Nitrogen-doped carbon coated SnO2 nanoparticles embedded in a hierarchical porous carbon framework for high-performance lithium-ion battery anodes
Graphical abstract
Introduction
To meet the ever growing demands of portable electronics and electric vehicles, searching for alternative electrode materials with high specific capacity is urgently needed for developing next-generation lithium-ion batteries with high energy density [1]. As one of the most promising anode candidates, tin dioxide (SnO2) has attracted great attention due to its high theoretical capacity, safe working potential, abundant resource and environmental friendliness [[2], [3], [4], [5], [6], [7], [8]]. The electrochemical interaction between SnO2 and Li can be described in two steps. During the lithiation process, SnO2 first undergoes a conversion reaction to produce Sn and Li2O, and the subsequent reduction of Sn results in the formation of LixSn alloy (0 ≤ x ≤ 4.4) [[9], [10], [11], [12], [13]]. The maximum capacity arising from the above two steps is 711 mAh g−1 and 783 mAh g−1, respectively. As a result, SnO2 is able to reach a high theoretical capacity of 1494 mAh g−1, which is about four times that of commercial graphite anodes. However, the practical application of SnO2 still remains great challenge owing to its inherent low electronic conductivity and extremely large volume expansion of 358% during the lithiation process [[14], [15], [16], [17], [18]]. These short-comings often lead to the formation of unstable solid electrolyte interphase (SEI) layer, rapid agglomeration of intermediate product (metal Sn particles), mechanical fracture and loss of interparticle electrical contact, thereby resulting in dissatisfactory electrochemical properties especially severe capacity fading [2,4].
Many efforts have been devoted to address these failure modes. One of the most popular approaches is to construct SnO2 nanomaterials with various morphologies [[19], [20], [21], [22], [23], [24], [25]], which can effectively accommodate huge volume change and shorten the transport length for lithium ions and electrons. It is widely recognized that conversion from Sn to SnO2 is irreversible for bulk SnO2, but can be reversible for nanostructured SnO2 owing to the vastly increased volume fraction of Sn/Li2O interfaces [26,27]. In this case, nanostructured SnO2 materials are expected to provide relatively high specific capacity. Unfortunately, the improvement in the cycling performance is still limited. One of the most important reasons is that the intermediate produced Sn nanoparticles prefer to agglomerate to larger electrochemically inactive clusters during the repeated lithiation/delithiation process [27]. Furthermore, the direct contact between nanosized SnO2 and electrolyte will lead to continuous growth of SEI layer [[28], [29], [30]]. The generated thick SEI layer may function as an additional barrier, which would hinder the lithium-ion transportation through the electrode/electrolyte interface.
Another effective way is to fabricate SnO2/carbon composite materials [[31], [32], [33], [34], [35]]. The role of carbon materials in the composites can be divided into two types. Firstly, carbon materials are employed as matrixes to support SnO2, which could hinder the agglomeration of SnO2 and Sn particles during the electrochemical process. Among various carbon matrixes, conductive carbon materials with porous structure are of particular interest, owing to the internal void space for buffering volume change of nano active materials [[36], [37], [38], [39]]. Recently, some research groups have improved the electrochemical performance of SnO2 anodes by adding conductive porous carbon as the growth matrix [[40], [41], [42], [43]]. However, the synthetic methods can't be applied to the actual production due to either expensive reagents or complicated procedures. Secondly, carbon materials are used as coating layers to modify the surface of SnO2 [26,[44], [45], [46]]. The carbon coating could provide effective pathways for electron transfer and prevent the formation of thick SEI layer. Moreover, it would also contribute to limiting the agglomeration of SnO2 and the produced Sn particles. Nevertheless, the electrochemical performance of carbon-coated SnO2 is still unsatisfactory due to the broken carbon layer resulting from the large volume change of SnO2, as previously observed in some similar kinds of large-volume-change materials [47,48].
Herein, we have reported a new design of SnO2-based composite by embedding SnO2 nanoparticles in the pores of a hierarchal porous carbon (HPC) framework and further introducing a completely coated N-doped carbon (NC) layer on the surface (Fig. 1). The as-prepared SnO2@HPC@NC composite simultaneously combines several appealing rationales including ultrafine active SnO2, highly conductive and porous carbon matrix, and uniform N-doped carbon coating. This novel structure affords the following advantages: (1) The nano SnO2 particles maintain the advantages of nanostructure materials, including avoided mechanical fracture and decreased lithium ions and electrons transport length. (2) The essential electrical contact between active SnO2 can be guaranteed by the highly conductive network arising from the synergic effect of both HPC framework and N-doped carbon coating. (3) The internal void space of the pores in HPC framework are helpful to accommodate the volume expansion of nano SnO2 during the lithiation process. (4) The HPC framework combined with the N-doped carbon coating could also act as obstacles to reduce the agglomeration of the intermediate products (nano Sn), thus significantly increase the reversibility of the conversion reaction between SnO2 and Sn. (5) The SEI layer mainly tend to form on the outer surface of the N-doped carbon coating instead of on the SnO2 particles, which is beneficial to achieve more stable SEI layer and thus higher Coulombic efficiency (CE). As expected, the obtained SnO2@HPC@NC composite delivers remarkable electrochemical properties with high specific capacity and extremely long-term cycling stability.
Section snippets
Synthesis of HPC
Typically, the pretreated macro-porous weak acidic type cation exchange resin (10 g) was impregnated with cobalt ions in 0.05 M cobalt acetate solution (100 mL) for 6 h. After washed with deionized water and dried at 80 °C for 12 h, the exchanged resin was added into a 400 mL KOH/ethanol solution (containing 10 g KOH) and stirred at 80 °C until the “slurry-like” mixture was formed. Then the mixture was dried at 80 °C for 48 h, smashed by a disintegrator, and heated at 800 °C for 2 h under N2
Results and discussion
The overall synthesis process of SnO2@HPC@NC composite is illustrated in Fig. 2. Firstly, in order to achieve the hierarchical porous structure, HPC was synthesized via a controllable annealing treatment of resin with the synergic effect of both KOH activation and Co2+-induced catalysis. Subsequently, the as-prepared HPC was employed as an effective matrix to hold the SnO2 nanoparticles via a simple low-temperature hydrolysis reaction of SnCl2·2H2O, resulting in the successful formation of SnO2
Conclusions
In summary, we have successfully developed a scalable, cost-effective method to synthesize SnO2@HPC@NC composite with novel structure, in which SnO2 nanoparticles are embedded in the HPC framework via an in-situ hydrolysis of Sn salts and further coated by the uniform N-doped carbon layer derived from the carbonation of PAN. The as-obtained SnO2@HPC@NC composite exhibits superior specific capacity (up to 1100 mAh g−1 at 0.1 A g−1) and long-term cycling stability (500 cycles at 1 A g−1 without
Acknowledgements
The authors would like to acknowledge the support by the National Natural Science Foundation of China (NSFC) (No. 51602058, 51702103), Special Support Plan for High-Level Talents of Guangdong Province (No. 2016TQ03N558, 2017TQ04N840), and Science and Technology Planning Project of Guangdong Province (No. 2017A010103011, 2017A030313081).
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These authors contribute equally to this work.