**1. Introduction**

With the rapid development of bendable electronic products, such as flexible displays, wearable electronics and medical electronics, flexible energy storage systems with higher energy density have become an urgent demand [1,2]. Lithium-ion batteries, as energy storage devices, have attracted enormous attention due to their long cycle life, fast charge–discharge, high energy density, and no memory effect [3–6]. Flexible anode materials are a particularly important factor to obtain lithium-ion batteries with high electrochemical performance [7,8]. Therefore, the development of flexible anode materials with bendable function and excellent electrochemical performance has become one of the research hotspots.

**Citation:** Chen, Y.; Wang, J.; Wang, X.; Li, X.; Liu, J.; Liu, J.; Nan, D.; Dong, J. Constructing High-Performance Carbon Nanofiber Anodes by the Hierarchical Porous Structure Regulation and Silicon/Nitrogen Co-Doping. *Energies* **2022**, *15*, 4839. https://doi.org/10.3390/en15134839

Academic Editor: Daniel T. Hallinan, Jr.

Received: 3 May 2022 Accepted: 29 June 2022 Published: 1 July 2022

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Carbon fibers are one of the most widespread anode materials of flexible lithium-ion batteries [9,10]. However, untreated carbon fibers make it difficult to achieve the high performance demands of lithium-ion batteries due to their low reversible capacity. To obtain high electrochemical performance, it is common to fabricate holes of different sizes in carbon fibers [11–14]. Wang et al. reported that micropores can increase the number of active sites of lithium storage and the ability of ion adsorption [15]. Guo et al. reported that mesopores greatly enhance the capability rate and facilitate the transport of ions [16]. Chen et al. reported that abundant meso/macropores in carbon nanofibers can offer more active sites for Li storage and facilitate electrolyte penetration of the inner part of carbon nanofibers, improving the electrolyte/electrode contacting area [17]. In addition, element doping is another measure of modifying electrode materials. Silicon materials have been widely concerned because of their high theoretical capacity of 4200 mAh g−<sup>1</sup> [18,19]. However, silicon materials undergo serious volume changes during the charging–discharging process, which leads to particle pulverization and rapid capacity decay [20,21]. Silicon– carbon composites can make use of the structural strength of carbon materials, so that the volume change in silicon materials can be alleviated during the charging–discharging process [22–25]. For example, Jang et al. prepared a pyrolytic carbon-coated silicon nanofiber anode, which has high capacity and excellent cycling performance [26]. Xu et al. reported a flexible 3D Si/C fiber paper anode with capacity of 1600 mAh g−1, which was synthesized by simultaneously electro spraying nano-Si and polyacrylonitrile fibers, followed by carbonization [27]. Traditionally, carbon materials are used to modify silicon materials to obtain silicon-based anode materials with high electrochemical properties. Similarly, it may be effective to use silicon materials to dope carbon fibers to obtain high-capacity carbon fiber anodes. In addition, N doping was also used to improve the conductivity of carbon materials [28–30]. However, it is difficult to fully meet the requirements of high electrochemical properties using a single modification strategy. Therefore, the synergistic effect of various modification methods may be effective for the preparation of high-performance carbon nanofiber anodes.

Based on the above analysis results, we prepared a hierarchical porous and Si/N codoped carbon nanofiber anode by novel gas–electric co-spinning technology in this work. Si doping can improve the specific capacity, N doping can improve the conductivity, and the fabricated micropores, mesopores and macropores can increase the number of reactive sites, improve the ion transport rate, and enable the electrolyte to penetrate the inner part of carbon nanofibers to improve the electrolyte/electrode contacting area during charging– discharging processes. This modified carbon nanofiber anode does not require a binder, and has the advantages of flexibility and foldability. Moreover, it exhibits a high initial reversible capacity of 1737.2 mAh g<sup>−</sup>1, good capacity retention and outstanding rate ability. This work provides a new avenue for the development of flexible lithium-ion batteries.
