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多途径实现高体积能量密度锂硫电池

2016-12-23 10:24:00来源:20区编辑:转角遇见你

多途径实现高体积能量密度锂硫电池

锂硫电池有望实现商业化应用并成为锂离子电池的理想替代品,但诸多问题限制了其进一步的实用化生产,其中其体积能量密度是最容易被忽视的问题之一。通常,正极中碳材料比例较高,加之硫密度较低(2.07 mg cm?3),使得正极中单位体积中发挥作用的活性物质量有限。其次,为保证活性物质的充分发挥,电解液与硫的比例想多较高,这在降低了成品电池的质量能量密度的同时,也极大的降低了其体积能量密度。因此,如何实现紧实的锂硫电池正极设计,从而提高电极的体积能量密度极为重要。

Figure 1. A schematicillustration of the concept of coupling hierarchical sulfur composite based ona hybrid host with in situ cross-linked binderin order to fabricate stablehigh-loading cathodes. Step 1 starts with hybridizing two individual 2Dnanosheets (graphene and g-C3N 4) to form thenanosheet-type host material; instep 2, hierarchical sulfur composites are formed as large multi-micrometersized secondary particles formed as aconsequence of evaporation inducedself-assembly and sulfur infusion; step 3 involves in situ cross-linking of thecarboxymethyl cellulose binder inthe presence of the sulfur composites.

近日,滑铁卢大学的Nazar教授课题组采用上图一所示的方法,通过原位交联的方式复合粘结剂CMC与正极物料,再通过传统涂覆的方式实现了14.9 mgcm?2高面负载硫电极的制备,更重要的是其电解液/硫的比例仅需3.5:1 (uL:mg),极大的解决了体积能量密度低的问题,在稳定循环的前提下可实现14.7 mA h cm ?2的面比容量。

Figure 2. a) Aschematic illustration of the cross-linking of CMC binder with CA as thelinker, based on the esterifiation of the –OH groups in CMCand the –COOH groupsin CA at 150 °C. b) The FTIR spectra of the pristine CA, CMC, cross-linkedCMC-CA (without the sulfur composite) and thein situ cross-linked sulfurelectrodes. c–e) The SEM images of the surface of sulfur cathodes fabricatedusing (c) PVDF binder and (d,e) cross-linkedCMC-CA binder.

2 (a)表明了反应的原理是通过CMC中的-OHCA中的-COOH酯化作用进行的,对应的红外光谱显示了CACMC和在极片中交联后粘结剂的特征峰的存在。而SEM表征则更为直观的显示出原位交联粘结剂在高负载的情况下仍然保持十分完整紧实的形貌,对应的PVDF粘结剂涂覆的电极则出现常见的开裂问题。

Figure3. a) The discharge/charge voltage profies (at C/20) and b) cycling stability(at C/2) of the NG-CN/PVDF, NG-CN/CMC-CA, g-C3N 4/PVDF, andNdC/PVDF sulfurcathodes with a low sulfur loading of ≈2.0 mg cm ?2; electrodes wereconditioned at C/20 before cycling at C/2. c,d) The voltage profies of theNG-CN/CMC-CA sulfur cathodes with varied sulfur loadings as a function of (c)mass specifi capacity and (d) areal capacity at 0.5 mA cm ?2.e) The cyclingstability of the NG-CN based sulfur cathodes fabricated with PVDF, non- andcross-linked CMC-CA binders with sulfur loading of5.2 mg cm ?2 at 1.0 mA cm ?2.f) The cycling stability of NG-CN/CMC-CA cathodes with sulfur loadings of 10.2and 14.9 mg cm ?2 at 1.0 mA cm ?2; theinset shows the voltage profies of the14.9 mg cm ?2 electrode at 11th cycle, with signifiant voltage flctuationindicating dendrite formation.

电化学表征进一步佐证了通过原位交联剂交联多功能化和多级结构硫正极复合物的方式可实现高电化学表现。氮掺杂的石墨烯和石墨化C3N4有很高的导电性和很强的多硫化物吸附性能。微米级颗粒的正极材料形成紧实的高负载量电极,硫比例下可达到14.9mgcm?2。其次,研究人员指出锂负极的问题是进一步实现更高负载量锂硫电池稳定循环的关键。

石墨烯广场(GrapheneSquare)

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