Electrocatalytic Selenium Redox Reaction for High-Mass-Loading Zinc-Selenium Batteries with Improved Kinetics and Selenium Utilization

The consensus practice in research of high energy density energy storage devices is to simultaneously achieve high areal capacity and high intrinsic specic capacity. Increasing the areal capacity of batteries necessitates the maximization of their mass loading. However, batteries usually deliver mass loading-dependent electrochemical performance. Take selenium (Se) cathode with a theoretically high specic capacity as an example, Se reaction kinetics, utilization and cycling lifespan seriously deteriorate with increased Se mass loading. Here, we propose an electrocatalytic Se reduction/oxidation reaction strategy to realize high-Se-loading Zn||Se batteries with fast kinetics and high Se utilization. Specically, the synergetic effects of Cu and Co transition-metal species inside channel structure of host can effectively immobilize and catalytically convert Se n during cycling, which thus facilitates Se utilization and 6-electron (Se 4+ ↔ Se 2– ) conversion kinetics. In particular, the Cu[Co(CN) 6 ] host exhibits a remarkably low energy barrier (1.63 kJ·mol −1 ) and low Tafel slope (95.23 mV·dec −1 ) for the Se reduction, and highest current response for Se oxidation. Accordingly, the Zn battery employing Se-in-Cu[Co(CN) 6 ] cathode delivers a capacity of 664.7 mAh ⋅ g −1 at 0.2 A ⋅ g −1 , an excellent rate capability with 430.6 mAh ⋅ g −1 achieved even at 10 A ⋅ g −1 , and long-cyclic life over 6000 cycles with 90.6% capacity retention. Furthermore, an A-h-level (~1350 mAh) Zn||Se pouch-type battery with high Se loading (~12.3 mg (Se) ⋅ cm −2 ) shows a high Se utilization of 3.3 % and outstanding cyclic stability with 9.4 % initial capacity retained after 400 cycles at exceedin 98 % Coulombic eciency.


Introduction
The exploration of new electrode materials is highly sought-after for high-energy-density, excellent rate capability and long-lifespan electrochemical energy storage technologies [1][2][3][4][5] . For sake of high energy density of devices, on one hand, intrinsic high speci c capacity of electrode materials is essential.
Compared with ions insertion-type materials, sulfur, selenium, and oxygen/air cathode have potential to provides two to ve times capacity based on conversion chemistry. Nevertheless, coupled with Zn metal anode, the Zn||S battery delivers very low output voltage (below 0.6 V) 6-8 . The Zn||O 2 (air) battery systems suffer from poor cycling performance (typically less 100 cycles) [9][10][11][12] . Selenium (Se) is a potential high performance cathode material for rechargeable Zn/Al batteries based on Se 4+ /Se 2− conversion reaction, because of the high theoretical gravimetric capacity of 678 mAh⋅g −1 and volumetric capacity of 3268 Ah⋅L −1 based on a 6-electron conversion process 13−18 .
On the other hand, most reported high performance cathode materials were evaluated based on an ultralow mass loading (usually less than 1 mg·cm −2 ), in contrast, the high mass loading of electrode to achieve high areal capacity is essential for practical applications (more than 10 mg·cm −2 ). However, high mass loading usually leads to sluggish electrochemical kinetics and low active-materials utilization, especially for the conversion-type materials such as Se 13,[19][20][21] .
We scrutinize the electrochemical performances of Se cathodes (with 40% Ketjen black added) with various Se loading levels of 1, 3, 5, 10 and 15 mg⋅cm −2 . With a low Se loading of 1 mg⋅cm −2 , Zn||Se cell delivers a high speci c capacity of 638.4 mAh⋅g −1 at 0.1 A⋅g −1 and 385.6 mAh⋅g −1 even at high current density of 2.0 A⋅g −1 . Once increasing the loading mass to 5 mg⋅cm −2 , almost no capacity can be delivered at 1.0 A⋅g −1 (Figure 1a). The decreased rate capabilities of the high Se-loading cathode are reasonable considering the insulating nature of elemental Se and the sluggish charge transfer kinetics.
The variations of initial areal and speci c capacities with increase of Se loads are depicted to examine the Se utilization. As shown in Figure 1b, the increase rate of areal capacity drops signi cantly and there is almost no capacity improvement with Se loads increased from 10 to 15 mg⋅cm −2 . The speci c capacity decreases almost linearly with increase Se loading. Meanwhile, the cycling performances of Zn||Se batteries also seriously deteriorate with increase of Se loading. The battery with 15 mg⋅cm −2 Se loaded only deivers 9.3 % initial capacity after 25 cycles (Figure 1c). The above-mentioned phenomenon is caused by the more severe incomplete conversion of Se 8 molecules at high mass loading, which not only lows Se utilization but also shuttles across the separator to react with the Zn anode, leading to lowenergy batteries and rapid capacity fading.
Herein, we present an electrocatalytic strategy of Se redox reaction to enable a high-energy, high-rate and cathode shows long-cyclic stability over 6000 cycles with 90.6% capacity retention, and the best rate performance with speci c capacity of 430.6 mAh·g −1 at 10 A·g −1 . We also demonstrate an A-h-level (~1350 mAh) Zn||Se pouch cell with only 200% excess Zn, which attained a high Se utilization of ~83.3% even under ultra-high Se loading of ~12.3 mg⋅cm −2 , and excellent cyclic stability over 400 cycles with a Coulombic e ciency > 9 %.

