Carbon/Sulfur Nanocomposites and Additives for High-Energy Lithium Sulfur Batteries

Chengdu Liang Oak Ridge National Laboratory

Vehicle Technologies Program Annual Merit Review and Peer Evaluation Meeting, May 2012

Contributors: Zhan Lin, Nancy Dudney, and Jane Howe

Project ID: ES105

“This presentation does not contain any proprietary, confidential, or otherwise restricted information”

Managed by UT-Battelle for the Department of Energy Overview

• Timeline • Budget – Start June, 2010 – $220k FY10 – $350k FY11 – $350k FY12 • Technical barriers for EV and PHEV – Low energy density • Partners – High cost of materials and – Oak Ridge National Laboratory processing – Center for Nanophase Materials – Fundamental research of high- Sciences, ORNL energy materials for Li-S – High Temperature Materials Lab, batteries ORNL • In situ SEM

2 Managed by UT-Battelle for the Department of Energy Objectives and relevance

• Objectives: – Improve the electronic conductivity of sulfur by using high surface area mesoporous carbon materials – Extend the cycle-life Li-S batteries by creating C-S nanocomposites with protective layers

– Explore electrolyte additive to catalyze the electrochemical reaction of Li2S • Relevance: – Enables high-energy Li-S battery chemistry for EV and PHEV batteries – Addresses the cost barriers for automobile batteries

3 Managed by UT-Battelle for the Department of Energy Milestones

Milestones: Target:

1. Confirm the earlier observation of long cycle life in Sep, 2010  half cells and expand the synthesis of sulfur/carbon composite materials of various sulfur loading 2. Compare the performance for different Jan, 2011  concentrations of additives to the electrolyte

3. Investigate additives to the cathode, including Sep, 2011  catalysts and alternative sulfur compounds

4. Design new liquid electrolytes, considering both Sep, 2011  poor/good solvents for Li polysulfides 5. Synthesize novel composite cathodes to improve Sep, 2012 cyclability Sep, 2012 6. Explore full cell configuration to minimize excess lithium at the anode

4 Managed by UT-Battelle for the Department of Energy Approach to fundamental research of Li-S batteries.

Goal: Develop advanced materials for Li-S batteries • Design novel composite materials for high energy sulfur cathode − C/S composite materials with optimized porous structures − Solid electrolyte coatings to retain sulfur at the cathode • Tailor electrolytes for Li-S batteries − Reduce the polysulfide shuttle − Electrolyte additives to catalyze the electrochemical reactions of Li2S • Explore approaches for anode protection − Suppress dendritic growth for Li-plating − Passivation of Li with designed SEI • Develop advanced diagnosis tools for the elucidation of battery reaction mechanisms − In situ SEM − Air-sensitive-sample handling

5 Managed by UT-Battelle for the Department of Energy Technical Progress – next 11 slides. Porosity is key to capacity retention. Double templating method has been developed to synthesize porous carbon with controlled porosity: • Two pore sizes were obtained by using PEO-PPO-PEO based triblock copolymers (7nm) and silica nanospheres (13nm) • Total porosity was controlled by the ratio of carbon precursors to the

1400 templates: two porosities of 1.12 and 3 100 2.35 cm /g. were demonstrated 1200 Pore volume 2.35 )

-1 1000 80 The C/S composites with 70 wt.% of sulfur loading showed that the sample 800 60

with large pore volume has better 600 40 capacity retention than the one with small 400 Charge, pore volume 2.35 pore volume.

Capacity ( mAh g Discharge, pore volume 2.35 20 200 Charge, pore volume 1.12

Discharge, pore volume 1.12 (%) Coulombic Efficiency 0 0 5 10 15 20 6 Managed by UT-Battelle for the Department of Energy Cycle Number Polysulfide shuttle is still the bottle-neck of cycle-life.

3.2 0.5

3.0 ) -2

) + 2.8 0.4 Before cycling mA cm

2.6 ( 0.3 2.4 V vsV Li/Li ( 2.2 Discharge 0.2 Charge 2.0 Voltage 0.1

1.8 Current Density After cycling 1.6 0.0 0 200 400 600 800 1000 1200 1400 Capacity (mAh g-1) The first charge and discharge curves SEM of the lithium anode before and of the C/S composite electrode with a after cycling: lithium sulfide deposition pore volume of 2.35 cm3/g. Two was observed on the lithium anode plateaus were observed with a ratio of • Polysulfide shuttle still exist in large 3:1 of the high to low plateaus. Current pore volume C/S composite tailing at the end of charging indicates • Capacity decay could be linked with possible polysulfide shuttle. sulfur loss through polysulfide shuttle.

