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锂离子电池界面反应研究

Investigation of Electrode/Electrolyte Interfacial Reactions Involving in Li Ion Batteries

【作者】 李君涛

【导师】 孙世刚; P.Marcus;

【作者基本信息】 厦门大学 , 物理化学, 2009, 博士

【摘要】 锂离子电池的界面反应包括锂离子的嵌脱,电解液的分解,和固体电解质界面膜(Solid electrolyte interphase,SEI)的形成等过程。这些界面反应对电池的循环性能、寿命、化学和物理稳定性、以及不可逆容量有重要的影响,是锂离子电池的研究热点之一。目前,随着各种新型电极材料的开发以及对原有电极材料的改性、修饰和惨杂,使得研究这些新型电极材料的界面反应变得十分必要。此外,关于研究锂离子电池的界面反应也有助于发展和建立相关的非水电解质理论和模型。本论文主要集中在对锂离子电池界面的研究,通过显微傅里叶变换红外光谱,(Microscope FTIRS reflection spectroscopy,MFTIRS),电化学石英晶体微天平(Electrochemical quartz crystal microbalance,EQCM),x射线光电子能谱(X-ray photoelectron spectroscopy,XPS),和飞行时间二次离子质谱(Time-of-flight secondary ion mass spectrometry,ToF-SIMS)系统、深入地研究了Sn负极,Sn-Co合金负极(原子比约为2:1),石墨负极,Cr2O3负极,Cr2S3负极和V2O5正极材料的界面反应和过程。通过FTIRS和XPS这两种灵敏的表面分析技术,可以非原位地检测电化学循环后电极材料的变化和指认SEI的化学组份。原位FTIRS主要用于电极极化时电解液的分解、SEI膜形的成和Li离子的嵌脱等过程。EQCM通过记录单位电荷转移引起电极上纳克级的质量变化,原位地跟踪充放时电极材料发生的不同的电化学过程。ToF-SISM通过记录不同剥离时间的碎片离子强度,对嵌脱锂后电极材料进行深度组份分布分析,从而研究Li离子在固相材料的嵌脱、扩散过程。本论文重点探讨了在充放电过程中,SEI膜的化学组份和变化,以及Li离子在不同电极材料上嵌脱过程和机制。首次实现了运用原位红外光谱通过溶剂化/去溶剂化效应表征Li离子的嵌脱过程,并将研究从红外光反射率较高金属电极扩展到反射率较低的粉末材料电极。主要研究结论如下:1.通过电镀的方法制备Sn,Sn-Co合金负极。对Sn负极的循环伏安研究表明,当电位正向、负向扫描时,在1.40-1.10V电位区间都能观察到电解液还原的不可逆电流峰,归因于当电位正向扫描时,Li离子与Sn负极去合金化过程所引起的Sn薄膜体积变化,导致SEI破碎脱离,新鲜的Sn暴露在电解液中,从而使得电解液在此电位区间继续还原。在经过第一周循环后,在Sn-Co电极上锂离子的合金化/去合金化的电流峰位与Sn电极上相似,表明在经过电化学循环后Sn、Co原子排列发生了改变,导致部分Sn聚集发生。Sn-Co合金负极上更好的电化学性能证实加入Co惰性组份有利益缓冲在合金化/去合金化过程中体积变化,进而改善其电化学性能。2.运用MFTIRS和EQCM原位技术,研究了锡薄膜负极在1M LiPF6/EC+DMC电解液中的界面反应。EQCM研究表明,在电解液分解的电位区间,mpe(1摩尔电子转移所引起的电极上物种质量的改变)小于其理论数值。这是因为部分固体还原产物溶解在电解液而不参与SEI的形成,和在电解液还原前一些溶剂分子吸附在Sn电极,并作为电解液还原的前置步骤。然而在Li离子与Sn负极合金/去合金的电位区间,mpe数值大于理论值(6.9g·mol-1),对应于Li离子的合金化使得Sn薄膜的体积膨胀,导致在高电位形成的SEI膜破裂,使得SEI膜在低电位继续生成。在电位阴极极化过程中,Li离子与Sn负极合金化,原位显微红外光谱给出正向和负向的红外吸收峰。在阳极极化过程时,去合金化过程发生,原位红外光谱在相同的波数给出与阴极极化谱峰方向相反的红外吸收峰,表明电解液薄层中物种在合金化/去合金化过程中发生可逆的变化。Li离子在电解液中是以溶剂化的形式存在(Li(sol)n+),合金化时,Li离子首先是从Li(sol)n+去溶剂化,然后与Sn电极反应,这会导致薄层溶液中自由的溶剂分子(sol:EC,DMC)的增加,而Li(sol)n+减少。同时由于Li(sol)n+中Li+…C=O间的相互作用,使得Li(sol)n+中C=O键相对于sol中的C=O被减弱,C=O的非对称伸缩振动频率相对于自由溶剂分子发生红移动。然而Li(sol)n+中的C-O,C-H键被加强。因此在Li离子与Sn合金化/去合金化过程中,相应物种浓度以及红外吸收峰位置的变化,是原位红外光谱给出正向、负向谱峰的原因,同时也为原位红光谱检测Li离子嵌脱过程(Sn电极上为合金化/去合金过程)提供了基础。基于对Sn电极上原位红外光谱的分析,我们首次提出运用红外光谱对锂离子嵌入脱出过程进行表征。而此前原位外原位光谱常常仅用于电解液的分解和SEI薄膜的形成过程研究。为了证实上述分析,我们采用透射红外光谱研究含有不同LiPF6浓度的电解液(xMLiPF6/EC+DMC)来模拟Li离子嵌脱过程时的溶剂化效应,得到了与原位红外相似的光谱特征和变化。