Introduction Motivation for Li-O 2 Batteries: • The electrification of transportation would decrease the consumption of fossil fuels, reducing CO 2 emissions and dependence on foreign oil imports • Li-ion is currently the most widely used rechargeable battery in electric vehicles • Li-ion is limited by an inadequate specific energy density • The Li-O 2 battery has a potentially much higher specific energy density Motivation for project: • Electrolyte selection is a critical barrier preventing the Li-O 2 battery from commercial viability • Decomposition reactions involving the electrolyte during discharge and charge limit cycling of the battery, and high overpotentials limit the cycling efficiency • LiNO 3 salt in N,N-dimethylacetamide (DMA) solvent is a promising electrolyte because of its stability at 1 M concentration • Preliminary results for higher [LiNO 3 ] in DMA show greater stability during cycling and larger cell capacity Results (cont.) Charles T. Wan 1 , Colin M. Burke 2 , & Bryan D. McCloskey 2 , PhD. 1 Cornell University, Ithaca, NY. 2 University of California, Berkeley, Berkeley, CA Examining the Performance of the Electrolyte LiNO 3 in N,N-Dimethylacetamide in Li-O 2 Batteries Electrolyte (e - /O 2 ) dis (e - /O 2 ) chg OER/ORR 1 M LiNO 3 DMA 2.06 2.67 0.77 3 M LiNO 3 DMA 2.09 2.51 0.83 5 M LiNO 3 DMA 2.03 2.46 0.82 References •Burke, C., Pande V., Khetan A., Viswanathan V., and McCloskey B. PNAS. 2015, 30, 9293-9298. •Girishkumar, G.; McCloskey, B. et al. J. Phys. Chem. Lett. 2010, 1, 2193-2203. •McCloskey, B. D. et al. J. Phys. Chem. Lett. 2013, 4, 2989- 2993. •Walker, W., Giordani, V., et.al. J. Phys. Chem. Lett. 2013, 135, 2076-2079 Acknowledgements Conclusion • There exists a positive correlation between capacity and [LiNO 3 ] in DMA • SEM images show Li 2 O 2 toroid formation at higher [LiNO 3 ]. This indicates that a solution-mediated mechanism is taking place, which could explain enhanced capacity • NMR measurements suggest Li + solvation does not vary over the [LiNO 3 ] range, contradicting toroid evidence of [LiNO 3 ] dependence • Pressure decay/rise experiments and titration yields point toward increased cycling stability as [LiNO 3 ] nears the saturation point in the electrolyte Methods and Materials Electrochemical tests to determine stability of electrolyte and cell capacity were conducted on custom Swagelok batteries (see right). +H 2 SO 4 → + 2Li + + SO 4 2− (1) + 2KI + H 2 SO 4 → +K 2 SO 4 + 2H 2 O (2) + → Na 2 S 4 O 6 + 2NaI (3) Iodometric titrations to determine Li 2 O 2 yield after discharge follow a similar procedure as described previously. The reactions take place as shown below: Quantitative analysis of O 2 consumed and evolved was gathered on this equipment, which samples and quantifies gas evolved in real time. Battery cell design: Custom-built differential electrochemical mass spectrometer: This work was funded by the Amgen Scholars program. I would like to thank Professor McCloskey and the McCloskey lab for this research opportunity. I would also like to acknowledge Colin Burke for his excellent mentorship and guidance throughout this experience. Many thanks to Jessica Nichols and the rest of the lab for their advice and support as well. Improvement in cycling stability with increasing [LiNO 3 ]: Galvanostatic discharge/charge profiles with corresponding pressure decay/rise graphs for Li- O 2 batteries (250 μA cycle for 1 M and 5 M, 250 μA discharge and 100 μA charge for 3 M). The chart is a summary of Faradaic charge compared to moles of O 2 consumed and evolved for discharge and charge, respectively. Results Cells were discharged at 250 μA in ~1.5 atm O 2 atmosphere for 1mAh. Scale bars all 1 μm. Insets are to make Li 2 O 2 toroids easier to see. Representative galvanostatic discharge profiles for Li-O 2 batteries (250 μA in ~1.5 atm O 2 atmosphere until 2V cutoff). Preliminary data shows an increase in cell capacity with higher [LiNO 3 ] : Scanning Electron Microscopy (SEM) images of discharged cathodes show toroid formation at higher [LiNO 3 ]: 1 M LiNO 3 DMA 5 M LiNO 3 DMA 3 M LiNO 3 DMA Li + solvation not affected by salt concentration: Low Li 2 O 2 titration yields: 7 Li Nuclear Magnetic Resonance (NMR) spectroscopy of electrolytes versus a 3 M LiCl in D 2 O reference show no significant change in chemical shift as a function of [LiNO 3 ]. The 7 Li chemical shift can be correlated to LiO 2 intermediate solvation, which in turn is related to toroid formation. Titrated cells were discharged at 250 μA in ~1.5 atm O 2 atmosphere for 1 mAh. Titrations reveal information about possible parasitic chemistry during discharge. 0 0.2 0.4 0.6 0.8 1 0 1 2 3 4 5 6 7 Li chemical shift (ppm) [LiNO 3 ] (M) 0 20 40 60 80 100 0 1 2 3 4 5 6 % Yield Li 2 O 2 [LiNO 3 ] (M) 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 U (V vs Li/Li+) Q (mAh) 1 M LiNO3 in DMA 3 M LiNO3 in DMA 5 M LiNO3 in DMA 1 M LiNO 3 DMA 3 M LiNO 3 DMA 5 M LiNO 3 DMA 0 0.25 0.5 2 2.5 3 3.5 4 4.5 U (V vs Li/Li+) Q (mAh) Discharge Charge 0 0.25 0.5 -10 -8 -6 -4 -2 0 ΔmO 2 (umol) Q (mAh) Discharge Charge 0.00 0.25 0.50 0.75 1.00 2 2.5 3 3.5 4 4.5 U (V vs Li/Li+) Q (mAh) Discharge Charge 0.00 0.25 0.50 0.75 1.00 -20 -15 -10 -5 0 ΔmO 2 (umol) Q (mAh) Discharge Charge 0 0.25 0.5 2 2.5 3 3.5 4 4.5 U (V vs Li/Li+) Q (mAh) Discharge Charge 0.00 0.25 0.50 -10 -8 -6 -4 -2 0 ΔmO 2 (umol) Q (mAh) Discharge Charge