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The Sudden Death Phenomena in Nonaqueous Na-O Batteries Jessica E. Nichols, and Bryan D. McCloskey J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09663 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016
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The Journal of Physical Chemistry
The Sudden Death Phenomena in Nonaqueous Na-O2 Batteries Jessica E. Nicholsa,b, Bryan D. McCloskeya,b*
a
Department of Chemical and Biomolecular Engineering, University of California Berkeley, Berkeley, CA 94720
b
Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
*
[email protected] ACS Paragon Plus Environment
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Abstract Metal-air (O2) batteries have been intensely studied over the past decade as potential high-energy alternatives to current state-of-the-art Li-ion batteries. Although Li-O2 batteries possess higher theoretical specific energies, Na-O2 cells have been reported to achieve higher capacities on discharge and exhibit much lower overpotentials on charge than analogous Li-O2 cells. Nevertheless, sudden and large overpotential increases (“sudden deaths”) occur in Na-O2 cells on both discharge and charge, substantially limiting achievable capacity on discharge and increasing the average charge voltage, thereby reducing round-trip energy efficiency. In this work, we unravel the origins of these sudden death phenomena, which have been previously linked to the electrochemistry occurring at the cathode. On discharge, the maximum capacity was limited by pore clogging at low current densities and by surface passivation at high current densities, with concentration polarization playing only a small role in limiting the achievable capacity. On charge, the discharge and charge current densities were both found to influence the attainable capacity prior to sudden death. We propose a charge mechanism consistent with our data, where a concerted surface oxidation mechanism and a dissolution-oxidation mechanism both contribute to the observed overpotentials. Sudden death on charge is proposed to occur when these two pathways cannot support the applied current rate.
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Introduction Among the various “beyond Li-ion” battery technologies, the highest potential theoretical specific energies are those of nonaqueous metal-oxygen batteries, such as Li-O2 and Na-O2 batteries. While nonaqueous Li-O2 batteries, in which lithium peroxide (Li2O2) is formed as the discharge product, have very high theoretical specific energy,1-4 the nonaqueous Na-O2 battery has been reported as a possible alternative.5-8 Despite its lower theoretical specific energy (1100 Wh kg-1 for Na-O2 vs. 3450 Wh kg-1 for Li-O2), sodium’s natural abundance and lower cost make large-scale implementation economically attractive.9 Recent studies on the Na-O2 battery chemistry have reported potential advantages over analogous Li-O2 systems, including improved stability and lower overpotentials, particularly on charge.5,
10-14
These advantages are at least
partially attributed to the differences in discharge products between the two systems. In batteries employing an electrolyte with an anhydrous ethereal solvent, the Na-O2 battery reaction has been convincingly shown to be a one-electron reduction and oxidation of oxygen to NaO2:10, 15-18 Na(s) + O2(g) ⇄ NaO2(s), E0 = 2.27 V In addition, subsequent studies have shown that Na-O2 batteries are highly sensitive to operating conditions and contaminants, leading to changes in capacity, discharge product morphology, and overall cell performance.15, 19-23
Typical Na-O2 cells studied to-date are fed with an unlimited or excess supply of O2, such that discharges should theoretically be capable of proceeding until all Na metal at the anode is consumed.15 However, Na-O2 cells undergo a “sudden death” phenomenon on discharge at a capacity well below what would be expected from complete Na metal conversion.9, 15-16, 24 This “sudden death” is a precipitous decrease in potential that signifies the end of discharge and is
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reminiscent of a similar phenomenon observed during discharge in Li-O2 batteries.25 However, unlike in Li-O2 cells, a poorly understood “sudden death” is also observed on charge in Na-O2 cells, where a precipitous increase in voltage occurs and does not necessarily correspond to the full discharge capacity, as will be shown later.9, 20 Delaying the charge sudden death to allow a low charging voltage for the full capacity attained during discharge is desirable to improve energy efficiency and rechargeability.
In this work, we examine this sudden death mechanism on the first discharge and charge of NaO2 batteries. First, we consider the maximum cell capacity on discharge and the associated deposition of NaO2 at various current densities. By examining the discharge product and cell behavior preceding and following sudden death, we provide insight into the factors limiting overall cell capacity. In particular, our evidence suggests that sudden death on discharge is caused primarily by pore blocking/clogging due to large NaO2 crystal formation at low current densities and by passivation due to a more conformal NaO2 film deposition at high current densities, consistent with our recent report.26 Second, we elucidate factors that influence sudden death on charge, such as the charge and discharge current densities. We find evidence that sudden death on charge is likely related to an ever-decreasing rate of NaO2 thin film oxidation and NaO2 large crystal dissolution. At some point during the charge process, the combined overall rates of these mechanisms cannot support the applied current density, which results in the charge sudden death.
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Experimental Methods Materials and Cell Assembly 1,2-Dimethoxyethane (DME) was purchased from BASF and used as received. Sodium trifluoromethanesulfonate (NaOTf) was purchased from Sigma Aldrich. To remove residual water from the NaOTf, it was dried in a vacuum oven (80°C,
