Original subtitles

all right so today we're going to talk a

bit about electrochemistry

and how we use electrochemistry with

battery

materials at least for battery research

and the different types of tests some of

the different tests

we do i do in part with my battery

research

um so electrochemistry it's a

it's a pretty broad field um

i i've only ever taken the fresh me

freshman chemistry courses as an

undergraduate

so before starting my research on

batteries i knew almost nothing about

electrochemistry so all of this i had to

learn on my own

and there's not really any any good

courses within our department that cover

the basics

of electrochemistry some of our classes

do

deal with the concepts of

electrochemistry for example there's a

senior level

corrosion class dealing with the

corrosion of

different materials and that that

heavily heavily relies on some of the

concepts i'll share with you today

and i believe there's some other uh

metallurgy

processing classes uh that also deal

with these kind of reactions

um so my definition of electrochemistry

and i'm sure it's it's a shared

definition is that you know it's the

study of any chemical reactions that

involve uh the electron transfer

and so in other words if you have a

metal ion or it could even be an organic

molecule that has a change in valence

then you can assume it's going to be an

electrochemical reaction

uh so the basis of electrochemistry is a

reduction

and oxidation reaction oh uh

i see a chat uh could you tell professor

mckinsley how to edit the

video lectures sure i

i can i can i can give them some tips i

actually

i'm not i'm not the best at it either

it is definitely a skill that you have

to practice and acquire i was just using

the

microsoft video editor which is a very

basic editor editor it doesn't it's

missing a lot of

uh stuff like uh you can't make any

transitions between

cuts so but sure i can i can send them

an email so anyways back to it

just a reminder reduction reaction is uh

the gain

of electrons so for example if you have

a metal ion

in this case a metal two plus ion and uh

you gain an electron that goes from

metal two plus to metal

one plus okay that's a reduction on the

other hand oxidation is the opposite is

the loss of electrons

uh if you go from a metal two plus ion

to metal three plus it's giving away an

electron

it's uh increasing its valence state so

that's oxidation

so in electrochemical reactions you have

to have both

okay so oftentimes we we only talk about

one

part of the reaction like a reduction or

an oxidation separate but in

every case you know you have to ask

yourself where do those electrons come

from

if you're receiving an electron there

has to be an opposite reaction somewhere

else in the system that's giving away

electrons you can't just get electrons

from nothing

so here's an example of a redox reaction

is the oxidation of iron and metal into

iron ii oxide feo

and so the reduction half reaction of

this uh

this chemical reaction is the oxidation

excuse me the reduction of oxygen

gas all right so oxygen o2 molecule

which a neutral molecule is giving away

excuse me it's receiving electrons and

becoming ionized it's becoming a o2

minus

ion and so the question is where does it

receive those electrons from

and that's the other half of the

reaction is the oxidation reaction

the iron metal which is a neutral

atom is oxidizing it's giving away its

electrons and becoming

iron two plus and then the overall

reaction is the iron metal is oxidized

by oxygen

becomes iron two oxide okay

and of course there's this uh chemical

reaction also depends on other factors

like

the oxygen content you know we can get

different

different iron oxides depending on how

how much oxygen the

oxygen there is available the

temperature for example

presence of water and acidity can give

you different

chemical compounds like different types

of oxides so i iron three oxide iron two

three oxide in different phases of those

oxides um so here's a

anti example this is not a a

redox reaction if you remember last

lecture

um i was asking you guys about this this

type of reaction we said it was a

precipitation reaction it's not a

it's not combustion it's not oxidation

reaction and

the reason i said that is because well

we don't have any electron transfer so

it's it's not going to be an

electrochemical reaction

so in this example we had a dissolved

species of iron two plus like iron

chloride

ferrous chlorides for example and uh

iron three plus like ferric chloride for

example

in solution and then so in solution iron

two plus and i and three plus are at

equilibrium they

they have no problem being together uh

even though there's two different

valence states of the two ions uh but

then we would add a base

such as ammonium hydroxide and we're

shifting the ph the higher

ph where these ions are no longer

stable they're no longer at equilibrium

in that solution and so what happens is

they precipitate out

and that forms this iron 304

also known as a spinel oxide

this is actually let's see iron

three plus and two plus coexist in this

solid material and in fact the crystal

structure is a inverse spinel

a spinel is a type of crystal where you

have a

lattice of oxygen you have many

different interstitial sites you have a

a variety of tetrahedral interstitials

and octahedral interstitials and so in

this inverse

spinel crystal the iron three plus

occupies tetrahedral sites

which is given by these parentheses and

then also

it occupies octahedral sites given by

the brackets and then all the iron two

plus also

occupies the octahedral sites so that's

an example

of a precipitation reaction without

electron transfer

okay so how do we

how do we find out what

what material oxidizes and what material

reduces in the given reaction

one way to look at it is the standard

reduction potential the

these are empirically determined values

of different

species and their reduction potential

or in other words you know what what's

its potential to be reduced

so on this list the the species at the

the bottom of the list have a higher

potential of being reduced in comparison

to the species at the top of the list

so like lithium plus to lithium metal

does not have a very high

potential compared to gold two plus the

gold

plus as far as reducing on the contrary

lithium metal has a very high potential

to become oxidized

it's just the in the negative of this

number um

and so the one thing i should point out

is that this

standard reduction potential chart is uh

is the reference to the she

she is standard hydrogen electrode

it's just a reference electrode where

you have a hydrogen

uh reaction h plus uh

it turns into h2 gas it's just a

standard

uh potential so we we just arbitrarily

mark that as zero if we're going to

reference it

uh every other reaction to that standard

hydrogen reaction so i kind of made this

kind of

this diagram of different mountain peaks

you could say

you know so it depends on your

perspective of where you are on the

mountain

you know you can either go uphill or

downhill and then everything is relative

to where you are so you know if we were

to start at lithium for example

you know everything's uphill from us as

far as reduction

potential and so you we could easily say

all these numbers are referenced to

lithium which we often do in lithium-ion

batteries

especially in research when we have our

one side of the battery

electrode is lithium metal and so you

want to you want to measure your voltage

compared to lithium rather than she

so that's just an explanation of that

okay but

overall the species lower on the list

have a higher potential of being reduced

in comparison to the species above it so

let's see

uh here's a kind of another explanation

of this

you know if we had a solution of iron

three plus

in the solution and we wanted to measure

the voltage

between two electrodes platinum

electrodes we use platinum because

platinum is an

inert electrode there's not going to be

any chemical change

with the electrode itself so what we're

really doing is measuring the potential

of any reaction

on the surface of the platinum so if if

we just have one

uh solution with two platinum electrodes

and try to measure

the voltage between it like with the

multimeter we're going to get zero volts

right there's there's no difference in

