A “Gold Rush” for High EnergyA “Gold Rush” for High EnergyA  Gold Rush  for High Energy A  Gold Rush  for High Energy Batteries ?Batteries ?

John MuldoonJohn Muldoon Toyota Research Institute of North 

A iAmerica

Moving Away From Alloys:  Towards Metal AnodesMoving Away From Alloys:  Towards Metal Anodes

9

6

7

8

e (p

pb)

3

4

5

Abu

ndan

ce

0

1

2log

Li Na K Be Mg Ca Zn Al

Li Na K Be Mg Ca Zn Al

MWMW (g/mol) 6.94 23.0 39.1 9.01 24.3 40.1 65.4 27.0

Density (g/cm3) 0.53 0.97 0.86 1.85 1.74 1.55 7.14 2.7

A magnesium anode is very attractive due to high capacities, reductive potential and abundance in the earth crust

Reduction at the Anode: the SEIReduction at the Anode: the SEIReduction at the Anode: the SEIReduction at the Anode: the SEI

Mg2+ Mg2+

SEI conducts Li+

Li+

Li+

Li+

Passivating layer

Mg Mg2

Mg2+Li+ X

SEI conducts Li

Li/C

Li+ Passivating layer

Mg

• In contrast to Li, an SEI (solid‐electrolyte interface) on Mg precludes the use of many electrolytes.

• Reversible Mg deposition can be observed in Grignard‐based electrolytesReversible Mg deposition can be observed in Grignard based electrolytes, first shown by Overcash and Mathers in 1933.

Overcash, D. M.; Mathers, F. C. Trans. Electrochem. Soc. 1933, 64, 305.Gregory, T. D.; Hoffman, R. J.; Winterton, R. C. J. Electrochem. Soc. 1990, 137, 775‐780.Lu, Z.; Schechter, A.; Moshkovich, M.; Aurbach, D. J. Electroanal. Chem. 1999, 466, 203‐217.

Deposition at the Deposition at the Magnesium AnodeMagnesium Anode

Highly dependent ong y pelectrolyte

MagnesiumLithium ag es uSEM resolution: 5000XDeposition rate: 2.0 mAcm−2

SEM resolution: 5000XDeposition rate: 2.0 mAcm−2

Mg is less reductive than Li =>

•Mg does not form SEI in ether solvents•Mg does not form dendrites

Dey, A.N.; Sullivan, B.P. J. Electrochem. Soc. 1970, 117, 222Matsui, M., J Pow. Sou. 2011, 196, 7048–7055Gregory, T.D.; Hoffman, R.J.;Winterton, R.C. J. Electrochem. Soc. 1990, 137, 775

g

Mg Battery with Mg Organohaloaluminate ElectrolytesMg Battery with Mg Organohaloaluminate Electrolytesg y g g yg y g g y

Cathode: MgxMo3S4Anode: Mg metal

Electrolyte: in situ generated Mg organohaloaluminate 2:1 Bu2Mg : AlClEt2

• First demonstration of rechargeable Mg battery system:• Proven >2000 cycle with <15% capacity fade, 100% DOD, ‐20~80oC • Proposed as a potentially higher energy battery than Ni Cd and lead acid batteries

Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 407, 724‐727.

• Proposed as a potentially higher energy battery than Ni‐Cd and lead‐acid batteries

Roadblocks Towards a High Energy Magnesium Roadblocks Towards a High Energy Magnesium BatteryBattery

Q

WhLvoldqqVvol 1][/)(/ WhLvoldqqVvol0

][/)(/

1) 2e‐ transfer to the same metal center

Increase voltage Increase capacity

dilithium in Li2NiO2, LiVSe2/Li2VSe2

2) Facile solid state diffusion of magnesium in the cathode) g

3) High voltage, non‐corrosive electrolyte

Whittingham, S., Chem. Rev., 2004, 104, 4271‐4301.Dahn, J.R, U. von Sacken, Michal, C.A, Sol. State. Ion., 1990, 44, 87‐97.

In Situ Generated Mg OrganohaloaluminatesIn Situ Generated Mg Organohaloaluminates

Black: 1:2 Bu2Mg and EtAlCl2Black: 1:2 Bu2Mg and EtAlCl2Red: 1:2 AlCl3 and PhMgCl

THFBu2Mg + 2 EtAlCl2Bu2Mg + 2 EtAlCl2

Crystallization product was electrochemically inactive when re-dissolved in THF.

