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To address the tradeoff between mechanical strength and Li+ conductivity in Poly(Ethylene Oxide) (PEO)-based electrolytes, a rigid nonconductive polymer is frequently added to the electrolyte via blending or copolymerization. The ionic conductivity of mixed PEO electrolytes is generally lower than that of unmixed PEO electrolytes. The suppressed ionic conductivity is attributed to the reduced segmental mobility and connectivity of the conductive PEO cites. Most experimental systems make it difficult to decouple the two mechanisms and accurately examine their impact on conductivity. We compare two symmetric polymer mixtures (50:50 wt%): a miscible polymer blend PEO/PMMA and a disordered block copolymer (BCP) PEO-b-PMMA, both with the same amount of Li salt. Because their chemical and physical properties are the same, changes in ionic conductivity can be attributed solely to local changes in PEO network connectivity. We discover that the mixtures' immediate Li+ solvation sites (+ transport within and across PEO clusters at the appropriate length scales. This new understanding of network connectivity in polymer electrolyte mixtures is critical for the design of future multiphase polymer electrolyte systems.

The tendency of metals to form uncontrolled dendritic morphologies during electrodeposition hinders the development of safe and reliable metal batteries. Multiphase nanostructured electrolytes can suppress dendritic growth if the mechanical modulus of the electrolyte is high relative to that of the metal or if the conducting channels are confined to nanoscale dimensions. Direct visualization and analysis of electrodeposition within polymeric nanostructures elucidates the structure−property relationships and mechanisms underlying the suppression of dendrite growth. To overcome these challenges, we introduce here a generalizable platform by which we can directly characterize electrodeposited metal morphologies in well-defined, multiphase nanostructured electrolytes by top-down SEM imaging [1]. This approach requires no destructive sample preparation techniques between electrodeposition and imaging. The platform consists of coplanar electrodes coated by polymeric structures consisting of alternating ion conducting and nonconducting material which are precisely created by lithography and etch techniques. These fabricated polymeric structures are designed to carefully model deposition of metals in nanostructured polymeric separators and electrolytes. In this study we track the electrodeposition of silver ions (Ag+) dissolved in dry, aprotic poly(ethylene oxide) (PEO) polymer electrolyte confined within channels that are defined by a microfabricated nonconductive polystyrene structures. We find that Ag metal electrodeposition occurs only in the conductive domains with no significant deformation of the adjacent low-modulus nonconductive and confining domains. Moreover, we have shown that the Ag metal deposit morphology was strongly dependent on the dimensions of the conducting domains. When confinement width was similar in scale to the critical length scale for unstable deposition, the metal morphology was dense, unbranched filaments. Gradually increasing the confinement width led to branched dendritic deposition, although the side branch feature size was limited by the size of the confinement. These results demonstrate that in addition to mechanical and transport properties, the nanodimension of the structured electrolyte can also serve to stabilize dendritic morphology growth. This work highlights critical parameters for design of structured battery components such as solid-electrolytes, separators, and current collectors. [1] Sharon, D.; Bennington, P.; Patel, S.N.; Nealey, P.F.*, “Stabilizing Metal Electrodeposition by Limiting Spatial Dimensions in Nanostructured Electrolytes,” ACS Energy Lett. 2020, 5, 2889−2896. Figure 1

In case of soluble electrolyte catalysts for Li-O2 batteries as a redox mediator, they were utilized to facilitate the exchange of electron and lithium ion, which can help formation and decomposition of discharge product. Although further researches are needed for finding proper catalysts, there is a promising approach fortunately, using lithium halides as a redox mediator in Li-O2 batteries. Related researches have been progressed vigorously. Previous paper reported in our group demonstrated the reaction mechanism and effect of the lithium iodide (LiI) in Li-O2 battery.1 Lim et al. exhibited the superior performance of Li-O2 battery using LiI additive.2 Recently, Liu et al. developed this concept and exhibited Li-O2 battery having better performance with LiOH as a discharge product by adapting more advanced electrodes.3 However, further research with LiI is necessary for clear comprehension because there is no accurate elucidation on the reaction mechanism which can be activated gradationally in Li-O2 battery system yet. Therefore, here in, we has progressed comparing and contrasting researches using LiI and lithium bromide (LiBr) for clearly understanding the role of lithium halide materials in Li-O2 battery system. As a redox mediator, LiBr catalyzes the reversible formation and decomposition of lithium peroxide (Li2O2), but hygroscopic LiI generated the side reaction making lithium hydroxide (LiOH) as a byproduct. Especially, when Li-O2 batteries are operated with LiBr and LiI until enough amount of capacities for practical using, LiOH made by LiI included electrolyte couldn’t be reversible compared to Li2O2 made by LiBr included electrolyte. Therefore, in our research, LiBr is much reliable redox mediator to operate the Li-O2 battery system. References Kwak, W.-J. et al. J. Mater. Chem. A 3, 8855–8864 (2015). Lim, H. et al. Angew. Chem. Int. Ed. Engl. 53, 3926–31 (2014) T. Liu. et al. Science, 350, 6260, 530-533 (2015).

