With recent advances in research, Lithium-ion (Li-ion) batteries have certainly become one of the most reliable renewable energy sources for powering automobiles, electronics (mobiles, laptops, tablets, etc.) and standalone energy storage equipment across the world. Li-ion batteries do stand out in comparison with fossil fuels, such as petrol and diesel. Unsurprisingly, with the current pace of advancement and focused priorities of several governments around the world, it does seem that Li-ion may likely replace various forms of non-renewable energy sources in many applications by 2030.

From automotive industry’s standpoint as well, Li-ion batteries have had tremendous impact so far. However, there still remain few gaps in exploring other Li-ion chemistries (besides those that are frequently used today), implementing effective battery thermal management procedures (as ineffective procedures can potentially lead to hazards associated with batteries catching fire), and addressing EV range anxiety.

So, what is battery thermal management and how can li-ion batteries catch fire?

Li-ion battery delivers optimum, safe, and long lasting performance when its temperature is controlled between 20-40⁰C. Controlling battery temperature within this range is commonly known “Battery thermal management.” High rates of charging and discharging the battery in less than 30 minutes, also referred to as “beyond 2C”, increases the heat load of a battery, for instance, during high acceleration and deceleration while driving in city traffic, fast charging of automobiles, etc. As the heat load increases, battery temperatures also increase until the charging and discharging rates are severe enough for battery to catch fire.

The major heat sources of this safety hazard (battery catching fire) are the polarization and ohmic heat (heat generated due to the movement of Lithium ions across the electrode and electrolyte). Former is due to the overpotential between operating potential and open circuit potential of batteries. It leads to the increase of charge transfer resistance at electrode-electrolyte interface, otherwise called as Solid Electrolyte interface (SEI), as explained in the table below. During the movement of Lithium ions from one electrode to another, they have to overcome this resistance at the interface, thus generating heat.

Ohmic heating occurs in both the electrode and electrolyte due to the resistance that they offer to the Li-ion movement through them. As the cell temperature rises beyond 80⁰C, SEI layer breakup starts. At higher battery temperatures between 120⁰C-200⁰C, there is anode-electrolyte reaction and electrolyte decomposition. Above 200⁰C, Oxygen is released from cathode, followed by combustion. Finally, the battery catches fire.

Apart from the above mentioned electrical and thermal causes for battery catching fire, mechanical causes can also be responsible for a battery to catch fire. Mechanical battery failures like bending and puncture of the battery geometry can also cause the fire hazards within the battery.

Lower temperatures reduce performance of the batteries as well

As the battery temperature goes below 20⁰C like during winters and in temperate countries, the lower freezing point of the electrolyte decreases the movement of Li-ion between the electrodes and thus slowing down the kinetics of the electrochemical reactions. Moreover, anode resistance towards the movement of Li-ion increases. Also during charging, side reactions of anode with electrolyte lead to the growth of dendrites (branches like projections), thus shortening the battery life and causing safety problems. With increase in temperature, these dendrites ultimately pierce the separator and this leads to internal short circuit — thus creating safety hazard. On the contrary, with reduction in temperature, metallic lithium gets deposited on the anode surface forming a lithium plating, which also reduces the performance of the battery. Thus, we realize the importance of controlling battery temperature between 20-40⁰C for safety, sustained performance and longevity.

Leveraging heating and cooling systems for effective battery thermal management

When battery temperatures fall below 20⁰C, a heating system shall be activated to maintain battery temperatures above 20⁰C. In order to elevate battery temperatures above 20⁰C, this heating system can either leverage the electrochemistry generated heat (without cooling system) namely polarization and ohmic heat, or can route a portion of exhaust air (coming out of AC cabin) over the battery. To maintain the battery temperatures below 40⁰C, a cooling system shall be used. The three most commonly used types of cooling systems are Air, Phase change material (PCM), Liquid-based cooling systems in increasing order of heat dissipation capacity and cost.

Battery thermal management can be done majorly by selecting the right Li-ion battery cell chemistries, having the right safety features in Battery management system (BMS), selecting and deploying the right cooling/heating systems. Choosing the right cell chemistry is effective to avoid the heating and cooling effects.

