Single-crystal LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) with a grain size of 2-3 μm was compared to conventional polycrystalline uncoated NMC532 and polycrystalline Al 2 O 3 -coated materials in this work. Studies were made to determine how single crystal NMC532 material with large grain size could be synthesized. Ultra high precision coulometry (UHPC), in-situ gas measurements and isothermal microcalorimetry were used to make comparative studies of the three materials in Li-ion pouch cells. All the diagnostic measurements suggested that the single crystal material should yield Li-ion cells with longer lifetime. Long-term cycling tests verified these predictions and showed that cells with single crystal NMC532 exhibited much better capacity retention than cells with the polycrystalline materials at both 40 • C and 55 • C when tested to an upper cutoff potential of 4.4 V. The reasons for the superior performance of the single crystal cells were explored using thermogravimetric analysis/mass spectrometry experiments on the charged electrode materials. The single crystal materials were extremely resistant to oxygen loss below 100 • C compared to the polycrystalline materials. The major drawback of the single crystal material is its slightly lower specific capacity compared to the polycrystalline materials. However, this may not be an issue for Li-ion cells designed for long lifetime applications. Lithium ion batteries with high energy density, long lifetime and low cost need to be developed for applications in electric vehicles and stationary energy storage. The family of Li(Ni x Mn y Co z )O 2 (x + y + z = 1) (NMC) materials with high nickel and low cobalt are used as positive electrode materials in lithium ion cells.1,2 One simple way to increase the energy density of NMC lithium ion cells is to increase their upper cutoff voltage which gives access to higher specific capacity from the positive electrode.3,4 However, increasing the upper cutoff voltage usually decreases the lifetime of cells due to an acceleration of 'unwanted' parasitic reactions between the electrolyte and the delithiated positive electrode surface at high voltages. Such reactions include oxidation of species found in the electrolyte, transition metal dissolution, etc. [5][6][7] In addition, structural reconstruction of the positive electrode surface can occur which can contribute to impedance growth and capacity loss. 3,4 The by-products of oxidation at the positive electrode can migrate to the negative electrode surface and be reduced there. 8,9 Such reactions can lead to the consumption of lithium ions from the electrolyte, (to maintain charge neutrality in the electrolyte), a reduction in lithium inventory, as well as a thickening of the negative electrode solid electrolyte interface (SEI) which together ultimately cause cell-failure.10,11 These processes are accelerated by higher charging potentials and higher temperatures.Methods such as modification of the positive electrode surface with coatings or dopants 12,13 and/or modification of electr...

Li [Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442)/graphite pouch cells demonstrate superb performance at high voltage when ethylene carbonate (EC)-free electrolytes, using a solvent mixture that is >95% ethyl methyl carbonate (EMC) and between 2 and 5% of an "enabler", are used. The "enablers", required to passivate graphite during formation, can be vinylene carbonate (VC), methylene-ethylene carbonate (MEC), fluoroethylene carbonate (FEC) or difluoro ethylene carbonate (DiFEC), among others. In order to optimize the amount of "enabler" added to EMC, gas chromatography coupled with mass spectrometry (GC-MS) was used to track the consumption of "enabler" during the formation step. Storage tests, electrochemical impedance spectroscopy (EIS), ultrahigh precision coulometry (UHPC), long-term cycling, differential voltage analysis and isothermal microcalorimetry were used to determine the optimum amount of enabler to add to the cells. It was found that the graphite negative electrode cannot be fully passivated when the amount of "enabler" is too low resulting in gas production and capacity fade. Using excess "enabler" can cause large impedance and gas production in most cases. The choice of "enabler" also impacts cell performance. A solvent blend of 5% FEC with 95% EMC (by weight) provides the best combination of properties in NMC442/graphite cells operated to 4.4 V. It is our opinion that the experiments and their interpretation presented here represent a primer for the design of EC-free electrolytes. Lithium-ion batteries (LIB) are now widely used in electrified vehicles and energy storage systems.1 These applications require longer calendar and cycle lifetime as well as higher energy density. In order to increase the energy density of LIB, researchers focus on developing electrode materials with high specific capacity that may involve charging to increased upper cutoff potentials.2,3

