Dramatic Effects of Low Salt Concentrations on Li-Ion Cells Containing EC-Free Electrolytes

Abstract

Different concentrations of LiPF 6 (0.3 M-2 M) in ethyl methyl carbonate (EMC) electrolyte and ethylene carbonate (EC)-based electrolyte were studied in LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442)/graphite pouch cells. Fresh cells containing 0.3 M LiPF 6 in EMC electrolyte showed extremely large charge transfer resistance while those with 0.3 M LiPF 6 in EC/EMC electrolyte did not. Impedance spectra taken on symmetric cells and ionic conductivity measurements suggest this difference is due to difficulty in dissociating and desolvating Li + ions from the EMC-based electrolyte to intercalate into both the electrodes. After elevated temperature storage experiments at 4.5 V, cells with 0.3 M LiPF 6 in EC/EMC showed a large increase in positive electrode charge transfer impedance, presumably caused by electrolyte oxidation. With salt concentrations greater than 1 M, charge transfer resistance was much smaller in EMC-based electrolytes and was stable during storage for both electrolyte types. Conductivity and cycle testing measurements suggest that 1.5 M LiPF 6 should be used in EC-free EMC-based electrolytes to optimize cell performance. Using high potential positive electrodes can increase the energy density of Li-ion cells. [1][2][3][4] Layered LiNi x Mn y Co z O 2 (x + y + z = 1) (NMC) electrodes have been extensively studied because they are cheaper materials than LiCoO 2 (LCO) and can operate at high working potentials.5-15 LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442) is a possible choice for high voltage application due to its high working potential, high specific capacity, moderate cost and excellent safety. [16][17][18][19] NMC442 has been shown to be structurally stable up to 4.7 V 16,17 and its specific capacity increases almost linearly as the upper cutoff voltage increases from 4.1 V to 4.7 V. At 4.7 V, NMC442 can deliver a reversible specific capacity of ∼207 mAh/g. NMC442 shows attractive safety features compared to other NMC grade materials as well. 19It is difficult to create NMC442/graphite cells that show long lifetime when operated continually to an upper cutoff potential above 4.4 V. Electrolyte additives such as pyridine boron fluoride (PBF), 20,21 triallyl phosphate (TAP) 22 and additive blends such as prop-1-ene-1,3-sultone (PES) + 1,3,2-dioxathiolane-2,2-dioxide (DTD) + tris-(trimethyl-silyl) phosphite (TTSPi) [23][24][25][26][27] in EC-based electrolyte have been used to improve charge-discharge cycle life and calendar life of cells operated to 4.4 V. Sulfone-based and fluorinated electrolyte systems have also shown to be of value in getting NMC442/graphite cells to operate beyond 4.4 V. 28,29 In spite of these improvements, further improvement is required. Recently, it was found that EC-free electrolyte using only ethyl methyl carbonate as the solvent can enhance high voltage operation and lifetime of NMC442/graphite pouch cells compared to EC-based electrolyte.30-32 However, only a salt concentration of 1 M LiPF 6 in this novel electrolyte was investigated.The concentration of LiPF 6 in EC-based electrolyte...

The Interplay between Salt Association and the Dielectric Properties of Low Permittivity Electrolytes: The Case of LiPF6 and LiAsF6 in Dimethyl Carbonate

“…Our work highlights the importance of including the salt while considering electrolyte overall dielectric behavior, even at low concentrations. Furthermore, we suggest that similar behavior should be expected in other linear carbonates such as diethyl carbonate (DEC) and ethyl methyl carbonate (EMC), which have similar dielectric constants to DMC . This finding bears significant relevance for designer higher conductivity Li-ion electrolytes as linear carbonate solvents with low dissociative behavior have recently attracted considerable attention. − …”

Process‐Structure‐Formulation Interactions for Enhanced Sodium Ion Battery Development: A Review

“… [210] A technique employed to improve electrolytic performance is to “degass” it; in Li‐ion cells this enables the efficient movement of the lithium ions, thereby improving charging and discharging performance. [211] For example, Xiong, Hynes & Dahn [212] performed degassing of Li‐ion pouch cells by a 2‐step process: by cutting the pouch cell open and then resealing it under vacuum after the cell was charged to 3.5 V at C/20 and held for an hour, this process was repeated after the cell was charged to 4.5 V and held for an hour again. The authors have pointed out that these voltages were determined based on in‐situ measurements during the first charging cycle.…”

“…The authors have pointed out that these voltages were determined based on in‐situ measurements during the first charging cycle. [212] …”

All-Climate High-Voltage Commercial Lithium-Ion Batteries Based on Propylene Carbonate Electrolytes

“…Lithium-ion batteries (LIBs) have developed rapidly in the past few decades, have occupied the market of portable devices and electric vehicles, and now are moving toward a large-scale energy storage market. − Recently, LIBs are required to have higher energy density, longer lifespan, and wider operating temperature to meet the application requirements. Nevertheless, the currently used ethylene carbonate (EC)-based electrolytes become the bottleneck for next-generation LIBs because EC has a high melting point of 36.4 °C, which hinders the low-temperature performance of LIBs. − More critically, EC has the limited electrochemical window of 4.3 V; thus, it is easily oxidized on the surface of the high-nickel cathodes with strong catalytic activity, leading to the decomposition of electrolytes, gas evolution, and the performance degradation of batteries. − So, it is crucial to develop EC-free electrolytes with modified properties for the purpose of next-generation LIBs. − …”