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Why do lithium-ion batteries die so quickly?


imageHow the battery goes dead: we all have seen how this happens in the phones, laptops, cameras, and the electric vehicles. It is painful process - if you are lucky – it could be slow. Over the years, the lithium-ion battery, which once powered your devices for the several hours (even days!), gradually loses its ability to hold the charge. After all we accept it, maybe curse Steve Jobs, and then we buy a new battery or a new gadget.

But why is this happening? What happens in a battery that causes it to go dead? The short answer lies in the fact that due to the damage from continuous effect of high temperatures and a large number of charging and discharging cycles, it eventually begins to break down the process of moving ions between the electrodes of lithium.

A more detailed answer, which lead us through the description of unwanted chemical reactions, corrosion, or a threat of high temperatures and other factors affecting the performance, It begins from an explanation, what is happening in the lithium-ion batteries, when everything works well.

Introduction to the lithium-ion batteries

imageWe find the cathode (or negative electrode) that is made of the lithium oxide, such as lithium oxide with cobalt in a typical lithium-ion battery. We also find the anode or positive electrode, which usually is made of the graphite. Thin porous separator keeps the two electrodes from each other to prevent the short circuits. The electrolyte is made of organic solvents that are based on the lithium salts, which lets lithium ions move inside the cell.

An electric current moves the lithium ions from the cathode to the anode during the charging (in other words, when using the battery) ions move back to the cathode.

A scientist Daniel Abraham from the Argonne National Laboratory, who was conducting the research of degradation of lithium ion cells, compared this process with the water hydropower system. Moving up water requires energy, but it is very easy flows down. In fact, it supplies the kinetic energy, says Abraham. In a similar way, the lithium-cobalt oxide “does not want to give its lithium” in the cathode. It is like the moving up water requires energy to move atoms of lithium from the oxide and move them to the anode.

Ions are placed between the sheets of graphite that make up the anode during charging. Abraham said, “they do not want to be there at the first chance they will move back”, as the water flows downhill. This is the discharge. The long-lived battery will stand a few thousand of the charge-discharge cycles.

When the battery dead is really dead?

imageWhen we talk about “dead” battery, it is important to understand two metrics of the performance: energy and power. In some cases, the speed is very important, so you can get energy from the battery. This is the power. The high power could allow the rapid acceleration as well as braking in the electric vehicles, when the battery needs to get charge for a few seconds.
On the other hand, the high power is less important than the capacity, or the amount of energy that can hold a battery in the cell phones. High-capacity batteries last longer on a single charge.

Within time the battery is degraded in the several ways, which may affect the capacity and power until it simply could not perform the basic functions.

Think about this using the other similar analogy, water-related: the battery charging, as filling a bucket with water from the tap. The volume of the bucket represents itself the capacity of battery, or the capacity. The speed with which we can fill it up - turn the faucet on the full power or thin stream – it is the power. But the time, high temperatures, multiple cycles and other factors that ultimately form a hole in the bucket.

The water leaks in analogy with a bucket. Lithium ions are removed or “linked up” in a battery, says Abraham. As a result, they lose the chance to move between the electrodes. So after several months a cell phone needs to be charged every day, which originally needed to be charged every couple of days. Then, it is twice a day. Finally, too much lithium ions “linked up” and the battery will not hold any useful charge. The bucket stops to keep the water.

What breaks down and why

imageThe active portion of the cathode (the source of lithium ions in the battery) is designed with a specific atomic structure to ensure the stability and performance. When ions move toward the anode and then return back to the cathode, ideally, we would like them to return back to their original location to maintain a stable crystalline structure.

The problem is that the crystal structure can change with each charge and discharge. It is not necessarily that the Ions from a cell A come back home, but they can move into a cell B nearby. Then the ion from the cell B finds its place occupied by another ion it decides to move into a next cell. And so on.

Gradually, these “substance phase transitions” convert cathode to the new crystal structure of a crystal with the different electrochemical qualities. The exact location of atoms is changed that initially providing the performance.

The batteries of hybrid cars, which are only required to supply the power when the vehicle accelerates or slows down, Abraham says, these structural changes happen considerably slower than in electric vehicles. This is due to the fact that, the system moves only a small fraction of lithium ions in each cycle. As a result, it is easier to return to their original positions.

The corrosion problem

imageDegradation can also occur in other parts of the battery. Each electrode connected to the current collector, which is essentially a piece of metal (usually copper for the anode, the aluminum for the cathode), which collects the electrons and moves them to an external circuit. So, we have clay of such “active” material as lithium-cobalt oxide (which is a ceramic and not a very good conductor) and binding material that is applied on a piece of metal.

If the binding material is destroyed, it leads to “flaking” of the surface current collector. If the metal is corroded, it cannot effectively move electrons.

The corrosion inside the battery may appear as a result of interaction of the electrolyte and electrodes. The graphite anode is “easy to return” i.e., it easy “returns” the electrons in the electrolyte. This can lead to unwanted coating on the graphite surface. Meanwhile, the cathode is very “oxidized”, which means that it easily accepts electrons from the electrolyte. It may erode the aluminum current collector or form a coating on the parts of the cathode in some cases, said Abraham.

Here are too many good things

imageGraphite is a material that commonly used for the manufacture of anodes - thermodynamically is unstable in the organic electrolytes. This means that from the very first charge of the battery, graphite reacts with the electrolyte. This creates a porous layer (called a solid electrolyte interface or SEI), which ultimately protects anode from further attacks. This reaction also consumes a small amount of lithium. In an ideal world, this reaction would occur once to create a protective layer, and that's all over.

Actually, SEI is a very unstable defender. It protects well the graphite at room temperature, says Abraham, but at the high temperatures or when the battery charge decreases to zero (“total discharge”), SEI can partially dissolve in the electrolyte. At the high temperatures electrolytes also have a tendency to decompose and the side reactions are accelerated.

When favorable conditions will return, the other protective layer will form, but it swallowed up a part of lithium, leading to the same problems as with the leaky bucket. We will have to charge the cell phone more often.

Thus, we need SEI to protect the graphite anode, and in this case, here can be really too many good things. If the protective layer thickens too much, it becomes a barrier for the lithium ions, but they are required to move freely back and forth. According to Abraham this affects the power, which is “extremely important” for the electric vehicles.

Creating the best batteries

imageSo what can we do to make longer the life of our batteries? The researchers are searching for the electrolytic additives in the laboratories that would work like vitamins in our diet i.e., that let the battery will work better and last longer by reducing the harmful reactions between the electrodes and the electrolyte, said Abraham. In addition, they are looking for new, more stable crystalline structures for the electrodes, and a more stable binding materials and electrolytes.

Meanwhile, the engineers that make batteries and electric cars at companies work on the casings and thermal control systems in an attempt to keep the lithium-ion batteries in a constant, favorable temperature range. We as consumers should avoid the extreme of temperatures and total discharge, and continue to grumble about the batteries that always seem to die too quickly.
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Pirat 18 september 2011, 18:06
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