Marshall C. Smart

Irreversible Capacities of Graphite in Low‐Temperature Electrolytes for Lithium‐Ion Batteries

Carbonaceous anode materials in lithium-ion rechargeable cells exhibit irreversible capacity, mainly due to reaction of lithium during the formation of passive surface films. The stability and kinetics of lithium intercalation into the carbon anodes are determined by these films. The nature, thickness, and morphology of these films are in turn affected by the electrolyte components, primarily the solvent constituents. In this work, the films formed on graphite anodes in low-temperature electrolytes, i.e., solutions with different mixtures of alkyl carbonates and low-viscosity solvent additives, are examined using electrochemical impedance spectroscopy (EIS) and solid-state ^(7)Li nuclear magnetic resonance techniques. In addition, other ex situ studies such as X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy were carried out on the graphite anodes to understand their microstructures.

Use of Organic Esters as Cosolvents in Electrolytes for Lithium-Ion Batteries with Improved Low Temperature Performance

The electrolyte composition plays a strong role in determining the low temperature performance of lithium-ion cells, both in terms of ionic mobility in the electrolyte solution, as well as forming suitable surface films on the electrode surfaces A series of ester solvents was chosen for incorporation into multicomponent electrolyte formulations due to their favorable physiochemical properties (i.e., low viscosity, low melting point, and high permittivity), as well as good compatibility with carbonaceous anodes and mixed metal cathodes (i.e LiCoO 2 and LiNiCoO 2 ). In addition to determining the relative facility of lithium intercalation and deintercalation in Li-carbon cells as a function of temperature, a number of conventional electrochemical methods were employed to further enhance the understanding of the nature of the electrode surface films in these ester-based electrolytes, including do polarization and ac impedance measurements A distinct trend was observed with respect to the stability of the surface films formed. In solutions containing low molecular weight cosolvents (i.e., methyl acetate and ethyl acetate) the surface films appear resistive and inadequately protective, whereas electrolytes containing higher molecular weight esters resulted in surface films with more desirable attributes. Promising electrolyte formulations were further evaluated in prototype lithium-ion cells (AA-size) and fully characterized in terms of their low temperature discharge performance.

Improved low temperature performance of lithium ion cells with quaternary carbonate-based electrolytes

Abstract In order to enable future missions involving the exploration of the surface of Mars with Landers, and Rovers, NASA desires long life, high energy density rechargeable batteries which can operate well at very low temperature (down to −40xa0°C). Lithium-ion technology has been identified as being the most promising chemistry, due to high gravimetric and volumetric energy densities, as well as, long life characteristics. However, the state-of-art (SOA) technology is not sufficient to meet the needs of many applications that require excellent low-temperature capabilities. To further improve this technology, work at JPL has been focused upon developing electrolytes that result in lithium-ion cells with wider temperature ranges of operation. These efforts have led to the identification of a number of ternary and quaternary, all carbonate-based electrolytes that have been demonstrated to result in improved low-temperature performance in experimental three-electrode MCMB–carbon/LiNi0.8Co0.2O2 cells. A number of electrochemical characterization techniques were performed on these cells (i.e. Tafel polarization measurements, linear polarization measurements, and electrochemical impedance spectroscopy (EIS)) to further enhance our understanding of the performance limitations at low temperature. The most promising electrolyte formulations, namely 1.0xa0M LiPF6 EC+DEC+DMC+EMC (1:1:1:2 v/v) and 1.0xa0M LiPF6 EC+DEC+DMC+EMC (1:1:1:3 v/v), were incorporated into SAFT prototype DD-size (9xa0Ah) lithium-ion cells for evaluation. A number of electrical tests were performed on these cells, including rate characterization as a function of temperature, cycle life characterization at different temperatures, as well as, many mission specific characterization tests to determine their viability to enable future missions to Mars. Excellent performance was observed with the prototype DD-size cells over a wide temperature range (−50 to 40xa0°C), with high specific energy being delivered at very low temperatures (i.e. over 95xa0Wh/kg being delivered at −40xa0°C using a C/10 discharge rate).

