safety mechanisms.pdf

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Safety mechanisms in lithium-ion batteries

Introduction

Lithium-ion battery hazards

Conventional safety devices

Safety vents

Thermal fuses

Other circuit breakers

Self-resetting devices

Ceramic PTC materials

Conductive-polymer PTC devices

Shutdown separators

Electrolytes

Non-flammable electrolytes

Redox shuttles

Halide shuttles

Metallocene shuttles

Aromatic redox shuttles

Shutdown additives

Ionic liquids

Electrolyte salts

LiPF3(C2F5)3

LiN(SO2CF3)2

LiBC4O8

Active materials

Carbon anode

LiCoO2

LiNiO2

LiMn2O4

LiFePO4

Coatings

Conclusions

References

Journal of Power Sources 155 (2006) 401–414 Review Safety mechanisms in lithium-ion batteries P.G. Balakrishnan, R. Ramesh, T. Prem Kumar ∗ Electrochemical Power Systems Division, Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu, India Received 18 October 2005; received in revised form 1 December 2005; accepted 2 December 2005 Available online 28 February 2006 Abstract With increasing use of lithium-ion power packs, reports of occasional incidents of severely debilitating and sometimes fatal tragedies appear in the news. This review analyzes possible scenarios that trigger such hazards before proceeding to discuss safety mechanisms such as pressure release valves, one-shot fuses, reversible and irreversible positive temperature coefﬁcient elements, shutdown separators, chemical shuttles, non-ﬂammable electrolytes and coatings. © 2006 Published by Elsevier B.V. Keywords: Lithium-ion batteries; Safety; Battery hazard; Non-ﬂammable electrolytes; Thermal runaway Contents 4. 5. 6. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. 2. Lithium-ion battery hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Conventional safety devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Safety vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. 3.2. Thermal fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Other circuit breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self-resetting devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Ceramic PTC materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Conductive-polymer PTC devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shutdown separators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Non-ﬂammable electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Redox shuttles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. Halide shuttles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Metallocene shuttles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Aromatic redox shuttles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shutdown additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ionic liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolyte salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LiPF3(C2F5)3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1. LiN(SO2CF3)2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2. 6.5.3. LiBC4O8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Active materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Carbon anode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. LiCoO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. 6.4. 6.5. 402 402 403 403 404 404 404 405 405 406 406 407 407 407 408 408 408 408 409 409 409 409 409 410 410 ∗ Corresponding author. Tel.: +91 4565 227888/227550–9; fax: +91 4565 227779. E-mail addresses: premlibatt@yahoo.com, prem@cecri.res.in (T. Prem Kumar). 0378-7753/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jpowsour.2005.12.002

