Battery thermal management

Recent technological advancement in the electrical energy storage system has led the development of Electric vehicles. In the current battery technologies, Lithium-ion (Li-ion) batteries have depicted their immense potential to power current electric vehicles (EVs) and hybrid electric vehicles (HEVs).  It is projected that the EVs and Hybrid Electric vehicles (HEVs) could majorly contribute to the transportation sector. Few distinguished qualities of these batteries are lightweight, high specific energy, lesser self-discharge, long lifecycle, and negligible memory effect.  

During a discharge cycle, lithium atoms in the anode are ionized and separated from their electrons. The lithium ions move from the anode and pass through the electrolyte until they reach the cathode, where they recombine with their electrons and electrically neutralize. The lithium ions are small enough to be able to move through a micro-permeable separator between the anode and cathode. In part because of lithium’s small size (third only to hydrogen and helium), Li-ion batteries are capable of having a very high voltage and charge storage per unit mass and unit volume. Li-ion batteries can use a number of different materials as electrodes. The most common combination is that of lithium cobalt oxide (cathode) and graphite (anode), which is most commonly found in portable electronic devices such as cellphones and laptops. Other cathode materials include lithium manganese oxide (used in hybrid electric and electric automobiles) and lithium iron phosphate. Li-ion batteries typically use ether (a class of organic compounds) as an electrolyte. 

In order to supply the required power to the EV, these cells are connected in various series and parallel configurations to form cell modules and packs. During vehicle use, the batteries undergo several charging and discharging cycles that leads to a series of electrochemical reactions inside the cells producing a significant amount of heat energy. This heat generated has to be dissipated from the battery compartment as the performance, life cycle, and safety of Li-ion batteries are highly susceptible to operating temperature. It has been reported that low operating temperature severely affects the performance of the Li-ion batteries. Various parameters such as charge transfer capacity, power, and energy capacity are significantly reduced. 

At low temperatures, the resistance to charge transfer between electrode and electrolyte increases, the ionic conductivity of electrolyte solution and solid-state diffusivity of Li-ion decreases significantly. This accelerates the aging of Li-ion batteries, especially when the temperature is below 0ºC. To improve the performance at low temperatures, presently, the research is focused on developing new materials to be used in the batteries in cold climate conditions. The factors affecting the performance of the batteries are significantly prominent at a higher temperature. The Li-ion batteries (Make – Sony, type-18650 cylindrical cells) report a capacity loss of about 36% for 800 cycles at 45ºC, and the loss increases up to 70% at 55 ºC even below 500 cycles. Several similar studies have been reported in the literature; the probable reason for such behavior could be loss of active material and loss of Li-ion inventory inside the cell. 

In addition, the increase in temperature alters the electronic conductivity and increases the self-discharge rate of the batteries. A further increase in the cell temperature can trigger disastrous phenomena like thermal runway (TR), leading to fire or explosion in the cells. At about 120ºC, the electrolytic layer starts to decompose, which triggers an exothermic reaction and further increases the temperature; this melts the separator between anode and cathode and cells become unstable. At this moment, the temperature reaches about 150-170ºC and TR condition is attained that leads to fire. The heat release during TR can reach up to 10^7 W/m3.

To address the above listed thermal issues, a battery thermal management system (BTMS) is employed which keeps the temperature and temperature gradient of the cells within the optimum temperature range. Using the appropiate thermal management techniques, the safety and reliability of the batteries can be established. In view of this, several thermal management systems are being used to maintain and regulate the temperature of battery modules. The thermal management systems are divided into two broad categories, namely, passive (interact with environment) and active (interact with built-in source for heating and cooling). The former doesn’t need any external power source to operate. In the active system, one can use either air cooled system or liquid cooling system. In the passive cooling system one can employ PCM based techniques.