Study of reaction characteristics and controlling mechanism of chlorinating conversion of cathode materials from spent lithium-ion batteries

Highlights

• Nearly 100% metal leaching rates are achieved by chlorinating conversion.

• Contribution of sub-reactions is used to determinate reaction controlling mechanism.

• The reaction controlling mechanism is determined to explain activity energy change.

Abstract

Spent lithium-ion batteries (LIBs) recycling has attracted much attention because it is highly favorable to environment protection and sustainable development. Developing a cleaner method for metals extraction can greatly reduce risk of secondary pollution. Chlorinating technology has been proved as an efficient method for metals extraction instead of traditional hydrometallurgy. In this paper, cathode materials from spent LIBs could be rapidly converted into metal chlorides by NH₄Cl roasting at 623 K for 20 min. The results indicated nearly 100% metal leaching rates were achieved. Further, in-depth study is performed to obtain the mechanism function of chlorinating conversion based on roasting and TGA experiments. The apparent activation energy as 73.40 kJ/mol was firstly obtained, and then the reaction model of chlorination reaction was determined by model fitting and verifying. Herein, sub-reactions of chlorination reaction were figured out and their contributions were used to determinate reaction controlling mechanisms of chlorination reaction. The results indicated that nucleation reaction played a leading role in the initial stage (0.05 < α < 0.43) while phase boundary reaction took the control in next stage (0.43 < α < 0.95), which gave a good explanation to activation energy change. Finally, our findings provided inspirations for studying the controlling mechanism of gas-solid reaction.

Introduction

Lithium transition metal oxide (LTMO) batteries including LiCoO₂ (LCO), LiMn₂O₄ (LMO) and LiNi_xCo_yMn_1−x-yO₂ (NCM) have occupied the major market share due to their excellent properties (Dunn et al., 2011, Goodenough and Park, 2013). Recently, spent LTMO-type batteries processing has attracted significant attention because it can greatly alleviate the problems of environmental pollution and resource shortage (Li et al., 2010, Xiao et al., 2019). Many promising technologies have been studies to recover metal products from spent LTMO-type batteries to realize the closed-loop process (Fu et al., 2020a, Zhang et al., 2018). Among these technologies, hydrometallurgy is regarded as one of the most competitive technologies of recycling high-purity products ready for the reproduction of new batteries (Fu et al., 2020b, Li et al., 2018). However, heavy use of chemicals in the metal extraction has limited its practical application from the perspective of cleaner production and sustainable development (Xiao et al., 2017).

Great efforts have been made to reduce chemicals consumption in extracting metals from spent LTMO-type batteries. By co-grinding method, waste polyvinyl chloride (PVC) was used to extract lithium (Li) and cobalt (Co) from LCO cathode materials in replacement of traditional leaching agents (Shu et al., 2004). Furthermore, to improve the extracting efficiency and avoid secondary organic pollution, supercritical water technology was reported to obtain over 95% metal extracting efficiencies without releasing toxic chlorinated organics (Liu and Zhang, 2016). Recently, subcritical water treatment was also applied in metal extraction from spent LCO batteries using the thermoplastic materials as chlorinated polyvinyl chloride (CPVC) as the additive (Nshizirungu et al., 2020a). The results indicated that more than 97.69% Li and Co was recycled in single step at 523 K in 60 min. Further study revealed that Ni²⁺ as the catalyst could accelerate the de-chlorination of CPVC to produce HCl and higher metals extracting rates were realized (Nshizirungu et al., 2020b). Nevertheless, in the metal extracting process, it is still of great interest to develop more applicable technologies for improving the handling capacity and reducing equipment costs.

Chlorinating metallurgy is known as an operable technology which is a traditional industrial method for metal extraction, where chlorine (Cl₂), hydrogen chloride (HCl), chlorides (CaCl₂, NaCl), and organic chlorides (PVC, CCl₄) can be used as the chlorinating agents (Jena and Brocchi, 2008, Manukyan and Martirosyan, 2003). By chlorinating roasting, metals can be efficiently extracted out from metal ores or wastes. However, the production of harmful gases and the introduction of impurities have greatly limited its practical application (Xing et al., 2020). Recently, NH₄Cl has been proved as a promising agent to provide NH₃ and HCl for metal extraction, reducing environmental hazards and avoiding impurities introduction (Panda et al., 2020). Besides, the spare NH₃ and HCl can regenerate as NH₄Cl for the reuse. NH₄Cl roasting was used by Fan et al. (2019) to recover high-purity products from spent LTMO-type batteries instead of wet-leaching process. According to Qu’s study, the recovered products could be used to regenerate LCO which delivered over 139.8  mAh/g at 0.5 C and kept a 99% capacity retention rate after 100 cycles (Qu et al., 2020). In addition, Xiao et al. (2020) proposed an O-layer cutting mechanism to explain the dry-extraction of metals from spent LTMO-type batteries. In a word, chlorinating technology with NH₄Cl as the only additive has been proved as a promising method to replace wet-leaching process of spent LTMO-type batteries processing, and less waste is produced. However, lots of efforts are still needed to realize the practical application of NH₄Cl roasting in spent LTMO-type batteries processing.

Until now, little attention has been paid on the in-depth study of chlorinating conversion of cathode materials from spent LTMO-type batteries. In-depth understanding the reaction controlling mechanism is important to the practical application of chlorinating technology. It should be highly noted that kinetic study on chlorinating conversion of spent LTMO-type batteries, has of great significance for increasing main reaction rate, controlling reaction processes, improving conversion efficiency (Ašperger, 2003, Chang et al., 2015). Therefore, the study of reaction controlling mechanism is becoming much necessary to further development of chlorinating technology for spent LTMO-type batteries recovery.

Accordingly, this study aims to investigate the thermal transformation mechanism of cathode materials from spent LTMO-type batteries. In previous works, the chlorinating conversion of cathode materials from LCO and LMO batteries was studies in detail and the results showed that three types LTMO batteries had similar properties in the processing (Xiao et al., 2021, Xiao et al., 2020). Therefore, NCM materials were chosen as the experimental materials in this study. Roasting and thermogravimetric analysis (TGA) experiments were first performed for the determination of chlorination reaction. The iso-conversional method was then applied to conduct the kinetic analysis. Finally, the reaction models and reaction controlling mechanisms were determined to better understand the chlorinating conversion.