EMC Electrolyte Research Articles

Performance Degradation Characteristics of Layered Oxide//Silicon-Graphite Full Cells

High-energy and high-capacity positive and negative electrode materials are needed to reduce the cost and weight per kWh of Li-ion batteries. The use of silicon, mixed with various amounts and types of carbon, can greatly enhance the specific capacity of the negative electrode. Lithium-rich layered oxides can provide high capacities, as well as the high voltages, required to increase energy densities. The performance degradation of these material types has been shown to be influenced by cell cycling conditions. However, detailed investigations of electrode performance in a full cell using this combination of materials are rarely reported, and the degradation behavior is harder to explain because of the difficulty of interpreting full cell cycling and impedance data. Furthermore, side reactions cause a relative state-of-charge shift of the positive and negative electrodes in a full cell, so that electrochemical cycling between given potential limits in reality exerts varying conditions on the electrodes themselves as the cell ages. In order to understand the degradation behavior of this combination of high-energy high-capacity materials, we conducted extensive tests with 2032-type coin cells and reference electrode (RE) cells; the results of these tests will be reported. The high-capacity negative electrode is a mixed silicon-graphite (SiGr) electrode, and the positive electrode has 90 wt% Li1.03(Ni0.5Co0.2Mn0.3)0.97O2 (NCM523). The coin cells contain 1.58 cm2 area electrodes with 1.2 M LiPF6 in EC:EMC (3:7 w/w) electrolyte (Gen 2). The RE cells contain 20.3 cm2 area NCM523-positive and SiGr-negative electrodes, two Celgard 2325 separators enveloping a LixSn RE, and a Li-metal RE external to the electrode sandwich. Cell cycling includes formation cycles followed by aging cycles; pulse-power and AC impedance measurements are made periodically throughout the cycling. The effects of various system variables on cell performance and aging have been explored and will be discussed. These variables include upper and lower cut-off potentials for cell cycling, electrode coatings, and electrolyte additives. In addition to the electrochemical results, diagnostic techniques such as X-ray photoelectron spectroscopy and scanning electron microscopy were performed on the electrodes in order to gain a mechanistic understanding into the capacity fade and impedance rise of the NCM523//SiGr cells. Our results provide insight into the performance and limitations of various cell chemistries that are being considered for use in high-energy high-capacity cells. Acknowledgements: Financial support from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.

Use of phosphoranimines to reduce organic carbonate content in Li-ion battery electrolytes

The use of phosphoranimines (PAs), a class of linear, monomeric phosphazenes, as electrolytes for Li-ion battery applications has been investigated as a route to improve safety and stability for Li-ion batteries. Of the potential PAs for use in battery applications, this work focuses on the initial synthetic preparation and analysis of N-trimethylsilyl-P,P-bis((2-methoxyethoxy)ethoxy)-P-ethylphosphoranimine (PA-5). PA-5 has high LiPF6 solubility in excess of 2M, high thermal stability with a melting point below −80°C and high thermal stability as a neat compound to at least 250°C. As part of electrolyte blends, the inclusion of PA-5 shifts the onset of thermal degradation by close to 40°C at 35% loading and by 20°C at a 10% loading, improves the low temperature performance of the electrolyte, and when used as a primary solvent leads to increases in the flash point (by 20°C) when compared to more traditional EC:EMC blends. Cycling capabilities of full-coin cells with graphite negative electrodes and Li1+w[Ni0.5Mn0.3Co0.2]1-wO2 positive electrodes using PA-5:EC:EMC electrolyte blends are comparable with the performance seen for traditional EC:EMC blends. Analysis of the impact of the use of additives such as vinylene carbonate in PA-5:EC:EMC blended electrolyte results in enhanced capacity retention and improved coulombic efficiency.

