C–SnO2/MWCNTs composite with stable conductive network for lithium-based semi-solid flow batteries

Lithium-based semi-solid flow batteries (LSSFBs) could potentially be applied in large-scale energy storage systems due to their high safety and relatively independent equipment units. However, the electrochemical performance of LSSFBs is limited by the unstable contact between conductive additives and active materials, as well as the poor conductivity of active materials. Therefore, it is necessary to develop semi-solid electrodes with high stability and specific capacity to obtain LSSFBs with satisfied energy density. Herein, carbon-coated SnO₂/multi-walled carbon nanotubes (C–SnO₂/MWCNTs) composite was designed as the anode material of LSSFBs. In such composite, SnO₂ nanoparticles are uniformly distributed on the surface of MWCNTs and coated with carbon layer, which was identified by field-emission scanning electron microscopy, transmission electron microscopy and X-ray diffraction (XRD) results. In general, the traditional SnO₂ as active material in electrodes will suffer from volume expansion and collapse of structure, which will decline the cycle life of batteries. In this composite, the nanoparticle structure endows SnO₂ with more reaction active sites. Furthermore, MWCNTs and carbon layer can construct a stable conductive network, which enhances the electron transport in SnO₂-based electrodes. Simultaneously, MWCNTs and carbon layer also achieve an integrated architecture. Thus, the electron transfer dynamics of SnO₂-based electrodes could be improved and their volume expansion is effectively suppressed during charging/discharging process, resulting in improved rate and cycling performance. The coin-type batteries based on C–SnO₂/MWCNTs can maintain a discharge capacity of 725 ​mAh ​g⁻¹ after 100 cycles under a current density of 0.5 ​A ​g⁻¹. On the contrary, the discharge capacity of the batteries based on bulk SnO₂ almost disappears after 100 cycles, which is attributed to the poor conductivity and excessive volume expansion of electrode materials. In addition, the MWCNTs will enhance the suspension stability of the semi-solid electrode. When the mass fraction of the C–SnO₂/MWCNTs in the semi-solid electrode is 8.0 ​%, the semi-solid electrode has superior suspension and electron conductivity, as well as suitable viscosity. Furthermore, the lithium storage mechanism of the semi-solid electrode was explored by ex situ XRD and X-ray photoelectron spectroscopy. The results show that, in C–SnO₂/MWCNTs composite, SnO₂ has a dual Li ​⁺ ​ions storage mechanism involving conversion and alloying reactions. When the flow rate is controlled with 5 ​mL ​min⁻¹, the conduction network reaches a dynamic balance, and the semi-solid electrode exhibits low charge transfer resistance. These advantages endow the LSSFBs with superior rate and cycling performance. The semi-solid flow batteries could maintain 42.4 ​% of their initial capacity (690.8 ​mAh ​g⁻¹) after cycling for 962 ​h at 0.2 ​mA ​cm⁻². This work provides a promising strategy for optimizing the semi-solid electrode of LSSFBs.