【作者】 张睿；

【导师】 温广武； 黄小萧；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2019， 博士

【摘要】 随着动力汽车、便携式电子设备以及能量储存器件等的快速普及和发展,人类对锂离子二次电池的需求日益增多,需要具备更高的能量密度、功率密度以及安全性。相对于常见的过渡族氧化物负极材料,过渡族碳酸盐负极具有比容量高、储量丰富和安全无毒等优点,并且制备工艺简便、制备成本更低。然而,过渡族碳酸盐导电性差和充放电时体积变化大等问题成为其应用和发展的瓶颈。与导电性高的碳材料复合是解决上述问题的有效途径。其中与石墨烯复合是一种常见的方法,这是由于石墨烯具有超高的电子导电性和优异的机械强度。本文针对碳酸盐负极材料的倍率性能和结构稳定性的改善需求,将碳酸盐与石墨烯进行复合,同时通过对碳酸锰的成分以及结构进行调控,改善其电化学反应活性、反应动力学和结构稳定性等,并对其储锂机理进行深入分析。利用钴元素掺杂和石墨烯复合对碳酸锰的组成和结构进行改性,合成出双金属碳酸盐/石墨烯(Mn_xCo_1-x-x CO₃/RGO)复合材料。通过调节原料中Mn/Co摩尔比可以得到不同比例的多孔结构的Mn_xCo_1-x-x CO₃固溶体,并紧密嵌在石墨烯骨架中。系统地研究了Mn_xCo_1-x-x CO₃/RGO复合材料的物相组成和结构以及单相结构的双金属碳酸盐的形成机理。对Mn_xCo_1-xCO₃/RGO、CoCO₃/RGO和MnCO₃/RGO的储锂性能和交流阻抗进行测试,并结合第一性原理计算,发现双金属元素的存在使得碳酸盐晶胞的带隙变窄,提高了碳酸盐电极的本征导电率,有利于促进电化学反应过程中电子的传输。由于不同金属间的协同作用以及丰富的多孔结构,Mn_xCo_1-x-x CO₃/RGO复合电极表现出比CoCO₃/RGO和MnCO₃/RGO更优异的循环性能和倍率性能。其中,Mn_0.7Co_0.3CO₃/RGO复合电极在2000 mA g^-1下循环1500圈,能够达到981 mAh g^-1的比容量,在5000mA g^-1下循环1500圈后依然能保持与商用石墨负极相当的可逆比容量。利用碳酸镍构筑异质结构,同时复合石墨烯对碳酸锰负极进行改性,制备出Ni-MnCO₃@Mn-NiCO₃/RGO复合材料。在该复合材料中,以Ni掺杂的MnCO₃作为核结构,同时以Mn掺杂的NiCO₃作为壳结构,构成壳核分级结构的MnCO₃@NiCO₃微米颗粒,并分散在石墨烯骨架中。这是首次报道合成出壳核结构的碳酸盐基复合材料作为锂离子电池负极材料。对水热反应时间、石墨烯的加入以及镍元素对于壳核结构的影响进行研究,发现碳酸锰先于碳酸镍在水热过程中形核和生长。这是由于MnCO₃具有较小溶度积常数和形成能,能够在溶液中迅速形核,而在随后的形核生长过程中,Ni掺杂的MnCO₃晶体与Mn掺杂的NiCO₃晶体结构上的相似性使二者间的过渡层错配度较小,碳酸镍可以在碳酸锰表面外延生长并形成较强的相互作用,最终形成具有壳核结构的复合碳酸盐。MnCO₃@NiCO₃/RGO独特的结构和成分优势,使其表现出优异的储锂容量、高倍率以及高循环稳定性能。提出一种高效、简便的锰基化合物/石墨烯复合材料的制备方法,并利用四氧化三锰和石墨烯对碳酸锰负极进行改性。以Hummer’s法制备氧化石墨所产生的酸性溶液为原料,通过加入不同的碱性溶液（Na₂CO₃或KOH）以及控制最终溶液的pH值,得到MnCO₃/RGO、Mn₃O₄/RGO以及MnCO₃/Mn₃O₄/RGO等复合材料。碱性溶液的加入不仅可以中和制备氧化石墨过程中产生的废酸,还能够与溶液中的锰离子产生沉淀反应,从而得到不同种类的锰基化合物,提高原料的利用率和制备效率。MnCO₃/Mn₃O₄/RGO作为负极材料时,表现出较高的可逆比容量以及优异的循环性能,在100 mA g^-1下循环200圈后,其比容量保持为988 mAh g^-1,在1000 mA g^-1下循环800圈后,仍然能保持532 mAh g^-1的可逆比容量。通过对电极材料的循环伏安曲线、充放电曲线以及容量电压微分曲线进行深入分析,发现由于电化学过程中电极极化的相对减弱,使得MnCO₃/Mn₃O₄/RGO复合材料展现出优于两相复合材料的性能。更多还原