chemistry

between the surface of this platinum and

the surface of that platinum it's all

mixed together so

we get zero volts on the other hand if

we were to replace one of these platinum

electrodes with a standard hydrogen

electrode which kind of looks like this

it's encapsulated in its own uh it gets

cell where this is a glass cell and then

at the bottom

there's this micro porous frit so it's a

very

very small pore porous ceramic

which allows ionic diffusion between the

two

so you'll have some kind of standard

electrolyte

in this solution that's different than

this iron to

the three plus but over time if you

leave it in here for like a couple days

you'll have some interes some some

exchange between ions between this fritz

so it's not good to have it for a long

time

and you'll have to constantly be

cleaning this and

refilling it anyway so if we use a

standard hydrogen electrode

now we have a different chemical process

happening at the the surface of this uh

this uh electrode all right and that

will

now we can measure a potential between

that so that we're measuring the

potential

of the iron three plus to be reduced to

iron two plus

uh i should i should make a note that

when we measure potential like using a

multimeter

ideally there's no electric current

going between

the two electrodes if we were to short

circuit this with a wire

then yeah definitely the hydrogen here

would be oxidized the h plus and the

iron would be reduced to the

two plus the reaction would go forward

because there's that that thermodynamic

driving force

where the iron three plus has a higher

reduction potential than the hydrogen

gas

being oxidized or excuse me the hydrogen

ions being reduced

uh but that's not good especially for a

a hydrogen

a standard reference electrode you don't

want to change the chemistry because if

you change the chemistry inside that

reference cell

then you're changing the reference

potential and you'll you'll see that

this this potential

would change uh so ideally when you

measure voltage across a

use like a multimeter there's very

little little current i

like nominally might as well be zero

very little current that goes through it

that wire so it's we call this an

open circuit voltage anyways

so back to this uh list of standard

reduction potentials

so again like i was saying this is this

is done these are measured in the

standard

state so it'd be like a one atmosphere

room temperature and then you know the

standard the activities of these are at

equilibrium so at equilibrium the

concentration of the reductant

is the same as the concentration of the

the

oxidant okay so that's that these are

equilibrium potentials

in other words uh so if you change if

you change the amount of

reductant or oxidant you're going to

change the voltage

so here's an example you know what if we

have iron three

plus and iron two plus and zinc two plus

and zinc metal in solution so we have

four different species

in the same solution i thought

i meant for this to be just going

so you could think about it but i guess

i forgot to add the animations

anyways so you have iron three plus iron

two plus zinc two plus and zinc

metal and the question is which one of

these species is going to oxidize

and which one of these species will be

reduced in this solution

and so if we look at the chart we see

that iron three plus the iron two plus

has a higher reduction potential than

the zinc two plus the zinc metal

so iron three plus is going to be the

species that's reduced

and zinc metal is going to be the

species that's oxidized

all right so nothing is going to happen

in the zinc 2 plus and nothing will

happen to the iron two plus we're just

gonna be

we're going to be making producing more

iron two plus and we're gonna be

producing more zinc two plus in our

solution

uh and those have their standard

reduction potentials right for these

different

these uh separate redox reactions uh the

overall reaction

is given here the zinc metal is

dissolving into the solution

and the iron three plus is reducing in

the solution

so you're going to accumulate more zinc

two plus and more iron two plus in

solution

you have an overall cell potential the

electric potential is related to the

gibbs free energy change of the reaction

so

you guys know that if you have a

negative gibbs free energy change

then the reaction is thermodynamically

uh

it will will go forward as well there's

a thermodynamic driving force

for the reaction to proceed so the same

idea if you have this cell potential

which is positive then there will be a

thermodynamic driving force

for this reaction to occur all right so

it's just a simple it's negative

z is the number of electrons i believe f

is uh faraday's constant

which is the number of coulombs per mole

of electrons

and then uh e is the the standard

reduction potential or that we

calculated for the reaction

excuse me the reaction potential okay

so let's look at this a little bit more

so this is like the setup that we just

described where we have a single beaker

of solution that contains these ions and

this metal

and i i said well the potential the

potential for this reaction is 1.5

volts but you know how do you measure

that

physically how can you measure that in a

single solution and the answer is

you can't measure that it's it's the

potential between the the surface of the

zinc

and the ions in solution right at the

interface

right so you just can't can't measure

that and this will hap this reaction

will proceed

and it proceeds at the interface of the

zinc where the zinc dissolves in the

iron three plus at that interface

of receive those electrons and turn into

iron two plus

let's see moving forward however we can

measure this potential using like a

multimeter

if we separate these two cells or the

these these two uh sets of species into

two cells okay and

separate electrolytes so in one cell we

have iron two plus an iron three plus

species

um in the other cell we have zinc two

plus and zinc species

also i want to point out i have i'm not

including any uh

counter ions such as chlorine or or any

whatever salt that these these

cations came from those can also affect

the redox potential and the activity of

these ions but just for simplicity i'm

leaving them out of the equation

those ions don't won't uh change their

valence

j we're just kind of folk we're just

interested in the metal ions in this

uh these reactions and so

here we have our iron species our zinc

species our zinc metal as an electrode

and we want to measure the potential

between the two solutions we have to add

an extra electrode for our iron

uh or iron solution which is platinum

again because platinum is inert and we

don't want to

to influence the chemistry of the

reaction but it's acting as a catalyst

so it's

we're measuring the potential at the

surface of the platinum and we also need

to include what's called a salt bridge

so salt bridge is just

uh something that can balance the charge

in both of these solutions so in

it's it's the golden rule for all these

systems that you

must maintain charge neutrality if we

take an electron away

from one of these beakers we have to we

have to give it

we also have to take a a positive ion

away

or in other words or if we add a

positive ion to a solution

we also have to add an electron to the

solution you know the overall charge

must must maintain zero okay

so for example if we're if we're

reducing iron three plus to iron two

plus

and we're receiving an electron from the

platinum

okay but now our solution has just

become

less positive or more negative because

we we've reduced the number of

three plus ions so we have to balance

that solution

by adding in some counter ions or taking

away counter ions

that and that's achieved by the salt

bridge it's just it's

maintaining ionic continuity

so we'll learn later that the essential

things for a battery

or you know the cathode anode and

electrolyte but also we need electrical

continuity through the circuit

and then ionic continuity between the

anode and cathode and this is achieved

by the salt bridge

anyways so at this point what we can do

is put a voltmeter between these two

electrodes the platinum the zinc

and that voltmeter should read the

standard reduction potential of our

reaction 1.5 volts

okay again these these two solutions by