Fluorinated Electrolyte for High Voltage Li(Ni0.4Mn0.4Co0.2)O2 (NMC442)/Graphite Pouch Cells

Introduction One of the best ways to increase the energy density of Li-ion batteries is to utilize high voltage cathode materials. Li(Ni0.4Mn0.4Co0.2)O2 (NMC442) has been proposed as a promising cathode material for high voltage Li-ion batteries. However, the instability of conventional organic carbonates towards high voltage is a great challenge [1, 2]. Searching for an appropriate electrolyte system is essential for application of these materials at high potential. In this presentation, fluorinated carbonate mixtures which are composed of fluoroethylene carbonate (FEC) and bis(2,2,2-trifluoroethyl) carbonate (TFEC) are shown to be viable in NMC442/graphite pouch cells that can be cycled to at least 4.5 V. The introduction of proper electrolyte additives has been shown to improve the cell performance. Long-term cycling results were also made to compare with 1 M LiPF6 EC/EMC (3:7 wt.%) with and without additives. Experimental The pouch cells employed in this study were Li[Ni0.4 Mn0.4Co0.2]O2 (NMC442)/graphite cells with a capacity of 240 mAh and coated NMC442/graphite cells with a capacity of 180 mAh. The positive electrode material in the coated cells has been coated with 3 wt% of LaPO4 which appears as nanoparticles on the NMC particle surfaces. Both types of cells were balanced for 4.7 V operation. 1 M LiPF6 EC/EMC (3:7 wt.% ratio, BASF, 99.99%) was used as the control electrolyte. The NMC442/graphite pouch cells were filled with 0.75 mL of electrolyte (0.90 g for EC:EMC=3:7 based electrolyte and 1.15 g for FEC:TFEC=1:1 based electrolyte). The cells then underwent a formation protocol during which they were opened and re-vacuum sealed at 3.5 V and again at 4.5 V to remove any gas generated during the first charge. In-situ gas measurements were made using the apparatus and procedure described in reference [3]. After formation, cells were then moved to the Ultra High Precision Charger (UHPC) for cycling. Storage tests were also used as were frequency response analyzer/cycling (FRA) experiments to measure the impedance growth during cycling [4]. The detailed testing method used in the FRA experiments is described in ref [3]. After these tests, cells were charged or discharged to 3.8 V where impedance spectra and gas volume produced were measured. Results and discussion Figure 1a shows the cell voltage versus time of two NMC442/graphite pouch cells with control and 2% VC in EC:EMC 3:7 electrolyte. Data for three other cells with, 2% PES, 2% PES + 1% MMDS and 2% PES + 1% TTSPi in FEC:TFEC 1:1 electrolyte are also shown. Figure 1b shows the in-situ gas volume versus time during the first two cycles for the same cells as shown in Figure 1a. Figures 1a and 1b show the FEC:TFEC electrolyte system containing PES as an electrolyte additive produces much less gas during the first a few cycles compared to the EC:EMC electrolyte system when charged to 4.5 V. However, a steady increase in gas volume produced occurs. Figure 1c shows the resistance, Rct, as a function of voltage for the NMC442/graphite pouch cells with 2% PES in FEC:TFEC 1:1 electrolyte at 40. ± 0.1ºC measured using the FRA.Figure 1c shows that Rct of the cells with FEC:TFEC:PES electrolyte does not increase when charged from 3.8 V to 4.5 V in each separate cycle. However, Rct increases as the cycle number increases. This suggests that FEC:TFEC electrolytes will not solve the problems associated with high voltage NMC/graphite cells. References L. Ma, J. Xia and J. R. Dahn, J. Electrochem. Soc., 161, A2250–A2254 (2014).L. Ma, J. Xia, and J. R. Dahn, Promising sulfur-containing electrolyte additives or ternary Electrolyte additive mixtures for Li-ion cells that promote long lifetime and better safety, 2015, submitted to J. Electrochem. Soc C. P.Aiken, J. Xia, D. Y. Wang, D. A. Stevens, S. Trussler and J. R. Dahn, J. Electrochem. Soc., 161, A1548–A1554 (2014).K. J. Nelson, G.L. d’Eon, A.T.B. Wright, L. Ma, J. Xia and J. R. Dahn, Studies of the effect of high voltage on the impedance and cycling performance of Li[Ni0.4Mn0.4Mn0.2]O2/graphite lithium-ion pouch cells, 2015, submitted to J. Electrochem. Soc. Figure caption Figure 1. (a) Cell voltage versus time during the first 260 h for the 240 mAh NMC/graphite pouch cells in FEC:TFEC 1:1 electrolyte with different additives cycled at C/20 (12 mA) and 40.0ºC; (b) Gas volume versus time measured using the Archimedes’ in-situ gas analyzer during the first ~260 h (2 continuous cycles and 2 cycles with 10 hour holds at 4.5 V). (c) Rct as a function of voltage measured every 5 cycles from 3.8 to 4.5 V on FRA. Rct increases with cycle number Figure 1

