Lithium ion (Li-ion) batteries are a commonly used type of rechargeable battery with a global market estimated at $11bn and predicted to grow to $60bn by 2020.
The popularity of the Li-ion battery is due to the advantages offered over other secondary (or rechargeable) batteries:
- Lighter than other rechargeable batteries for a given capacity
- Li-ion chemistry delivers a high open-circuit voltage
- Low self-discharge rate (about 1.5% per month)
- Do not suffer from battery memory effect
- Environmental benefits: rechargeable and reduced toxic landfill
However Li-ion batteries have also struggled with issues such as:
- Poor cycle life, particularly in high current applications
- Rising internal resistance with cycling and age
- Safety concerns if overheated or overcharged
- Applications demanding more from Li-ion battery capacity
In Li-ion batteries, lithium ions move from the anode to cathode during discharge, and from cathode to anode when charging. The materials used for the anode and cathode can dramatically affect a number of aspects of the battery’s performance, including capacity.
In our previous battery example – the zinc carbon battery – while discharging, the anode would oxidize (lose electrons) naturally and those electrons would make their way towards the cathode through the electrolyte and wire/closed circuit.
In a lithium ion battery, graphite (carbon) is most commonly used for the anode, and lithium cobalt oxide (LiCoO2) is the most common cathode material. You may also see it in other sites written as carbon being used for the negative terminal and LiCoO2 as the positive terminal.
During discharge (or use of the battery):
- Li+ ions go from the carbon to the LiCoO2 or from anode to cathode
During recharging of the battery:
- Li+ ions go from the LiCoO2 to the carbon or from cathode to anode
I had some questions from this:
- How do Li+ ions move?
- What happens to the electrons?
- What is chemically happening?
I have found that each question answers the other:
QUESTION 1: How do Li+ ions move?
So it took a bit of reading, but it involves a process known as intercalation. Simply put, intercalation is the ability of a molecule to be inserted and extracted from two other molecules. Specifically with Li-ion batteries, we’re referencing graphite intercalation and graphite intercalation compounds. The carbon rod anode would be considered a graphite intercalation compound.
The above shows the cathode but not the anode (where the Li is coming from)
Graphite intercalation is where molecule X is inserted/intercalated between the graphite layers. In this type of compound, the graphite layers remain largely intact and the guest molecules or atoms (Li+) are located in between. So it’s like checking into a hotel, then checking out without the hotel falling on your head.
- Both the anode and cathode are materials into which, and from which, lithium can migrate. During insertion/intercalation lithium moves into the electrode. During the reverse process, extraction/deintercalation, lithium moves back out.
- When a lithium-based cell is discharging, the lithium is extracted from the anode and inserted into the cathode. When the cell is charging, lithium is extracted from the cathode and inserted back into the anode.
The concept of intercalation however, only says that, yes – a molecule can be inserted and extracted from other molecules. This still does not answer how.
Question 2: What happens to the electrons?
Therefore, to explain this, we may need to consider the electrolyte separator. In the zinc-carbon battery, it was a middle man, a second step that facilitated the passing on of electrons.
In the Li-ion battery, the highly permittive electrolyte is typically a mixture of ethylene carbonate or diethyl carbonate with varying lithium complexes such as LiPF6, LiAsF6, LiClO4, LiBF4, and LiCF3SO3. This, as with other batteries, separates the anode from the cathode, the carbon from the LiCoO2.
The electrons still move from anode to cathode as helped by the electrolyte, but because Li as an ion is able to intercalate in both anode and cathode materials, it finds itself following the electrons it gives up to keep electrical neutrality. This is why sometimes you might hear explanations where they say the lithium ion carries the current. It’s not so much that it is the current, as it is following the electrons it gave up. Kind of like a bad girlfriend/boyfriend who won’t leave you alone.
Question 3: What is chemically happening?
During discharge (use of the battery) – equations simplified to concentrate on only what the lithium and electrons are doing:
Anode: CLix → C + xLi+ + xe–
- This seems to indicate that the lithium ions attached to the carbon come (deintercalate) off of the carbon as ions, and the electrons that would otherwise be on them, tend to travel separately.
- According to the orbital configuration for Li (which has 3 electrons for its 3 protons): 1s2 2s1 – it would seem easy for a single electron in its outer 2s shell to be sucked away by something more powerful.
Cathode: Li1-xCoO2+ xLi++ xe–→ LiCoO2
- So during discharge and looking at it from the LiCoO2 ‘s point of view, the metal oxide is getting extra lithium ions and electrons. All it does, though, is suck them up and create more of itself.
During charging, the charger applies a higher voltage (but of the same polarity) than that produced by the battery, forcing the current to pass in the reverse direction. The lithium ions then migrate backwards from cathode to anode, where they, once again, become intercalated in the electrodes.
Anode: C + xLi+ + xe–→ CLix
- The electrons and lithium ions that may have just separated from the carbon rod, turn around and check back in.
Cathode: LiCoO2→ Li1-xCoO2+ xLi++ xe–
- The metal oxide gives up some of its lithium ions and electrons which then go back through the electrolyte separator to attach back with the carbon anode.