themselves are at equilibrium

okay but compared to each other

uh there's that thermodynamic driving

force of 1.5 volts

for them to for the zinc to to oxidize

and for the iron 3 plus to reduce

okay so if we were to put a wire a short

circuit

this reaction then this this reaction

would go forward

but like i said before the multimeter

ideally

there's no current going through the

multimeter

so this is an open circuit voltage

that we're measuring and it also again i

want to emphasize that we're measuring

the potential at the surface of these

electrodes right

so these ions in the middle of the

solution there nothing is going to

happen to them

right they have to be at the surface of

the electrode in order to receive

an electron and to be reduced and the

same with the zinc the zinc

at the surface is dissolving into the

solution

okay uh so this is a an example of like

a two electrode cell

um oftentimes in electrochemical work

we use what's called a three electrode

cell

where we'll have a working electrode and

that's

that's where the reaction that we want

to study is happening at and this is a

half reaction remember so just

it's only oxidation or it's only

reduction happening at that

interface and then we have a reference

electrode that we're measuring the

voltage

against okay so like in the previous

example

we had uh the reference electrode right

the voltage

of the iron three plus the iron two plus

reduction compared to the

she raised 0.77 volts so that's what

this circuit is it's just measuring

voltage no current is going through

there

and then we have a counter electrode to

provide

the necessary electrons for this redox

or this reduction or oxidation reaction

either giving electrons or taking

electrons

away so there's another reaction that's

happening

at the surface of this platinum if we

were going to proceed you know drive

this

this uh reaction forward or or backwards

by changing the voltage uh there's a

there has to be a counter

reaction remember i said you you can't

just get electrons from nowhere

so there has to be a reaction happening

at the counter electrode that's

either providing or taking electrons but

you don't measure the voltage

of that reaction so you don't really you

don't really care as long as

your solution is large enough uh you

have a large enough quantity of this

electrolyte

that whatever reaction is happening here

does not influence

for example the concentration or the the

ph of the solution then it's it's it's

fine it's negligible

okay here's some different diagrams some

phase diagrams are useful to

electrochemistry one's the poor bay

diagram

so i've used this quite a bit kind of

shows you what's the stable species

at a given ph so this is this is

primarily just for

aqueous work so a lot of the battery

research i do is also a non-aqueous

battery so this is not relevant too much

but

you know different ph uh for different

different

uh species that are stable and what

their reduction potential is

uh and so it also useful for the you

know

predicting different chemical reactions

in solution

and then the other one is the ellingham

diagram and i think you guys have seen

this before it's like the first or

second page in your kinetics textbook i

believe

that you've used the last quarter and

hopefully you've done some calculations

using that

if not i think later this quarter when

we do thermal properties lab

we might take another look at this

because we talk about the oxidation of

different metals

but basically you can you can use this

diagram to show you know what species

you would need

to reduce uh metal oxide so it's very

useful for

smelting of ores or refining of of

metal oxides right so most cases you're

using carbon or carbon monoxide

as a reducing agent to reduce metal

oxide such as iron oxide or aluminum

well aluminum oxide is a different

process but like iron oxide

to reduce it into iron metal

okay so let's talk about batteries now

um like i said before i started any

research on batteries i had

i knew very very little about batteries

so i've had to learn everything on my

own

uh and these are kind of like the

the basics of how batteries work and

specifically i

i say ion batteries but i mean all

batteries

need ions i at least i haven't found any

batteries that don't use ions

but there's three essential parts of all

batteries and that's the cathode

the anode and the electrolyte all right

so there was going to be a

a posi a potential difference between

your

cathode and anode just like in that cell

we saw before

you know there's this potential

difference between this solution and

this solution because of this redox

reaction

because of the reduction potential

between the two okay

and then the electrolyte is the medium

that allows

ions to transfer in and out of your of

your

electrodes um a common well i wouldn't

say common but

sometimes it's misconceived that you

know

you have an ion that's in your cathode

and when you

when you dis or when you charge the

battery you're taking an ion out of your

cathode

and it goes into the anode and

oftentimes i see these diagrams where it

shows the ion

moving all the way from the cathode all

the way to the anode it's the same ion

that goes back and forth

in reality that's that's very unlikely

that you're going to have the same ion

go all the way to the anode in reality

all these ions that come in and out of

these different materials

are probably going to be really

relatively close to the surface and not

travel too

far in distance to go to all the way to

the other side

another misconception that i i've seen

is that

all lithium-ion batteries commercial

lithium-ion batteries that are

rechargeable like your cell phone

battery or or

your computer battery or the batteries

in electric cars

they do not contain any lithium metal

right

they instead for an anode they they all

contain graphite

okay lithium metal is just too dangerous

of a material to be using in a lithium

a rechargeable lithium-ion battery right

as the more

cycles you you charge or charge and

recharge

uh the lithium metal will will start

precipitating dendrites

and those dendrites can grow and over

many many cycles these dendrites grow

eventually they touch the other side and

that's when you get a short circuit

right so if the cathode and anode are

touching each other

then you're going to have a complete

short circuit and you can't have any

electron you won't have any electrons go

through an external circuit

they'll just go straight to each other

and so that can generate a lot of heat

it's very high current it generates a

lot of heat and that can cause problems

and so in order to to prevent that

in in all batteries there's a permeable

separator is another important part

that's often left out of these diagrams

the permeable separator can be uh for

sodium ion batteries we use like a glass

fiber so you know it allows the

diffusion of liquids through it but

it prevents it prevents uh the anode and

cathode touching

in lithium ion batteries we use a

polymer

porous membrane so it has a very small

micro pores that still allow the

liquid electrolyte to diffuse through

and allows

ionic continuity but no uh

electrical continuity because when

commercially when you make these

materials you want to

make them as compact as possible and so

the anode and cathode are going to be

very close to each other

but you don't want them to be touching

so there'll be a separating

material in between and then i think for

the

most of the work you'll see is going to

be for non-aqueous ion batteries but

both

is they're all it's the same same

concept

for both for aqueous and non-aqueous

okay

so here's some definitions before i go

forward about uh

you know different definitions of the

things we use in batteries such as

capacity of the battery so capacity is

equivalent to

how many electrons are transferred

you know between the anode and the

cathode or between the reduction and

oxidation

uh process it is exactly equivalent to

the units of capacity are coulombs so

one mole of coulombs is 96 000

oh sorry one mole of electrons is 96 000

coulombs and that that

that is called a faraday's constant is

this

this relation however the conventional

unit for capacity

and you'll see this written on you know

a lithium-ion battery

if you have a cell phone that you can