Aurbach, D et al, Nature, 2000, 407, 724‐727.Aurbach, D et al, Chem. Record  2003, 3, 61‐73.

Aurbach, D et al, Adv. Mater., 2007, 19, 4260‐4267.

Crystallization of 1Crystallization of 1stst Generation ElectrolyteGeneration Electrolyteyy yy

O MgO

Cl Mg

Cl

OO

+

NClSiSiTHF

NSiSi

+ AlCl3 gO

g

Cl O AlCl Cl

Cl24 hrs

NMgCl

3

HMDSMgCl

Kim H S et al Nat Commun 2:427 doi: 10 1038/ncomms1435 (2011)Kim, H.S.et al.  Nat. Commun.  2:427 doi: 10.1038/ncomms1435 (2011) .

Electrochemistry of crystalcrystal

In situelectrolyte

HMDSMgCl

Formation of (Mg (μ Cl) 6THF)[(HMDS)AlCl ]HMDSMgCl + AlCl3 → MgCl+ + HMDSAlCl3- Transmetallation (1)

2HMDSMgCl HMDS2Mg + MgCl2 Schlenk equilibrium (2)

Formation of (Mg2(μ-Cl)3·6THF)[(HMDS)AlCl3]

2HMDSMgCl HMDS2Mg + MgCl2 Schlenk equilibrium (2)

MgCl+ + MgCl2 Mg2Cl3+ (3)

3HMDSMgCl + AlCl3 → Mg2Cl3+ + HMDSAlCl3- + HMDS2Mg (4)g 3 g2 3 3 2 g ( )

• Transmetallation is key reaction in the formation of the product• (Mg2(μ-Cl)3·6THF) is the electrochemically active species

The Role of Frontier Orbitals in Electrochemistry of ElectrolytesThe Role of Frontier Orbitals in Electrochemistry of ElectrolytesThe Role of Frontier Orbitals in Electrochemistry of ElectrolytesThe Role of Frontier Orbitals in Electrochemistry of Electrolytes

(eV)

Lowest Unoccupied ( )

Reduction is the addition of electron to LUMO

Oxidation is the loss of electron to HOMO

Energy 

Molecular Orbital (LUMO)

Highest Occupied Molecular Orbital (HOMO)

of electron to LUMOorbital

of electron to HOMOorbital

• Reductive stability may be predicted by calculating LUMO energy value• Oxidative stability may be predicted by calculating HOMO energy value

More negative HOMO energy  → higher oxida ve stabilityMore posi ve LUMO energy  →  higher reduc ve stability

DFT Prediction of Electrochemical PropertiesDFT Prediction of Electrochemical Properties

Table 1 Summary of HOMO and LUMO energy levels for the anion component of the

crystallized electrolytes. 20Electrolyte Anion HOMO (eV) LUMO (eV)

*(HMDS)2AlCl2- --- ---

11015

mA

/cm

2• Based on this logic, our DFT calculations predict the electrolyte order of oxidative 

(HMDS)AlCl3- -5.670 0.061

Ph4Al- -5 384 0 182505

J, m

stability to be 4>2>1>3. 

• Based on the assumption that 

Ph4Al -5.384 0.182

Ph3AlCl- -5.678 0.047

Ph2AlCl2- -6.045 0.058 2

-51 1.5 2 2.5 3 3.5 4 4.5

E , V vs Mgthe HOMO energy level gap between the (HMDS)AlCl3‐ and (HMDS)2AlCl2‐ anions is similar to h b PhAlCl

PhAlCl3- -6.402 -0.062

Cl4Al- -6.742 -1.384

Fig. 5 Linear scan voltammograms depicting typical voltage stability of (Mg2(μ-

Cl)3·6THF)(HMDSnAlCl4-n) (n=1,2) (blue), (Mg2(μ-Cl)3·6THF)(PhnAlCl4-n) (n = 1 – 4)

the energy gap between PhAlCl3‐and Ph2AlCl2‐ anions.

