Direct regeneration of degraded lithium-ion battery cathodes with a multifunctional organic lithium salt

The escalating demand for lithium-ion batteries (LIBs) in electric vehicles and energy storage systems necessitates effective recycling strategies to address environmental concerns and resource scarcity. Lithium iron phosphate (LiFePO4, LFP) batteries are widely used due to their inherent advantages, including structural stability and longevity. However, capacity fade in LFP cathodes is primarily caused by lithium loss, leading to the formation of the Fe(III) phase and poor electrical conductivity, limiting rate capability. Current end-of-life battery recycling methods such as pyrometallurgical and hydrometallurgical recycling suffer from drawbacks like high energy consumption, use of harsh chemicals, and low economic value of the byproducts. Direct regeneration offers a more sustainable alternative, focusing on restoring the original composition and structure of the degraded cathode. This study proposes a novel direct regeneration method leveraging the multifunctional properties of Li2DHBN to address the key challenges of lithium loss, Fe(III) formation, and poor conductivity simultaneously, offering a potentially more efficient and cost-effective recycling strategy.

Previous research has explored direct recycling of degraded LFP cathodes using various approaches. These include targeted healing methods, the use of prelithiated separators, and molten salt processes. While these methods have shown some success, they often require additional steps, such as the introduction of extra carbon sources, to improve conductivity. In contrast, the current study aims to achieve complete restoration by using a single multifunctional lithium salt capable of addressing multiple degradation issues concurrently. Inorganic lithium salts, such as Li2CO3, LiOH, and Li2SO4, have been explored as lithium sources, but their monofunctional nature limits their effectiveness in creating a reductive atmosphere or providing a carbon source. The use of conductive agents like carbon and conducting polymers is common to improve rate performance; however, these coatings can degrade over time, hindering long-term performance. The existing literature underscores the need for a more integrated and efficient approach to direct regeneration, capable of simultaneously restoring lithium content, reducing Fe(III), and enhancing conductivity.

The study utilized spent LFP battery cathodes from 18650 cylindrical cells (LG manufacturer). The cathodes were disassembled to obtain the active material, which was characterized to assess its degradation. Inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were employed to analyze the composition and phase structure. The multifunctional organic lithium salt, Li2DHBN, was synthesized using a previously reported method involving 3,4-dihydroxybenzonitrile and lithium hydride in tetrahydrofuran. The regeneration process involved mixing the degraded LFP with Li2DHBN, followed by heat treatment at 800 °C under an Ar/H2 atmosphere. The optimal amount of Li2DHBN and the sintering temperature were determined through single-factor experiments. For comparison, inorganic lithium salts (Li2CO3 and LiOH) were also used. The regenerated cathodes were characterized using ICP-OES, XPS, XRD, high-resolution transmission electron microscopy (HRTEM), and energy dispersive spectroscopy (EDS). Electrochemical characterization included galvanostatic charge-discharge cycling, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) to assess the performance of the restored cathodes. In-situ XRD was used to monitor phase changes during heat treatment. Electron energy loss spectroscopy (EELS) was utilized to study the surface valence states and the concentration distribution of elements in single particles. Techno-economic analysis was performed by comparing the hydrometallurgical and direct regeneration routes, considering factors such as raw material costs, reagent costs, labor costs, and revenue generated from recovered materials.

ICP-OES results revealed a significant lithium deficiency in the degraded LFP, consistent with the formation of the Fe(III) phase, as confirmed by XPS and XRD. HRTEM and EDS analysis showed that the Li2DHBN-treated cathodes exhibited a uniform carbon coating (4-5 nm thick) derived from the pyrolysis of the salt. XPS depth profiling demonstrated the elimination of the Fe(III) phase after regeneration with Li2DHBN. Electrochemical tests showed that the regenerated cathode using Li2DHBN exhibited superior performance compared to the inorganic salt-treated cathodes. It displayed higher specific capacity (157 mAh g⁻¹ at 0.1 C), lower polarization voltage difference, and a faster lithium-ion diffusion rate. The Li2DHBN-regenerated cathode demonstrated excellent rate capability, maintaining high capacities even at 10 C. The superior low-temperature performance (-20 °C) and long-term cycling stability (88% capacity retention after 400 cycles at 5 C) were attributed to the enhanced conductivity due to the carbon coating. In-situ XRD confirmed that Li2DHBN facilitated lithium insertion and the reduction of the Fe(III) phase. EELS analysis of single particles revealed that the degraded LFP displayed inhomogeneous phase distribution, with Fe(III) concentrated near the surface. The Li2DHBN treatment eliminated Fe(III) and achieved a homogeneous LFP phase. Techno-economic analysis showed that the direct regeneration method using Li2DHBN was significantly more profitable than traditional hydrometallurgical recycling, especially when the residual lithium content in the spent cathodes was low. This method also required significantly less energy and resulted in lower greenhouse gas emissions compared to pyro- and hydro-metallurgical recycling.

The findings demonstrate the efficacy of the proposed direct regeneration method using the multifunctional organic lithium salt Li2DHBN. The ability of Li2DHBN to simultaneously compensate for lithium loss, reduce Fe(III), and enhance conductivity through carbon coating represents a significant advancement in spent LFP cathode recycling. The superior electrochemical performance of the regenerated cathode, coupled with the economic and environmental advantages, makes this approach highly promising for large-scale application. The insights gained from the EELS analysis regarding the inhomogeneous phase distribution in degraded LFP particles provide valuable information for developing improved regeneration strategies. While the focus was on LFP cathodes, the successful application of Li2DHBN to other cathode materials suggests its broader applicability.

This study presents a highly efficient and sustainable strategy for direct regeneration of degraded LiFePO4 cathodes from spent LIBs. The multifunctional organic lithium salt, Li2DHBN, successfully addresses the critical challenges associated with lithium loss, Fe(III) formation, and poor conductivity, leading to significantly improved electrochemical performance. Techno-economic analysis demonstrates the economic and environmental advantages of this method compared to traditional recycling techniques. Future research should focus on optimizing the process for highly degraded cathodes and extending the methodology to a wider range of cathode chemistries. Large-scale implementation requires further investigation into practical considerations such as spent battery collection and transportation.

The current study focused primarily on spent LFP cathodes from a specific source. While the successful regeneration of LiCoO2 and LiNi0.5Co0.2Mn0.3O2 cathodes indicates broader applicability, further investigation is needed to validate the method's effectiveness with a wider range of cathode materials and degradation levels. The techno-economic analysis provides a valuable assessment but may not encompass all real-world factors, such as transportation and preprocessing costs. Although this study provides evidence of the effectiveness of Li2DHBN, large-scale implementation may necessitate optimization of the synthesis and regeneration process to ensure cost-effectiveness and scalability.