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Surfactant-mediated synthesis of graphene-TiO2 nanocomposites for lithium-ion batteries.

机译：表面活性剂介导的锂离子电池石墨烯-TiO2纳米复合材料的合成。

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摘要

Metal oxides are attractive electrode materials for lithium-ion batteries due to their inherent safety from Li electroplating and their mechanical and chemical robustness during operation. However, poor electronic conductivity limits bulk metal oxides to slow charging rates. In addition to nano-sized oxide particles, a conductive phase is often used to boost high-rate Li-storage performance. Functionalized graphene sheets (FGSs), which have a high electrical conductivity and theoretical specific surface area, are ideal for this purpose. In a proof-of-concept study, hybrid nanocomposites of titanium dioxide (TiO 2) and FGSs were produced via an aqueous surfactant-mediated approach using sodium dodecyl sulfate (SDS). The Li-storage capacities of FGS-TiO 2 nanocomposites were significantly enhanced compared to TiO2 nanoparticles, particularly at high chargedischarge rates. Nevertheless, neither the synthesis nor the mechanisms for enhanced performance are well understood.;In the synthesis of FGS-TiO2 nanocomposites, SDS acts to disperse FGSs in the aqueous reaction medium and promote the growth of TiO2 on FGSs. The key process for both is the adsorption of SDS onto FGSs, which is investigated in the first part of this thesis using a conductometric surfactant titration technique. Based on an analysis of the titrations of deionized water and aqueous FGS suspensions, a four-stage adsorption model is proposed: (i) adsorption of isolated monomers at low concentrations; (ii) formation of an adsorbed monolayer at full coverage, above a bulk concentration of ~12 muM; (iii) formation of hemi-cylindrical surface micelles, above a critical bulk concentration of ~1.5 mM; and (iv) micelle formation in the bulk solution, above a concentration of ~8 mM SDS in the bulk.;In the second section of this thesis, SDS adsorption is directly related to the colloidal stability of FGSs. Optical microscopy was used to observe FGSs dispersed in solutions with a range of SDS concentrations: branched aggregates formed in the absence of SDS, a transition to compact aggregate formation was observed as the concentration increased to 10 muM, and dispersed FGSs were imaged at concentrations ≥ 20 muM SDS. UV-Vis absorbance was used to quantify FGS settling during centrifugation, and the largest decreases in absorbance were measured in suspensions where compact aggregates were observed. Persistent settling was observed at SDS concentrations ≥ 20 muM, indicating compact aggregates had formed either through restructuring of branched aggregates or aggregation of initially dispersed FGSs. These results were supported by a simple interaction energy calculation. Ultimately, it was determined that stably dispersed FGSs can be obtained at SDS concentrations above ~40 muM. Higher SDS concentrations are necessary to stably disperse FGSs at the ionic strength of the FGS-TiO2 reaction.;The last section of this thesis investigates the influence of reaction parameters on the properties of several FGS-TiO2 samples. The high-rate Li-storage performance was characterized by estimating the amount of power lost to energy dissipation, and clear differences between the samples were observed, particularly at high mass loadings. This is attributed to differences in the resistive losses that occur in the cells during the transport of lithium ions and electrons in FGS-TiO2 as well as the transport of lithium ions in the electrolyte. Characterization of the electrode materials showed that electron transport might not be the main source of performance enhancement; instead, Li+ transport in both the electrode and the electrolyte likely has a greater impact on electrochemical performance of this system.
机译：金属氧化物是锂离子电池极具吸引力的电极材料，这是因为它们具有锂电镀的固有安全性以及操作过程中的机械和化学稳定性。但是，较差的电子电导率限制了块状金属氧化物以降低充电速度。除纳米级氧化物颗粒外，导电相通常用于提高高速率锂存储性能。具有高电导率和理论比表面积的功能化石墨烯片（FGS）是实现此目的的理想选择。在概念验证研究中，二氧化钛（TiO 2）和FGS的杂化纳米复合材料是使用十二烷基硫酸钠（SDS）通过表面活性剂介导的水性方法制备的。与TiO2纳米颗粒相比，FGS-TiO 2纳米复合材料的锂存储能力显着提高，尤其是在高充电率下。然而，无论是合成还是增强性能的机理都没有被很好地理解。在FGS-TiO2纳米复合材料的合成中，SDS的作用是将FGS分散在水性反应介质中，并促进TiO2在FGS上的生长。两者的关键过程是SDS在FGS上的吸附，这是在本文的第一部分使用电导表面活性剂滴定技术进行研究的。在对去离子水和FGS水性悬浮液的滴定度进行分析的基础上，提出了一个四阶段吸附模型：（i）低浓度下分离出的单体的吸附； （ii）在〜12μM的体积浓度以上完全覆盖形成吸附单层； （iii）超过〜1.5 mM的临界堆积浓度时，形成了半圆柱形表面胶束； （iv）在本体溶液中浓度大于约8 mM SDS时，在本体溶液中形成胶束。在本论文的第二部分，SDS的吸附与FGS的胶体稳定性直接相关。光学显微镜用于观察分散在一定浓度SDS溶液中的FGS：在没有SDS的情况下形成支链聚集体，当浓度增加到10μM时观察到向紧密聚集体的过渡，并且在浓度≥ 20μMSDS。 UV-Vis吸光度用于量化离心过程中的FGS沉降，在观察到紧密聚集体的悬浮液中，吸光度下降最大。在SDS浓度≥20μM时观察到持久沉降，表明紧密的聚集体是通过重组分支聚集体或最初分散的FGS聚集而形成的。这些结果得到了简单的相互作用能计算的支持。最终，确定了在〜40μM以上的SDS浓度下可以获得稳定分散的FGS。为了使FGS稳定地分散在FGS-TiO2反应的离子强度下，需要较高的SDS浓度。本论文的最后一部分研究了反应参数对几种FGS-TiO2样品性能的影响。高速率锂存储性能的特征在于估计能量耗散的功率损失，并且观察到样品之间的明显差异，尤其是在高质量负载下。这归因于在FGS-TiO2中锂离子和电子的传输以及电解质中锂离子的传输过程中，电池中发生的电阻损耗差异。电极材料的表征表明，电子传输可能不是提高性能的主要来源；反之亦然。相反，在电极和电解质中的Li +传输都可能对该系统的电化学性能产生更大的影响。

著录项

  • 作者

    Hsieh, Andrew.;

  • 作者单位

    Princeton University.;

  • 授予单位 Princeton University.;
  • 学科 Engineering Chemical.;Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 136 p.
  • 总页数 136
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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