LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) can deliver a high capacity of ∼200 mAh/g with an average discharge potential of ∼3.8 V (vs. Li + /Li), making it a promising positive electrode material for high energy density lithium ion batteries. However, electrochemical tests from half cells and full cells show poor cycling performance when charged to potentials above 4.2 V. The calendar and cycle lifetimes of cells are affected by the structural stability of the active electrode materials as well as the parasitic reactions that occur in lithium ion batteries. In order to explore the major failure mechanisms of the material, half cells (coin cells) with control electrolyte and full cells (pouch cells) with control electrolyte and with selected electrolyte additives were tested over four different potential ranges. Isothermal microcalorimetry was used to explore the parasitic reactions and their potential dependence. In-situ and ex-situ X-ray diffraction and scanning electron microscopy were used to investigate the structural and morphological degradation of the materials over cycling. It was found that the dramatic c-axis change of the active material during charge and discharge may not be the major problem for cells that are cycled to higher potentials. The parasitic reactions that arise from the interactions between the electrolyte and the highly reactive delithiated cathode surface at high potentials are suggested as the main reason for the failure of cells cycled above 4.2 V. It should be possible to further improve the performance of NMC811 at high potentials by modifying the cathode surface and/or identifying and using electrolyte blends which reduce parasitic reactions. High energy density lithium ion batteries (LIBs) that are cheaper, safer, and with longer lifetimes need to be developed in order to meet the increasing demand for applications such as electric vehicles and large scale stationary energy storage. The energy density of LIBs can be increased by increasing the specific capacity and average potential of the cells. The calendar and cycle lifetimes of cells are affected by the structural stability of the active electrode materials as well as the parasitic reactions that occur in lithium ion batteries. The degree of lithium utilization of LiCoO 2 is limited to ∼70% in order avoid the O3 -H1-3 -O1 phase transformation when charged above 4.45 V.1 Parasitic reactions such as electrolyte oxidation at the cathode-electrolyte interface can ultimately cause cell failure.2-5 The rate of the parasitic reactions is related to both the catalytic role of the cathode surface which depends on its composition and surface area, 3,6 as well as on the stability of the electrolyte.2-5 Methods such as the use of electrolyte additives 7-11 and core-shell positive electrode materials [12][13][14] have been developed and studied to reduce the rate and extent of parasitic reactions, and hence increase capacity retention and lifetime of high-voltage Li-ion cells. The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1...

Battery and EV manufacturers carry out extensive long-term tests to estimate the lifetime of the battery and base warranty durations on those tests. The long duration of these tests slows progress in the research and development required to improve the lifetime of Li-ion batteries. This paper shows that accurate measurements of coulombic efficiency (CE) and impedance spectra of Li-ion batteries, that take a few weeks to acquire, can be used to rank the resulting lifetime of Li-ion cells. Adding one or more electrolyte additives to Li-ion batteries that act synergistically can dramatically improve the CE and long-term tests show corresponding ten-fold improvements in lifetime.In a recent review of battery technologies for electric vehicle applications, Bruce et al. 1 argue that technologies with greater energy densities than those achievable by Li-ion batteries are required in order to reach driving ranges greater than 200 km. Bruce et al. picked the Nissan Leaf as their canonical example which has an approximate 160 km range. The view that Li-ion batteries cannot lead to electric vehicles with widely acceptable driving ranges is also held by others. 2,3 Recently, the Tesla Model S, a Li-ion battery powered electric vehicle, was named Motor Trend car of the year for 2013. 4 The Tesla S has a driving range of 425 km when equipped with an 85 kWhr Li-ion battery. The Tesla S demonstrates that Li-ion batteries can power EV's for distances over 200 km in an elegant design. However, the vehicle is expensive and the 85 kWh battery must contribute at least $25,000 to the price assuming the $300/kWh USDOE target for EV battery costs. 5 No one would argue that increased energy density would not be an advantage, but the real issues with Li-ion batteries are cost and lifetime, not energy density, as far as automotive applications are concerned.Recently, a class action lawsuit was brought against Nissan by Nissan Leaf owners alleging that the Li-ion batteries in the Leaf can lose as much as 27% of their energy storage capacity within one year of use. 6 The Nissan Leaf uses a different Li-ion battery technology than the Tesla Model S. Nevertheless, EV manufacturers and users are worried about battery lifetime since the cost of a replacement battery is large.Testing the lifetime of a Li-ion battery under realistic conditions for an EV application takes years. Research and development to improve the lifetime of Li-ion batteries cannot have iterative cycles of several years before the outcome of experiments are known. We recently proposed the use of high precision measurements of the coulombic efficiency (CE) of Li-ion cells as a rapid way to screen and rank new electrode materials, electrolytes, and electrolyte additives for their impact on Li-ion cell lifetime. 7-10 In this paper it is demonstrated that short-term CE measurements coupled with initial impedance measurements can serve as a good predictor for cell lifetime and also dramatically demonstrate the beneficial impact of multiple electrolyte additives on lifetime. Figure...

