Li [Ni 0.4 Mn 0.4 Co 0.2 ]O 2 (NMC442)/graphite pouch cells containing various electrolyte additives, either singly or in combination, were studied using cycling experiments up to 4.4 and 4.5 V coupled with simultaneous electrochemical impedance spectroscopy (EIS) measurements. The impedance of most cells increased dramatically at 4.4 and 4.5 V, but was nearly reversible over one cycle. However, during continued cycling, the impedance of all cells slowly increased at all potentials. Electrolyte additives were found to dramatically affect this behavior. The impacts of adding prop-1-ene-1,3-sultone (PES), vinylene carbonate (VC), triallyl phosphate (TAP), methylene methane disulfonate (MMDS), ethylene sulfate (DTD) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) to 1M LiPF 6 ethylene carbonate:ethyl methyl carbonate (EC:EMC) electrolyte were studied. PES-containing cells had dramatically lower impedance and better capacity retention than VC and TAP-containing cells during both 4.4 and 4.5 V experiments. When MMDS, DTD and/or TTSPi were added in combination with PES, the performance was improved further. Finally, continuous charge-discharge cycling was compared to cycling with a 24-hour hold applied at the top of charge at 4.4 V. The high voltage hold led to severe impedance growth which could be partially overcome through the use of optimal additive combinations. Lithium-ion (Li-ion) batteries are currently used in phones, laptop computers and, more recently, electric vehicles. It is well known that electrolyte additives can have a dramatic effect on the performance and lifetime of Li-ion batteries.1,2 Vinylene carbonate (VC) is perhaps the most famous and widely used additive and has been shown to improve cycle and calendar life of Li-ion cells.3 VC is less effective, however, when used in cells cycling to potentials above 4.2 V 4 or at elevated temperatures.5 Sulfur-containing additives have recently been investigated by several research groups in the hopes of overcoming the temperature sensitivity of VC and extending the usable voltage range of Li-ion cells. [6][7][8] Prop-1-ene-1,3-sultone (PES) has been shown to function as a stable solid electrolyte interphase (SEI)-forming additive that improved coulombic efficiency (CE), reduced charge end point capacity slippage and self-discharge rates. 8,9 PES nearly eliminated all gas production during storage at 4.2 V and 60• C, whereas VC did not. 9,10The work by Xia et al. 9 and Nelson et al. 10 demonstrated the superiority of PES over VC as an electrolyte additive in NMC/graphite cells. Methylene methane disulfonate (MMDS) has been shown to reduce electrolyte oxidation at the positive electrode and reduce the volume of gas produced, as well as decrease the impedance and rate of parasitic reactions when compared to cells without MMDS.6,11 The additive ethylene sulfate or 1,3,2-dioxathiolane-2,2-dioxide (DTD) has been shown to function as a film-forming additive for the SEI on the negative electrode.12,13 The additive tris-(trimethyl-silyl) phosphite (TTSPi) has bee...

A technique has been developed to allow for in-situ observation of the state of the liquid electrolyte present in an electrochemical device through exploiting the nature of solid-liquid phase transitions. An apparatus was developed to perform differential thermal analysis (DTA) on complete NMC/graphite lithium-ion pouch cells. The DTA apparatus monitored the temperature of a sample cell alongside that of a thermally inert reference cell during a controlled temperature scan through the melting point of the electrolyte in the sample cell, producing a thermal signature of the sample electrolyte without damage to the cell performance. This signature holds information pertaining to the composition and amount of liquid electrolyte present in the electrochemical device, allowing for detailed study of electrolyte evolution at various points of life and indications of the state-of-health of the device. In recent years, lithium-ion batteries have surpassed other competing technologies as the pre-eminent choice in a wide range of energy storage applications. With expansion beyond use in personal electronics into mainstream automotive applications and grid storage, a more complete understanding of the complex processes evolving in these devices over a decade of life is of utmost importance. A generic lithium-ion cell comprises two electrodes and a liquid electrolyte (itself composed of ever-increasingly complex systems of lithium-based salts, organic solvents and a blend of electrolyte additives intended to stabilize and improve the cell operation and lifetime). Parasitic reactions are known to coincide with regular operation of the device, leading to reduction and oxidation of the liquid electrolyte through development of solid electrolyte interface layers (SEI) and also other reactions at the electrodes.1-3 Depending on the specific electrolyte chemistry and cycling protocol, this may result in substantial changes to the chemistry of the electrolyte compared to that introduced to the cell at time of manufacturing, significant electrolyte consumption, or both.To date, studies of this evolution have been limited to ex situ post mortem studies of the cells. Possible methods include X-ray photoelectron spectroscopy (XPS) 2 and gas chromatography coupled with mass-spectrometry (GC/MS). 4 These studies are limited as XPS studies the SEI directly as opposed to the electrolyte itself and GC/MS is insensitive to the electrolyte salt. Furthermore, all such schemes are inherently destructive, precluding intermittent study of a single cell at arbitrary points of life.A thorough understanding of the evolution and loss of liquid electrolyte are critical to both determining chemistries for study in research programs, as well as establishing the value and remaining lifetime of an used lithium-ion cell. The strict capacity requirements for Li-ion cells in electric vehicles necessitating battery disposal at an intermediate state of life has resulted in the recent emergence of a used-battery market. 5,6 This further underscores the value of...