still take the battery out it'll

probably say how many amp hours of

capacity it has

and so they use this convent this unit

and one amp remember amp is a

coulomb per second so one amp hour is

one coulomb per second times

at one hour which is 36 100 seconds

so that's one amp hour is equivalent to

3 600 coulombs

okay so this is the conventional you'll

see this written a lot amp hour milliamp

hour

is another that we'll use um

so that's capacity so again how many

electrons are being transferred

and energy is the capacity of a battery

times the voltage that the redox

reaction is occurring at

right so for like in our previous

examples

um

well i'll make it simple if you if you

have a redox reaction and it happens at

one volt

versus versus that zinc metal right

um and you're you you uh

it lasts for one hour and the current

that you're discharging it at

is one amp so one amp of current

for one hour the capacity is one amp

hour and if that redox potential was one

volt

then it'd be a one times one to be one

watt hour

of energy right so the the units of

energy is joules but the conventional

units that we use is watt

hour so remember a watt is a form of

is the unit of power which is the

current

times the voltage the watts are joules

per second

so current is coulombs per second volts

are

joules per coulomb okay and so that

makes a watt so one

watt hour is one joule per second times

3 600 seconds it's 3 600 joules

so that's energy okay so again energy

has to do with voltage and capacity oh

another another thing to watch out for

is the term capacity versus capacitance

capacitance is is different the units of

capacitance is a

ferrad i believe f ferrad and those are

used for capacitors and i believe

capacitance is uh

coulombs divided by volts the

the the i guess capacity divided by the

voltage

i i'm not too familiar with capacitors

actually although capacitors are very

similar to batteries

in fact some supercapacitors are

basically just batteries

that discharge and charge at very quick

rates is the the point of the capacitor

um so here's an example calculation for

uh

the theoretical capacity so theoretical

capacity is you know if we were to

reduce all of the material the valence

change the valence state of all the ions

in this material

you know what how much capacity would

that be

for that material as a battery and so

here's the example of this material

vanadium uh penta oxide v2o5

so the you know the first question asked

is what's the starting valence

of this material all right the so the

starting valence is a

vanadium five plus um and the next

question is you know how much are we

going to

uh reduce it by vanadium is a very

unique material

ion it can have multiple valence states

that are in this

very stable so it goes from vanadium

five plus the name four vanadium three

vanadium two i don't think there's a

vanadium one is

very stable but all those four valence

states five to two

are stable forms so you could you could

reduce this

material all the way down to two i'm not

sure if it would retain the same crystal

structure if you did that

because there's only a limited number of

space for for ions to get inside

but anyways so in this case let's just

say we're going from vanadium five plus

to vanadium four plus

okay so the first question is how many

moles

of electrons are transferred if we

reduce all the vanadium 5 plus to

vanadium 4 plus

i'm going to go grab myself a coffee

actually you guys

spend the next five minutes trying to

calculate the theoretical capacity

in milliamps amp hours per gram

all right when i come back we'll see if

anyone's uh

progressed from there i'll do the next

slide so

the answer here is 2. the next question

will be 2 moles of electrons is how many

coulombs of charge

all right i'm gonna pause the recording

actually and uh i'm just gonna go grab

myself a coffee

five minutes i'll be back let's see if

anyone's come up with an answer

and if not i'll get it

did anyone come up with a uh answer for

the theoretical capacity

of v205 if we reduce all the vanadium

five plus the video four plus

okay i don't think anyone did it

that's okay all right so hopefully you

guys are still listening

because i have no way of knowing if

you're listening or not anyways

so uh we've we've determined that

there's going to be two moles of

vanadium

or two moles of electron per formula

unit of v2o5 that are transferred for

this reduction process

so how many coulombs is two moles of

electrons

remember we use faraday's constant

right we use uh this number here

faraday's constant

so one mole of electrons 96 000 coulombs

all right so now we have the number of

coulombs

of charge that's been uh transferred in

this reduction process per mole of v205

and then we can convert that to milliamp

hours per gram

okay so uh one

uh coulomb divided by

let's see how did i do this i already

forget

coulombs all right so one amp

hour of capacity is equivalent to 3 600

coulombs

right because 1 amp is a coulomb per

second

and an hour is uh 3 600 seconds

okay and then you multiply by or the

inverse of the molecular weight

then that will give you actually it'll

be in amp hours but you multiply by a

thousand to get milliamp hours per gram

okay so the the theoretical capacity per

gram

of this material for just one one

electron

transfer 295 milliamp hours per gram

that has a pretty good capacity for one

electron

transfer for a electrode material

anyways okay

so one more definition is definition of

power

for batteries so the power is the amount

of energy transferred

divided by how long it takes to transfer

that energy

right so we could have a battery that uh

the current in the battery if we're

drawing current for the

from the battery very slowly at a really

low current

so it takes a long time for that energy

to

deplete it has you could say that that's

a very low power

okay on the other hand if we take if we

can discharge

that battery in a very short amount of

time then it has very high power

the problem is as typically in battery

materials as we

increase the the current or in other

words if we're decreasing the amount of

time to discharge the battery or

increasing the current the

increasing the number of coulombs of

charge

per second uh oftentimes

that that the consequence is that we

have lower

voltage of the battery and then also

lower capacity of the battery which

results in lower

energy so as we if we try to discharge

the the battery faster

our power density tends to go down and

so that's a kind of a big problem with

with batteries and it's what separates

batteries from super capacitors

that super capacitors they might have

very low energy

but they can they can discharge and

charge very quickly

and so that they should have a higher

power where batteries are

more limited by kinetics like for

example the kinetics of

ion transport within the material you

know you have to rely on the diffusion

of ions within the solid

or also electrical conductivity a lot of

these battery materials have very poor

electrical conductivity not only do you

have to transport ions in the material

but you have to diffuse the electrons

the material

and so if you try to do that too quickly

you're going to result in

a higher resi higher impedance and so

that's going to decrease your your

capacity and

energy anyways the units of power is

watts of course

joules per second so what's nice about

these conventional units when we took

energy in watt hours and we just divide

it by the amount of time it takes

to achieve that energy then we just get

watts

so i'll i'll briefly talk about the

three

mechanisms of charge storage in a

battery

so the first uh mechanism is called

intercalation intercalation if you were

to look it up in a dictionary

i believe at least at least maybe 20

years ago it would say

uh something about taking a day out of

the calendar

year or putting a day into the calendar

year so it has to do with the calendar

um and particularly it's talking about

the february 29th a leap day year where

we're taking a day

out of the that calendar and then every

four years we put it back into the

calendar intercalate

um but we use it the same

idea for for materials

that we're inserting an ion into the

material or we're taking an ion

out of the material but the overall

structure

of the material the crystal structure

