Methods and criteria for assessing the commercial viability of Si-based materials are discussed and demonstrated with the 3M V6 alloy and 60 nm nano Si powder. These materials are firstly evaluated through the cycling of neat electrodes containing only alloy and binder to characterize the capacity, first cycle efficiency, binder compatibility, and microstructure stability of the material. The alloy displays higher first cycle efficiency, lower fade, and a more stable amorphous microstructure compared to the nano Si, which displays a variable microstructure with a rate dependent presence of crystalline Li 15 Si 4 . The materials are then evaluated in graphite-containing composite electrodes having high areal capacities (> 2 mAh/cm 2 ). In a well designed composite electrode including carbon nanotubes, 3M V6 material was found to cycle with little fade and high coulombic efficiency (∼99.8%) while maintaining a stable dQ/dV. A composite electrode of equivalent volumetric capacity with nano Si powder shows similar capacity retention over 50 cycles but an unacceptably low coulombic efficiency (∼99.2%). High precision coulometry and calorimetry results show surface area as the dominant factor in levels of parasitic reactions with Si based materials.

A method for measuring the energy produced from parasitic cell reactions in lithium ion cells by electrochemical calorimetry is described. Negative electrode symmetric cells were charged and discharged by high precision current sources in an isothermal heat flow calorimeter while the cell voltage was accurately measured. Two sources of graphite of different BET surface areas were investigated. Symmetric cells of Li 4 Ti 5 O 12 and lithium/graphite half cells were also measured by this method. The measured parasitic energy was well correlated to the loss of active Li, or coulombic efficiency, confirming the source of the parasitic energy as the heat of reaction occurring between the lithiated electrodes and the electrolyte. The effect of electrode formulation was also explored. Electrochemical calorimetry of symmetric cells is an excellent method to study new material sets to determine which will lead to extended cell lifetime.

Symmetric Li-ion cells" are assembled using two identical electrodes, one previously charged and one discharged. For example, a graphite/graphite symmetric cell is assembled using one lithiated graphite electrode and one graphite electrode, both of which have the same mass of graphite per unit area. Symmetric cells allow study of electrodes, electrolyte and electrolyte additives in only the limited potential range of the electrode contained in the symmetric cell, rather than in the presence of both a high potential positive electrode and a low potential negative electrode as in a full lithium ion cell. Graphite/graphite and lithium titanate/lithium titanate symmetric cells were studied and the results are reported with a major focus on the interpretation of the symmetric cell results. We conclude that symmetric cells are well-suited for fundamental studies of the impact of electrolyte additives and new electrode materials on cell degradation.Lithium-ion batteries are used in portable electronics such as laptops and cell phones where they deliver several hundred chargedischarge cycles over their lifetime of a few years. Automotive and grid energy applications for Li-ion batteries, however, are much more demanding where thousands of cycles and lifetimes of at least a decade are required. Researchers around the world are addressing this need. [1][2][3] It is well known that electrolyte additives, such as vinylene carbonate (VC), can improve the cycle and calendar life of lithium-ion batteries. 4-7 It was recently shown that this difference in long-term cycle life can be inferred in a much shorter time by accurately comparing the coulombic efficiency (CE) of cells with and without VC. 6 The work in Ref. 6 showed that the major impact of VC was to reduce the rate of electrolyte oxidation at the positive electrode. This is in contrast to the suggestion of other researchers 8, 9 that the major impact of VC was to modify the SEI on the negative electrode. In order to learn whether the impact of a particular electrolyte additive is actually at one electrode or the other, the use of symmetric cells is proposed here.Symmetric Li-ion cells are assembled using two identical electrodes, one previously charged and one discharged. Symmetric cells have previously not been used extensively in lithium ion battery research 10 but have been used for studies of electrode impedance using impedance spectroscopy. 11-13 A symmetric cell must be assembled with one of the electrodes in a lithiated state, while the other is in a delithiated state. This arrangement allows a limited supply of lithium to be transferred between the two electrodes during cycling. Without the presence of excess lithium, such as the lithium foil in a half cell, any parasitic reactions that consume lithium result in capacity loss during cycling similar to a full cell. In symmetric cells the only electrode potentials present in the cell to induce parasitic reactions are limited to the range of the single electrode being tested. For example, in a graphite/graphite sym...

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...

Carbon dioxide is shown to be an effective additive to standard Li-ion electrolyte for extending the cycle life of full pouch cells containing an engineered silicon alloy. CO 2 was introduced to pouch cells by adding a few milligrams of dry ice to cells before sealing. The cells contained composite negative electrodes formulated with 15 to 17 wt% of an engineered silicon alloy and a LiCoO 2 positive electrode. Parasitic electrolyte reactions were measured in-situ by isothermal micro-calorimetry and high precision coulometry and compared to cells containing 1-fluoro ethylene carbonate (FEC). Extended cycling of cells containing CO 2 were compared to cells containing FEC. Cell gas generation and gas consumption were measured by applying the Archimedes principle. A new approach using small tubes in pouch cells to differentiate volume changes from gassing and solid expansion is introduced. Cells with CO 2 showed significantly lower parasitic thermal power, improved coulombic efficiency and better capacity retention compared to cells containing electrolytes with FEC. The gas generation/consumption experiments showed that Si alloy reacts with CO 2 during cycling until it is fully consumed. Combining FEC and CO 2 reduces the consumption rate of CO 2 . Microscopy of cross-sectioned cycled electrodes showed a thin SEI layer and minimal silicon alloy erosion. The combined work establishes CO 2 as a powerful precursor to an effective SEI layer on silicon alloys. Finally, CO 2 is shown to be an effective SEI former for graphite in EC-free electrolytes. The desire to increase energy density in Li-ion cells has fueled the research and development of silicon and silicon-based materials as a component of Li-ion negative electrodes. In most cases, however, only small amounts of silicon or silicon alloy are used largely because of the generally inferior cycle life of silicon compared to graphite. While there can be several reasons for the inferior cycle life of silicon and silicon alloys one important contributor to poor cycle life is the erosion of the silicon particle by parasitic electrolyte reactions. [1][2][3] Because of the volume changes Si experiences during cycling, continuous passivation is required at the surface and the demands on the electrolyte system are greater than for graphite.Carbon dioxide (CO 2 ) has been described as an effective electrolyte additive in metallic Li cells many years ago. 4 In that work it was shown that cycling efficiencies of the Li electrode were greatly increased in the presence of CO 2 . With the development of Li-ion cell chemistry the use of CO 2 was also explored early on. [5][6][7] In a few papers on the subject it was shown that the CO 2 generators, known as pyrocarbonates, were effective in forming SEI layers on graphite. 8,9 More recently a patent by workers at Sanyo has described the effect of electrolytes containing CO 2 on the cycle life of sintered negative electrodes containing silicon. 10In the course of our work on the erosion of silicon alloys our interest was drawn to the ...