| Recently, spinel LiMn2O4and α-MnO2have been extensively studied as the electrode materials for the lithium ion batteries and electrochemical capacitors, respectively, due to their low costs, environmental harmlessness and better safeties. However, their low electronic conductivities are one of the factors which lead to the poor electrochemical performance. In view of this, in this paper, we aim at using some simple and effective synthetic methods to prepare the LiMn2O4and α-MnO2materials with good crystalline structures, homogeneous morphologies and excellent electrochemical performance, studying the relationship between the electrochemical performance and the electronic conductivities of the two materials, and using some methods to increase their electronic conductivities and electrochemical performance, to provide the theoretical basis for the extensive applications of the two materials in the lithium ion battery and electrochemical capacitor regions. The main contents of this paper include:(1) For the first time, using PVP as the reducing agent to prepare the LiMn2O4nanoparticles with good crystalline structures and homogeneous morphologies through the hydrothermal reaction between LiOH and MnO2. It was proved that the hydrothermal reaction could not happen without PVP but the reaction time could be reduced to24h while the PVP reducing agent was used, much less than the typically reported3-7days. The most appropriate amount of the PVP reducing agent was determined by comparing the electrochemical performance of various LiMn2O4nanoparticles which were prepared with different amounts of PVP. The LiMn2O4nanoparticles (10-LMO) prepared with the appropriate amount of PVP (10mL PVP solution) displayed the best electrochemical performance. The initial specific discharge capacities of10-LMO at high C-rates (5C and10C) were about112and102mAh g-1, respectively. After100charge/discharge cycles, the capacity retentions at5C and10C were about93%and94%, respectively. The results of electrochemical impedance spectroscopic (EIS) measurements demonstrated that the10-LMO electrode had the lowest solid state electrolyte interface (SEI) film resistance (Rs), charge-transfer resistance (Rct) and diffusion resistance of Li+(Warburg impedance, Zw). Meanwhile,10-LMO sample had the highest diffusion coefficient of Li+, which was calculated by means of the EIS method. So the electrochemical performance of10-LMO was better than others. For this reason, the PVP-assisted hydrothermal method was effective to prepare the LiMn2O4material with excellent performance. (2) Using the sucrose-assisted combustion method to mix the excellent conductive Ag in the LiMn2O4material, to prepare the LiMn2O4/Ag nanocomposite material and increase the electronic conductivity of LiMn2O4. It was proved that the doped Ag metal was uniformly dispersed on the surfaces of the LiMn2O4nanoparticles instead of entering the crystal lattice of LiMn2O4due to the overlarge radius of Ag+ions. The appropriate amount of the doped Ag was determined by comparing the electrochemical performance of various LiMMn2O4/Ag composites in which different amounts of Ag were contained. Besides, the electrochemical performance of the LiMn2O4/Ag nanocomposite material was firstly measured at55℃. When appropriate amount of Ag (5wt%) was doped, LiMn2O4/Ag nanocomposite showed much better electrochemical performance at both25℃and55℃. The specific discharge capacity of the LiMn2O4/Ag nanocomposite was always maintained above120mAh g-1while the capacity retention was98.8%during the50charge/discharge cycles at0.5C at25℃, much better than those of the LiMn2O4material prepared through the same synthetic method. LiMn2O4/Ag nanocomposite also had better performance at different C-rates (2C and5C) at55℃. The EIS results demonstrated that for one thing, the doped Ag could effectively decrease the SEI film resistance (Rs), the charge-transfer resistance (Rct) and the diffusion resistance of Li+(Zw); for another thing, Ag could also suitably alleviate the dissolution of Mn3+into the electrolyte at high temperature. But, because Ag was just dispersed rather than being densely coated on the surfaces of the LiMn2O4particles, which was different from the dense coating for LiMn2O4by oxides, so the dissolution of Mn3+could not be completely inhibited by the doped Ag, leading to the inferior performance of the LiMn2O4/Ag nanocomposites at high temperature, compared with those at room temperature. Besides, it was found that both LiMn2O4and LiMn2O4/Ag composite showed better cycling performance at higher C-rate than those at lower C-rate at55℃. The reason was firstly studied in this paper and showed that the essence of the dissolution of Mn3+into the electrolyte at high temperature was the easy reaction between the Mn3+and the HF in the electrolyte. At higher C-rate, due to the faster charge/discharge rates, the transformation between the Mn3+and the Mn4+during the reaction was also faster, so the Mn3+had been transformed into the Mn4+before its reaction with the HF. As a result, the dissolution of Mn3+was alleviated and the cycling performance of the two materials at higher C-rate were improved.(3) For the first time, using the excellent conductive graphene sheets (GNs) as the conductive additives in the LiMn2O4and α-MnO2electrodes to increase their electronic conductivities while the conventional acetylene black (AB) conductive agent was simultaneously used. It was proved that the electrochemical performance of the two materials could be effectively improved when the appropriate amount of GNs were used in the electrodes. The specific discharge capacity and cycling performance of LiMn2O4at various C-rates (0.5-20C) were significantly enhanced when GNs (5wt%) were used in the LiMn2O4electrode (LMO-G (5wt%)) and they were much better than those of the LiMn2O4electrode without GNs. Similarly, the specific discharge capacitance and cycling performance of α-MnO2at various current densities (100-1000mA g-1) were also better when GNs (10wt%) were used in the α-MnO2electrode (α-MnO2-G (10wt%)). This was caused by the unique "plane-to-point" conducting mode of the GNs and the "filling" mode of the AB conductive agent during the conducting process of the electrode. For one thing, the conductive surfaces of the GNs could bridge many LiMn2O4or α-MnO2particles through the "plane-to-point" conducting mode, so the conducting efficiency of the electrode was significantly improved; for another thing, some AB particles might serve as the fillings in the gaps between the GNs and the isolated LiMn2O4or α-MnO2particles, bridging the isolated materials which were not connected by the graphene conductive agent, so the conducting efficiency of the electrode was further improved. The EIS results demonstrated that the synergy effect between the "plane-to-point" conducting mode and the "filling" mode had effectively decreased the charge-transfer resistance (Rct). As a result, the electrochemical performance of the LiMn2O4and α-MnO2materials was enhanced. It needed to be noticed that excess graphene additives would lead to the decrease of the electrochemical performance of the materials. This was because when excess GNs exsited in the electrode, they tended to agglomerate together due to the van der Waals force, so the "plane-to-point" conducting mode was weakened. Especially when the AB conductive agent did not exist in the electrode, the "filling" mode and the synergy effect no longer existed, so the conductivity of the electrode was decreased, leading to the macro reduction of the electrochemical performance. |
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