Towards Moisture Tolerant Lithium-Ion Batteries: A Systematic Investigation on the Effect of Moisture on Ni-Rich NMC Cathodes

Abstract

In the ever-expanding lithium-ion battery (LIB) industry, lithium hexafluorophosphate (LiPF6) has been the most dominant lithium salt for preparing electrolytes. However, the challenges related to its moisture sensitivity (manifested by the formation of hydrofluoric acid) and thermal instability are yet to be addressed. Due to its improved ionic conductivity, reduced risk of producing hydrofluoric acid and better thermal and chemical stability, Lithium bis(fluorosulfonyl)imide (LiFSI) has been considered as a potential alternative salt to replace LiPF6. Nonetheless, LiFSI salt is currently not available in volumes relevant to industrial production; hindered by its high cost and the presence of trace quantities of chloride impurities. In addition, for operating cells at upper cut off voltages higher than 4.1~4.2 V, oxidative aluminum current collector corrosion [1] is a challenge.Among the high energy density LIB cathode materials, Nickel-rich LiNixMnyCozO2 (NMCs) are the most promising choices to fulfil the long mileage per charge requirements for powering electric vehicles [2]. In this study, the effect of moisture on the electrochemical performance of NMC622/Li and NMC622/graphite pouch cells was systematically investigated by adding water into 1 M LiFSI electrolyte. To better understand the interaction and effect of the added H2O with other electrolyte additives such as fluoroethylene carbonate (FEC), the electrochemical performance of the NMC622/Li half cells were tested with four different electrolytes; 1 M LiFSI in EC/DMC (50/50, v/v, control), 1 M LiFSI + 1000 ppm water, 1 M LiFSI + 10 wt% FEC, and 1 M LiFSI + 1000 ppm water + 10 wt% FEC. Double layered ultra-high molecular weight polyethylene separator (Solupor 3P07B) was used in all measurements. The water content in the electrolytes with 1000 ppm water was regularly monitored using the Karl-Fisher titration technique.It is intriguing that the addition of 1000 ppm water into the 1 M LiFSI electrolyte improved the specific discharge capacity and capacity retention of the NMC622/Li pouch cells. Furthermore, to investigate the morphological, microstructural, and chemical evolutions of the electrodes after short- and long-term cycling performance tests, postmortem characterizations such as ex situ XRD, S(T)EM, SEM/EDS, FTIR and XPS are being conducted.As expected, the oxidative aluminum current collector corrosion was scrutinized by running cyclic voltammetric measurements with the different electrolytes and the results were compared with that in 1 M LiPF6 (Figure 1a). It should be noted, however, that the voltammograms are obtained in the voltage range of 3.0 – 4.9 V. Ex situ SEM images of the cycled aluminum electrodes exhibited the formation of a thin film on the surface (Figure 1b, 1c), and the film formed on the aluminum electrode cycled with 10 wt% FEC was thicker compared to those with 1 M LiFSI and 1 M LiFSI + 1000 ppm H2O electrolytes. It was evident from the oxidative aluminum corrosion study that the current density and pitting corrosion were relatively smaller in the 10 wt% FEC containing electrolytes (Figure 1a). This is in agreement with the formation of relatively thicker films on the electrode surface with FEC containing electrolytes, as discussed above.Overall, it was evident from this study that addition of 1000 ppm water enhanced the discharge capacity and capacity retention of NMC622/Li pouch cells, and such moisture tolerant LiFSI based lithium-ion batteries can open new pathways which can ultimately enable the development of relatively higher dew point and less energy intensive dry rooms. However, for cell operations above ca. 4.3 V, proper prevention mechanisms are required for the oxidative aluminum current collector corrosion in such systems.