In recent years, with the rapid development of new energy vehicles, increasingly higher requirements have been imposed on various performance aspects of lithium-ion batteries, with the enhancement of battery energy density being the most urgent.
In the current lithium-ion battery system, on the one hand, energy density can be improved by optimizing battery structure, such as through technologies like CTP, CTC, and CTB. On the other hand, through the iteration of positive and negative electrode materials, such as the use of high-nickel ternary cathodes, high-voltage nickel-manganese materials in the positive electrode, and high-capacity silicon and tin-based alloy negative electrodes, a significant increase in battery energy density can be achieved.
Furthermore, lithium replenishment technology is also an important means to improve battery energy density. During the initial charging process of lithium-ion batteries, the organic electrolyte will undergo reduction and decomposition on the surface of the negative electrode material, such as graphite, forming a Solid Electrolyte Interface (SEI) film. This permanently consumes a large amount of lithium from the positive electrode, leading to a low initial cycle efficiency (ICE) and reducing the capacity and energy density of lithium-ion batteries.
Additionally, processes such as the deactivation of negative electrode material particles due to detachment and the irreversible deposition of lithium metal will also consume active lithium from the positive electrode, further lowering the battery’s capacity and energy density.
Lithium replenishment, also known as “pre-lithiation” or “pre-insertion of lithium,” involves adding lithium to the interior of the lithium-ion battery before it operates to supplement lithium ions. By replenishing lithium through pre-lithiation on the electrode materials, irreversible lithium losses are offset, enhancing the overall capacity and energy density of the battery.
Currently, the irreversible capacity loss of the widely used graphite negative electrode is greater than 6%. For negative electrodes based on high-capacity silicon and tin alloys, the irreversible capacity loss can be as high as 10% to 30%. Coupled with lithium replenishment technology, this can address the shortcomings of low initial efficiency, fully leveraging their high-capacity advantages.
Lithium replenishment technology is classified into negative electrode replenishment and positive electrode replenishment.
Negative electrode replenishment technology has been under research and development for a longer time and includes various replenishment methods such as physical mixing replenishment based on metallic lithium, vacuum winding plating replenishment, self-discharge lithiation, chemical replenishment, and electrochemical lithiation. Currently, negative electrode replenishment is still constrained by several challenges in battery manufacturing processes, including issues related to the use and production environment of metallic lithium, conventional solvents, binders, air, and heat treatment processes, making the road to replenishment for the negative electrode fraught with difficulties.
Positive electrode replenishment is typically accomplished through electrochemical methods. This involves adding replenishment materials to the positive electrode of lithium-ion batteries, where the replenishment material decomposes and releases active lithium during the battery charging process, compensating for the irreversible active lithium loss caused by the growth of the negative electrode’s Solid Electrolyte Interface (SEI). Positive electrode replenishment materials exhibit stable chemical properties, ease of synthesis, low cost, and high replenishment capability. Simultaneously, the positive electrode replenishment process can be well-compatible with existing lithium-ion battery manufacturing processes, providing a new solution for the commercial application of replenishment technology.
I. Negative Electrode Replenishment
Negative electrode replenishment involves introducing active lithium into the negative electrode to compensate for capacity loss caused by SEI growth. The main methods for negative electrode replenishment include physical mixing, vacuum winding plating, chemical lithiation, self-discharge mechanism lithiation, and electrochemical lithiation.
- Physical Mixing Lithiation
In the early stages, researchers directly pressed lithium foil onto the surface of the negative electrode to compensate for active lithium loss, simultaneously improving initial Coulombic efficiency and cycle life. In 2003, Kulova and others directly pressed lithium foil onto the surface of graphite negative electrodes to compensate for capacity loss, indicating that the reduction of irreversible capacity loss depends on the mass ratio of metallic lithium to graphite. Subsequently, they used the same method to compensate for capacity loss in amorphous Si.
In 2019, Tongji University reported a roll-to-roll negative electrode sheet pre-lithiation method that could be applied in batches. They rolled metallic Sn foil and metallic lithium foil together, and under the mechanical force, alloying reactions occurred on the surface of Sn foil, forming LixSn. This pre-lithiated Sn foil maintained good stability in the air, with a slight change in color after 48 hours of exposure to normal environmental conditions and still retaining 90% of the initial capacity after 12 hours of exposure to 79% humidity air. Assembled LFP|Sn batteries with pre-lithiated Sn foil achieved an initial Coulombic efficiency of 94%, with stable cycling for 200 cycles. This replenishment method is also applicable to Al foil and conventional silicon-carbon negative electrode sheets.
