Ni-Ion-Chelating Strategy for Mitigating the Deterioration of Li-Ion Batteries with Nickel-Rich Cathodes

The proliferation of electric vehicles has increased exponentially over the past few years, and the ban on petrol and diesel vehicles in European nations has caused automakers to switch their primary focus toward electric vehicles. Engines, which are the most critical components of automobiles, have been replaced by motors with batteries; specifically, batteries containing Ni-rich cathodes are essential in ensuring high performances, e.g., in extending the mileage of electric vehicles. However, high-valence Ni4+, which is formed in a highly charged state in such batteries, is prone to reduction to Ni3+ and Ni2+, resulting in oxygen loss and cation mixing. In addition, residual Li species, such as LiOH and Li2CO3, induce parasitic reactions in electrolytes. Furthermore, Ni2+ dissolved from Ni-rich cathodes by acidic compounds, such as HF formed by LiPF6 hydrolysis in the electrolyte, induces structural deterioration by forming an inactive rock-salt phase and the loss of the Li storage sites of the cathode. Further, transition metals electrodeposited at the anode surface via the dissolution–migration–deposition of transition metal ions (TM-DMD) hinder the intercalation of Li+ within the anode structure, catalyze undesirable electrolyte decomposition reactions, and act as solid–electrolyte interphase (SEI) components. The electrodeposited transition metals also increase the probability of the formation of dendritic Li, which threatens battery safety. Thus far, surface coating of the cathode and forming a protective film on the cathode using functional electrolyte additives have been proposed as viable solutions to minimize transition-metal-ion dissolution from LiNi0.85Co0.1Mn0.05O2 (NCM85) cathodes. However, Ni2+, which is similar to Li+ in size, can easily penetrate the surface coating layers and cathode protective films because they should guarantee facile Li+ transport while blocking electron transfer to prevent electrolyte decomposition. Electrolyte additives that suppress HF generation or scavenge HF do not completely remove HF, and thus, the cells exhibit Ni-dissolution- and deposition-related problems. Chelation of the dissolved transition metal ions may prevent electrodeposition on the surface of the anode. However, transition-metal chelating agents can hardly be applied as electrolyte additives because they decompose electrochemically at the electrodes, resulting in shortened battery lifespans. Studies regarding chemically active separators with insoluble bipyridine (C-N) ligands and gel polymer electrolytes based on polymer matrices containing pyrrolidone (C-N-C=O) moieties as chelating functional groups have been conducted in efforts to avoid undesirable decomposition of the chelating agents at electrodes. Nevertheless, the incorporation of a chelating agent into the electrolyte without additional processing is clearly a more efficient method of capturing Ni ions from scalability and techno-economic standpoints. Further, the microquantity of chelating agent as an electrolyte additive does not cause significant changes in the rheological, chemical, or electrochemical properties of the electrolyte, which may increase the cell impedance. With the aim of enhancing cell performance, we report the use of a tricoordinate phosphorous compound, 1,2-bis(diphenylphosphino)ethane (DPPE), to provide effective donor ligands that are capable of forming complexes with Ni2+ dissolved in electrolytes, thereby preventing the electrodeposition of Ni2+ on the anode surface. Further, DPPE as a Lewis base additive can deactivate Lewis acidic PF5, which can generate corrosive HF, mitigate the damage of the SEI and cathode electrolyte interface (CEI), and alleviate PF5-driven electrolyte solvent decomposition. DPPE, as an electrolyte additive, imparted a remarkable cycling stability on a Li-ion battery (LIB)composed of an NCM85 cathode and a graphite anode. DPPE chelated Ni2+, which may occur in the electrolyte, and blocked the generation of undesirable species, which cause Ni2+ dissolution from the NCM85 cathode via the destabilization of PF5, which leads to HF generation. With the optimized binding force between Ni2+ and DPPE, dissolved Ni2+ could be effectively trapped, reducing the overpotential of lithiation of graphite caused by electrodeposited Ni. Severe structural deterioration of the NCM85 cathode, including microcracking and phase transition to the rock-salt phase, was significantly suppressed using DPPE. The results of this study will contribute to significant advances in the development of electrolyte additives, which may selectively trap transition metal ions dissolved in the electrolyte and eliminate the detrimental substances causing transition metal dissolution, thus realizing high-energy-density LIBs.

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