同时在不能和Li离子合金化的Cu惰性电极上,没有观察到相关的红外谱峰的变化。原位红外光谱研究还发现:循环伏安曲线上在1.1-1.4V出现的电流峰是由于电解液的还原引起。在这个电位区间,阳极极化和阴极极化的红外吸收峰在相同的波数给出相同方向的谱峰,这表明电解液还原反应的不可逆。原位MFTIRS研究证实是溶剂化的分子,而不是自由的溶剂分子在电极表面还原,其还原产物为ROCO2Li。3.基于Sn电极上原位红外光谱,我们采用MFTIRS对较为复杂的Sn-Co合金以及石墨膜负极在1 M LiPF6/EC+DMC界面性能进行研究。在Sn-Co负极上Li离子的合金化/合金化过程中,原位MFTIRS给出了与Sn电极上相似的结果。即:在合金化/去合金化过程中,Li离子可逆的去溶剂/溶剂化导致溶剂和Li(sol)n+在薄层中浓度的变化,以及相关的红外吸收峰发生位移。我们用旋转套膜的方法制备了具有较高红外反射率的石墨薄膜电极,以获得具有较好信噪比的原位MFTIRS光谱。由于在石墨电极上,电解液的还原和Li离子的嵌入过程的电位区间有所重叠,因此原位红外光谱给出的红外吸收峰通常被归属于电解液还原和SEI膜形成过程。然而,我们的结果表明,红外光谱的变化主要是由于Li离子的嵌脱过程引起,在石墨电极上电解液的还原过程很难被观测到。与Sn电极上相比较不同的是,其原位红外谱峰的强度变小,且PF6-1吸收峰的变化规律不明显,这与石墨负极表面较大的粗糙度和和较低的反射率有关。电化学阻抗谱的研究结果表明,在循环过的Sn-Co合金和石墨负极上都形了一层SEI膜。在密闭条件下,非原位MFTIRS检测SEI膜的主要化学组成是ROCO2Li。同时也能观察到残留在电极表面电解液的红外吸收峰(主要是EC分子)。4.由于与Li离子的发生还原时还原电位较低,Cr2O3被认为一种很有前途的锂离子电池的负极材料。我们通过热处理方法在Cr金属上生长一层Cr2O3薄膜,并研究其在1 MLiClO4/PC中的电化学性能和界面反应。将Cr2O3薄膜厚度控制在纳米尺度范围,是为了减少Li离子嵌脱时的体积效应,以提高其电化学性能,也为了方便对Li离子嵌脱过程的机理进行研究。循环伏安研究表明,在首次电位负向扫描时,电解液在1.1 V开始分解,在0.59V给出阴极电流峰。Li离子与Cr2O3薄膜负极的嵌脱电流峰出现在0.02 V(负向扫描)和1.27 V(正向扫描)。充放电实验表明初始放电容量大于其理论容量,初始不可逆容量是首次放电容量的70%,主要归因于电解液的分解导致。由于Cr2O3薄膜的厚度为纳米级厚度,电解液还原是初始不不可逆容量的主要因素。从第二周到第十周,其充放电容量稳定在460 mAh·g-1左右。XPS和PM-IRRAS研究表明经过电化学循环的Cr2O3电极形成了一层较厚的SEI膜。SEI膜的化学组份为Li2CO3,且比较稳定,但其厚度或者密度在充放电过程中会变化。嵌Li过程时,Cr2O3的体积膨胀,导致SEI薄膜裂缝,使电解液继续分解,增加Li2CO3的量。脱Li时Cr2O3薄膜体积收缩,产生的应力使得SEI上部分Li2CO3脱落,从而减小其厚度。由于电化学循环后Cr2O3薄膜被一层较厚的SEI膜覆盖,通过IR和XPS不能给出关于Li离子与Cr2O3反应过程的信息,因此我们采用ToF-SIMS对其进行深度组份分析。ToF-SIMS结果证实了由Li离子的嵌脱过程所导致Cr2O3薄膜的体积变化,和Li离子在Cr2O3薄膜电极上的嵌脱过程受到限制(即使是纳米级厚度)。研究表明,在嵌Li后的Cr2O3薄膜电极上,Cr2O3外层(与电解液接触)和内层(与Cr金属基底接触)的化学组份不同,外层主要是由Li2O和微量的Cr组成,说明外层Cr2O3能够与Li完全反应,内层由Cr2O3,Cr的低价氧化物和Li2O组成,这说明内层Cr2O3不能和Li完全反应。在内外层之间,有一层由Cr金属(Li与Cr2O3反应生成)形成的致密的阻隔层,阻止了Li离子向内层扩散。5、Cr2S3的密度比Cr2O3小,这可能减小Li离子嵌脱过程中引起的体积变化效应。我们通过在H2S气氛中热处理Cr金属,制备了Cr2S3薄膜材料,并首次用于锂离子电池负极材料的研究。在研究其在1 M LiClO4/PC电解液中的电化学性能和界面反应中发现,首次电位负向扫描过程中,当电位低于0.85 V时候,给出阴极电流,在随后的电位负向扫描中,这个电位提高至1.12 V并在大约0.6 V给出一个电流峰。阴极电流峰主要对应于Li离子嵌入。在电位正向扫描的过程中,一个较宽的电流峰出现在2.0 V左右,对应于Li从Li2S中脱出。阴极和阳极电流峰的强度随扫描圈数增加而减少,指认为在循环过程中一些活性物质从电极上脱离引起。在Cr2S3薄膜电极上,没有观察到电解液的不可逆还原峰,这有助于降低Cr2S3电极上的不可逆容量。XPS研究发现,电化学循环后的Cr2S3被SEI膜覆盖,其主要组份是Li2CO3,但厚度比电化学循环后的Cr2O3上的SEI膜薄,因为在循环后的Cr2S3薄膜能检测到Cr的XPS信号,而在循环后的Cr2O3却不能检测到Cr的信号。电化学循环后的Cr2S3电极上,S的XPS信号衰减,主要由以下两个原因导致:1.部分Cr2S3颗粒在电化学循环过程中脱落;2.表面被SEI膜覆盖。XPS研究同时指出,Cr2O3经过电化学循环后部分Cr3+转换为Cr2+,说明Li不能完全脱出。