CATL Patent: Lithium Powder Rolling Device and Method for Electrode Sheet, Lithium Powder Rolling Device for Electrode Sheet, comprising an unwinding mechanism, a winding mechanism, a roller mechanism, and a wrapping roller; the roller mechanism is positioned between the unwinding mechanism and the winding mechanism, and the wrapping roller is positioned between the unwinding mechanism and the winding mechanism. The roller mechanism includes an upper-pressure roller and a lower-pressure roller coordinated with the upper-pressure roller, and the unwinding mechanism includes an unwinding roller. The upper edge of the wrapping roller is higher than the upper edge of the unwinding roller and the tangent line of the lower edge of the upper-pressure roller. The present invention also provides a method for rolling lithium powder on the electrode sheet.
- Vacuum Winding Plating Lithiation Utilizing vacuum coating and automated design for roll-to-roll cycling operation. This method can achieve uniform lithiation and large-scale batch production but is still a distance away from commercialization, requiring market demand to drive it further.
Adding lithium-rich materials during stirring is the safest lithium replenishment method. The use of lithium powder or lithium foil is relatively dangerous and expensive, with the price of lithium foil ranging from 100,000 to 500,000 per roll. Currently, vacuum winding vapor deposition for lithium replenishment is focused on the passivation of the lithium metal surface and the inhibition of lithium dendrite growth.
Companies involved in the layout of winding plating lithium equipment include Jia’s Automation, Xiandao Intelligent, Xinjiatuo, etc. Due to variations in the processes of different power battery companies, equipment needs to be adjusted to varying degrees. Currently, most of these processes are still in the research and trial production stages and have not been fully implemented.
- Chemical Lithiation Chemical lithiation involves a chemical reaction between a low-potential lithium-containing chemical reagent (replenishing agent) and the negative electrode material to achieve reduction and lithium replenishment. Common replenishing agents include lithium powder, molten lithium, lithiated silicon powder, LiOH at high temperatures, thermally evaporated lithium, lithium-organic composite solution, etc. The chemical stability of the replenishing agent in this method is poor, and it is incompatible with polar solvents and air, requiring encapsulation for improved stability. FMC Corporation in the United States was the first to develop stabilized lithium metal powder (SLMP) products, which are added to the negative electrode through processes such as spraying or slurry coating for replenishment.
- Self-Discharge Lithiation Self-discharge mechanism lithiation involves direct contact between the negative electrode and lithium foil in the presence of an electrolyte, resulting in spontaneous lithium insertion through a thermodynamic reaction. Self-discharge mechanism lithiation does not alter the morphological characteristics of the active substance, making it suitable for studying the relationship between morphology and electrochemical performance. Liu et al. directly brought the Si negative electrode into contact with lithium metal foil in the presence of electrolyte and slight pressure, achieving spontaneous insertion of a capacity of 2000 mA·h/g after 20 minutes.
Electrochemical prelithiation is a common method for prelithiating the negative electrode sheet of lithium-ion batteries. In the current lithium-ion electrochemical system, prelithiation of the negative electrode sheet can be achieved by introducing metallic lithium with the negative electrode to control the depth of electrochemical charge and discharge. However, this method involves battery pre-assembly and disassembly, leading to a complex battery preparation process.
Huawei’s patent provides a method for preparing a prelithiation agent (Publication number: CN112542581A). The method involves selecting an active material to prepare an electrode as a working electrode, incorporating metallic lithium as the counter electrode, adding an electrolyte to assemble the battery. The battery is then subjected to discharge treatment to lithiate the working electrode. Finally, the battery is disassembled, and the material obtained after the transformation of the active material is separated and collected. After cleaning and drying, the prelithiation agent material can be obtained.
Tesla’s patent on pre-doped anodes and methods and devices for manufacturing them involves coating metallic lithium powder or a mixture containing lithium powder on the surface of the negative electrode and replenishing lithium through electrochemical lithiation.
In December 2021, Xiaomi achieved the application of power battery-level high-silicon replenishment technology for the first time in mobile phones. The silicon content increased threefold, combined with upgraded packaging technology, resulting in a 10% increase in battery capacity and a 100-minute improvement in endurance under the same volume. The replenishment technology for electrode sheets was applied to mobile phone batteries for the first time. This technology compensates for the initial capacity loss of the battery by directly supplementing the lost lithium during the activation process through the composite ultra-thin lithium foil on the surface of the negative electrode sheet.
II. Positive Electrode Replenishment
Compared to the complex and less secure negative electrode replenishment, positive electrode replenishment materials can be directly added during the slurry coating process of the positive electrode without additional process improvements and at lower costs. Therefore, positive electrode replenishment is considered the most promising replenishment technology for current lithium-ion battery manufacturing processes.
