Preparation Of LiMn₂O₄ Cathode Materials And Its Properties Of Electrode/Electrolyte Interfaces For Lithium Ion Batteries

In this paper the structure, physicalchemical properties, existing problems and their solutions of spinel LiMn2O₄ have been introduced with emphasis. On the basis of these, the electrochemical performances for the LiMn2O₄ with different preparation methods, doped LiMxMn2-xO₄ (M=Co, Ni, Al, Cr, x=0.05, 0.1, 0.2, 0.3, 0.4, 0.5), Al2O3 coated LiMn2O₄ and graphene doped LiMn2O₄ were researched, especially focusing on their dynamic characteristics of electrode process. Meanwhile the electrochemical performances of spinel LiMn2O₄ cathode and LiTi2(PO₄)3 anode in aqueous electrolyte were also explored. The main results are summarized below.(1) Electrochemical performances of spinel LiMn2O₄ depend strongly upon the synthesis methods. LiMn2O₄ sample prepared by hydrothermal route has higher specific capacity and better cycling performance than the one synthesized from sol-gel method. The former delivered a discharge capacity of 114.36 mAhg-1 and retained 99.78 mAhg-1 at the 100th cycle, its retention of 86.28%. However the later had only discharge capacity of 98.67 mAhg-1, after over 100 cycles the capacity fading to 60.25 mAhg-1 and 61.06% of their max discharge capacity.(2) Electrochemical impedance spectroscopy of spinel LiMn2O₄ was researched at different potentials and different temperatures in detail. To amazing a new semicircle at high to middle frequency emerge in the Nyquist diagram, to date which never been reported in the literatures as for our knowledge. It was found that the high frequency semicircle and the middle to high frequency semicircle begin to overlap or compress each other at with the variation of environmental temperature, which means the effects of the electronic and ionic transport properties of lithium intercalation materials clearly appear as separate features in the spectra. Therefore the high to middle frequency semicircle wad assigned to electronic conductivity of material. In 1 mol/L LiPF6-EC: DEC electrolyte solutions, the energy barriers for the ion jump relating to migration of lithium ions through the SEI film of the spinel LiMn2O₄ electrode were determined to be 15.49 kJ/mol, the thermal active energy of the electronic conductivities to be 24.21 kJ/mol, and the intercalation-deintercalation reaction active energies to be 53.07 kJ/mol, respectively.(3) Micro-size doped LiMxMn2-xO₄(M=Co, Cr, Cu, Ni, Al,x=0.05, 0.1, 0.2, 0.3, 0.4, 0.5 ) samples were prepared by the sol-gel and their physical properties, electrochemical performance and impedance spectroscopy were investigated in detail. The cell parameters of LiMxMn2-xO₄ (M=Co, Cr, Cu, Ni, Al ) decrease with the increase in the contents of the metal ions, which indicated the structure of substituted electrode more stable. All doping metal ions could improve the cycle performance of spinel LiMn2O₄, at same time make its capacity to decrease. The capacity of LiMxMn2-xO₄ slowly decreased with the respect to the content of doping metal ions (0.2, 0.3, 0.4, 0.5). Of all them, the LiNi0.2Mn1.8O₄ showed 117.0 mAhg^-1 discharge capacity, reaching to 105mAhg^-1 after 100 cycles, its retention of 93.00%; whereas for LiCu0.05Mn1.95O₄ material, its discharge capacity got to only 93 mAhg^-1, its retention of 82.00% after 25 cycles, which was same for both routes. At high temperature (60℃), the electrochemical performance of electrode materials decreased significantly. For example LiCo_0.1Mn_1.9O₄ showed 111 mAhg^-1 initial discharge capacity, its retention of 88.30% after 50 cycles at room temperature, however, only 108 mAh·g^-1 initial discharge capacity, the retention of 77.20% after 20 cycles at 60℃high temperature. 1.5%Al₂O₃ coated LiMn₂O₄ and LiAl_0.2Mn_1.8O₄ among all LiAl_xMn_2-xO₄ samples had the same initial discharge capacity of 99.7 mAhg^-1, capacity retain of 87.4% and 88.4% after 50 cycles respectively. The analysis from the EIS equivalent circuit showed SEI film formed mainly because of the spontaneous reaction between the electrolyte components and electrode active material in the contact process of the electrode and the electrolyte, which was little affected by the charge and discharge, the electronic resistance Re of active material decreased with the increase of of electrode potential overall and the charge transfer resistance Rct firstly decreased and then increased with the increase or decrease of electrode potential.