7 Managed by UT-Battelle for the Department of Energy Solid electrolyte coated cathode for Li-S batteries.

Li2S core A new core-shell structure overcomes the key challenges for Li-S cells: • Poor ionic and electronic conductivities • Low utilization of sulfur • Sulfur dissolution • Polysulfide shuttle • Low coulombic efficiency e • Corrosion of Li anode Li+ graphene • Poor cycling performance • Change of microstructure

Concept: • Impart ionic and electronic conductivities to the cathode through intimate contacting with conductors Solid electrolyte shell • Prohibit sulfur dissolution and shuttle prevent direct contact of phenomenon by solid electrolyte coating Li2Sx with liquid electrolyte 8 Managed by UT-Battelle for the Department of Energy Li2S@Li4P2S7 Core-shell Nanoparticles were synthesized through facile solution chemistry approach.

D=13 nm 111

220 Intensity (a. u.) 200 311 222

10 20 30 40 50 60 2θ (degrees) SEM of solid electrolyte coated XRD pattern of Li2S@Li4P2S7 electrode with a composition of 60 Core-shell Nanoparticles: particle wt.% Li4P2S7 coated Li2S size of Li2S was 13 nm based on nanoparticles, 30 wt.% carbon peak width. Li4P2S7 coating was black, 10 wt.% PVC confirmed by Raman spectrum. The coating was too thin to show significant XRD peaks

9 Managed by UT-Battelle for the Department of Energy Solid electrolyte coated cathode has superior cycling performance in liquid electrolytes.

3.2 40 Charge • The cathode

) 3.0 + Discharge consisted of pre- 2.8 30

) lithiated nanoparticles A

2.6 µ ( • No shuttle V vs. Li/Li vs. V ( 2.4 20 phenomenon was 2.2

Current observed in any cell Voltage 2.0 10 • No sulfur dissolution 1.8 observed in the 1.6 0 0 200 400 600 800 1000 1200 electrolytes (good 1100 Capacity (mAh g-1) solvents, DME, 1000 charge discharge TEGDM) 900 • Capacity varies with 800 700 the electrode 600 preparation from 200

500 mAh/g to 1100 mAh/g 400 Capacity (mAh) 300 • Close to 100% 200 coulombic efficiency 100 0 0 5 10 15 20 Cycle number 10 Managed by UT-Battelle for the Department of Energy P2S5 as novel electrolyte additive for Li-S batteries. The photo presents the solubility of a series of mixtures of

Li2Sx (1≤x≤8) with and without P2S5 in TEGDME

P2S5 forms complex with Li2Sx and facilities the dissolution of lithium sulfide/polysulfides in ether based electrolytes.

200 Li2S/P2S5 Cyclic voltammograms (CV) of Li2Sx/P2S5 Li S /P S 150 2 2 2 5 complexes: Li2S4/P2S5 100 Li S /P S • No redox peak was observed in the Li2S/P2S5 2 6 2 5

A) Li S /P S

µ 2 8 2 5

complex between 1.7 and 3.2 V. This complex

I ( 50 can be reduced at potential below 1.7 V and be oxidized at potentials over 3.2 V. 0

• Complexes of Li2Sx/P2S5 (x≥2) have redo peaks -50 between 1.7 and 3.2 V. -100 1.5 2.0 2.5 3.0 3.5 4.0 11 Managed by UT-Battelle for the Department of Energy E (V) P2S5 improves cell performance.

Capacity (mAh g-1) 1200 1000 800 600 400 200 0 3.2 1600 3.0 100 with P2S5 2.8 ) Discharge -1 1400 2.6

) Charge without P2S5

+ 80 2.4 1200 2.2 mAh g ( Li/Li 2.0 1000 with P S 1.8 2 5 60 vs. (

1.6 800 1.4 1.2 600 40 1.0 Voltage 0.8 400 without P S 0.6 2 5 20 0.4 B 200 Coulombic efficiency (%) 0.2 Discharge capacity 0.0 0 A 0 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 -1 Capacity (mAh g ) Cycle number Charge discharge curves showed that Comparison of cycling performance of the polysuflide shuttle was suppressed: cells with/without P2S5. Much higher Nonporous carbon was used to capacity retention was observed in the demonstrate the efficacy of additive cell with the additive. effect.