Lithium-ion cells produce a considerable amount of gas in their first cycle. If the gases are not removed in a degassing step, most are consumed by the cell over time. This phenomenon has never been investigated explicitly in the literature. In this paper, the evolution and subsequent consumption of gas in typical lithium-ion cells are measured by Archimedes' principle and gas chromatography. It is found that all evolved gases are subsequently consumed to some degree, except for saturated hydrocarbons. The consumption of gas occurs predominantly at the negative electrode, where the gases are reduced to form part of the solid-electrolyte interphase (SEI). Changes to the negative electrode SEI upon gas consumption are investigated using X-ray photoelectron spectroscopy. The effect of gas consumption on cell performance is studied with ultra-high precision charging and high voltage storage experiments. It is found that gas consumption does not result in measurable adverse effects to cell performance. Lithium-ion cells can produce a significant amount of gas during the first charge (in the formation cycle), as electrolyte and additives react at the surfaces of the charging electrodes to form passivating films. If lithium-ion cells are packaged in a flexible casing, these gases are normally removed by the manufacturer in a degassing step, to prevent deformation of the cell and to ensure uniform stack pressure on the electrodes. If the degassing step is omitted, a large portion of the gas evolved is consumed over time.1 The reactions that consume gas are presumably prevalent in hard-cased cylindrical cells, such as 18650 s, which are often hermetically sealed before the first charge, and therefore cannot be easily degassed. The reactions that consume gas are presumably less prevalent in pouch-type cells, which are degassed.Several authors have speculated about the fates of gases in lithiumion cells.2-5 There has been no work explicitly dedicated to understanding the phenomenon of gas consumption. There is no consensus as to whether the effects of gas consumption are beneficial or harmful to cell performance. For example, it has been argued by some that the consumption of CO 2 is beneficial to cells, as it reacts to form a passivating film on the negative electrode. 3,4,6 However it has also been argued that the consumption of CO 2 is detrimental to cells, as it may reduce at the negative electrode to form Li 2 C 2 O 4 , which causes continual self-discharge at high voltage. 2It is important for both scientists and manufacturers of lithium-ion cells to understand the causes and the effects of gas consumption. If gas consumption is quick, benign, or even beneficial to cell performance, then the time-consuming degassing step for lithium-ion pouch cells might be skipped. 7 The gases evolved in lithium-ion pouch cells could be left for consumption within the cell, perhaps leaving the pouch cell flat and rigid after several hours if all the gases were consumed. If gas consumption in a cell produces undesirable effects, such...

The chemical and electrochemical reactions at the positive electrode–electrolyte interface in Li-ion batteries are hugely influential on cycle life and safety. Ni-rich layered transition metal oxides exhibit higher interfacial reactivity than their lower Ni-content analogues, reacting via mechanisms that are poorly understood. Here, we study the pivotal role of the electrolyte solvent, specifically cyclic ethylene carbonate (EC) and linear ethyl methyl carbonate (EMC), in determining the interfacial reactivity at charged LiNi 0.33 Mn 0.33 Co 0.33 O 2 (NMC111) and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathodes by using both single-solvent model electrolytes and the mixed solvents used in commercial cells. While NMC111 exhibits similar parasitic currents with EC-containing and EC-free electrolytes during high voltage holds in NMC/Li 4 Ti 5 O 12 (LTO) cells, this is not the case for NMC811. Online gas analysis reveals that the solvent-dependent reactivity for Ni-rich cathodes is related to the extent of lattice oxygen release and accompanying electrolyte decomposition, which is higher for EC-containing than EC-free electrolytes. Combined findings from electrochemical impedance spectroscopy (EIS), TEM, solution NMR, ICP, and XPS reveal that the electrolyte solvent has a profound impact on the degradation of the Ni-rich cathode and the electrolyte. Higher lattice oxygen release with EC-containing electrolytes is coupled with higher cathode interfacial impedance, a thicker oxygen-deficient rock-salt surface reconstruction layer, more electrolyte solvent and salt breakdown, and higher amounts of transition metal dissolution. These processes are suppressed in the EC-free electrolyte, highlighting the incompatibility between Ni-rich cathodes and conventional electrolyte solvents. Finally, new mechanistic insights into the chemical oxidation pathways of electrolyte solvents and, critically, the knock-on chemical and electrochemical reactions that further degrade the electrolyte and electrodes curtailing battery lifetime are provided.