(4) It was surprised found that the LiNi_0.5Mn_1.5O₄ had high 4.7V electrochemical properties. A higher voltage plateau around 4.7V and a lower voltage plateau around 4.V at 3.3-5.0V emerged in the charge-discharge curves of LiNi_0.5Mn_1.5O₄, corresponding to two redox peaks of Ni²⁺/Ni⁴⁺ and Mn³⁺/Mn⁴⁺ of its CV respectively. The LiNi_0.5Mn_1.5O₄ with ordered P4332 space group has 150 mAhg^-1 charge capacity and 149 mAhg^-1 discharge capacity, above 110mAhg^-1 after 42 cycles at 0.4C, almost 100% charge-discharge efficiency. A short arc at more high frequency zone from 4.4V potential emerges in electrochemical impedance spectroscopy (EIS) of LiNi_0.5Mn_1.5O₄, attributing to oxidative decomposition of the electrolyte at high voltage. Before 4.35V the impedance of SEI almost no change and gradually increase with the increase of electrode potential, which maybe related to breakage on the surface of the electrode caused by the oxidative decomposition of the electrolyte.(5)Electrochemical performances of spinel LiMn₂O₄ doped by graphene were also researched. Graphene nanosheets were synthesized via reduction of exfoliated graphite oxides, which were few-layer graphenes with less than 10 layers of graphene layers. The graphene could significantly improve the special capacity and cycle performance of spinel LiMn₂O₄ material. The max discharge capacities of spinel LiMn₂O₄ doped by graphene before and after were 96.44 mAhg^-1 and 111.87 mAhg^-1 respectively and after 100cycles the capacity faded to 71.67 mAhg^-1 and 108.13 mAhg^-1, capacity retain of 74.31% and 96.66%,respectively.At room temperature the high to middle frequency semicircle related to the electronic conductivity separated from the high frequency semicircle in the EIS of spinel LiMn₂O₄ doped by graphene, and huge semicircle in middle to low frequency had still not isolated the inclined line of Warburg impedance assigned to the solid state diffusion of Lithium ion in the matrix, all which were connected with the high conductivity of graphene. In the polarization potential 3.95V before, a reactance appeared in the EIS, which were caused by the difference of local conductivity because of semiconductor LiMn₂O₄ unevenly distributing on the surface of graphene nanosheets. A new model generating reactance was proposed.(6)Electrochemical performances of spinel LiMn2O₄ cathode and LiTi₂(PO₄)₃ anode in aqueous electrolyte. It was found the open voltage of in aqueous electrolyte was unstable, rising with store time, which was due to the chemical reaction of LiTi₂(PO₄)₃ and the residual oxygen left in the aqueous electrolyte. A pair redox peaks(-0.674V/-0.864V) in the CV curve of LiTi₂(PO₄)₃ in 1M Li₂SO₄ aqueous electrolyte corresponded to the phase transition between the LiTi₂(PO₄)₃ and Li₃Ti₂(PO₄)₃. The EIS of LiTi₂(PO₄)₃ anode in aqueous electrolyte was studied firstly, mainly consisting of a small semicircle in high frequency region and a large semicircle gradually moving from high to middle frequency to low frequency, the former related to lithium ion migration through SEI film of LixTi₂(PO₄)₃, the later attributed to charge-transfer through the electrode/electrolyte interface. In addition, it was also found that before the charge-discharge cycle, the surface of LiTi₂(PO₄)₃ electrode had already covered by the original SEI film, which was consistent with the formation of SEI film of negative electrode for lithium ion battery in the organic electrolyte. Even in the open circuit potential the intercalation/deintercalation of lithium ion occurred because of the excellent conductivity of the aqueous electrolyte. The intercalation/deintercalation of lithium ion greatly affected the SEI film stability of LiTi₂(PO₄)₃ in the aqueous electrolyte. In aqueous electrolyte the intercalation/deintercalation of lithium ion in spinel LiMn₂O₄ was still two steps, but there was no SEI film on its surface. Whether in store or in the charge-discharge process in aqueous electrolyte, the electrode was subject to continous erosion of electrolyte, resulting in the increasing of charge transfer resistance with storage time and so the deintercalation of lithium ion was difficult. In the charge process the charge transfer resistance first increased and then decreased with the polarization potential.