High Energy Rechargeable Li-S Cells for EV Application. Status, Challenges and Solutions Yuriy Mikhaylik, Igor Kovalev, Riley Schock, Karthikeyan Kumaresan, Jason Xu and John Affinito Sion Power Corporation 2900 E. Elvira Rd. Tucson AZ 85756 USA www.sionpower.com 1
Outline Why lithium-sulfur technology? Specific energy. Rate capability. Low temperature performance. Status of lithium-sulfur technology. Addressing the challenges. New approach pursued by Sion in collaboration with BASF for EV applications. Conclusions. 2
Why Lithium Sulfur Technology? Anode (-) Li 0 Charge (Li plating) Discharge (Li stripping) Li + Li + Li + Li + Li + S S S S Li Li S S S S S Li Li + Li S S S Li Li S S S S S Li Li S S S S Li Li S S Li Li S S S S S Li Li S S S Li + Li + Li + S S Li S Li Li + Li + Li + Polysulfide Shuttle S S Li S Li S Li Li S S Li Li Li + Cathode (+) Li + S 8 Li 2 S 8 Li 2 S 6 Li 2 S 4 Li 2 S 3 Li 2 S 2 Li 2 S Lithium ions are stripped from the anode during discharge and form Li-polysulfides in the cathode. Li 2 S in the cathode is the result of complete discharge. On recharge the lithium ions are plated back onto the anode as the Li 2 S x moves toward S 8 High order Li-polysulfides (Li 2 S 3 to Li 2 S 8 ) are soluble in the electrolyte and migrate to the anode scrubbing off any dendrite growth. - Load / Charger + Theoretical Energy: ~2800Wh/l and 2500 Wh/kg 3
Why Lithium Sulfur Technology? Specific Energy 2.5 2.4 Typical experimental discharge and charge profiles with strong shuttle. Following recharge 2.5 2.4 Charge and discharge profiles with shuttle inhibitor. Following recharges 2.3 2.3 Voltage 2.2 2.1 First discharge Voltage 2.2 2.1 First discharge 2.0 1.9 Second and following discharges 2 1.9 Second and following discharges 1.8 0 200 400 600 800 1000 1200 Specific capacity mah/g 1.8 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Specific capacity, Ah/g With NO 3 - additives Sion Power controls shuttle and achieves 100% of high plateau sulfur utilization, 99.5% charge efficiency and 350 450 Wh/kg 4
Why Lithium Sulfur Technology? Rate Capability 450 Specific Energy, Wh/kg 400 350 300 250 200 150 100 50 0 Li-ion 18650 Ni-MH Ni-Cd Sion Power Li-S Li-ion High Power 0 1000 2000 3000 4000 Specific Power, W/kg 5
Why Lithium Sulfur Technology? Low Temperature Performance Work partially support by NASA Glenn Contract NNC06CA85C 2.5A Discharge Profiles Charge and Discharge Profiles at -60 oc 3.0 3.0 2.5 2.5 2.0 + 20 o C 2.0 Charge 50 ma to 2.95 V Voltage 1.5 1.0 0.5-70 o C - 60 o C Voltage 1.5 1.0 0.5 Discharge 2500 ma to 1.0 V 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Discharge Capacity, Ah 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Ah Batteries with optimized solvent and salt concentrations delivered: 1)~160 Wh/kg at -60 o C at 1C, 2)~130 Wh/kg at -70 o C at 1C, 3) The battery can be recharged at -60 C. 6
Status of Lithium Sulfur Technology Power Density (W/L) 400% Cycle Life 300% 200% Specific Power (W/kg) USABC Sion Rate Cap @1C (%) 100% Recharge Time (hr) 0% Upper Temp(oC) Specific Energy (Wh/kg) Lower Temp (oc) Energy Density (Wh/L) Limiting Mechanisms: 1) Rough lithium surface during cycling 2) Li/electrolyte depletion. 7
Addressing the Challenges Keys to the EV Market for Lithium-Sulfur Challenges - cycle life and high temperature stability: Dynamics of lithium surface roughness and cycling. Solvent depletion chemistry. 8
The Dynamics of Lithium Surface Roughness Monte-Carlo Simulation 0.2 Surface Roughness vs Li DoD Initially, surface roughness increases in direct proportion to Li depth of discharge (DoD). Surface Roughness 0.1 0 0 0.2 0.4 0.6 0.8 1 Li DoD Maximal surface roughness can be observed at ~50-70% of Li DoD. The typical scenario is cycling at low Li DoD. The best scenario is cycling Li anodes at 100% DoD but only with a current collector. 9
The Dynamics of Lithium Surface Roughness Experimental Observations 30 cycles 26% Li DoD 330cycles 100% Li DoD 352 cycles 100% Li DoD Cycling at 100% DOD of lithium prevents surface roughness but lithium/electrolyte depletion still occurs. 10
