Design, Construction And Electrochemical Properties Of Manganese Oxide Electrode Materials

Since the beginning of this century, with the development of energyrequirements, sustainable development and green energy gradually becomechallenging but inevitable topics around the world. However, as one importantapproach to solve the energy problem, current performance of Li-ion batteries andNa-ion batteries is far from enough when it comes to the large-scale energy supply.Therefore, searching for the ideal electrode materials with high energy density andpower density is very meaningful. Manganese oxides appear to be very goodelectrode materials in Li-ion battery due to their natural abundance and hightheoretical capacity. Meanwhile, exploring manganese oxides as sodium-ion batteryelectrode material is of great scientific significance. However, there still exist manyproblems when it comes to the performance of manganese oxides as electrodes andone important reason leading to these problems is manganese oxides’ low electronicand ionic conductivities. Another part of this thesis is further optimizing the electrodematerials based on the problem of large volume changes of transition metal oxidesincluding manganese oxides in conversion reaction, resulting in poorcharge-discharge cycle life. In this paper, the strategies like nanotechnology andpre-lithiation to optimize the performance of electrode materials in conversionreaction have been adopted.In this dissertation, the manganese oxides as electrode materials were studied aselectrode materials in Li-ion batteries and Na-ion batteries. Firstly, MnO2nanowires,Li-enrichment Li2MnO3nanowires and MnO2nanorods were synthesized, and thenthe electrode materials of Li-ion batteries and Na-ion battery were characterized andconcluded by presenting their electrochemical performances. Secondly, themechanism of optimization with strategies of nanotechnology and pre-lithiation bycomparison were investigated. The main resultss are as follows:1. Hydrothermal method combined with solid-phase reaction was introduced tocontrollably synthesize MnO2nanowires, Li-enrichment Li2MnO3nanowires andnanorods, through which the cyclic performance and capacities were enhanced. Andthe samples were characterized by TG, XRD, SEM, EDS etc., and the optimalsintering temperature of Li-enrichmentLi2MnO3was confirmed. 2. Electrochemical performance of Li-enrichment Li2MnO3and MnO2nanowires anode materials were characterized and compared with each other,demonstrating that Li-enrichment Li2MnO3nanowires as electrode materials canexhibit better reversible specific capacity and better cycling stability. High capacity of1279mAh g-1was obtained at the current of500mA g-1after500cycles. Besides theLiMn2O4/Li2MnO3full-batteries for applications were investigated. Based on theresults of XPS and XRD, electrochemical reaction mechanism of Li-enrichmentLi2MnO3nanowires and mechanism of performance improvement were studied.Excellent electrochemical performance is mainly derived from these two effectsinduced by Li-enrichment Li2MnO3nanowires:(1) intermediate divalent manganeseacting as buffer in the electrochemical charge and discharge process;(2)pre-intercalated lithium ions’effect of relieving the volume change caused by strain.3. Preliminary exploration of electrochemical performance of the MnO2nanorodsas sodium ion battery electrode materials was conducted. The results showed that theMnO2nanorods sodium ion battery electrode materials demonstrate uniqueelectrochemical performances. This material has two pairs of redox peaks, so it canbe utilized as both cathode and anode materials. When the current density is100mAg-1, it can achieve the initial discharge capacity of668mAh g-1, after the8cycles ofreversible charge and discharge, capacity still reached496mAh g-1.