Si-based composite interconnected by multiple matrices for high-performance Li-ion battery anodes
Graphical abstract
Introduction
Recently, the demand for Li-ion batteries (LIBs) with high energy densities has been rapidly increasing to meet the needs of energy storage systems and electric vehicles [1], [2], [3], [4], [5], [6], [7]. Although graphite has been commercialized as an anode in LIBs, its theoretical capacity (LiC6: 372 mAh g−1) falls short of the requirements for higher-energy-density LIB anodes. Therefore, Si-based anodes have been proposed as an alternative high-capacity anode for next-generation LIBs [2], [8], [9], [10], [11]. Si-based anodes have various advantages, including a high theoretical capacity of 3578 mAh g−1 (for Li3.75Si at room temperature), non-toxicity, appropriate operating potential (Li+/Li), and accessibility due to the abundance of Si in nature. Although Si-based anodes have many advantageous features, they suffer huge volume variations (>300%) during cycling which results in poor cycle performance. Therefore, a significant amount of research effort is focused on mitigation of this volume expansion during cycling in consideration of next-generation Si-based anodes [12], [13], [14].
Various attempts have been made to mitigate the volume expansion of Si-based anodes used in LIBs during cycling. A typical improvement involves enhancing the structural control of the Si-based material. Such structural changes include reducing the crystallite size, applying a carbon coating, as well as the use of porous structures, nanofibers, nanotubes, nanowires, etc [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. These approaches elicit better electrochemical performance than that of counterpart anodes based on pure Si. However, commercial application is restricted because these synthetic materials require a long and expensive manufacturing process. To address these issues, the fabrication of nanostructured composites via simple solid-state methods, particularly those based on heat-treatment (HT) and ball milling (BM), has been proposed as a solution for the development of Si-based anodes. The products produced through such synthetic methods also have excellent electrochemical properties [31], [32], [33], [34], [35], [36], [37]. In addition, composites fabricated by simple solid-state methods have a higher initial coulombic efficiency (ICE) than those synthesized by chemical methods, because the chemically fabricated composites contain several residual impurities. The ICE is a very important factor in the electrochemical performance of LIB anodes. Because the anode receives Li from the relatively expensive cathode, an anode with low ICE results in losses of the cathode. In general, solid-state synthesis methods can be applied in current production processes, imparting advantages such as enhanced simplicity of the process and the possibility for mass production. Among the transition metals, Cu has various advantageous features, including environmental compatibility, inexpensiveness, and high electronic conductivity. Cu can form several compounds with Si which can consequently be used as conducting Li-inactive matrices in composites, because Cu-Si compounds are known to be electrochemically Li-inactive materials [38], [39], [40], [41], [42], [43]. Therefore, the use of Cu-Si compounds can complement the poor electrical conductivity and large volume variations of Si, and are thus suitable conductive Li-inactive matrices for the commercialization of Si-based anodes.
The purpose of this research is to manufacture a Si-based nanostructured composite with excellent electrochemical performance characterized by high initial reversible capacity (IRC), high ICE, long cycle durability, and high rate capability. Based on the opinions of researchers from representative battery manufacturers (i.e., Samsung SDI and LG Chem.), we established a specific target for the electrochemical performance (IRC: >1000 mAh g−1; ICE: >80%; capacity retention after 100 cycles: >85%), with the focus being suitability for commercialization. Si-Cu alloy (Cu3Si) and various structured carbon-based materials have been adopted to improve the electrochemical performance of Si-based anodes, playing key roles as conducting and buffering matrices, enhancing structural stability during cycling, and supporting better conductivity than that of bulk Si. A combination of simple solid-state synthetic methods was adopted to manufacture a high-capacity Si-based composite anode. Therefore, we herein propose an optimized and practical nanostructured Si-based composite anode for LIBs with superior performance.
Section snippets
Materials synthesis
Four Si-Cu alloys with different weight compositions (Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40) were synthesized by the following solid-state synthetic route. The composition of each of these alloys is marked with a red line in the Si-Cu binary phase diagram presented in Fig. S1. The Si (Aldrich; 99.9%, average size: ~150 μm) and Cu (Kojundo; average size: ~75 μm) powders (Si/Cu = 90:10, 80:20, 70:30, and 60:40 wt%) and stainless steel balls (diameters: 3/8 and 3/16 in.) were placed into a
Results and discussion
Various binary compounds such as Cu3Si, Cu4Si, and Cu5Si are shown in the binary Si-Cu phase diagram as shown in Fig. S1 [44], [45], [46]. On the basis of the Si-Cu phase diagram, four SixCuy alloys with different weight compositions were synthesized using a simple solid-state BM process. Their compositions are marked with red lines on the Si-Cu binary phase diagram (Fig. S1). Fig. 1 shows the XRD patterns of the synthesized Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40 alloys. All of the XRD
Conclusion
The novel composite Si-Cu3Si-CNT/G-C has been proposed as a high-capacity Si-based anode material for LIBs. The Si-Cu3Si-CNT/G-C was fabricated by a combination of two simple solid-state synthesis methods (BM and HT), which makes mass production feasible. Within the composite, multiple matrices were interconnected. The nanocrystalline Cu3Si contributed to improving the IRC and ICE via its role as a conducting and Li-inactive matrix. The CNT and graphite contributed to improved cycling
Acknowledgement
This work was supported by a National Research Foundation of Korea grant, funded by the Korean Government (MSIP) (NRF-2018R1A2B6007112, NRF-2018R1A6A1A03025761).
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