Rate-Dependent, Li-Ion Insertion/Deinsertion Behaviour of LiFePO4 Cathodes in Commercial 18650 LiFePO4 Cells

Although the performance of LiFePO4 has been improved significantly, the underlying mechanism of lithium ion insertion and de-insertion in FePO4/LiFePO4 (FP/LFP) is still not clearly understood1-4. There are two basic mechanisms occurring during charge/discharge process: solid-solution reaction mechanism and two-phase reaction mechanism. In the first case – the solid solution reaction – only a single phase is involved, and as a result the lattice parameters and unit cell volumes would be expected to change continuously during the charge/discharge cycle. In the second case – associated with a first order phase transition – the unit cell volumes of both phases remain nearly constant, with the unit cell volume difference of two phases varying by only DV=6.5%. The distinct nature of these two mechanisms is thus apparent using X-ray diffraction, and as a result, in operando synchrotron HEXRD technique is particular amenable to revealing the underlying mechanisms.In this study, we have investigated the structural changes that occur in LiFePO4 electrodes in commercial 18650 cells during the charge/discharge process using in operando synchrotron HEXRD.The 18650 cell (APR18650M, 1.1Ah) was provided by A123 Systems with a graphite anode, a LiFePO4 cathode, and a 1.20 M LiPF6 in EC: EMC electrolyte. No special modification was needed for the 18650 cell before characterization. Four equivalent cells have been cycled and in situ characterized under different cycling rates from 0.1 C, 1C, 3C to 5 C.The results show that (1) the phase fractions of both LiFePO4 and FePO4 (Fig. 1 a and b) change with SOC at both low rates (i.e. 0.1 C) and high rates (i.e. 1 C), suggesting that the LiFePO4 electrode is undergoing a two-phase reaction mechanism in the entire flat voltage plateau; (2) the unit cell volume of both LiFePO4 and FePO4 (Fig. 1 c and d) changes with the SOC at both low rates (i.e. 0.1 C) and high rates (i.e. 1 C), indicating that the electrode is experiencing the dual-phase solid-solution reaction mechanism, and (3) the difference (DV ) in the unit cell volume between the LiFePO4 phase and the FePO4 phase reduces as the rate increases (not shown here due to the page limit), implying that the region over which the dual-phase solid-solution exists will be compressed with increased rates and, eventually, may be diminished at extremely high rates5. On the other hand, our results suggest that the insertion/deinsertion process is rate dependent and the two different mechanisms, two-phase and solid-solution, co-exist during the charge/discharge process. At very low over-potential (i.e. very low rate charge/discharge), the process is dominated by the two-phase mechanism while it might be dominated by the solid solution mechanism at very high over-potential (i.e. very high rate charge/discharge). Between the two extremes, the process is controlled by both mechanisms with different ratios depending on the rate. The proposed dual-phase solid-solution mechanism may explain the remarkable rate capability of LiFePO4 in commercial cells.