402 P.G. Balakrishnan et al. / Journal of Power Sources 155 (2006) 401–414 7.3. 7.4. 7.5. LiNiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LiMn2O4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LiFePO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 410 411 411 411 411 1. Introduction Perhaps the word lithium itself has questions of safety tagged to it. In fact, safety is a recurring theme even with lithium-ion cells where metallic lithium is replaced with lithium-insertion active materials. Ridden with a poor understanding of the ﬂedgling lithium-ion battery technologies, what manufactur- ers and consumers fear are accidents during use or inadvertent abuse. For example, in an incident that occurred at Apple in 1995, lithium-ion batteries got overcharged during an in-house testing of a newly manufactured PowerBook 5300 portable com- puter [1]. Apple then removed all lithium-ion power packs from their product lines [2]. Hereabouts, Ericsson announced that its mobile phones and other portable electronic applications would wean away from lithium-ion batteries [3]. In fact, several other OEM manufacturers have also been proactive in recalling their products. In 2000, in cooperation with the U.S. Consumer Product Safety Commission, Dell voluntarily recalled 27,000 lithium-ion batteries, manufactured by Sanyo Electric Co. Ltd., and sold in notebook computers. Compaq also recalled 55,000 notebook lithium-ion batteries manufactured by Sony Corpora- tion because of a defect in the circuit board that controls the recharge and discharge processes. One of the recent lithium-ion battery recalls with the USPSC was in 2002 when, upon receiv- ing ﬁve reports of batteries overheating (in three of the instances they caught ﬁre), EV Global Motors Company announced the recall of 2000 batteries in their electric bicycles. Withdrawal of products, loss of market and even a ban on lithium-ion batteries were part of a backlash prompted by these incidents. Thus arose the need for safety in commercial lithium- ion battery applications. Today, lithium-ion batteries are the state-of-the-art power sources for a variety of portable electronic devices. They combine high energy density and excellent cycle life, and have no memory effect. That no lithium battery-related accident has been reported in the recent past is testimony to improved safety characteristics of present-day lithium battery products. The excellent safety record has been brought about by regulations governing the safety of the cells [4]. Continual improvements in safety are being made especially with large battery packs as for electric traction and load leveling [5]. The gravity of the situation becomes evident considering the market share for lithium batteries. Of the US$ 37 billion battery market in 2000, about US$ 2.9 billion was shared by lithium batteries, the share for primary and secondary lithium batteries being US$ 1.1 and 1.8 billion, respectively [6]. According to a prediction rechargeable lithium battery market should grow to more than US$ 2.1 billion by 2009 [7]. Lithium-ion batteries combine highly energetic materials in contact with a ﬂammable electrolyte based on organic solvents. They can suffer premature failure if subjected to conditions for which they are never designed. Any abuse, including dispos- ing in ﬁre, overcharging, external short circuiting or crushing, can trigger spontaneous heat-evolving reactions, which can lead to ﬁre and explosion. Lithium-ion batteries must pass a num- ber of safety tests before they can be certiﬁed for use by a consumer. The tests include electrical tests such as external short circuit, mechanical tests such as nail penetration, crush- ing, dropping to the ground, and environmental tests such as heating in a microwave oven, throwing into a hot liquid, and leak tests in a vacuum. Several techniques have been devised to improve safety. They include use of safety vents, positive tem- perature coefﬁcient (PTC) elements, shutdown separators, more oxidation-tolerant or less ﬂammable electrolyte constituents and redox shuttle mechanisms. In this paper we review safety mech- anisms adopted in commercial lithium-ion batteries. 2. Lithium-ion battery hazards Apart from the fact that lithium batteries have highly oxidiz- ing and reducing materials, their safety is compounded by the fact that the design of these non-aqueous cells has an inherent drawback of poor heat dissipation. Compared to lithium metal- anode batteries, lithium-ion cells are considered to be safer. The redox potentials of metallic lithium and lithiated carbons (LixC6), for example, are similar. The reactive surface area of the carbonaceous anode with a typical particle size of about 10 ␮m is large. Although the speciﬁc surface area of the lithi- ated carbon electrode has been demonstrated to increase by a factor of ﬁve upon cycling [8], the reactivity of anode is kineti- cally limited by the slow transport of lithium from the galleries to the surface of the graphitic electrode [9–11]. Another important factor that contributes to enhanced safety of lithium-ion batteries vis-`a-vis lithium metal anode batteries is the much higher melt- ing point of LixC6 as compared to that of lithium metal. The low melting point of lithium (180 C) poses an additional risk of ﬁre hazard from molten lithium generated by inadvertent over- heating. However, exothermic reactions between LixC6 and the electrolyte can be triggered by the application of heat [12,13]. The potential ranges experienced in common 4-V lithium- ion cells are beyond the thermodynamic stability windows of the electrolytes. Electrolytes, therefore, decompose upon con- tact with the charged active materials, both anodes [14–19] and cathodes [20–24]. The interface between the cathode and the electrolyte is further complicated by partial dissolution of the positive active materials [25–27]. This is particularly a problem at the end of charging and at elevated temperatures, conditions under which electrolyte oxidation can proceed at accelerated rates [28–34]. ◦