The effects of electrolyte additives singly or in combination on Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (NMC)/graphite pouch cells have been systematically investigated and compared using the ultra high precision charger (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS), an automated storage system, gas evolution measurements and selected long-term cycling experiments. The results of testing Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (NMC)/graphite pouch cells with different electrolyte additives singly or in combination were measured and the results for over 110 additive sets are compared. A "Figure of Merit" approach is used to rank the effectiveness of the additives and their combinations. The combination of vinylene carbonate (VC) and/or prop-1-ene-1,3 sultone (PES), a sulfur containing additive, such as methylene methane disulfonate (MMDS), as well as either tris(-trimethly-silyl)-phosphate (TTSP) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) as additives in the electrolyte can give cells with extremely high coulombic efficiency, excellent storage properties, low impedance and superior long term cycling at 55 • C. Additive mixtures such as 2% PES + 1% MMDS + 1% TTSPi are especially excellent in all respects. It is hoped that this comprehensive report sets a benchmark for future studies by others and can be used as a guide and reference for the comparison of other electrolyte additives singly or in combination.

The measured heat flow of graphite/NMC lithium ion cells under charging conditions show a characteristic and easily identifiable signal at the onset of lithium plating on the graphite electrode. A marked decrease in heat flow signals the full lithiation of the graphite host. The origin of this signal is shown to arise from the combined effects of entropy and cell over potentials. This signal allows for an accurate measure of the maximum amount of lithium intercalation possible in the host. Metallic lithium deposition begins within 5-7 mAh/g after the heat flow begins to decrease. Two different types of graphite were examined; G25 and MCMB. The onset of lithium plating was detected at 336 mAh/g for the G25 graphite and 297 mAh/g for the MCMB graphite, yielding empirical formulas of Li 0.888 C 6 and Li 0.804 C 6 , respectively. The effect of plated lithium on the electrode/electrolyte reactivity was also examined by precise measurement of the coulombic efficiency, parasitic thermal energy and cell capacity fade. These measurements then allowed for the calculation of the efficiency of lithium plating on the graphite surface: 0.98 and 0.97 for G25 and MCMB graphites, respectively.In Li-ion battery chemistry the amount of lithium stored in the positive electrode material should not exceed the amount of lithium that can be fully intercalated into the negative electrode material, typically graphite. If this balance is exceeded, metallic lithium can deposit, or plate, at the graphite electrode with potentially serious consequences for reliability and safety. 1-5 Meanwhile, portable electronics are demanding increasingly higher energy densities for longer run times and higher power at low cost. To meet these demands the amount of excess material in the cell is minimized. This closer balance combined with potentially poor current distribution, high charging rates, 3,6 or low operating temperatures 7-9 can increase the risk of lithium deposition at the negative electrode. Therefore it is essential to accurately detect the onset of lithium plating in real-time with a high degree of sensitivity.Recently, the integration of isothermal heat flow calorimetry with electrochemical cycling of Li-ion cells, known as electrochemical calorimetry, has been used to examine the thermal behavior of several Li-ion chemistries. [10][11][12][13][14][15][16][17] In this paper, this technique is used to show that the thermal power from a graphite/NMC 442 Li-ion cell produces a characteristic pattern immediately prior to the deposition of metallic lithium at the graphite electrode. This characteristic thermal signal is explained through the study of graphite/lithium half cells where the graphitic electrode is intentionally driven to sufficiently negative potentials to induce lithium plating.An earlier paper 18 describes how to use electrochemical calorimetry to separate the various sources of thermal power. Those methods are applied in this paper to examine the effects of plated lithium on the coulombic efficiency, capacity fade, and the heat p...

The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4̅2(1)m and consists of π-stacked radicals, tightly clustered about 4̅ points and running parallel to c. The β-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = (1)/(2) centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S-S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-1a behaves as a Mott insulator, but the ambient-temperature conductivity σ(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of σ(RT) for β-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ(RT) increases in a manner which parallels the behavior of α-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S-S···S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S···Se-Se···S σ-bonds.