Fundamental Chemistry of Sion Power Li/S Battery. Yuriy Mikhaylik Sion Power Corporation, 9040 South Rita Road, Tucson, Arizona, 85747, USA

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1 Fundamental Chemistry of Sion Power Li/S Battery Yuriy Mikhaylik Sion Power Corporation, 9040 South Rita Road, Tucson, Arizona, 85747, USA

2 Outline Thermodynamics of Li-S Discharge-charge mechanism in the organic solvents medium: ambient temperatures and Li-S electrochemistry at temperatures below 40 o C. Polysulfide electrochemical shuttle and impact on charge efficiency and specific capacity. Specific energy beyond 300 Wh/kg. Short-chain polysulfides solubility and impact on sulfur utilization. Specific energy beyond 400 Wh/kg. Ragone plots and specific power-energy limitations

3 Why Lithium Sulfur? Theoretical Energy Density Comparison S + 2Li = Li 2 S ΔG ~ kj/mol OCV ~ 2.2 V Energy density ~ 2800 Wh/L Specific Energy ~ 2500 Wh/kg Compare Specific Energy with: Li-ion ~ 580 Wh/kg TNT equivalent ~ 1280 Wh/kg

4 Theoretical Volume Changes at Discharge S + 2Li = Li 2 S cm 3 /Ah % Li+S % Li % S % S cathode has to have high porosity to accommodate bigger volume discharge products Li 2 S Cell mechanical design has to sustain volume changes during discharge-charge cycles cm 3 /Ah

5 How it works? Active Materials Transformation Diagram Discharge Charge Cathode 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 Porous Separator Polysulfides Diffusion through Separator Shuttle insoluble compounds S 8 Li 2 S 8 Li 2 S 6 Li 2 S 4 Li 2 S 3 Li + Anode Polysulfides reduction on the Anode surface Li o Lithium plating-stripping

6 Li-S cell s first discharge at RT High 418 mah/g Low 1256 mah/g Sub-Low e - /S High plateau fast kinetics S 0 +! 8 e = 2 4 S 2! 4 Low plateau moderate kinetics S 2!! 2! 4 + 4e = 2S + S 2! 2 Sub-Low plateau very slow kinetics S 2!! 2 + e = 2 2 S Electrons per S atom R.D. Rauh, K.M. Abraham, J. Electrochem Soc, ! E. Peled, H. Yamin, J. Electrochem Soc, 1989

7 2.6 Discharge profiles at C/3 rate 300 Specific energy at different temperatures C/3 rate o C - 50 o C - 40 o C - 30 o C - 20 o C - 10 o C +65 o C +25 o C Specific Energy, Wh/kg Ah Temperature, o C Only two discharge plateau can be observed at medium or high discharge rates High plateau discharge demonstrates very low polarization down to 60 o C

8 Discharge profilers at low rate and temperatures below 40 o C showed multi-step sulfur reduction o C -20 o C +25 o C o C Ah

9 Differential capacity vs at different temperatures Evidence of multi-step process dq/dv o C dq/dv o C dq/dv o C dq/dv o C

10 Thermal effects Discharge Charge Cell temperature Ambient temperature Time, min Maximal exothermic effects at transition from high to low plateau and at end of discharge Temperature Cell temperature Ambient temperature Time, min Inhibited shuttle. No overcharge. Endothermic effects at high plateau Temperature Charge Cell temperature Ambient temperature Temperature Charge at strong shuttle. Significant overcharge. Exothermic effects at high plateau

11 Polysulfide Shuttle Understanding and Control The rate of polysulfide reduction on the Li surface is not limited by diffusion. It is limited by the rate of heterogeneous reaction on the anode surface. d[ S dt H ] = I q H! k s [ S [S H ] high plateau polysulfide concentrations S 8, Li 2 S 8, Li 2 S 6. I charge current k s - the heterogeneous reaction constant or Shuttle constant. q H - high plateau specific capacity H ] Charge profiles at different Charge-Shuttle factor f c ksqh S I mah/g C total = f C J. Electrochem Soc. 151 A1961-A1976 (2004) 4.0

12 Polysulfide Shuttle Understanding and Control Available S capacity at different Charge- Shuttle factors Retained High Plateau capacity after 30 days storage at different shuttle constants ks mah/g mah/g Charge-Shuttle factor Shuttle constant Log k S

13 Typical experimental discharge and charge profiles with strong shuttle. Charge and discharge profiles for cells with strong lithium protection Following recharge Following recharges 2.2 First discharge 2.2 First discharge Second and following discharges Second and following discharges Specific capacity mah/g Specific capacity, Ah/g Newly discovered shuttle inhibitors allowed us to control shuttle and achieve 100% of high plateau sulfur utilization and 350Wh/kg

14 OCV vs Time Protected Lithium First Month Self-Discharge 6% OCV, V 2.2 First Day Self-Discharge 15% Unprotected Lithium Time, h

15 Very strong shuttle. No charge voltage termination After 2 days storage Self Discharge = 11% Charge Efficiency = 65% Immediately after charge Ah Partially inhibited shuttle. Charge voltage termination High self-discharge After 2 days storage Self Discharge = 24% Charge Efficiency = 79% Immediately after charge Ah Completely inhibited shuttle. Charge voltage termination High charge efficiency Low self-discharge Charge Efficiency = 99.7% After 2 days storage Self Discharge = 0.2% Immediately after charge Ah Self-discharge, % B C A D Charge Efficiency, %

16 Increasing sulfur utilization at Low discharge plateau The sulfur cathode internal shuttle mechanism with Red-Ox mediator (M) mah/g or 75% e - /S Sub-Low plateau very slow kinetics. Discharge stops with insoluble Li 2 S 2 S 2!! 2 + e = 2 2 S 2!

17 Sion Power Li/S cells specific capacity evolution 2.4 Protected Li + Capacity Promoter 2.2 Protected Li 70% utilization 90% utilization Theoretical Unprotected Li 50% utilization Specific capacity mah/g

18 Sion Power Li-S Rechargeable Cells Weight g Dimensions 52 x 38 x 10 mm Capacity Ah V Specific energy Wh/kg

19 Specific Energy, Wh/kg Ragone plots for different rechargeable systems Li-S Protected Li & Capacity promoter Ni-MH Li-ion Ni-Cd Li-S Protected Li Specific Power, W/kg

20 Arrhenius plots for cell polarization resistance R p Λν (Ρπ, Οηµ) o C Q ~0.25 ev R Ohm Rp /Τ, Κ 1 R A R p = d V /d I = = R o A *exp (Q/kT) + R Ohm (1+r*T) At room temperature the electrochemical processes became so fast that construction materials not chemistry control the polarization resistance.

21 Conclusion: Polysulfide shuttle understanding and control through shuttle inhibitors lead to: - sulfur utilization 75% and 350 Wh/kg - charge efficiency ~99.7% - self-discharge ~ 4-6% per month Low plateau discharge limitations understanding and improvement with homogeneous catalysts lead to 85-90% of sulfur utilization and paves the way achieving 450 Wh/kg. Areas of continuing developing: Increase cycle life Extend temperature range Increase rate capability

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