Intercalation

Overview

Intercalation inserts guest atoms or ions into the interlayer spaces of a layered host material. This is fundamental to battery electrode design, where ions shuttle between electrodes during charge/discharge cycles.

Common applications:

• Li-ion batteries: Li intercalation in LiCoO2, LiFePO4, graphite
• Na-ion batteries: Na intercalation in layered oxides, MXenes
• Supercapacitors: ion insertion in MXenes, layered hydroxides
• Catalysis: intercalated species modifying interlayer chemistry
• 2D materials: ion intercalation in MoS2, graphite for exfoliation

MCP Tool: catgo_intercalation

{"tool": "catgo_intercalation", "arguments": {
  "structure": "<current structure dict>",
  "species": "Li",
  "position": "auto",
  "n_intercalants": 1
}}

Parameter	Description	Default
structure	Layered host structure dict	(required)
species	Intercalant element symbol	(required)
position	Placement strategy	"auto"
n_intercalants	Number of intercalant atoms to insert	1

Position Modes

Mode	Description
auto	Finds the largest gap in z-fractional coordinates and inserts at the midpoint. Best default for layered materials.
tetrahedral	Places at approximate tetrahedral interstitial sites (frac coords ~0.25).
octahedral	Places at approximate octahedral interstitial sites (frac coords ~0.5).
custom	For manual positioning (specify coordinates separately).

Router: /build/intercalation (POST)

Complete Workflow: Li Intercalation in LiCoO2

Step 1: Fetch the host structure

{"tool": "catgo_fetch", "arguments": {
  "action": "crystal", "formula": "CoO2", "source": "mp"
}}

Or fetch the fully lithiated phase:

{"tool": "catgo_fetch", "arguments": {
  "action": "crystal", "formula": "LiCoO2", "source": "mp"
}}

Step 2: Create a supercell for dilute intercalation

{"tool": "catgo_structure", "arguments": {
  "action": "supercell",
  "scaling": [2, 2, 1]
}}

Step 3: Intercalate Li

{"tool": "catgo_intercalation", "arguments": {
  "species": "Li",
  "position": "auto",
  "n_intercalants": 1
}}

For higher concentrations, increase n_intercalants. Multiple intercalants are distributed across the interlayer plane.

Step 4: Verify

{"tool": "catgo_view", "arguments": {"action": "get_state"}}

Check: Li atom is between layers, not overlapping with host atoms, reasonable interlayer distance preserved.

Step 5: Relax and compute intercalation voltage

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "create", "params": {"name": "Li-CoO2 intercalation"}
}}

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "add_task", "params": {
    "workflow_id": "<wf_id>",
    "task_type": "geo_opt",
    "params": {"software": "vasp", "ENCUT": 520,
               "LDAU": true, "LDAUU": {"Co": 3.32},
               "system_name": "LixCoO2 relax"}
  }
}}

Intercalation Voltage Calculation

The average intercalation voltage is:

V = -(E[Li_x2 Host] - E[Li_x1 Host] - (x2 - x1) * E[Li_metal]) / ((x2 - x1) * F)

where F is the Faraday constant. In practice:

1. Relax the empty host (x = 0)
2. Relax the fully intercalated structure (x = 1)
3. Optionally relax intermediate compositions
4. Compute V from the energy difference

Common Intercalant Species

Species	Application	Typical Hosts
Li	Li-ion batteries	CoO2, FePO4, MnO2, graphite, TiS2
Na	Na-ion batteries	MnO2, V2O5, Prussian blue analogues
K	K-ion batteries	Graphite, MoS2
Mg	Mg batteries	V2O5, MoS2, TiS2
H	Proton intercalation	MnO2, WO3 (electrochromics)

DFT Considerations

• DFT+U: Required for transition metal oxides (Co, Mn, Fe, Ni). Common U values: Co (3.32 eV), Mn (3.9 eV), Fe (5.3 eV), Ni (6.2 eV).
• Van der Waals: DFT-D3 corrections important for layered materials where interlayer bonding is weak.
• Spin polarization: Always use ISPIN=2 for transition metal oxides.
• K-points: Sufficient sampling in the layer plane; fewer points needed perpendicular to layers.

Common Pitfalls

1. The auto position mode finds the largest z-gap. For materials with multiple interlayer gaps of similar size, verify the intercalant ended up in the correct gap.
2. Always relax after intercalation. The host lattice expands to accommodate the guest species.
3. For concentrated intercalation (multiple atoms), ensure intercalants are not placed too close together. Check with catgo_view after insertion.
4. The host structure should be a bulk layered material, not a slab. Slabs have vacuum that will confuse the auto-positioning algorithm.
5. Transition metal oxidation states change upon intercalation (e.g., Co4+ to Co3+ when Li is inserted). This affects magnetic moments and DFT+U parameters.
6. For accurate voltage predictions, use the same ENCUT, k-points, and DFT+U parameters for all compositions in the voltage calculation.