Ionic Liquid-Based Electrolytes for High Energy, Safer Lithium

Chapter 4

Ionic Liquid-Based Electrolytes for High Energy, Safer Lithium Batteries Downloaded by DUKE UNIV on January 18, 2013 | http://pubs.acs.org Publication Date (Web): December 18, 2012 | doi: 10.1021/bk-2012-1117.ch004

G. B. Appetecchi,*,1 M. Montanino,1 and S. Passerini*,2 1ENEA

(Italian National Agency for New Technologies, Energy and Sustainable Economic Development) UTRINN-IFC C.R. Casaccia, Via Anguillarese 301, 00123 Rome, Italy 2Westfälische Wilhelm Universität Muenster, Institute of Physical Chemistry, Corrensstrasse 28/30, MEET, Corrensstrasse 46, D-48149 Muenster, Germany *E-mail: [email protected] (S.P.); [email protected] (G.B.A)

Ionic Liquids (ILs), salts molten at room temperature, have very interesting properties such as high chemical, thermal and electrochemical stability, high conductivity, no measurable vapor pressure and non-flammability. Because of these characteristics, ILs were proposed as electrolyte components for replacing the hazardous and volatile organic solvents used in commercial electrochemical devices, particularly rechargeable lithium batteries. In the last ten years ionic liquid-based electrolytes in combination with lithium battery electrodes were extensively investigated with the aim to realize safer devices without hindering their electrochemical performance. Here, a review of best promising uses of ionic liquid-based electrolytes in lithium batteries is reported.

© 2012 American Chemical Society In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Ionic Liquid Electrolytes for Lithium Batteries Lithium Batteries

Downloaded by DUKE UNIV on January 18, 2013 | http://pubs.acs.org Publication Date (Web): December 18, 2012 | doi: 10.1021/bk-2012-1117.ch004

Rechargeable lithium batteries are excellent candidates for the next generation power sources (1, 2) because of their high gravimetric and volumetric energy with respect to other cell chemistries as reported in Figure 1 (left panel).

Figure 1. Gravimetrical energy of different cell chemistries. Schematic illustration of a lithium battery. The lithium battery technology (Figure 1, right panel) is based on the use of electrode materials able to reversibly intercalate lithium cations. Lithium metal is used for the negative electrode (anode), whereas a lithium transition metal oxide (LiMO) is used for the positive electrode (cathode). During the discharge step, electrons are generated at the anode to do external work whereas positive ions migrate to the cathode through the electrolyte. The electrode reactions during discharge are:

However, lithium metal cells exhibit poor safety because of the high reactivity of Li anode towards the electrolyte, this leading to the growth of a porous passive layer (which is sensible to mechanical, thermal and electrical abuse) and/or dendrites (which might lead to internal short-circuit) onto the lithium electrode. Therefore, the lithium anode has been replaced with a negative electrode (generally a carbonaceous material) able to reversibly intercalate Li+ cations:

The graphite/LiyMO cell system is called “lithium-ion” since the lithium is present just as a cation. The overall electrochemical process may be schematized as the following:

68 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Safety Drawbacks in Lithium Batteries Rechargeable lithium battery systems commonly use small amount of electrolytes based on suitable lithium salts (generally LiPF6) and organic solvents (generally alkylcarbonates as EC, DEC, DMC), volatile and flammable, which represent a major problem for device safety (2–4). The lithium-ion battery configuration improves the safety of lithium batteries because lithium is only contained within a host structure, both at the anode and at the cathode. During charge and discharge, lithium is simply transferred between one host structure and the other, with concomitant oxidation and reduction processes occurring at the two electrodes. However, in their charged state, the lithium-ion cells are inherently unsafe. The lithiated graphite electrode is strongly reducing as it operates close to the potential of metallic lithium. The interface between lithiated graphite and the electrolyte is stable only because of the formation of a solid electrolyte interphase (also called SEI) that is generated during the first charge of the battery. It is typically formed by the electrolyte decomposition products and has a mixed organic-inorganic nature. The SEI is a pure Li+ ion conductor, and, once formed, prevents the electrolyte from being further reduced. However, if the SEI is degraded then the electrolyte is further degraded with heat release and generation of flammable gases, such as H2, CO, C2H4 and C2H6. A delithiated Li1-xMO2 electrode is, on the other hand, an extremely strong oxidant. The presence of flammable and volatile organics (even if trapped in polymeric hosts as in a few commercial lithium-ion polymer batteries), dangerous events such as heat generation, thermal runaway, cell venting, fire, and rapid disassembly may occur. Therefore, whereas lithium-ion batteries are rapidly replacing NiMH batteries in consumer market devices (e.g., portable and hand-held devices), the main drawback for their large introduction in automotive products (electric and hybrid vehicles) continues to be the safety related to thermal effects. As is well known, the safety (but also the performance and life) of lithium-ion batteries is affected by their storage and operative temperatures (5). If heat developed during the charge step is not effectively dissipated, the battery internal temperature increases rapidly which triggers heat-generating exothermic reactions, such as the SEI breakdown, which, in turn, raise the temperature further and triggers more deleterious reactions, often leading to destructive results. Such a process, considered as a sort of uncontrolled positive feedback, is known as “thermal runaway” (Figure 2, left panel). Rechargeable lithium batteries can experience thermal runaway, especially when handled improperly (overcharged) or manufactured defectively. Several stages are involved in the build up to thermal runaway, each one resulting in progressively more permanent damage to the cell: i)

