Decoding Li-ion Cell Voltages Of Different Chemistries

While deciding an optimal Lithium-ion cell chemistry for different applications, I came across this question quite a few times, 'Why different Li-ion cell chemistries have different terminal voltages?'

There are numerous LiB chemistry types available in the market. e.g. Lithium Cobalt Oxide (LCO), Li Nickel Cobalt Aluminum Oxide (NCA), Lithium Tantalate Oxide (LTO), Lithium Iron Phosphate (LFP), etc to name a few. However, each and every battery type has its positive-negative points.

Although the working principle of all these chemistries is the same, their terminal voltages are different. For example, LCO cells can go up to 4.25V, NCA can be charged up to 4.2V whereas LFP and LTO type cells can have a maximum of 3.8V and ~2.5V respectively. This difference in the voltage has a very large effect on the battery capacity (since energy capacity is directly proportional to the battery voltage).

In this blog, I will answer 'Why different LiB chemistries have different terminal voltages?'

Before starting with this blog, I would recommend going through my previous blog post 'How Does The LiB Work?' if you are not familiar with the basic LiB working mechanism.

Different LiB Chemistries and Their Discharge Curves

As mentioned before, LiB comes with a variety of flavors (chemistries). I don't want to go through positive and negative points of each and every chemistry since a large number of online resources are already available for that information. I would recommend going through the following webpage if you want to compare different LiB chemistries by their performance:

• Types of Lithium-ion batteries by Battery University

Figure 1 shows the discharge curves of different LiB chemistries (it is not real data.)

Figure 1: Discharge Voltage curves of different LiB chemistries

As can be seen in the above figure, LTO has the lowest cell voltage over the discharge curve whereas NCA and LCO have higher cell voltages. Cell voltage has very serious implications on the energy and power capacity of a cell as shown in the below equations:

For the same current over time, if the cell has a lower voltage, total energy capacity is proportional to the voltage. This means LTO has the least energy capacity whereas LCO has highest energy capacity for the same Lithium intercalation/de-intercalation capacity (same 'Ampere-hour' capacity in simple words!) 

Seems interesting, isn't it? Let's dig a bit deeper and see why LTO and LFP have significantly less cell voltage as compared to NCA and LCO.

The Concept of Half Cell Voltage

In the previous blog, I indicated LiB as a simple construction with 2 electrodes (Anode and Cathode) placed in an electrolyte, as shown in figure 2. (Actual Li-ion cell is not at all as shown in this figure. I will post another blog addressing the internal structure of a Li-ion cell)

Figure 2: Simplified diagram of Li-ion cell

We can measure the cell voltage as a voltage difference at positive and negative (cathode and anode) terminals. Remember this voltage is not absolute, the cell voltage is measured at the positive terminal with respect to the negative terminal. However, there is a way to determine the voltage of anode and cathode separately with respect to the reference electrode. This type of anode/cathode characterization gives us deep insight into their behavior at different stages of charge/discharge. Li-ion cell shown in figure 2 can be divided into 2 'half cells' as shown below

Figure 3: Half cell

Since Lithium has very high standard electrode potential (-3.04V), in the anode half cell, graphite or LTO anode works as +ve terminal. In the cathode half cell, cathode remains +ve terminal. When we measure the voltages of these type of half cells with different anode and cathode materials, we get data something as shown below (figure 4).

Figure 4: half cell voltage (this is not real data I have fabricated it for the purpose of understanding)

From figure 4, we can see that an anode (graphite and LTO) voltage increases with respect to Lithium metal electrode when the cell is near to complete discharge whereas cathode voltages fall down. From this data, we can have a fair idea of voltage curves of a full cell.

For example, we want to know cell voltage which has graphite as an anode and NCA as a cathode. We simply subtract graphite curve from NCA shown in figure 4 and we get cell terminal voltage as shown in figure 1! (this method does not give 100% accurate data. there are few other factors which affect the cell terminal voltage. However, these type of half cell voltage curves give fair enough idea of what will the cell voltage be..)
Similarly, if we want to calculate cell terminal voltage with LTO as an anode and LFP as a cathode. We can get it by subtracting those two curves from figure 4.

One more important observation from figure 4, Since LTO as an anode has higher voltage with respect to Lithium metal, LTO based lithium ion cells have lesser terminal voltages as compared to graphite-based anode cells. Similarly, LFP cathode has lesser voltage w.r.t Li metal electrode as compared to NCA/LCO. Hence LFP chemistry based cells have smaller terminal voltage as compared to NCA or LCO. 

This explains why LTO and LFP type cells have significantly less energy density as compared to their NCA/LCO counterparts.

In a nutshell, the electrode potential voltage of used anode and cathode material with respect to Li metal decides the terminal voltage of the Lithium-ion cell.

I hope you find this blog interesting. Feel free to post questions and suggestions in the comments.

Keep exploring!
--Harshad