6、制备了纳米结构V2O5正极薄膜材料,并研究了其在1 M LiClO4/PC电解液中的电化学性能和界面反应。当在2.8 V到3.8 V的电位区间扫描时,循环伏安曲线在3.39,3.42,3.18,3.22 V给出两对稳定的阴极/阳极峰,说明Li离子此电位区间的嵌脱是一个可逆的过程。在电化学循环后V2O5电极上,XPS只能观察到两种价态的V氧化物(V5+和V4+)。随着扫描周数的增加,V4+的比例提高,对应部分V5+还原到V4+。对于没经过电化学循环V2O5材料,V5+所占的比例为94%,经过1个循环后,V5+下降到90.1%。15周循环后,这一比例下降到83.3%。XPS和红外研究结果表明SEI膜的主要Li2CO3组成。V2O5材料上SEI膜的厚度远小于Cr2O3上SEI膜的厚度。而且其形成也较为困难,因为在循环1周后,XPS和EIS都很难检测到SEI膜的存在。本论文的研究工作,对深入认识锂离子电池的界面反应,发展相关的非水电解质理论具有重要的基础理论意义和应用价值。本论文首次提出并实现了原位红外光谱研究表征锂离子的嵌脱过程,并将这种方法由红外光反射率较高金属电极扩展到反射率较低的粉末材料电极。本论文还运用XPS和ToF-SISM对Cr2O3电极上SEI膜的形成变化及Li离子的嵌脱过程进行了研究,这对深化认识当前被广泛研究的氧化物负极的界面过程和进一步提高其性能具有指导意义。

【Abstract】 The interfacial reactions are the key issues that relate to cycling ability, lifetime,chemical and physical stability, and irreversible capacity of a lithium ion battery (LIB). Inaddition, studies of interfaces of LIB are of significance in revealing the structure ofnonaqueous interfaces and developing relevant models and theories. The main scope of thisdissertation is developing an approach to use FTIRS (in situ, ex situ), EQCM, XPS, andToF-SIMS to investigate the interfacial reactions of LIB. We have tried to investigate theinterfaces of Sn, Sn-Co alloy, graphite, Cr2O3 and Cr2S3 anodes as well as V2O5 cathode. Themain experiments and results are given follow:1.Sn and Sn-Co alloy anodes were prepared by electroplating. The betterelectrochemical performance on Sn-Co alloy than Sri anode confirms that inactive Cocomponent can buffer against volume change of Sn component during the alloying/dealloyingprocess.2. The interfacial reactions of Sn thin film anode in 1 M LiPF6/EC+DMC were in situinvestigated by MFTIRS and EQCM. When electrolyte is reduced, the measured massaccumulated per mole of electrons (mpe) values are smaller to the theoretical ones. However,in alloying/dealloying process, the measured mpe values are higher than the theoretical values.The lithiation/delithiation process was characterized by MFTIRS through the desolvation/solvation effect. The solvation/desolvation effect varies the concentration free solvent (sol:EC, DMC) and solvated solvent (Li(sol)n+) as well as causes the shifts of IR bands (C=O, C-O,C-H). In situ MFTIRS studies revealed that Li(sol)n+ species rather than free solvent wasreduced on Sn anode, and the reductive products of electrolyte