Isothermal microcalorimetry has been previously used to determine the voltage dependent parasitic heat flow for individual lithium ion cells. While a decrease in parasitic heat flow over time was observed, no attempts were made to quantify the time dependence. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the components of the total heat flow were isolated as functions of time and state of charge. By fitting the measured total heat flow to a simple empirical model, the polarization and entropic heat flows were determined as a function of state of charge, while the heat flow resulting from parasitic chemical reactions was determined as functions of both state of charge and time. The time and state of charge dependent parasitic heat flow was determined for high voltage LiCoO 2 /graphite and Li[Ni 0.4 Mn 0.4 Co 0.2 ]O 2 (NMC442)/graphite pouch cells, with particular emphasis on high voltage operation. The effects of electrolyte additive blends containing combinations of vinylene carbonate, prop-1-ene sultone, tris(trimethylsilyl)phosphite, and methylene methanedisulfonate are also shown. ■ INTRODUCTIONApplications such as electric vehicles and grid-scale energy storage are putting an increased demand on lithium ion batteries to have higher energy densities and dramatically longer lifetimes than cells typically used in portable electronics. The calendar and cycle lifetimes are well known to be largely affected by parasitic reactions such as electrolyte oxidation occurring in the cell. To extend the lifetimes, these parasitic reactions must be reduced, and therefore, it is of utmost importance to be able to measure and quantify these reactions in a precise manner.One common technique for improving cell lifetimes by increasing coulombic efficiencies, increasing capacity retention, and reducing parasitic reactions is with the addition of electrolyte additives. 1−3 Extensive studies have been performed investigating and cataloging the benefits of electrolyte additives, including studies by Wang et al. 4,5 of 55 and 110 additive combinations. However, the mechanisms of how additives provide such benefits are not well understood and are frequently debated in the literature. Furthermore, transitioning to increasingly higher upper cutoff voltages has proved difficult as many additives and solvents are unstable at such high potentials, and the parasitic degradation of these components results in severely decreased lifetimes. It is therefore important to examine the voltage dependent impact of electrolyte additives on parasitic reactions.One technique well suited for the quantification of parasitic reactions is isothermal microcalorimetry. Many reported isothermal calorimetry studies on various lithium ion chemistries 6−15 interpret the measured heat flow based on the Newman energy balance, 16,17 a model which assumes that the heat flow due to side, or parasitic, reactions is negligible. However, with the introduction of microcalorimeters with nanowatt-scale sensitivities, it ha...

Core-shell positive electrode materials with a core:shell mass ratio of 2:1 and 4:1 were synthesized in a two-step reaction. Powder X-ray diffraction, SEM and spatial EDS measurements were used to characterize the core and shell phases in the precursors and lithiated products. It was determined using EDS that the precursor and lithiated products are both core-shell and the two phases can be easily resolved with laboratory grade XRD equipment. Two phase Rietveld refinement was completed on the core-shell lithiated products. The results of these refinements in conjunction with contour plots of the lattice parameters within the Li-Ni-Mn oxide layered single phase region were used to position the core and shell of each sample on the Li-Ni-Mn-O phase diagram as a function of the amount of Li 2 CO 3 used in synthesis. The shell phase retained an approximately fixed amount of Li while the Li content of the core phase increased as the overall Li content of the core-shell sample increased. Both the core and shell were electrochemically active. A specific capacity of 220 mAh/g was achieved in a core shell material between 2.5-4.6 V vs. Li/Li + . © The Author ( There has been an extraordinary effort to synthesize and characterize different materials for the positive electrode of lithium-ion batteries.1,2 A primary motivation of this research is to improve the performance of lithium-ion cells to meet requirements for electric vehicle (EVs) and grid storage applications. As the choices of positive electrode materials move away from current commercial staples such as LiFePO 4 5 the cell must be charged to higher potentials, sometimes as high as 4.8 V vs. Li/Li + , to access the additional reversible capacity. At higher potentials, degradation of the electrolyte through oxidation on the positive electrode may vastly reduce the lifetime of the cell. For the high energy density of new materials to be fully utilized, oxidation of the electrolyte must be minimized. Otherwise, these materials are of little commercial interest for EVs and grid storage where lifetime requirements of decades of useful service would be difficult to achieve.The rate of parasitic reactions that degrade the electrolyte at the positive electrode's surface are affected by the composition of the electrolyte, the potential of the positive electrode, the composition of the positive electrode and many other parameters in the construction and operation of the cell. Commonly, researchers are addressing this by improving electrolyte formulations via additives and solvent blends.6,7 A complementary approach is examining the positive electrode surface composition and its effect on the degradation of the electrolyte. [8][9][10][11][12] Rowe et al. 13 showed via high precision charger measurements that the columbic efficiency and charge end point capacity slippage were dependent on the composition of the positive electrode of various LiNi-Mn layered oxides. Samples in that study were cycled up to 4.6 V vs. Li/Li + with the same additive-free electrolyte formulation. ...

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