Pre-lithiation (also known as lithium replenishment) is a technique that actively introduces additional active lithium into the electrode system before a lithium-ion battery officially participates in charge-discharge cycles. This compensates for the irreversible lithium loss that is unavoidable during the initial formation and subsequent cycles, thereby improving initial efficiency, energy density, and cycle life.
It is a crucial step that high-energy-density batteries (especially silicon-based anode batteries) cannot bypass in their transition from the laboratory to industrialization.
I. The Fundamental Problem to be Solved by Pre-Lithification: Initial Irreversible Lithium Loss
To understand why pre-lithiation is necessary, we must first understand why “active lithium is lost.”
The capacity of a lithium-ion battery is carried by the reversible insertion and extraction of Li⁺ between the positive and negative electrodes. Initially, all the Li⁺ required by the negative electrode comes from the positive electrode. However, during the initial charging formation stage, a series of irreversible side reactions occur on the surface of the negative electrode, “locking” some lithium ions:
SEI Film Formation Consumption: The electrolyte is reduced and decomposed at the low potential of the negative electrode, generating inorganic/organic products such as Li₂CO₃ and LiF, which are deposited as films. This process irreversibly consumes a large amount of active lithium and electrolyte, and is the main source of initial efficiency loss.
Electrode Structure Changes and Irreversible Lithium Intercalation: After lithium extraction from the positive electrode, the crystal structure changes, and some Li⁺ in the negative electrode (such as irreversible lithium intercalation in graphite) cannot be extracted.
Secondary losses during cycling: The SEI film repeatedly ruptures and regenerates due to volume fluctuations in subsequent cycles, continuously consuming Li⁺ and electrolyte; the high-nickel cathode also loses some reversible capacity due to irreversible changes in crystal structure and diffusion kinetics.
Low initial efficiency is essentially the price that a battery must pay to build a stable and usable negative electrode interface. The core idea of pre-lithiation is to use an external lithium source to “pay” for this start-up cost for the positive electrode, so that the Li⁺ extracted from the positive electrode is no longer wasted on the SEI and can be fully retained in subsequent reversible cycles.
II. Mechanism of Pre-lithiation: More Than Just “Lithium Replenishment”
Mechanistically, pre-lithiation’s role goes far beyond simply “replenishing lost lithium.” It’s a multi-effect coupled process:
1. Compensating for Active Lithium, Enhancing Initial Efficiency and Energy Density
Externally introduced Li⁺ is preferentially used to construct the SEI and fill irreversible lithium intercalation sites, ensuring that almost all Li⁺ in the cathode enters the reversible cell. This significantly improves initial efficiency, leading to increased usable capacity and energy density.
2. Pre-forming a Controllable SEI Film
The pre-lithiation stage artificially induces SEI formation, offering opportunities to optimize its composition and density, reducing ineffective electrolyte consumption during subsequent formation and cycling.
3. Pre-expansion, Stabilizing Electrode Structure
For silicon-based and other high-expansion anodes, pre-lithiation is equivalent to performing a “pre-expansion” of the electrode before assembly, preventing particle pulverization, conductive network breakage, and structural collapse caused by drastic volume changes during subsequent cycling.
4. Reduced Internal Resistance and Improved Rate Performance
Pre-lithiation results in a more balanced Li⁺ concentration and a more stable interface within the electrode, leading to reduced internal resistance and improved rate and low-temperature performance.
5. Widened Voltage Window
Lowering the negative electrode operating potential allows for a wider voltage window for the entire cell, further increasing energy density.
However, it’s important to emphasize that more pre-lithiation is not always better. Taking contact pre-lithiation as an example, the lithium metal source cannot be 100% converted. The unconverted portion loses its electronic conductivity, becoming “dead lithium,” which accumulates at the negative electrode interface, hindering Li⁺ diffusion and increasing polarization, inducing lithium plating. Industrially, lithium source utilization is often below 65%, and improving utilization is a core engineering challenge.
III. Mainstream Pre-lithiation Technology Routes
Based on the location and method of lithium source introduction, they can be summarized into three main categories: negative electrode pre-lithiation, positive electrode pre-lithiation, and electrolyte pre-lithiation.
Directly introducing active lithium into the negative electrode is a currently active research and industrialization area, mainly involving three mechanisms:
Contact pre-lithiation: The lithium source (lithium foil, stable lithium metal powder SLMP, lithium alloy) is physically contacted with the negative electrode. Spontaneous discharge occurs due to the potential difference generated by the extremely low potential of Li (-3.04 V vs. SHE), oxidizing Li to Li⁺ which is then embedded in the negative electrode.
Electrochemical pre-lithiation: A half-cell consisting of a negative electrode and a lithium source is constructed. An external current is applied to drive the directional migration of Li⁺ from the lithium source to the negative electrode. This can be divided into in-situ (pre-lithiation followed by disassembly and reassembly of the full cell) and in-situ processes.
Chemical pre-lithiation: The negative electrode is immersed in a lithium-containing reagent solution with strong reducing properties (such as lithium biphenyl, lithium naphthylene, etc.). The active lithium is transferred to the negative electrode material through a redox reaction using the redox potential difference. This method is gentle, has good uniformity, and is suitable for roll-to-roll processes.
Cathode Pre-lithiation (Indirect Lithium Supplementation)
Lithium-rich sacrificial additives (such as lithium-rich compounds, conversion-based nanocomposites, and binary lithium compounds) are introduced during cathode slurry preparation. During the first charge, these additives decompose and release additional Li⁺, which migrates to the anode to compensate for SEI losses.
Advantages include minimal modification to existing production lines and superior safety compared to directly using metallic lithium. Disadvantages include the potential impact of high-temperature residues on long-term cycling. Considering safety, operational complexity, and cost, cathode lithium-rich additives are considered one of the most promising directions for large-scale application.
Electrolyte/Other Component Pre-lithiation
Active lithium or SEI composition is introduced or controlled through lithium-containing electrolyte additives, functionalized binders, and modifications to the separator or current collector. This represents a more upstream, systematic design.
