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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/68209

    Title: 鉬系材料應用於鎂電池正極之性質研究;Mo-based materials as cathodes for magnesium batteries
    Authors: 徐晟睿;Hsu,Cheng-Jui
    Contributors: 化學工程與材料工程學系
    Keywords: 鎂電池;硫化鉬;鎂鋰雙鹽系統
    Date: 2015-08-24
    Issue Date: 2015-09-23 10:53:23 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 鎂金屬的熱力學性質使之成為吸引人的二次電池負極選項,相比於鋰和鉛,鎂有著低價位、低汙染和高安全性的特點。但鎂電池面臨著兩大困難,其一為鎂的高化學活性,使電解液的選擇受到相當程度的限制;其二為正極材料的選擇,鎂離子在很多材料中都不易完成嵌入嵌出,Mo6S8在2000年被提出,是能夠讓鎂離子嵌入嵌出的正極材料,但仍面臨鎂離子本身性質所造成之性能表現不如預期。
    本研究選用Mo6S8作為第一樣探討之活性物質,先以固態反應法(solid state reaction)合成Cu2Mo6S8,經6 M鹽酸去除銅離子,獲得活性物質Mo6S8。在APC電解液中研究其室溫及高溫下之電化學性質,在室溫下僅約60 mAh/g,升溫至60℃時,性質提升至117 mAh/g,在電解質中加入鋰鹽,改變了工作離子,使其電化學性質在室溫下即可達127 mAh/g,故在此材料中離子的移動率(mobility)扮演著左右其儲能性質的角色。
    在APC電解質中加入鋰鹽,改變了正極端的工作離子,可以使一些無法讓鎂離子嵌入的材料使用在此系統中,故本實驗選用之第二樣活性物質為MoS2,在僅有鎂離子存在之電解液時,幾乎無法儲能,但添加鋰離子於電解液中便可使此材料進行儲能,且其電化學行為與MoS2作用於鋰離子電池十分類似,在經過工作離子的改變後,電容值由極低推升至159 mAh/g,當充放電速度達500 mA/g時下仍可維持110 mAh/g的電容值。
    為提升MoS2導電性,以球磨的方式加入10 wt%石墨烯及奈米碳管改善其導電性,在含Li之APC電解質中,電容值皆有顯著的進步,在25 mA/g由原材之159 mAh/g提升至210 mAh/g及190 mAh/g,當充放電速度提升至500 mA/g時仍維持有151 mAh/g及134 mAh/g之電容值。
    ;Thermodynamic properties of magnesium make it an attractive anode material in secondary battery. As compared to lithium and lead, magnesium has advantages of low cost, environmental friendliness, high safety and easy handling. But there are still two major challenges in developing magnesium batteries. The first one is the high chemical activity of magnesium, leading to the formation of passive layer on Mg surface (obstructing Mg ions transportation) and limiting the selection of a suitable electrolyte. The other one is the selection of the cathode materials because magnesium ions are difficult to intercalate/deintercalate into the structure in most of cathode materials. Up to now, Mo6S8, reported in 2000, is one of a few cathode materials that can accommodate magnesium ions, but still faces the drawback of the magnesium ions.
    In this study, Mo6S8 is chosen as the first active material for establishing the Mg battery system. Mo6S8 is fabricated by solid state reaction synthesis of Chevrel-phase Cu2Mo6S8, followed by removing copper ions in 6 M HCl. At room temperature, the capacity of Mg//Mo6S8 cell in APC electrolyte is about 60 mAh/g. In contrast, the cell capacity significantly increases to 117 mAh/g at elevated temperature of 60 C. Besides, adding lithium salt into APC electrolyte improves the cell capacity to 127 mAh/g even at room temperature. The changes of working ion and ion mobility are believed to play important roles in electrochemical and energy storage performance.
    After showing the remarkable benefits of lithium salt addition in Mg//Mo6S8 cell, it is worth expanding this concept to other cathode materials, which are not capable of being intercalated/deintercalated by magnesium ions before. Among them, we introduce another active material MoS2, which magnesium ions cannot intercalate/deintercalate into the structure in APC electrolyte. After adding the lithium ion in the APC electrolyte, the Mg//MoS2 cell starts storing energy and its electrochemical behavior is very similar to MoS2 in lithium-ion system. The cell delivers impressive capacities of 159 mAh/g (at 25 mA/g) and 110 mAh/g at higher current rate of 500 mA/g, respectively.
    To further improve the conductivity of MoS2, 10 wt% carbon nanotubes or graphene is incorporated into MoS2 via a ball milling process. The capacity of Mg//MoS2 cell is thus significantly advanced. Specifically, 190, and 210 mAh/g discharge capacities are obtained at 25 mA/g for the cells consisting of MoS2 cathode with carbon nanotubes and with graphene, respectively. At a higher current rate of 500 mA/g, the cells with carbon nanotubes and with graphene still deliver 134 and 151 mAh/g, respectively, outperforming 110 mAh/g obtained for the cell without carbon addition.
    Appears in Collections:[National Central University Department of Chemical & Materials Engineering] Electronic Thesis & Dissertation

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