P.G. Balakrishnan et al. / Journal of Power Sources 155 (2006) 401–414 403 ◦ The temperature of a cell is determined by the heat balance between the amount of heat generated and that dissipated by the cell. When a cell gets heated above a certain temperature (usu- ally above 130–150 C), exothermic chemical reactions between the electrodes and electrolyte set in, raising its internal temper- ature. If the cell can dissipate this heat, its temperature will not rise abnormally. However, if the heat generated is more than what can be dissipated, the exothermic processes would proceed under adiabatic-like conditions and the cell’s temperature will increase rapidly. The rise in temperature will further accelerate the chemical reactions, rather than the desired galvanic reactions, causing even more heat to be produced, eventually resulting in thermal runaway [9,35,36], whose onset temperature determines the safety limit of the device. Any pressure generated in these processes can cause mechanical failures within cells, triggering short circuits, premature death of the cell by irreversible inter- ruptions in the current path, distortion, swelling and rupture of cell casing. It is clear that the thermal stability of batteries depends on its ability to dissipate the heat. The ability of an object to absorb heat is deﬁned by its thermal capacity. Obviously, for a given amount of heat, bigger and heavier objects would suffer less temperature rise than would a similar object that is smaller and lighter. Thus, for lithium-ion batteries, which are designed for applications where size and weight are a premium, a decrease in the thermal capacity is an unavoidable penalty. Thus, heat dissi- pation in lithium-ion batteries turns out to be a major engineering challenge, especially for those designed for high power appli- cations. Designs for effective heat dissipation must be adopted both at the cell and battery pack levels. Heat dissipation can occur by convection and radiation at the surface of the cell. Heat dissipation by convection depends, among other things, on the external surface area and geometry of the cell. However, heat dissipated by radiation depends on the nature of the surface of the cell and makes up nearly 50% of the dissipation [37]. Radiation dissipation can be improved by use of cell cases that have high thermal conductivity and labels that have high emis- sivity. Thermal performance is rarely a cause for cell failure in low-power cells that have simple designs. However, thermal design of high-power cells is not that simple. Poor designs can result in localized hotspots within the cell, which can lead to cell failure. Possible exothermic reactions that trigger thermal runaway include [36,38]: (i) thermal decomposition of the electrolyte; (ii) reduction of the electrolyte by the anode; (iii) oxidation of the electrolyte by the cathode; (iv) thermal decomposition of the anode and cathode; and (v) melting of the separator and the con- sequent internal short. Moreover, high-voltage metal cathodes are known to release oxygen at elevated temperatures [39,40]. Thermal runaway is often caused under abuse conditions, which can be thermal (overheating), electrical (overcharge, high pulse power) or mechanical (crushing, internal or external short cir- cuit) [36,41]. It must be noted that the release of materials from batteries can be benign, mild or violent. Battery hazards are classiﬁed accord- ing to the damage they cause [35]. Physical hazards involve a simple rupture of battery case; chemical hazards result from leakage or venting of corrosive or toxic materials in the bat- tery; both chemical and physical hazards can cause equipment damage due to breakage or corrosion of electrical/electronic components; environmental hazards arise from the reactive and ﬂammable nature of lithium and/or leakage of toxic materials from batteries that are improperly disposed. An area that has often been overlooked is the possible embrit- tlement of container metal with lithium (similar to hydrogen embrittlement). This can happen if the metal in question is capable of alloying with lithium. In such a case, a spontaneous transfer of lithium to the alloying metal casing can occur [42]. This can lead to a structural destruction of the container mate- rial, resulting in leakage paths. Lithium embrittlement at highly stressed regions of battery containers can accelerate crack prop- agation. Although lithium battery leakages have been observed, no conclusive evidence is available to merit extensive research in this direction. 3. Conventional safety devices A predominant mechanism by which lithium batteries are rendered safe involves limiting the current passing through them. Current-limiting devices such as positive thermal coefﬁcient devices are designed to respond to high temperatures. Several factors play a role in the operation of these devices: the ambient temperature, thermal insulating properties of the container, heat generated in the equipment, cumulative heat in the battery pack, and rate and duration of discharge. Thus, it becomes necessary to consult the manufacturer or conduct tests in order to determine the suitability of a battery pack for a particular application. Apart from preventing ﬂow of excessive currents that can potentially damage cells, current-limiting protection devices must withstand continuous ﬂow of the load’s design current and tolerate normal surges and transients. Furthermore, safety devices must also ﬁt into very small spaces and must be rela- tively cheap. For acceptance in commerce, the current-limiting device must be fail-proof, which also means that it should not be prone to false tripping, factors that can decide customer dis- satisfaction. It must be pointed out that batteries regulated with external electronic devices such as PTC elements and integrated circuits would not only have higher manufacturing costs but also lower energy density. 3.1. Safety vents Conventional safety mechanisms include such devices as vents and current-limiting devices like fuses and circuit break- ers. Safety vents open in response to a sudden increase in cell pressure, allowing gases to escape. If the pressure inside a cell builds up, a plastic laminate membrane is punctured by a spike incorporated in the vent in the cell top. A safe release of internal pressure precludes dangerous rupture of the cell casing. Safety vents can be designed to operate at pre-set internal cell tempera- tures. Today, vents are a back-up safety device. During instances of electrical abuse, other devices such as a positive temperature coefﬁcient device (described below) override the vent. If bat- teries are subjected to severe mechanical abuse conditions, the

