Effect of Water Contamination on the Transition Metal Dissolution in Water-Enriched Electrolyte: A Mechanistic Insight into a New Type of Dissolution

Abstract

Transition metal (TM) dissolution from cathode active materials is a major factor in accelerating Li-ion battery aging. Residual moisture in the battery is suspected to enhance this degradation mechanism by reacting with the electrolyte, generating acidic species and other decomposition products. In previous studies, we presented a method to track the dissolution in real-time. In the present study, we demonstrate with this method the effect of water on the dissolution of TMs from LiNi0.33Co0.33Mn0.33O2 (NCM111) and LiCoO2 (LCO) used as thin-film model cathodes. We show highly increased dissolution rates in electrolytes with 100 and 1000 ppm of water added and a concomitant decline in electrode performance. More interestingly, we observe a novel hitherto undisclosed dissolution phenomenon during the cathodic back scan and with that give insight into a new dissolution mechanism. This study further reveals that the impact of moisture on TM dissolution may be mitigated by replacing the standard conductive salt, LiPF6, with hydrolysis-stable LiC2NO4F6S2 (LiTFSI).

Contribution of Electrolyte Decomposition Products and the Effect of Temperature on the Dissolution of Transition Metals from Cathode Materials

“…Surprisingly, even the significantly aged electrolyte sample does not increase the dissolution overall, as shown in Figure . This is in contrast to decomposition products from addition of water to the electrolyte, which we have shown in our previous publication …”

“…In the last set of experiments, cyclic voltammograms (CV) were performed in the range of 3.0–5.5 V vs Li + /Li, with a scan rate of 1 mV s –1 . As stated in our previous paper, these scan rates result in a very high effective cycling rate of ca. 2C.…”

“…This is in contrast to decomposition products from addition of water to the electrolyte, which we have shown in our previous publication. 17 When increasing the temperature of the cathode film (heated plate) to 60 °C, dissolution levels increase markedly. The peak concentrations of Mn and Ni rise from ca.…”

“…The setup adapted from Wachs et al 15 and Ranninger et al 16 has been described in our previous publication. 17 The translational stage (Physik Instrumente PI) was set to different temperatures (25, 50, 60, 70, and 80 °C in this work), with the help of a homemade Peltier-controlled electrode holder platform. Both the cathode and the electroanalytical flow cell were pressed on top.…”

“…Determination of TM concentration levels in the electrolyte of cycled cells, however, does not allow the evaluation of the impact of these compositional changes conclusively. In this study, we tackle the issue by employing our electroanalytical flow cell (EFC), which we have presented in previous publications. − As a model electrode system, we chose LiNi 0.33 Co 0.33 Mn 0.33 O 2 (NCM111) thin-film cathodes, as employed in our previous publication. , The cathode working electrode is the only component directly heated in our setup as the entire cell volume of liquid is constantly replaced by electrolyte at ambient temperature (25 °C) in less than 2 s. We find that already a slight increase of the cathode temperature to 50 °C facilitates TM dissolution under moderate voltages of 4.0 vs Li + /Li, which drastically increases for even higher temperatures. Analysis of electrolytes preheated at 60 °C for 18 h reveals no significant changes in the electrolyte composition; however, some literature reports suggest that even low quantities of decomposition products in LiPF 6 electrolytes, which might not be recognized with GC-MS in the bulk electrolyte, can have a significant impact on surface reactions with the cathode .…”