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发展可再生能源是改善能源结构的重要途径，是解决目前环境问题的有效手段，但是风能、太阳能等可再生能源发电存在不稳定、不连续的问题，需要配套稳定、高能量密度的储能系统。锂硫液流电池是锂硫电池与液流电池相结合的一种新型液流电池体系，以金属锂为负极，硫或者硫复合材料的悬浮溶液为正极（流体正极），具有能量密度高、成本低、工作温度范围广、无毒害等优点，是一项非常有前景的储能技术。然而硫电极导电性差、多硫化锂的穿梭效应等锂硫电池的主要问题，在锂硫液流电池中依然存在，特别是穿梭效应，在液流电池体系中由于需要使用更多的电解液来形成流动体系，被进一步放大。同时，流体正极还存在稳定性差、粘度高等问题。本论文通过对正极材料进行结构优化设计，提高了流体正极的稳定性；首次探讨了通过离子液体桥接作用调控多硫化物的穿梭效应，并研究了相关机理；通过聚合物对正极材料进行功能化处理，提高了流体正极的能量密度和低温性能；研究了流动模式和流动速度对流体电极充放电性能的影响，主要的创新性成果如下：（1）高能量密度流体正极的设计和制备。通过原位氧化还原法制备了硫-科琴黑（S-KB）复合材料，相对于机械混合硫和KB的方式制备的流体电极，采用S-KB复合材料的流体电极表现出了较高的放电比容量，突出的倍率放电性能。通过在S-KB流体电极中加入聚乙二醇辛基苯基醚（Triton X-100）来降低流体电极的粘度，制备了硫含量高、流动性强的流体电极，比能量达到600 Wh L-1。同时，Triton X-100抑制了活性物质流失，提高了循环稳定性。（2）调控硫复合材料微观结构，设计制备具有自稳定特性的流体电极。通过控制Zeta电位，原位制备了三明治结构的S-KB-rGO复合材料。当S-KB-rGO分散在电解液中，超支化的KB和层层交织的rGO形成了三维的导电和负载网络，浸渍在其中的硫具备了较强的悬浮性、流动性和电化学稳定性。S-KB@rGO流体电极可以搁置30天而不发生沉降；硫的放电比容量达到1532 mAh g-1，1C循环超过1000次，自放电低至1.1%/天。S-KB@rGO流体电极在流动模式下具有较高的电化学活性和循环稳定性；液流电池的放电平台随着流速的升高而提高，说明S-KB@rGO悬浮液具有非牛顿流体的特性，其粘度随着流速的升高而降低，有利于锂离子的迁移。自稳定流体电极的设计概念为其它半固态流体电极的研究提供了参考。（3）通过离子液体纳米颗粒“桥接”作用调控多硫化锂的穿梭效应，提高锂硫液流电池的循环性能。多硫化锂在电解液中的溶解是一把双刃剑，一方面它是穿梭效应的根源，这是锂硫液流电池失效的主要原因；另一方面多硫化锂提高了硫的电化学反应活性，因此，只有将溶解的多硫化锂有效的控制在正极区域才能同时提高流体电极的活性和循环稳定性。本文中采用一种特殊结构的离子液体纳米颗粒SiO2-PPCl与载体材料复合，它可以通过甲氧基和胺基同时与多硫化锂和碳负载材料成键，作为硫和负载材料之间“桥梁”，在充放电过程中调控多硫化物的溶解和沉积，抑制了多硫化物的迁移，并提高了硫电极的反应动力学。SiO2-PPCl功能化的流体电极循环1000次后仍有95.4%的容量，且库伦效率保持在99%。离子液体桥接多硫化物的策略也为锂硫电池的发展提供了新的思路。（4）设计制备了低温、高能量密度流体正极。采用PVP对硫复合材料进行表面功能化处理制备低温S-KB-G@P流体电极，该电极在-30℃的能量密度和峰值功率密度分别为445 Wh L-1、22.5 mW cm-2，且具有稳定的循环性能（-30℃循环200次容量几乎没有衰减）。S-KB-G@P流体电极优良的低温性能源于硫复合材料表面的PVP降低了悬浮颗粒的作用力，抑制了团聚，从而降低了悬浮液的粘度、促进了离子迁移；同时PVP加强了石墨烯和KB在悬浮液中的分散，形成了连续的导电网络，因此S-KB-G@P流体电极在低温下具有较高的离子和电子电导率。双亲性的PVP可与非极性碳负载材料和极性多硫化锂形成较强的相互作用，抑制多硫化物迁移，从而提高了电极的循环稳定性。;Developing renewable energy is an important strategy to improve energy structure and solve environmental problems. High-energy-density and stable energy storage systems are crucial for extensive deployment of renewable energy due to its intermittent and unstable natures, such as solar and wind systems. Lithium-sulfur flow battery is a new type of flow battery system combining lithium-sulfur battery and flow battery, its anode is lithium, and its cathode is sulfur or sulfur composite suspension (catholyte). Lithium-sulfur flow battery has the advantages of high energy density, low cost, wide working range and non-toxic etc., and is a very promising energy storage technology. However, the main problems in lithium-sulfur battery, such as poor conductivity of sulfur electrode and shuttle effect of lithium polysulfides (LPS), still exist in lithium-sulfur flow battery, especially shuttle effect is more serious in the flow system as more electrolyte must be used to form flow configuration. Meanwhile, the sulfur suspension catholyte also has the problems of poor stability and high viscosity. In this dissertation, the stability of the suspension catholyte was improved by the structure optimization design of catholyte materials; the shuttle effect was modulated by ionic liquid polysulfieds bridgebuilder, and the related mechanism was analyzed for the first time; high-energy-density and low-temperature suspension catholyte was achieved by the functional processing of the catholyte materials; the effects of flow mode and flow velocity on the charge/discharge performance of the suspension electrode were studied. Major innovative results are summarized as follows:(1) High-energy-density sulfur suspension catholyte was designed and prepared, and the effects of surfactant on the physical and electrochemical properties of the suspension catholyte were studied. The sulfur-Ketjenblack (S-KB) composite was prepared by in-situ deposition to improve the conductivity of sulfur. Compared with the sulfur suspension catholyte prepared by mechanical mixing sulfur and KB, the S-KB catholyte showed higher discharge capacity and better discharge performance. The viscosity was reduced by introducing Triton X-100 into the S-KB suspension catholyte, meanwhile, Triton X-100 adsorbed polysulfide and sulfide species, thus inhibited the loss of active substances and improved the cycle stability of suspension catholyte.