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

Abstract

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 ...

An Analysis of Artificial and Natural Graphite in Lithium Ion Pouch Cells Using Ultra-High Precision Coulometry, Isothermal Microcalorimetry, Gas Evolution, Long Term Cycling and Pressure Measurements

“…An earlier microcalorimetry study on the parasitic heat flow in various coated NMC/NG cells with PES211 electrolyte found that NMC622A/NG cells had a much larger parasitic heat flow than NMC622B/NG cells but performed similarly in long term cycling (just as for the blue data in Figures 13b and 13e), suggesting the presence of shuttle-like reactions in NMC622A/NG cells. 24 These shuttle-like reactions may be responsible for keeping the cell impedance small (See Figure 14c). The entire suite of experiments presented in this paper for the NMC532 cells have not yet been applied to the NMC622 cells.…”

Probing the thermal effects of voltage hysteresis in anionic redox-based lithium-rich cathodes using isothermal calorimetry

“…where to other chemical reactions. These formulations have extensively been used for investigating the entropic heat of intercalation materials 45 and more recently the parasitic heat in Li-ion cells 30,[46][47][48][49] .…”

“…Such behaviour is typical for classical electrochemical intercalation reactions. 26,30 Understanding the exact shape of entropic heat flow is beyond the scope of this study, as it may be related to many aspects of the cathode material, such as the changes in Ru-Ru bond distance ordering 23 , charge ordering, Li vacancy ordering, or the shape of the ⁄ profile (Supp.…”

High-Precision Coulometry Studies of the Impact of Temperature and Time on SEI Formation in Li-Ion Cells

“…The same well-established conclusions can be made 10 cycles after the preferred protocol (rank the red points bottom to top) and 20 cycles after the standard formation (rank the points left to right): i) cells with single crystal NMC532 positive electrodes and artificial graphite negative electrodes are better than all those with polycrystalline NMC622 positive electrodes and natural graphite negative electrodes; 35 ii) cells with the ternary additive blend PES211 outperform cells with 2% VC; 29 iii) within cells of the same chemistry, the CIE increases with increasing cycling voltage; 29 iv) in cells with PES211, the Al 2 O 3 -coated positive electrodes outperform those with the proprietary "high voltage" coating. 36 These results suggest that a UHPC experiment, to rank new cells with different chemistries, can be shortened to 10 cycles, if the preferred formation protocol were used. Figure 2 shows that after 20 cycles, most cells formed with the preferred protocol have lower CIEs than the cells cycled with the standard protocol (all the blue points are below the dashed line).…”