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Lithium ion full battery production process–Soft-Pack Battery Formation – Part 2

Key Factors Influencing Formation: Mechanism

Generation Process of SEI Membrane:

  1. Electrons are transferred from the current collector, through the conductive agent, to point A inside the graphite particles where the SEI membrane is to be formed.
  2. Solvated lithium ions, wrapped in the solvent, diffuse from the cathode to point B on the surface of the SEI membrane that is currently being formed.
  3. The electrons at point A diffuse to point B through the electron tunneling effect. The electrons that jump to point B react with lithium salt, solvated lithium ions, film-forming agents, etc., to continue generating the SEI membrane on the surface of the existing SEI membrane.
  4. This process results in the continuous increase of the SEI membrane thickness on the surface of the graphite particles, ultimately leading to the formation of a complete SEI membrane.

SEI film-forming reaction:

  1. Two-electron reaction: This type of reaction requires the participation of two electrons simultaneously. Under such conditions, it is easier to form inorganic lithium salt components.
  2. One-electron reaction: This reaction occurs with the participation of only one electron. In this case, it is more likely to generate organic lithium salt components.

Note: The electrolyte is prepared with PC as the solvent, ES as the additive, and 1M LiPF6 as the lithium salt. Graphite is used as the active material.

Key factors influencing SEI film composition:

  1. Electron density
  2. Concentration of reactants

Which process parameters affect electron density and reactant concentration? 
Electron density:
    Current magnitude
    Pressure (applied between the positive and negative electrodes)
Reactant concentration:
    Temperature
    Composition and concentration of electrolyte
Others:
     Formation cutoff voltage

Let the formation current be I:

  1. Electrons are transferred from the current collector to the conductive agent and then into the interior of the graphite particles to point A, where the SEI film is to be formed.
  2. For a flat interface: The number of electrons reaching point A will be determined by the formation current I. The larger the formation current, the greater the current passing through point A on the electrode.
  3. Different current densities will produce SEI films with different structures.

Formation current

Formation Pressure (Current Uniformity)

Formation pressure P:

  1. When the interface is not flat, the current distribution at the electrode interface is uneven, with a higher current density at points (a) that are closer together and a lower current density at points that are further apart. This uneven current distribution leads to an inhomogeneous SEI film structure.
  2. By applying a pressure of P, the electrode interface can be flattened, enabling a uniform distribution of current across the interface and the formation of a uniform SEI film.

Formation Temperature

Formation temperature:

  1. For the same electrolyte, increase the temperature, the viscosity of the electrolyte decreases, reducing the resistance to the transport of reactants (film-forming agents/lithium ions) within the electrolyte.
  2. For the same electrolyte, higher temperature leads to an increase in electrical conductivity and a decrease in polarization, further reducing the resistance to the transport of reactants in the electrolyte.
  3. More film-forming agents and solvated lithium ions can reach point B within a given time, thereby influencing the SEI film-forming reaction.

Composition and Concentration of Electrolyte

Electrolyte Components:

  1. A wide variety of electrolyte solvents, additives, and salts.
  2. Different electrolyte components participate in different film-forming reactions, resulting in differences in the composition of the SEI film formed.

Formation Cutoff Voltage

Formation Cutoff Potential: The electrochemical windows of various components can be obtained through CV testing:

  1. Different electrolyte components have different electrochemical windows.
  2. There are also subtle differences in different anode-cathode systems.

Pouch Battery Formation Process Parameters

Charge Current 1

Current of step 1→form SEI

  1. Current ↗, reaction peak →, polarization↗, effect SEI form
  2. All data of step 1 use 05 or 1C and 02C is in the same level by now.

Suggested formation current of 1st step =0.5~1.0C

Charge Current 2

Current of step 2→for Charging

  1. Current ↗, polarization↗.
  2. 2C of step 2 is OK.

Suggested formation current of 2nd step =1.0 ~ 2.0C

Formation Temperature

Temp.=90℃, D0 ↘;

Suggested formation Temp = 70~85 degC

Formation Pressure

Pressure↗, THK ↘ af. Cap

Suggested formation pressure : 0.6~1.0 MPa

Suggested formation pressure : 0.6~1.0 MPa

SOC↗,THK ↘,
For rippled cell, High SOC is a must

Suggested formation SOC = 66%~83%

Coin Cell Formation

Key Parameters: Typically focused on performance with less attention paid to efficiency.

  1. Charge 1/Charge 2 Rate: Low-current charging (0.02C).
  2. Formation Temp: Room temperature (some laboratories may adopt 45℃ under certain conditions).
  3. Pressure,Double gasket structure is used for coin cell assembly, with high flatness and rigidity of the gaskets to maintain a flat interface.
  4. SOC (Time),A complete charging (cathode/full cell) or discharging (anode) process.

Determination Method 1 for Formation Current/Cutoff Potential

Remarks:

  1. Lithium cobalt oxide/graphite system;
  2. Electrolyte: Electrolyte with VC and FEC as additives;
  3. dQ/dV curve, similar to CV curve; with voltage as the horizontal axis and the integral of voltage with respect to energy as the vertical axis;

Determination Method 2 for Formation Current/Cutoff Potential

Method for Determining Formation Current and Cutoff Potential: Trial-and-Error Method.

Experimental Process:

  1. Conduct constant-current charging at 0.02C until the battery is fully charged (4.2V). Calculate the dQ/dV value and plot a dQ/dV-V curve. The position of the first reaction peak (~2.8V) represents the potential at which the film-forming additive decomposes to form the film. The formation cutoff potential is ~2.92V.
  2. Select a current of 0.03C to form parallel samples. Obtain the position of the first reaction peak (~2.8V), which represents the potential at which the film-forming additive decomposes to form the film. The formation cutoff potential is ~2.94V.
  3. Select a higher current (0.05C) to form parallel samples. Obtain the position of the first reaction peak (~2.9V), which represents the potential at which the film-forming additive decomposes to form the film. The formation cutoff potential is ~3.16V.

Experimental Conclusion:

  1. When forming at 0.02C and 0.03C, the positions of the film-forming reaction peaks overlap, indicating that the entire SEI film-forming reaction is the same. When forming at 0.05C, the peak position shifts significantly to the right, indicating increased polarization, which will affect the SEI film-forming effect. Therefore, from the perspective of saving formation time and obtaining battery cells with excellent performance, selecting a formation current of 0.03C is a preferable result.
  2. When forming with different formation currents, the cutoff potentials for the film-forming reaction are 2.92V, 2.94V, and 3.16V, respectively. Therefore, we can select a formation cutoff potential of 3.5V. In other words, the formation process is 0.03C CC to 3.5V.

Company introduction

Welcome to Canrd Company (Canrd stands for “Creating Avenues for New Research Development”,website: www.canrd.com ). We specialized in:

  1. Electrochemical experiment consumables, including coin cell cases, cathodes, anodes, electrodes, electrolytes, and various other materials.
  2. Battery cell research and development outsourcing, offering Custom-made electrodes, dry cells, finished battery cells, and battery cell testing.
  3. Test line equipment, such as coin cell equipment lines, pouch cell test lines, cylindrical cell test lines, and more.

We have strong research and development capabilities.If you are interested, please feel free to contact us at any time.
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