remains the same or relatively

unchanged uh so just like we're changing

the calendar

year the the calendar structure doesn't

change anyways

so basically we're yeah we're this

happens uh

in a lot of the different types of

crystal structures uh but primarily

layered crystal structures uh it's easy

to do this because you're just inserting

an ion

in between the layers for example

graphite which is a layered crystal

structure

you can easily in insert ions and that's

why why we use a graphite as a

anode material because the the potential

to insert

ions into like lithium ions into

graphite is relatively low so it makes

it a good anode material you want to

have low

redox potential for anode materials and

then metal

transition metal oxides that are layered

like vanadium oxide

or uh manganese oxide these these have

relatively higher

um redox potentials for these these

transition metal oxides so we use those

as cathode materials

so this is just an example again the

v2o5 if we

intercalate two lithiums and also we add

two electrons

then the this new formula is

lithium two v205 but it retains

more or less the same crystal structure

but we're just expanding

or and sometimes it contracts actually

uh

the inner interlayer spacing and so

obviously you know if you insert an ion

in between layers

you you can conceptualize why it would

expand right we're putting material in

between

but in some instances you'll actually

see a contraction of the layers and why

could that be

and the reason is that these layers are

are layers of

like vanadium and oxygen or a different

transition metal and oxygen the oxygen

is negatively charged

right so you have two kind of negatively

charged uh

planes uh kind of against each other

and then you insert a positive ion in

between and that the coulombic forces of

the positive ion

bring the layers closer together so they

actually get smaller even though you're

in

putting putting material in

yeah so intercalation is one of the main

mechanisms for

for lithium-ion batteries in fact the

nobel prize winner for

uh lithium-ion batteries um dr

whittingham

and amongst other others but dr

whittingham

got it because of his discovery of the

intercollation process for

the modern lithium-ion batteries

actually another note on that dr

whittingham uh

made this discovery while working at

exxon

the gas company so they were doing a lot

of research into a

lithium-ion batteries they're just kind

of kind of funny the big gas

it's not just gas it's a energy an

energy company you know is investing

in uh lithium-ion batteries

that was that was during the 70s though

so the

the next uh charge storage mechanism is

called conversion so this is just just

like

we're breaking down the bonds of a of a

material and we're

we're significantly changing the crystal

structure of that material

uh so for this example we have tin oxide

uh or any kind of metal oxide like that

uh adding lithium

and electrons and um then that

turns the tin oxide into tin metal so it

went from tin four plus to 10

0 or neutral metal and the lithium

becomes a lithium oxide a solid

so this this is this tends to happen

at lower voltages so a lot of anode

materials

that are being researched so if we go

back to the this uh

standard this standard right if we look

at these

these materials um right we're turning

them into metals

right so like uh where's the tin tin tin

two plus the tin metal

tin four plus those ten two plus um

so these often happen at lower voltages

so they're

they're more prevalent for anode

materials um

if i actually go back uh the highest one

the highest voltage would be copper so

there has been a bit of research of

trying to utilize

copper two plus the copper metal as part

of

different materials to help increase the

capacity like if we could

if we could make copper two plus we

would copper one through intercalation

and then copper one plus the copper

metal that could increase the

capacity even further but the problem is

you're just significantly

changing the structure of the material

and that's going to make it more

difficult

for a reversible reaction if we want to

you know we can discharge the battery

but

the question is can we charge the

battery and retain the same capacity

so oftentimes in conversion reactions

capacity cyclic capacity

over many cycles the capacity gets

smaller and smaller and smaller because

you're you're losing

material or the material is uh you know

it's

the you're losing continuity electrical

continuity for example with the

electrode

so yeah the crystal completely changes

and then the last form of charge storage

is alloying

so in this case you might have a metal

that can make an alloy

with lithium for example or whatever ion

you're using

so in this case again tin metal can

can be lithiated with lithium and so in

this case we're going from metal but

then we have lithium ions

the lithium ions is what's changing

their valence state and so

now just have a neutral metal this can

this these reactions these alloying

reactions

they can have a very high capacity and

also very low voltage which is

which is good for anode materials if you

want to increase the energy density of

your battery

the problem is that there's very

significant volume change in these

materials

tin for example has a very large volume

change another material

that has been a lot of research is a

silicon as an anode material because

silicon can

can absorb a lot of lithium but

again the volume change is very

significant i forget the exact number

but like

over over 500 percent change in volume

so you can imagine if you're changing

that much of your volume

you're gonna have a lot of mechanical

deformation uh and so

over many cycles you get significant

loss of capacity

because because of the mechanical

deformation that's occurring in your

electrode that the pieces are breaking

off

perhaps and yeah

so there's some some research that's

looking at you making very nanoscopic

uh particles of tint uh or of uh

of uh silicon or nanowires so they they

can they can

take they can have space for that volume

change without breaking

um and then another thing to consider is

uh

the selection of your electrolyte um and

i don't talk about

too much about the electrolytes and the

solvents that are used

but the electrolyte also has a

limit of you know if if you have a

cathode inside

your electrolyte it's going to have a

limit where it starts to reduce or it

starts to oxidize in contact with your

cathode or

anode and so it's depending on the lumo

the lowest

unoccupied molecular orbital in the homo

uh

highest occupied molecular orbital of

your electrolyte

uh solvent and so if your cathode has a

lower energy

or excuse me uh yeah lower energy than

the homo then you'll have a reduction

and if it has a higher energy than the

lumo the anode has higher energy than

luma they don't have

oxidation so for example for aqueous

batteries this severely limits

the selection of materials and

electrochemical processes for

aqueous batteries because you have to

deal with what's called the her the

hydrogen evolution reaction

that's where water breaks down into

hydrogen ions and the hydrogen

ions turn into hydrogen gas so that

happens

at a pretty a low voltage and then

so that's about point it also depends on

the the ph of the

the solvent as well and then on the

other end you have oxygen reduction

reaction

so that's when water breaks down and

you get oxygen ions that turn into

oxygen gas

so that's one of the limits for working

with aqueous ion

batteries and

also non-aqueous ion batteries so

there's some common electrolytes

for non-aqueous at least the solvents so

i should say the electrolyte is made out

of a solvent

and a a salt so the solvent for

non-aqueous electrolytes are typically

different types of carbonates like

propylene carbonate ethylene carbonate

dimethyl carbonate so these organic

materials they have a very high working

potential

so we can allow for higher voltages or

voltages as low as the lithium

without it breaking down and even even

then some of the materials do break down

and it forms what's called a electrolyte

interface

so on the surface of your material

you'll have a like a thin

layer of a byproduct of your electrolyte

that's broken down on the surface

but that that creates like a passivation

for further breaking down but still