The Chemistry of Solvent Depletion Experimental Observations Weight (g) Solvent and metallic Li mass vs. Cycle Number (2.5 Ah Li-S battery). 2.5 2.0 1.5 1.0 0.5 0.0 Lithium DOL DME 0 20 40 60 80 100 Cycle 1,2-Dimethoxyethane(DME) is mainly responsible for depletion. Mass of metallic Li in the cell did not change dramatically. However, visually Li looks completely depleted at 60-80 cycles due to roughening and disintegration of Lithium foil. The slopes suggests that Lithium and DME may react in a molar ratio of 1:1 to 1:2. Several Lithium alcoholates can form by reaction with DME. 11
The Chemistry of Solvent Depletion Products and Effects Identified depletion products and their impact on battery performance. O OLi High amount, highly soluble and highly detrimental for S cathode performance. O O Li/Li 2 S x MeOLi MeS x Li Moderate amount, low solubility, neutral. Small amount, soluble, consumes S. DME CH 4 O Traces. Traces. Identified at Sion Power O OLi Highly soluble and highly detrimental for S cathode performance. Li/Li 2 S x O O DOL R O H 2 O O Li n Increases anode polarization Traces Identified at Sion Power and by D. Aurbach, J. Electrochem. Soc.156,8. 2009. 12
New Approaches Pursued by Sion in Collaboration with BASF for EV Application Reduction of lithium roughness. Proprietary anode design. Development of innovative materials Structurally stable cathodes. Materials developed by Sion/BASF Physical protection of lithium with multi-functional membrane assemblies. 13
Lithium Roughness Development Proprietary Anode Design 1600 Charge Current Changed from: an 8-hour charge to a 2.5-hour charge Specific Capacity, mah/g 1200 800 400 Conventional design Proprietary design 47 um Proprietary design 60 um Conventional design 0 0 50 100 150 Cycle Experimental batteries cycling behavior Proprietary design allowed for increased charging rate without increase in surface roughness. 14
30 Lithium Roughness Development Proprietary Anode Design 25 20 mah 15 10 Conventional design Proprietary design 24 μm 5 0 FoM ~34 FoM ~110 0 100 200 300 400 Cycle Experimental batteries cycling behavior Li anode after 450 cycles. Initial and final Li thickness ~24 µm. Li Figure of Merit (FoM) exceeds 100 at Li DoD ~26-30%. FoM = DoD x Number of Cycles. 15
Development of Innovative Material Structurally Stable Cathodes Specific Capacity (Ah/g) 1.6 1.4 1.2 1.0 0.8 0.6 Improved structure Conventional cathode 0 20 40 60 80 Cycle No. Cathode structure improvement resulted in sulfur utilization increase from 1.2 Ah/g to 1.45 Ah/g. This development paves the way to increasing specific energy from the current 350 Wh/kg to the 550 Wh/kg needed to achieve a 500 km EV range Experimental batteries cycling behavior 16
Development of Innovative Material Multi-functional Membrane Assemblies Thermal Ramp Test of Fully charged Li-S batteries after 20 cycles at 5 o C/min. 95 75 Conventional design Proprietory design T cell - T Heater, o C 55 35 15 Sulfur melting Li melting Sion-BASF Protective Layer -5 100 120 140 160 180 200 220 240 Heater Temperature, o C With Sion-BASF protective layer on anode, there is no thermal runaway. 17
Conclusions Reduction of lithium surface roughness with new anode design, and better cathode structure, resulted in: Recharge time reduced to less than 3 hours. Substantial cycle life increase if lithium surface roughness suppressed. Sulfur utilization increased to 87%, or 1.45 Ah/g, paving the way to 550 Wh/kg Li-S battery. Innovative anode design, and Sion Power-BASF protective membranes, increased thermal stability of Li-S cells eliminating thermal runaway. Batteries passed the melting point of the Li without violent events. 18
Takeaway Sion Power Corporation, in collaboration with BASF, is very optimistic that the future of all electric EV applications will be dominated by Sion Power s lithium-sulfur technology. 19
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