During the first charge and discharge of the lithium battery, a solid electrolyte phase interface membrane (SEI) will be formed on the surface of the negative electrode material, permanently consuming lithium from the positive electrode, resulting in low initial Cullen efficiency (ICE) and energy density. In particular, in the process of delamination/embedded lithium of silicon-based materials, the volume of silicon changes greatly, which is easy to cause structural collapse and capacity attenuation, resulting in the instability of the solid electrolyte interface membrane, and the continuous format ion and destruction of the SEI membrane will continue to consume lithium ions. The stable SEI membrane is the main factor to extend the battery cycle life, so Silicon-based materials still face huge challenges.
For the above problems, the most effective solution is to use pre-lithium technology to add a small amount of lithium sources before the electrode is officially charged and discharging the cycle to make up for the excessive consumption of lithium in the reaction. It supplements the side reaction and cathode lithium consumption during the formation of the SEI membrane, which reduces the volume expansion to a certain extent and improves the overall performance of lithium-ion batteries. This paper summarizes the research progress of pre-lithium technology in the positive and negative electrodes of batteries, summarizes the challenges and advantages of various cutting-edge methods, and looks forward to the future development direction of pre-lithium technology.
Positive pre-lithium
The technology of using metal lithium powder and lithium foil to directly supplement the negative electrode is relatively mature, but safety issues and high costs are still a major obstacle to its commercialization. In contrast, the positive lithium supplement process is safe, has the advantages of simple operation and low cost, and is compatible with the existing process. The disadvantage is that the technical maturity is low. Positive pre-lithium usually adopts the chemical synthesis method to add lithium sources in the process of synthesis materials. This method is suitable for commercial applications, but how to find a stable lithium source is the direction to break through now. The following are some main methods of positive lithium supplementation additive.
1. Lithium-rich additives are used as pre-lithium reagents
The concept of cathode pre-lithium comes from Giuliob Gabrielli. However, so far, no reports have been found that this method can be applied to other materials, so the practical value is not very high. For example, LiNiO2, Li2CuO2 and Li2CoO2 are also a commonly used lithium-rich additives. However, the surface of LiNiO2 is unstable in the air, and its surface will react with carbon dioxide and water in the air to form lithium carbonate and lithium hydroxide. The lithium sources of Li2CuO2 and Li2CoO2 in the preparation process are usually LiOH and Li2CO3. LiOH is unstable in the air, while lithium carbonate produces gas during the preparation of the battery, which affects the battery performance. Kim and others used isopropyl alcohol aluminum to modify LiNiO2 and synthesized LiNiO2 material coated with stable alumina in the air with excellent lithium supplementation effect. However, as a cathode pre-lithium reagent, these lithium-rich transition metal oxides have transition metal oxide residues after melting, resulting in a slight decrease in the energy density of the battery. The above shortcomings seriously hinder its practical application as a cathode pre-lithium reagent.
2. Binary lithium compounds
The lithium supplement effect of this kind of positive lithium supplementation additive is much higher than that of lithium-rich compounds, and a small amount of such additives can complete the compensation for the first irreversible capacity loss of the battery. The theoretical ratio capacity of commonly used Li2O2, Li2O and Li3N reaches 1168mAh/g, 1797mAh/g and 2309mAh/g respectively. Theoretically, the residues of these materials after lithium supplementation are O2, N2, etc., which can be discharged from the gas during the formation of the SAI film.
Li2O and Li2O2 have been reported as cathode pre-lithium reagents, which are decomposed into O2 after pre-lithium. Abouimrane et al. studied the micron-sized Li2O as a positive lithium supplement additive. Bie and others used a mixture of commercial Li2O2 and NCM to compensate for the lithium loss during the first charging of the graphite anode. Although Li2O and Li2O2 are compatible with the conventional binder PVDF using NMP solvents. However, Li2O and Li2O2 need to be activated at a high voltage of 4.7V to pre-lithium, which may lead to serious decomposition of the electrolyte. Park and others grind the commercial Li3N into powder with a particle size of 1µm to 5µm, which is used as a lithium supplement additive. Li3N is stable in dry air, but unstable in humid air and is incompatible with the current slurry process based on polar solvents (i.e. NMP and water), so it is difficult to achieve commercial application.
Li2O and Li2O2 have been reported as cathode pre-lithium reagents, which are decomposed into O2 after pre-lithium. Abouimrane et al. studied the micron-sized Li2O as a positive lithium supplement additive. Bie and others used a mixture of commercial Li2O2 and NCM to compensate for the lithium loss during the first charging of the graphite anode. Although Li2O and Li2O2 are compatible with the conventional binder PVDF using NMP solvents. However, Li2O and Li2O2 need to be activated at a high voltage of 4.7V to pre-lithium, which may lead to serious decomposition of the electrolyte. Park and others grind the commercial Li3N into powder with a particle size of 1µm to 5µm, which is used as a lithium supplement additive. Li3N is stable in dry air, but unstable in humid air and is incompatible with the current slurry process based on polar solvents (i.e. NMP and water), so it is difficult to achieve commercial application.
3 Nanocomposites of reverse reaction
M/Li2O (M=Fe, Co, Ni, Mn, etc.) nanocomposites prepared by MxOy in reaction with molten Li metal are also reported as cathode pre-lithium reagents. M/Li2O nanocomposites have a high theoretical specific capacity. Co/Li2O composites of different scales show different pre-lithium capabilities. The de-lithium potential of nano-Co/Li2O composites is lower than that of micron-level and sub-micron-level Co/Li2O composites, and has a strong dissolution capacity. This is because the close contact between nano-Co and Li2O is conducive to the release of Li. The specific capacity of the first charge ratio of nano Co/Nano Li2O composites synthesized by Sun and others is 619mAh/g; after 8 hours of exposure in ambient air, the loss is only 51mAh/g, indicating that the nano Co/Nano Li2O has good environmental stability. Similarly, LiF and Li2S are also excellent cathode lithium supplement materials. Synthetic M/LiF nanomaterials can improve the problem of low LiF conductivity and ion conductivity. Although the theoretical capacity of Li2S reaches 1166mAh/g, there are still many problems when used as a lithium supplement additive, such as compatibility with electrolyte: the reaction between intermediate polysulfide and carbonate-based solvents is incompatible with the carbonate-based electrolyte in existing commercial lithium-ion batteries. Insulation, strong toxicity, will react with moisture in the environment, which hinders the practical application of M/Li2S composite materials. In a word, due to the use of molten lithium metal in the preparation of M/Li2O, M/LiF and M/Li2S nanocomposites, it is still necessary to explore simple and safe material synthesis methods for large-scale application. In addition, these nanocomposites leave a large amount of residues after pre-lithium, which will also reduce the energy density of the battery and may adversely affect the performance of the battery. Although the cathode pre-lithium reagent has a high redox potential, its stability is better than that of the anode pre-lithium reagent. Using the existing NMP solventbased slurry electrode preparation technology, a variety of cathode pre-lithium reagents can be evenly dispersed into the cathode. However, the release of gas in the pre-lithium process and the residue of metal oxides after lithium ions are provided, which have brought many obstacles to the practical application of cathode pre-lithium reagents.
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