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Rachel Carter1 Danniel Reed2 Ryan DeBlock3 Megan Sassin1 Corey Love1 Partha Mukherjee2

1, U.S. Naval Research Laboratory, Washington, District of Columbia, United States
2, Mechanical Engineering, Purdue University, West Lafayette, Indiana, United States
3, Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States

Replacing lithium ions with sodium ones in energy-storage systems is highly attractive due to the ~1300× enhancement in material abundance However, the 25% larger volume of sodium ions prevents a drop-in substitution. For example, the typical graphite-based anode used in Li-ion systems are not amenable to reversible Na-ion intercalation and cathodes designed for Li-S cannot accommodate the larger volume expansion. Herein, we employ a specially designed in-situ electrochemical cell developed at the Naval Research Laboratory1 to probe the optical characteristics of electrodes under charge–discharge operation, yielding key information on electrochemical mechanisms.
For Sodium-ion anodes, disordered “hard carbon” anodes show more promise than graphitic carbons for Na-ion cells, but their charge-storage mechanisms are more complex, involving surface association, bulk cation-insertion reactions, and micropore-based deposition. Using the in-situ cell, we examine the sodiation of carbon nanofoam papers (CNFPs), which were recently reported for their high-capacity and high-rate charge-storage properties in nonaqueous Na-ion electrolytes.2 Imaging the CNFP during galvanostatic “discharge” (reduction in half-cell configuration vs. Na metal) reveals distinct color changes from grey to black to bronze to blue, caused by carrier concentration variation with increasing Na-ion association/insertion at the carbon electrode. We correlate these optical changes with variations in the complex discharge profile of the CNFP, which arises from multiple defect- and porosity-enable Na+-storage mechanisms that are supported by this electrode. This fundamental understanding will enable optimization of defect concentration and porosity of this anode material for the realization of this system.
Alternatively, the RT Na-S system that boasts higher energy than Li-ion, in addition to high material abundance, is stalled by material challenges at the metal anode and conversion cathode. The soluble nature of discharge products and their interaction with the metal limit energy and cyclablity.3 However, using the in-situ cell, careful mapping of discharge mechanisms is achieved in conjunction with anode observation. The discharge products (Na2Sn, 8<n<1) and relative quantities are determined, due to their distinct colors, with UV-vis spectroscopy. Since these behaviors prove strongly dependent on electrolyte solvent, the study is valuable to selection of optimal composition for anode stability and cathode performance.

1. Love, C. T.; Baturina, O. A.; Swider-Lyons, K. E., Observation of Lithium Dendrites at Ambient Temperature and Below. Ecs Electrochem Lett 2015, 4 (2), A24-A27.
2. DeBlock, R. H.; Ko, J. S.; Sassin, M. B.; Hoffmaster, A. N.; Dunn, B. S.; Rolison, D. R.; Long, J. W., Carbon nanofoam paper enables high-rate and high-capacity Na-ion storage. Energy Storage Materials 2019, 21, 481-486.
3. Carter, R.; Oakes, L.; Douglas, A.; Muralidharan, N.; Cohn, A. P.; Pint, C. L., A Sugar-Derived Room-Temperature Sodium Sulfur Battery with Long Term Cycling Stability. Nano Letters 2017, 17 (3), 1863-1869.

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