3 Ph4B- -4.819 -0.536

(turquoise), (Mg2(μ-Cl)3·6THF)(BPh4) (red) and (Mg2(μ-Cl)3·6THF)[B(C6F5)3Ph] (green)

on a Pt working electrode with a surface area of 0.02 cm2. Scan rate for all scans is 25

4 (C6F5)3BPh- -5.559 -0.422

*The structural flexibility of (HMDS)2AlCl2- makes its geometry difficult to optimize.

mV s-1; magnesium reference and counter electrodes are used at a temperature of 21 �°C.

Problem Charging in a 2025 Coin CellProblem Charging in a 2025 Coin Cell

• why cannot charge above 2 2V?• why cannot charge above 2.2V?

• voltage stability of gen1 on Pt working electrode (w. e.) is 3.2V 

Voltage Stabilities of Voltage Stabilities of Gen 1 Gen 1 on Various Working Electrodes on Various Working Electrodes 

20

15

2

NiSS Pt CAu

A B5

10

J, m

A/c

m

-5

0

SEM of stainless steel before (A) and after (B) 

51.5 2 2.5 3 3.5 4 4.5

E vs Mg, V( ) ( )

exposure to 1st generation electrolyte for 1 week

• Crystallized magnesium organohaloaluminates are corrosive in nature

Voltage Stabilities for Voltage Stabilities for Gen 2  Gen 2  ElectrolyteElectrolyte

2030

m2

Pt working electrode

3PhMgCl + BPh3 → [Mg2Cl3+][BPh4‐] + Ph2Mg

100

1020

J, m

A/c

g

-101.25 1.75 2.25 2.75 3.25 3.75

E , V vs Mg

J

15SS ki l d

, g

05

10

mA

/cm

2 SS working electrode

-50

1.25 1.75 2.25 2.75 3.25 3.75

J,

E , V vs Mg

C i t f 400 V i lt t bilit

No pitting observed

• Causes an improvement of 400  mV in voltage stability

Muldoon, et al, Energy and Environ. Sci., 2012. 5, 5941

• Anion has dramatic effect on corrosion

Voltage Stabilities for Voltage Stabilities for Gen 3  Gen 3  ElectrolyteElectrolyte

352 Pt working electrode

3PhMgCl + B(C6F5)3 → [Mg2Cl3+][B(C6F5)Ph3‐]+ Mg(C6F5)2

51525

J, m

A/c

m2 Pt working electrode

-51 2 3 4 5

E , V vs Mg

J

352 SS ki l d

5152535

, mA

/cm

2 SS working electrode

Hysteresis-5

0 1 2 3 4 5E , V vs Mg

J

y

• 1.0V improvement in voltage stability on Pt w.e. : 3.7V

Corrosion observed• Corrosion on SS limits potential window to 2.2V

Muldoon, et al, Energy and Environ. Sci., 2012. 5, 5941

Chlorine Free Magnesium OrganoboratesBPh +B M (BPh B ) M

Mixture and FilterBPh3 +Bu2Mg (BPh3Bu)2Mg Soluble in THF <0.1M

Gen 4

THFBPh3 +Ph2Mg (BPh4)2MgTHF

Electrolyte Solvent Solubility

[Mg2Cl3+][BPh4‐]  (gen 2) THF >0.4M

(BPh3Bu)2Mg (gen 4) THF <0.1M

(BPh4)2Mg (gen 5) THF <0.01M

(BPh4)2Mg (gen 5) Acetone >0.5MGen 5

16Chlorine free magnesium organoborates lack adequate solubility in low dielectricSolvents compatible with Mg such as THF

Electrolyte Preparation by Ion Switching y p y g

gen3On SS316

gen2gen1

genX

genX

• Solubility of genX is >0.5M in glyme• Chlorides Are the Culprit of Corrosion• Its reductive stability must be improved

Muldoon et al, Energy Environ. Sci., 2013, 6, 482–487

Membrane Encapsulated Sulfur Cathodes

membrane

Bl 1C bBlue: 1C – membrane Black: 1C – non‐membraneRed: 5C – membrane 0.1C ‐membrane

Improved lifetime and C – rates on encapsulation with a selective membrane

ConclusionsConclusions

Anion structure as well as combination between cationand anion strongly affected g ythe oxidation stability and corrosion of the electrolyte.

There is a need for developing high dielectric solvents with a higher reductive stability such that they are compatible with Mgy p g