Methyl Acetate as a Co-Solvent in NMC532/Graphite Cells

Abstract

One goal of researchers focusing on lithium-ion batteries for electric vehicles is to decrease the time required for charging. This can be done by several methods, including increasing the electrolyte transport properties. Methyl acetate, used as a co-solvent in the electrolyte, has been shown by a number of researchers to increase cell rate capability dramatically but careful considerations of the impact of methyl acetate on cell lifetime have not been published to our knowledge. The impacts of methyl acetate as a co-solvent in NMC532/ graphite cells were systematically studied in this work. Ex-situ gas evolution measurements, electrochemical impedance spectroscopy, high rate charging tests, ultra-high precision coulometry, isothermal microcalorimetry and long term cycling at both 20 and 40 • C were used to probe the impacts of including methyl acetate as a co-solvent. This work will be of great interest to Li-ion battery scientists developing cells that can support rapid charge and still maintain long lifetime. Lithium ion cells for electric vehicles should have long lifetime, high energy density and be able to support high rate charging. If cells are charged too rapidly for a given temperature, it is possible that unwanted lithium plating on the graphite negative electrode can occur and can accelerate cell capacity loss.1 There are a number of factors that influence the ability of cells to be charged rapidly. These include the thicknesses of the electrodes and the separator, the porosity and tortuosity of the electrodes and the separator and the electrolyte transport properties. In addition, Liu et al.,2 recently highlighted the importance of the negative electrode solid electrolyte interphase (SEI) resistance on the ability of cells to be charged rapidly without unwanted lithium plating during charge.For any given cell design, the easiest change to make in Liion cell manufacturing is a change in the electrolyte. All that entails, apart from possible materials compatibility issues, is filling cells with a different fluid. Electrolytes that promote high lithiumion diffusion constants, low viscosity, high conductivity, a high lithium ion transference number and promote low resistance negative electrode SEI layers are preferred for cells designed for high rate charging. Esters are beneficial co-solvents that lower freezing points, increase ionic conductivity and lower viscosity. Esters have been applied to improve the low temperature rate capability of Liion cells and also improve rate capability at room temperature. [3][4][5][6][7][8][9][10] Li-ion batteries with esters and positive electrodes of LiCoO 2 were studied in References 4-7 while those with LiNi x Co 1-x O 2 positive electrodes were studied by Smart et al. lifetime with an electrolyte containing 2 wt% prop-1-ene-1,3 sultone (PES) + 1 wt% tris (trimethylsilyl) phosphite (TTSPi) + 1 wt % ethylene sulfate (DTD) in 1 M LiPF 6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 by weight). More than 92% capacity was maintained after testing for ...

Ester-Based Electrolytes for Fast Charging of Energy Dense Lithium-Ion Batteries

“…Methyl acetate (MA), the simplest organic ester, is an aprotic liquid over a wide temperature range (−98 to 57 °C) with two weakly basic sites and modest polarity (ε 25 °C = 6.9). − It has found extensive use as a cosolvent for low-temperature electrolytes and for high-power cells on account of its low viscosity and large polarity relative to linear carbonate esters (cf. dimethyl carbonate, ε 25 °C = 3.1). , The use of methyl acetate as the sole solvent for lithium-ion battery electrolyte solutions has not been explored, although studies have explored the use of other esters as sole electrolyte solvents, specifically ethyl acetate (EA) and methyl propionate (MP). However, these solvents by themselves show limited improvement to ionic transport compared to a traditional EC:DMC system. , …”

“…However, the UHPC test results show that there will be an inherent loss in lifetime associated with using the MA-based electrolytes, given that the CE values are lower than the EC/DMC counterparts. This is not surprising, given that the introduction of MA as a cosolvent to conventional electrolytes is well-known to have an associated loss in CE and cell lifetime. ,,, The negative impact of MA on the lifetime of cells can also be seen in the long term cycling data (Figure a–c) where the MA-based electrolyte with 10% FEC loses just under 2% capacity after ∼1400 cycles, while the EC:DMC-based electrolytes show negligible capacity loss.…”

Sustainable Li‐Ion Batteries: Chemistry and Recycling

“…The early practical applications that utilized Li x Ni 1/3 Co 1/3 Mn 1/3 O 2 (NMC333) showed a small volume change upon cycling with ≈1-2% in the range of ≈0 < x ≤ 0.7, [70,71] and high capacities in the range of 200 mAh g −1 at 4.5-2.5 V were demonstrated for this compound. [69] Further investigations in the field led to the use of higher nickel derivatives NMC532, [72,73] NMC622, [74,75] and NMC811, [75,76] though the highest nickel content of member of the family has still problems to be solved for large-scale implementation in batteries. There, layered nickel-rich oxides such as Li[Ni 0.80 Co 0.15 Al 0.05 ]O 2 [77] and Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 [78] were widely searched and shown to yield capacities in the range of ≈200 mAh g −1 ; however, they suffer largely from poor thermal properties as a result of oxygen release in highly delithiated state, which could lead to thermal runaways and fire hazards.…”

Design aspects of electrolytes for fast charge of Li‐ion batteries

“…For the mixture of solvents, cyclic ethylene carbonate (EC) is considered to be an indispensable co-solvent for the formation of robust solid-electrolyte-interphase (SEI) on the graphite anode, and the other solvent can be a linear dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), or their mixture for reduced viscosity and low freezing temperature of the electrolytes. In some cases, small amounts of esters [7][8][9] or ethers 10,11 that have a low boiling point and acceptably chemical stability are added to enable operations at low temperatures or/and high current rates. However, such benefits often come with a trade-off in reversibility and lifetime.…”