Prelithiation technology for lithium-ion batteries

Technology Sharing | A&S POWER | Sep 01, 2023

Research on lithium-ion batteries has always focused on the high energy density and long cycle life of electrode materials. Lithium-ion battery materials, led by high-nickel ternary cathodes, silicon-based anodes, etc., while showing higher lithium storage capacity, also face huge applications Problem: During the first charge and discharge process, the electrolyte decomposes, resulting in the irreversible loss of lithium ions, which greatly reduces the first Coulombic efficiency and energy density. Therefore, reducing the loss of active lithium is the most direct solution, and the technology used in this is prelithiation.


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What is prelithiation? What's the point?


Pre-lithiation is actually a lithium replenishment technology that "prevents problems before they occur". By adding active lithium in advance, additional lithium is released during the first charge and discharge process to supplement losses, increase capacity and energy density, and improve first Coulombic efficiency. , and stores additional lithium in the positive and negative electrode systems. Before formally introducing these strategies, we might as well figure out where the lost lithium ions go? What are the root causes of prelithiation?


Taking half-cells with common positive electrode materials such as ternary and lithium iron phosphate as examples, the first charge and discharge curves are as follows.


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The first charge capacity of the two cathode material half-cells is greater than the first discharge capacity. In other words, the lithium ions deintercalated from the cathode during charging do not return 100% to the cathode during discharge. So where does the lost capacity go? After the first charge and discharge, the structure of the cathode material changes due to delithiation, thereby reducing the number of lithium-intercalable sites in the material. When the lithium ions return, they find themselves "homeless", resulting in capacity loss.


Negative material half-cells are also affected by first-time efficiency. Taking the graphite anode half-cell as an example, its first charge and discharge curve is as follows.


The first charge capacity of this half-cell is significantly lower than the first discharge capacity, which means that lithium ions come to the graphite layer during the discharge process and are not 100% deintercalated from the graphite layer during subsequent charging. The reason is that before lithium ions are embedded in the graphite layer, they will first form a solid electrolyte interface film (SEI film) on the surface. The lithium ions dedicated to the SEI film are "confined" and cannot return to the positive electrode, thus causing the irreversible loss of lithium. loss.


Introduction to prelithiation technology

In order to supplement the lithium loss caused by the change of electrode structure and SEI film consumption, researchers have introduced many pre-lithiation strategies. Next, we will explore them from the aspects of advantages and classification.


1. Advantages of prelithiation technology

In addition to increasing energy density, expanding capacity, and replenishing lithium losses, prelithiation technology also has many benefits for improving battery electrochemical performance:


1.The prelithiated electrode reduces the internal resistance of the battery and has better rate performance.


2.For materials with large volume changes during cycling (such as silicon), prelithiation can cause the electrode to expand in volume in advance, preventing the electrode structure from collapsing and electrode materials from falling off during the battery's cycling process.


3. Promote the formation of SEI film in advance. With artificial control, a more stable SEI film can be formed, which helps to reduce electrolyte consumption and improve electrochemical performance.

 

2. Classification of prelithiation technology


Seizing the key points from the advantages, the key point of prelithiation technology is the "electrode", which also corresponds to the current research focus. Starting from battery materials, prelithiation technology can be divided into positive electrode prelithiation, negative electrode prelithiation, separator prelithiation and electrolyte prelithiation. Among them, the prelithiation of positive and negative electrodes is the most extensive and in-depth research, and will be used as Highlights.


(1) Negative electrode prelithiation

Existing research focuses on three key directions: adding negative electrode lithium replenishing agents, electrochemical prelithiation and chemical prelithiation.

1. Negative lithium supplement


When it comes to lithium supplementation, the most direct way is to prescribe the right medicine - adding lithium powder. This is theoretically feasible because metallic lithium has a high theoretical capacity of 3860mAh/g and there is no residue or impurities after lithium supplementation. However, due to the excessive properties of metallic lithium Lively, it has very strict requirements on production, transportation, storage and application environment.


To this end, researchers have developed stabilized metal lithium powder (SLMP) coated with lithium carbonate. SLMP has a core-shell structure with a capacity of about 3600mAh/g. The outer protective effect of the lithium carbonate layer makes the lithium powder Can be used in a dry environment. SLMP is generally added through homogenization or directly loaded on the surface of the negative electrode piece. The degree of pre-lithiation can be adjusted by controlling the amount. When used, the surface coating layer must be crushed with pressure to activate lithium. SLMP is simple to use, has good lithium replenishment effect, can be commercially produced, and has been widely used in silicon and carbon-based anode materials.