allows ions to diffuse through that

layer

um but some cases that doesn't happen so

then you're you're

constantly breaking down the electrolyte

which is not good

and then the salt that's used is also

important the salt determines you know

what

what ions your ion battery is so for for

non-aqueous lithium ion batteries a

typical

salt is lithium hexafluorophosphate

it's lithium h excuse me

pf6 hexafluorophosphate is a common salt

um and then for sodium ion batteries

non-aqueous systems the sodium

perchlorate is another

common sodium salt so they they dissolve

in the electrolyte excuse me

they dissolve in the solvent and that's

what makes up the electrolyte

okay so some different properties of

these battery materials and

what they how they influence the

different performance

of the batteries so for the electric

material of course composition or in

other words the chemistry of the

material

that will highly affect things like the

electrochemical potential

so right you know if we're changing from

iron

ions to vanadium ions that's going to

change the voltage of our system

um you know even if we keep everything

the same if we just replace

if we have the same crystal structure

and we replace iron with vanadium that's

going to change our

our crystal structure i'm assuming that

forms the same stable complex which

might not

um and not only that but you know you

can have the same transition metal ion

like iron three plus iron two plus

and change its local environment you

know whether it's surrounded by

uh octahedral coordination of oxygen or

a tetrahedral coordination of oxygen

you still have the same transition metal

but whether it's not whether or not it's

octahedral or tetrahedral that will

change the potential of your battery as

well

and it goes even further than that it's

not only the nearest neighbor but

it could be the next nearest neighbor so

for example iron phosphate

lithium iron phosphate compared to

lithium iron sulfate

so in both cases iron is i think uh

i forget if it's octahedral or or

tetrahedral

but in both cases the iron is

surrounded by oxygen but in one the

oxygen is coordinated to phosphate

or phosphorus another one that is

coordinated to oxygen

well i say oxygen is coordinated the

sulfur and so just by changing those

next nearest neighbors you can also

change the electrochemical potential of

the battery or the discharge potential

of the battery

uh so it all has to do with the energy

levels of these of these

transition metal uh elements and how

near the local environment affects that

energy

and then like crystal structure can

affect things like uh

the cyclic stability right

like i said if you if you have um or

capacity is a better example

if you have a nice layered structure

lithium ions or sodium ions can diffuse

into the layer structure pretty easily

but there's other crystal structures

that are not layered or they don't have

good

channels for lithium-ion diffusion so

that can that can

severely affect the capacity and or like

things like power density because

uh you're the diff if your diffusion is

limited

you're eliminating you're limiting the

kinetics of your reaction so

you can't you can't just uh increase the

current of your battery there's going to

be a

much higher impedance to that so that's

going to decrease capacity and

power density morphology is also a big

factor about

with that like i said um kinetics is a

big part

of of the pro performance of the battery

you know how fast can you diffuse

lithium

and also electrons electrical

conductivity within your material

and just by changing the morphology you

know say you're changing it from

a micro particle to a nano particle

but you keep everything else the same

the crystal structure same composition

is the same just changing the size of

the particle

can change the capacity and the power

density because you're changing the

kinetics

i have an example of that next

the electrolyte right so what solvent

you choose like i said that changes uh

your electrochemical window

whether you're using water or something

else also some of the solvents might

react with the electrode material

um and then the ion choice also so

uh depending on what your ion is uh you

know

or depending on what your solvent is

some solvents can't dissolve certain

salts

and so on okay

so here's an example of changing the

morphology

uh in titanium oxide so titanium oxide

is not

a very good def uh doesn't have a very

high diffusion

of lithium ions and it doesn't it has a

very poor

electrical conductivity you know it's

it's a wide band gap insulator right

so if you you start with micro particles

you know you can you can reduce the

surface of the titanium four plus the

titanium

titanium three plus and intercalate a

bit of lithium

but it only diffuses into the surface so

only the surface

uh is is reduced um and so that severely

limits the specific capacity or the the

amount of electrons per gram of material

transferred

but if you were to take the same

material and make it on the nano scale

now you're really decreasing the excuse

me

really just decreasing the diffusion

distance of the lithiums the lithium has

no problem just diffusing a

few nanometers and then you can fully

uh achieve the the full or near

theoretical capacity in that case if you

make

very small materials and so there's an

example of this from this paper

and where they have titanium oxide micro

particles is about diameters about one

micron

and then compared it to a different type

of titanium oxide particle they call

this the the urchin

particle so it's basically a a sphere

but it's been etched away so you have a

very high

surface area they're just kind of flat

sheets that make up the sphere

and nanowires and so it has very high

surface area

and then the thickness of the the sheet

is very small so the diffusion distance

from like the lithium outside to inside

is very small

and so you see just the difference in

specific capacity

you know for the high surface area

urchin type titanium oxide compared to

just the bulk tio2

so just by changing the morphology

making it smaller

you're increasing the capacity also

you're increasing the the

performance i'll talk a bit about this

type of data a little bit later

so this is an example of how

i make batteries in my research it's a

bit different than like if you're making

batteries from

commercial applications obviously in

commercial applications

everything's mass produced you have

these printers that print out the

electrode material

very quickly right so just for research

we use these little button cells

the coin cells um so we do we start with

our material

so either a cathode material and a

material it's whatever our active

material is that we're studying

we mix that material by weight or these

numbers can change but i use 70

and it's fairly standard and then you

add carbon

we use a black carbon there's different

types of carbon that have different like

surface areas and

you could also use some people use

graphite that the purpose of adding

carbon

is because a lot of these materials have

low

uh electrical conductivity for example

my material here is a semiconductor

and the purpose of adding carbon is to

increase the electrical continuity of

your electrode so you can

help deliver the electrons throughout

your electrode but the carbon itself

does not participate

in the electrochemical reaction unless

you have graphite

and you're working at very low potential

then you could have

at low potentials you can have low

lithium intercalation but

for my material as a cathode i work at

higher potentials it's above the

potential for intercalation

so it doesn't happen so it's inert

basically it's just

added to increase cu electrical

continuity and then we keep it all

together using a binder

so a common binder that we use is called

a pvdf

um polyvirum bean difluorine

some something like that basically it's

a carbon chain

and then um you have fluorine attached

to each carbon and also hydrogen

attached to each carbon

so very similar to teflon which is a

carbon chain with

two fluorines attached to each carbon

and so it's it's also chemically inert

but we can dissolve it we dissolve it in

a solvent

called nmp nmp is a very it's very nasty

solvent

it's uh it eats through a lot of

different polymers including like

