Intro to First Cycle Efficiency (Part II) — A Visual Approach

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

Introduction

This is a sequel to Intro to First Cycle Efficiency (Part I).

To understand first cycle efficiency (FCE), we must first understand the numbers that impact the calculation. Cells typically immediately go into the formation process after the electrolyte fill step. A simple formation process involves 2 steps: charging and discharging the cell to its designated voltages.

First cycle efficiency = Discharge Capacity / Charge Capacity

First charge (blue) and discharge (orange) curves for LCO.

In industry, formation processes are often more complicated. Different cell manufacturers have their own “secret” optimized procedures, which balances forming a stable solid electrolyte interface (SEI) layer and minimizing formation time to decrease the operational cost.

Now that we understand the formation process, let’s take a deeper dive into what determines the first cycle efficiency. Assume that we are designing a battery where the cathode’s FCE is 85% and the anode’s FCE is 90%. For this cell, what would be the FCE? We will use the following diagram to illustrate this case.

In an unformed electrode, the Li resides in the cathode structure. In the first charge step, we lose 3 active sites in the cathode due structural changes in the material, also known as its irreversible capacity. SEI is formed in the anode, using up 2 Li+. During the discharge step, there is insufficient active sites for all the Li+ to intercalate back into the cathode. The full cell FCE will be a smaller value of the respective half cell FCE, which is 85% in this example.

Let’s look at another case. Assume that we are designing a battery where the cathode’s FCE is 95% and the anode’s FCE is 90%. For this cell, what would be the FCE?

Using the same analysis, there are 2 positions in the anode that consumes Li in the first charge. Once again, the full cell FCE will be the smaller value of the respective half cell FCE, which is 18/20=90% in this case.

Note that FCE is also impacted by side reactions and the voltage range selected for the first charge and discharge. However, these factors will not significantly affect the conclusions above.

In Intro to First Cycle Efficiency (Part I), the editor introduced FCE values for various types of materials. Besides materials, what other factors will impact the first-cycle efficiency of the full battery?

1. Anode specific surface area. Higher specific surface area for graphite materials consumes more lithium ions during the SEI formation process, lowering overall FCE (provided that FCE of the negative electrode is lower than the positive electrode). The following graph depicts first cycle efficiency as a function of surface area for an LCO-graphite system.

First cycle efficiency as a function of surface area of anode

2. Formation charging protocol. When the formed SEI is thinner and denser, less lithium ions are consumed in the process, which increases the first-cycle efficiency. Even when using the same electrolyte formulation, the quality of the SEI formed is dependent on various factors such as current density, cut-off voltages, and temperature.

3. Electrolyte compatibility. For example, adding propylene carbonate (PC) widens the operating temperature window. However, without the proper selection of electrolyte additives to tune the SEI, the co-intercalation of PC-solvated Li+ results in the exfoliation of graphite layers, thereby impacting FCE and long term performance.

Let us summarize what we have learned today. First, FCE can be calculated by Discharge Capacity / Charge Capacity. Second, the electrode with the lower FCE will determine the full cell FCE. Finally, we introduced several factors that will impact a full cell’s FCE.

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