404 P.G. Balakrishnan et al. / Journal of Power Sources 155 (2006) 401–414 safety vent provides a means of releasing internal pressure and prevents the cell from reaching excessively high temperatures. Kato et al. [43] developed a safety mechanical link by which a concave aluminum disk welded to the cathode would break the circuit upon release of gas. In this design, lithium carbonate deliberately added to the LiCoO2 cathode mix would decompose to yield CO2 when the cell is overcharged to greater than 4.8 V. The built-up pressure would push the aluminum disk, discon- necting the cathode lead from the circuit. This simple mechanism prevents the cell from the thermal runaway caused by an exces- sive overcharge. Choi et al. [44] have shown that in addition to providing safety, the added lithium carbonate can suppress the initial irreversibility of the carbon anode. Since the safety vent opens up the cell, spewing out a signif- icant quantity of volatile organics, it is used as a back-up safety device. In fact, other safety devices such as PTC elements over- ride the safety vent during abuse. Under severe mechanical and electrical abuse conditions, the vent provides a safe means of releasing internal pressure before the cell reaches excessively high temperatures. 3.2. Thermal fuses The oldest and most common current limiter is the one-shot fuse, which is a wire of a fusible alloy with resistance and ther- mal characteristics that allow it to melt when a pre-set current ﬂows through it. Some fuses require several seconds to trip, but they are inherently fast-acting. The advantages of the fuse as a safety device lie in its simple construction, low cost and avail- ability in a wide range of currents and voltages ranges. Fuses act by destroying themselves, thereby positively and permanently opening the circuits they protect. Thus, they must be replaced once blown, which is another advantage (as it draws the atten- tion of the user to take action for resuming service) although the mechanical action involves labor. However, fuses can pre- maturely blow under other conditions such as pulse discharges (or repeated pulse discharges that can degrade the alloy), which are normal operational modes of batteries. Moreover, there is the possibility of inadvertent replacement with fuses with higher or lower current ratings, which can result in improper use of equip- ment. Fuses are wired in series with the cell stack and will open when a pre-set cell temperature is reached. Thermal fuses are employed as protection against thermal runaway and are usually set to open at 30–50 C above the maximum operating temper- ature of the battery. Fuses are cheap and are ideal for low-cost, throwaway products with limited warranties. ◦ 3.3. Other circuit breakers Other circuit breakers such as magnetic switches, bimetallic thermostats and electronic protection circuit modules can be used to protect power packs and to monitor their temperature. They must also tolerate continuous design current as that of the load as well as occasional current surges, without tripping. However, their size and cost often rule out the application of the ﬁrst two in many onboard circuits, especially where space is at a premium. Thermistors sense the internal temperature of the battery, and provide information to an external control through a cali- brated resistance. Thermistor controls may be located in a battery charger. The thermistor is attractive as the control can be set to meet speciﬁc conditions of charge and to regulate input current to the battery. This device can also be used to control the bat- tery through T/t control, where T and t are the temperature and time, respectively. PTC thermistors have a positive temper- ature coefﬁcient, as will be described below. Similarly, thermis- tors whose resistances decrease with increasing temperature are called negative temperature coefﬁcient (NTC) thermistors. Both are used for monitoring and protection of control circuits. The thermostat or temperature cut-off (TCO) devices oper- ate at a ﬁxed temperature, and can be used to terminate charge (or discharge) when a pre-set internal battery temperature is reached. TCOs are usually resettable. They are connected in series with the cell stack. Electronic safety circuits, commonly referred to as protection circuit module (PCM), are usually attached to battery packs as separate modules. In the event of a wrongful condition, such as short circuit, the PCM opens the battery circuit and prevents damage to the pack. Some groups believe that the cell chemistry in lithium-ion cells can be modiﬁed and safety levels raised, rendering PCMs redundant [45]. Unlike aqueous electrolyte cells, which have an inher- ent balance-adjusting mechanism such as gas recombination, lithium-ion cells require an external overcharge/overdischarge protection system, particularly those for use in specialized appli- cations as in electric traction and spacecraft. This can be pro- vided through an electronic control circuit. However, the cost component of the circuits is kept small as compared to the cost of the batteries themselves. The basic circuitry consists of a bypass circuit controlled by a microchip based on MOS- FET. The bypass circuit gets activated when a cell in a pack reaches a given state-of-charge/discharge earlier than other cells. Thus, the charge/discharge process is terminated until balance is regained. Open-circuit voltage of lithium-ion cells can be used as indicators of their state-of-charge, electronic controllers can be designed to sense voltages and, thereby, switch on or off the charging/discharging circuit. This ensures charge balance among cells in a pack and damage by overcharge/overdischarge of individual cells. In specialized applications, battery packs come with protection circuits that monitor cell temperature and activate cooling gadgets such as fans. 4. Self-resetting devices Factors such as inconvenience of replacement and prema- ture failure of fuses (which call for time-consuming technical services), unsuitability of integrating devices such as mag- netic/thermal switches onboard, size restrictions and cost led to a search for a self-resetting, fuse-like device. Thus, emerged safety devices called positive temperature coefﬁcient devices based on materials whose resistance increases dramatically with a rise in temperature. For example, if a large current ﬂows across the PTC element, as during external short circuiting, its temper- ature rises up abruptly up due to Joule heat evolution within the

