[Reason for lithium supplementation].
The most widely used graphite anode has an irreversible capacity loss of 6%, while for silicon-based and tin-based alloy anodes with high specific capacity, the irreversible capacity loss can even be as high as 10% to more than 20%.
[Lithium supplementation technology].
In the case of the addition of some high-capacity silicon-based anode materials, the coulombic efficiency and cell capacity of the cell are low in the first week of the battery. Supplementation of activated lithium is an effective means to solve this problem, and there are many ways to supplement activated lithium, mainly in the negative electrode and positive electrode.
Lithium supplementation at the negative electrode includes the physical mixing of lithium metal, such as adding lithium metal powder to the negative electrode or rolling lithium metal foil on the surface of the electrode piece; Chemical lithiation, using lithiating agents such as butyllithium to chemically pre-intercalate lithium in the anode; Self-discharge lithiation, the contact between the negative electrode and the metal lithium in the electrolyte completes the self-discharge lithiation; In electrochemical prelithiation, lithium metal is introduced into the battery as the third electrode, and the negative electrode and the lithium metal third electrode are charged and discharged to complete the pre-lithiation.
According to the different types of compounds, it can be divided into binary lithium-containing compounds represented by Li2O, Li2O2 and Li2S, ternary lithium-containing compounds represented by Li6CoO4 and Li5FeO4 and organic lithium-containing compounds represented by Li2DHBN and Li2C2O4. The application of lithium supplementation technology not only improves the capacity of lithium-ion batteries, but also improves the cycle life of silicon-containing anode batteries.
[Classification of lithium supplementation].
1) Lithium foil type lithium supplementation.
The pre-lithiation method of the roll-to-roll negative electrode piece is applied in batches, the metal SN foil and the metal lithium foil are rolled against the roll, and the lithium metal undergoes an alloying reaction with the surface layer of the SN foil under the action of mechanical force to form LixSn, the pre-lithiated SN foil maintains good stability in the air, the surface of the pre-lithiated tin foil is slightly discolored after 48 h of exposure to normal environment, and the pre-lithiated tin foil can still maintain 90% of the initial capacity after 12 h of exposure to 79% humidity air. Pre-lithiated SN foil assembly lfp|The coulombic efficiency of the SN battery reaches 94% in the first week, and it can be stably cycled for 200 cycles. This method of lithium supplementation is also suitable for AL foil and conventional silicon-carbon anode electrodes.
2) Lithium powder supplementation.
Drip emulsion technology is a very effective method for synthesizing stabilized lithium powder, the principle is shown in the figure, the surface of the lithium powder is coated with a stable layer of lithium powder, the composition of the stable layer is lithium carbonate and lithium fluoride, the results show that the lithium powder containing the stable layer can not only improve the cycle life of the lithium anode, but also can effectively inhibit the formation of lithium dendrites. Stabilized Lithium Metal Powder (SLMP) is a commercial product manufactured by FMC Corporation with a core-shell structure consisting of 97% lithium and 3% lithium carbonate cladding. The lithium carbonate protective layer is evenly coated on the surface of the lithium powder, which prevents the adverse side effects of the lithium powder, so that the SLMP can be used in a dry environment without having to be in an inert atmosphere. SLMP has a specific capacity of up to 3600 mA·h g, and its use for lithium supplementation at the anode can not only increase the capacity of lithium-ion batteries, but also improve their first-week storage capacity efficiency and cycle life.
3) Electrochemical lithium supplementation.
In the existing lithium-ion electrochemical system, the pre-lithiation of the negative electrode piece can be completed by introducing lithium metal and the negative electrode to form a counter electrode, and controlling the electrochemical charge-discharge depth. Electrochemical prelithiation can be divided into non-situ and in-situ electrochemical prelithiation according to the implementation method. As shown in Figure A, in the conventional in-situ electrochemical pre-lithiation process, a half-cell is assembled with a metal lithium anode piece that needs to be pre-lithiated, and after a specific charge-discharge cycle, the negative electrode piece reaches a set pre-lithiation level, and then the pre-lithiated anode piece and the new positive electrode piece are assembled into a battery.
In order to simplify the process of disassembling and reassembling batteries in the non-in-situ electrochemical lithiation method, the in-situ electrochemical lithiation method was developed step by step. As shown in Figure 5b, the in-situ electrochemical lithiation method requires the redesign of the lithium-ion battery, in which lithium metal is added as the third electrode in advance during the assembly process, and the negative electrode and metal electrode are discharged as the counter electrode, and the in-situ lithium supplement for the negative electrode can be realized. Firstly, the hard carbon anode and the metal lithium electrode were discharged as the counter electrode to supplement the lithium for the hard carbon anode, and then the NCM cathode and the lithium hard carbon anode were combined to form an electrochemical system for charging and discharging.
4) Lithium supplementation at the cathode.
The cathode lithium supplement material can be added directly in the homogenization process of the cathode slurry without additional process improvement and low cost, so it is more suitable for the current lithium-ion battery manufacturing process and is known as the most promising lithium supplement technology. From the perspective of application, the perfect cathode lithium supplement material needs to meet the following four basic requirements:
The irreversible delithiumization process of the cathode lithium supplement material should be within the working voltage range of the cathode, that is, its delithiumization potential is lower than the upper voltage limit of the cathode material, and the lithium intercalation potential is lower than the lower voltage limit of the cathode material.
Lithium-supplemented materials should exhibit sufficiently high specific energy and volumetric energy densities, usually with irreversible capacities greater than 350 mA·h g to meet the requirements of efficient pre-lithiation.
The cathode lithium supplement material should be compatible with the current general manufacturing process and battery system, and will not react with NMP and binders during the production of the electrode piece, and there will be no adverse side reactions with the electrolyte during the cycle process, and its decomposition products will not affect the battery cycle after the first cycle cycle;
The cathode lithium supplement material has good environmental stability and can remain stable in air or dry environment.
Note: Reference [Lithium-ion battery supplementation technology].