Exploiting Deep Eutectic Solvents in LiFePO4 Battery Recycling: From Cathode Leaching to Resynthesis

Abstract

Nowadays, the increasing demand for electronic devices and electric vehicles influences the request for Lithium-Ion Batteries (LIBs), which are worldwide the most exploited electrochemical energy storage systems for portable devices and are expected to completely dominate also the sector of transportation soon. In particular, the use of LIBs with ordered-olivine structure LiFePO4 (LFP) cathodes stands out ever since their invention in 1997, being a valid solution in terms of durability, reliability, and safety, essential characteristics in the electric mobility application combined with the benefit of low content of critical raw materials, CRMs. Considering that the average life of LFP cathodes is about 5-8 years, the management of these materials as waste becomes essential: the growing number and volumes of LIBs that are currently produced is inevitably intended to correspond to the number of LIBs that will reach the end of their life, becoming waste that are accumulating, posing a staggering danger. [1,2] An also important consideration concerns the conditions required to synthesize LFP for electrochemical purposes, as it is necessary to coat the surface with conductive species, controlling the morphology and reducing the particle size, thus complicating the synthesis and making it economically more impactful. [3]In such a context, recycling combines the necessity to produce new LIBs with the need to cope with the increase in batteries’ waste accumulation: this can limit the disposal of waste batteries into landfills, thus decreasing the inevitable hazardous environmental consequences, while reducing the demand for new mining, re-assigning value to raw materials, in an attempt to minimize the costs that would be required to synthesize pristine LFP-based cathodes. Today, recycling is already implemented at the industrial level, exploiting the pyrometallurgy approach, which involves thermal treatments at very high temperatures (>1000°C) allowing for the recovery of only a fraction of valuable metals; nevertheless, this approach has a strong environmental impact due to the high energy demand and high carbon footprint. The hydrometallurgical approach is also employed, exploiting the leaching process with strong inorganic acids; however, this brings problematic issues, because producing such acids is itself a polluting process, and acidic wastewater management is a demanding problem. Moreover, today these processes involve the recycling of the most diffused cathode, LiCoO2, Li(Ni/Mn/Co)O2, LiMn2O4, and the sustainability of recycling relies on the possibility to recover valuable CRMs such as Co and Ni.In the proposed work we are currently investigating the degradation properties of Deep Eutectic Solvents (DESs), cheap and greener mixtures of compounds forming a eutectic system, liquid at room temperature. [4] In particular, the focus is on the leaching of LFP with a DES obtained by mixing different molar ratios of hydrogen bond acceptor and various hydrogen bond donors, in order to assess the best combination to leach the elements out of LFP structure. The key point of such approach is the design of a degradation system that can enable the conditions for the direct re-synthesis of new LFP material. Indeed, here the sustainability of the process cannot be driven by the recovery of CRMs, thus must rely on the close-loop recycling scheme. For this reason, we specifically designed DES compositions that allow for this process.The proposed DESs, which have been easily prepared at mild heating conditions, have been investigated through different techniques; the results obtained on the chemical-physical characterization (via FTIR spectroscopy, Solid-State NMR, viscosity, thermal analysis) will be presented. Among the different DESs, those with the best combination of properties (e.g. low viscosity, high thermal stability) have been explored for the leaching of LFP cathode. The leaching process has been optimized with respect to the temperature, duration, and cathode-to-DES ratio for different DESs, and the leaching yields have been evaluated through ICP-OES analysis. The obtained leached solution has been considered as the starting point for a sol-gel-like synthesis of new LFP material, finally characterized through XRD, TGA, ICP-OES, and SEM-EDX analysis. Tuning the parameters of the synthesis (temperature and duration of the heat treatment, ratio between LFP mass and conductive species to realize the coating) enables to control the morphology and the dimension of the synthesized particles, to find the best condition for the electrochemical application. The electrochemical performances of the resynthesized materials will also be presented.[1] Sun et al., J. Alloys Compd., 818 (2020)[2] Lv et al. ACS Sustain. Chem. Eng., 6 (2018)[3] Yuan et al., Energy Environ. Sci, 4 (2010)[4] Zhu et al., Resour. Conserv. Recycl., 188 (2023)