Nanoflake δ-MnO2 deposited on carbon nanotubes-graphene-Ni foam scaffolds as self-standing three-dimensional porous anodes for high-rate-performance lithium-ion batteries
Introduction
Amongst all the various energy storage devices, lithium ion batteries (LIBs) have generally been regarded as one of the most widely used secondary energy sources for portable electronic equipment like smartphones, tablet computers and mobile power packs. However, large scale applications on the electrification of transportation and MW-class battery storage units are growing rapidly. To meet the requirement for electric vehicles (EVs), it is essential to explore novel anode materials with a high specific capacity as well as superior rate and cycling performance as alternatives to the current graphite anodes in LIBs [[1], [2], [3]]. Hence, in recent years, high-capacity anode materials such as transition-metal oxides [[4], [5], [6], [7], [8], [9]], silicon [[10], [11], [12], [13], [14]] and metal Li [15,16] have been widely investigated as alternatives for these traditional carbon-based anodes.
Manganese dioxide (MnO2), as one of the most potential transition-metal oxides, has arisen increasing attention owing to its high theoretical capacity (∼1230 mAhg−1), low equilibrium voltage vs. Li/Li+ [17,18], low-cost and environmental benignity [19,20]. However, several acknowledged challenges still exist and block the application of MnO2 anodes in LIBs. These include rapid capacity fading, poor rate performance and low columbic efficiency caused by large volume expansion during the cycling processes and low electrical conductivity which is intrinsic.
In-depth studies have been carried out to ameliorate the above-mentioned problems of MnO2-based anode materials, such as different MnO2 polymorphs (α-MnO2 [[21], [22], [23], [24]], β-MnO2 [25,26], γ-MnO2 [[27], [28], [29]], δ-MnO2 [30,31]), morphology control to buffer volume change (nanocrystal [25], nanorods [21], nanoflakes [30]), composite design with carbon-based nanomaterials to enhance its conductivity [18,22,27,32], and combination with three-dimensional (3D) conductive matrixes to prepare self-standing electrode materials (steel plate [33], nickel foam [23,28,34], graphene [24,30,31]). Zhang et al. [32] reported an optimized microstructure δ-MnO2/carbon nanotubes (CNTs) composite which delivered 903 mAh g−1 reversible capacity at 0.24 A g−1 and exhibited a good rate capacity of 540 mAh g−1 at a current density of 2.4 A g−1. Yu et al. [24] prepared free-standing graphene/α-MnO2 nanotube films as LIBs anodes, which delivered 686 mAh g−1 of reversible capacity at 0.1 A g−1 and 208 mAh g−1 at a higher current density of 1.6 A g−1. However, considering the poor conductivity and mediocre rate performance of MnO2, the improvement on its ion diffusion, electrical conductivity is still worthy to be anticipated.
Herein, we construct a novel 3D self-standing composite and consider using δ-MnO2 to grow on conductive nanocarbon materials and making the porous nickel foam as framework to achieve self-stand electrodes, which avoids the use of non-conducting binder. In the design, we use porous, conductive and robust NF to load nanocarbon and δ-MnO2, fabricate in-situ grown δ-MnO2 to enhance connection with carbon materials and increase composite conductivity, choose porous δ-MnO2 nanosheets to enable more catalytic sites and shorter ion diffusion path. Besides, δ-MnO2 nanoflakes and the overlapped δ-MnO2-CNT clusters provide enough room for product accumulation and volume changing during the cycling process. Compared with previous works, nanoflake δ-MnO2 has grown simultaneously on the CNTs and the graphene layer below, which indicates a larger electrode/electrolyte contact area and better connection between the active materials and the conductive matrix. In contrast to other MnO2 polymorphs, δ-MnO2 with low crystallinity is able to naturally assemble into lamellar nanoflake in hydrothermal reaction under certain conditions. Owing to the uniform 3D porous structure of δ-MnO2-CNTs-G-NF, the assembled Li-ion battery displays a prominent rate captivity and excellent cycling performance.
Section snippets
Growth of the CNTs-G-NF scaffold
The CNTs-G-NF scaffold was prepared by a two-step CVD technique on NF using ethanol as the carbon source [35]. First, acid-treated NF was pressed to a thickness of 0.4 mm and stamped into small disks (R = 12 mm) as templates. Then, the disks were placed into a tube heating furnace, and ethanol was bubbled into the quartz tube at 1000 °C under an atmosphere of Ar/H2 (160 sccm: 80 sccm) to grow graphene for 5 min. Finally, after immersing the graphene–NF substrate into a 10 mM Ni(NO3)2 ethanol
Morphological and structural characterization
The SEM image of untreated NF (Fig. 2a) shows surface is rough and covered with oxide bumps. After the growth of Graphene, wrinkles and ripples, formed due to different thermal expansions, are easily to observe (Fig. 2b). The diameter of such grown CNTs are about 50 nm (Fig. 2c). Low and high magnifications of the synthesized δ-MnO2-CNTs-G-NF hybrid exhibit that δ-MnO2 are uniformly deposited on the CNTs and underlying graphene (Fig. 2e and f). Moreover, the δ-MnO2-CNT clusters with diameters
Conclusions
In summary, hierarchical 3D porous δ-MnO2-CNTs-G-NF composites were synthesized by two-step CVD growth and a facile hydrothermal process, and were investigated as binder-free anode materials for LIBs. From this work, it is concluded that the unique structure of the composite shortens transport paths of lithium ions, enlarges electrode/electrolyte contact area, improves electrical conductivity of the composite and accommodates the volumetirc change during cycling process. Thereby, the assembled
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2022, Materials Today ChemistryCitation Excerpt :One strategy is compounding MnO2 with conductive additive (typically carbonaceous matrices) that can not only improve overall electrical conductivity of the electrode but also serve as a buffer layer to effectively alleviate large volume expansion during the cycling [22–24]. Typically, Zhai et al. prepared hierarchical 3D porous δ-MnO2-CNTs-G-NF composites by a two-step chemical vapor deposition method, which show excellent performances for lithium storage [25]. Li et al. designed sandwich-like carbon nanotube/rGO@MnO2 composites with 3D multilevel porous conductive architecture via a vacuum freeze-drying method, which display enhanced performance as anodes for LIBs [26].