Necessary functionalities required of cell materials and BMS to control battery temperature between 20-40⁰C are mentioned in the table below:

Li-ion cell deterioration effects for cell temperature < 200C	Remedies to control cell temperature (to maintain it above 200C)	Li-ion cell deterioration effects for cell temperature > 400C	Remedies to control cell temperature (to maintain it below 400C)
1) Anodes are porous substances with electrolyte filled in them. Thus solid electrolyte interface (SEI) is formed within them. Frequent cell temperatures below 200C would result in unstable or too thick SEI layer formation with low conductivity. This leads to lithium plating.	Synthetic graphite like highly oriented pyrolytic and highly porous graphite delivers a stable SEI layer & higher reversible capacities at low temperatures (LT)	1) SEI Layer breakup	Aluminum Oxide (Al2O3) coatings of Graphite anode
2) Graphite anode & electrolyte pose higher restriction towards movement of Li-ion	Remedy for effects 2, 3, and 4: Several modifications like controlling the morphology and microstructure, doping (addition of different substance(s) to alter the properties of the parent material) and coating, modifications to electrode composition, coupling carbon-based anode materials with alternate anode materials (binder-free LTO composite electrode with highly conductive agents, such as carbon nanotubes (CNT) and silver nano crystals.	2) Anode electrolyte reaction	Remedy for effects 2 and 3: Additives addition to electrolyte multilayer films separators, coatings to increase melting point of separators, and alternate separator material
3) During charging, anode-electrolyte side reactions cause dendrite growth		3) Electrolyte decomposition
4) Dendrites (or branches like projections) from anode surface penetrate through separator to the cathode side and cause short-circuit & safety problems.		4) Oxygen released from Cathode & Combustion	Alternate Cathode materials, coatings & doping of Cathode

 

Functionalities of Battery Management System (BMS):

1. Battery temperatures below 20⁰C and above 40⁰C can be detected and prevented by BMS
2. BMS should manage thermal effects and update main vehicle display dashboard regarding onboard events that occur in the system
3. Once a fault event (that decreases battery temperature below 20⁰C or increases it above 40⁰C) is initiated and identified, BMS is required to take necessary steps, which include isolating faulty cell/cells/module & disconnecting them, and informing the user of the damage.
4. If the severity of the fault requires, the entire battery pack should be cut-off from the electrical system

How to address range anxiety?

Range anxiety is referred to as the EV owner’s or operator’s worry of not able to reach their destination with the available battery pack/module charge. Let us reflect on some of the common range anxiety challenges in India and proposed solutions, mentioned in the table below

Typical current scenario	Ideal scenario	Proposed solution
1. A 4/5 seater Car (PV) in INR 10-20 lakh segment with 30kWh battery capacity can last 300kms in a single charge.	1. Need 4/5 seater car (PV) in INR 4-10 lakh segment with 50kWh battery that can last 500kms range of travel in a single charge	1) A standard swappable 50kWh battery capable of both fast charging (50kWh/3hrs) and slow charging (50kWh/10hrs) that has a combination of a) Power batteries, such as Lithium ferro phosphate (LFP), which have high specific power, suitable for high acceleration, deceleration without overheating. However, they cannot give long range. b) Energy batteries, such as Lithium cobalt oxide (LCO) have high specific energy, suitable for covering long range without overheating. However, they cannot give high acceleration, deceleration.
2. Complete charging takes 12 hours at home	2. Fast charging battery i.e. 50kWh/3hrs at INR less than 500	2) Solar energy (through solar panels fitted on the car) as auxiliary charging source on the go
3. Single charge barely lasts 7 days of travel	3. Fast charging infrastructure at affordable cost every 5kms within city and every 100kms on highways	3) Swappable batteries available at charging stations

Let us understand some of the proposed solutions for range anxiety:

• Combination (in ratio of 3:7) of Power and Energy batteries will help to reduce the scaling cost and packaging volume of cooling system, while scaling battery capacity from 30kWh to 50kWh.
• Cooling system for batteries can be indirect liquid cooling. Cooling system should only be diverted to only those Energy/Power battery modules which are engaged during charging or discharging processes.
• Power batteries should be charged first, during need of quick charge. Energy batteries can be charged either with solar energy through solar panels fitted on the car and/or with slow charging station (preferably powered by solar energy) at home.
• Standard swappable batteries are ones that have identical modular configuration, dimensions, sizes, chemistries, capacities, etc. of power and energy batteries each.

How can academia, organizations, and working professionals address key challenges?

For areas within battery and EV domains that are well understood, expert faculties can train working professionals and help them understand the underlying theory in such areas, besides also guiding them to implement such principles in practical battery and EV designs at their respective organizations.

For remaining areas of battery and EV domains that are less well understood, organizations can sponsor well-defined projects at academic institutions that are actively pursuing research in such areas. Organizations can also consider allocating their specific working professionals to such projects for pursuing research and in-turn be trained in those specific areas of battery and EV domains.

Source: https://www.cxotoday.com/story/how-to-address-battery-thermal-management-and-ev-range-anxiety/