Applications of electrospinning in post-lithium-ion batteries

The worldwide energy consumption is growing rapidly nowadays and renewable energies are playing a critical role in alleviating this energy crisis. Batteries, as sustainable energy storage systems, are emerging as the key solutions to effectively integrate renewable resources. The lithium-ion battery is the most widely used rechargeable battery nowadays; however, limitations in uneven distribution of raw materials and high risk of increasing costs have motivated researchers to seek for alternatives such as sodium-ion batteries or metal sulfur batteries, mostly summarized as post-lithium-ion batteries. Different storage mechanisms and new materials are potentially able to maintain structural stability and offer high energy density, among which nanomaterials have attracted a lot of attention. As a low-cost, controllable and scalable method of nanomaterial fabrication, electrospinning was applied in three different aspects in post-lithium-ion batteries in this work: (i). Carbon nanofibers (CNFs) were synthesized as anode material in sodium-ion batteries; (ii). Nanocomposites from maricite NaFePO${}_4$ and carbon nanofibers were developed as cathode material in sodium-ion batteries; (iii). Modified separator aided with electrospinning was applied in magnesium-sulfur batteries. (i). Free standing carbon nanofibers in the form of fiber mats were successfully prepared by electrospinning and directly used as negative electrode in sodium-ion batteries. Different carbonization temperatures were carried out to generate various carbon structures containing both graphitic domains and defects. Morphologies of the electrodes before and after cycling were analyzed. From extensive electrochemical analysis such as cyclic voltammetry and in situ Raman spectroscopy, the behavior of each kind of CNFs was characterized. The CNFs carbonized at 600 and 800 °C exhibited a capacitive behavior from surface charging while the CNFs treated at 1000 °C followed a mixed diffusion-controlled and surface-controlled behavior. In situ Raman spectra showed that the G-band was shifted upon discharge, which proved an intercalation of sodium ions into the carbon host. However, the irreversible high capacity loss in the first cycle is the main reason of hindering the application of such CNFs as anode materials in sodium-ion batteries. (ii). A self-standing, collector-free, binder-free maricite NaFePO${}_4$ / carbon nanofiber hybrid positive electrode was successfully prepared via electrospinning. An increased capacity was observed in the initial cycles and finally the electrode reached a capacity of 108 mAh g${}^{-1}$ based on the mass of active material NaFePO${}_4$ (30 wt%) at a 0.1C rate after 200 cycles. In comparison, a slurry-based NaFePO${}_4$ electrode yields a capacity of only around 20 mAh g${}^{-1}$. A comprehensive study of the hybrid electrode's morphology, structure, and electrochemical performance was conducted. Mössbauer spectra showed an increasing Fe${}^{3+}$ content from 43.2% at the first charged state to 63.0% at the charged state after 100 cycles, demonstrating an activating process of maricite NaFePO${}_4$ upon extensive charge-discharge cycling. This study on self-standing positive electrodes revealed the change in activity of maricite NaFePO${}_4$ from electrochemical inactive to active within the network of carbon nanofibers upon charge-discharge cycles. (iii). Magnesium-sulfur (Mg-S) batteries, with a high theoretical volumetric energy up to 3200 Wh/L, are considered to be competitive post-lithium battery systems. However, the known "shuttle effect" of soluble polysulfides during cycling typically leads to rapid capacity fade that greatly hinders their practical applications. Although considerable efforts have been made in cathode design and electrolyte development, the functionalization of separators is often neglected. In this study, a one-side coated glass fiber separator with polyoxometalate/carbon composite materials was prepared via electrospinning as a "polysulfide-phobic" shield. Mg-S batteries assembled with such a coated separator showed significantly enhanced cycling stability with a capacity of 350 mAh g${}^{-1}$ for over 100 cycles. Furthermore, a compact and low-cost battery design was realized with conductive carbon/S as cathode, Mg foil as anode and additive-free Mg[B(hfip)${}_4$]${}_2$ as electrolyte. This electrospun functional separator is expected to enlighten future separator design for metal-sulfur batteries towards practical applications.