those the purple nitrile gloves that can

eat through those purple nitrile gloves

so we have to wear

special gloves when handling it and so

we

we mix that together and either like in

a mooring pestle like this where we

we kind of mix it all together or we can

put it into a

little a little container and we

ultrasonicate it with a very powerful

ultrasonic probe

much more powerful than a regular like

little container that ultrasonicates it

and then we spread it out the slurry

onto

the electrode or what we call the

current collector now depending on if

you're working with an anode or

cathode you'll either use aluminum foil

or copper foil as you're you're a

current collector

the reason is for for lithium-ion

batteries and as well as sodium ion

batteries if we use

aluminum lithium and sodium will

alloy with the aluminum so it's an

active material

but it alloys only at low voltages that

we would use

like for anode materials so for anodes

instead of aluminum we use

copper and lithium and sodium do not

alloy with copper

so we use copper foil for that however

at higher voltages copper

will oxidize in the copper ions so we

don't use copper for the cathodes we

only use it for the anodes

aluminum on the other hand aluminum also

oxidizes but aluminum has a passive

oxide layer on it

which passivates it from being further

oxidized at higher voltages

so that's why we use aluminum foil for

cathodes and a copper foil for anodes

anyways so we we doctor blade it onto

this aluminum foil i do mine just by

hand i take like a glass rod

and i just i put it onto the aluminum

foil and i just spread it out

and then i let it dry and after it's

dried like in this picture

i'll cut out little little circles with

a punch we have a

little like a lever punch that cuts out

these circles

um and then we weigh those and then

everything gets transferred to

the glove box this is a argon glove box

so the atmosphere is ultra pure argon in

fact it's 99.999 percent

argon and the the expensive stuff

and uh this glovebox also has a filter

system where it filters out oxygen

and water through if there's any leaks

in there

and so the oxygen water content of this

glove box is always

less than 0.5 parts per million

every now and then it will kind of bump

up to it'll start going up

over time because the the cattle there's

a

in the circulation system of the

glovebox there's a copper catalyst

the copper will will absorb the oxygen

and water water from the the atmosphere

and turn into copper oxide and then over

time that catalyst

depletes itself and it has to be

regenerated and then the oxygen starts

to go up so every every

two months or so we'll we'll do a

regeneration process

where we we attach the the catalyst to

uh

hydrogen so we have a five percent

hydrogen argon mixture

and we run hydrogen through the copper

ox which is now copper oxide

catalyst and the machine heats it up and

it reduces the copper oxide

back into copper and then then it's good

for another

couple months before we have to do it

again so

that's the regeneration process there's

a lot of upkeep that has to go on with

this glove box

another thing about the working with the

glove box is that it's uh

it's not very comfortable because it's

under it's under positive pressure

uh and so it kind of feels like you're

working under water

and also you know along with the the lab

coat we also wear like the purple

nitrile gloves and i also wear

like these chemical resistant sleeves

just to keep my lab coat on

and then we put that into these big

rubber gloves okay

and then on the other side inside the

glove box we also put on extra large

purple nitrile gloves

and so we got right already three layers

of gloves and you can imagine during the

summer it gets kind of toasty in there

and then additionally if if we're

working with sodium

i work with sodium a lot and the sodium

comes in chunks of sodium i have to

actually cut the sodium

so i use a knife and we've had instances

in the past where people accidentally

cut the glove and then

we start leaking argon out so now the

rule is we have to wear these giant

rubber gloves you see them in the

picture these uh kind of yellowy

pale gloves that go on top of these

already

big rubber gloves so it's it's very

uncomfortable

when we're trying to cut things and

and um the lithium on the other hand the

lithium we purchased lithium chips

they're already in the little cert

pre-cut circles

so it's they're very convenient to work

with but sodium

although however sodium is a much more

reactive metal than lithium

and so it oxidizes more easily

so there's i don't think there's a

manufacturing method to make the pre-cut

sodium to be able to ship out so we get

the blocks of sodium or maybe there's no

demand

as well that could also be it but we get

the blocks of sodium we have to

slice the sodium and then we have a

inside the glovebox we have a

pasta roller just like uh you know

making like

pasta from scratch rolling out the dough

into a sheet

and so we put the sodium chunk uh in the

pasta roller and we roll it out into a

flat sheet of sodium

and then we take a circle punch and we

punch out

the little circle chips of sodium

anyways so then we assemble the

the battery inside the glove box we have

the working electrode on one side that's

connected to

the top or bottom of this uh coin cell

the coin cell is stainless steel so you

need to make sure that whatever

electrolyte you use some of these

electrolytes are very corrosive

you want to make sure that the

electrolyte does not corrode the

stainless steel

and also you want to make sure that the

stainless steel does not react

with your you know when you're applying

a voltage to it there might be some

other chemical reaction happening at the

surface of the

stainless steel and that's going to

influence your results but the stainless

steel it's it you know has a chromium

oxide

passive layer so it's it's pretty

chemically inert

and then there's the separator that we

add

at this point we would add the

electrolyte so we have like a beaker of

our electrolyte some of them are

pre-made that we purchase

and then some of them we have to make

ourselves so we have our solvent and our

salt

and we add the salt to the solvent and

mix it up and then we

pipet it into this uh this coin cell

then we add the counter electrode which

is probably just lithium or sodium

and then in some of the cells we add

stainless steel spacers and springs

and then also importantly is that the

top of the cell

we you need to make sure that the the

counter electrode and the top cell does

not make any

electrical connection with the bottom

otherwise you short circuit the cell

so there's a a plastic plastic gasket

that protects the two

and then once you have that assembled

there's a device inside the glove box

that that's a press that crimps

crimps the the button cell so we put in

the press and we

we press it down and it compresses the

button cell so everything's

uh sealed in and then it can be taken

out

so i'll talk uh i'll i'm about to wrap

it up just a few more slides i believe

um actually i think i've added more so

we'll go through it though

uh some of the different tests we do for

batteries

um the machine we use is called the

potentiostat or galvanostat

and um this is an example of a

galvanostag

we have the same exact one in our lab

and uh so it has

many different channels that you can put

your batteries in and you program it

um so each channel actually has four

wires right like i said before for a

three electrode cell

you know you have voltage between the

working electrode and reference

electrode

and you have the current between the

working electrode and the counter

electrode

however for the two electrode cell like

our button cell

the reference electrode and counter

electrode will be the part of the same

same cell so we only have uh you know

two clamps

okay um so this is

one of the most common type of test

cyclic

voltometry where voltometry is uh

basically

we're we're controlling the voltage of

the system

while we measure the current response at

these different voltages okay

so this shows uh what at what voltages

different redox reactions happen at so

in this example

we have a sodium vanadium phosphate

material

now when you make this material more

very likely there will already be sodium

inside the material when you synthesize