P.G. Balakrishnan et al. / Journal of Power Sources 155 (2006) 401–414 405 PTC element. A concomitant and abnormally high resistance of the PTC element prevents current ﬂow. Thus, upon activation, the resistance of the PTC element shoots up, leading to a precip- itous fall in the current, which limits heat generation in the cell. Once the cause for alarm is removed, the cell and PTC element cool and the resistance of the latter drops, allowing resumption of charge/discharge. PTC elements are generally installed inside cells. The temperature above which the resistance of the PTC element jumps to an inﬁnite value is called the “trip tempera- ture,” whose value is generally set at about 100 C. ◦ Although the primary purpose of PTC devices is to protect batteries against external short circuits, they also provide pro- tection under certain other electrical abuse conditions. This is accomplished by limiting current ﬂow when the cell temperature reaches the designed activating temperature of the PTC device. For extended equipment life, the PTC must work reversibly. Although PTC devices can operate in this way several times, it will not reset indeﬁnitely. Fortunately, when they cease to reset, they remain in their high-resistance condition, rendering the cell unusable. PTC devices usually come as surface-mountable units and are compatible with pick-and-place equipment. Thus, they carry little assembly-costs. But because they are costlier than fuses, they become economically attractive only when used in equipment that are costly or demand long-term warranties. 4.1. Ceramic PTC materials Ceramic materials with fuse-like action were the materials of choice for early PTC elements. Ceramic PTC devices can operate under high voltages and can return to their normal resis- tance mode with great accuracy. Thus, they are attractive for application in several high-voltage circuits although their rela- tively large sizes preclude their use in miniature high-component density gadgets. It must be noted that their applicability in low- voltage circuits is undermined by their high inherent resistance, the high voltage drop across which can cause problems with the operation of the gadget. Another intrinsic disadvantage with ceramic PTC materials is their high thermal mass, which ren- ders their reaction time to moderate over-currents longer than those of the components in the gadget. The sluggish response can damage costly equipment. 4.2. Conductive-polymer PTC devices ◦ Conductive-polymer PTC devices are non-linear PTC ther- mistors based on a composite of polymers and conductive par- ticles. It is known that above their glass transition temperatures (Tg) polymers transform into an amorphous state and return to their crystalline state upon cooling to temperatures below their Tg. At normal operating temperatures, the conductive particles embedded in a crystalline polymer matrix provide a low resis- tance path for current ﬂow. At elevated temperatures (typically ∼125 C), the polymer’s structure changes to an amorphous state. The accompanying expansion of the matrix breaks the conductive pathway between the embedded particles, rapidly increasing the device’s resistance by several orders (Fig. 1). This reduces the current to a relatively low and safe level. An advan- tage of PTC devices is that this trickle current maintains the internal temperature of the cell high, prevents the conductive chains from returning to their original state. In other words, the trickle current “latches” the PTC device in its tripped state. Upon opening the circuit the device cools, allowing the polymer matrix to return to its normal state and returns the resistance of the device to its normal low value. Fig. 2 shows the variation of the resistance of a PTC device as a function of temperature. Conductive-polymer PTC devices are made from a blend of plastics and conductive materials. The temperature of the conducting-polymer PTC device is determined by the ambient temperature and heat generated by internal I2R losses. Under normal operating conditions, the I2R losses are too low to gen- erate enough heat to transform the polymer into its amorphous state. However, under abuse conditions when large currents ﬂow through the device, the I2R losses become sufﬁciently high, increasing the temperature and hence the resistance of the PTC element. The reduction in the current in turn reduces the I2R losses. Upon regaining thermal equilibrium, the PTC device Fig. 1. Principle of a conductive-polymer PTC device. Distribution of ceramic particles at: (a) normal operating temperature and (b) trip temperature.

