2D Black Phosphorous as a Polysulfide Trapping Agent in Lithium-Sulfur Batteries

Nikhil KoratkarJohn A. Clark and Edward

T. Crossan ProfessorEditor CARBON (Elsevier)

Department of Materials Science and Engineering

Rensselaer Polytechnic Institute110 8uth Street, Troy,

New York, USA.

e− e−

Li+

Li+

+−

Lit

hiu

m Poly-sulfides

Li2S6 catholyte

Before we look at Li-S, lets review Li-ion Batteries

Liquid Electrolyte: Solution of lithium-salt electrolytes, such as LiPF6, LiBF4, or LiClO4, in an organic solvent such as alkene carbonates

Specific Capacity ~ 370 mAh/gSpecific Capacity ~ 250 mAh/g

Do Lithium-Ion Batteries (LIBs) need to be improved ?

Source: Chem. Rev. 2004, 104, 4245.

Li-S Battery Chemistry Shows Great Potential

Sulfur cathode:S8+16Li++16e-

8Li2SLithium anode:

Li++e- Li

Charge: 8Li2S S8+16LiDischarge:S8+16Li 8Li2S

a b

Theoretical specific capacity: 1675 mAh/g Energy density of ~2,600 Wh/kg

5X higher than Lithium-ion batteries (~387 Wh/kg).

S8 + 16Li ↔ 8Li2S 2 electron transfer per S atom !

Source: Chem. Soc. Rev. 2016, 45, 5605–5634.

The challenge with Li-S technology- very poor cycle stability

Intermediate reaction products (lithium polysulfides- LiPS) are soluble in the liquid electrolyte (~1.0 m lithium bistrifluoro-methane-sulfonylimide in 1,3-dioxolane & 1,2-dimethoxyethane)Soluble LiPS include: Li2S2, Li2S4, Li2S6, Li2S8

Source: Chem. Soc. Rev. 2016, 45, 5605–5634.

S8 + 16Li ↔ 8Li2S

Fast Capacity Fade with Charge/Discharge Cycling

N-G-SA-G-S

Rapid Capacity Fade (~0.25% per cycle) due to Loss of S (LiPS Dissolution)

One solution strategy: Add a polysulfide immobilizer (trapping agent) to S electrode

0 20 40 60 80 1000.00

0.05

0.10

0.15

0.20

Deca

y ra

te/c

ycle

(%)

Mass ratio of adsorbent/S or Li2S(%)

500 cycles(S loading: 3.3 mg cm-2)

300 cycles(S loading: ~1.5 mg cm-2)

200 cycles

400 cycles(Li2S loading: 1 mg cm-2)

300 cycles(S loading: 0.7 to 1.2 mg cm-2)

500 cycles(S loading: 1 mg cm-2)

500 cycles

1500 cycles(S loading: 1.5 mg cm-2)

Baseline capacity fade rate ~ 0.25% per cycle

Common Trapping AgentsMetal and Carbon basedOxides, Sulfides andNitrides

Our Objective: To investigate new classes of trapping agents.We studied how Phosphorene (or 2D Black Phosphorous) performs as a S trapping agent in Li-S batteries

Phosphorene- a monolayer of black phosphorous (BP)“also called 2D Black Phosphorous”

Red Phosphorous (RP) to BP Conversion by Chemical Vapor Transport (CVT) Method

10 20 30 40 50 60

Inte

nsity

2θ (degrees)

(020

)

(040

)

(060

)

500 μm Energy (keV)

ba

c

Solution Phase Exfoliation of BP into Few Layer Phosphorene (FLP) Nanosheets

10 μmPuckered Honeycomb Structure of Monolayer BP

Characterization of Few-Layer Phosphorene (FLP) sheets

8 nm6 nm

100 nm

ea

f

500 nm

Inte

nsity

(a.u

.)

0 nm

200

002

250 nm

d

b

c

2 nm

Li-S Battery Assembly: FLP Pre-Dispersed in a Carbon Nanofiber (CNF) Electrode, Followed by S Impregnation

a b c

e− e−

Li+

Li+

+−

Lith

ium Poly-

sulfides

CNF

FLP

FLP+Li2Sx

dLi2S6 catholyte

C mapping P mappinguFLP

Cross-Sectional Imaging of Electrode

5 μm

5 μm

80 μm

5 μm5 μm

a b

c d

Electrochemical Performance

Incorporation of FLP (~10% of S weight), results in very significant improvement in Cycle Stability

cFLP-CNFCNF

FLP-CNF CNF

57% 41%

Utilization ratio of sulfur

Electrochemical Performance

FLP-CNFCNF

a

FLP-CNF

CNF

E

b

Effect of Charge/Discharge Rate

Typical Voltage Profile

Understanding the ability of FLP to bind with Lithium Polysulfides (LiPS)

Phosphorene+Li2S

Phosphorene+Li2S2

Phosphorene+Li2S3

Phosphorene+Li2S4

Phosphorene+Li2S6

a

b

c

d

e

Li2S2Li2S Li2S3 Li2S4 Li2S6

f Binding energy to phosphorene

Binding energy to carbon

Density Functional Theory with Corrections for van der Waals Forces (DFT-D)

S-P: BondingLi-P: Electrostatic Interactions

Post-Cycling Characterization

2 μm

c

5 μm 5 μm

e f

Comparison with state-of-art S trapping agents

500 cycles(S loading: 3.3 mg cm-2)

300 cycles(S loading: ~1.5 mg cm-2)

200 cycles

400 cycles(Li2S loading: 1 mg cm-2)

300 cycles(S loading: 0.7 to 1.2 mg cm-2)

500 cycles(S loading: 1 mg cm-2)

500 cycles

1500 cycles(S loading: 1.5 mg cm-2)

N-doped carbon nanotubes

N-doped grahene

TiS2

CaO

TiN

Carbon Nitride

C3N4

FLP: Comparable fade rate at 3X lower loading !S loading in our battery is 2-3 X greater than other work

FLP: Comparable fade rate at 3X lower loading, Why ?

2D Nanosheet Geometry (maximizes surface atoms) when compared to 3D particles

FLP density is ~ 2.69 gm/cm3

Lighter than:Metal oxides (CaO: 3.35 g/cm³), Sulfides (TiS2: 3.22 g/cm3), & Nitrides (TiN: 5.4 g/cm³).

Publication: Advanced Materialsdoi:10.1002/adma. 201602734 (2017)

Takeaway Points• Lithium polysulfides bind strongly to phosphorene Strong bonds formed between S and P atoms Electrostatic interaction between Li and P atoms

• Phosphorene excels as a polysulfide trapping agent Phosphorene addition (~10wt% of S) into the electrode

reduces the capacity fade rate of the Li-S battery to ~0.05% per cycle (5X lower than the baseline).

• Phosphorene outperforms other competing polysulfide trapping agents such as nitrogen doped nano-carbons, as well as metal oxides, sulfides and nitrides.

Future Research Directions• Vary loading fraction of FLP

• Disperse thinner FLP (ideally monolayers)

• Defect engineering of FLP to enhance LiPSbinding

• Explore Red Phosphorous (RP) as a polysulfide trapping agent. RP is earth abundant and relatively inexpensive ($50/Kg: comparable to graphite). However nano-size particles of RP must be produced, which is non-trivial.

Acknowledgements

Funding Support: NYSERDA & NSF

My Students: Lu Li, Shravan Suresh & Tushar Gupta

THANK YOU FOR YOUR ATTENTION !

Collaborators: Hui-Ming Cheng (Shenyang, China)Chandra Veer Singh (Toronto, Canada)