Lithium oxygen (Li-O2) research generates much interest and many expectations among electrochemists. The high theoretical specific energy, the simplicity of preparation and operation of electrochemical cells, the sizable commercial possibilities, and the ever-increasing number of new publications, has directed many groups to Li-O2 battery research. Over the past decade, significant progress in the study of possible Li-O2 battery systems has also prompted interest from the chemical and automotive industries. Nevertheless, practical Li-O2 batteries are far from realization. Many scientific challenges are related to the oxygen reduction reactions (ORR). In aprotic Li-O2 cells the general assumption is that during discharge, oxygen is reduced on the cathode to form insoluble lithium oxide compounds. Experimental evidence support that the major final product is lithium peroxide (Li2O2). Nevertheless, the electrolyte solution and the carbon cathode instability toward reduced oxygen species lead to intrinsic instability and irreversible formation of many side products. Electrochemical formation of Li2O2 can proceed in two ways: (i) One electron transfer to molecular oxygen to form LiO2, which continues to react by receiving another electron and Li ion to form Li2O2. (ii) Two electron transfer pathway that skip over the formation of LiO2, and forms directly Li2O2. Besides the electrochemical routes, Li2O2 can be formed chemically by disproportionation of Li-superoxide to form molecular oxygen as the second product. In fact, electrochemically formed LiO2 is unstable, its further reaction to form Li2O2 is fast1. The possibility of isolating lithium superoxide and to assess its lifetime in different aprotic media are still under investigation. The rout which Li2O2 is being formed can affect the growth mechanism. Li2O2 is insoluble in aprotic solvents, thus during ORR, crystalline Li2O2 is deposit on the cathode surface. When fully covered by ORR products, the continuation of the process depends on electrons crossing the product layer to reach fresh oxygen molecules. Due to poor electronic conductivity of the oxygen reduction product layer, the cathode is deactivated and the ORR is terminated after the deposition of a few layers of Li-peroxide. The stabilization of superoxide intermediate can change the growth mechanism of the Li2O2 layer. In contrast to lithium peroxide, superoxide radical can be soluble in aprotic solvents that contain lithium cations. During discharge, oxygen is reduce to superoxide through a free cathode site. Then, the charged radical can diffuse in the solution and transfer charge or to disproportionate in all dimensions, leading to a thick deposits of Li2O2. This mechanism can be considered as a top down growth, as superoxide can disproportionate on top of a Li2O2 particle. Using aprotic solvents with high Guttman donor number such as DMSO, DMF, and DMA in Li-oxygen cells can help to extend the superoxide lifetime. The presence of high DN solvent reduces the electrophilicity (Lewis acidity) of the Li cations and thereby helping to extend the stability of O2 -· before disproportionating to Li2O2. In practice using high donor number solvents in Li-O2 cells is challenging due to their reactivity toward the lithium metal anode and reduced oxygen species. Recently it was reported that superoxide radicals can be stabilized indirectly by the electrolytes’ counter anions. In highly associated electrolytes the counter anions are coordinated to the solvated lithium cations, these interactions practically reduce the lithium cations Lewis acidity and thereby temporarily preventing them to bind to the reduced oxygen moieties. As a result, soluble species such as superoxide can take part in the growth mechanism of Li-peroxide deposits on the cathode surface. In this manner we can stabilize superoxide moieties, lead to massive Li2O2 deposition by in a top-down mechanism (i.e. high ORR capacity) although using relatively low DN number solvents such as glymes - CH3O(CH2CH2O)nCH3. Glymes are suggested to be relatively stable towered lithium metal, lithiated carbons and reduced oxygen species.

