Pre-lithiation technology for lithium ion batteries

Lithium-ion battery (LIBs) has become one of the most widely used electrochemical energy storage systems due to its characteristics of high energy density, high working voltage and no memory effect. Its commonly used graphite anode is difficult to fully meet the increasing market demand due to its relatively low capacity (372 mAh g-1).

Therefore, the pre-lithiation technology before battery assembly is particularly important. This article will introduce you to the pre-lithiation technology.

Research background

Over the past few decades, a variety of new cathode materials have been proposed that show ideal potential ranges, higher capacity, excellent rate performance and long cycle life, but have the disadvantage of high ALL (Active Lithium Loss).

Therefore, eliminating ALL before full battery assembly is crucial to achieve high performance lithium ion batteries. In recent years, new anode materials for the next generation of lithium ion batteries have gradually begun to be commercialized. Therefore, the research on pre-lithiation technology, which is crucial for eliminating ALL, has become an important research direction.

Recently, Zheng Junsheng, associate professor of Tongji University in China, and Zheng Jianping, Professor of State University of New York at Buffalo, reviewed the research progress of pre-lithiation technology in the international top journal Advanced Science. The review firstly theoretically explained the main causes of anode capacity loss and its impact on the performance of the whole battery.

Then, the advantages and disadvantages of various pre-lithiation technologies are summarized, classified and compared in detail, and the research progress of representative electrochemical pre-lithiation strategies is reviewed in detail. This review reevaluates the importance of pre-lithiation technology from a new perspective, compares the proposed pre-lithiation technology, and puts forward the research direction for the subsequent research on the pre-lithiation technology of lithium ion batteries.

What is pre-lithiation technology

During the first charging process of lithium battery, electrolyte will be reduced and decomposed on the surface of the anode, forming solid electrolyte interface (SEI) film, consuming lithium ions, resulting in low first cycle and low coulomb efficiency (ICE), reducing the capacity and energy density of lithium battery. pre-lithiation is to reduce the loss of lithium ion by replenishing lithium to achieve the effect of improving battery capacity and energy density.

Why do we need pre-lithiation technology

The cause of high active lithium loss in anode

The high initial ALL of the anode occurs in the first few cycles, and the coulomb efficiency is low (CE < 100%), which indicates that some Li+ remains in the anode, leading to a decline in the amount of recyclable Li+ in the LIBs. When matched with a cathode, the reduced recyclable Li+ will inevitably result in a lower energy density for the entire battery.

The figure below shows typical embedding/insertion, conversion, and alloying lithium storage mechanisms for cathode materials. These materials mainly show relatively low potential and much higher capacity than commercial graphite and Li4Ti5O12, but the first-cycle coulomb efficiency of these materials is usually less than 80%, resulting in a low coulomb efficiency mechanism. The causes of initial anode ALL can be divided into SEI formation, loss of active material and appearance of dead lithium.

Effect of anode active lithium loss

In practical applications, some of the recyclable Li+ is consumed to form SEI on the anode surface, resulting in a lower first coulomb efficiency, which in turn leads to a rapid capacity decay of the battery. As shown in the figure below, the reversible capacity of the electrode is not reduced during this process, and when additional lithium sources are added to the system, the specific capacity of the battery will return to the ideal condition.

The introduction of additional lithium sources will offset the specific energy brought by the additional lithium sources. The influence of higher initial ALL on the specific capacity loss of the whole battery is elaborated through theoretical calculation and analysis, and the specific energy based on the total mass of the cathode, the cathode and the lithium source can be obtained as follows.

The figure below shows the effect of different additional lithium sources on contrast energy. For lithium sources with initial CE of 50%, 70% and 90%, R is shown as a specific capacity function of lithium source(cls). It can be seen that the R-factor increases with the increase of cls, while the decrease of CE will lead to a lower R-factor. It can also be seen that when cls is larger than cc, the energy density can be effectively improved by using lithium sources. The analysis of these results can add more detailed parameters for different systems.

