Modification Of Lithium-rich Cathode Materials For Lithium-ion Batteries

Lithium-rich manganese-based cathode materials have been widely studied because of their high specific capacity,wide voltage range and low cost.However,it has low initial coulomb efficiency,rapid voltage decay and poor cycling performance.In order to solve these problems,the electrochemical performances of lithium-rich manganese-based cathode materials were improved based on structure regulation,defect design and element doping in this thesis.The work is mainly reflected in the following aspects:（1）The Li_1.2Mn_0.54Ni_0.13Co_0.13O_2-xF_x（F-O2-LRO）is designed and prepared on the basis of P2 type sodium ion cathode material Na_5/6Li_1/4(Mn_0.54Ni_0.13Co_0.13)_3/4O₂（Na-LRO）through ion exchange.Due to the synergistic effect of fluoride doping and O2 type structure,the surface and phase structure stability of the material are increased,and the voltage attenuation is inhibited.The results show that the prepared F-O2-LRO has a good electrochemical performance.The specific discharge capacity at 0.1 C is 280 m Ah g^-1 and the initial coulomb efficiency is 94.4%,which is significantly higher than that of the original LRO（77.2%）.After 100 cycles at 1 C,F-O2-LRO still has a high capacity retention of 95%,while the capacity retention of LRO is only 69.1%at the same magnification.In addition,the voltage difference（ΔV）between F-O2-LRO before and after 100 cycles at 1 C is only 0.268 V,which is less than that of LRO（0.681 V）,indicating that the polarization of F-O2-LRO is lower.What’s more,F-O2-LRO has a specific discharge capacity of 210 m Ah g^-1 and a excellent capacity retention of 90.1%after 100 cycles at a high current density of 5 C.Therefore,the work in this paper provides a new idea for the development and industrial application of lithium-rich cathode materials in high-energy lithium ion batteries.（2）Excess lithium is not conducive to kinetics and hinders the transport of lithium ion.An appropriate amount of lithium vacancies can improve the kinetic characteristics and induce in-situ spinel coating and in-situ nickel doping.Sodium doping can increase the interlayer spacing of lithium,accelerate the diffusion of lithium ions and stabilize the layered structure.The Na doped lithium-rich cathode material with lithium vacancies Li_1.17Na_0.02Mn_0.54Ni_0.13Co_0.13O₂（PN-LMNCO）is prepared via co-precipitation and high temperature solid phase methode.The results show that the prepared PN-LMNCO has good electrochemical performance.PN-LMNCO has a specific discharge capacity of 280 m Ah g^-1 at 0.1 C,with an initial coulomb efficiency of 91.2%,which is significantly higher than that of the original LMNCO（76.1%）.After 100 cycles at 1 C,PN-LMNCO still has a high capacity retention of 94.7%,while the capacity retention of LMNCO is only 69.1%.In addition,the voltage difference（ΔV）of PN-LMNCO after 200 cycles at 1 C is only0.3 V,which is less than that of LMNCO（0.682 V）,indicating that the polarization of PN-LMNCO is lower.What’s more,at the high current density of 5 C,the discharge capacity of PN-LMNCO is 200 m Ah g^-1,and the capacity retention is 88.7%after 100cycles.The calculated lithium ion diffusion rate(D⁺_Li)of PN-LMn CO is much higher than that of LMNCO.The average D⁺_Li of PN-LMNCO and LMNCO are 7.26×10^-11cm² s^-1 and 2.26×10^-11 cm² s^-1,respectively.Therefore,Na⁺doping and lithium deletion can improve the reaction kinetics of lithium-rich Mn-based cathode materials.Therefore,this work offers a promising approach to high-energy lithium-ion batteries.（3）Single crystal lithium-rich cathode materials are prepared by different molten salts.Through testing,the lithium-rich single crystal cathode material with sodium chloride as molten salt（LMRO-NC）has excellent cycling stability.In addition,such regular single crystal particle is conducive to the transport of lithium ions,which makes the material has a better dynamic performance and rate capacity.The reversible specific capacity at 0.1 C is 270 m Ah g^-1,and the capacity retention is95.6%after 100 cycles at 1 C.What’s more,LMRO-NC shows excellent lithium-ion diffusion kinetic.Its average D⁺_Li is 9.62×10^-11 cm² s^-1,resulting in a high reversible capacity of 201 m Ah g^-1 at 5 C.Therefore,the preparation of lithium-rich single crystal cathode materials by using sodium chloride as molten salt will provide a new idea for the research of long cycle life lithium ions batteries.