The Impact of Electrolyte Composition on Parasitic Reactions in Lithium Ion Cells Charged to 4.7 V Determined Using Isothermal Microcalorimetry

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

Section: Resultsmentioning

confidence: 99%

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

Glazier

Petibon

Xia

et al. 2017

Self Cite

“…1 They also demonstrated a strong voltage dependence on the parasitic thermal power; it increases significantly at higher voltages due to electrolyte oxidation. 1 In their method, a mathematical model was developed and fit to experimental thermal data.…”

mentioning

confidence: 99%

“…1 In their method, a mathematical model was developed and fit to experimental thermal data. By simultaneously fitting multiple experimental datasets obtained at different currents, the authors were able to separate the parasitic thermal power from the total cell thermal power.…”

mentioning

confidence: 99%

Measurement of Li-Ion Battery Electrolyte Stability by Electrochemical Calorimetry

Krause

Jensen

Chevrier

2017

Recent work describing the use of high precision coulometry combined with isothermal heat flow calorimetry has shown promise in studying electrolyte reactivity in Li-ion batteries. In this paper we describe what we term an "integration/subtraction" technique for determining the electrolyte reactivity as a function of cell voltage in Li-ion full pouch cells. We apply this method to the characterization of a base electrolyte blend of ethylene carbonate and ethyl methyl carbonate (EC/EMC 3/7 w/w) with 1 M LiPF 6 . We then show how the parasitic thermal power and coulombic efficiency are affected by the addition of the reactive carbonates vinylene carbonate and 1-fluoro-ethylene carbonate to the base electrolyte. We show how this method can discriminate the effectiveness of additives used in Li Li-ion cell technology has reached a level of maturity witnessed by the pervasive applications of the energy storage technology in everything from cell phones to electric vehicles. However there will always be a need and desire to achieve higher energy density, longer cycle life and better safety. Inevitably the cell electrolyte chemistry is a key factor in achieving these goals. Presently there is considerable effort applied to higher voltage Li-ion cells utilizing different positive electrodes. There is also considerable effort being applied to the incorporation of silicon and silicon-based materials into the negative electrode to increase cell energy density. In these examples the electrolyte chemistry and its stability are critical concerns. Therefore techniques or tools that enable sensitive measurement and discrimination of different electrolyte chemistries are very useful and important. For example, cell chemistries beyond Li-ion, such as Li-air, Li-S or Mg-based cell chemistries will rely on unique passivating layers that attempt to protect the reactive metal electrode. Sensitive thermal techniques that can measure the heat produced by irreversible electrolyte reactions with highly reducing or oxidizing electrodes will be important in determining the effectiveness of engineered passivation layers.Recently Dahn and co-workers have shown how isothermal heat flow calorimetry can be used to discriminate the effects of electrolyte additives in Li-ion cells.1 They also demonstrated a strong voltage dependence on the parasitic thermal power; it increases significantly at higher voltages due to electrolyte oxidation. 1 In their method, a mathematical model was developed and fit to experimental thermal data. By simultaneously fitting multiple experimental datasets obtained at different currents, the authors were able to separate the parasitic thermal power from the total cell thermal power. While the method described in the present paper, which we term "integration/subtraction", also discriminates parasitic thermal power from the total thermal power, we also simultaneously acquire useful coulombic efficiency data as well as other cell parameters.In an earlier publication we described a method for extracting the parasitic therm...

“…Downie et al also probed the effect of different additives in LiCoO 2 /graphite and NMC442/graphite pouch cells on parasitic heat flow. 13,14 The heat flow due to polarization varies as the current squared (I 2 ), the heat flow due to entropy changes in the intercalation electrodes varies as I, heat flow from inherent hysteresis varies as |I| and heat flow from unwanted parasitic reactions is believed to have no current dependence * Electrochemical Society Student Member.* * Electrochemical Society Fellow. z E-mail: jeff.dahn@dal.ca at low rates.…”

mentioning

confidence: 99%

The Effect of Different Li(Ni_1-x-yMn_xCo_y)O₂Positive Electrode Materials and Coatings on Parasitic Heat Flow as Measured by Isothermal Microcalorimetry, Ultra-High Precision Coulometry and Long Term Cycling

Glazier

Nelson

Allen

et al. 2017

Self Cite

Isothermal microcalorimetry is used to investigate the effect of different Li(Ni 1-x-y Mn x Co y )O 2 materials (NMC442, NMC532, NMC622) and coatings (Al 2 O 3 and a proprietary high voltage coating) on parasitic reactions that occur in Li-ion pouch type cells. NMC/graphite pouch cells were prepared with a typical organic carbonate-based electrolyte containing a well-known additive blend and were tested up to 4.4 V at 40 • C. A new method of extracting the parasitic heat flow during both charge and discharge is introduced. Differences between charge and discharge parasitic heat flow yielded more insight into the behavior of high voltage parasitic reactions. Ultra-high precision coulometry, long-term charge discharge cycling, in-situ gas measurements, and electrochemical impedance spectroscopy were also used to compare the observed heat flow to well-known performance metrics. All coated cell types performed significantly better than uncoated NMC442/graphite cells. It was found that the magnitude of the parasitic heat flows did not correlate as expected to the precision coulometry results nor to the long term cycling results. In particular, cells with Al 2 O 3 -coated NMC622 had the highest parasitic heat flow among the cells with coated electrodes but competed for best performance in the cycling tests. High voltage lithium ion batteries have become a focus of academic and industrial research in order to meet the increasing demand for high energy density, low cost and long lifetime energy storage applications. In order to increase the operational voltage of lithium ion cells, all components must be as electrochemically stable as possible in order to prevent unwanted parasitic reactions within the cell, especially at high potentials. Arguably one of the most effective solutions to minimizing parasitic reactions and improving cell lifetime has been the addition of small amounts of additives to typical electrolytes already used in research and industry.1-3 These additives typically: are incorporated at a few percent by weight, introduce little to no change in the manufacturing process, and form protective passivating films known as the solid-electrolyte interphases (SEI) on both the positive and negative electrodes during cell operation. Although extensively studied, the mechanisms and chemical pathways responsible for the SEI films and the observed changes in performance caused by electrolyte additives are relatively poorly understood.Another common technique to reduce parasitic reactions is to alter the electrode materials in order to create a more electrochemically stable interface between the electrolyte and the highly delithiated positive electrode during high voltage operation. Metal oxide surface coatings such as Al 2 O 3 have been found to improve the stability of this interface, mitigate electrolyte oxidation, improve cycling performance and scavenge HF.4-8 Selected ratios of transition metals in Li(Ni 1-x-y Mn x Co y )O 2 (NMC) positive electrodes can also lower the reactivity of the electrode surface with ...