Transition Metal Dissolution and Degradation in NMC811-Graphite Electrochemical Cells

Abstract

Nickel-rich lithium nickel-manganese-cobalt oxide cathodes, in particular Li(Ni 0.8 Mn 0.1 Co 0.1 )O 2 (NMC811), are currently being commercialized as next generation cathode materials, due to their increased capacities compared to current materials. Unfortunately, the higher nickel content has been shown to accelerate cell degradation and a better understanding is needed to maximize cell lifetimes. NMC811/graphite cells were tested under stressed conditions (elevated temperature and cell voltages) to accelerate degradation focusing on transition metal (TM) dissolution from the cathode. Increasing the cell temperature, upper cutoff voltage (UCV) and number of cycles all accelerated capacity fade and diffraction studies showed that under stressed conditions, additional degradation mechanisms beyond lithium loss to the SEI are present. Significant TM dissolution and subsequent deposition on the graphite anode is seen, particularly at stressed conditions. The concentration of TMs in the electrolyte remained invariant with cycling conditions, presumably reflecting the limited solubility of these ions and emphasizing the role that TM deposition on the anode plays in continuing to drive dissolution. Significant deposits of metals from the cell casings and current collectors were also detected at all cycling conditions, indicating that corrosion and metal leaching can be as important as TM dissolution from the active material in some cell formats.

Electrolyte reactivity at the charged Ni-rich cathode interface and degradation in Li-ion batteries

“…20,68 Oxygen release also leads to under coordinated TMs at the surface of NMC, which will be easier to dissolve. 44,69 Furthermore, the strong electrolyte dependence, and specifically the significantly higher TM dissolution/deposition for EC electrolyte, is consistent with the enhanced lattice oxygen release (and hence H2O and H + formation) observed for EC electrolyte compared to EMC electrolyte.…”

Two electrolyte decomposition pathways at nickel-rich cathode surfaces in lithium-ion batteries

“…After cycling the NMC/LFP cells, the 1 H NMR signals have broadened, most likely as a result of an increased transition metal concentration in the electrolyte due to the high HF concentration. 14,[81][82][83] For the cells cycled below the gas evolution onset potential (4.1 and 4.3 V), a weak signal assigned to formaldehyde (9.58 ppm) appears (Fig. 10b and c).…”

Identifying surface degradation, mechanical failure, and thermal instability phenomena of high energy density Ni-rich NCM cathode materials for lithium-ion batteries: a review

“…The root cause of transition metal dissolutions are the high state of charge and high temperature. 104 Transition metal dissolution is ascribed to a combined effect of the cation mixing and oxygen evolution reaction, in which the cation mixing accompanies the entire life of the battery while the oxygen evolution occurs mainly in the H3 phase. On the one hand, the cation mixing and the oxygen evolution reaction result in the formation of low-valence transition metal oxide (MO) by the loss of oxygen.…”