|Cycling performance of Si, Si@C, and Si@C−CNTs at 100 mA g−1 and within a voltage window of 0.02−2.0 V. Credit: ACS, Xue et al. Click to enlarge.|
Researchers at North Carolina State University (NCSU) have combined carbon coating and a carbon nanotube (CNT) framework to improve the cycling stability of Si (silicon) anodes for Li-ion batteries. In a paper published in the journal ACS Applied Materials & Interfaces, they report carbon-coated Si nanoparticles dispersed in CNT networks show capacity retention of 70% after 40 cycles—a much better rate of retention than carbon-coated Si nanoparticles without CNTs.
In this novel structure, the carbon layer can improve electric conductivity and buffer the severe volume change, whereas the tangled CNT network is expected to provide additional mechanical strength to maintain the integrity of electrodes, stabilize the electric conductive network for active Si, and eventually lead to better cycling performance.
As a promising anode material for lithium-ion batteries (LIBs), Si provides the highest known lithium storage capacity (4200 mA h g−1), which is more than 10 times greater than that of commercialized graphite (372 mA h g−1). However, the limited cycling life of Si anodes, resulting from the large volume change upon lithium insertion and extraction, greatly restricts their practical use in LIBs. Studies have shown that each silicon atom can theoretically accommodate up to 4.4 lithium atoms to form Li22Si5 alloy, accompanied by a volume expansion of about 400%. Such huge volume change leads to the pulverization of electrodes, which in turn causes the breakdown of electric conductive network and insulation of active material, eventually resulting in rapid capacity fading.
To address the large volume change, researchers have employed Si nanoparticles because they generate less stress during cycling and can better accommodate the repeated volume expansion and contraction. For further improvement, the nanosized Si was often encapsulated in a carbon shell, which not only provides greatly improved electric conductivity, but also helps buffer the severe volume change. However, the unstable electrode integrity due to the insufficient binder strength and significant volume change of Si is still a problem.
...During the electrode fabrication process, active material such as carbon-coated Si is first made into slurry by using a polymer binder and corresponding solvent. After drying, active material particles are then connected with each other by the polymer binder to form an electrode. Upon repeated volume expansion and contraction, however, the polymer binder does not have sufficient mechanical strength to maintain the integrity of the Si electrode, leading to the crack, pulverization, and breakdown of the electric conductive network of the electrode. Finally, the insulation of active Si results in rapid capacity loss.—Xue et al.
The NCSU researchers suggest that their approach can avoid those issues. In the study, they report on their preparation process, and on the electrochemical performance of the resultant Si@C−CNT electrode along with two other Si-based anode materials: Si and Si@C.
The Si electrode exhibited poor cycling performance. Although it begins with a very high reversible capacity (2780 mA h g−1) in the first cycle, it delivers only 100 m Ah g−1 capacity after 40 cycles.
After carbon coating, the resultant Si@C shows much better cycling stability because there is a homogeneous carbon layer on the surface of the Si nanoparticles to buffer the volume change and provide good electric conductivity. As a result, the Si@C anode shows capacity retention of 48% after 40 cycles.
For Si@C−CNTs, the researchers observed a small increase of reversible capacity during the first 5 cycles, followed by slow fase. Although Si@C−CNTs show much lower capacity (699 mA h g−1) than the other two electrodes due to the introduction of carbon layer and CNTs, the Si@C−CNTs material exhibits the best cycling stability among all three anodes and the capacity retention after 40 cycles is 70%.
...any anode capacity greater than 1000 mA h g−1 will not lead to improvement in the battery performance due to the limitation in the cathode capacity (<250 mA h g−1), which is significantly lower than the anode capacities. Therefore, the capacity (699 mA h g−1) achieved by Si@C−CNTs is compatible with the current cathode materials.—Xue et al.
Leigang Xue, Guanjie Xu, Ying Li, Shuli Li, Kun Fu, Quan Shi, and Xiangwu Zhang (2012) Carbon-Coated Si Nanoparticles Dispersed in Carbon Nanotube Networks As Anode Material for Lithium-Ion Batteries. ACS Applied Materials & Interfaces doi: 10.1021/am3027597