delivered the optimal electrochemical performance.Its capacity retention after 20cycle was 12%higher than the undoped Li2MoO3,indicating that Co-doping can stabilize the capacity and increase the median discharge voltage to².3V.Subsequently,carbon-coated Li₂MoO₃ were synthesized by two different carbon sources（glucose,sucrose）,XRD manifested that the crystal structure of Li₂MoO₃ was not affected by carbon coating.Performance tests showed that the initial coulombic efficiency of Li₂MoO₃/C were all over 100%.When the glucose and sucrose coating amount were 15wt%and 20wt%,respectively,and they both calcined at 700°C for2h under reducing atmosphere were optimized.The initial discharge capacity of 15G and 20S was 299.6 mAh/g and 263.5 mAh/g at 5 mA/g,respectively,indicating carbon coating significantly upgrade capacity of Li2MoO3.When current density increased to 20 mA/g,the initial discharge capacity of 15G and 20S was 220.9 mAh/g and 212.4 mAh/g,and the capacity retention was 75.5%and 74.2%,respectively,increased 26%and 24.9%compared to Li₂MoO₃;The polarization represented by the difference between the average charge and discharge potentials decreased with carbon coating.These results showed that Li2MoO3 coated with glucose and sucrose significantly increased the conductivity,cycling and rate performance.The combination of above two modified means effectively decreased average size of particles and the charge transfer resistance,improved rate performance,confirming that cobalt-doping and carbon-coating in combination significantly enhanced the conductivity and reversibility of Li₂MoO₃.