锂离子电池锡基负极材料的合成及性能表征

【作者】 崔王君；

【导师】 夏永姚；

【作者基本信息】 复旦大学 ， 物理化学， 2011， 博士

【摘要】 目前,商用的锂离子电池主要采用石墨及改型石墨作为负极材料,然而石墨的理论容量偏低,其利用率已经达到了它的极限(372 mAh/g),开发具有更高容量的新一代锂离子电池已经成为当前锂离子电池领域研究的热点。与石墨材料相比,合金负极材料理论贮锂容量大,贮锂电位安全。如金属Sn,它具有高达990 mAh/g (7200 mAh/L)的理论容量,其质量比容量是石墨的2.7倍,而体积比容量达到石墨的8.8倍。然而合金负极材料在锂离子嵌入脱出过程中伴随着巨大的体积膨胀收缩,这将导致材料受内部应力的作用而龟裂,从集流体上剥落,进而失去电化学活性,最终导致材料的循环性能下降。如何提高合金负极材料的循环性能成为开发高性能合金锂离子电池材料的关键。基于以上的研究背景,本论文的研究致力于改善锡基合金材料的循环性能,创新性地提出了将合金结构抑制作用与纳米包覆技术形貌抑制,两种体积缓冲方式相结合的新方法,开发出碳包覆的纳米核壳结构锡基合金负极材料,使材料的循环性能相对于一般纳米合金材料得到明显改善。此外,本论文还对基质材料的选择进行了探索,合成了以磷酸锂为基质的磷酸锡锂负极材料,并对材料的电化学性能及结构转变机理进行了深入研究。对于三维有序结构对电极材料电化学性能的影响,本论文也有所涉及。本论文通过对锡基负极材料制备,电性能等各方面的研究,对于制备高循环性能的锂离子电池用合金负极材料起到一定的指导作用。本论文的具体内容如下：在论文的第三章,我们首次成功实现了纳米锡基合金材料的碳包覆改性。在合成过程中,克服了锡基合金材料熔点低(一般低于232℃),难于制备纳米尺度高导电性碳包覆材料的难点。创新性地提出采用原位乳液聚合技术,对无机纳米粒子进行表面改性,实现油相／水相相反转,进而成功实现碳包覆的新方法。该法制备得到纳米碳包覆核壳结构Cu6Sn5具有优异的电化学性能,其可逆容量在100 mA/g的恒流测试条件下为437 mAh/g (0 V～2.0 V vs. Li+/Li),50圈循环后其容量为可逆容量的93%,循环性能明显优于未包覆的纳米Cu6Sn5粒子。通过TEM表征发现,该材料具有典型的核壳结构,其中壳为5纳米厚度的石墨化碳层,核为Cu6Sn5合金粒子。通过对充放电后体积收缩膨胀的合金粒子进行TEM表征发现,该材料所包覆的碳层具有一定的伸缩性,无论在充电状态还是放电状态,碳层都保持完整。该材料循环性能的改善主要是由于该材料在结合了合金的结构抑制作用与纳米材料绝对体积膨胀小的优点的同时,具有了完好的约5纳米的碳包覆层。该碳层有效地防止高温处理过程中低熔点锡基合金的熔出,保证了锡基材料的纳米尺寸,还能抑制锂离子嵌入脱出过程中锡基材料的粉化过程。另外,高电导性的碳包覆也有助于材料整体电化学性能的提高。该材料由于性能优异制备方法简单,是具有广阔应用前景的锡基负极材料,且该方法也易于拓展至其他合金体系。采用硼氢化钠还原法制备纳米合金粒子有其局限性。氧化物纳米粒子作为一种廉价易制备的前躯体,如果能用于纳米合金负极材料的制备,将简化制备流程,有着较高的实用意义。