Over the last decade extra efforts were invested in the development of aprotic Li-O2 cells. Early research presented optimistic results that showed the great potential of this system. However in recent years more research works observed that it is hard to find suitable cell components that will enable prolong cycling of Li-O2 cells. Many challenges need to be addressed. Two dominant subjects were given special attention: the carbon cathodes and the electrolyte solutions. These factors governed Li-O2 cells’ operation during both the oxygen reduction reaction (ORR) and the oxygen evaluation reaction (OER). Despite many attempts to find solvents that are stable toward active oxygen species formed by oxygen reduction (super-oxide, peroxide moieties) , no solvent was found to be fully stable during ORR & OER. Several sovents were explored and although none of them was found to be stable, they presented difference features that can affect positively the ORR. One parameter is the Guttmann number of the solvent. High donor number solvents like dimethyl sulfoxide (DMSO), dimethylacetamide (DMA) and dimethylformamide (DMF) are able to stabilize the soluble superoxide intermediate during ORR. Stabilization of superoxide moieties in the course of oxygen reaction can help the growth mechanism of the main product of ORR in aprotic media, namely, Li2O2. Similar effect was accomplished by adding water contaminations to the electrolyte solution consequently enhancing formation of soluble species. However, solvents and solution additives that stabilize superoxide species can undergo parasitic, side reactions. Soluble charged species were investigated as red-ox mediators that can reduce the over-potential of OER. Redox mediators used as additives in solution can easily oxidize the Li2O2 by accepting charge at relatively low potentials and transfer it to the carbon cathode. In this work we show that by using lithium nitrate (LiNO3) as an electrolyte, we can positively affect ORR mechanism and reduce OER over-potential. LiNO3 is a well-known passivating agent for lithium metal anodes. This phenomenon can be used in Li metal based batteries such as Li-S systems. In Li-O2 cells LiNO3 was first introduced as a passivating agent for Li metal in cells based on amide solvents like DMA1. We show that LiNO3 by-product NO2 -, from the reaction of NO3 - with lithium metal, can behave as a redox mediator that reduces OER over-potential below 4V. We suggest that the oxidation of NO2 - to NO2 forms a NO2/NO2 - couple which serves as a good red-ox mediator. . NO2 can oxidize the Li2O2 and, as a result, the NO2 is reduced back to NO2 -. As with other redox mediators, only a small amount is needed to complete the process. The ability of the soluble NO2 - anions to be oxidized on the carbon surface and then oxidize thick Li2O2 surface films, enables to reduce pronouncedly the OER over-potential. Recent work2 suggested that anions that can complex with the lithium cations reduce their Lewis acidity. As a result, soluble species like superoxide can take part as intermediates in the Li2O2 formation. Lithium nitrate moieties are associated in poly-ethers more than any other lithium salt used in lithium batteries. This means that nitrate anions can be coordinate strongly to solvated Li cations thus enabling formation of mobile superoxide moieties. We show that even using cathodes with flat area, we can achieve high capacity of ORR, including formation of submicron Li2O2 particles due to the influence of the NO3 - ionson the growth mechanism as presented in Figure. 1. Figure 1. HR-SEM images of gold electrodes discharged to 2 V in 1M LiNO3 diglyme electrolyte solution. References (1) Walker, W.; Giordani, V.; Uddin, J.; Bryantsev, V. S.; Chase, G. V; Addison, D. J. Am. Chem. Soc. 2013, 135 (6), 2076. (2) Gunasekara, I.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. a.; Abraham, K. M. J. Electrochem. Soc. 2015, 162 (6), A1055. Figure 1

Non aqueous Li-O2 battery would seem to be a most promising energy storage and conversion battery system. However, it has practical challenges such as solvent and electrode stability, pronounced overvoltage for oxygen evolution reactions, limited cycle life and poor rate capability. One of the strategies is the use of electrocatalysts. These were used to facilitate oxygen reduction, and the oxidation of the Li2O2 precipitate.1-2 One of the approaches suggested to the fast oxidation of Li2O2 is the use of redox mediators such as iodine. Redox mediators in electrolyte can easily be electrochemically oxidized and transfer electrons to Li2O2. The latter step considerably reduces the high over-voltage required to oxidize solid Li2O2. One of the redox mediators recently studied in Li-O2 batteries was iodine.3 Iodine and its reduced forms I3- and I- are soluble in polar aprotic solvent. Considering that upon oxidation iodide forms iodine which oxidation, which in turn catalyzes the oxidation of Li-peroxide, the development of Li–oxygen cells with LiI as an additive would look quite promising.4 At very low concentrations of LiI, the iodine formed serves as a redox mediator for Li2O2 oxidation, oxygen is reduced to Li2O2. However, in high concentrations of LiI, the presence of the salt promotes a side reaction that forms LiOH as a major product. References 1. B. D. McCloskey, R. Scheffler, A. Speidel, D. S. Bethune, R. M. Shelby, and A. C. Luntz, J. Am. Chem. Soc., 2011, 133, 18038–18041.` 2. S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé, and P. G. Bruce, Angew. Chem. Int. Ed. Engl., 2011, 50, 8609–8613. 3. H.-D. Lim, H. Song, J. Kim, H. Gwon, Y. Bae, K.-Y. Park, J. Hong, H. Kim, T. Kim, Y. H. Kim, X. Lepró, R. Ovalle-Robles, R. H. Baughman, and K. Kang, Angew. Chem. Int. Ed. Engl., 2014, 53, 3926–3931. 4. S. Meini, N. Tsiouvaras, K. U. Schwenke, M. Piana, H. Beyer, L. Lange, and H. A. Gasteiger, Phys. Chem. Chem. Phys., 2013, 15, 11478–11493.

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