Are Electrospun Fibrous Membranes Relevant Electrode Materials for Li‐Ion Batteries? The Case of the C/Ge/GeO₂ Composite Fibers

Abstract

Self-supporting paper-like membranes consisting of carbon/germanium dioxide (C/GeO 2 ) fibers are prepared via electrospinning of solutions with different germanium load (2.50−4.25 wt%), followed by carbonization at 550−700 °C, and are evaluated as anode materials in lithium ion batteries. The investigation of the physicochemical properties of the membranes by several characterization techniques shows that, as expected, with increasing carbonization temperature better graphitized and less nitrogen-rich C fibers are obtained, containing Ge 0 and/or reduced oxide phases along with GeO 2 nanoparticles. These characteristics, combined with the cold pressing of the as-spun membrane that noticeably reduces the hollow space within the fibres giving rise to a more compact and tight structure, lead to initial discharge volumetric capacities (≈1390-3580 mAh cm −3 ) much higher than commercial graphite anodes. In particular, the membrane prepared from solution with 4.25 wt% Ge-load by cold-pressing and carbonization at 700 °C, is able to deliver ≈1500 mAh cm −3 after 50 cycles at 50 mA g −1 with a Coulombic efficiency close to 100%. Nevertheless, the anodes exhibit poor rate capability. This is because the carbonization at high temperature promotes outward diffusion and subsequent coalescence of Ge-clusters in larger particles, with the structure of the fibers made fragile by the formation of voids within them.promoted the search for clean renewable energy sources. However, the intermittent nature of these sources calls for the use of efficient energy storage systems. The requirements that electrochemical energy storage devices, ranging from coin cells to large-scale energy storage systems, have to meet are ever increasing. They include high energy density, long durability, low costs, and eco-friendliness. For this reason, the advanced research on electrode materials is continuously progressing.To date, lithium-ion batteries (LIBs) represent the most popular energy storage systems, widely utilized to power small/ portable electronic devices, as well as hybrid and fully electric vehicles. The gravimetric capacity (372 mAh g −1 ) of the commonly used graphite anodes is inadequate to meet the current requirements for energy storage systems. In the search for alternative anodes materials, the focus is mainly on transition metal oxides MO x , [1][2][3][4][5][6] which store Li + ions through the conversion reaction MO x + 2x Li + + 2xe − ↔ M 0 +xLi 2 O, and IV group elements M 0 [7][8][9] that are able to store larger amount of Li + ions through the alloying reaction M 0 + yLi + + ye − ↔ Li y M. [10] Among the latter, germanium is the one endowed with by far superior properties (higher Li diffusivity and electronic conductivity, and smaller volume changes suffered during lithiation/delithiation Electrode Materials

Electrospun Nanomaterials for Energy Applications: Recent Advances

“…At last, a single-step (calcination) or multi-step post-spinning heat treatment (stabilization and carbonization, sometimes followed by activation) upon different atmospheres (air, inert gases, gas mixture, activating gases) can be necessary to finalize the production process of the various fiber typologies (Figure 3). Polymer fibers do not require any treatment [46,47]; pure (and doped) oxide fibers are obtained by removing the organic component of the precursor(s) via an oxidative process [48][49][50][51], whereas stabilization of the polymer enabling its subsequent processing at higher temperature for graphitization is needed to obtain carbon and composite carbon-based fibers [52][53][54][55][56][57][58]. Activation upon gaseous atmosphere [53] or chemical etching to remove sacrificial templating agents [59] are utilized to enhance the fiber porosity and surface area.…”

“…It is widely recognized that carbon incorporation and nano-sizing improve the electrochemical and mechanical performance of the electrode materials [72,[76][77][78][79], providing them with enhanced electronic conductivity and increased resistance against the large volume changes, from which they suffer during the lithiation/de-lithiation process. It has been further shown that the use of binder-free self-supporting flexible carbon-based membranes as electrode materials [56][57][58]66,[80][81][82][83][84] translates into a beneficial reduction of the inactive weights with a consequent gain in the effective gravimetric and volumetric capacities of the batteries. These benefits are of crucial importance in manifold applications including transportation, aerospace, and portable and wearable electronics.…”

“…Electrospinning is a viable route to simultaneously implement all these ameliorative strategies. Therefore, it has been increasingly and extensively utilized for the production of all the battery components-not only anodes [12,[56][57][58]72,74,81,82,[84][85][86][87][88][89][90][91] and cathodes [12,[92][93][94][95], but also electrolytes [12,[96][97][98] and separators [12,99,100], which makes it feasible to manufacture secondary batteries by assembling components produced exclusively by electrospinning ( Figure 5). Actually, the increasing number of published papers focused on this subject witnesses the growing interest of the scientific community for the use of this technique in the manufacturing of LIB components.…”

Structural Strategies for Germanium‐Based Anode Materials to Enhance Lithium Storage

“…Self‐supporting paper‐like membranes consisting of C/Ge dioxide (C/GeO 2 ) fibers are prepared via electrospinning of PAN/DMF/Ge(OCH(CH 3 ) 2 ) 4 germanium(IV) isopropoxide gel solutions with different Ge load (2.50–4.25 wt%), followed by carbonization at 550–700 °C. The membrane anode prepared from solution with 4.25 wt% Ge‐load by cold‐pressing and carbonization at 700 °C delivered about 1500 mAh cm −3 after 50 cycles at 50 mA g −1 with a Coulombic efficiency close to 100% . 3D interconnected Ge/C‐NCFs composites were prepared by annealing GeO 2 ‐polyvinyl alcohol (PVA) gel at 800 °C for 5 h, GeO 2 ‐PVA gel be coated onto NCFs surface through steeping the NCFs into the above gel solution.…”

“…This enhance capacity performance can be attribute to the following the catalytic effect of Ge and promoting the decomposition of Li 2 O, which also enables to improve the conversion reaction of GeO 2 . C/Ge/GeO 2 composite fibers were prepared via electrospinning of solutions with different Ge load (2.50–4.25 wt%), followed by carbonization at 550–700 °C . With increasing carbonization temperature better graphitized and less N rich carbon fibers were obtained, containing Ge 0 and/or reduced oxide phases along with GeO 2 NPs, combined with the cold pressing of the as‐spun membrane that the hollow space within the fibers had been noticeably reduced to rise more compact and tight structure, C/Ge/GeO 2 composite fibers showed initial discharge volumetric capacities of 1390–3580 mAh cm −3 .…”