406 P.G. Balakrishnan et al. / Journal of Power Sources 155 (2006) 401–414 permeability, these microporous separators display a protective property during cell abuse. For example, if the cell temperature rises abnormally because of an excessive overcharge, for exam- ple, the heat generated softens PE and closes the micropores in the ﬁlm. This is called separator “shutdown” [50,51]. Once shutdown occurs, ionic transport between the electrodes is effec- tively stopped and current ceases to ﬂow [51]. If the separator can retain mechanical integrity above its shutdown temperature, it can provide a margin of safety to the device; otherwise, the electrodes can come into direct contact, react chemically, leading to thermal runaway. However, it is possible that due to thermal inertia the temperature can continue to rise even after shutdown. Under such conditions the separator would melt and short the electrodes, leading to violent reactions and heat generation. This phenomenon is called “meltdown” or “breakdown” of the sep- arator [48]. Therefore, in order to ensure safety of the cell, the difference between the “shutdown” and “meltdown” tempera- tures should be as large as possible. ◦ ◦ ◦ Separators made entirely of high-density polyethylene melt at 135 C and lose mechanical integrity above this temperature. However, separators made by laminating layers of polypropy- lene and polyethylene maintain mechanical integrity at least up to 165 C, the melting point of polypropylene. It is interesting to note that although ultrahigh molecular weight polyethylene melts at 135 C, separators made from this material retain their mechanical integrity up to at least 180 C as the viscosity of the material is such that it maintains physical integrity. Shutdown separators are reliable and lithium-ion battery manufacturers are increasingly opting for their incorporation in their products. The most common shutdown separators have high molecu- lar weight polypropylene blended with super-high molecular weight polyethylene [45]. Here, the unique shutdown property of polyethylene is combined favorably with the high mechanical integrity of polypropylene at elevated temperatures. Because the shutdown is irreversible, once actuated, these separators leave the cells permanently damaged. ◦ 6. Electrolytes The key to a safe high-performance lithium-ion cell lies in the identiﬁcation of a suitable electrolyte. Lithium is intrinsi- cally unstable with any commonly known electrolyte. More- over, lithium battery electrolytes based on alkyl carbonate solvents are known to react vigorously at elevated temper- atures with lithiated graphite and delithiated cathodes (e.g., LixCoO2 (x < 0.5)) [19,52–54]. At elevated temperatures, the SEI on the graphite anode gets destroyed, allowing rapid and direct reaction with the lithiated graphite underneath the pas- sivating layer. In their delithiated forms, cathodes are highly oxidizing and enter into exothermic reactions with alkyl car- bonates, especially at elevated temperatures. Careful calori- metric studies have thus become mandatory to determine the safety of electrode–electrolyte combinations. According to Aurbach et al. [55], commonly used electrolytes such as LiPF6 in EC–DEC–DMC are only a compromise. They are ﬂammable and their electrochemical windows are limited to about 4.5 V. Alternatives to such alkyl carbonate solvents are Fig. 2. Representation of the variation in the resistance of a PTC device as a function of temperature. allows a circuit current insufﬁcient to cause damage but enough to maintain the device’s trip temperature. The safety device thus gets latched in its tripped state. It must be pointed out that conductive-polymer PTC devices allow a small residual leakage current through the circuit after its tripped state. The resulting voltage drop across the device can be a concern in certain gad- gets, especially those that demand precise power requirements. Conductive-polymer PTC current-limiters still have some inherent problems. Although they can trip in a few millisec- onds’ time, their response times are still inferior to those of fuses. However, they are suitable for applications where a slow- blow fuse-like characteristic is tolerated. They are also costlier than common fuses. Moreover, the maximum current and volt- age they can tolerate are also limited. However, as compared to their ceramic counterparts, conductive-polymer PTC current limiters have low normal resistances, which means that they do not inﬂuence the normal operation of the battery. Moreover, because of their low thermal mass, polymeric PTC devices react quickly to over-current conditions. Additionally, small surface- mount polymeric PTC devices can be constructed. As noted above, an important concern with external safety mechanisms such as PTC devices is their ability to respond when hazardous reactions occur at high rates [46]. In order to obviate this problem, Feng et al. [47] developed an internal self-actuating thermal cut-off mechanism. Here, the electrodes themselves would have a PTC effect, an effect that is achieved by coating the current collector with a suitable PTC material. According to Feng et al. [47], this would be much more responsive to inter- nal heat changes than external PTC devices, providing cut-off at a pre-set activation temperature. Moreover, these internal PTC devices are reversible, which ensures reusability of the batteries once the reason for heat build-up is removed. 5. Shutdown separators Separators for lithium-ion batteries are polyoleﬁn microp- orous ﬁlms and are generally uniaxially drawn polyethylene (PE) and polypropylene (PP), biaxially drawn PE or multi- axially drawn PP/PE/PP [48,49]. In addition to conventional characteristics such as good mechanical strength, electrolyte

