The Impact of Varying the Concentration of Vinylene Carbonate Electrolyte Additive in Wound Li-Ion Cells

120

Vinylene Carbonate (VC) is an effective electrolyte additive to produce a stable solid electrolyte interphase (SEI) on graphite anodes, increasing the capacity retention of lithium-ion cells. However, in combination with LiNi 0.5 Mn 1.5 O 4 (LNMO) cathodes, VC drastically decreases cell performance. In this study we use on-line electrochemical mass spectrometry (OEMS) and electrochemical impedance spectroscopy (EIS) with a micro-reference electrode to understand the oxidative (in-)stability of VC and its effect on the interfacial resistances of both anode and cathode. We herein compare different VC concentrations corresponding to VC to graphite surface area ratios typically used in commercial-scale cells. At low VC concentrations (0.09 wt%, corresponding to 1 wt% in commercial-scale cells), an impedance increase exclusively on the anode and an improved capacity retention is observed, whereas higher VC concentrations (0.17 wt -2 wt%, corresponding to 2 wt -23 wt% in commercial-scale cells) show an increase in both cathode and anode impedance as well as worse cycling performance and overcharge capacity during the first cycle. By considering the onset potentials for VC reduction and oxidation in graphite/LNMO cells, we demonstrate that low amounts of VC can be reduced before VC oxidation occurs, which is sufficient to effectively passivate the graphite anode. During the first charge of a lithium ion battery (LiB), the so called solid electrolyte interphase (SEI) 1 is formed on the surface of the negative electrode. The standard electrolyte for LiBs consists of a mixture of cyclic and linear carbonates, e.g., ethylene carbonate (EC) and ethyl methyl carbonate (EMC), typically with lithium hexafluorophosphate (LiPF 6 ) as salt. Starting from a potential of ∼0.8 V vs. Li/Li + , EC is reduced electrochemically into ethylene gas and lithium ethylene dicarbonate (LEDC), which is a key component of the SEI.2,3 Vinylene carbonate (VC) is one of the most effective additives to modify the SEI on graphite anodes, as it is reduced at potentials more positive than 1.0 V vs. Li/Li + and hence suppresses the reduction of EC. 4,5Aurbach et al. have used VC as electrolyte additive in an EC/DMC (dimethyl carbonate) based electrolyte and that time reported a reduction of the irreversible capacity in the first cycles and an improved cycling stability at elevated temperatures for graphite anodes. The SEI resulting from the reduction of VC consists mainly of poly (vinylene carbonate) (poly(VC)). 4,6Important studies on the impact of different VC concentrations in graphite/NMC pouch cells have been carried out by the Dahn group. For example, Burns et al. 7 investigated the effect of different concentrations of VC (0, 1 and 2 wt%) on cycle life and impedance growth of full-cells with graphite anodes and either LCO (LiCoO 2 ) or NMC (Li(Ni 0.42 Mn 0.42 Co 0.16 )O 2 ) cathodes, employing galvanostatic cycling experiments coupled with high precision coulombic efficiency and electrochemical impedance spectroscopy (EIS) measurements. For cel...

Section: Discussionmentioning

confidence: 99%

Analysis of Vinylene Carbonate (VC) as Additive in Graphite/LiNi_0.5Mn_1.5O₄Cells

Pritzl

Solchenbach

Wetjen

et al. 2017

120

“…5,6,[8][9][10] These side reactions lead to marching of the charge and discharge endpoints. 11,12 Such side reactions and their effects on the cell life are relatively less explored.…”

mentioning

confidence: 99%

Electrode Side Reactions, Capacity Loss and Mechanical Degradation in Lithium-Ion Batteries