Determination of the Voltage-Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Parasitic reactions that occur in lithium ion batteries are well known to result in cell failure [1], and therefore being able to measure and reduce these reactions is of utmost importance. Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition [2]. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources as a function of state of charge can be isolated. When a current is applied to a cell there are three sources of heat flow [3]: Joule heating due to polarization, changes in entropy, and parasitic reactions. Polarization produces a non-reversible heat flow that is proportional to the square of the current, changes in entropy during intercalation and deintercalation produces a reversible heat flow that is proportional to the current, and parasitic reactions produce a heat flow that is thought to be independent of the current. The relative effect of entropy, polarization, and parasitics can therefore be determined over small voltage ranges by varying the current. The data can be fit using a simplistic model where each contribution is modeled with a simple function, typically linear, of the state of charge. The fitting results then give the relative contributions of each term, with particular importance to the function associated with the parasitic heat flow for an individual cell, allowing for the extraction of the voltage-dependence of the parasitic heat flow for an individual cell.Figure 1 shows an example of such an analysis for a 180 mAh machine-made high voltage-LiCoO2/graphite pouch cell with a 1 M LiPF6 in 3:7 EC:EMC electrolyte. The heat flow was measured using a TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters with an accuracy of < ±1 mW. There is an excellent agreement between the simplistic model and the experimental data, showing the ability of isothermal microcalorimetry to accurately extract the voltage-dependence of parasitic heat flow in individual cells.This analysis is particularly useful for drawing conclusions about the behaviours of electrolyte additives across different cell chemistries where the method of relative differences in heat flow cannot be used. Results will be presented for three different machine-made pouch cell chemistries: high voltage-LiCoO2/graphite to 4.4 V, Li[Ni0.33Mn0.33Co0.33]O2 (NMC)/graphite to 4.2 V, and NMC/graphite to 4.4 V. The effect of a variety of different additives such as vinylene carbonate, methylene methanedisulfonate, ethylene sulfate, trimethylene sulfate, etc, will also be explored.

Determination of the Voltage-Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Parasitic reactions that occur in lithium ion batteries are well known to result in cell failure [1], and therefore being able to measure and reduce these reactions is of utmost importance. Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition [2]. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources as a function of state of charge can be isolated. When a current is applied to a cell there are three sources of heat flow [3]: Joule heating due to polarization, changes in entropy, and parasitic reactions. Polarization produces a non-reversible heat flow that is proportional to the square of the current, changes in entropy during intercalation and deintercalation produces a reversible heat flow that is proportional to the current, and parasitic reactions produce a heat flow that is thought to be independent of the current. The relative effect of entropy, polarization, and parasitics can therefore be determined over small voltage ranges by varying the current. The data can be fit using a simplistic model where each contribution has one linear term and one that scales with the state of charge. The fitting constants then give the relative contributions of each term, with particular importance to the parameters associated with the parasitic heat flow for an individual cell, allowing for the extraction of the voltage-dependence of the parasitic heat flow for an individual cell.Figure 1 shows an example of such an analysis for a 225 mAh machine-made LiCoO2/graphite pouch cell with a 1 M LiPF6 in 3:7 EC:EMC electrolyte. The heat flow was measured using a TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters with an accuracy of < ±1 mW. There is an excellent agreement between the simplistic model and the experimental data, showing the ability of isothermal microcalorimetry to accurately extract the voltage-dependence of parasitic heat flow in individual cells.This analysis is particularly useful for drawing conclusions about the behaviours of electrolyte additives across different cell chemistries where the method of relative differences in heat flow cannot be used. Results will be presented for four different machine-made pouch cell chemistries: LiCoO2/graphite to 4.2 V, Li[Ni0.33Mn0.33Co0.33]O2 to 4.2 V, Li[Ni0.33Mn0.33Co0.33]O2 to 4.4 V, and Li[Ni0.4Mn0.4Co0.2]O2 to 4.7 V. The effect of a variety of different additives such as vinylene carbonate, methylene methanedisulfonate, ethylene sulfate, trimethylene sulfate, etc, will also be explored.

Preparation and characterization of trifluoroethyl aliphatic carboxylates as co-solvents for the carbonate-based electrolyte of lithium-ion batteries

In this work, a series of trifluoroethyl aliphatic carboxylates with different carbon-chain lengths in acyl group are prepared and investigated as the co-solvents for the carbonate-based electrolyte of lithium-ion batteries. The trifluoroethyl aliphatic carbonates are synthesized by a modified one-step approach, using aliphatic carboxylic acid and trifluoroethanol as the raw materials (molar ratio, 1.2:1), hydrogen ion exchange resin as the catalyst and silica gel drier as the de-hydration. The structure and electrochemical properties of the final products have been characterized by FTIR, 1H NMR, GC–MS, viscosity, conductivity meter and electrochemical measurements. The structure characterizations show that the final products have high purity. Electrochemical tests present that the co-solvents are able to improve the electrochemical performances of graphite electrode at low temperature. In particular, we find that an addition of trifluoroethyl n-hexanoate (TFENH) into 1M LiPF6/EC+EMC electrolyte can significantly decrease the Li de-intercalation potential of graphite by 540mV and achieve a high capacity retention of 92% at 218K. The electrochemical impedance spectroscopy (EIS) measurements indicate that the observed performance improvement at low temperature is associated with the decreased surface film resistance (RSEI) by the addition of co-solvents.

