Complete structure introduction of prismatic lithium iron phosphate (LiFePO4) square cell

I. Overall External Structure (from outside to inside)

1. Aluminum Metal Shell (Casing)

Material: 3003 aluminum alloy stamped and formed, square hard case

Functions:

  • Seals and contains the electrode assembly and electrolyte;
  • Thermal conductivity and heat dissipation, rapid cooling under high-current conditions;
  • Mechanical support, resistant to compression and minor impacts;
  • The shell is at negative potential; PACK assembly must be insulated to prevent grounding short circuits.

2. Top Cap Assembly (most critical external component)

The top cap integrates all safety and conductive parts, including:

  • Positive and negative terminals
  • Positive aluminum post, negative copper nickel-plated post, for high-current output; starter cell terminals are thickened to reduce heating and voltage drop;
  • Explosion-proof safety valve (pressure relief valve)
  • Automatically ruptures to release pressure when gas is generated internally or thermal runaway occurs, preventing casing rupture; essential safety structure for automotive starter batteries;
  • Electrolyte filling sealing pin
  • Electrolyte is injected during production, then laser welded to seal after filling;
  • Insulating plastic gasket
  • Isolates positive and negative terminals to prevent short circuits on the top cap;
  • Sealing ring
  • Seals between terminal and top cap to prevent electrolyte leakage.

3. External Insulation Components

  • PVC/PP flame-retardant heat-shrink sleeve/film: wraps the entire aluminum shell for insulation and scratch protection;
  • Epoxy insulating sheet: placed between cells in parallel to separate adjacent aluminum shells and prevent shell-to-shell short circuits.

II. Internal Core Electrode Assembly (winding or stacking processes)

Prismatic cells are divided into wound type (mainstream for power/starter cells) and stacked type (energy storage slow-charge cells)

1. Wound Electrode Assembly (preferred for automotive starter lithium batteries)

Four layers of material continuously wound into a square jelly roll:

  • Positive electrode sheet

Aluminum foil substrate, double-sided coated with lithium iron phosphate active material; intercalates/deintercalates lithium ions, absorbs lithium during charge, releases lithium during discharge;

  • Separator

Microporous PP/PE composite separator, isolates positive and negative electrodes to prevent direct contact and short circuit; allows lithium ions to pass through while blocking electrons; automatically shuts down pores at high temperature, interrupting current for safety protection;

  • Negative electrode sheet

Copper foil substrate, double-sided coated with graphite; stores lithium ions;

  • Tabs

Positive aluminum tabs, negative copper tabs, multiple thin strips bundled and welded to the top cap terminals; high-rate starter cells use multi-tab design to reduce internal resistance and withstand kiloamp pulse currents.

2. Stacked Electrode Assembly

Positive and negative sheets are alternately stacked, offering lower internal resistance and better cycle life, but weaker large-current pulse performance, mostly used in energy storage, not suitable for automotive starting.

III. Internal Electrolyte and Auxiliary Materials

  • Electrolyte

Lithium salt + organic solvent, medium for lithium ion conduction, soaks all electrode sheets and separators;

  • Insulating tape, fixing adhesive

Secures the electrode assembly so that electrode sheets do not shift or tabs tear under vehicle vibration;

  • Insulating gaskets

Insulate the top and bottom ends of the electrode assembly to prevent contact short circuits with the inner wall of the aluminum shell.

Prismatic cell structure diagram.jpg

IV. Current Conduction Path (key for high starting current)

Discharge path: Negative graphite deintercalates lithium → electrolyte → separator → positive LFP → positive tab → top cap positive terminal → external load (starter motor)

Charge path is reversed: generator current enters through positive terminal, lithium ions intercalate into negative graphite.

V. Special Structural Optimizations for High-Rate Starter Cells

  • Multi-tab structure
  • Dozens of tabs in parallel dramatically reduce DC internal resistance, briefly withstand 1000A pulses without overheating; ordinary energy storage cells with single or double tabs cannot be used for engine cranking;
  • Thickened tabs and enlarged top cap terminals
  • Reduce contact voltage drop, resulting in smaller voltage sag at startup;
  • Reinforced explosion-proof valve
  • Copes with gas generation from high-current heating, stronger pressure relief capability;
  • Wide-format electrode design
  • Increases active material contact area, enhancing short-time discharge capability.

VI. Advantages and Disadvantages of Prismatic Cells vs. Pouch/Cylindrical Cells

Advantages

  • Aluminum shell with high rigidity, not easily deformed under vehicle vibration;
  • High heat dissipation efficiency, suitable for high-current starting conditions;
  • Large single cell capacity (100Ah/280Ah), simplifies PACK series/parallel connection;
  • Built-in explosion-proof valve, safer than pouch cells;
  • Regular arrangement, simple PACK casing design, easy replacement of original vehicle lead-acid batteries.

Disadvantages

  • Aluminum shell carries negative potential, requiring additional insulation boards during assembly, adding process steps;
  • Slightly heavier than pouch cells.