**1. Introduction**

Due to a deteriorating environmental situation (e.g., global warming, and the gradual depletion of oil and hard coal resources), the development of balanced and clean energy resources is extremely important. Clean energy sources such as wind, sun and tidal energy are preferred alternatives to fossil fuels, and they are best utilized with high-efficiency energy storage technologies. Lithium-ion batteries (LIBs)are appealing storage sources due to their unique characteristics, such as their extended life, energy density, low maintenance costs, environmental friendliness and lack of memory effect [1,2]. Although the performance of lithium-ion batteries continues to improve, their energy density, cycle lifetime and productivity remain insufficient for large-scale applications in consumer electronics, and transportation and storage of renewable energy. Much effort has been made to create new electrode materials or to design unique electrode architecture to address the everincreasing demand for batteries with higher energy density and longer cycle life [3–7]. The electrodes must maintain their integrity across multiple discharge–recharge cycles, which is one of the challenges in their design. Li-alloying agglomeration or the formation of passivation layers, which prohibit the fully reversible injection of Li ions into negative electrodes, reduce the life spans of electrode systems [8,9]. Transition metal oxides (TMOs) have recently found use as electrode material for energy storage devices including LIBs [10]. These materials exhibit a large theoretical specific capacity and high working potential for LIBs (ca. 500–1000 mAhg−1) [11]. This is an advantage of TMO application due to the prevention of lithium dendrite formation, which increases safe use.

**Citation:** Wenelska, K.; Trukawka, M.; Kukulka, W.; Chen, X.; Mijowska, E. Co-Existence of Iron Oxide Nanoparticles and Manganese Oxide Nanorods as Decoration of Hollow Carbon Spheres for Boosting Electrochemical Performance of Li-Ion Battery. *Materials* **2021**, *14*, 6902. https://doi.org/10.3390/ ma14226902

Academic Editors: Ioannis F. Gonos, Eleftheria C. Pyrgioti and Diaa-Eldin A. Mansour

Received: 13 September 2021 Accepted: 3 November 2021 Published: 15 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

TMOs such as SnO2 [12], Fe3O4 [13] and MnO2 [14] are characterized by a very high theoretical capacity. Furthermore, due to its low cost and low environmental impact, iron oxide is one of the most promising materials. Its disadvantage is that the Li reactivity mechanism of transition metal oxides requires the formation and the decomposition of Li2O, accompanying the redox reactions of metal nanoparticles [15]. During this conversion reaction, there is usually a large change in volume, which can cause electrode fracture or deterioration of electrochemical efficiency [16]. In order to address these issues, a carbon material can be used, which can prevent both volume change and aggregation of the nanoparticles [17]. In order to obtain high electrochemical performance, the carbon material should possess advantages such as high surface area and high electronic conductivity. A high surface area provides active sites for the pinning on or embedding of nanoparticles on the carbon surface [18].

Mesoporous carbon materials are characterized by a large specific surface area, which can reduce current density per area unit. Another advantage is the thin walls shorten the diffusion paths. In addition, they are an acceptable electrode material due to low cost, high chemical stability and good processing ability [19,20]. Mesoporous hollow carbon nanospheres fully meet these requirements, and therefore, its application as a carrier of metal oxide nanoparticles appears to be reasonable.

Herein, we present a facile synthesis method of hollow carbon nanospheres (HCS) with two stages of functionalization using transition metal oxides (iron oxides in the form of spherical nanoparticles and rod-like manganese dioxide) as advanced anode material for high-performance LIBs. The prepared FexOy/MnO2/HCS nanocomposite combines the advantages of empty carbon spheres, such as stability or adaptation to expanding volumes during the cycle, with a high specific capacity of transition metal oxides.
