Novel Electrolytes Enabling High Efficiency

Cycling of Rechargeable Li Metal Batteries

The 10th Symposium on Energy Storage beyond Li-Ions

IBM Research - Almaden

June 27, 2017

Ji-Guang Zhang

Pacific Northwest National Laboratory, Richland, WA

1. Dendrite Free Li Deposition Using Salt Additives

2. High Rate Li Deposition with High Coulombic Efficiency (CE)

3. Accurate Determination of CE

4. Long Term Cycling of High Voltage Li Metal Batteries

5. Summary

2

Outline

3

(a) Li metal

batteries

(b) The typical morphology

of Li dendrite

(Chianelli,1976)

(c) Main problems related

with dendrite and low

Coulombic efficiency.

Challenges on Li Metal Anode

Two main barriers :

1. Dendrite growth; 2. Low Coulombic efficiency

Cathode Anode

Li metalLiCoO2

Sulfur

Oxygen

.

+ -

Dendrite

Short cycle Life

“Dead Li”

Short circuit

Consuming Li& electrolyte

Low energy density

Safety hazards

High surface

Low CE

Co

nseq

uen

ces

Wu Xu, Jiulin Wang, Fei Ding, Xilin Chen, Eduard Nasybulin, Yaohui

Zhang and Ji-Guang Zhang, Energy Environ. Sci., 2014, 7 (2), 513 – 537.

High surface area

1. Dendrite Free Li Deposition Using Salt Additives

Fei Ding, Wu Xu, Gordon L. Graff, Jian Zhang, Maria Sushko, Xilin Chen, Yuyan Shao,

Mark H. Engelhard, Zimin Nie, Jie Xiao, Xingjiang Liu, Peter V. Sushko, Jun Liu, and

Ji-Guang Zhang, J. Am. Chem. Soc., 2013, 135 (11), pp 4450–4456, 4

Li+ Cs+ Rb+

Stand reduction

potential (1M)-3.040 V -3.026 V -2.980 V

Effective reduction

potential

at 0.05M*

- -3.103 V -3.06 V

Effective reduction

potential

at 0.01M*

- -3.144 V -3.098 V

𝑬𝑹𝒆𝒅= 𝑬𝑹𝒆𝒅∅ −

𝑹𝑻

𝒛𝑭𝒍𝒏𝜶𝑹𝒆𝒅𝜶𝑶𝒙

Nernst Equation:

An cation may have an ERed lower

than those of Li+.

5

20 µm

a

20 µm

b

20 µm

c

20 µm

d

20 µm

e

• Control electrolyte: 1 M LiPF6 in PC.

• CsPF6 concentration in the electrolyte: (a) 0 M, (b) 0.001 M, (c) 0.005 M,

(d) 0.01 M, and (e) 0.05 M.

Cs+ additive can effectively suppress Li dendrite growth.

Effect of CsPF6 Additive on

The Morphology of Li Deposition

6

1 M LiPF6 in PC 1 M LiPF6 in PC+ 0.05 M CsPF6

Dendrite Free Li Deposition with Salt Additive

Surface

Cross

section

Dendritic surface

Random growth Smooth surface

Highly ordered growth

1m

1m

Zhang et al, Nano Lett., 2014, 14 (12), pp 6889–6896

2. High Rate Stable Li Deposition using

High Concentration LiFSI-DME Electrolyte

7

Li deposited in 4M LiFSI-DME Electrolyte exhibits a nodule structure with

much smaller surface area as compared to those deposited in carbonate

based electrolyte.

J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin & J.G. Zhang

Nature Communications, 2015, DOI: 10.1038/ncomms7362.7

Effects of Current Density on the Cycling Stability

The average Coulombic efficiency

of the cycling is >99% (0.2 mA cm-

2), >98% (2.0 and 4.0 mA cm-2) and

>97% (8.0 and 10.0 mA cm-2).

CE is stable up to1000 cycles.

4M LiFSI in DME

8

4M LiFSI in DME

4M LiFSI in DME

J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin & J.G. Zhang

Nature Communications, 2015, DOI: 10.1038/ncomms7362.

Long Term Cycling Stability of Li|Li cells Using

Electrolyte E1

Current density: 10 mA/cm2.

Stable cycling for more than 6,000 cycles.

No short, no increase in impedance or cell voltage.

