Self-Assembled Porous-Silica within N-Doped Carbon Nanofibers as Ultra-flexible Anodes for Soft Lithium Batteries

摘要

class="head no_bottom_margin" id="sec1title">IntroductionThe evolving soft electronics and portable electronics require soft, lightweight, and durable lithium (Li)-ion batteries (LIBs) as power sources (, , , , , , ). SiO2 has been highlighted as a promising anode material owing to its large specific capacity as part of the development of high-energy-density LIBs (, , ). Although the huge volume changes of SiO2 and the side reactions with the electrolytes during cycling were identified as the main reasons of short cycle lives of SiO2 anodes (, , ), it has been recently recognized that nanoscale designs contributed substantially to improve the cycle lives (, , href="#bib43" rid="bib43" class=" bibr popnode">Wu et al., 2012, href="#bib46" rid="bib46" class=" bibr popnode">Yang et al., 2014, href="#bib48" rid="bib48" class=" bibr popnode">Yuan et al., 2016).However, achieving long-term stability still remains challenging for the nanoscale anodes with commercial level of SiO2 loadings (e.g., >1 mg/cm²). For one thing, SiO2 nanoparticles (NPs) were susceptible to separate from the conductive additives during the huge volume changes, which easily led to structural degradation and unstable solid-electrolyte interface (SEI) (href="#bib3" rid="bib3" class=" bibr popnode">Aricò et al., 2005, href="#bib34" rid="bib34" class=" bibr popnode">Sasidharan et al., 2011, href="#bib38" rid="bib38" class=" bibr popnode">Tu et al., 2014, href="#bib45" rid="bib45" class=" bibr popnode">Yan et al., 2013). For another, SiO2 was commonly known to be an intrinsic electronically insulating material owing to its wide band gap (href="#bib36" rid="bib36" class=" bibr popnode">Sun et al., 2008). Although coating SiO2 with carbon materials was a solution to such problems, the capacities of the LIBs that contained high SiO2 loading anodes also fade quickly (href="#bib1" rid="bib1" class=" bibr popnode">An et al., 2017, href="#bib21" rid="bib21" class=" bibr popnode">Kim et al., 2014). In addition, from a practical perspective, it is still a great challenge to manufacture large-scale flexible electrodes with low resistances by using the available thin-film technologies. The conventional SiO2 anodes utilizing binders and metal current collectors were typically too rigid and heavy, and therefore could not meet the requirements of soft battery applications (href="#bib9" rid="bib9" class=" bibr popnode">Dirican et al., 2015, href="#bib17" rid="bib17" class=" bibr popnode">Jang et al., 2009, href="#bib50" rid="bib50" class=" bibr popnode">Zhou et al., 2012).Here, we propose a scalable strategy of manufacturing hierarchical structured anodes with commercial level of SiO2 loadings and durable flexibility to tackle the aforementioned problems. Having noticed that the SEI stability and stress dissipation during the volume expansion of SiO2 are key to the robust operation of SiO2 anodes (href="#bib7" rid="bib7" class=" bibr popnode">Chen et al., 2017), porous SiO2 (p-SiO2) NPs were first synthesized via a sol-gel technique by employing the positively charged cetyltrimethyl ammonium bromide (CTAB) as a soft template (href="/pmc/articles/PMC6545390/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figure 1A) (href="#bib40" rid="bib40" class=" bibr popnode">Wang et al., 2016a). We did not wash the residual CTAB off after the reactions because a small amount of CTAB made these p-SiO2 NPs easier to be dispersed in the spinning solutions. The p-SiO2 NPs shared the characteristics of limited volume changes of nanoscaled SiO₂ and sufficient porosity for SiO₂ volume expansions (href="#bib1" rid="bib1" class=" bibr popnode">An et al., 2017). Then a sol-gel electrospinning technique was developed to assemble these NPs into polyacrylonitrile (PAN) precursor nanofibers (NFs). This strategy allowed the positively charged p-SiO₂-CTAB composites to electrostatically attract the negatively charged PAN in the spinning solution as shown in href="/pmc/articles/PMC6545390/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figure 1A, which led to uniformly distributed p-SiO₂ nanoclusters within the matrix of PAN NFs. During the subsequent calcination process, the CTAB was decomposed and the PAN was converted into N-doped carbon nanofibers (N-CNFs), consequently forming overlapped p-SiO₂ nanoclusters within N-CNFs (href="#bib35" rid="bib35" class=" bibr popnode">Sui et al., 2016, href="#bib47" rid="bib47" class=" bibr popnode">Ye et al., 2017).href="/pmc/articles/PMC6545390/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">class="inline_block ts_canvas" href="/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=6545390_gr1.jpg" target="tileshopwindow">target="object" href="/pmc/articles/PMC6545390/figure/fig1/?report=objectonly">Open in a separate windowclass="figpopup" href="/pmc/articles/PMC6545390/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">Figure 1Fabrication of the Ultra-flexible Anodes of p-SiO₂@N-CNF and Their Applications in Soft LIBs(A) Schematic illustration for the electrostatic assembly of p-SiO₂ NPs containing CTAB with PAN followed by electrospinning and carbonization to fabricate flexible p-SiO₂@N-CNF anode.(B) TEM image of the honeycomb-structured p-SiO₂ NPs.(C) An SEM image of the nanofibrous film of p-SiO₂@N-CNF with interconnected architectures. The inset figure is a single CNF that contained overlapped p-SiO₂ nanoclusters.(D) The inner structures of typical soft Li-batteries that contained the flexible p-SiO₂@N-CNF anode and LiFePO₄ (LFP) cathode.(E) The digital image of the soft Li-battery.