are ROCO2Li.3. The interfacial properties of Sn-Co alloy and graphite film anodes in 1 MLiPF6/EC+DMC were investigated by using in situ and ex situ MFTIRS. In situ MFTIRSresults on Sn-Co alloy anode confirm that FTIRS is efficient to characterize thelithiation/delithiation process through desolvation/solvation effects. Taking the advantage ofgraphite thin film electrode with a high IR reflectivity which is prepared by spin coating, insitu IR spectra with an excellent signal-to-noise ratio are obtained. As the potential regions of electrolyte reduction and lithiation processes overlap partly on graphite anode, the change ofin situ IR spectra is frequently described to reduction of electrolyte and formation of SEI layer,rather than lithiation process. However, our in situ IR results suggest the variations of spectraare caused by the intercalation process. EIS studies confirm that the SEI layer is formed on acycled Sn-Co and graphite anodes, and ex situ MFTIRS determines that the layer consists ofROCO2Li.3. Cr2O3 thin films grown by thermal oxidation of Cr metal were investigated as anodematerial for LIB in 1 M LiClO4/PC. The initial capacity is larger than the theoretical capacitybecause of the decomposition of electrolyte. The stable charge/discharge capacity of460 mAh·g-1 was obtained in the 3rd-10th cycles. XPS and PM-IRRAS reveal the maincomposition of SEI layer is Li2CO3. This chemical composition is stable but there arevariations of the surface contents during the conversion/deconversion process. The volumeexpansion on the lithiated sample, evidenced by ToF-SIMS, presumably generates cracks inthe SEI layer that are filled by the immediate decomposition of electrolyte, thus increasing thesurface content in Li2CO3. The volume shrink of the delithiated oxide, also evidenced byToF-SIMS, is thought to generate the loss of fragments of the SEI layer due to compressivestress. ToF-SINS results demonstrate that the conversion/ deconverson processes of Li withCr2O3 are limited, most likely by mass transport, even for ultra-thin films.4. Cr2S3 film grown by thermal treatment of Cr metal under H2S atmosphere was testedas an anode material for LIB for the first time. The intensities of both cathodic and anodiccurrent peaks in CV curves are declined with increasing cycling