9

J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin & J.G. Zhang

Nature Communications, 2015, DOI: 10.1038/ncomms7362.

10

3. Accurate Determination of CE

Different Li CEs were reported even for the same system.

There is an urgent need to identify an general methodology to measure

CE for Li metal anode.

11

Factors Affecting Measurement of Li CE

Substrate selection

Substrate treatment approaches

Accuracy of the instrument

Cell design

Measurement protocols

12

Method 1: Li|Cu cells – Full stripping for Each Cycle

Average CE = 98.8% for 100 cycles

𝐶𝐸 =𝑄𝑆

𝑄𝑃

𝐶𝐸𝑎𝑣𝑔 = 𝑄𝑆𝑄𝑃

𝑛Average CE:

Single Cycle CE:

Current Density = 0.4 mA/cm2

QP = 0.5 mAh/cm2

Electrolyte: 4M LiFSI in DME

Method 2: Li|Cu cells – Partial Stripping

13

𝐶𝐸𝑎𝑣𝑔 =𝑛𝑄𝑐+𝑄𝑆

𝑛𝑄𝑐+𝑄𝑇

Average CE = 99.2% for 100 cycles

Current Density = 0.4 mA/cm2

QT = 4 mAh/cm2

QC = 0.5 mAh/cm2

Alternative Equation

when voltage exceeds

upper limit in N cycles :

𝐶𝐸𝑎𝑣𝑔 = 1 −𝑄𝑇

𝑁𝑄𝑐 + 𝑄𝑇

Proposed Universal Approach-

Method 3: Combination of Conditioning Cycle and Partial Stripping

Conditioning cycle

not included in

calculation of CE

14

𝐶𝐸𝑎𝑣𝑔 =𝑛𝑄𝑐+𝑄𝑆

𝑛𝑄𝑐+𝑄𝑇

Current Density = 0.4 mA/cm2

QT = 4 mAh/cm2

QC = 0.5 mAh/cm2

Eliminate the uncertainty related to substrate material

and treatment conditions

Average CE = 99.4% for 100 cycles

15

Effect of Li Deposition Rate in Carbonate Electrolytes

CE and cycle life of Li/NCA cell can be improved by slow charge (Li deposition).

Li||NCA cells using 1M LiPF6

EC:EMC (4:6 wt.) electrolyte

Li deposition~ charge process

Lv & Xiao et al., Adv. Energy Mater. 2015, 5, 1400993

Cross-sectional SEM images of the Li anodes obtained from the

cells after 100 cycles at a) 0.2C charge/1C discharge, b) 0.5C

charge/discharge, c) 1C charge/discharge, and d) 2C

charge/discharge.

16

Effect of Li Stripping Rate in Carbonate Electrolytes

Li||NMC cells using 1M LiPF6/EC-DMC (1:2 in volume) electrolytes

Li stripping ~ Discharge process

Zheng & Xu et al., Adv. Energy Mater. 2016, 1502151.

CE and cycle life of Li/NMC cell can be improved by fast discharge (Li stripping).

Fast discharging formes a transient highly concentrated Li+ ion solution in the

vicinity of Li surface and reduce the interaction between fresh Li metal and

electrolyte.

High

17

Effect of Charge/Discharge Protocol on

CE of Li Cycling in Ether Based Electrolyte

a. CE of Cu||Li cells.

b. Charge/discharge voltage profiles

of Cu||LiFePO4 cells.

c. Discharge capacity and CE of

anode-free Cu||LiFePO4 cells.

The CE of Li cycling can be increased to 99.8% with the combination of high

concentration electrolyte (4M LiFSI/DME) and low rate Li deposition/high rate Li

stripping protocols.

Anode-free Cu||LiFePO4 cell can retain 54% capacity after 100 cycles.

a

b

c

Cu||LiFePO4

Electrolyte: 4M LiFSI-DME

Qian et al, Adv. Funct. Mater. 2016,DOI:10.1002/adfm.201602353

4. Long Term Cycling of High Voltage Li Metal Batteries

18

0 100 200 300 400 500

0

50

100

150

200

Dual-salt (LiTFSI + LiBOB)

E-control (1M LiPF6)

Sp

ecific

ca

pa

city (

mA

h g

-1)

Cycle number

0.175 mA cm-2

1.75 mA cm-2

0.0

0.5

1.0

1.5

2.0

Are

al ca

pa

city (

mA

h c

m-2)

18

0 50 100 150 2002.5

3.0

3.5

4.0

4.5E-control (1M LiPF6)

Vo

lta

ge

(V

vs.

