Micro Mechanism Of Monovalent Cation Doping To Improve The Electrochemical Properties For Li_xMn₂O₄ Cathode

In order to reveal the microscopic mechanism of monovalent cation doping to improve the structure and performance of spinel lithium manganate（Li_xMn₂O₄,LMO）cathode materials,LMO with different Li/Mn ratios were prepared by high-temperature solid-phase reaction method,and monovalent cations（K⁺,Na⁺）doping was performed.The mechanism of doping modification was systematically investigated by combining SEM,XPS,CV,EIS,and non-in situ XRD during charging/discharging.It was found that Li_1.1Mn₂O₄ has the best overall electrochemical performance among LMO with different Li/Mn ratios,such as an initial discharge specific capacity（0.2C）of 107.69 m Ah·g^-1,capacity retention of 96.8%after 100 cycles at 1C,and 34.6%of the initial（0.2C）discharge specific capacity at 10C.That can be attributed to the appropriate amount of Li-rich could enhance Li⁺-ions diffusion,reduce electrode polarization,weaken the interaction of Mn-O,and inhibit the effect of Jahn-Teller and the disproportionation of Mn³⁺.Basis of Li_1.1Mn₂O₄,the doped of monovalent cations（K⁺,Na⁺）was performed.The results showed that the high-temperature solid-phase method could achieve uniform doping of K⁺and Na⁺,and the doped ions were introduced into the lithium sites.The appropriate amount of K⁺and Na⁺can effectively improve the rate and cycle performance of LMO.At the discharge rate at 10C,the discharge specific capacity of Li_1.1Mn₂O₄ was37.31 m Ah·g^-1,while that of the optimal K⁺or Na⁺doped sample(Li_1.09K_0.01Mn₂O₄ and Li_1.085Na_0.015Mn₂O₄)were 56.59 m Ah·g^-1 and 70.20 m Ah·g^-1,which were improved by51.7%and 88.2%,respectively.More notably,the Na⁺-doped cathode Li_1.085Na_0.015Mn₂O₄retained a discharge specific capacity of 66.97 m Ah·g^-1 after 200 cycles of a high rate（10C）,which was much higher than the initial discharge specific capacity of Li_1.1Mn₂O₄(37.10 m Ah·g^-1).And the capacity retention rate of Na⁺-doped material was increased by47.5%.The reasons for improving the performance of lithium manganate can be attributed to the following points:after monovalent cations（such as K⁺,Na⁺）doped,the cell volume and crystal plane spacing of LMO expanded,which extended the diffusion channels of Li⁺ions;Li and Mn cation mixing decreased,Li-O bonding grown,which improved the diffusion ability of Li⁺ions and reduced the polarization of the electrode;the amount of carriers(Mn³⁺)in the lattice increased,which enhanced the electrical conductivity of the material;and the[Mn₂]O₄ skeleton was more stable that enhanced the cycling performance.In addition,K⁺or Na⁺doped could alleviate the change of electrochemical internal stress due to the extraction-insertion of Li⁺ions and suppress the crystal structure change and grain fragmentation due to lattice expansion/contraction,thus making the surface organization of the doped material more intact during the charging/discharging,mitigating the erosion of the electrolyte and the dissolution of the surface Mn.This paper has 34 figures,20 tables,and 115 references.