Natural graphite (NG) negative electrode materials can perform poorly compared to synthetic, or artificial, graphite (AG) negative electrodes in certain lithium ion cells. LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532)/(AG or NG) pouch cells were tested with various loadings of an electrolyte additive blend to study the effect of the graphite type as well as the formed solid electrolyte interphase (SEI). Cells underwent testing using ultra-high precision coulometry, isothermal microcalorimetry, in-situ pressure measurements, long term cycling and in-situ gas measurements. In short term experiments NMC532/AG and NMC532/NG cells showed similar coulombic efficiencies, parasitic heat flows, and gas production with large electrolyte additive loadings, but NG cells showed worse capacity retention in long-term tests. With low additive loadings NMC532/NG cells showed lower coulombic efficiency, higher capacity fade, more parasitic heat flow, and more gas production. In-situ cell stack pressure measurements showed that NMC532/NG cells irreversibly expanded during cycling while NMC532/AG cells did not. Although these results lead one to propose a simple model for the poor performance of NMC532/NG cells, NMC622/NG and NMC622/AG cells showed very different behavior in long term tests suggesting that positive/negative interactions play a strong role in governing the behavior of graphites in Li-ion cells. Next generation lithium ion batteries require higher energy density, longer life, better safety, and lower cost to fulfill the ever-increasing demand for electric vehicles and renewable grid-level energy storage. By increasing the energy density of cells while keeping lifetime consistent, one can in turn decrease the cost of Li-ion cells. Much work has been focused on increasing the upper cutoff voltage of cells in order to achieve this increase in energy density. However, increasing the upper cutoff potential increases the rates of unwanted reactions in cells which can compromise lifetime.1-3 These unwanted reactions are commonly termed parasitic reactions.Another way of addressing the issue of cost is to use higher energy density materials, such as natural graphite (NG) as a negative electrode material instead of synthesized graphite, here called artificial graphite (AG). NG is known to perform poorly in some cells, which has in the past been attributed to surface exfoliation and cracking of particles.4-7 Park et al. found spherical natural graphite showed signs of particle swelling and cracking caused by mechanical strain during cycling, which could be suppressed using a carbon coating process. 5Carbon coatings on natural graphite negative electrodes have been studied in the past to avoid exfoliation from propylene carbonatecontaining electrolytes, but these coatings may decrease the energy density.4,6 AG performance reported in the literature appears to outperform natural graphite, however, few direct comparisons of artificial and natural graphite exist in the literature. Lee et al. 8 found that plasma treated AG performed better i...