Li/L

i+)

Specific capacity (mAh g-1)

1st

5th

25th

50th

100th

150th

200th

250th

300th

350th

400th

450th

500th100

th cycle

0 50 100 150 2002.5

3.0

3.5

4.0

4.5

450th cycle

Dual-salt (LiTFSI + LiBOB)

Specific capacity (mAh g-1)

1st

5th

25th

50th

100th

150th

200th

250th

300th

350th

400th

450th

500th

0 50 100 150 2002.5

3.0

3.5

4.0

4.5

500th cycle

Dual-salt + 0.05 M LiPF6

Specific capacity (mAh g-1)

1st

5th

25th

50th

100th

150th

200th

250th

300th

350th

400th

450th

500th

0 100 200 300 400 500

0

50

100

150

200

Dual-salt + 0.05 M LiPF6

Dual-salt (LiTFSI + LiBOB)

E-control (1M LiPF6)

Sp

ecific

ca

pa

city (

mA

h g

-1)

Cycle number

0.175 mA cm-2

1.75 mA cm-2

0.0

0.5

1.0

1.5

2.0

Are

al ca

pa

city (

mA

h c

m-2)

LiTFSI-LiBOB dual salt electrolyte with LiPF6 additive shows better stability

with Li metal.

Electrochemical behaviour of Li||NMC cells

Zheng, M. H. Engelhard, D. Mei, S. Jiao, B. J. Polzin, J.-G. Zhang, and W. Xu,

Nature Energy, 2017, 2, 17012.

Effects of testing temperature and charge current density

19

LiPF6 additive (optimum 0.05M) improves the cycling performances at

high temperature and low charge current.

0 100 200 300 400

0.0

0.5

1.0

1.5

2.0 1.75 mA cm-2

Dual-salt + 0.05 M LiPF6

Dual-salt (LiTFSI + LiBOB)

E-control (1M LiPF6)

Are

al c

ap

aci

ty (

mA

h c

m-2)

Cycle number

60 oC

0 200 400 600 8000.0

0.5

1.0

1.5

2.0

2.5

3.030

oC

Electrolyte: Dual-salt + 0.05 M LiPF6

, discharge: 1.75 mA cm-2

Charge: 0.58 mA cm-2

Efficiency

Capacity

Are

al c

ap

aci

ty (

mA

h c

m-2)

Cycle number

0 200 400 600 800

0

20

40

60

80

100

120

Cou

lom

bic

effic

ien

cy (

%)

Zheng, M. H. Engelhard, D. Mei, S. Jiao, B. J. Polzin, J.-G. Zhang, and W. Xu,

Nature Energy, 2017, 2, 17012.

20

5. Summary

Most reliable method for CE measurement is by combining a conditioning

cycle and partial plating/stripping of Li metal using Li/Cu cells.

The combination of high concentration electrolyte (4M LiFSI/DME) and low

rate Li deposition/high rate Li stripping protocols can further increase the

CE of Li cycling to 99.8%.

LiPF6 additive (0.05M) in LiTFSI-LiBOB dual salt electrolyte can largely

enhance long-term cycling stability (> 800 cycles) of high voltage Li metal

batteries.

A new electrolyte enables high efficiency cyling of both Li metal anode (up

to 99.5%) and stable cycling of Li/NMC cells (>95% capacity retention after

300 cycles and 85% capacity retention after 600 cycles).

21

Acknowledgments

PNNL: Wu Xu, Shuru Chen, Jianming Zheng, Fei Ding, Jiangfeng Qian,

Yaohui Zhang, Brian D. Adams, Wesley Henderson, Ruiguo Cao,

Shuhong Jiao, Jie Xiao, M. H. Engelhard, M. E. Bowden, D.H. Mei, J. Liu

ARL: Oleg Borodin, Kang Xu

Financial Support

DOE/OS/BES/The Joint Center for Energy Storage Research

(JCESR)

DOE/EERE/OVT/Advanced Battery Materials Research Program

(BMR) and Battery 500 Program