Improved performance of lithium-ion cells with the use of fluorinated carbonate-based electrolytes

Abstract There has been increasing interest in developing lithium-ion electrolytes that possess enhanced safety characteristics, while still able to provide the desired stability and performance. Toward this end, our efforts have been focused on the development of lithium-ion electrolytes which contain partially and fully fluorinated carbonate solvents. The advantage of using such solvents is that they possess the requisite stability demonstrated by the hydrocarbon-based carbonates, while also possessing more desirable physical properties imparted by the presence of the fluorine substituents, such as lower melting points, increased stability toward oxidation, and favorable SEI film forming characteristics on carbon. Specifically, we have demonstrated the beneficial effect of electrolytes which contain the following fluorinated carbonate-based solvents: methyl-2,2,2-trifluoroethyl carbonate (MTFEC), ethyl-2,2,2-trifluoroethyl carbonate (ETFEC), propyl-2,2,2-trifluoroethyl carbonate (PTFEC), methyl-2,2,2,2′,2′,2′-hexafluoro- i -propyl carbonate (MHFPC), ethyl-2,2,2,2′,2′,2′-hexafluoro- i -propyl carbonate (EHFPC), and di -2,2,2-trifluoroethyl carbonate (DTFEC). These solvents have been incorporated into multi-component ternary and quaternary carbonate-based electrolytes and evaluated in lithium–carbon and carbon–LiNi 0.8 Co 0.2 O 2 cells (equipped with lithium reference electrodes). In addition to determining the charge/discharge behavior of these cells, a number of electrochemical techniques were employed (i.e. Tafel polarization measurements, linear polarization measurements, and electrochemical impedance spectroscopy (EIS)) to further characterize the performance of these electrolytes, including the SEI formation characteristics and lithium intercalation/de-intercalation kinetics.

Li ion batteries for aerospace applications

Rechargeable Li ion batteries are perceived as likely substitutes for conventional nickel systems in an effort to minimize the mass and volume of the power subsystems in aerospace applications. The on-going consortium of NASA and DoD, after 2 years of existence, has propelled the advancement of aerospace Li ion technology in the US. Prototype cells of different sizes have been built by domestic manufacturers and are being evaluated both by NASA and Air Force. The early versions of these prototypes catered to needs of imminent NASA missions, i.e. Mars landers and rovers. Developmental efforts are underway to further improve the technology to meet the demands of long calendar life, as in the geosynchronous earth orbit (GEO) and the outer planets missions, and long cycle life as in the low earth orbit (LEO) missions. In this paper, we will briefly describe the objective and progress of this joint effort.

Effects of SEI on the kinetics of lithium intercalation

The electrochemical stability of electrolytes at lithium, or lithium-intercalating anodes, is achieved via ionically conducting surface films termed as solid electrolyte interphase (SEI). Since the lithium deposition or intercalation process occurs on the electrode covered with the SEI, the characteristics of the SEI determine the kinetics of lithiation/delithiation, stability of the interface, and thus, the overall cell performance, especially at low temperatures. In this paper, we have reiterated the significance of the SEI characteristics over the solution properties, using a few illustrative examples from our research on low temperature Li ion battery electrolytes at JPL. The examples specifically include the beneficial aspects of a ternary carbonate mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) compared to the binary mixtures (of EC and either DMC or DEC) and quaternary solutions with appropriate co-solvents, such as alkyl esters.

LITHIUM BATTERIES FOR AEROSPACE APPLICATIONS: 2003 MARS EXPLORATION ROVER

Future NASA planetary exploration missions require batteries that can operate at extreme temperatures and with high specific energy and energy densities. Conventional aerospace rechargeable battery systems, such as Ni-Cd, Ni-H;! and Ag-Zn, are inadequate to meet these demands. Lithium ion rechargeable batteries are therefore being chosen as the baseline for these missions. The 2003 Mars Exploration Rover mission plans to deploy twin rovers onto Mars, with the objectives of understanding its geology, climate conditions and possibility of life on Mars. The spacecraft contain various batteries, i.e., primary batteries on the lander, thermal batteries on the back shell and rechargeable batteries on the Rovers. Significant among them are the Li ion rechargeable batteries, which are being utilized for the first time in a major NASA mission. The selection of the Li ion battery has been dictated by various factors, including mass and volume constraints, cycle life, and its ability to operate well at sub-zero temperatures (down to -30°C), at moderate rates. This paper describes the selection criteria, design and performance of the three battery systems on 2003 MER mission.

