High temperature molten salts mediated deep regeneration and recrystallization of ternary nickle-rich cathodes

Within the framework of carbon neutrality, the use of lithium-ion batteries (LIBs) is rising along with the growth of the green and clean energy sector. However, the extensive application of LIBs with limited lifespan has brought about a significant recycling dilemma. Traditional hydrometallurgical or pyrometallurgical strategies are not capable of maximizing the output value of spent LIBs to minimize potential environmental hazards. This drives interest in "direct regeneration" techniques that repair cathode bulk structures to boost recycling efficiency.

Meanwhile, low-eutectic-temperature molten salts for lithium replenishment have gained traction due to reduced energy demands and eco-friendly processes. Yet, their low-temperature operation struggles to fully restore severely degraded LIBs. Substantial active lithium loss, impurity phases and structural defects like grain cracks remain inadequately addressed at eutectic-point temperatures.

“These limitations hinder performance recovery in heavily damaged batteries, demanding urgent optimization of molten-salt systems,” shares Gen Chen, corresponding author of a new study published in Advanced Powder Materials. “Enhancing defect-repair capabilities while maintaining energy efficiency is crucial to broaden this method's applicability across varied battery degradation levels.”

In the study, the authors proposed an operable molten-salt regeneration strategy at high-temperature conditions. Compared with prior work, the high-energy medium environment at elevated temperature provides a powerful driving force for complete proceeding of the regeneration reaction. Simultaneously, with the incorporation of high-melting-point KCl salt, the molten-salt system can form homogeneous fluid, leading to sufficient ions exchange and mass transport that is crucial for the secondly growth of grains.

“As a result, the regenerated ternary cathodes, noted as R-NCM, grow into stable single grains with complete defects repair from severely degraded crystal structure,” says Chen. “After deep regeneration and recrystallization, the highly degraded S-NCM can restore the layered structure, and the surface oxygen vacancies and O-TM content can be reduced significantly,” adds Chen. “Additionally, the upcycled cathode with single-crystalline structure effectively suppresses particle cracks and harmful side reactions during cycling, delivering a capacity retention of 81.2% after 200 cycles at 1 ^oC, which is even significantly superior to standard C-NCM.”

Notably, this method can be readily extended to the direct recovery of ternary cathodes with different transition metal ratio, presenting high applicability and compatibility.

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Contact the author: geenchen@csu.edu.cn (G. C.), School of Materials Science and Engineering, Hunan Provincial Key Laboratory of Electronic Packaging and Advanced Functional Materials, Central South University, Changsha, Hunan 410083, P. R. China.

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