Pre-lithiation strategies

Add lithium source to anode

The initial ALL is caused by an irreversible electrochemical process on the anode, so the most direct strategy to eliminate the initial ALL is to prepare a lithium-preintercalated anode through electrochemical and chemical strategies before pairing with the cathode.

Strategies on the anode can be divided into three categories: half-cell electrochemical method (HC-EM), short-circuit electrochemical method (SC-EM), and chemical method (CM) as shown in Figure 3. After the lithium is pre-intercalated in the cathode, the problem of high initial ALL can be well solved, and the first cycle CE of the entire battery can be effectively improved.

HC-EM

HC-EM is a pre-lithiation strategy widely used in laboratory research, which can be achieved by a half-cell structure with the anode material as the cathode and lithium metal as the anode. After the pre-intercalation of lithium is completed, the anode of pre-intercalated lithium is removed from the half-cell and reassembled with the cathode to form a complete battery. The most important advantage of this strategy is the simplification at the laboratory scale, which can be controlled by current and voltage termination after pre-intercalation of lithium.

SE-EM

In order to avoid the complicated operation and extensive use of electrolyte caused by HC-EM, a strategy of directly contacting the anode with lithium metal foil is proposed, and the end point of pre-intercalation of lithium is determined by the battery voltage or pre-intercalation time of lithium in the whole process. Compared with HC-EM, SC-EM can not only generate SEI with very similar characteristics on the surface during the lithium-pre-intercalated process, but also without sacrificing the structural stability of the anode.

CM

In recent years, various chemical methods have been proposed to directly produce pre-intercalated lithium materials as anode candidates to match cathode, and as the result, the pre-intercalated lithium are more stable than those produced by electrochemical pre-intercalated lithium strategies. However, the electrode preparation of lithium-pre-intercalated anode materials obtained by this method is very difficult.

Since the lithium-pre-intercalated anode materials have high chemical activity, anhydrous solvents must be used and carried out in a dry atmosphere. Similar to HC-EM and SC-EM, CM still requires a dry atmosphere to assemble the whole battery due to the chemical instability of the pre-lithiation anode. Therefore, the method of adding lithium source to the anode faces the challenge of harsh conditions, which will inevitably increase the manufacturing cost.

Add lithium source to cathode

The high reactivity of lithium-pre-intercalated anodes cannot be effectively solved, making large-scale applications difficult to achieve. Therefore, researchers have also made great efforts to preload the lithium source to the cathode to alleviate the initial ALL during the first cycle of charging. Under this strategy, all methods can be mainly divided into two categories, that is, the preparation of lithium-pre-intercalated cathode materials with additional Li+ (OL-C) or the addition of lithium-containing additives LA-C in the cathode to reduce the initial ALL.

OL-C

Excessively lithium-pre-intercalated cathodes (also known as lithium storage) refer to materials that release additional Li+ during charging to mitigate initial ALL during the first cycle of charging. Anode materials with excessively preintercalated Li can deliver more Li+, which are stored in unoccupied crystallographic sites, such as typical cathode materials Li1+xMn2O4, Li1+xMn1.5Ni0.5O4 and Li3+xV2(PO4)3, etc. Li1+xMn1.5Ni0.5O4 is a novel cathode material with excessively pre-intercalated lithium, which can store excess Li+ at potentials below 3 V.

The OL-C can provide additional Li+ by itself to alleviate the initial ALL during the first cycle of charging, that is, there is no need to add additional materials (weight) in the anode to reduce the energy density of the device. However, OL-C has a relatively low providing ability of lithium ions and cannot fully alleviate the initial ALL of the anode, which limits its application field in LIBs.

LA-C

LA-C materials need to meet the following characteristics.

Firstly, a good cathode additive should have a higher lithium storage capacity by weight and volume than the existing cathode materials.

Secondly, the charging potential of the additive must be lower than the maximum cathode potential, and the discharge potential of the additive must be lower than the minimum cathode discharge potential.