12 Managed by UT-Battelle for the Department of Energy P2S5 passivates the surface of Li anode.

SEM images of (A) the surface of pure metallic Li, (B, C) the passivation layer morphologies with different magnifications, and (D) the cross- section of the passivation layer peeled off from Li surface after the 1st cycle when using Li2S8/P2S5 as the catholyte in 1 m LiTFSI @ TEGDME.

13 Managed by UT-Battelle for the Department of Energy Composition of surface passivation layer on Li anode.

Li (002) 418 XRD Raman Li (011) The surface pasivation

layer is Li3PS4

Peaks matchs Intensity (a.u.) Intensity (a. u.) 184 Li3PS4 305 229 257

10 20 30 40 50 60 500 450 400 350 300 250 200 150 -1 2θ (Degrees) Wavelength (cm ) The function of the additive is two-fold:

– P2S5 promotes the dissolution of Li2S and alleviates the loss of capacity caused by the precipitation of Li2S; and

– P2S5 passivates the surface of lithium metal and therefore eliminates the polysulfide shuttle phenomenon.

14 Managed by UT-Battelle for the Department of Energy Air-transfer sample holder for cell diagnosis.

A new sample transfer stage has been developed for taking samples from glovebox to a scanning electron microscope (SEM) without exposure to air and moisture. The transfer stage operates by the difference of pressure inside and outside the SEM chamber: when a sample is loaded in a glovebox and pumped at mild vacuum, the atmosphere pressure pushes the lid against the o-ring seal. The sample is protected by the transfer stage when it is transferred in air. After the sample stage is loaded inside the SEM chamber, the lid pops open when the SEM chamber is pumping down to a lower vacuum for imaging. This is a useful tool for imaging air-sensitive materials used in Li-S batteries and other lithium-ion batteries. The vacuum chamber transfer stage with the lid in (a) a closed and (b) open position; and (c) Exploded view showing the O-ring seal and hinged lid.

15 Managed by UT-Battelle for the Department of Energy Air-sensitive sample holder enables imaging of lithium anode materials.

SEM image of stabilized lithium SEM of SLMP in contact with metal powder (SLMP) with/without moisture at different duration: (A) 0 exposure of air. The contrast shows min, (B) 15 min, (C) 30 min, and the difference by surface reaction (D) 45 min. The contrast of the with moisture in air. SEM images indicates the extent of reaction.

16 Managed by UT-Battelle for the Department of Energy Future work

• Develop solid electrolyte coating on cathode to prevent the dissolution of polysulfides in liquid electrolytes • Explore other electrolyte additives and solvents that are compatible with P2S5 to optimize the additive effect • Optimize the electrolyte composition to form rigid SEI for lithium anode • Investigate solid electrolyte coatings on lithium metal anode for Li-S batteries • Evaluate the full cell performance of Li-S batteries with limited usage of solvents and lithium metal

17 Managed by UT-Battelle for the Department of Energy Summary • Relevance: Exploratory research of Li-S battery chemistry leads to discoveries of advanced materials for high-energy batteries with potential use in EVs and PHEVs. • Approach: – Carbon-sulfur nano-composites with optimized porosity retains sulfur at the cathode

– Electrolyte additives facilitate the electrochemical cycling of Li2S

– Solid electrolyte coating on the Li2S cathode surface mitigates the dissolution of sulfur during battery cycling – Surface passivation on lithium anode improves the cycle performance of Li-S batteries • Accomplishment and progress: – Optimized the porosity of C/S composite electrode

– Discovered new electrolyte additive of P2S5 and elucidated the mechanism of additive effect of P2S5 on cell performance

– Synthesized a solid electrolyte coated Li2S cathode for Li-S batteries. – Developed a novel sample transfer stage for the diagnoses of Li-S batteries – Understood the importance of lithium protection by SEI • Future work:

– Optimize the electrolyte composition to facilitate electrochemical reaction of Li2S. – Explore solid electrolyte coatings for the protection of both cathode and anode – Evaluate the full cell performance of Li-S batteries with optimized cell components 18 Managed by UT-Battelle for the Department of Energy Technical Back-Up slides

19 Managed by UT-Battelle for the Department of Energy Business Sensitive Challenges for Li-S Battery

2.4 V 1.8 V discharge charge

Cathode: S8 Li2S8 Li2S6 Li2S4 Li2S2 X Li2S solid soluble/liquid form S migration through diffusion insoluble solid /irreversible deposition

+ Anode: Li Li X Li2S solid • Intrinsic sulfur migration: liquid phase diffusion

• Irreversible Li2S formation: both cathode and anode

• Poor Li anode cyclability: corrosion/ Li2S deposition/ dendrites

20 Managed by UT-Battelle for the Department of Energy