Detailed Explanation of the First Cycle Efficiency of Lithium Batteries: Principles, Influencing Factors, and Improvement Methods

Detailed Explanation of the First Cycle Efficiency of Lithium Batteries: Principles, Influencing Factors, and Improvement Methods

I. Definition and Importance of First Cycle Efficiency

• Definition: The first cycle efficiency (First Cycle Coulombic Efficiency, FCCE) of lithium batteries refers to the ratio of the amount of lithium ions actually intercalated into the negative electrode to the amount of lithium ions extracted from the positive electrode during the first charge. It is usually expressed as a percentage.
• Importance: First cycle efficiency is one of the key indicators for measuring battery performance and directly affects the energy density, cycle life, and safety of the battery. Higher first cycle efficiency means higher energy utilization and less irreversible capacity loss.

II. Principles of First Cycle Efficiency

1. Intercalation and Deintercalation of Lithium Ions:
   • During the first charge, lithium ions are extracted from the positive electrode material, pass through the electrolyte, and are intercalated into the negative electrode material.
   • During the first discharge, lithium ions are deintercalated from the negative electrode and return to the positive electrode.
   • The first cycle efficiency reflects the efficiency of this lithium ion transfer process.
2. Irreversible Capacity Loss:
   • During the first charge, some lithium ions form a solid electrolyte interphase (SEI) film on the surface of the negative electrode, resulting in irreversible capacity loss.
   • The formation of the SEI film is necessary because it can protect the negative electrode material from further erosion by the electrolyte, but it also leads to a reduction in first cycle efficiency.

III. Factors Affecting First Cycle Efficiency

1. Electrode Materials:
   • Positive Electrode Materials: Different positive electrode materials (such as lithium iron phosphate, ternary materials, etc.) have different lithium ion extraction capabilities and structural stabilities. For example, the first cycle efficiency of lithium iron phosphate is usually higher than that of ternary materials.
   • Negative Electrode Materials: The surface characteristics and structure of the negative electrode material have an important impact on the formation of the SEI film. For example, the first cycle efficiency of graphite negative electrodes is usually lower than that of silicon-based negative electrodes, but the expansion problem of silicon-based negative electrodes needs to be solved additionally.
2. Electrolyte:
   • The composition and properties of the electrolyte directly affect the transfer efficiency of lithium ions and the formation of the SEI film. For example, adding specific electrolyte additives can improve the stability of the SEI film, thereby increasing the first cycle efficiency.
3. Battery Manufacturing Process:
   • Electrode Sheet Design: The thickness, porosity, and coating uniformity of the electrode sheet affect the transfer path and diffusion efficiency of lithium ions.
   • Battery Assembly: The battery assembly process (such as winding, welding, etc.) affects the contact between electrodes and the wettability of the electrolyte.
4. Environmental Conditions:
   • Temperature has a significant impact on the first cycle efficiency. Higher temperatures can accelerate the diffusion of lithium ions, but may also lead to an increase in side reactions.
   • Humidity and impurities can affect the stability of the electrolyte, thereby affecting the first cycle efficiency.

IV. Methods to Improve First Cycle Efficiency

1. Optimizing Electrode Materials:
   • Surface Modification: Improve the surface characteristics of the negative electrode material through surface coating or nanostructuring to reduce the formation of the SEI film.
   • Composite Materials: Develop new composite materials, such as silicon-carbon composite negative electrodes, combining the high capacity of silicon and the stability of carbon to improve the first cycle efficiency.
2. Improving Electrolyte Formulation:
   • Additives: Add specific electrolyte additives (such as film-forming agents, antioxidants, etc.) to improve the stability and uniformity of the SEI film.
   • New Electrolytes: Develop high-voltage, high-stability electrolytes to improve the transfer efficiency of lithium ions.
3. Optimizing Battery Manufacturing Process:
   • Electrode Sheet Design: Optimize the thickness, porosity, and coating process of the electrode sheet to improve the diffusion efficiency of lithium ions.
   • Battery Assembly: Improve the battery assembly process to ensure good contact between electrodes and full wetting of the electrolyte.
4. Controlling Environmental Conditions:
   • Temperature Control: Conduct battery manufacturing and testing within an appropriate temperature range to avoid side reactions caused by high temperatures.
   • Humidity Control: Manufacture batteries in a low-humidity environment to reduce the impact of impurities on the electrolyte.

V. Summary