Comparative study of electrolyte additives using electrochemical impedance spectroscopy on symmetric cells

The effect of various electrolyte additives and additive combinations added to a 1 M LiPF6 EC:EMC electrolyte on the positive and negative electrodes surface of 1 year old wound LiCoO2/graphite cells and Li[Ni0.4Mn0.4Co0.2])O2/graphite cells was studied using electrochemical impedance spectroscopy (EIS) on symmetric cells. The additives tested were: vinylene carbonate (VC), trimethoxyboroxine (TMOBX), fluoroethylene carbonate (FEC), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and H2O alone or in combination. In general, compared to control electrolyte, the additives tested reduced the impedance of the positive electrode and increased the impedance of the negative electrode with the exception of LiTFSI in Li[Ni0.4Mn0.4Co0.2]O2/graphite wound cells. Higher charge voltage led to higher positive electrode impedance, with the exception of 2%VC + 2% FEC, and 2% LiTFSI. In some cases, some additives when mixed with another controlled the formation of the SEI at one electrode, and shared the formation of the SEI at one electrode when mixed with a different additive.

Effect of sulfolane on the morphology and chemical composition of the solid electrolyte interphase layer in lithium bis(oxalato)borate‐based electrolyte

To discuss the source of sulfolane (SL) in decreasing the interface resistance of Li/mesophase carbon microbeads cell with lithium bis(oxalate)borate (LiBOB)‐based electrolyte, the morphology and the composition of the solid electrolyte interphase (SEI) layer on the surface of carbonaceous anode material have been investigated. Compared with the cell with 0.7 mol l−1 LiBOB‐ethylene carbonate/ethyl methyl carbonate (EMC) (1 : 1, v/v) electrolyte, the cell with 0.7 mol l−1 LiBOB‐SL/EMC (1 : 1, v/v) electrolyte shows better film‐forming characteristics in SEM (SEI) spectra. According to the results obtained from Fourier transform infrared spectroscopy, XPS, and density functional theory calculations, SL is reduced to Li2SO3 and LiO2S(CH2)8SO2Li through electrochemical processes, which happens prior to the reduction of either ethylene carbonate or EMC. It is believed that the root of impedance reduction benefits from the rich existence of sulfurous compounds in SEI layer, which are better conductors of Li+ ions than analogical carbonates. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Study of Electrolyte Additives Using Electrochemical Impedance Spectroscopy on Symmetric Cells

Wound LiCoO2/graphite cells with 1 M LiPF6 EC:EMC electrolyte containing 1 wt%, 2 wt% vinylene carbonate (VC), 0.3 wt% trimethoxyboroxine (TMOBX) and 2 wt% VC + 0.3 wt% TMOBX were subjected to extended storage studies. After storage, the electrodes were studied using the symmetric cell and electrochemical impedance spectroscopy (EIS) approach described by previous workers. This approach allows the impact of an additive on the impedance of the negative and the positive electrode to be distinguished. Compared to the control cells, adding 1 wt% VC reduced the positive electrode impedance and only slightly affected the negative electrode impedance. Adding 2 wt% VC reduced the positive electrode impedance and greatly increased the negative electrode impedance. An addition of 0.3 wt% TMOBX greatly decreased the positive electrode impedance and slightly increased the negative electrode impedance. Compared to the cells with 2% VC only, adding 2% VC + 0.3% TMOBX decreased the positive electrode impedance without affecting the negative electrode impedance leading to a significant reduction in full cell impedance. These results help explain why the combination of VC and TMOBX additives can be effective in LiCoO2/graphite cells designed for long life time.