Deshpande

Pan

et al. 2015

171

124

For advancing lithium-ion battery (LIB) technologies, a detailed understanding of battery degradation mechanisms is important. In this article, experimental observations are provided to elucidate the relation between side reactions, mechanical degradation, and capacity loss in LIBs. Graphite/Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 cells of two very different initial anode/cathode capacity ratios (R, both R > 1) are assembled to investigate the electrochemical behavior. The initial charge capacity of the cathode is observed to be affected by the anode loading, indicating that the electrolyte reactions on the anode affect the electrolyte reactions on the cathode. Additionally, the rate of "marching" of the cathode is found to be affected by the anode loading. These findings attest to the "cross-talk" between the two electrodes. During cycling, the cell with the higher R value display a lower columbic efficiency, yet a lower capacity fade rate as compared to the cell with the smaller R. This supports the notion that columbic efficiency is not a perfect predictor of capacity fade. Capacity loss is attributed to the irreversible production of new solid electrolyte interphase (SEI) facilitated by the mechanical degradation of the SEI. The higher capacity fade in the cell with the lower R is explained with the theory of diffusion-induced stresses (DISs). Lithium-ion batteries (LIBs) are widely used in small, portable, electronic devices due to their high energy density, high voltage, low self-discharge rate, and good cycle performance.1,2 Yet, for automotive propulsion applications, LIBs need improvement in volumetric and gravimetric energy density and with performance over the life of the vehicle. This is one of the major motivations of researchers worldwide focusing on developing cheaper and more durable LIBs for applications such as hybrid vehicles, electric vehicles, and other large-scale energy storage systems. 3Capacity decay during storage and charge-discharge cycling is one of the major challenges that limit the life of LIBs. Instability of the electrolyte at the operating potentials results in side reactions on the electrode surfaces, part of which lead to the formation of solid electrolyte interphases (SEIs). [4][5][6][7] Side reactions may result in lower coulombic efficiency, loss of usable capacity of the cell, and increase of cell impedance. Cell performance degradation due to side reactions is termed as chemical degradation, which is known to be the main cause of lithium loss in well-made LIBs. 5,6,[8][9][10] These side reactions lead to marching of the charge and discharge endpoints.11,12 Such side reactions and their effects on the cell life are relatively less explored.The solid state diffusion of lithium atoms in and out of host electrode particles results in diffusion induced stresses (DISs) and volume changes in electrode particles during charge and discharge. Depending upon the operating conditions these stresses may have various effects on the electrodes such as mechanical fatigue and fracture of the electr...

“…Petibon and Xia et al 8 found that 2% (wt) VC [9][10][11] in EMC yielded improved performance at 4.4 V compared to EC-containing electrolytes. This work investigated the effects of adding 1% (wt) of tris(trimethylsilyl)phosphate (TTSP), 12 pyridine phosphorus pentafluoride (PPF), 13 or trialyll phosphate (TAP), 14 to EMC:VC (98:2 wt).…”

mentioning

confidence: 99%

Measuring the Parasitic Heat Flow of Lithium Ion Pouch Cells Containing EC-Free Electrolytes

Glazier

Petibon

Xia

et al. 2017

Self Cite

A recent study has shown that removing ethylene carbonate (EC) from electrolytes in Li [Ni 0.4 Mn 0.4 Co 0.2 ]O 2 /graphite lithium ion pouch cells significantly increases the cycle life and lifetime at high voltage operation. This work investigates the performance of EC-free electrolytes by adding small amounts of electrolyte additives to 1 M LiPF 6 ethyl-methyl carbonate in Li [Ni 0.4 Mn 0.4 Co 0.2 ]O 2 /graphite pouch cells and measuring the parasitic heat flow during high voltage operation. EC-free electrolytes yielded higher parasitic heat flow at lower voltages but performed better than EC-containing electrolyte above 4.3 V. Additionally, EC-free electrolytes were able to recover to a lower parasitic heat flow after high voltage exposure than those containing EC. Most EC-free electrolytes had lower impedance and acceptable gas evolution compared to electrolytes containing EC throughout the experiment. EC-free electrolytes shed light on a new area of high voltage electrolyte development, using a simple and cost effective chemistry. Future and present day applications for lithium ion batteries such as electric vehicles and grid energy storage demand increasingly longer lifetimes, lower costs and higher energy density. One promising way to achieve these goals is to develop an electrolyte capable of performing well at high voltages for long periods of time. A significant amount of work has been focused on electrolyte additives, which can improve the voltage tolerance of traditional carbonate solvents. [1][2][3][4] Other works have used alternative solvent systems, such as fluorinated carbonates, which are more stable at high voltages, but produce large quantities of gas during operation. 5-7Recently it has been discovered that ethylene carbonate (EC), which has been thought to be necessary for good lithium ion battery performance, is not required at all for high voltage operation and is in fact detrimental to performance.8 By creating a simple electrolyte containing ethyl methyl carbonate (EMC) and a small amount of vinylene carbonate (VC) as a passivating agent, cell cycle life and coulombic efficiency (CE) during 4.4 V cycling was greatly improved over electrolytes containing EC. Co-additives were found to further improve aspects of cycle performance and safety.The relative performance of EC-free electrolytes with different additive combinations is studied here by measuring the parasitic heat flow of cells during high voltage operation using isothermal microcalorimetry. Petibon and Xia et al. 8 found that 2% (wt) VC 9-11 in EMC yielded improved performance at 4.4 V compared to EC-containing electrolytes. This work investigated the effects of adding 1% (wt) of tris(trimethylsilyl)phosphate (TTSP