Creating Safer Lithium-Ion Batteries

The technical and performance characteristics lithium-ion batteries offer make them the preferred power source for all kinds of consumer, commercial and industrial applications — from electric vehicles to satellites and spacecrafts. Compared to other kinds of batteries, they excel in charge rate, discharge rate, charge cycle durability, specific energy (Wh/kg), energy density (Wh/L), power density (W/L) and specific power (W/kg). Yet, they have been in the news in recent years because they can overheat and catch fire..

Because so many electronics manufacturers focus on rechargeable, battery-powered products and devices, the search for a safer lithium-ion battery is consuming researchers around the world.

The Evolution of Lithium-Ion Batteries

Batteries of every type are made from two dissimilar metal terminals separated by an electrolyte, the resulting electrochemical reaction is what provides electricity. One terminal is connected to a material that easily gives up electrons from its outer valence shell, anode or negative electrode. The other terminal is connected to a material that needs to acquire electrons to fill its outer shell, cathode or positive electode. The electrolyte is essentially a transmission medium that allows the flow of ions to pass from one electrode to another inside the battery. The electrons flow through the external circuit effectively powering the device.

The earliest lithium-ion battery made by Sony used a lithium-cobalt-dioxide (LiCoO2) cathode (positive electrode). The anode (negative electrode) was fabricated using coke, a form of carbon. Later, research found that a graphite anode provided the battery with a more linear discharge curve and significantly higher energy, which led to the adoption of graphite as the anode in most lithium-ion batteries.

The basic electrochemistry of Li-ion batteries is much the same as any other type. When a battery is connected to an electrical load, an electrochemical reaction begins.

• Positive ions — those that are missing some of their valence shell electrons — begin moving from the cathode terminal through the electrolyte toward the anode where those ions can attract electrons into their valence shell. They are “seeking” electrons to balance their positive electrical charge and reset their net charge to zero.
• At the anode terminal, electrons are freed and pass through the electrical load, causing a current to flow.
• When the battery is being recharged, the direction of ion and electron flow is reversed.

It's About the Cathode

Much of the research that has brought advances in Li-ion batteries has focused on squeezing more energy into a given battery package. Consequently, scientists have mixed lithium with other elements to make the cathode more chemically active. A variety of lithium oxides and lithium phosphates have given rise to batteries whose characteristics vary — giving each type of Li-ion battery a distinct “personality.”

For example, the table below illustrates how various cathode compositions affect three battery characteristics:

• Specific energy — which refers to how much energy a battery can store. Specific energy is measured in watt-hours per kilogram of battery mass.
• Specific power — which indicates how much current a battery can deliver, measured in watts per kilogram.
• Safety — which is a measure of how likely a given battery type can be expected to operate without thermal runaway that leads to overheating, fire or explosion.

The values shown in the table below are dimensionless. On a scale of 0 to 4, they show relative performance on those three parameters, with larger numbers indicating better performance:

It is clear that changing the chemistry used in a lithium-ion battery affects its overall performance in several ways. Generally speaking, there is always a trade-off between energy and safety. The higher the energy content the greater the risk of a safety event. This is driven by the market where the demand is for a smaller battery that delivers more energy.

Insuring safe operation has become a key objective of every manufacturer as each struggles to also optimize these characteristics.

It is About the Electrolyte, Too

Another area of research in lithium-ion batteries is using different electrolytes. The most significant difference between lithium-ion cells and other rechargeable technologies is the electrolyte. Other rechargeable batteries, such as lead acid, nickel cadmium and nickel metal hydride, use an aqueous or water based electrolyte; lithium ion uses a non-aqueous electrolyte. Conversely, a non-aqueous electrolyte uses organic solvents. A rechargeable cell with an aqueous electrolyte has chemical protection against overcharge due to the water splitting reaction which generate Oxygen and Hydrogen. Lithium-Ion cells with a non-aqueous electrolyte have no internal chemical protection against overcharge.

The largest contributor to safety events for lithium-ion cells is overcharging; this is why lithium-ion batteries have more complex electronics than other cell chemistries, called a battery management system (BMS). The BMS ensures all the cells are protected from operating outside safe operating conditions, including protection from overcharge, overdischarge, high current and balancing the individual cell capacities. Also, it is important to note that a non-aqueous electrolyte is flammable; therefore if there is a safety event, the flammable electrolyte will add to the intensity of the event.

Researchers are actively working on non-flammable electrolytes that are compatible with lithium-ion cells and do not reduce the positive attributes of the cells. If the electrolyte is non-flammable than the safety of the lithium-ion cells and batteries will be greatly enhanced.

Lithium-ion batteries are made of several individual cells connected in a series-parallel fashion. For instance, a laptop battery may have three or four cells, while a battery used to power an automobile has hundreds of cells.

The anode and cathode in each cell need to be separated from each other to prevent an internal short circuit. Today's batteries often use a polymer-separating material. However, if a battery undergoes a mechanical shock, such as being dropped, the separating material can be damaged. When that happens in even one cell, the short circuit can cause its electrolyte to heat. When one cell overheats and dissipates its heat into a neighboring cell, the battery can go into thermal runaway, which can boil the electrolyte, leading to fire or explosion.

Making lithium-ion batteries safer relies on improving the internal mechanical structure of each cell and its separating material. In addition, a good deal of research is bringing new electrolytes, which do not overheat or boil, to the forefront. Various solid-state electrolytes are beginning to replace the liquid or gel electrolytes with a solid material. The composition of the solid depends on the cell chemistry selected. In all cases, the solid electrolyte will add to enhanced safety. The trick is to ensure the changes do not detract from the performance attributes of the Lithium-Ion cell.

These solid-state electrolytes eliminate the problem of thermal runaway while also giving the battery assembly more mechanical strength and, in some cases, improved energy density.

In addition to improvements at the cell level, there are significant changes that can be done at the battery level to improve safety. EaglePicher addresses battery design with a layered and integrated sub-system approach. At the cell level, EaglePicher balances the available energy with the failure propagation effects of a given cell within the battery. This trade-off looks at cell chemistry and design. At the battery level, mechanical and electrical safety measures are implemented. Mechanical measure may include thermal barriers to prevent propagation and material selection for the battery case.

Electronics oversight can provide a practical means to avoid possible cell abuse through integration of on-board electronics to monitor and control the individual cells. The battery electronics can readily become complicated and is more appropriate to discuss on an application specific level.

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