A systematic thermal reactivity analysis to reveal thermal safety of Si–C anodes

A team of researchers has revealed the material factors responsible for the unstable thermal safety of silicon–carbon (Si–C) anodes through systematic thermal stability tests of silicon–carbon materials with electrolyte. This safety analysis provides the material design direction to avoid the occurrence of thermal runaway in silicon-based batteries.

The thermal safety of lithium-ion batteries remains a critical concern. Compared to the traditional graphite anode, the Si–C anode poses a higher risk of thermal runaway due to its dramatic volume expansion and more violent reactivity with the electrolyte. At present, strategies to improve the thermal stability of Si–C anodes mainly focus on two key directions: electrolyte optimisation and solid electrolyte interphase (SEI) modification. However, researches into the contribution of intrinsic properties of Si–C materials to the thermal runaway generation mechanism are still insufficient.

To comprehensively elucidate the factors influencing the material aspects of the thermal safety of the Si–C anodes, the team designed and synthesised three types of silicon–carbon materials by modulating the porous hard carbon (pHC) substrate materials and the preparation process: the silicon-carbon (C@pHC–SiC), silicon-carbon with smaller pore volume (C@pHC'–SiC), and silicon-carbon with more floating silicon (C@pHC–SiC').

The team compared the heat release of three types of Si–C anodes in contact with electrolyte at 100% state of charge through the differential scanning calorimetry (DSC) tests. The results indicate that the pHC substrate does not exert a significant influence on the thermal safety of Si–C anodes, whereas floating silicon without a carbon layer directly triggers thermal runaway.

In addition, the team conducted DSC tests by combining graphite anodes with different blending ratios of C@pHC–SiC with electrolyte. The results show that the amount of exothermic heat is almost positively correlated with the blending ratio of C@pHC–SiC, further illustrating that the thermal safety risk increases with the proportion of C@pHC–SiC.

Accelerating rate calorimeter (ARC) tests show that the maximum temperature of the pouch cells with floating silicon achieved 875.2 °C (532.1 ℃ for samples without floating silicon). The effect of floating silicon on thermal safety is further demonstrated.

"This study provides important insights into the structural design of silicon–carbon composites and offers guidance for evaluating the thermal safety of commercially available anodes with high silicon–carbon content," said Xin-Bing Cheng, a professor at Southeast University.

"Design strategies for the high thermal stable silicon–carbon materials can be explored from these three aspects:" said Dr. Xin Shen, a postdoctor at Southeast University. "First, a vase-shaped micropore design of the hard carbon substrate is able to enhance the spatial confinement of nano-silicon particles; The second is to optimize the thickness of carbon coating layer, balance the interface ion transport and structural stability; Finally, the difference in expansion coefficients between the hard carbon substrate and nano-silicon particles upon lithiation should be addressed to minimize the dynamic escape of nano-silicon particles."

This work was supported by the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities, SEU Innovation Capability Enhancement Plan for Doctoral Students.

Other contributors include Zhi-Jun Jiang, Zhen-Hui Luo, Jia-Xin Guo, Yun-Fei Du, Feng Jiang, Nai-Lu Shen, Tao Wang, Xu Liu, Yang Zhou, Zhiyang Lyu, Yuping Wu from Southeast University; Jie Huang from Physcience Golden Silicon New Material Technology Co., Ltd.; and Wen-Han Chen from Contemporary Amperex Technology Co., Ltd..

Particuology (IF=4.1) is an interdisciplinary journal that publishes frontier research articles and critical reviews on the discovery, formulation and engineering of particulate materials, processes and systems. Topics are broadly relevant to the production of materials, pharmaceuticals and food, the conversion of energy resources, and protection of the environment. For more information, please visit: https://www.journals.elsevier.com/particuology.