A Deep Look into the Thermochemical Reactions Between Lithium and Carbonate Electrolytes at Elevated Temperature

Lithium metal batteries (LMBs) offer significant potential for enhancing energy density compared to lithium-ion batteries (LIBs) due to their superior theoretical capacity. However, thermal runaway (TR) remains a critical safety concern in LMBs. Understanding the thermochemical interactions between lithium and the electrolyte, especially at elevated temperatures, is essential for identifying the primary trigger reactions responsible for TR. Despite considerable research, controversies persist regarding the chemical reactions between Li and electrolytes, specifically the boiling, thermal decomposition of electrolytes, and Li metal reduction, which hinder a comprehensive understanding of the underlying reaction mechanisms. This study investigates the fundamental reaction mechanisms in Li and 1M LiPF6 EC:DEC (1:1 v/v) systems based on thermochemical and structural data.

Differential scanning calorimetry (DSC) tests of individual components and their combinations revealed the influence of various components on the thermal reactivity of LMBs at elevated temperatures (~400°C). X-ray diffraction (XRD) analysis of the heated samples at 350°C provided further insight into the reaction products involved in the chemical reactions.

The DSC results showed that solvents are crucial in forming the solid electrolyte interphase (SEI). Li+DEC failed to form a stable SEI layer, instead producing ethyl lithium, which dissolved the SEI and induced exothermic reactions before Li melting. In contrast, Li+EC formed a stable SEI layer composed of Li2CO3 and Li2O, preventing Li from reacting with the electrolytes until it melted. Thus, Li+EC:DEC initiated the first exothermic reaction after Li melting, indicating that molten Li completely disrupted the SEI layer. This makes Li melting a key trigger for TR. XRD analysis showed that Li+EC primarily contained organics due to polymerization at high temperatures, while Li+DEC was dominated by Li2CO3. Meanwhile, Li+EC:DEC showed the presence of LiH and organic compounds. Given that exposed Li2CO3 in the outer SEI layer can react with otherwise stable electrolytes [1], the results suggest that Li2CO3 could react with EC.

LiPF6 significantly increased the heat of reaction between Li and EC-containing electrolytes. For Li+EC, the heat increased from -34.6 kJ/g Li to -70.4 kJ/g Li, and for Li+EC:DEC, it rose from -40.5 kJ/g Li to -78.0 kJ/g Li. A secondary exothermic reaction occurred around 230°C, coinciding with the decomposition point of LiPF6. Although LiFP6 can assist in creating a stable SEI layer by forming LiF at room temperature, it becomes evident that at elevated temperatures, LiPF6 increases the probability of thermal hazard by reacting with Li and EC through decomposition. XRD results confirmed the presence of LiF as a major reaction product in Li+1M LiPF6/EC:DEC, supporting the involvement of Li2CO3+PF5 and Li+POF3 reactions during exothermic reactions. Additionally, the decomposition of LiPF6 at elevated temperatures releases PF5, a strong Lewis acid, leading to the decomposition of the otherwise stable solvents. Also, HF can be formed by trace water, which can attack the SEI layer and expose Li metal to the electrolyte.

Furthermore, this study demonstrates that the concentration of electrolytes critically influences heat release. A linear increase in heat was observed until the electrolyte-to-Li ratio (mol/mol) reached approximately 2.5, after which the heat converged to around -66.64 kJ/g Li. This suggests carbonate electrolytes can function as oxidizers, analogous to propellant systems.

In conclusion, this study has identified the primary reactions triggering TR in LMB systems: Li melting and LiPF6 decomposition. Also, the electrolyte concentration and the presence of LiPF6 play crucial roles in determining the intensity of reaction heat. These findings are vital for addressing TR issues and optimizing LMB design and performance.

References
[1] Han, Bing, et al. Advanced Materials 33, 2100404 (2021).