(2) A self-stabilized suspension catholyte was designed and prepared by optimizing the structure of sulfur composite. Sandwich-structure S-KB-rGO composite was prepared in situ by controlling Zeta potential. When S-KB-rGO was dispersed in the electrolyte, the hyperbranched KB and the interconnected rGO sheets formed a three-dimensional conductive and load network, The impregnated sulfur in the network had good suspensibility, flowability and electrochemical stability. No deposition was found in the S-KB@rGO suspension after 30-days rest, the specific capacity of the catholyte was 1532 mAh g-1, upon 1000 cycle was achieved, and the self-discharge rate was 1.1% per day. The S-KB@rGO catholyte was operated in the flow battery equipmennt and showed high electrochemical activity and stability at flow mode. Meanwhile, the discharge voltage rose with the increase of flow rate, indicating that the S-KB@rGO suspension had the characteristics of non-newtonian fluid, whose viscosity reduced with increasing flow velocity, which facilitated ion transfer. The design concept of self-stabilized catholyte provides references for other suspension electrodes. (3) The shuttle effect was modulated and cycle performance of the suspension catholyte was effectively improved by ionic liquid polysulfides bridgebuilder. The dissolution of LPS is a double-edged sword, on one hand, it is the root cause of shuttle effect, which leads to the failure of flow cell; on the other hand, it improves electrochemical reactivity of the catholyte. Therefore, the kernel is controlling LPS migration in cathode zone to balance cycle performance as well as kinetic performance. An ionic liquid nanoparticles SiO2-PPCl was adopted to control shuttle effect, it formed chemical bonding with polysulfides and carbon load materials by its methoxyl and amino groups, which made SiO2-PPCl act as a bridge between carbon loading materials and polysulfides, the polysulfides dissolution and consequent shuttle effect were modulated, and the kinetic reaction of catholyte was improved. The SiO2-PPCl functionalized catholyte showed over 1000 cycels with a capacity retention ratio of 95.4%, and the coulombic ration was about 99%. The approach of exploiting polysulfides bridgebuilder to control LPS shuttle also offers a new direction to develop lithium-sulfur batteries.(4) A high-energy-density, low-temperature sulfur suspension catholyte was designed and prepared. The S-KB-G@P suspension catholyte was prepared by surface functionalization treatment of PVP on sulfur composite. The energy density and peak power density of the S-KB-G@P suspension catholyte was 445 Wh L-1, 22.5 mW cm-2 respectively at -30℃, and the cycle was stable (200 cycles without obviously dacay). The excellent low-temperature performance of the S-KB-G@P catholyte stems from the fact that PVP on the surface weakens agglomeration between materials, and thus reduces the viscosity of the suspension, which is beneficial for ion transfer; meanwhile, PVP enhances the dispersion of rGO and KB in the suspension, thus a continuous conductive network is formed by rGO and KB, therefore, S-KB-G@P catholyte has high conductivity at low temperature. Amphipathic PVP formed chemical bonding with carbon loading materials and polysulfides, which inhibited the migration of polysulfides and improved the cyclic stability.
|徐松. 锂硫液流电池流体正极设计、制备及性能研究[D]. 中国科学院大学,2018.|
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