it so

let's say it starts with it probably

starts in the vanadium

of let's see yeah vanadium four plus

is that right yeah starts out as

vanadium four plus with one sodium in it

i'd say

so in that case if you were to take this

material

and it's freshly made and it's inside

your battery and you were to measure the

open circuit voltage of this battery

this material is in the vanadium four

plus state

the open circuit voltage would say it's

around this this

would be right here so right there's no

current going through the battery

you're just measuring voltage it'd be

right in between here and so that's it's

at

the equilibrium state at that potential

okay and so what this test will do is it

sweeps the voltage for example let's we

start with

decreasing the voltage and as it

decreases the voltage you start to get

a reduction reaction if it's negative

current that's reduction

so we're giving electrons to the cathode

of this material

and we're taking electrons away from the

counter electrode so in this case the

counter electrode

our voltage is v versus sodium metal

so the counter electrode is just the

sodium redox reaction

so we're taking electrons away from

sodium sodium is dissolving

into the electrolyte and the vanadium

four plus

is uh reducing the vanadium three plus

and at the same time we're intercalating

sodium ions into the crystal structure

okay so that's what this current this

peak represents

okay and then and then after that peak

uh

we we keep on pushing the voltage and

there's nothing no reaction happening

because we haven't met that uh redox

potential yet for the next

uh reduction reaction which is an am3

plus and then the same thing happens so

we and we

intercalate another sodium ion into the

structure

okay and then at this point now we've

fully reduced

vanadium four plus into vanadium two

plus so you could say this is the

discharged state

of the battery and then we reverse the

voltage sweep

and then the opposite happens the

oxidation potential

the oxidation reaction happens on the

cathode

so now sodium is sodium ions in

electrolyte are plating onto the sodium

metal so they need to receive an

electron

and the cathode uh the sodium vanadium

phosphate

is is oxidizing it's giving away an

electron

right and that's given that's indicated

by the positive current

all right so basically what this shows

is which uh what potentials

what reactions are happening although

you have to have a bit of knowledge on

like what

you know what species is being reduced

in what species it doesn't tell you

obviously but it can tell you a bit of

information like the kinetics like how

how high the current is um and the

reversibility so this might happen over

many cycles and see if it's

repeatable or if the chemistry changes

you know after if you cycle

the battery many times perhaps the local

environment of the vanadium might change

and what you would see is that these

peaks begin to shift and

in potential you say oh it's not it's

not stable it's not a reversible

reaction because things are shifting

around

right which is not ideal for a battery

if you want to put in a consumer

electronic

so that's the kind of information you

can get so cyclic photometer again

controlling voltage measuring current

i had some examples of psychophotometry

i think we might skip through it because

uh we're a bit over time

on this lecture and my voice is starting

to go so i'll just

briefly introduce the second most common

test is galvanostatic cycling so if

cyclic

voltometry was controlling voltage and

measuring current galvanostatic cycling

is controlling current and measuring

voltage response

okay so this is the same example with a

sodium vanadium phosphate

and this this type of test you'll say

discharge at a certain current so for

example

uh 10 milliamps per gram all right

so you you know the the amount of

material in your

battery because you weighed it before

you assembled the battery

so you can calculate what 10 milliamps

per gram is

for that that battery right and give it

a

current oftentimes we use what's called

a c

rate uh so this symbol here 1c

1c is equivalent to the current density

needed to completely reach the

theoretical capacity of the battery in

one hour that's so that's one

c so 0.1 c or c over 10

would be what that'd be six minutes

so it that would be it would take six

minutes

to reach the theoretical capacity

at whatever current that is anyway so c

rate is often used

uh and the other common uh current rate

is milliamps per gram is also the common

they're they're basically equivalent

um so in this type of test you know

let's say we start with our material and

it's in the charge state

all right it's fully charged that means

it's fully oxidized

vanadium four plus so as soon as we

start the test

you know then the voltage will will

undergo it'll reach its

uh this is the the this plateau

the voltage of this plateau is

equivalent

to this peak this voltage

right so you see this is the redox

reaction happens at this voltage

vanadium four plus

the radium three plus that's essentially

the same as what this plateau is showing

is that

this is the reaction happening and then

capacity

this x-axis is essentially the same as

time

so remember we're discharging at a

constant current

and so for example uh 100 milliamps

and then this is just measuring the time

and so you just multiply by time as

milliamp hours and that's capacity

so this kind of shows how much capacity

there is in that material

and then it reaches a certain voltage

and uh this is a bit arbitrary we

we decide as a user when to stop the

test

you know say like oh this this person

could have stopped the test at 3 volts

or it could have stopped at 3.2 volts

and it would have been

more or less the same although this

this study didn't go to the further

reduction state of vanadium iii plus the

venem 2 plus so that's and this is the

fully discharged state just from

vanadium for the video three

and then what you'll do is you'll

reverse the current so if we this was

negative current to discharge

then you'll reverse it the positive

current the charge and so you'll

you'll go back to the origin and it'll

go up and this is the

the charge potential the plateau

representing the vanadium three plus the

m4 plus reaction and then again it's

arbitrary when you want to stop the

test so that's a galvanostatic rate

stability is another uh

very similar test where you're changing

the current rate that you're discharging

or charging battery at so that's again

with the c rate

representing the current rate so like i

said before

as you increase the current rate the

your your re

you're bumping into certain kinetic

limitations of this

uh electrochemical cell for example the

lithium diffusion

in your material or the electrical

conductivity of your material

or perhaps the the there's an energy

barrier

for electron transfer between your your

electrode and the material

so if you try to increase the current

higher and higher you're reaching those

kinetic limitations which is going to

start

dropping the voltage all right because

there's going to be a voltage drop if

you have a resistance it's essentially

the same as v equals ir

if you have a resistance and you

increase current the voltage

drop in this case a voltage drop

increases so if we were to take this

plot

and increase the current we would expect

this plateau to get smaller and smaller

and smaller and smaller

and then also the the capacity would get

smaller and smaller and smaller as well

that's what we see exactly here as we

increase the current our capacity is

getting lower and lower

okay um yeah i think we'll stop there

we covered a lot and now it's uh

uh some of it's a bit complex but i i

hope you guys kind of get a

broader understanding of kind of battery

materials energy materials and

electrochemistry is there any questions

about what we covered

all right if there's no questions i'll

see you guys thursday um i haven't

planned out

exactly what we're going to cover

there's a lot of slides i haven't

included

um about some of my own research in the

battery results

some of it's kind of interesting but i

still want to talk more about maybe the

the data analysis a bit so i'll find

something to talk about on thursday

and if there's no questions you guys uh

are good to go i'll see you on thursday

you