Flexible electrodes with high areal capacity based on electrospun fiber mats

The ever-growing portable, flexible, and wearable devices impose new requirements from power sources. In contrast to gravitational metrics, areal metrics are more reliable performance indicators of energy storage systems for portable and wearable devices. For energy storage devices with high areal metrics, a high mass loading of the active species is generally required, which imposes formidable challenges on the current electrode fabrication technology. In this regard, integrated electrodes made by electrospinning technology have attracted increasing attention due to their high controllability, excellent mechanical strength, and flexibility. In addition, electrospun electrodes avoid the use of current collectors, conductive additives, and polymer binders, which can essentially increase the content of the active species in the electrodes as well as reduce the unnecessary physically contacted interfaces. In this review, the electrospinning technology for fabricating flexible and high areal capacity electrodes is first highlighted by comparing with the typical methods for this purpose. Then, the principles of electrospinning technology and the recent progress of electrospun electrodes with high areal capacity and flexibility are elaborately discussed. Finally, we address the future perspectives for the construction of high areal capacity electrodes using electrospinning technology to meet the increasing demands of flexible energy storage systems.

Taming Polysulfides in an Li-S Battery With Low-Temperature One-step Chemical Synthesis of Titanium Carbide Nanoparticles From Waste PTFE

In this work, titanium carbide (TiC) nanoparticles have been successfully synthesized at much lower temperatures of 500°C using cheaper starting materials, such as waste polytetrafluoroethylene (PTFE) (carbon source) and titanium and metallic sodium, than the traditional carbothermal reduction of TiO₂ at 1,800°C. An XRD pattern proved the formation of face-centered cubic TiC, and TEM images showed the obtained TiC nanoparticles with an average size of approximately 50 nm. In addition, the separator coated with TiC nanoparticles as an active material of interlayer effectively mitigates the shuttling problem by taming the polysulfides in Li–S batteries compared with a traditional celgard separator. The assembled cell realizes good cycling stability with 501 mAh g⁻¹ and a low capacity fading of 0.1% per cycle after 300 cycles at 1 C due to high utilization of the sulfur-based active species.

Manipulating Particle Chemistry for Hollow Carbon-based Nanospheres: Synthesis Strategies, Mechanistic Insights, and Electrochemical Applications

Hollow carbon-based nanospheres (HCNs) have been demonstrated to show promising potential in a large variety of research fields, particularly electrochemical devices for energy conversion/storage. The current synthetic protocols for HCNs largely rely on template-based routes (TBRs), which are conceptually straightforward in creating hollow structures but challenged by the time-consuming operations with a low yield in product as well as serious environmental concerns caused by hazardous etching agents. Meanwhile, they showed inadequate ability to build complex carbon-related architectures. Innovative strategies for HCNs free from extra templates thus are highly desirable and are expected to not only ensure precise control of the key structural parameters of hollow architectures with designated functionalities, but also be environmentally benign and scalable approaches suited for their practical applications.In this Account, we outline our recent research progress on the development of template-free protocols for the creation of HCNs with a focus on the acquired mechanical insight into the hollowing mechanism when no extra templates were involved. We demonstrated that carbon-based particles themselves could act as versatile platforms to create hollow architectures through an effective modulation of their inner chemistry. By means of reaction control, the precursor particles were synthesized into solid ones with a well-designed inhomogeneity inside in the form of different chemical parameters such as molecular weight, crystallization degree, and chemical reactivity, by which we not only can create hollow structures inside particles but also have the ability to tune the key features including compositions, porosity, and dimensional architectures. Accordingly, the functionalities of the prepared HCNs could be systematically altered or optimized for their applications. Importantly, the discussed synthesis approaches are facile and environmentally benign processes with potential for scale-up production.The nanoengineering of HNCs is found to be of special importance for their application in a large variety of electrochemical energy storage and conversion systems where the charge transfer and structural stability become a serious concern. Particular attention in this Account is therefore directed to the potential of HCNs in battery systems such as sodium ion batteries (NIBs) and potassium ion batteries (KIBs), whose electrochemical performances are plagued by the destructive volumetric deformation and sluggish charge diffusion during the intercalation/deintercalation of large-size Na⁺ or K⁺. We demonstrated that precise control of the multidimensional factors of the HCNs is critical to offer an optimized design of sufficient reactive sites, excellent charge and mass transport kinetics, and resilient electrode structure and also provide a model system suitable for the study of complicated metal-ion storage mechanisms, such as Na⁺ storage in a hard carbon anode. We expect that this Account will spark new endeavors in the development of HCNs for various applications including energy conversion and storage, catalysis, biomedicine, and adsorption.

MOF-derived fluorine and nitrogen co-doped porous carbon for an integrated membrane in lithium–sulfur batteries

The F and N co-doped porous carbon derived from ZIF-8 is used as a membrane in Li–S batteries with enhanced capacity and cycling stability.

Frogspawn inspired hollow Fe3C@N-C as an efficient sulfur host for high-rate lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries with high theoretical energy densities of ∼2600 W h kg^-1 have been recognized as a promising energy storage device. However, the practical application of Li-S batteries is still limited by the cycle stability and rate capability, which is highly relied on the well-designed cathode material. Inspired by the unique structure of frogspawn in Nature, a hollow Fe₃C@N-C with frogspawn-like architecture was successfully constructed as a highly efficient sulfur host in this paper. Derived from a Prussian blue self-template, Fe₃C@N-C possesses a metal-like Fe₃C spawn core and the high conductivity of an N-doped carbon shell. This unique structure enables a large surface area, fast e^-/Li⁺ transport, as well as a large hollow space for the volumetric expansion of the sulfur cathode. Moreover, with the N-doped carbon shell and the polar Fe₃C core, the trapping and catalytic conversion of intermediate polysulfides are also facilitated. The strongly coupled interaction of polar Fe₃C and polysulfides is confirmed by both theoretical calculations and electrochemical performance. Specifically, the Fe₃C@N-C/S electrode presents a high capacity of 1351 mA h g^-1 at 0.1C with the Fe₃C@N-C as an integrated sulfur host. In particular, the rate capability and cycling stability of the Fe₃C@N-C/S electrode is outstanding. It displays a high capacity of 792 mA h g^-1 at 5C and a low capacity decay rate of 0.08% per cycle at 0.5C after 400 cycles. This work opens a convenient and economical avenue to design a frogspawn-like hollow metal carbide/carbon as an efficient sulfur host for advanced Li-S batteries.