There can be a trade-off between the lifetime and energy density of LiNi x Mn y Co 1-x-y O 2 (NMC)-containing cells that depends on their upper cutoff voltage. This work applies thermogravimetric analysis coupled with mass spectrometry (TGA-MS) to measure the release of oxygen from delithiated NMC electrode materials at high electrode potentials, i.e., low lithium content. This release is observed at relatively mild temperatures as low as 40 • C. The amount of oxygen released is greatly limited using single crystal NMC particles. Electrochemical measurements demonstrate a correlation between TGA-MS results and the cycling performance of NMC/graphite cells. X-ray photoelectron spectroscopy complements the TGA-MS results and provides evidence of the solid electrolyte interphase decomposition. The results in this work offer strong support that the release of oxygen from NMC can cause oxidative decomposition of the electrolyte and is a major reason why high voltage cells can generate gas and can have poor capacity retention. There is a need for longer-lasting and higher energy density lithium-ion cells for electrified vehicles and grid energy storage applications.1-3 Layered LiNi x Mn y Co 1-x-y O 2 (NMC) materials make good positive electrodes on account of their well-balanced properties of high specific capacity, moderate cost, and good safety.4-9 As a result, NMC/graphite cells have become a ubiquitous part of our society. However, to avoid electrolyte and material degradation, commercially available NMC-containing cells normally operate at a much lower voltage (typically ≤ 4.2 V) than their theoretical limit. This is because trade-offs exist between the lifetime and the energy density of a NMC cell that depends on its upper cutoff voltage. 10,11 One method to improve existing NMC technology is the development of new electrolyte solution chemistries. The use of new solvent blends and the introduction of electrolyte additives are two practical ways to improve the lifetime of high voltage NMC Li-ion cells. 12-22Ma et al. and Nelson et al. found that a ternary additive combination can greatly improve NMC442/graphite cell performance at high voltage.11-13 Xia et al. reported that fluorinated electrolyte and ECfree electrolyte also enhance NMC/graphite cell performance. [16][17][18][19] Another approach to improve cell stability is the application of an inorganic coating onto the NMC surface. [23][24][25][26][27][28][29] For example, Arumugam et al. reported that a surface coating of Al 2 O 3 greatly improves NMC622/graphite cell performance. 24 Whereas both of these approaches have proven to be quite valuable, further improvements to the lifetime of NMC-containing cells at high voltages are still desired.Great effort has been put forward to understand the chemical mechanisms behind the degradation of high voltage NMC cells. Analytical approaches have included monitoring impedance growth, 12,13,30-38 the volume and composition of gases produced, 39-43 the heat flow [44][45][46][47][48] and the leakage current 49,50 du...

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 ...

Electrolyte additives are a promising route to stable solution chemistries needed for improved and next-generation lithium-ion cells. Yet the underlying chemistry remains unknown for most additives and additive blends in use. This work presents possible reaction pathways for solid-electrolyte interphase formation in lithium-ion cells from ethylene sulfate (DTD), prop-1-ene-1,3-sultone (PES) and the binary PES/DTD blend. Pathways are supported by theoretical calculations (density functional theory) and experimental results (electrochemistry, gas chromatography thermal conductivity detection, X-ray photoelectron spectroscopy, isothermal microcalorimetry). A hypothesis to understand the synergistic chemistry of the blend is proposed: Reduction of PES, the 'primary additive', at the negative electrode forms a nucleophile that reacts with electrophilic DTD, the 'secondary additive', to produce a passive solid-electrolyte interphase that inhibits direct reduction of DTD or the solvent. The results are further discussed in the contexts of future mechanistic studies, computational additive discovery, and the development of improved lithium-ion cell chemistries. Stable electrolyte solution chemistries are essential for the development of higher voltage, faster charging, and longer-living lithium-ion cells as well as next-generation cell technologies (e.g., sodium-ion and lithium-air).1-4 To avoid electrolyte decomposition, many batteries in use today operate at much lower cell voltages than the capability of the electrode materials, often leaving a significant fraction of the theoretical capacity unused. [5][6][7] In recent years, chemical additives have been used in research and commercially available cells to increase cell stability limits to higher voltages and temperatures, creating industrial and academic interest in further additive development. 5,6,[8][9][10][11][12][13][14][15][16][17][18] The potential to achieve cutting-edge performance with minimal changes to supply chains for electrolyte salts and solvents has tremendous practical appeal. However, the design of new additives for different applications still poses significant scientific challenges to which the emergence of additive blends in recent years adds further complexity. [19][20][21][22] Whereas the selection of new solution chemistries has traditionally followed a lengthy trial and error method, there is an increasing desire to streamline the process by applying modern calculations. High-throughput computational methods for the discovery of new compounds are by now well-developed, for example in the pharmaceutical field of drug discovery. [23][24][25] Quantum mechanical and molecular mechanics calculations have been suggested to offer similar potential for energy storage technology research, including the development of new electrode materials 26-28 and electrolyte solutions 29-32 for batteries. Yet before the computational discovery of new electrolyte additives may be realized, there is a need for a greater understanding of how additives function, at...