Effect of surface preparation and R-group size on the stabilization of lithium metal anodes with silanes

As new applications for lithium-ion batteries emerge into the marketplace, a new emphasis is being placed on developing higher capacity electrodes. Two of the higher capacity technologies under development are lithium–sulfur and lithium–air batteries, both of which, in most configurations, use a lithium metal anode. Building on our previous work extending the cycle life of lithium metal anodes via surface functionalization with silane groups, we have identified two separate regimes for the cycle life enhancements based on size of the silane R-groups. Very small R-groups (TMS) and R-groups bulkier than triphenyl show enhanced cycle life compared to control samples while R-groups between these in size show reduced cycle life. Additionally, we present a comparison between different cleaning methods to optimize the hydroxyl functionalized layer on the lithium metal and the influence of these methods on the stability of lithium metal in EC:EMC electrolyte. A solvent based cleaning approach is shown to substantially improve stability when combined with chlorotrimethyl silane treatment.

Impedance Reducing Additives and Their Effect on Cell Performance: II. C3H9B3O6

Wound LiCoO2/graphite and Li[Ni0.42Mn0.42Co0.16]O2 (NMC)/graphite cells with 1 M LiPF6 in EC:EMC (3:7 by wt) electrolyte containing 2 wt% vinylene carbonate (VC) and/or 0.3 wt% trimethoxyboroxine (TMOBX) were studied using the High Precision Charger at Dalhousie University, automated cycling/storage, AC impedance and long-term cycling at elevated temperature. The additive, VC, improves lifetime performance by increasing coulombic efficiency and decreasing charge and discharge end point capacity slippage. The impact of TMOBX on cell performance depends on electrolyte and electrode choices within the cell. When TMOBX is added to control electrolyte in LiCoO2/graphite cells, the cycle life is improved and the impedance reduced. When added to VC-containing electrolyte the impedance is also reduced with the impact on lifetime not being clear at this time. When TMOBX is added to NMC/graphite cells with control electrolyte the impedance is decreased but when added to VC-containing electrolyte the impedance is unchanged. The impact on cycle life when TMOBX is added to NMC/graphite cells is a decrease in capacity fade but a lower measured coulombic efficiency. Therefore TMOBX appears to be an interesting additive with potential to decrease impedance and/or extend cycle life based on the electrolyte formulation and electrodes used in a Li-ion cell.

Impedance Reducing Additives and Their Effect on Cell Performance: I. LiN(CF3SO2)2

Wound LiCoO2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite cells with 1 M LiPF6 in EC:EMC (3:7 by wt) electrolyte containing 2 wt% vinylene carbonate (VC) and/or lithium bis (trifluoromethanesulphonyl) imide (LiTFSI or 3M Fluorad Lithium HQ-115) were studied using the High Precision Charger at Dalhousie University, automated cycling/storage, AC impedance and long-term cycling at elevated temperature. The additive, VC, improves lifetime performance by increasing coulombic efficiency and decreasing charge and discharge end point capacity slippage, while the impact of HQ-115 alone on lifetime is minimal. However, HQ-115 reduces cell impedance when added to either control or VC-containing electrolyte. Therefore it serves as a useful additive for improving the rate capability of cells and capacity retention during high rate cycling at elevated temperature. It is our opinion that Li-ion cells incorporating VC can benefit from a simultaneous addition of HQ-115 due to a synergistic effect observed between HQ-115 and VC additives. This report is the first of two companion reports studying impedance reducing additives and their impact on cell lifetime.

The Impact of Varying the Concentration of Vinylene Carbonate Electrolyte Additive in Wound Li-Ion Cells

Wound LiCoO2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite cells with 1M LiPF6 EC:EMC electrolyte containing either 0, 1 or 2 wt.% vinylene carbonate were studied using the High Precision Charger at Dalhousie University, automated cell storage and AC impedance. Vinylene carbonate (VC) was found to improve the coulombic efficiency of the cells, decrease charge endpoint capacity slippage and decrease self discharge, in all cases primarily by slowing electrolyte oxidation at the positive electrode. The beneficial impacts of VC are greater in LiCoO2 cells than in Li[Ni0.42Mn0.42Co0.16]O2 cells. One percent VC is enough to derive the benefits without causing an impedance rise in the cells.

