Thin film coatings on lithium metal for Li-S batteries AIMCAL 2016 Memphis, TN

Transcription

1 Thin film coatings on lithium metal for Li-S batteries AIMCAL 2016 Memphis, TN Stephen Lawes, Research Scientist

2 OXIS Company Background OXIS have been working on Li-S since 2005 at Culham Science Centre (Oxfordshire, UK) $70 million raised to date Expanding rapidly: 3 fold increase in the number of employees since 2012, 60 today Highly trained staff (14 PhDs, 13 MSc/MA) Cutting edge R&D facilities (i.e. second largest high specification dry room in Europe) Strong patent portfolio protecting IP (79 patents granted, 97 pending, encompassing 27 families) 2

3 Benefits of Li-S technology High gravimetric energy Theoretical: 2500 Wh/kg vs. 500 Wh/kg for Li-ion Practical: 400 Wh/kg achieved vs. 250 Wh/kg for Li-ion Low cost No expensive cathode material Environmentally friendly No heavy metals such as Cobalt and Nickel Safety Tolerant to mechanical abuse, nail and bullet penetration, overcharge, short circuit 3

4 4 REVB

5 Introduction to Li-S batteries Discharge Elemental sulfur Current collector Li Separator S 8 Current collector Conductive carbon Binder Li-S Li-ion Specific capacity 1675 mah/g 200 mah/g Theoretical energy density Achievable energy density 2500 Wh/kg 2800 Wh/L 500 Wh/kg 700 Wh/L 500 Wh/kg 1800 Wh/L 300 Wh/kg 900 Wh/L Current cycle life ~ (-) (+) - Load / Charger + 5

6 Introduction to Li-S batteries e - e - e - Li anode Electrolyte S/C cathode 6

7 Introduction to Li-S batteries e - e - e - Li anode Electrolyte S/C cathode 7

8 Problems with lithium metal anodes 1. Dendrites/mossy lithium 2. Dead lithium 3. Large volume change 4. Electrolyte decomposition 5. Irreversible Li corrosion G. Zheng et al. Interconnected hollow carbon nanospheres for stable lithium metal anodes. Nature Nanotechnology,

9 Lithium protection Less electrolyte needed Electrolyte can represent up to 50% of the weight of a cell! Successful lithium protection will mean less electrolyte is required Electrolyte 50% Other, 5% Cathode, 25% Separator, 5% Lithium, 15% A typical distribution of masses in an Li-S cell 9

10 Lithium metal protective coatings "The ideal protective layer for a lithium metal anode needs to be chemically stable to protect against the chemical reactions with the electrolyte and mechanically strong to withstand the expansion of the lithium during charge. Prof. Yi Cui, Stanford Coating requirements: Stable against lithium and electrolyte Mechanically strong Uniform thickness Flexible Ionically conductive Electrically insulating High transference number 10

11 Lithium protection extends cycle life 80% BoL Protected lithium Increase in surface area leads to electrolyte depletion and cell failure Unprotected lithium 11

12 Lithium protection at OXIS Energy Unprotected lithium 50 cycles Protected lithium 50 cycles High surface area Electrolyte depletion Less mossy growth Longer cycle life 12

13 Polymers vs. ceramics Polymer coatings: Flexible Easy to process Ionically conductive Low interfacial resistance Swells in electrolyte Delamination Low shear modulus Ceramic coatings: Hard Ionically conductive No swelling in electrolyte Single-ion transport Brittle High interfacial resistance Hard to process 13

14 Hybrid coatings Combining the benefits of both polymers and ceramics Flexible to withstand large volume changes Hardness to prevent mossy lithium growth Ionically conductive to allow fast lithium transport Stable against electrolyte Processable for low-cost, scalable coatings Polymer/ceramic nanocomposite 14

15 Coating techniques Requirements for coating technique: Low temperature (< 150 C) Dry atmosphere (0.25% RH) Thin coatings (100nm 5µm) Pinhole-free coatings Double-sided coatings Able to handle flexible, soft substrate Scalable 15

16 Conclusions Reduced mossy lithium growth and electrolyte depletion with protective coating on lithium Improved cycle life for high energy density lithium-sulfur cells How do we maintain/improve cycle life when going to higher energy density? Developing new materials for lithium protection Optimizing coating technique for uniformity, thickness, etc. All components are strongly interdependent (anode, electrolyte, cathode) 16

17 Thank You For Listening