A Study of Three Ester Co-Solvents in Lithium-Ion Cells

Abstract

The effects of three esters incorporated as co-solvents in 1.2 M LiPF 6 EC:EMC:DMC (25:5:70 by volume %) electrolyte were studied in Li[Ni 1-x-y Co x Al y ]O 2 /Graphite-SiO pouch cells. The esters: methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB) were compared in a variety of tests on the cells. Storage tests at 60 • C at both 4.2 V and 2.5 V demonstrated that MB and MP outperformed EA and cells containing 20% MP showed the least voltage drop during the 500 h storage period. In long-term cycling tests at 40 • C, cells containing up to 20% ester exhibited similar capacity retention compared to cells with only EC:EMC:DMC solvent. Unwanted lithium plating could be suppressed in cells with 20% MP during charging above 2C compared to ester-free cells, which showed the onset of unwanted lithium plating at 1.8C. This improvement is due to the increased Li + conductivity of electrolytes containing MP. In addition, the use of up to 40% MP did not enhance reactivity between the charged electrode materials and electrolyte at elevated temperatures according to accelerating rate calorimetry measurements. To further satisfy the development of electric vehicles (EVs), Liion batteries need to be designed with higher volumetric energy density, longer cycling life, higher rate capability, lower cost and so forth. These improvements can be fully or partially achieved by applying new positive and/or negative electrode materials, suitable electrolytes and additives. For instance, Li[Ni 0.85 Co 0.10 Al 0.05 ]O 2 (NCA), as a typical positive electrode material, exhibits high energy density and capacity, and has been used in many EVs 1,2 and power tools. Esters are beneficial co-solvents that can improve the physical properties of carbonate-based electrolytes due to their low freezing points, high ionic conductivity of the resulting electrolytes and low viscosity. Esters have been mainly studied for the low temperature application of Li-ion cells, and compounds of interest include, ethyl acetate (EA), methyl butyrate (MB), ethyl butyrate (EB), methyl propionate (MP), ethyl propionate (EP), etc.4-12 Of these ester solvents, it has been reported that those with low molecular weight (e.g. EA) performed well initially but degraded during longterm cycling due to high reactivity with the negative electrode. 4,5 The most studied positive electrode materials in combination with esters in Li-ion batteries were LiCoO 2 5-8 and LiNi x Co 1-x O 2 , and the latter was mainly studied by Smart et al. 4,9 In our group, EA and MP used as a sole electrolyte solvent have been studied in Li [Ni 0.33 Large fractions of esters are now appearing in commercial lithium-ion cells. For example we have performed analysis on some LiCoO 2 /graphite Li-ion cells, rated for 4.35 V, and found an electrolyte that was LiPF 6 dissolved in MP:FEC:PC in ratios of 56:37:7 by weight. Presumably this novel electrolyte system was selected to improve high voltage performance and improve rate capability. It is important to determine if MP would be the best...

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.…”

“…This viewpoint can be supported by the results of impedance analyses, showing that the reduction in charge-transfer resistance (R ct ), achieved by the above strategies, is much more significant than that in the bulk resistance (R b ). 8,10,40 The R ct of a Li-ion cell corresponds to the kinetics of Faradic processes occurring at two electrodes in relation to the solvation and desolvation of the Li + ions and the charge transfering. Furthermore, the structure and stability of the Li + ion solvation shell are very sensitive to foreign cations with multiple valances 41,42 and low/non-polar solvents.…”

“…In many cases, this strategy is also beneficial to increasing ionic conductivity of the electrolytes so that the significance of the Li + ion solvation and desolvation in dictating the rate capability of Li‐ion batteries has long been ignored. Many successful improvements by introducing a third co‐solvent such as carbonate, 35 ester, 8,35,36 and ether, 10,11 or a second Li salt such as LiBF 4 , 37 lithium bis(oxalato)borate (LiBOB), 38 lithium difluoro(oxalato)borate (LiDFOB), 39 and lithium bis(fluorosulfonyl)imide (LiFSI) 40 are actually more related to the alteration on the structure and stability of the Li + ion solvation shell, rather than the enhanced ionic conductivity. This viewpoint can be supported by the results of impedance analyses, showing that the reduction in charge‐transfer resistance ( R ct ), achieved by the above strategies, is much more significant than that in the bulk resistance ( R b ) 8,10,40 .…”

“…Many successful improvements by introducing a third co‐solvent such as carbonate, 35 ester, 8,35,36 and ether, 10,11 or a second Li salt such as LiBF 4 , 37 lithium bis(oxalato)borate (LiBOB), 38 lithium difluoro(oxalato)borate (LiDFOB), 39 and lithium bis(fluorosulfonyl)imide (LiFSI) 40 are actually more related to the alteration on the structure and stability of the Li + ion solvation shell, rather than the enhanced ionic conductivity. This viewpoint can be supported by the results of impedance analyses, showing that the reduction in charge‐transfer resistance ( R ct ), achieved by the above strategies, is much more significant than that in the bulk resistance ( R b ) 8,10,40 . The R ct of a Li‐ion cell corresponds to the kinetics of Faradic processes occurring at two electrodes in relation to the solvation and desolvation of the Li + ions and the charge transfering.…”

Challenges of Fast Charging for Electric Vehicles and the Role of Red Phosphorous as Anode Material: Review

“…Some researchers have also noted that enhancing electrolyte properties is important to enable XFC within high-energy density cells. Methyl acetate (MA) and methyl propionate (MP) have been used as electrolyte additives to support fast-charging without plating [72,73]. To improve the fast-charging performance, developing 3D current collector is also a promising direction, some of the results presented point in this direction already [74,75], since the 3D current collector can increase the contact surface area between the active material and the collector and benefiting more mass loading.…”