P.G. Balakrishnan et al. / Journal of Power Sources 155 (2006) 401–414 407 not on the horizon although alternative salts such as lithium bis(oxalato)borate, LiBC4O8 (LiBOB) [56], and lithium ﬂu- oroalkylphosphates (e.g., Li[PF3(C2F5)3]) [57–59] are being considered in place of LiPF6. Aurbach et al. [55] suggest that under the circumstances, it is only prudent that additives that can protect electrode-active materials even at high temperatures by forming highly protective ﬁlms on the electrodes be investi- gated. In fact, new formulations of solvents and salts are unveiled continually with an eye on safety and performance. A number of additives are also being investigated to make up for problems due to protective ﬁlms at the positive and negative electrodes. Additives have also been sought to lower electrolyte ﬂammabil- ity under cell venting. Redox couples that shuttle back and forth as additives to limit overcharge and additives that produce gas for activating current interrupter devices have also attracted interest. 6.1. Non-ﬂammable electrolytes ◦ Solvents used in lithium-ion batteries are typically low- boiling and have ﬂash points around 30 C. Thus, a major danger from a cell that vents or explodes arises from the ﬂammabil- ity of the hot electrolyte vapors that are spewed out. Although identiﬁcation of a solvent–salt combination that not only pos- sesses desirable properties for use in batteries but also has the ability to resist combustion under heat or in the presence of an external ﬂame may only be a dream, it is possible to develop elec- trolytes that are not easily ﬂammable [60–67]. Thus, the aim is to look for “low ﬂammability” or “ﬂame retarding” electrolytes that do not support continued combustion when the source of heat, spark or ﬂame is withdrawn. An important consideration here is that the heat of reaction of the electrolyte with the charged electrode materials should also be low so that a self-sustaining combustion reaction does not occur under accidental heating. Present-day electrolyte formulations are a trade-off between the electrolyte’s ﬂammability and performance in the cell. The reduced battery performance is due either to electrochemical instability (which leads to capacity fading) or increased viscos- ity of the additive (which affects capacity utilization and power). Since performance cannot be sacriﬁced, studies mostly focus on ﬂame-retardants as the additives or co-solvents in known elec- trolytes [60–64]. Fluorinated compounds [61] and organophos- phorus compounds [61,62,68] are among the most investigated as co-solvents to decrease ﬂammability. For example, trimethyl phosphate, a popular ﬂame retardant, has been studied for its electrochemical stability on the positive and negative electrodes of lithium-ion cells [60–63]. However, it is important to note that since electrolytes react with the active materials in lithium-ion batteries, the surface chemistry at the anode and cathode is a key factor that decides cell performance. Therefore, the design of new electrolytes must also consider the properties of the SEI formed with the electrolyte. 6.2. Redox shuttles Fig. 3. A schematic showing the working of a redox shuttle. Compound R gets oxidized at the positive electrode to O, which diffuses to the negative electrode and gets reduced to the original molecule. positive electrode at potentials slightly higher than the typi- cal charging plateau. The oxidized forms of these molecules diffuse to the negative electrode, where they get reduced with- out side-reactions back to the starting neutral molecules, which then shuttle back to the positive electrode (Fig. 3). Thus, redox shuttles shunt the excess charge injected into the cell during overcharge. In this way, redox shuttles can indeﬁnitely ‘lock’ the cathode potential at the oxidation potential of neutral molecules until termination of the charge. In principle, all the Faradaic cur- rent goes for the reversible reactions, which means that the redox couple acts as a controlled internal short. A necessary condition is that both the oxidized and reduced forms of the molecules be mobile in the electrolyte. It is possible to visualize scenarios under which the over- charge current becomes too high for the redox couple to carry, letting the excess current to delithiate the cathode and causing irreversible decompositions. To avoid consequent safety hazards arising under this condition, the current limit that can be shunted should be maximized by employing large concentrations of the shuttling molecules [70]. The identiﬁcation of such a redox species is fraught with several conditionalities: (i) both the oxidized and reduced forms of the redox molecule must not only be inert towards cell constituents, but also have sufﬁcient thermal stability; (ii) the solubility and diffusion coefﬁcient of the shuttling molecules in the non-aqueous battery electrolyte should be high; (iii) the oxidation potential of the redox couple must be lower than the decomposition potential of the electrolyte solvents but slightly higher than the overcharge cut-off voltage; (iv) the shuttle must be electrochemically reversible and must not enter into side-reactions; and (v) the reversibility of the couple should last for the entire lifetime of the cell. A number of soluble redox couples have been suggested as shuttles for overcharge protection, but they work only at high charging voltages, which means they actually do not respond to heat generation in batteries. Redox shuttles are among the most promising mechanisms for overcharge protection [69]. The working of redox shuttles, added to electrolytes, involves electrochemical oxidation at the The earliest shuttles, employed for 3-V lithium metal batteries, were based on halides [71,72]. For example, the iodine/iodide couple can be oxidized at the cathode at 3.20 V 6.2.1. Halide shuttles

