【作者】 王东；

【导师】 杜高辉；

【作者基本信息】 陕西科技大学 ， 材料科学与工程， 2021， 博士

【摘要】 锂离子电池(LIBs)技术日益成熟,商业应用越来越广泛。而探究其他新型的电池体系例如钠离子(SIBs)、钾离子(PIBs)以及锂-硫(Li-S)电池等,不仅能扩充能量存储的后备,也能节约生产成本,促进能源与环境的友好关系。开发具有高比容量、高倍率以及循环稳定性的材料是每个电池体系中共同的目标。钼基材料由于结构和存储机制的不同,表现出不同的比容量和循环性能。金属氧化物,尤其是钼基双金属氧化物,在SIBs的应用报道较少,因此对于它们的放电机理研究显得十分必要。这有助于改善此类材料的电化学性能。钼元素的价态变化范围大,因此使得含有钼元素的化合物具有很好的离子存储能力。钼基化合物的放电电压虽然比碳材料的高,但这也避免了锂枝晶的产生,安全性比较好。目前,钼基化合物在储能方面的表现并不理想,尤其是钼基材料储钠/钾的机理不明确,作为电池负极时循环稳定性差。本论文以钼基纳米材料为基础,深入研究它们的储能机制,并揭示了它们在电化学循环过程中可能发生的相变,最终得到电化学性能稳定的电极材料。通过研究本论文得到以下结论:(1)使用静电纺丝法结合烧结处理,制备了钼酸锌(ZnMoO4)纳米管。所制备的纳米管由钼酸锌小颗粒组成,纳米管的平均直径为463 nm。当用作SIBs负极时,在0.1 A g-1的电流密度下,初始放电容量为548 mAh g-1,初始的充电比容量为330 mAh g-1,之后它的比容量缓慢降低,经过500圈循环后,稳定在190 mAh g-1。此外,利用非原位XRD和TEM技术研究了它的放电机理。在放电过程中,当电压在1.5 V以上时,仅有0.4 mol Na+嵌入;当放电至1.5 V以下时,ZnMoO4转变为Na2MoO4,随着进一步放电至0.5 V以下,Na2MoO4被还原成Mo。最终钼酸锌被还原成Zn以及Mo单质。(2)使用不同的温度热分解壳聚糖得到了不同形貌的的碳材料。当热解温度为1000℃时,所制备的碳材料为多孔结构,拥有最高的比表面积482.4 m2g-1。当用作SIBs和PIBs的负极材料时,该材料在PIBs中表现出更好的电化学性能。在0.2 A g-1的电流密度下拥有217 mAh g-1的比容量,即使在大电流密度2 Ag-1的条件下依旧具有146 mAh g-1的比容量。以该条件为基础,通过球磨法制备碳包覆钼酸锌复合物。当电流密度为0.5 Ag-1时,作为PIBs负极材料,首次放电为1106 mAh g-1,经过500圈循环后,稳定在585 mAh g-1。作为SIBs负极材料,首次放电为982 mAhg-1,经过500圈循环后,稳定在352 mAh g-1。复合材料展现出优异的电化学性能,利用CV和TEM研究了钾离子存储机制,钾离子首先嵌入ZnMoO4中,形成KxZnMoO4,最终被还原成Zn以及Mo单质。(3)利用静电纺丝法制备出了荆棘状的NaVMoO6/C复合纳米纤维。该复合物的平均直径为320 nm,NaVMoO6的颗粒尺寸小于100 nm。作为LIBs的负极材料时,在电流密度为0.1 A g-1循环500圈之后,它的容量稳定在1013 mAh g-1。在大电流密度1 Ag-1下循环500圈后,容量保持在682 mAh g-1以上。这种荆棘状结构能够提高电导率,缩短离子传输距离,为锂化提供更多的反应位点。通过非原位XRD以及XPS对放电产物进行了表征,结果表明,放电过程中NaVMoO6发生晶型转变,该过程中生成的V2O5以嵌入-脱出机理进行存储锂离子,最终生成LiVO2;而钼的化合物则是以转换型机理储能。(4)以静电纺丝法为基础加入造孔剂制备了多孔碳纤维与硒化钼纳米片的复合材料(MoSe2/PCFs),并探究了它的SIBs性能。作为SIBs负极时,当电流密度为0.1 A g-1时,它的初始放电容量为618 mAh g-1,循环200圈之后,容量稳定在371 mAh g-1左右。当电流密度为5Ag-1时,MoSe2/PCFs电极的比容量循环1200圈之后稳定在200 mAh g-1,这说明MoSe2/PCFs具有优异的储钠性能。通过非原位XRD和TEM分析发现,硒化钼深度放电后转变为3-5 nm的硒化钠和钼单质。此外设计的多孔结构能有效的抑制充放电过程中产生的硒化物的穿梭效应,因此它具有极佳的SIBs性能。(5)以氯化钠为模板,制备具有高比表面积的海胆状二硫化钼-空心碳纳米球(MoS2-CHNs)。所制备的空心碳球直径在100-300 nm之间,毛刺状的MoS2分布在碳层表面。当MoS2-CHNs负载高含量的S时(5 mg cm-2),在0.2 C下循环100圈拥有1139 mAh g-1的容量。在5 C下循环500圈拥有601 mAh g-1的容量。除此之外,使用三电极体系研究了 MoS2-CHNs对于锂多硫化物的催化性能。结果显示,MoS2-CHNs体系中锂离子扩散系数高达2.6×10-9 cm2 s-1。它的恒电位极化曲线表明MoS2-CHNs具有更小的催化电压,塔菲尔斜率为67 mV dec-1;双向催化以及Li2S的成核测试,进一步证明MoS2-CHNs对于多硫化物的转化具有更好的催化性能。更多还原

【Abstract】 Lithium-ion batteries(LIBs)technology and its commercial application are becoming more and more mature.Exploring other new battery systems,such as sodium ion(SIBs),potassium ion(PIBs)and lithium sulfur(Li-S)batteries,can not only expand the backup of energy storage,but also save production costs and promote the friendly relationship between energy and environment.The development of materials with high specific capacity,super rate and cycle stability are the common goal of each battery system.Different molybdenum-materials present distinct specific capacity and cycling performance due to the unique structure and storage mechanism.Metal oxides,especially molybdenum-based bimetallic oxides,are