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Li-ion electrolyte MD on a cloud LAMMPS runner

QuickMDSim 7 min read

In a lithium-ion cell, Li⁺ ions move through a carbonate electrolyte between electrodes. Continuum models give conductivity. Molecular dynamics shows whether a given Li⁺ is coordinated by carbonyl oxygens, paired with PF₆⁻, or sitting in a dense layer at a wall.

Side view of a LiPF6 / ethylene carbonate cell between graphite-like walls. Large amber Li+ ions with gold trails, green PF6, dim solvent, dark electrodes.
Mid-run still. Amber: Li⁺. Green: PF₆⁻. Dust: ethylene carbonate. Slate bands: graphite-like walls.
Fixed camera, 10 ps of production. Li⁺ trails mark drift toward the negative electrode.

Background

Working cells dissolve LiPF₆ in cyclic and linear carbonates, typically ethylene carbonate (EC) plus dimethyl carbonate, and shuttle Li⁺ between a graphite anode and an oxide cathode. Xu, Chem. Rev. (2014) reviews that liquid. Soetens, Millot, and Maigret simulated EC/DMC + Li⁺ in 1998. Borodin and Smith built polarizable force fields for bulk transport. Tenney and Cygan (2013) classified Li–solvent clusters in LAMMPS. Boyer, Vilčiauskas, and Hwang (2016) put the electrolyte against a graphite edge.

Methods

An educational-scale cell (liquid-density EC, ~1 M LiPF₆, two graphite-like walls) was submitted from app.quickmdsim.com and run with LAMMPS in a cloud container.

870
Atoms (72 EC + 6 LiPF₆ + walls)
−3.6 Å
Li⁺ mean-z drift in 10 ps
3.7
Li–O coordination at the end
  • atom_style full for bonds and angles (EC and octahedral PF₆⁻)
  • pair_style lj/cut/coul/long and kspace_style pppm with a slab correction for non-periodic z walls
  • fix efield on the mobile group after 2 ps NVT equilibration
  • Project files: input.lammps and data.lammps

Li⁺ Lennard-Jones from Åqvist (1990). PF₆⁻ from Canongia Lopes and Pádua (2004). EC charges after Soetens et al. (1998), with OPLS-AA Lennard-Jones and harmonic springs on a 6-site united-atom ring (no explicit hydrogens; the image is not built with SHAKE).

The applied field is larger than in a working cell so that drift is visible in 10 ps. At experimental fields the same displacement would take nanoseconds. The movie is rendered from dump.electrolyte.lammpstrj.

What the dump shows

  • Mean Li⁺ z falls from 25.1 Å to 21.5 Å after the field turns on (drift toward the negative electrode).
  • Each Li⁺ keeps a first-shell oxygen coordination of about 3–4 (carbonyl and ether oxygens of EC within 2.8 Å).
  • PF₆⁻ remains octahedral. Some Li⁺ share an anion (contact ion pairs of the kind Tenney and Cygan counted in bulk).

Product notes

Electrolytes need a read_data file with atom types, charges, bonds, and angles. QuickMDSim copies that file into the working directory with the input. Long-range Coulomb (PPPM) and bonded molecule styles are compiled in. A slab correction (kspace_modify slab 3.0) allows walls on boundary p p f.

Files

Repo path demos/li-ion-electrolyte/. Regenerate data with python3 scripts/gen_li_ion_electrolyte.py. The app starter is named Li-ion electrolyte.

  1. Open app.quickmdsim.com
  2. Pick Li-ion electrolyte (or upload the two demo files)
  3. Run
  4. Watch movie.mp4, then color dump.electrolyte.lammpstrj by type in OVITO. Li is type 1.

Educational cell, sized for the free tier. Cite Xu (2014) for the electrolyte; Åqvist, Lopes and Pádua, Soetens, and OPLS-AA for the numbers; Tenney and Cygan and Boyer et al. for what to measure in the dump.