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

Abstract

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

Comparison of Single Crystal and Polycrystalline LiNi0.5Mn0.3Co0.2O2Positive Electrode Materials for High Voltage Li-Ion Cells

“…42,46 Since the entropic heat flow is reversible between charge and discharge, the average parasitic heat flow at each voltage point can be found by taking the average of charge and discharge heat flow and subtracting the average overpotential heat flow. The details of this method was introduced by S. Glazier et al 47 Figure 8a • C were observed for all the charged electrode materials and the peak intensity increases with increasing of state of charge of the electrode materials (higher upper cutoff voltage of the cells). The release of oxygen from the bulk of the charged materials is associated with the degradation of NMC532 from a layered (Li x MO 2 , M = Ni, Mn and Co) structure to a rock-salt (Li x M 1-x O) structure.…”

Ethylene Carbonate‐Free Propylene Carbonate‐Based Electrolytes with Excellent Electrochemical Compatibility for Li‐Ion Batteries through Engineering Electrolyte Solvation Structure

“…However, these cathodes suffer from poor cyclability and thermal abuse tolerance due to their high reactivity with the commonly used ethylene carbonate (EC)‐based electrolyte solution. [ 7–11 ] As well recognized, the EC‐based electrolyte is easily oxidized on the surfaces of high nickel cathodes at high voltages, resulting in electrolyte dryout, gas evolution, and impedance growth and consequently a continuous degradation of battery performance. [ 12–14 ] On the other hand, the high melting point of EC is a major cause for the poor low temperature performance of current LIBs.…”

“…Recent works have demonstrated that simply removal of EC from the currently used electrolytes can enable considerably improved high‐voltage cyclability and high temperature stability of LIBs. [ 9,10,12–15 ] Therefore, development of advanced electrolyte systems is crucial for overcoming the challenges of conventional EC‐based electrolyte on new LIB electrodes.…”

The Effect of Different Li(Ni1-x-yMnxCoy)O2Positive Electrode Materials and Coatings on Parasitic Heat Flow as Measured by Isothermal Microcalorimetry, Ultra-High Precision Coulometry and Long Term Cycling

“…This method has been used to determine the benefit of removing EC from NMC442/graphite cells above 4.3 V, 27,28 as well as measuring the effect of vinylene carbonate and 1-fluoro-ethlyne carbonate as electrolyte additives on the parasitic heat flow in large voltage ranges. 29 Figure 4a demonstrates this method to calculate the parasitic heat flow of one of the 442 cells during the first protocol step between 3.9 V and 4.2 V and will be called the "average method" throughout this work.…”