number, which is partly dueto the exfoliation of active species. The strong irreversible cathodic peak, assigned to thereductive electrolyte decomposition, is not observed on Cr2S3 film. XPS studies indicate thatthe main composition of the SEI layer is Li2CO3 species, and its thickness is thinner than thatof cycled Cr2O3 anode. The XPS signal of S2p on the cycled sample is dramatically attenuatedfor the following two reasons: 1 .part of Cr2S3 particles are exfoliated from the sulfide film; 2.the surface is covered by SEI layer. The XPS studies suggest that Li is trapped and thevalence of Cr decreases for 3+ to 2+ after the electrochemical cycles.5. Interfacial reactions on nanostructured V2O5 thin film cathode were ex situinvestigated by utilization of the XPS technique. The CV results evidence that Li intercalation is quasi reversible in this range of potential 2.8-3.8 V. With the increasing of cycle number,the change of the V2p3/2 core level peak results from the decrease of the higher bindingenergy peak at 517.9 eV (V5+)and the increase of the lower binding energy peak at 516.5 eV(V4+), which is due to the partial reduction of V from 5+ into 4+. This proportion decrease ofV5+ and V4+ reveals that Li is trapped in the oxide film. XPS and PM-IRRAS of V2O5 thinafter 15 charge-discharge cycles in 1 M LiClO4/PC suggest the SEI layer consist of mainlyLi2CO3 species. However the formation of SEI is not as easy as on Cr2O3 anode, for both XPSand EIS do not detect the formation of SEI layer after 1 CV cycle.The results of this dissertation throw insight into electrode/electrolyte interfacialreactions, and are of significance in developing relevant fundamental theory. The present IRresults provide firstly an approach to probe the lithiation/delithiation process of LIB by in situFTIR reflection spectroscopy, and are also of importance for the analysis of other LIB systems,especially the powder electrode materials. The detail research on Cr2O3 anode by XPS andToF-SIMS, is helpful to understand the electrochemical processes and improve theirelectrochemical performance of transition oxide materials, which are intensively studiedrecently.

  • 【网络出版投稿人】 厦门大学
  • 【网络出版年期】2009年 11期
  • 【分类号】TM912
  • 【被引频次】4
  • 【下载频次】1866
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