Safety lithium polymer batteries

The substitution of the liquid electrolyte with the less reactive polymer electrolyte has led to lithium-polymer batteries, among the most likely to be commercialized for electric vehicles [89]. It must be stressed that the lithium-polymer battery is still a lithium-metal battery and not a lithium-ion one. Lithium-polymer batteries are solid-state, in that their electrolyte is a solid. A great safety advantage of this type of battery is that the electrolyte will not leak out if there is a rupture in the battery case. Furthermore, it can be assembled in any size and shape, allowing manufacturers considerable flexibility in cell design for electric vehicle or electronic equipment. 

This type of Li battery has already widely diffused in the electronic consumer market, however for automotive applications the presence of a liquid electrolyte is not considered the best solution in terms of safety, then for this type of utilization the so-called lithium polymer batteries appear more convenient. They are based on a polymeric electrolyte which permits the transfer of lithium ions between the electrodes [21]. The anode can be composed either of a lithium metal foil (in this case the device is known as lithium metal polymer battery) or of lithium supported on carbon (lithium ion polymer battery), while the cathode is constituted by an oxide of lithium and other metals, of the same type used in lithium-ion batteries, in which the lithium reversible intercalation can occur. For lithium metal polymer batteries the overall cycling process involves the lithium stripping-deposition at the anode, and the deintercalation-intercalation at the anode, according to the following electrochemical reaction, written for a Mn-based cathode ... 

The lithium-polymer version of these batteries is another area where work is needed. Lithium-polymer batteries are being rapidly developed for portable consumer electronics applications and may be used in the future for EV/HEVs since the polymer design mitigates safety concerns regarding lithium metal in large cells. Some work to develop recycling processes is under way, but no details have been published and no process test data have been made available. Although many of the constituents are shared in common with the Li-ion battery system, the presence of a solid polymer... 

Lithium-polymer batteries do contain metallic lithium and require compliance with authorities for disposal and storage to preserve safety. 

The lithium-ion-polymer battery, which uses a cathode that contains lithium instead of cobalt, is likely to eventually replace lithium-ion. Lithium-ion-polymer batteries boast a longer life expectancy (over 500 charge-and-discharge cycles as opposed to around 400), much more versatility (they are flat and flexible and can be cut to fit almost any shape), and better safety (far less likely to vent flames while recharging). 

The solid polymer electrolyte approach provides enhanced safety, but the poor ambient temperature conductivity excludes their use for battery applications. which require good ambient temperature performance. In contrast, the liquid lithium-ion technology provides better performance over a wider temperature range, but electrolyte leakage remains a constant risk. Midway between the solid polymer electrolyte and the liquid electrolyte is the hybrid polymer electrolyte concept leading to the so-called gel polymer lithium-ion batteries. Gel electrolyte is a two-component system, viz., a polymer matrix... 

Apparently, these commercial lithium-ion polymer batteries have characteristics, such as reduction in thickness and improvements in safety, which make them very appealing for the modern consumer electronics markets, particularly the new generation of cellular phones. This may lead to the conclusion that the evolution of the lithium-ion battery technology will be focused on polymer configurations with an output that is expected to soon experience a substantial share in the electronics market. 

The evaluation of safety for the cathode material also is carried out in the polymer battery system. The Li CoO, Li NiO, and Li jjMiijO have exothermic peaks of about several W/g in the range of 200 to 300°C therefore, the reaction between the organic materials and the oxygen generated by the thermal decomposition of cathode material is basically inevitable. In addition, details in the evaluation of safety on lithium-ion batteries are referred to in the paper by Tobishima et al. ... 

Safety is a key issue for Li-Ion batteries. The U.S. Department of Transportation (DOT) and the United Nations classify Li-Ion and Li-Ion polymer batteries as hazardous materials for shipping. The DOT grants exemptions for shipping small Li-Ion cells, provided that the cells/battery with limited lithium-equivalent content... 

Recent demands of the market is for the mid- or large-sized lithium battery for the power-assisted bicycle, electronic bike, and hybrid vehicle. As the capacity of the battery increases, safety becomes very important. The gel polymer electrolyte contributes to keeping the battery safe even as the capacity of the lithium-ion battery increases. 

Initially, Li batteries used a liquid electrolyte, necessitating the use of a robust case for safety. It is now used in the ionized form. Figure 23.7 shows a typical Li ion cell utilizing a Li20 cathode and a carbon compound anode separated by a microporous membrane, using a non-aqueous electrolyte such as a Li salt dispersed in a mixture of alkyl carbonates. Since the non-aqueous electrolytes can be flammable. Valence Technology has developed the Li ion polymer battery using liquid lithium ion electrochemistry in a matrix of conductive polymers that eliminate free electrolyte within the cell. 

Polymer Electrolytes. An alternative to the liquid electrolytes is a solid polymer electrolyte (SPE) formed by incorporating lithium salts into polymer matrices and casting into thin films. These can be used as both the electrolyte and separator. These electrolytes have lower ionic conductivities and low lithium-ion transport numbers compared to the liquid electrolytes, but they are less reactive with lithium, which should enhance the safety of the battery. The use of thin polymer films or operation at higher temperatures (60-100°C) compensate in part for the lower conductivity of the polymer film. The solid polymers also offer the advantages of a nonliquid battery and the flexibility of designing thin batteries in a variety of configurations. 

A pol5uner lithium-ion battery has been assembled using LiNio.sMni 5O4 as the positive electrode, a Sn-C composite as the negative electrode, and a gel polymer electrolyte of 1M LiPFg solution in EC/PC (1 1) and PVDR The thermo gravimetric analyzer (TGA)-differential scanning calorimetry (DSC) analysis shows that there is no decomposition of the electrodes or the electrolyte at 200°C, which illustrates the excellent safety performance of this assembled battery. 

Possible shutdown behavior. When the temperature of a lithium-ion battery increases sharply to 120-140°C, plastic separators melt down, and as a result, the pores needed for electrolytes to pass through are closed. At this stage, ions would not be able to pass through, and the current would be reduced. However, when accidents such as fire or explosion happen, this shutdown behavior usually does not mean any safety improvement, since the temperature is usually lower than 120°C. As a result, if possible, polymer electrolytes instead of separators should be used. 

In relation to the safety problems of lithium-ion batteries, shutdown behavior has been a well-discussed topic. At elevated temperature, one polymer in a separator will melt down, and the porous structure is closed. As a result, ions cannot be transported, leading to a sharp increase in resistance. This temperature is known as the shutdown temperature. If this temperature is too low, the lithium-ion battery can simply stop working. If it is too high, there is the danger of sharp thermal production. It is important to select polymer materials to tailor the shutdown temperature for the separator. Optimizing the polymer components and porous structure can realize a separator that will shut down at a targeted temperature. 

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