Lithium battery pre lithiation technology (II): positive electrode pre lithiation

During the first charge and discharge of lithium battery, solid electrolyte phase interface facial mask (SEI) will be formed on the surface of anode material, which will permanently consume lithium from anode, resulting in low initial coulomb efficiency (ICE) and low energy density. In particular, in the process of lithium removal/insertion of silicon based materials, the volume of silicon changes greatly, which is easy to cause structural collapse and capacity degradation, leading to instability of the solid electrolyte interface facial mask, and the continuous formation and destruction of the SEI film will continue to consume lithium ions. The stable SEI film is the main factor to extend the cycle life of the battery, so silicon based materials still face huge challenges.
The most effective solution to the above problem is to use pre lithiation technology to add a small amount of lithium source before the electrode undergoes formal charge and discharge cycles, to compensate for the excess lithium consumed in the reaction. Supplementing the side reactions during SEI film formation and the consumption of cathode lithium to some extent reduces volume expansion and improves the overall performance of lithium-ion batteries. This article reviews the research progress of pre lithiation technology on 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 lithiation technology.

Positive electrode pre lithiation
The technology of directly supplementing lithium on negative electrodes using metal lithium powder and lithium foil is relatively mature, but safety issues and high costs are still major obstacles to its commercialization. In contrast, the positive electrode lithium replenishment process has good safety, advantages such as simple operation and low cost, and is compatible with existing processes. The disadvantage is that the technology maturity is relatively low. Positive electrode pre lithiation is usually achieved through chemical synthesis, which involves adding a lithium source during the material synthesis process. This method is suitable for commercial applications, but finding a stable lithium source is currently a breakthrough direction. The following are several main methods for adding lithium to positive electrodes.
1. Rich lithium additive used as pre lithiation reagent
The concept of positive electrode pre lithiation comes from Giuliob Gabrielli, however, so far there have been no reports that this method can be applied to other materials, so its practical value is not very high.
LiNiO2, Li2CuO2, and Li2CoO2 are also commonly used lithium rich additives. However, LiNiO2 is unstable in air and its surface reacts with carbon dioxide and water in the air to produce lithium carbonate and lithium hydroxide. The lithium sources for Li2CuO2 and Li2CoO2 during the preparation process are usually LiOH and Li2CO3. LiOH is unstable in air, while lithium carbonate produces gas during battery preparation, which affects battery performance. Kim et al. modified LiNiO2 with aluminum isopropoxide and synthesized a stable aluminum oxide coated LiNiO2 material with excellent lithium replenishment effect in air. However, these lithium rich transition metal oxides, as positive electrode pre lithiation reagents, have residual transition metal oxide after melting, resulting in a slight decrease in the energy density of the battery. The above drawbacks seriously hinder its practical application as a positive electrode pre lithiation reagent.

2 binary lithium compounds
The lithium replenishment effect of such positive electrode lithium replenishment additives is much higher than that of lithium rich compounds, and a small amount of such additives can compensate for the irreversible capacity loss of the battery for the first time. The theoretical specific capacities of commonly used materials such as Li2O2, Li2O, and Li3N reach 1168mAh/g, 1797mAh/g, and 2309mAh/g, respectively. In theory, the residual substances of these materials after lithium supplementation are O2, N2, etc., which can be released as gases during the formation of SEI film in the battery.
Li2O and Li2O2 have been reported as cathode pre lithiation reagents, which decompose into O2 after pre lithiation. Abouimrane et al. studied micrometer sized Li2O as a positive electrode lithium supplement additive. Bie et al. mixed commercial Li2O2 with NCM to compensate for lithium loss during the initial charging process of graphite negative electrodes. Although Li2O and Li2O2 are compatible with conventional bonding agents PVDF using NMP solvents. However, Li2O and Li2O2 need to be activated at a high voltage of 4.7V for pre lithiation, which may lead to severe decomposition of the electrolyte.
Park et al. ground commercial Li3N into powder with a particle size of 1 μ m to 5 μ m, which was used as a lithium supplement additive. Li3N is stable in dry air, but unstable in humid air and incompatible with current slurry processes based on polar solvents (i.e. NMP and water), making it difficult to achieve commercial applications.
Nanocomposite materials with 3 reverse conversion reactions
M/Li2O (M=Fe, Co, Ni, Mn, etc.) nanocomposites prepared by reacting MxOy with molten Li metal have also been reported as cathode pre lithiation reagents. M/Li2O nanocomposites have a high theoretical specific capacity.
Co/Li2O composite materials of different scales exhibit varying pre lithiation abilities. The delithiation potential of nano Co/Li2O composite materials is lower than that of micro - and submicron Co/Li2O composite materials, and their dissolution ability is stronger. This is because the close contact between nano Co and Li2O is conducive to the release of Li. The nano Co/nano Li2O composite material synthesized by Sun et al. has a specific capacity of 619mAh/g on the first charge; After being exposed to ambient air for 8 hours, the loss was only 51mAh/g, indicating that nano Co/nano Li2O has good environmental stability.

Similarly, LiF and Li2S are excellent positive electrode lithium replenishment materials. The synthesized M/LiF nanomaterials can improve the low conductivity and ionic conductivity of LiF. Although the theoretical capacity of Li2S reaches 1166mAh/g, there are still many issues when used as a lithium supplement additive, such as compatibility with electrolytes: the reaction between intermediate polysulfides and carbonate based solvents is incompatible with carbonate based electrolytes in existing commercial lithium-ion batteries. Insulation, high toxicity, and the ability to react with moisture in the environment all hinder the practical application of M/Li2S composite materials.
In summary, due to the use of molten lithium metal in the preparation of M/Li2O, M/LiF, and M/Li2S nanocomposites, there is still a need to explore simple and safe material synthesis methods to achieve large-scale applications. In addition, these nanocomposites leave a large amount of residue after pre lithiation, which can also reduce the energy density of the battery and may have adverse effects on its performance.
Although cathodic pre lithiation reagents have higher oxidation-reduction potentials, their stability is superior to anodic pre lithiation reagents. By utilizing the existing NMP based slurry electrode preparation technology, multiple cathode pre lithiation reagents can be uniformly dispersed into the cathode. However, the release of gas and the residual metal oxide after providing lithium ions during the pre lithiation process have brought many obstacles to the practical application of cathode pre lithiation reagents.