Revisiting the Mechanism Behind Transition-Metal Dissolution from Delithiated LiNi_xMn_yCo_zO₂ (NMC) Cathodes

Abstract

Dissolution of transition metals (TMs) from lithium-ion battery cathodes under high-voltage conditions is a major issue affecting battery performance that is not well understood mechanistically. Here, this phenomenon is studied by chemically aging pristine and charged LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) cathodes in the presence of different solutions. The solution composition was varied by 1) adding water to a standard electrolyte, 2) replacing LiPF 6 salt with lithium acetylacetonate (Li-acac), 3) and/or adding oxidatively unstable tris(2,2,2-trifluoroethyl) phosphite (TTFP) as an electrolyte additive. Our results demonstrate that while TM dissolution from pristine NMC532 cathodes is dominated by HF-attack, TM dissolution from charged NMC532 cathodes is affected by many other factors apart from HF-attack. We suggest that reduction of TMs due to chemical/electrochemical oxidation of the electrolyte at cathode/electrolyte interface, followed by formation of soluble TM-complexes with concomitant Li + intercalation into the cathode, is the dominant mechanism of TM-dissolution at high voltage.

Understanding the Stability of NMC811 in Lithium-Ion Batteries with Water-in-Salt Electrolytes

“…However, our leaching results are in line with a recent report about the compatibility of Ni-rich NMC materials with a saturated LiNO 3 aqueous electrolyte that identified Ni to be the most soluble species . The apparent discrepancy between organic and aqueous electrolytes is consistent with a report on the leaching behavior of NMC532 that showed that the nature of the electrolyte affects the ratio of dissolved transition metal ions . Further studies are needed to better understand the preference for Ni dissolution in WIS electrolytes.…”

“…30 The apparent discrepancy between organic and aqueous electrolytes is consistent with a report on the leaching behavior of NMC532 that showed that the nature of the electrolyte affects the ratio of dissolved transition metal ions. 31 Further studies are needed to better understand the preference for Ni dissolution in WIS electrolytes.…”

A Comprehensive Understanding of the Aging Effects of Extreme Fast Charging on High Ni NMC Cathode

“…Besides loss of Li, ICP-MS data in Figure 6 maps out the extent of transition metals (TM) (e.g., Ni, Co, Mn) that migrated from NMC811 to the anode for different XFC conditions at different cycles. TM dissolution in NMC chemistry has been reported to happen predominantly at a higher voltage (e.g., >4.3 V) [44][45][46] contributing to capacity fade and impedance rise. Here, the primary goal was to investigate whether different XFC conditions (e.g., 1C to 9C) with lower charge voltage (e.g., ≤4.1 V) would lead to any distinctly different TM dissolution, and if so, how that would affect the electrochemical data.…”

Enabling Extreme Fast‐Charging: Challenges at the Cathode and Mitigation Strategies

“…Transition metal dissolution in the NMC chemistry has been reported to occur predominantly at a higher voltage (>4.3 V) for the non‐XFC conditions. [ 143–145 ] Bulk measurement of TM dissolution through ICP‐MS remains mostly comparable for the charging rates between the 1C to the 9C with a 4.1 V charge cutoff for both the NMC532 and NMC811 cathodes. Similar to non‐XFC and higher voltage conditions, [ 143,144 ] the NMC532 cells had more Mn dissolution, while the NMC811 cells had a higher Ni dissolution than the other two metals.…”