The initial stage of lithium battery charging involves a low-current pre-charge process—known as Constant Current (CC) Pre-charge—designed to stabilize the cathode and anode materials. Once the battery state has stabilized, the system switches to a high-current charging phase, or CC Fast Charge. Finally, the process enters the Constant Voltage (CV) mode. For lithium batteries, CV mode begins when the system detects the voltage reaching 4.2V; the charging current gradually decreases, and charging terminates once the current drops below a specific threshold.

Standard charging currents vary depending on the battery type; for instance, batteries in 3C products typically use a standard rate of 0.1C–0.5C, whereas high-power traction batteries generally employ a standard rate of 1C. Lower charging currents are often selected to prioritize safety. Consequently, "fast charging"—as commonly understood—refers to charging at rates several to dozens of times higher than the standard current.

Charging a lithium battery is often likened to pouring beer: pouring quickly fills the glass fast but generates excessive foam, whereas pouring slowly yields more actual beer. While fast charging saves time, it can significantly damage the battery itself. Due to polarization phenomena within the battery, the maximum charging current the battery can accept decreases as charge-discharge cycles accumulate. During sustained high-current charging, ion concentrations at the electrodes rise and polarization intensifies, causing the terminal voltage to lose its direct linear relationship with the amount of charge or energy input. Simultaneously, high-current charging increases internal resistance, thereby intensifying Joule heating (Q=I²Rt) and triggering side reactions—such as electrolyte decomposition and gas generation. These issues sharply increase safety risks and inevitably lead to a significant reduction in the lifespan of non-power-optimized batteries.

Cathode Materials

The fast-charging process of a lithium battery essentially involves the rapid migration of lithium ions (Li+) from the cathode and their intercalation into the anode. The particle size of the cathode material influences factors such as electrochemical response times and ion diffusion paths; research indicates that reducing the material's grain size increases the lithium-ion diffusion coefficient. However, as the particle size decreases, slurry preparation encounters significant issues such as particle agglomeration and uneven dispersion. Furthermore, nanoparticles reduce the compacted density of the electrode sheet and increase the contact area with the electrolyte during charge-discharge cycles, leading to side reactions that impair battery performance.

A reliable approach is to modify the cathode material through coating; for instance, LFP inherently possesses poor electrical conductivity, but applying a surface coating of carbon or other materials can enhance its conductivity, thereby improving the battery's fast-charging performance.

Anode Materials

Fast charging in lithium-ion batteries implies the rapid de-intercalation of lithium ions and their swift migration toward the anode; consequently, the anode material must possess a high capacity for rapid lithium intercalation. Anode materials suitable for fast charging include carbon-based materials, lithium titanate (LTO), and other novel materials.

Regarding carbon-based materials, the potential for lithium intercalation is very close to the potential for lithium plating. Under standard charging conditions, lithium ions preferentially intercalate into the graphite structure; however, during fast charging or at low temperatures, lithium ions may deposit on the surface, forming lithium dendrites. These dendrites can puncture the Solid Electrolyte Interphase (SEI) layer, leading to a secondary loss of lithium ions and a reduction in battery capacity. If lithium accumulation reaches a critical level, the dendrites can grow from the anode toward the separator, creating a risk of internal short circuits.

As for LTO, it is an oxygen-containing anode material characterized by "zero-strain" properties; it does not form an SEI layer during operation and exhibits a stronger affinity for lithium ions, thereby meeting the requirements for rapid charge and discharge. However, the absence of an SEI layer means the anode material comes into direct contact with the electrolyte, promoting side reactions. The issue of gas generation in LTO batteries has proven difficult to resolve completely and can currently only be mitigated through surface modification techniques.

Electrolyte

As previously mentioned, the mismatch between lithium-ion migration speeds and electron transport rates during fast charging leads to significant battery polarization. To minimize the adverse reactions caused by this polarization, electrolyte research and development should focus on the following three areas: 

1. Electrolyte salts with a high degree of dissociation; 

2. Solvent combinations that offer lower viscosity;

3. Interface control—lower film impedance.

Relationship between production processes and fast charging

As previously mentioned, significant polarization occurs in batteries during fast charging due to the mismatch between lithium-ion migration speeds and electron transport rates. To minimize the adverse effects caused by this polarization, electrolyte R&D focuses on three key areas: 1. Electrolyte salts with high dissociation degrees; 2. Solvent compounding for lower viscosity; and 3. Interface control for lower film impedance.

(3) Electrode coating uniformity

It has been asked whether inconsistencies in electrode areal density affect battery performance. Regarding fast charging, the uniformity of the negative electrode is particularly critical. If the negative electrode's areal density is inconsistent, significant variations in the internal porosity of the active material arise after calendering (rolling). These porosity differences lead to uneven internal current distribution, affecting SEI formation and performance during the battery formation stage, which ultimately impacts fast-charging capabilities.

(4) Electrode compaction density

Why is electrode compaction necessary? It serves to increase specific energy and enhance overall battery performance. Optimal compaction density varies depending on the electrode material. Increasing compaction density reduces electrode porosity and tightens particle connections; for a given areal density, it results in a thinner electrode, thereby shortening the lithium-ion migration path. However, excessive compaction density hinders electrolyte wetting, potentially damages material structure and conductive agent distribution, and can lead to winding issues during later stages of production.

Formation, aging, and other factors

For batteries with carbon-based anodes, formation and aging are critical processes that determine the quality of the Solid Electrolyte Interphase (SEI). An uneven SEI thickness or unstable structure can compromise the battery's fast-charging capability and cycle life.

In addition to the factors mentioned above, cell manufacturing and charge-discharge protocols significantly influence performance. 

As the battery ages, the charging rate should be appropriately reduced to avoid exacerbating polarization.

The essence of rapid charge and discharge in lithium batteries lies in the ability of lithium ions to quickly intercalate and de-intercalate between the cathode and anode materials. Battery material properties, process design, and charge-discharge protocols all influence high-current charging performance. Structural stability in cathode and anode materials prevents structural collapse during rapid delithiation, while rapid lithium-ion diffusion within the materials enables them to withstand high-current charging. Mismatches between ion migration speeds and electron transport rates lead to polarization during charge-discharge cycles; minimizing polarization is essential to prevent lithium metal plating, which reduces capacity and compromises service life.