Intro to First Cycle Efficiency (Part I)

The content below has been translated from 知行锂电 with permission.

Introduction

For those in the lithium-ion battery industry, whether you are working in battery R&D or materials development, you will certainly encounter the first cycle efficiency problem: whether it is a full cell or a half cell, a cell’s coulombic efficiency from the first charge and discharge will be significantly different from its subsequent cycles. The parameter describing this characteristic is the “first cycle efficiency”, or FCE.

Discussions on FCE should be divided into half-cell and full-cell; we will also discuss a mitigation strategy for improving FCE: pre-lithiation.

Let’s get started!

After fabrication of the cathode half-cell (the positive electrode is made of cathode material and the negative electrode is made of metallic lithium), the cell must first undergo a charge-discharge cycle (formation): during the charging process, lithium ions are deintercalated from the positive electrode and plated onto the metallic lithium negative electrode. When discharging, the process works in reverse, where the lithium metal forms lithium ions after losing electrons and passes through the electrolyte, and then intercalates into the positive electrode.

Taking the data for LCO half-cell as an example, we made its first charge-discharge curve, as shown in the following figure:

Figure 1. Li(metal)||LCO first cycle charge (blue) and discharge (orange) potential vs. specific capacity

From the figure above, we can see that the first charge capacity of the half-cell is slightly higher than the first discharge capacity. That is to say, not 100% of the lithium ions released from the positive electrode during charging return to the positive electrode during discharge. The ratio of first discharge capacity/first charge capacity is the first cycle efficiency of this half-cell.

Not only for LCO, other common cathode materials such as NMC and LFP half-cells also show the phenomenon of FCE. The following are the formation cycles of NMC and LFP half cells:

Figure 2. Li(metal)||NMC first cycle charge (blue) and discharge (orange) potential vs. specific capacity

Figure 3. Li(metal)||LFP first cycle charge (blue) and discharge (orange) potential vs. specific capacity (note the x-axis unit should be x100)

As can be seen from the above several graphs, the first cycle efficiency of NMC is the lowest, generally 85~88%; LCO is second, generally 94~96%; LFP is slightly higher than LCO, 95%~97%.

So where did the lost capacity during the first charge and discharge go? For the cathode half-cells, the capacity loss is mainly caused by the change of the material structure after the first discharge: after the first discharge, the structure of the cathode material changes due to delithiation, this reduces the available lithium intercalation sites in the material, and the lithium ions cannot return to the positive electrode after the first charge.

Like the cathode half-cell, the anode half-cell will also be affected by first cycle efficiency. Taking the graphite half-cell as an example, the lithium ion deintercalation and intercalation potential of the graphite material is higher, so it is the positive electrode, and the metal lithium material is the negative electrode. During the first cycle, the lithium ions must be transported from the lithium metal (negative electrode) and then inserted into the graphite (positive electrode), so the half-cell is discharged first and then recharged. The first discharge and charging curve of the graphite material half-cell is as follows:

Figure 3. Li(metal)||Graphite first cycle discharge (blue) and charge (red) potential vs. specific capacity

As can be seen from the figure above, the first charge capacity of the half-cell is significantly lower than the first discharge capacity, which means that lithium ions do not come out of the graphite 100% during subsequent charging after they come in contact with the graphite material during the discharge process.

Where were the lithium ions lost during this period? When the graphite half-cell is discharged for the first time, before the lithium ions are embedded in the graphite, an SEI film will be formed on the graphite surface. The lithium ions lost during the formation of the SEI film cannot be returned during subsequent charging. To the negative electrode of the lithium metal, which results in the first discharge capacity of the graphite half-cell being greater than the first charge capacity.

First cycle efficiency is not 100% because of SEI formation

For the commonly used graphite anode materials, the first cycle efficiency is generally between 90 and 92%. For lithium titanate, a material that hardly forms an SEI film, the first cycle efficiency is significantly higher, about 97%. In addition, for the currently emerging silicon-carbon anode materials, since the first cycle efficiency of the silicon anode is only 50%, the first cycle efficiency of the silicon-carbon anode will gradually decrease as the silicon content increases.

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References

【新嘉拓】技术帮 ┃ Li-ion电池首次效率是个什么鬼？--深入解读首次效率（一）