408 P.G. Balakrishnan et al. / Journal of Power Sources 155 (2006) 401–414 and reduced at the lithium anode. Halide shuttles were, how- ever, abandoned as the volatility and reactivity of the oxidized forms (free halogens) rendered such cells impracticable. 6.2.2. Metallocene shuttles Metallocenes, which form redox pairs, MC/MC+, are among the earliest chemical shuttles investigated, and were tested in 3-V lithium cells [73–76]. The redox potentials of these couples can be tuned by varying the substituent groups at the cyclopen- tadienyl rings [77]. In fact, the potential can be tailored by as much as 450 mV by varying the number and electron-donating or electron-withdrawing nature of the substituents. Ferrocene shuttles have withstood over a hundred “turnovers” in lithium cells. However, ferrocenes can block ionic paths on the sur- face of the cathode, which can reduce the power capabilities of the cell. Moreover, their adsorption on the cathode can result in capacity loss [74]. The situation is more complex in the case of lithium-ion cells, for application in which the poten- tial of the redox couple must be higher than the end-of-charge electrode potential (say, 4.3 V versus Li+/Li). Additionally, the SEI-covered graphite electrode in lithium-ion cells does not sup- port the reduction of MC+, rendering the soluble redox couple inoperable. son they are called “shutdown additives” in the battery indus- try [81,83]. There are two classes of shutdown additives: one releases gases, which in turn activate a current interrupter device, while the other undergoes polymerization, thereby blocking ion transport in the electrolyte. Gas-releasing shutdown additives include biphenyl [81,84], cyclohexylbenzene [81], pyrocarbon- ates [81] and phenyl-tert-butyl carbonate [85]. Biphenyl and other substituted aromatic compounds constitute the polymer- izable class of shutdown additives [86–88]. It can be deduced that both the gassing and polymerizable additives are sure-shots against overcharging. Thus, an approach to a reliable line of defense against catastrophic failure due to overcharging would be to incorporate a redox shuttle and a shutdown additive in the cell such that the activation potential of the latter is higher. Nevertheless, given the paramount importance of safety, espe- cially in consumer gadgets and children’s toys, redox chemical shuttles, which can only provide limited overcharge protection [79,89] and cannot prevent catastrophic failure as obtained dur- ing severe overcharging, shutdown protection mechanisms such as polymerizable additives must be incorporated even at the cost of termination of useful life of the battery. Some studies have extended the polymerization reaction into the separator, immo- bilizing the electrolyte [90]. 6.2.3. Aromatic redox shuttles 6.4. Ionic liquids Redox shuttles based on aromatic compounds have also been investigated. They include 3-V cell shuttles such as tetracya- noethylene and tetramethylphenylenediamine [78]. The selec- tion of 4-V cell shuttles for lithium-ion cells based on LiMn2O4, LiCoO2 and LiNiO2, however, presents more challenges as only a handful of substances are amenable to reversible turnover at potentials around 4.0 V versus Li+/Li. These include complexes of cerium, iridium, iron or ruthenium with phenanthroline or bipyridine. Again, their redox potentials can be tuned by varying the number, nature and position of the substituents on the aro- matic rings [79]. However, their shunting currents are low due partly to their limited solubilities in non-aqueous electrolytes and partly to the low diffusion coefﬁcients resulting from their large sizes and molecular weights. According to Adachi et al. [79] anisole-based compounds, which have high solubility in lithium battery electrolytes, should make for a better class of redox shuttles. Anisole compounds with two methoxy groups at 1,2-(ortho-) and 1,4-(para-) posi- tions display reversibility at the 4-V region [79]. The authors suggest an empirical structure–property relationship for anisole- type shuttles, which can help in the design and selection of more efﬁcient redox shuttles [80]. Adachi et al. [79] conclude that 4-bromo-1,2-dimethoxybenzene provides the best shunt- ing performance among 4-V shuttles. Furthermore, anisole-type shuttles are stable against reduction at carbonaceous anodes. Other 4-V shuttles reported in the literature include bipyridyl and biphenyl carbonates as well as diﬂuoroanisoles [81,82]. 6.3. Shutdown additives Some less known additives, also intended for overcharge protection, terminate cell operation permanently. For this rea- Several room temperature ionic liquids (molten salts) present themselves as possible electrolytes in lithium-ion batteries [91–95]. Not only are they not prone to forming SEI on the electrodes, they have inherent safety characteristics by virtue of their thermal stability, non-ﬂammability, non-volatililty and low heat of reaction with active materials. The non- ﬂammability is effective in preventing batteries from catching ﬁre, while non-volatility prevents batteries from bursting. Fur- thermore, they have favorable electrochemical stability win- dows for application in lithium-ion cells. One of the central issues is identiﬁcation of ionic liquids with sufﬁcient lithium- ions to allow high ﬂux of lithium-ion through the electrolyte [96]. Ionic liquids based on the (1-ethyl-3-methylimidazolium) cation are particularly interesting because of their low viscosi- ties [97–99], but are not sufﬁciently stabile towards reduc- tion in the lithium-ion cell environment [95]. An other- wise potential electrolyte, the imidazolium salt, 1-ethyl-3- methylimidazolium tetraﬂuoroborate (EMI-BF4), has a reduc- tion potential of about 1 V versus Li+/Li, which is too high for lithium battery electrolytes [93,99–102]. However, with the hope that the reduction potential of the EMI+ cation can be tailored by incorporation of organic functional groups [100], Hayashi et al. [103] developed alkylated EMI-BF4 molten salts, 1-ethyl-2,3,4,5-tetramethylimidazolium tetraﬂuoroborate and 1,2-diethyl-3,4(5)-dimethylimidazolium tetraﬂuoroborate. Both salts exhibited very little decomposition at 0 V versus Li+/Li and a wide electrochemical window (up to 5 V ver- sus Li+/Li). Additionally, the latter electrolyte had a relatively ◦ low melting point of about 20 C and a good speciﬁc con- −1 at 20 ◦ ductivity of 1.44 mS cm C. Quaternary ammonium

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