Diffusivity of Alkali Metal Ions in Solid and Aqueous Electrolytes

Student: Bhavin ShahTeacher: Jeffrey SapinMentors: Michiel Niesen & the Miller Group

Chemistry Background

The Big Idea - Batteries

Depicted above are (a) a typical smartphone battery, (b) a Tesla Model S battery pack, and (c) Tesla Powerwall, a home battery pack.

A Molecular Insight

Electrolyte

Anode

Cat

hode

Li+

A lithium battery has two conducting plates

known as electrodes separated by a

conductive medium called an electrolyte.

E

In a lithium-ion battery, electrons split from lithium and

carry charge through the circuit, while the remaining

lithium ions (cations) move to the cathode.

Li+ Li+

Li+

E

The electrons from the anode then flow through the circuit

Electrodes are generally made of lithium salts, such

as LiFePO4

Image Credits: Islam, M. Saiful, and Craig A. J. Fisher. "Lithium and Sodium Battery Cathode Materials: Computational Insights into Voltage, Diffusion and Nanostructural Properties." Chem. Soc. Rev. 43.1 (2014)

Discharge of a Li-ion Battery

Liquid Electrolytes

● Widely used for their high ionic conductivitiesDownsides:

1) Are safety hazards, especially in vehicles such as cars (Leaking, evaporating, overheating)

2) Are chemically unstableImage Credit: Josie Garthwaite, for National Geographic, December 06, 2013. "While U.S. Probes Tesla, What You Should Know About Car Fires." National Geographic. National Geographic Society

A New Battery Electrolyte: Polymers

● Polymers are solid plastics composed of long molecular chains● Benefits: Won’t leak, lightweight, safe, long-lasting

Movers of Charge: IonsHighly

Reactive{ Highly Reactive

{

The Big Questions

(1) What trends can we observe in the behavior of alkali, alkaline earth, and halogen ions in different electrolytes?

(2) Do solid polymer electrolytes offer significant benefits over liquid electrolytes?

Technical Overview

Computational Chemistry

★ Use of computational tools to model molecular systems

★ Employs Molecular Dynamics○ Molecular Dynamics is the process of computing the

motion trajectories for molecules

t = 1

t = 2t = 4

θ

Tools of Molecular Dynamics: LAMMPS & VMD

LAMMPS● Molecular Dynamics Simulator● Continuously solving F=ma for all particles● Takes coordinates of atoms and, using various

equations, distributions, and parameters, produces a trajectory of the atoms’ motion

VMD● Visualizes the trajectory from LAMMPS● Used to render snapshots and videos

To the right is a portion of a LAMMPS input script used in our simulations

How MD Works

Start

Temperature

Temperature - the average kinetic motion of particles

Kinetic Energy =

Newton’s Second Law is at the heart of how simulations calculate motion trajectories for particles

VMD Movie

Temperature

Density

Volume

Lennard-Jones

Bonds Angles

Objectives

Using MD Simulations, we want to:● Determine the coordination effects of each

electrolyte structure on various ions● Determine the diffusivity, or mobility, of

various ions in both a polymer and liquid electrolyte

Simulating Ions in Electrolytes

Electrolyte Materials: PEO & Dimethyl Ether

● PEO - is a leading candidate for a polymer electrolyte○ Hindered by low conductivity at ambient temperatures

● Dimethyl Ether (DME) is a segment of PEO ○ Set as a liquid electrolyte for simulations

LIQUID

SOLID

Simulation - Setup

1

2

5

4

3

Chlorine

Lithium

Magnesium

Potassium

Sodium

PEO Box at 400 K with fixed density

DME Box at 400 K with fixed density

NOTE: Running both materials at the same density gives us a basis for

comparison

On the right is a box of PEO

On the right is a box of Dimethyl Ether

Simulation Analysis

Objective #1

Using MD Simulations, we want to:● Determine the coordination effects of each

electrolyte structure

● Determine the diffusivity, or mobility, of various ions in both a polymer and liquid electrolyte

Solvation Shells:Ion Coordination

● Particles surrounding an ion are known as solvation shells● Solvation Shells contribute to how ions move through a material:

aka coordination

This is the first solvation shell surrounding this atom. The

shell is said to be ‘coordinating’ the ion by

keeping it trapped within the ring-like structure.

This is the second solvation shell surrounding this atom. The shell

is actually made up of atoms bonded to the first shell. It lies

further from the atom because it does not coordinate the atom as

well as the first shell.

RDF - PEO

O

Solvation Structure - PEO

This Potassium ion in PEO is surrounded by oxygen atoms,

which form its first solvation shell.

O

O

O

O

O

O

O

Ion Predicted 1st Solvation

Simulated 1st Solvation

Li (+)

Mg (+2)

K (+)

Na (+)

Solvation Structure - DME

Oxygen AtomsSodium Ion

Ion Predicted 1st Solvation

Simulated 1st Solvation

K (+)

Na (+)

Cl (-)

O O

O O

CH2 CH2

Objective #2

Using MD Simulations, we want to:● Determine the coordination effects of each

electrolyte structure

● Determine the diffusivity, or mobility, of various ions in both a polymer and liquid electrolyte

MSD & the Diffusion Coefficient

● MSD is the displacement of an atom from a its starting point● Greater MSD means that atoms are diffusing freely in a given material

Dimethyl Ether MSD - 400 K Origin

1

4 2

3

MSD Visualized

Ion Diffusivity in PEOMost Diffusive

Least Diffusive

Cl-1

K+1

Na+1

Li+1

Mg+2 log t (Picoseconds)

Ion Diffusivity in DME

Cl-1

K+1

Na+1

Most Diffusive

Least Diffusive

log

MS

D (A

ngst

rom

s)

log t (Picoseconds)

Mean-Square-Displacement for Dimethyl Ether

Conclusions

The Big Answers

The Big Answers(1) What trends can we observe in the behavior of alkaline, alkali earth,

and halogens in different electrolytes?

IncreasedDiffusivity

IncreasedOxygen Coordination

IncreasedDiffusivity

IncreasedOxygen Coordination

Predicted Trend

The Big Answers(2) Do polymer electrolytes offer significant benefits over liquid electrolytes?

● Diffusivity of ions was approximately 50% as strong in polymer electrolyte compared to liquids

● Trends suggest study of new ions may be appropriate for polymer electrolytes ○ Fluorine (F-)○ Calcium (Ca+2)○ Chlorine (Cl-)

Next Steps

● Further simulations of Potassium & Calcium cations, Chlorine & Fluorine anions in polymers

● Look at new, synthetic polymers● Conduct feasibility studies of other ions for battery applications

Image Credit: “Polymers.” http://www.survivalworld.com/science/polymers N.p., n.d. Web. 30 July 2015

Computational Chemistry in the Classroom

From the Lab: Demos From Caltech: Programming

Observations and Analysis of various phenomena in physics, biology, and chemistry

Scripting: The new language requirement

Acknowledgements Michiel Niesen, Brett Savoie, Michael Webb, and the Miller Group @ Caltech

James Maloney and Julius Su, Program Directors

Questions?