Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin, 300071, P. R. China
E-mail: email@example.com; wangyj@ nankai.edu.cn
Rechargeable Li-ion batteries have dominated every aspect of our lives, such as phones, laptops and so on. 1, 2 These equipments facilitate and enrich our lives. However, their statements ”the power is low” let us very helpless from time to time, especially when the ball approaches the goal or the hero/heroine skirts danger. At this point, a high capacity Li-ion battery is eager for us. Owing to the high specific theoretical capacity (1018 mAh g-1, about three times for the graphite (372 mAh g-1) used currently), low cost, and environmentally benign nature, manganese oxide (Mn2O3) is believed to be the most promising alternative anode materials for next generation LIBs. 3, 4 However, the achieved capacity of this material is far lower than its theoretical value and the rate capability is not satisfactory. What is more, the fully charged production is ambiguous. Therefore, it is worthwhile to synthesize Mn2O3 LIBs anode with enhanced capacity and better rate capability by a new route. Meanwhile, gaining further understanding of the conversion mechanism of this material is also an urgent task.
Nanosize effect, a typical characteristic of transition metal oxide electrodes, always destroys the unstable electrode structure and lead to performance degradation. 5-8 Many merits, however, such as higher specific area, more active sites, and enhanced kinetics of the electrochemical activity, are also induced by the nanosize effect. Encouraged by these merits, we designed a mini-hollow Mn2O3 electrode to improve the electrochemical performance by utilizing the merits and inhibiting the disadvantages simultaneously, achieving high capacity and long cyclic stability.
As shown in Figure 1a and 1b, a hollow structure with a thick shell and mini-hollow cavity is observed. After the first cycle, the small interior cavity disappears, and a hierarchical nanostructure forms. The electrochemical performance of this mini-hollow Mn2O3 electrode is shown in Figure 1c. At high current densities of 1.0 and 2.0 A g-1 (Figure 2c), stable capacity of 795.3 mAh g-1 and 686.7 mAh g-1 can be obtained, respectively, and then recovered to 1164.1 mAh g-1 at 0.2 A g-1. Even after 1000 cycles, the capacity at 2.0 A g-1 is still stabilized at ~760 mAh g-1.
Figure 1. TEM images of the mini-hollow Mn2O3 electrode before (a) and after (b) cycling, (c) the electrochemical performance of the mini-hollow Mn2O3 electrode.
Different from the bulk material lacking room to hold the inward volume expansion and the conversional large hollow structure owning too large room for the inward volume expansion without confine, the small interior cavity of a mini-hollow structure is filled by the reformated nanoparticles caused by nanosize effect, leading to the formation of a hierarchical nanostructure with homogeneous dispersion of the nanoparticles. As shown in Figure 2a, a hierarchical nanostructure is formed, and the small interior cavity disappears after the first cycle for our mini-hollow polyhedron Mn2O3 electrode. This structural evolution is caused by the nanosize effect of the conversion mechanism, and the volume expansion cannot be avoided. Fortunately, the original small interior cavity offers room for the inward volume expansion, so that it is filled by the reformatted nanoparticles, leading to the formation of a hierarchical nanostructure with homogeneous dispersion of the nanoparticles. This formed hierarchical nanostructure keeps its structure without any serious “electrochemical sintering”, indicating its stability. Similarly, the nanosize effects also occure on the bulk polyhedron electrode (Figure 2b). Due to the lack of room to contain the inward volume expansion, the unbalanced expansion tension may lead to the formation of a hierarchical nanostructure with congested core. Obviously, this reconstructed structure is not on equilibrium stage and will aggregate easily and finally collapse. In contrast to our mini-hollow electrode, the conventional large-hollow electrodes reported in previous works mostly did not show comparable cycling stability though their capacity and rate capacity are excellent. As illustrated in Figure 2c, a hollow reconstructed hierarchical nanostructure was formed after the first cycle. Owing to the lack of confine from the interaction with each other, the nanoparticles near the inner cavity may keep expansing and lead to the structure collapse. Thus, this mini-hollow polyhedron Mn2O3 electrode could show a long cyclic stability and high capacity. This work suggests that it is important and practical to design and control the hollow size to improve the cycling stability of conversion mechanism electrodes, which offers a new perspective to design the structure of an electrode material with high-performance energy storage.
Figure 2. Schematics illustration of the structure evolutions of (a) mini-hollow, (b) bulk and (c) large-hollow polyhedron Mn2O3 electrodes on cycling.
This work was financially supported by the National Natural Science Foundation of China (51231003), the Ministry of Education of China (IRT-13R30), the 111 Project (B12015) and the Ph.D. Candidate Research Innovation Fund of Nankai University (68150003).
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