Development of low temperature Li-ion electrolytes for NASA and DoD applications

Abstract Both NASA and the US Army have interest in developing secondary energy storage devices with improved low temperature performance to meet the demanding requirements of space missions and man-portable applications. Lithium-ion systems have been identified as having the most promise due to their high specific energy density and wide operational temperature ranges from the use of organic solvent-based electrolytes, rather than aqueous-based systems. Initially, the SOA lithium-ion technology was limited to operation above −10°C, due primarilly to the fact that the electrolytes employed had high melting points and were highly viscous at low temperatures, resulting in low ionic conductivity. However, due to recent developments in electrolyte formulations at the Army and at JPL, improved low temperature performance of lithium-ion cells have been demonstrated, with efficient cell operation to temperatures as low as −30°C. This was achieved by developing multi-component solvent systems, based on mixtures of cyclic and aliphatic alkyl carbonates. In the course of investigating the viability of a number of advanced electrolyte systems, it was identified that the protective surface films which form on the carbonaceous-based anodes can strongly influence the low temperature capabilities of lithium-ion cells, in addtion to the ionic conductivity of the electrolyte. Thus, in order to optimize an electrolyte for low temperature applications, it is necessary to balance the inherent physical properties of the formulations (i.e. freezing point, viscosity, and ionic conductivity) with the observed compatibility with the chosen cell chemistry (i.e. the nature of the passivating films formed on the electrodes).

Electrochemical and solid state NMR characterization of composite PEO-based polymer electrolytes

A comprehensive matrix of composite poly(ethyleneoxide) (PEO)-based solid-state electrolytes was developed in order to systematically study a number of variables and their impact upon the electrochemical properties of the resulting materials. The different parameters studied in the fabrication of these materials include: (i) the lithium electrolyte salt type, (ii) the ether oxygen to lithium ratio, (iii) the molecular weight of PEO, (iv) the type of ceramic additive used, either aluminum oxide (Al 2 O 3 ), silicon oxide (SiO 2 ), or titanium oxide (TiO 2 ), (v) the particle size of the additives used, and (vi) the concentration of additive (wt.%). The standard lithium salt used for the preparation of these electrolytes was lithium trifluoromethanesulfonate (lithium triflate or LiSO 3 CF 3 ), which served as the baseline electrolyte salt. Other lithium salts investigated include: lithium perchlorate (LiClO 4 ) and lithium bis-trifluoromethanesulfonimide (LiN(SO 2 CF 3 ) 2 ). Conductivity measurements were performed for each electrolyte membrane over a wide temperature range (23-100 °C). In addition, cyclic voltammetry measurements were performed on selected PEO membranes as a function of temperature to determine the impact of various parameters upon the electrochemical stability. It was observed that the parameter that displayed the most significant effect upon the PEO-base polymer conductivity was the lithium salt type employed. The lithium triflate salt-containing PEO polymers demonstrated the best mechanical properties before and after heat treatment. Ceramic fillers also appear to enhance the mechanical properties of PEO polymer electrolytes at temperatures above the melting point of PEO (60-70 °C). In addition to investigating the electrochemical characteristics of the composite membrane, solid state 7 Li NMR characterization was performed to study ionic mobility by measuring spectral line widths and lithium self-diffusion coefficients. It was determined that ceramic nanoparticle additives can enhance the Li + diffusivity without a corresponding increase in polymer segmental mobility.

Surface Analysis of Electrodes from Cells Containing Electrolytes with Stabilizing Additives Exposed to High Temperature

We have conducted a detailed investigation of the effect of thermal stabilizing additives, including dimethyl acetamide (DMAc), N-methyl pyrrolidone, vinylene carbonate (VC), and vinylethylene carbonate (VEC), on the reactions of the electrolyte with the surface of the electrodes in lithium-ion cells. Cells were constructed with mesocarbon microbead anodes, LiNi 0.8 Co 0.2 O 2 cathodes, and 1.0 M LiPF 6 in 1:1:1 ethylene carbonate/diethyl carbonate/dimethyl carbonate electrolyte with and without electrolyte additives. The cells were stored sequentially at 55, 60, and 65°C for 10 days at each temperature. The cells were then dismantled, and the surfaces of the electrodes were analyzed via a combination of infrared spectroscopy with attenuated total reflection, X-ray photoelectron spectroscopy, and scanning electron microscope-energy dispersive spectroscopy. The surface of the electrodes extracted from cells containing the baseline electrolyte contained thick surface films composed of electrolyte decomposition products. The addition of 1% DMAc inhibits the reaction of the electrolyte with surface of the electrodes, especially on the anode. The addition of 1.5% VC results in the formation of poly(vinylene carbonate) on both electrodes and inhibits the reaction of electrolyte with the electrodes, especially the cathode. The addition of 1.5% VEC or 10% DMAc did not significantly impede the reaction of the electrolyte with the electrodes.