The first stage is the breakdown of the thin passive SEI layer on the (graphite) anode due to overheating or physical penetration. The initial overheating may be caused by excessive current flow, overcharging or high external ambient temperature. The SEI film (which normally protects the lithiated graphite from further reaction with the organic 69 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


ii)

iii) iv)

v)

electrolyte) contains both stable and metastable compounds, the latter decomposing exothermically when the temperature rises to a value between 85°C and 120°C (6, 7). Once breaching the SEI layer, the organic electrolyte begins to exothermically react (around 100°C) with the negative electrode just as it did during the formation process, but driving the temperature up to much higher and uncontrolled values (near 200°C). As the temperature builds up, heat from the anode reaction causes the breakdown of the organic solvents used in the electrolyte this releasing flammable hydrocarbon gases (ethane, methane and others) but no oxygen. This typically starts at 110°C, but with some electrolytes it can occur as low as 70°C. The gas generation due to the breakdown of the electrolyte causes pressure to build up inside the cell. Although the temperature increases to beyond the flashpoint of the vapors released by the electrolyte decomposition, the gases do not burn because there is no free oxygen in the cell to sustain a fire. The cells are normally fitted with a safety vent which allows the controlled release of the gases to relieve the internal pressure in the cell avoiding the possibility of an uncontrolled rupture of the cell - otherwise known as an explosion or more euphemistically "rapid disassembly" of the cell. Once the hot gases are released into the atmosphere, they can, of course, burn in air. At around 135°C the polymer separator melts, allowing the electrodes to short circuit. Heat from short circuits causes breakdown of the metal oxide cathode material, which releases oxygen, thus enabling the combustion burning of both the electrolyte and the gases inside the cell. Also, the cathode breakdown is highly exothermic, thus further increasing the internal temperature and pressure. This reaction starts above 180°C (8), e.g., around 200°C for LCO cells but at higher temperatures for other cathode chemistries such as LFP (9) and LiNi0.8Co0.15Al0.05O2 (7). By this time the internal pressure is extremely high, thus leading to the violent explosion of the cell if safety vents are overwhelmed or nonfunctional.

A similar pathway was proposed by Yang at al. (7) for thermal runaway in lithium-ion batteries. Once the SEI layer begins to exothermally decompose (T > 85°C), a secondary film starts to form, which is successively decomposed if the temperature increases above 110°C. The thermal energy released at this stage may be absorbed through electrolyte vaporization (about 140°C) or separator melting (130 - 190°C). Hazardously, the organic electrolyte can readily combust once vaporized if oxygen is available (e.g., from the delithiated positive electrode). In addition, the separator melting could cause short circuit of the battery electrodes, leading to additional heating. The negative graphite electrode begins to react at 330°C, releasing some additional heat. Eventually, the aluminum current collector can melt at 660°C, if catastrophic events (explosion) do not occur first. In addition, it is noteworthy that the electrolyte burning causes the decomposition of the LiPF6 salt, leading to the development of highly toxic HF gas. 70 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 2. Left panel: diagram of thermal runaway. Right panel: Lithium-ion cell operating window. Data from refs. (10, 11). To summarize, heat is a major danger for lithium-ion batteries. The effect of decreasing the operating temperature is to reduce the rate at which the active chemicals are transformed. This leads to a reduction in the current carrying capacity of the cell both for charging and discharging. Moreover, the operating voltage can affect the battery performance. If the charging voltage is increased beyond the recommended upper cell voltage (typically 4.2 V), excessive current flows giving rise to two problems, e.g., lithium plating and overheating. The diagram reported in the right panel of Figure 2 shows that the cell operating voltage and temperature must be kept within the limits indicated by the yellow box or permanent damage to the cell will be initiated. Obviously, this scenario can be quite dangerous for lithium-ion batteries, especially for large systems. Therefore, large efforts are currently aimed to increase battery safety. Air and/or liquid cooling can improve the cell temperature uniformity, even if poor thermal conductivity does not allow fast heat dissipation from the battery. For anodes, LTO results much safer than graphite since no growth passive layer occurs at interface with the electrolyte. For cathodes, LCO starts to break just above 150°C releasing very large amount of thermal energy whereas LFP breaks down with oxygen release a much higher temperature (> 225°C) since oxygen has a much stronger valence bond to phosphorous, resulting also in smaller exothermic heat release (9). The addition of flame-retardants, as aromatic phosphorous-containing esters, to the electrolyte was found to remarkably inhibit the thermal runaway, thus preventing potential fire and explosion (12). Finally, an appealing and very promising approach, which currently knows a true blowing-up, is represented by the replacement of organic hazardous solvents with non-volatile, non-flammable, thermally stable fluids known as “ionic liquids”. Viable Ionic Liquids for Lithium Batteries “Ionic liquid” (IL) is the commonly accepted term for low-melting salts, typically below 100°C (13). If molten at room temperature, they are called room temperature ionic liquids (RTILs). ILs, constituted by an organic cation and an 71 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