本论文的第四章,首先通过热力学计算对多种氧化物碳热还原体系的热力学性质进行研究,同时运用差热扫描量热法DSC对氧化物(CuO, SnO2)碳热还原反应的温度进行表征。采用纳米Sn02和CuO粒子为前躯体,利用十六烷基三甲氧基硅烷为表面改性剂(硅烷试剂能与氧化物粒子形成稳定的硅氧键,使硅烷有机链端朝外,从而改变氧化物粒子的表面性质),实现氧化物粒子的相反转。然后采用酚醛树脂包覆的方法,通过惰性气氛煅烧得到碳包覆的纳米Cu6Sn5合金材料。所得的材料可逆容量达到420 mAh/g,50圈后容量仍可维持在383mAh/g,容量维持率达到80%。材料循环型的提高同样得益于对合金粒子的碳包覆作用。本论文的第五章,对CoSnC体系进行了研究。文献报道的无定形CoSnC体系虽然具有较大的容量,然而在循环过程中由于纳米粒子的团聚,会形成电化学失活的晶簇,影响电化学性能。针对以上问题,并对上一章以氧化物为前躯体合成合金粒子的过程中,按化学计量比均匀混合氧化物前躯体操作繁琐的问题,我们首次采用COSnO3纳米粒子为前躯体,利用化合物中Co、Sn的化学计量比,合成同样化学计量比的CoSn合金。CoSnO3纳米粒子的碳热还原过程,由于还原过程中液态锡的存在,更利于合金的形成。CoSnC体系体现出特殊的性质,该体系由于Co-Sn体系存在较多热力学相近的合金相,得到是多种具有电化学活性的合金相混合物,然而由于碳与合金粒子能形成稳定的复合体系,在充放电过程中,存在的碳不仅能抑制合金粒子的体积膨胀,还能防止纳米合金粒子的团聚。因此该材料显示出优异的循环性能,可逆容量达到450mAh/g,50圈容量维持率为72%。论文第六章对磷酸锂为基质材料的锡基负极材料磷酸锡锂进行了系统研究。本工作以磷酸锂为基质,合成晶体材料磷酸锡锂。对磷酸锡锂的合成条件进行探索,确定了采用纳米粒子SnO2为前躯体,900℃空气气氛的合成条件,该条件下合成的材料具有典型的NASICON结构,且形貌尺寸均一。该材料表现出优秀的电化学性能,可逆容量320mAh/g,50圈容量维持率达85%。本工作还对该材料的充放电机理进行了深入研究。通过对不同充放电电位的产物进行非原味X射线衍射结表征,发现在锂离子嵌入过程中,磷酸锡锂的晶体结构会遭到破坏,从而形成磷酸锂基质包裹的纳米金属锡体系。生成的金属锡能进一步与锂离子发生反应形成Li4.4Sn。由于磷酸锂基质的存在,金属锡的体积收缩膨胀得到抑制,材料的循环性得到改善。此研究也对该类材料的不可逆容量进行了分析,加深了对造成该材料首圈不可逆容量的原因理解。具有三维有序结构的纳米电极材料由于具有稳定的锂离子扩散通道、较薄的适于锂离子传输的孔壁,因此具有更大的容量和更好的倍率性能。本论文第七章,对锂离子电池用三维有序电极材料进行了初步研究。本工作以有序阵列聚苯乙烯小球为模板,采用浸渍法合成了三维有序大孔磷酸铁材料。SEM表征发现,该材料具有立方密堆积的孔结构,平均孔径为250m。在不同温度煅烧条件下,虽然材料发生了晶型的转变,但仍然维持了有序孔结构。该化合物在工作区间2.5V-4.0V (v.sLi+/Li),0.05C的放电倍率下,具有120mAh/g的容量且具有良好的循环性能。在2C条件下仍然具有40mAh/g的比容量,与同类材料相比具有较好的倍率性能。更多还原