【Abstract】 The rapid development of the electric vehicles and portable electronic devices has prompted the demand for lithium-ion batteries（LIBs）with higher energy/power densities.Compared with the transition metal oxides,transition metal carbonates have attracted considerable attention due to their higher theoretical capacity,natural abundance,and environment friendliness.However,their practical application as LIB anodes suffers from the intrinsic poor electrical conductivity and structural degradation during charging/discharging processes.A popular strategy for addressing these issues is to hybridize carbonates with carbonaceous materials.Graphene,a two-dimensional（2D）material with sp²-bonded carbon,has been widely utilized as a promising composite because of its excellent conductivity,ultrahigh surface area and robust mechanical strength.Unfortunately,for the carbonates/graphene composites,the subpar rate performance and the structure instability still remain the bottleneck problems.Herein,by constructing the structures and constituents,the morphology and component of the metal carbonates are modified in order to improve their lithium storage properties.The single-phase mixed transition metal carbonate/graphene composites were synthesized by cobalt-doping of MnCO₃ and combining with reduced graphene oxide（RGO）.By adjusting the molar ratio of Mn/Co in the raw materials,the Mn_xCo_1-xCO₃ microparticles are obtained,which are tightly encapsulated by graphene.The chemical composition and structure features of Mn_x Co_1-xCO₃/RGO,as well as the formation mechanism of single-phase Mn_x Co_1-x-x CO₃ are investigated thoroughly.By analyzing the electrochemical performance,EIS spectra and DFT results,it is found that the improved conductivity and large surface area are benefical for rapid electron/ions transportation and providing sufficient active sites.Benefitting from the structural and componential advantages induced by the synergistic effect of Mn and Co,Mn_x Co_1-x-x CO₃/RGO possesses superior electrochemical performance than CoCO₃/RGO and MnCO₃/RGO.Especially,Mn_0.7Co_0.3CO₃/RGO exhibits excellent reversible capacity as high as 981 mAh g^-1at 2000 mA g^-11 after 1500 cycles,and a comparable capacity with commercial graphite at 5000 mA g^-11 after 1500 cycles.Taking the advantages of hierarchical structure and bimetal synergy,the RGO wrapped core-shell MnCO₃@NiCO₃（MnCO₃@NiCO₃/RGO）composites were rationally designed and synthesized to further improve the electrochemical performance of metal carbonates.Through a simple hydrothermal method,the core-shell structure carbonates are formed with Ni-doped MnCO₃ core coated by Mn-doped NiCO₃ shell.This is the first time for the fabrication of carbonates with core/shell structure as LIBs anode.We systematically investigated the evolution process of the products,as well as the effects of graphene and element-doping on the morphology of MnCO₃@NiCO₃/RGO.Due to the lower solubility and formation energy,the MnCO₃ is precipitated preliminarily with Ni elements doped and then Mn-doped Ni₂CO₃ nucleates and grows on the surface of MnCO₃ to form core-shell hierarchical structure.In virtue of the structural and component advantages,the obtained MnCO₃@NiCO₃/RGO composites manifest excellent Li-ion storage performance in view of high rate performance(300 mAh g^-1 at 10 A g^-1and 200 mAh g^-1 at 20 A g^-1)and remarkable cycling stability(capacity retention of75%after 1300 cycles at 2 A g^-1).In order to promote the practical application of manganese carbonate as LIBs anodes,a facile and efficient method is developed to synthesize the Mn-based compounds/RGO composites.Through adding different types of alkali liquor Na₂CO₃ or KOH)to GO suspension(GO/Mn suspension,including GO,Mn²⁺,SO₄^2-and H⁺)and controlling the pH values of the final solution,the MnCO₃/RGO,Mn₃O₄/RGO,and MnCO₃/Mn₃O₄/RGO composites are obtained.In this synthetic route,the residual acid in GO suspension is neutralized,and the in-situ precipitation of Mn-based compounds can dramatically improve the utilization of raw materials and simplify the operating procedure.When evaluated as LIBs anode,the ternary MnCO₃/Mn₃O₄/RGO exhibite high capacity with 980 mAh g^-1 at100 mA g^-1 after 200 cycle,and enhanced cyclability(532 mAh g^-1 at 1000 mA g^-1after 800 cycles).Moreover,cyclic voltammetry,charge/discharge curves,and corresponding dQ/dV profiles demonstrate that the synergistic effect of MnCO₃ and Mn₃O₄ as well as the decreased potential difference during cycling induced the better electrochemical activities and reaction kinetics of MnCO₃/Mn₃O₄/RGO than those of the binary composites.更多还原