Tag Archives: Li-Ion Batteries

Protecting the Li-ion Battery

For decentralization of the source of energy, it is hard to beat rechargeable lithium-ion batteries. A wide range of applications uses this electrochemical option of energy storage as a strategic imperative. That includes powering up units in the military sector,  storing and providing energy for personal use, keeping uninterruptible power supply systems operational for data centers and hospitals, storing energy from photovoltaic systems, and enabling the operation of battery electric vehicles and power tools.

The rechargeable battery pack is the most common design in the accumulator segment and accounts for the major share of battery-powered applications. Such a pack usually consists of multiple Li-ion cells. With continuous technological development, the economics of the Li-ion rechargeable battery pack is also becoming attractive enough to warrant a substantial increase in its use. This is also leading to the miniaturization of individual cells, resulting in an increase in their energy density.

However, even with the increased availability and use, the Li-ion rechargeable battery pack continues to carry a residual risk of hazards, especially due to the increase in energy density brought on by miniaturization. The disadvantage is in terms of safety.

The electrolyte in the Li-ion cells is typically a mixture of organic solvents and a conductive salt that improves its electrical conductivity. Unfortunately, this also makes the mixture highly flammable. During operation, the presence of an inordinate thermal load can lead to the point where the mixture becomes explosive. Furthermore, this safety hazard to the end-user is increasing with the constant efforts to further increase the energy density of Li-ion cells.

Most electric battery cells have a narrow operational temperature range, varying from +15 °C to +45 °C. That makes temperature the key parameter. When the cell exceeds this temperature range, its rising heat becomes a threat to its functional safety, and to the safety of the overall system.

Overcharging the battery substantially increases the statistical probability of the defect in the cell. This may lead to a breakdown of the cell structure, typically associated with the generation of fire and in some cases, an explosion.

Manufacturers of rechargeable battery packs try to mitigate this risk by including a battery management system, and primary and secondary protection circuits that they embed in the electronic safety architecture of the battery. This allows the battery to remain within its specified operating range during the charging and discharging cycles. But nothing is immune to failure, including components in the protection circuit, and the battery system can ignite and explode on an excessively high load.

As the battery powers up a load, excessive current flow can heat up the battery, and the primary protection circuit may not detect it even when it exceeds the permissible level. For the protection of batteries, RUAG Ammotec is offering a heat lock element, a pyrotechnical switch-off device that is entirely independent of the battery system. This comprises a physicochemical sensor to continuously monitor the environmental heat. As the temperature rises, the sensor blocks the flow of current permanently. The heat lock element causes an insulating piston to shear off a current conductor, thereby electrically isolating the battery.

Salt Water Makes Li-Ion Batteries Safer

So far, high-energy lithium-ion batteries were always a matter of concern on account of safety. If you wanted to remain safe from exploding batteries, an aqueous battery such as made from nickel/metal hydride would be preferable, but then it would give you lower energy.

Usually, 3-V batteries using aqueous electrolyte technologies are unable to achieve higher voltages because of the cathodic challenge. This happens as the aqueous solution degrades one end of the battery made from either lithium or graphite. One research team solved this problem by covering the graphite or lithium anode with a gel polymer electrolyte coating.

As the coating is hydrophobic, it does not allow water molecules to reach the electrode. However, when the battery charges for the first time, the coating decomposes, forming a stable layer separating the solid anode from the liquid electrolyte. The layer protects the anode from side reactions that could deactivate the anode. This allows the battery to use anode materials that are more effective, such as lithium metal or graphite, and allowing the battery reach higher energy densities and cycling abilities. The gel coating improves the safety of the battery, and is now comparable to the safety standards of non-aqueous lithium-ion batteries.

Organic solvents used in non-aqueous batteries are highly flammable. In comparison, aqueous lithium-ion batteries use water-based electrolytes that are non-flammable. Another advantage of this gel polymer coating is if this layer is damaged, the reaction with the lithiated graphite or lithium anode is very slow, preventing smoking, fire, or explosion that would normally happen if in the damaged battery the metal came into direct contact with the electrolyte.

This aqueous lithium-ion battery with the gel covering the anode has power and energy density matching its counterpart with non-aqueous electrolyte, and is suitable for commercial applications. However, the researchers intend to improve on the number of full-performance cycles the battery can complete. According to the researchers, this will reduce material expenses as far as possible. Although at present the battery is able to complete only 50-100 cycles, the team intends to increase that to 500 or more.

The researchers are also trying to manipulate the electrochemical process to allow the battery achieve 4 V on its terminals. According to the researchers, this is the first time they have been able to stabilize the reactive anodes such as lithium and graphite in aqueous media. They feel this opens a huge field of possibilities into several different topics in electrochemistry. For instance, this could cover not only lithium-ion batteries, but also lithium-sulfur, sodium-ion batteries, and other batteries using multiple ion chemistry technologies such as magnesium and zinc, and electrochemical and electroplating synthesis.

The researchers understand that interphase chemistry requires to be perfected before they can commercialize their product. They also feel that they need to work more towards scaling up the technology so that big cells can be used for testing. However, the researchers are confident they will be able to commercialize their product within the next five years, provided they are able to gather more funding.