From an application perspective, perfect positive electrode replenishment agents should meet the following four basic requirements:
- The irreversible delithiation process of positive electrode replenishment materials should be within the working voltage range of the positive electrode, meaning its delithiation potential is lower than the upper voltage limit of the positive electrode material, and its lithium intercalation potential is lower than the lower voltage limit of the positive electrode material.
- Replenishment materials should exhibit sufficiently high specific energy and volumetric energy density, typically with irreversible capacity greater than 350 mA·h/g to meet efficient prelithiation.
- Positive electrode replenishment materials should be compatible with the current production processes and battery systems. During electrode sheet production, they should not react with NMP, binders, etc., and exhibit no adverse reactions with the electrolyte during the cycling process. After the first cycle, their decomposition products should not affect battery cycling.
- Positive electrode replenishment materials should have good environmental stability and remain stable in air or relatively dry environments.
Generally, positive electrode replenishment agents can be classified into three main types:
- Binary lithium-containing compounds, such as Li2O, Li2O2, Li3N, Li2S, etc. Their surfaces are usually coated with carbon or used in combination with metal nanoparticles to catalyze the release of lithium ions from replenishment materials.
- Lithium-rich compounds, such as lithium-rich iron phosphate (Li5FeO4), lithium nickelate (Li2NiO2), etc.
- Lithium composites, such as Li2S/Co, LiF/Co, Li2O/Co, etc.
- Replenishment Agent – Binary Lithium-Containing Compounds
Li2O, as a replenishment agent, has high capacity but poor conductivity. Additionally, it may lead to the dissolution of metallic lithium at high potentials, affecting battery performance. Guoxuan High-Tech has developed a method for preparing rGO@Li2O/Co nanocomposites based on a conversion reaction (CN112290022A). Li2O/Co nanoparticles adhere to the surface of graphene, enhancing conductivity.
Yiwei Li Neng has obtained a patent (CN111193019A) for encapsulating Li2O and metal M in a shell of SiOx and carbon, improving the stability of the material and reducing its dissolution. This process is compatible with existing lithium-ion battery processing and manufacturing techniques.
In another patent by Huawei (Publication number: CN112542589A), a method for preparing positive electrode prelithiation material is described. By improving the overall process, reaction raw material composition, and ratio, the patent generates a prelithiation agent mainly composed of Me elemental substance, LiF, and Li2O. Adding this positive electrode prelithiation additive to the positive electrode can enhance the capacity and energy density of lithium-ion batteries.
Li2O2 can also be used as a positive electrode replenishment material. When fully delithiated, it produces oxygen, corresponding to a theoretical specific capacity of up to 1167 mA·h/g. It serves as sacrificial lithium salt to compensate for irreversible capacity loss in lithium-ion batteries. However, Li2O2 has low electrochemical activity, requiring a high decomposition potential. Catalysts are needed to reduce the decomposition potential.
Tesla’s patent (combination and method for preparing an energy storage device with prelithiation, Publication number: not provided) utilizes Li2O, Li2O2, Li2S, Li3N, LiN3, LiF, Li5FeO4, Li2NiO2, Li6CO4, and Li2MoO3, or their combination, as prelithiation materials to replenish lithium. The dry electrode preparation method provides a unique approach to incorporate prelithiation materials into the electrode film without exposure to solvents.
Li3N is an ion-conductive material with a capacity of 1400 mAh/g. It is thermodynamically unstable and exhibits ion conductivity but electronic insulation. When dispersed as an additive inside the electrode, it affects rate performance. Coating Li3N on the electrode surface has no impact on rate performance. However, Li3N particles are often too large to exhibit electrochemical activity and require grinding for use.
Li3N undergoes oxidation decomposition with no residue:（Li3N= N2+Li）,Li3N is stable in dry air and reacts with polar solvents: Li3N+H2O=LiOH+NH₃.
Li2S is the lithiated state of S, offering a theoretical specific capacity of 1167 mA·h/g when fully delithiated. Sun et al. synthesized Co/Li2S nanocomposites through a conversion reaction in an inert atmosphere. This structure, with a low decomposition potential below 3 V and good stability in the environment, exhibits a high irreversible capacity of 670 mA·h/g.
To explore the role of Li2S as a positive electrode lithium supplement material, carbon coating was applied to the surface of Li2S to produce Li2S/KB composite nanoparticles. Combined with ethanol and PVP, it was formulated into a lithium supplement slurry. This lithium supplement material exhibited a lithium supplement capacity of up to 1053 mA·h/g in the potential range of 2.5 to 3.6 V in a carbonate-based electrolyte, which is generally applicable to all traditional lithium-ion batteries.
