Fundamental Study on Li Metal Dissolution and Deposition on Cu Foil in Nonaqueous Electrolytes

with 3DOM Separator

Kiyoshi Kanamura, Naohiro Kobori, and Hirokazu Munakata

Department of Applied Chemistry, Graduate School of Urban Environmental Sciences

Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, 192-0397 Tokyo, Japan

Email address: kanamura@tmu.ac.jp

2017 BLI X, Symposium on Energy Storage, June 27-29, 2017, at IBM- Research Almaden in San Jose, CA, USA

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Li22

Si5

Li3As

Li22

Sn5

LiAl

LiC6

Li3Sb

Li

体積容量密度

(m

Ah

dm

-3)

重量容量密度 (mAh g-1)

Li2Pb

5

炭素系

合金系

リチウム金属

En

erg

y d

en

sity p

er

vo

lum

e

(mA

h d

m-3

)

Energy density per weight

(mA h g-1)

Carbon anode

Alloy anode

Li metal anode

Introduction : Lithium Secondary Battery

Lithium metal has high capacity density (3861 mA h g-1, -3.045 V vs. SHE) as

excellent anode material.

500 W h kg-1

Rechargeable batteries

3861 mA h g-1

Lithium secondary batteries are needed to realize high energy density.

< In the future >

Li metal is the ideal anode material for lithium secondary batteries.

Introduction : Problem of Lithium Secondary Battery

Li metal cannot be used in practical batteries due to low

cycleability and safety problem, which are related to the

morphology of Li metal deposited during charging process.

In order to use Li metal as the anode in rechargeable batteries, the

Li dendrite formation has to be suppressed.

Introduction : Lithium Dendrite

The formation of lithium dendrite is related to

non-uniform current distribution on a lithium

metal anode in the course of the charging

process of battery.

Low

Flat and smooth Dendritic growth

Uniform

Current density

Formation of SEI

SEM image of Li dendrite

High

Non-uniform

Behavior of Surface Film on Li metal

Before immersion in electrolyteAfter immersion in electrolyte

In Electrolyte

In Electrolyte

Surface film is changed by chemical reaction between electrolyte or impurities

and native surface film.

Surface Structure of Li Metal & Morphology

: Li+ ion (Current) flow

High ResistivityLow Resistivity

Li Metal

Surface Layer

Concentration of Current flow : Large current distribution during Li metal deposition

Surface film formed in electrolyte is not so uniform.

The morphology of Li metal depends on

current, concentration of slat and kind of

solvent. The surface structure of Li metal is

determined by chemical reaction between

electrolyte components and native surface film.

1M PF/EC/PC 2 mA cm-2

1M PF/EC/DEC 2 mA cm-2 1M PF/EC/DEC 0.5 mA cm-2

0.2M PF/EC/DEC 0.5 mA cm-2

Control for Surface Film with HF Additive

XPS spectra of Li metal surface in

propylene carbonate containing 1

mol dm-3 LiClO4.

XPS spectra of Li metal surface in propylene

carbonate containing 1 mol dm-3 LiClO4 with HF

additive.

LiF, Li2CO3, LiOH,

LiCl

Li2O, LiOH

Li Metal

LiF, Li2CO3, LiOH

Li2O, LiOH

Li Metal

With HF

Without HF

Dynamic Behavior During Discharge and Charge

Mass change during lithium metal

deposition on Ni substrate in propylene

carbonate containing 1 mol dm-3 LiClO4

with and without HF additive, which are

measured with EQCM .

300 mC20 mC

discharge

60 mC

discharge

110 mC

discharge

Figure 12 AFM i mages for Li metal during deposition and dissolution processes under

galvanostatic conditions at 0.2 mA cm-2

, (a) 300 mC deposition, (b) 20 mC dissolution, (c) 60

mC dissolution, and (d) 110 mC dissolution.

AFM images for Li metal surface deposited in

propylene carbonate /1.0 mol dm-3 LiClO4

with HF additive.

LiPF6 EC:DEC(=1:1), 2 mA/cm-2

Li metal react with electrolyte during

dissolution process.

Electrolyte penetrationCylindrical Shape

Interfacial Current Distribution Control by Separator Attached to Li Metal

Li+

Porosity (about 70 %)

Porous structure

Constant current density

Conventional separator

Three-dimensionally ordered macroporous (3DOM) polyimide (PI) separator

Porosity (about 30 ~ 40 %)

Columnar pore

1mm

Li+

9/19

1mm

Preparation of 3DOM Separator

Top side

(SEM)

Base side

Polyimide

Three Dimensionally Ordered

Macroporous (3DOM) structure

3DOM separator provides uniform current distribution to Li metal surface.

Properties of 3DOM Separator

3DOM PI separator has high affinity to electrolyte

solutions due to high hydrophilicity.