rarely reported for SIBs,so it is necessary to investigate their discharge mechanism.It is helpful to improve the electrochemical performance of these materials.The valence distribution of molybdenum is relatively wide,so the compounds containing molybdenum have excellent ion storage capacity.Although the discharge voltage of molybdenum-base compound is higher than that of graphite,it avoids the generation of lithium dendrites.At present,the performance of molybdenum based compounds in energy storage is not ideal,especially the mechanism of sodium/potassium storage of molybdenum based materials is not clear,and the cycle stability is poor as the negative electrode of battery.Based on the synthesis of different molybdenum-based nanostructures,this paper deeply investigated their energy storage mechanism,and revealed their possible phase transformation in the process of electrochemical cycle.Through the result of this paper,the following conclusions are obtained:(1)Zinc molybdate(ZnMoO4)nanotubes were prepared by electrospinning and subsequent sintering process.The nanotubes were composed by small zinc molybdate particles with an average diameter of 463 nm.When used as the anode of SIBs,the initial discharge capacity is 548 mAh g-1 and the specific charge capacity is 330 mAh g-1 at a current density of 0.1 A g-1.After 500 cycles,the specific capacity of ZnMoO4 stablizes to 190 mAh g-1.In addition,the discharge mechanism of ZnMoO4 was studied by ex-situ XRD and TEM.In the discharge process,when the voltage is above 1.5 V,only a little Na+ is embedded;when the voltage is below 1.5 V,ZnMoO4 transforms into Na2MoO4,and with further discharge to below 0.5 V,Na2MoO4 is reduced to Mo.Finally,ZnMoO4 is reduced to Zn and Mo.(2)Carbon materials with different morphologies were obtained by thermal decomposition of chitosan at different temperatures.When the pyrolysis temperature is 1000 ℃,the prepared carbon materials have porous structure with the highest specific surface area of 482.4 m2 g-1.When used as anode material of SIBs and PIBs,the material shows better electrochemical performance in PIBs.It presents a reversible specific capacity of 217 mAh g-1 at 0.2 A g-1 and 146 mAh g-1 at 2 A g-1.The subsequent pseudocapacitance tests show that the performance of the material is affected by the surface-controlled pseudocapacitance.Based on this condition,carbon coated ZnMoO4 composites were prepared by ball milling.When the current density is 0.5 A g-1,as the anode material of PIBs,the first discharge capacity is 1106 mAh g-1.After 500 cycles,it is stable at 585 mAh g-1.As the anode material of SIBs,the first discharge capacity is 982 mAh g-1.After 500 cycles,it is stable at 352 mAh g-1.The composite shows improved electrochemical properties.And its potassium ion storage mechanism was studied by CV and TEM.Potassium ions were first embedded in ZnMoO4 to form KxZnMoO4,which was finally reduced to Zn and Mo.