Some Effects of Intentionally Added Water on LiCoO₂/Graphite Pouch Cells

Abstract

LiCoO 2 /graphite pouch cells with 1 M LiPF 6 in EC:DEC (1:2 v/v) (control electrolyte) containing selected additives and different amounts of intentionally added water were studied. Archimedes principle was used to track gas evolution while an automated storage system, electrochemical impedance spectroscopy and cell cycling experiments were used to study electrochemical properties imparted by the addition of water. Water was added from 125 to 2000 ppm to control electrolyte and to electrolyte with 2% vinylene carbonate (VC) and/or 2% lithium bis(trifluoro methane sulfonyl imide (LiTFSI). The addition of water to either VC or VC + LiTFSI-containing electrolyte resulted in minimal or no changes in voltage drop and charge transfer resistance during storage while both added water and LiTFSI actually decreased swelling during storage. Added water to control electrolyte or VC-containing electrolyte did not impair cell cycling performance at 55 • C. Solution nuclear magnetic resonance (SNMR), in-situ gas measurements, gas chromatography coupled with a thermal conductivity detector (GC-TCD), and X-ray photoelectron spectroscopy (XPS) were employed to understand why added water did not affect cell performance. Lithium-ion batteries are generally used in power tools and portable electronics applications. Li-ion battery packs for electrified vehicles and grid energy storage need even longer lived and, preferably, less expensive cells. Water is commonly regarded as being detrimental to cell performance, because HF generated from water and LiPF 6 (widely used in lithium-ion batteries) can react with SEI components and corrode positive materials causing transition metal dissolution.1,2 Therefore, Li-ion cell manufacturers generally lower water contents in electrolyte solvents and electrolytes to less than 50 ppm. 3Stringent water content specifications lead to increased manufacturing cost. Recently, Burns et al. [4][5][6] found that water (up to 1000 ppm) added intentionally to the electrolyte of Li-ion prismatic cells with LiCoO 2 (LCO) or LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) positive electrodes and graphite or LTO negative electrodes did not have a detrimental effect on cell performance. In some cases, the addition of water was even beneficial in terms of coulombic efficiency, cell impedance and voltage drop during storage. They found that the added water caused cell swelling during the first cycle and generally the more water that was added the larger expansion the cells experienced. The swelling of cells can lead to mechanical stress of the devices containing the cells. Pouch cells are widely used in the Li-ion industry and are an excellent cell format to study gas evolution due to their flexible packaging. 7,8 In addition, pouch cells are degassed after the first formation step so any initial gases produced by the presence of water can be removed.In this study, pouch cells were chosen to study the impact of intentionally added water on their electrochemical properties and gas evolution. Various experimental techniques were ...

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“…Some amount of LiPO 2 F 2 is expected from the uncoated materials due to reaction of the LiPF 6 with (a) residual moisture at the surface and/or (b) residual Li 2 CO 3 from the lithiation process. There were no peaks present in the 19 F spectra at −190 ppm, indicating there was negligible HF, a known byproduct of the reaction of LiPF 6 and H 2 O . This is significant because HF can lead to cell degradation.…”

“…The sealed pouch bags were then stored at either 40 or 60 °C for 1 week. Given that the introduction of water to electrolyte solutions is known to produce LiPO 2 F 2 , 43 all of the materials in this work were meticulously dried and handled to avoid contamination. It is also known from previous work that solution NMR is a suitable method for the detection of small quantities of LiPO 2 F 2 based on the characteristic 19 F and 31 P chemical shift positions and multiplet splitting patterns (Figure 2).…”

“…There were no peaks present in the 19 F spectra at −190 ppm, indicating there was negligible HF, a known byproduct of the reaction of LiPF 6 and H 2 O. 43 This is significant because HF can lead to cell degradation. Further analysis was performed by comparing the integral of the PF 6 − and PO 2 F 2 − peaks at −75 and −85.8 ppm, respectively (Figure 7).…”

Formation of Surface Impurities on Lithium–Nickel–Manganese–Cobalt Oxides in the Presence of CO2 and H2O

“…It is generally believed that insulating materials can be produced on the cathode from the surface impurities reacting with the electrolyte leading to increased kinetic barriers in the Li + insertion/desertion process . However, Li 2 CO 3 as an additive in electrolytes or one component of a coating layer is found to be beneficial to mitigate side reactions at the cathode surface. , Although water is generally considered to be detrimental to LIB performance, the addition of trace amounts of H 2 O has been shown not to impair electrochemical performance, and the addition of 1000 ppm water is found to improve certain aspects of battery performance. , In addition, LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathodes formulated in water show excellent capacity retention after 1000 cycles …”

“…14,15 Although water is generally considered to be detrimental to LIB performance, the addition of trace amounts of H 2 O has been shown not to impair electrochemical performance, and the addition of 1000 ppm water is found to improve certain aspects of battery performance. 16,17 In addition, LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathodes formulated in water show excellent capacity retention after 1000 cycles. 18 The impurities of carbonate and hydroxide either come from unreacted precursors during synthesis or can be formed during storage in the presence of ambient CO 2 and H 2 O.…”