【Abstract】 Recently, graphite and modified graphite are mostly used as anode material in commercial lithium-ion batteries. However, the graphite materials have low theoretical capacity (372 mAh/g), and the current used graphite anode is close to its capacity limitation. Exploring new generation anode materials with higher capacity has become the focus of the research on lithium-ion batteries. It has been demonstrated that metals and alloys, present higher capacity and lower lithium intercalation potential. For example, Sn yields a maximum theoretical capacity of 990 mAh/g or 7200 Ah/L. However, one major problem preventing them from the commercial application is that they undergo large volume changes during cycling, which result in disintegration of the electrodes and subsequent rapid capacity fading. How to improve the cycleability of tin anode is the key point to develop high performance anode.This dissertation is on the foundation of existing research, and creatively introduces a new method that combines two existing ways. A core-shell carbon-coated nano-scale alloy anode has been prepared in this dissertation, and shows excellent electrochemistry performance. Moreover, in the preparation, we used in-situ polymerization technique and surface modification to avoid the low-melting point alloy pouring out from shell in heating process. This method can be applied extensively to other nano-alloy preparation work. In addition, this dissertation explores the matrix materials for tin-based anode, and synthesize a LiSn2(PO4)3 compound in which the Li3PO4 is used as matrix. The lithiuation mechanism of this material has also been investigated. The ordered three-dimension structure anode is also included in this dissertation.In Chapter 3, a core-shell-structure carbon-coated nanoscale Cu6Sn5 is prepared by using an in situ polymerization method integrated with a surface modification technology. The composite combines the merits of intermetallic compounds and nano-sized anode materials, and exhibits an excellent cycling stability with a reversible capacity of 437 mAh/g and no obvious fading after 50 cycles, which is the best cycle performance among alloy anodes reported up to date. The improvement in the cycling stability could be attributed to the fact that the well-coated carbon layer can effectively prevent the encapsulated low melting point alloy from out-flowing in a high temperature treatment process, as well as preventing aggregation and pulverization of nano-sized Cu6Sn5 alloy particles during charge/discharge cycling.It is convenient and has practical meaning to use oxides which is cheap and easily got as precursors to prepare nano-alloy anodes. In Chapter 4, the thermodynamic characters of carbothermal reduction of various oxides have been studied. And differential scanning calorimetry (DSC) is applied to identify the reduction temperatures. The nano CuO and SnO2 are used as precursors, and hexadecyltrimethoxysilane (HTMS) is used as surface modifier. A "Si-O-metal" bond which is detected by Fourier infrared spectrometry (IR) technique is formed during the modification to keep the oxide particles dispersing well in the organic phase. This is a key point to guarantee that polymerization occurs on a single particle surface and ensure that the polymer layer firmly coats the grains. The prepared Cu6Sn5 delivers a reversible capacity of 420mAh/g with capacity retention of 80% after 50 cycles.In Chapter 5, CoSnC system has been investigated. It is reported that although the amorphous CoSnC has large capacity, the aggregation of nano particles deteriorate the electrochemistry performance. To solve this problem, for the first time, stannate, CoSnO3, which mixes the elements Co and Sn evenly at the atomic level, is used as precursor to prepare CoSnC by a modified carbothermal reduction method. The alloy formation is easier due to the existence of liquid tin. However, the composition of the product consists of CoSn and CoSn2 due to inhomogeneous quenching process and the chemical formula can be expressed as CoSnC8 identified by Thermogravimetric (TG) measurements. The synthesized CoSnC delivers a reversible capacity of 450mAh/g with a capacity retention of 72% after 50 cycles.A systematic research of LiSn2(PO4)3 is presented in Chapter 6. Through the optimization of the synthesis condition, a NASICON structure LiSn(PO4) is prepared in air at 900℃with nano-SnO2 as precursor. This material delivers a reversible capacity of 320mAh/g with a capacity retention of 85% after 50 cycles. The lithiuation mechanism of this material is studied by ex-situ XRD and SEM, which confirms that a Li4.4Sn phase is eventually formed accompanying with the structure collapse. The formed Li3PO4 matrix restricts the volume expansion of the tin particles and greatly improves the cycleability of tin-based anode. This work analyzes the irreversible capacity in the first cycle and deepens the understanding of tin/matrix anode.The three-dimensional structure materials have some unique advantages and show great promising for enhancing the performance of rechargeable lithium-ion batteries. In Chapter 7, highly ordered three-dimensional macroporous 3DOM FePO4 material was prepared by using a colloidal crystal template. The effects of the annealing temperature on the morphology changes and the electrochemical properties of the composite were investigated. The 3DOM FePO4 prepared at 400℃shows the excellent cycling stability and good rate capability. This material delivers a reversible capacity of 120mAh/g at current density of 0.05C (2.5V-4.0V v.s Li+/Li). These improvements are mainly due to the following facts:first, the macropores can make the electrolyte solution infiltrate the electrode better. Second, the 3DOM structure with the wall thickness only several tens of nanometers, can reduce the diffusion distances of lithium ions markedly. Third, the continuous network of 3 DOM materials should have better electrical conductivity.更多还原