inorganic/organic anion, represent a new class of room temperature fluids with very interesting properties including non-flammability, negligible vapor pressure in conjunction with a ambient or sub-ambient melting point (14, 15), remarkable ionic conductivity, high thermal, chemical end electrochemical stabilities, low heat capacity, ability to dissolve inorganic (including lithium salts), organic and polymeric materials and, in some cases, hydrophobicity (16, 17). Because of these unique properties, the ILs have attracted growing attention as electrolyte components to replace organic solvents currently used not only for lithium batteries, but also in other electrochemical devices such as fuel cells, double-layer capacitors, hybrid supercapacitors, photoelectrochemical cells, and applications such as electrodeposition of electropositive metals (18–24), resulting in improved safety in case of overheating/overcharging (e.g., such as some spectacular flaming/explosions have occurred with conventional lithium-ion batteries in recent years).

Figure 3. Flammability tests performed on organic solvent mixture (EC-DMC, left panel) and IL (PYR14TFSI, right panel) for lithium conducting electrolytes. Flammability and volatility tests have evidenced the remarkably improved behavior of IL electrolytes with respect to organic ones. As shown in Figure 3, no flammability is observed for the IL material even upon prolonged exposition to fire. Figure 4 evidences the internal pressure increase of an organic electrolyte pouch cell (panel A) due to solvent evaporation after heating the electrochemical device up to medium-high temperature (≥ 50°C). Conversely, no volume change was exhibited by the IL-containing cell (panel B), which evidenced a very good vacuum retention. 72 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 4. Heating tests performed on vacuum-sealed cells based on organic- (A) and IL- (B) based electrolytes. An IL-lithium salt mixture needs to meet a few requirements to find application as an electrolyte component in lithium batteries: -

-

High ionic conductivity (≥ 10-3 S cm-1), even at sub-ambient temperature in order to assure sufficiently high Li+ transport properties through the electrolyte; Wide electrochemical stability to allow Li+ reversible intercalation, even at high voltages, without any relevant degradation; High thermal stability to allow safe utilization at high-temperature; No hydrolysis, thus minimizing the generation of protons; Hydrophobicity, minimizing the water incorporation into the battery.

Such properties are mainly satisfied by ILs formed by di- or trialkylimidazolium, saturated cyclic aliphatic quaternary ammonium as N-methylN-alkylpyrrolidinium (PYR1A where the subscripts indicate the number of carbon atoms in the alkyl side chains) or N-methyl-N-alkylpiperidinium (PIP1A), tetraalkylammonium (A4N) cations in combination with hydrophobic perfluoroalkylsulfonylimide anions, e.g., FSI, TFSI, BETI, IM14 (where the subscripts indicate the number of carbon atoms in the fluorine-containing side chains). Figure 5 depicts the chemical structure of the most common IL cations and anions used as electrolyte components for lithium batteries, while their physicochemical properties are summarized in Table 1. The room or sub-ambient melting point is addressed to cation/anion bulkiness and asymmetry, which hinder the crystal packing of the ions (13, 15–17). In particular, highly asymmetric anions such as IM14 do not allow the IL material to crystallize even in presence of symmetric cation (i.e., PYR11), resulting in a very low melting point, e.g., lower than -40°C (25). Also, the introduction of an oxygen atom in the side aliphatic chain was seen to sharply decrease the melting temperature as a result of the enhanced flexibility of the ether group, this further reducing the ion packing (26, 27) but also affecting the thermal and electrochemical stability of the IL (25, 28). 73 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 5. Chemical structure of the most common IL cations and anions used as electrolyte components for lithium batteries.

74 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

75


Table 1. Physicochemical properties of various ILs for lithium battery applications. Melting point: m.p.; thermal stability: ther. stab.; viscosity: η. Data from refs. (25, 28–30) Ionic Liquid

m.p./°C

Ther. stab. / °C N2

PYR11TFSI PYR12TFSI PYR13TFSI PYR1iso3TFSI

Air

131.0

405.1

390.0

91.8

----

----

11.4

----

----

6.0

----

----

σ / mS cm-1

η / mPa s 20°C solid solid 72 solid

ESW / V

60°C

- 10°C

20°C

20°C

solid

Ionic Liquid-Based Electrolytes for High Energy, Safer Lithium

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