The lithium supplement slurry was directly coated onto the surface of the positive electrode LiFePO4 electrode and assembled with a Si-C composite negative electrode to form a full cell. After lithium supplementation with the core-shell structure Li2S/KB, approximately 20% of the irreversible capacity loss in the LiFePO4(Li2S)|Si-C full cell was completely recovered in the first week. The cell exhibited excellent cycling performance and rate capability, with a discharge capacity of 150 mA·h/g after 200 cycles and a capacity retention of nearly 100%.
2 Lithium Supplement Agents—Lithium-Rich Compounds
Li2NiO2 is a commonly used lithium-rich compound for positive electrode lithium supplementation. However, its drawbacks cannot be ignored: 1) poor stability—Li2NiO2 has poor stability in the air, and its structure is unstable, prone to side reactions with the electrolyte at high potentials; 2) high impedance—the addition of Li2NiO2 can affect the cycling performance and rate capability of the battery. Reasonable modification of Li2NiO2 can enable it to exhibit excellent lithium supplementation effects. For example, M. G. Kim et al. used isopropanol aluminum to modify Li2NiO2, synthesizing Li2NiO2 material coated with stable alumina in the air, resulting in outstanding lithium supplementation effects.
Theoretically, each mole of Li5FeO4 can provide 5 Li+, and the specific capacity can reach up to 867 mAh/g. By incorporating a certain amount of Li5FeO4 into traditional positive electrode materials, the first-cycle efficiency and energy density of lithium-ion batteries can be significantly improved. The current focus of technical research is to enhance the chemical stability and conductivity of Li5FeO4 in ambient air.
3 Lithium Supplement Agents—Lithium Compounds
Li2S/Co exhibits high capacity of 670 mAh/g, improved conductivity, and stability. The composite structure can fix intermediate polysulfide products, preventing them from undergoing irreversible reactions with carbonate electrolytes. However, there is a trace amount of non-active CoS2 residue after delithiation.
Preparation method: CoS2 + 4Li → Co + 2Li2S. Co nanoparticles are uniformly embedded in the Li2S matrix. Lithium release during charging: Co + 2Li2S → CoS2 + 4Li+ + 4e-
Prepared at 240°C using a chemical method: CoF3 + 3Li → Co + 3LiF. Co nanoparticles are uniformly embedded in the LiF matrix. Lithium release during charging: Co + 3LiF → CoF3 + 3Li+ + 3e-. Lithium release capacity is 516 mAh/g, with non-active CoF3 residue after delithiation.
Chemical method for Li2O/Co preparation: Co3O4 + 8Li → 3Co + 4Li2O2. When Co particles are large, the composite exhibits higher charging voltage and lower capacity, indicating poor contact between large Co and Li2O particles. 3. Nanoscale Li2O/Co has a capacity of 619 mAh/g. Conversion reaction: 3Co + 4Li2O → Co3O4 + 8Li+ + 8e-. However, the non-active Co3O4 residue not only increases weight but also leads to side reactions.
The development of lithium supplementation technology has gradually shifted from scientific research to industrial application over the past 20 years. Lithium supplementation technology based on metallic lithium for negative electrode has received the most attention and research due to its high lithium supplementation capacity, clear principles, and processes. However, hindered by the safety risks associated with metallic lithium, the industrial application of negative electrode lithium supplementation technology has progressed slowly. Future research will focus on overcoming the safety risks in the manufacturing and usage of lithium supplementation batteries.
Positive electrode lithium supplementation technology, proposed as a new solution for lithium-ion battery lithium supplementation, has clear advantages in safety and process convenience compared to negative electrode lithium supplementation. However, there are still some challenges, such as low decomposition potential and poor stability of materials like Li3N and Li2O, leading to gas generation and safety hazards affecting battery performance. Materials like Li5FeO4 and Li6CoO4 exhibit high irreversible capacity, good environmental stability, but increase electrode mass with high decomposition voltage, making them incompatible with most positive electrode materials.
Positive electrode lithium supplementation technology needs to develop a positive electrode lithium supplementation material with stable chemical properties, low decomposition potential, no gas generation, high delithiation capacity, and reversible lithium intercalation. Simultaneously, it should be compatible with existing lithium-ion battery manufacturing processes.
In the context of slow development in the lithium-ion battery material system, the development of high-safety, low-cost lithium supplementation technology to enhance the energy density and cycle life of lithium-ion batteries is crucial. Lithium supplementation technology, as a means to address the shortcomings of lithium-ion batteries, is expected to break through the bottleneck of energy density and cycle life. It will become a key technology for the future development of lithium-ion batteries, making it a major focus of scientific research and technological development. The practical application of lithium supplementation technology will drive the collaborative advancement of various aspects of lithium-ion batteries, including materials, manufacturing, and equipment, promoting the development of the lithium-ion battery industry.