Performance of high-power lithium-ion cells under pulse discharge and charge conditions

Lithium-ion cells being designed for transportation applications must sustain high current pulses under rapid discharge and rapid charge conditions without significant degradation of cell performance. In this article we examine the pulse discharge and charge performance of cells, containing a LiNi 0.8 Co 0.15 Al 0.05 O 2 -based cathode, a graphite-based anode, and a LiPF 6 -bearing EC:EMC electrolyte, at current rates ranging from 3 to 25 C. Impedance data indicate that 18650-type cells containing this chemistry can withstand 18s discharge pulses at rates up to 17 C in the 3.7-4.0V voltage window. Data from cells containing a LiSn micro-reference electrode show that the positive electrode impedance increases, whereas the negative electrode impedance decreases, with increasing magnitude of the discharge current pulse. The discharge pulse-current that can be sustained by the cell is limited by lithium diffusion into oxide particles of the positive electrode.

Novel concept of pseudocapacitor using stabilized lithium metal powder and non-lithiated metal oxide electrodes in organic electrolyte

A concept of using two non-prelithiated metal oxides (e.g., MnO 2, V 2O 5, and FeO x ) in both positive and negative electrodes in organic Li-ion electrolytes has been proposed and tested to improve the energy density of pseudocapacitors. To take the advantages of this concept, additional lithium source is essential to provide lithium ions during the charge–discharge cycles. The stabilized lithium metal powder (SLMP ™) developed by FMC Corp., provides such an essential Li + source. Here we report the first result of the symmetric pseudocapacitor using two non-prelithiated metal oxide (i.e., manganese oxide/carbon nanotube (MnO 2/CNT)) electrodes, with added SLMP in one of them. The capacitor using the SLMP added MnO 2/CNT (positive) and pure MnO 2/CNT (negative) electrode in 1.2 M LiPF 6-EC:EMC electrolyte shows supercapacitive behaviors in 3.0 V voltage range. The addition of SLMP opens new opportunities of using the non-lithiated metal oxide electrodes in pseudocapacitors and hybrid electrochemical capacitors (ECs), which has not been possible before.

Electrochemical characterisation and modelling of the mass transport phenomena in LiPF 6–EC–EMC electrolyte

The conductivity, the salt diffusion coefficient, the lithium-ion transport number and the thermodynamic factor of the salt and the solvent were reported for LiPF 6 in EC:EMC (3:7) at 25 °C and for concentrations between 0.2 and 2.0 mol/dm 3. The mass transport in the electrolyte was fully characterised by combining three types of electrochemical experiments; concentration cells, galvanostatic polarisation experiments and electrochemical impedance measurements with a mathematical description of the mass transport in the electrolyte. The apparent salt diffusion coefficient had a local maximum in the concentration range, while the viscosity-dependent salt diffusion coefficient decreased from 4.1 × 10 −10 m 2/s at 0.2 mol/dm 3 to 4.4 × 10 −11 m 2/s at 2.0 mol/dm 3. Both the thermodynamic factor and the conductivity varied strongly with the concentration. The conductivity had a maximum of 9.5 mS/cm at 1.0 mol/dm 3. The lithium-ion transport number with respect to the room decreased with increasing salt concentration, with a maximum of 0.37 at 0.2 mol/dm 3 in the concentration range. The Maxwell–Stefan diffusivities and the electrolyte potential drop in a lithium-ion battery at steady state were lastly calculated from the obtained transport properties. An analysis of the characterisation method was also done on the basis of the characterisation results.

Thermal stability of LiPF 6–EC:EMC electrolyte for lithium ion batteries

A differential scanning calorimeter (DSC) was used to perform a thermal stability study of the LiPF 6–EC:EMC electrolyte. The effect of different variables on its thermal stability was evaluated: salt (LiPF 6) concentration effect, solvents, EC:EMC ratios, and heating rates. Hermetically sealed and crimped DSC pans were used during the experiments. The results indicate that the salt concentration, solvent concentration, and heating rates play an important role in the thermal stability of the LiPF 6–EC:EMC electrolyte.