(3)Bramble-like NaVMoO6/C composite nanofibers were prepared by electrospinning method.The average diameter of the composite is 320 nm,and the particle size of NaVMoO6 is less than 100 nm.As the anode material for LIBs,the electrochemical performance is relatively stable.After 500 cycles at a current density of 0.1 A g-1,its capacity remains at 1013 mAh g-1.After 500 cycles at a high current density of 1 A g-1,the capacity remains 682 mAh g-1.The advantages of this structure can improve the ionic conductivity,shorten the ion transport distance,and provide more reaction sites for Li+.The discharge products were characterized by ex-situ XRD and XPS.The results show that there are two types of mechanism in the discharge process.V2O5 generated in the discharge process stores energy by the mechanism of intercalationdeintercalation,and finally reduced to LiVO2;while molybdenum compounds store energy by the mechanism of conversion,and finally generates molybdenum.(4)Porous carbon fiber and molybdenum selenide composites(MoSe2/PCFs)were prepared by electrospinning with pore forming agent to explore the SIBs properties.When used as anode of SIBs,the initial discharge capacity of MoSe2/PCFs is 618 mAh g-1 at the current density of 0.1 A g-1.After 200 cycles,the capacity is stable at 371 mAh g-1.When the current density is 5 A g-1,the specific capacity of MoSe2/PCFs electrode is stable at 200 mAh g-1 after 1200 cycles,which indicates that MoSe2/CNs has excellent SIBs performance.Ex-situ XRD and TEM analysis showed that MoSe2 was transformed into Na2Se and Mo with the size of 3-5 nm after deep discharge.This porous structure can effectively inhibit the negative effect of selenides produced in the process of charge and discharge,so it has excellent SIBs performance.(5)Sea urchin-like molybdenum disulfide-carbon hollow nanospheres(MoS2-CHNs)with high specific surface area was prepared by using NaCl as core and sucrose as carbon shell after sintering and washing.The diameter of carbon sphere is between 100-300 nm,and the burr-like MoS2 is distributed on the surface of carbon layer.When MoS2-CHNs is loaded with high content of S(5 mg cm-2),it has a capacity of 1140 mAh g-1 after 100 cycles at 0.2 C.It has a capacity of 616 mAh g-1 after 500 cycles at 5 C.In addition,three electrode systems were used to study the catalytic performance of MoS2-CHNs for lithium-polysulfide.The results show that the MoS2-CHNs system exhibits a high Li+ diffusion coefficient of 2.6×10-9 cm2 s-1.After assembling the battery with lithium-polysulfide as electrolyte,the potentiostatic polarization curve showed that MoS2-CHNs had lower catalytic voltage and Tafel slope(67 mV dec-1),which proved that the material had excellent catalytic performance for polysulfide.Bidirectional catalysis and Li2S nucleation test further proved that MoS2-CHNs had better catalytic performance for the conversion of polysulfides.更多还原