Electrolyte solution 3DOM PI separator

Conventional PP separator

1 mol dm-3 LiPF6 in EC : DEC = 1 : 1 (in vol.) 8.0 mm 4.0 mm

1 mol dm-3 LiPF6 in EC 7.0 mm x

3DOM PI separator has high affinity to electrolyte solutions due to high hydrophilicity.*Ref.: J.-R. Lee et al. Journal of Power Sources 216 (2012) 42-47.

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2000cycle 3000cycle

Symmetrical Cell Li/Li (Utilization 30 %, 8 mA h, 16 mA)

EC EC : DEC = 1 : 1 EC : DMC = 1 : 1

SEM Images of Li metal During 1st Cycle

Li/Cu cell

Before the 1st cycle

After deposition

After dissolution

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EC EC : DEC = 1 : 1 EC : DMC = 1 : 1

Before the 1st cycle

After deposition

After dissolution

EISs of Li metal During 1st Cycle

Comparison of Separators

SEM images of lithium metal anode after 1st

charge with 3DOM polyimide separator (a)EC (b)EC : DMC = 7 : 3 (c) EC : DMC = 1 : 2, (d) EC : DEC = 1 : 2 (e) PC

‘ ‘

‘

‘

‘

SEM images of lithium metal anode after 1st

charge with polypropylene separator (a)’ EC (b)’ EC : DMC = 7: 3 (c)’ EC : DMC = 1 : 2, (d)’ EC : DEC = 1 : 2 (e)’ PC

PP separator3DOM separator

(a) (a)’(b)

(c)

(e)

(d)

(b)’

(c)’ (d)’

(e)’

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Volt

age

/ V

Time / Hr

EC EC : DEC = 1 : 1 EC : DMC = 1 : 1

1st cycle 96.25 93.47 95.17

2nd cycle 96.88 92.44 95.96

3rd cycle 96.89 91.81 95.93

4th cycle 97.32 91.88 96.27

5th cycle 97.57 92.04 96.54

Coulombic efficiency (%)

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EC EC : DEC = 1 : 1 EC : DMC = 1 : 1

Coulombic Efficiency

Full Cell Performance

• Cu : anode & current collector (18 mm)

• NMC : cathode

• 3DOM-PI : separator

• Electrolyte : EC, EC+EMC (3:7), EC+DMC (1:2), EC+DMC (1:1), EC+DMC (1:1), PC, EC+DEC (1:2), EC+PC (1:1) / 1.0 mol dm-3

LiPF6

• Laminated Cell : 3 cm × 4 cm

• Composite Electrode :

• NCM 92 %, AB 4 %, PVdF 4 %

• Thickness : 73 ± 2 mm

• Density : 2.8 ± 0.05 g cm-3

• Al current collector

Test Cells

• Cathode : NCM (Celion L-1013):92 %, AB (acetylene black) : 4 %, PVdF (KF Polymer) : 4 %, Thickness : 73±2 mm (Density：2.8±0.05 g cm-3)

• Anode : Cu foil (thickness : 18 mm, weight : 220±6 mg)

• Separator : 3DOM PI (50±3 mm)

• Electrolyte (320 mL) : EC, EC/EMC=3/7, EC/DMC=1/2, EC/DMC=1/1, EC/DMC=9/1, EC/DEC=1/2, EC/PC=1/1, PC

• Preparation of laminate cell

• Conditions of charge-discharge and rate performance tests• voltage : upper limit : 4.2V, lower limit : 2.0V, Current : 1-3 cycles ; 0.1 C,

4-6 cycles ; 0.2 C, 7-9 cycles ; 0.5 C

• Cyclic performance• voltage : upper limit ; 4.2 V, lower limit ; 2.0 V, Current : 1-10 cycles ; 0.1 C

(3.6±0.3 mA)

Cycleability of Cells

Cycleability of Cells

Cycleability of Cells

Cathode: High capacity NMC cathode 170 mA h g-1 250 mA h g-1

Anode: Li metal/Cu (6 mm thickness) foil

Electrolyte: EC based electrolyte (EC or EC/DMC)

Separator: 3DOM separator

Similar to LIB, but different.

Energy density estimated from basic research: more than 400 W h

kg-1 (near to 1000 W h L-1)

Li metal (20 mm) / Cu foil (utilization 40 ~ 60 %)

NMC (80 mm)

3DOM separator

Li Metal Battery with 400 W h kg-1

Summary for Li Metal Anode

• 3DOM separator suppress the dendrite formation of lithium metal.

• The suppression of dendrite formation depends on a kind of electrolyte. EC provides the best performance.

• When using extra amount of Li metal, cycleability of Li metal is improved very much. The utilization of Li metal anode is important.

• The full cell with NCM cathode and Cu anode can be cycled. However, the rechargeability of these cells is not so good depending on the kind of electrolyte. In this case, EC electrolyte exhibited the best performance.

• What is a criteria for choice of electrolyte?• Solvent + LiPF6 → HF or Other F compounds

Acknowledgement

• This work has been done by financial supports from NEDO, JST (ALCA-spring) and 3DOM Inc..

• Dr. Nishikawa provides useful SEM observation results.