Thirdly, the anode lithium-pre-intercalated additive should not have a negative impact on the electrode material, electrolyte and the stability of the whole battery.

Fourthly, the cathode lithium-pre-intercalated additive should be stable under environmental conditions and compatible with existing industrial battery manufacturing processes. Compared with OL-C, LA-C has a relatively high capacity. However, after the release of Li+, the remaining material will be inert material or even insulating material, which has a negative impact on the specific capacity of the system.

Add lithium source in the battery manufacturing process

At present, the pre-lithiation method of cathode and anode still faces the problem of chemical instability, which hinders its practical application. Therefore, the method of adding lithium source in the battery manufacturing process is proposed. In this method, a battery is assembled from an anode, a cathode and a diaphragm that are not pre-intercalated with lithium, and then a lithium foil is attached to the very edge of the battery.

When a lithium metal is connected to an anode that is not pre-intercalated with lithium, pre-lithiation of the anode begins.This method is relatively practical for pre-lithiation, where the degree of impingement can be well controlled by the potential of the anode, while the integrity of the electrode can be maintained throughout the pre-lithiation, and only one step of foil assembly can be performed in the drying chamber.

However, there are still some problems to be solved:

● Pre-lithiation efficiency is low, that is, complete pre-lithiation requires a long time, and the electrode with through-hole significantly increases the cost of the battery.
● Meanwhile, the degree of lithium embeddedness of each electrode is not uniform, so it takes a long time to balance the degree of lithium embeddedness.

Add additional lithium source

In addition to the above methods, the addition of additional lithium sources is an effective way to achieve efficient pre-lithiation without the use of through-hole electrodes. Stable Lithium Metal Powder (SLMP), manufactured by FMC Corporation (USA), is a special lithium source with a particle size of 10-20 µm that can be safely handled in dry air. In the pre-embedding process, the reaction between the SLMP and the electrolyte is minimized due to the inert Li2CO3 coating.

The SLMP can be used as a lithium source to achieve preinsertion of lithium after pressure activation. But because the particles are so small, it’s difficult to achieve a practical level of uniform dispersion on the electrode during the battery manufacturing process, leading researchers to look for better ways to incorporate SLMP into the electrode. In practical application, the method of using additional lithium sources is considered to be an operational pre-lithiation process.

However, there are still some challenges with this method:
● Even though several dispersion methods have been proposed, the dispersion problem of SLMP has not been solved.
● Since the SLMP purity is relatively low (< 98%), the method may cause short circuit.
● The SLMP with small particle size is difficult to control, and has the potential danger of explosion.
● Ultra-thin lithium films are a better option than SLMP, but are still impractical for mass production.

Comparison of representative pre-lithiation strategies

This article summarizes four types of pre-lithiation strategies, namely adding lithium source to the cathode, adding lithium source to the anode, adding lithium source in the battery manufacturing process, and adding additional lithium source. The figure below compares the main properties of these pre-lithiation strategies, including their operating conditions, controllability, practical potential, and impact on the energy density of the battery.

Overall, all the four pre-lithiation strategies still face different challenges. The pre-embedding of lithium source in the anode has excellent controllability and has no negative impact on the energy density of the full battery, making it suitable for use in laboratory research. Lithiated anodes and anode materials are very sensitive to water, thus requiring strict assembly conditions in all fabrication processes and making it difficult to achieve large-scale applications.

However, if the sensitivity of chemical lithium intercalation materials to water can be solved, the strategy of adding lithium sources to the anode still has the potential for commercial application.

Information sources

Liming Jin, et al. Pre-lithiation Strategies for Next-Generation Practical Lithium-Ion Batteries, Advanced Science, 2021, DOI:10.1002/advs.202005031.Liming Jin, Junsheng Zheng, Jim P. Zheng. Theoretically Quantifying the Effect of pre-lithiation on Energy Density of Li-Ion Batteries[J]. Journal of The Electrochemical Society, 2021, 168(1): 010532.