Results And Discussion
Design and structures of hosts for Se Figure 2a depicts the design rationale of the Se host with abundant continuous porosity and electrochemical catalytic functionality of double polar sites. The intrinsically porous structure of Prussian blue analogues enables a facile guests' ions in ltration and ion transportation, while the highly tunable transition metal species pledges the incorporation of electroactive sites for effective battery chemistries.
Such a structure can signi cantly promote the redox kinetics and greatly mitigate the shuttle effects under high Se loading, bene ting from the improved electrocatalytic and enhanced con nement effect. In comparison, the binding strength and conversion e ciency of frequently used porous carbon host towards pristine Se and reactive Se n are unsatisfactory.
The typical construction of Se-in-(Cu/Ni/Co)[Co(CN) 6 ] is obtained by extracting primary K ions from K(Cu/Ni/Co)[Co(CN) 6 ] frameworks and impregnating Se molecules into their pores. It should be mentioned that the (Cu/Ni/Co)[Co(CN) 6 ] exhibits good thermostability. Its structure is completely maintained even after heating treatment at 300 o C under N 2 atmosphere ( Figure S1). The Se molecules are successfully locked in (Cu/Ni/Co)[Co(CN) 6 ] pores in a quantitative manner via melt diffusion at 260 o C, as re ected in the drastic decrease in N 2 uptakes of these Se-in-(Cu/Ni/Co)[Co(CN) 6 ] samples in contrast to that of the pristine (Cu/Ni/Co)[Co(CN) 6 ] ( Figure S2). Meanwhile, the crystallinities of these (Cu/Ni/Co)[Co(CN) 6 ] are well-maintained as evidenced by the sharp peaks and systematic variation in the peak intensity of (Cu/Ni/Co)[Co(CN) 6 ] con rms the con nement of Se species within (Cu/Ni/Co)[Co(CN) 6 ] pores ( Figure 2b). The pinpoint management of Se content is conducted by veering from the feeding ratio, and the concrete loads is determined by thermalgravimetric analysis (TGA). In consideration of (Cu/Ni/Co)[Co(CN) 6 ] structure with high porosity and strong chemisorption characteristic to guest ions (  (Figure 2c, Figure S3).
The high Se loads with Cu[Co(CN) 6 ] framework can be attributed to that the synergetic effect of metallic Cu and Co species together with its effectively porous structure. The morphologies of (Cu/Ni/Co) [Co(CN) 6 ] are approximate cubic with a size of ~500 nm, as revealed by scanning electron microscope (SEM) (Figure 2d Figure 3c). We also plot the partial density of states (PDOS) curves before and after ZnSe adsorption in Figure S4. Hybridization of transition (Cu/Co/Ni) 3d and Se 4p orbitals with the energy ranging from -1.0 to 0.5 eV is observed, leading to changes in DOS around the Fermi level after ZnSe adsorption. For Se 4p orbital, the tendency of its downshift can be concluded for Cu[Co(CN) 6 ], manifesting a high level of ZnSe activation. The above results suggest that Cu and Co metal species serve as active sites to enhance the phase transformation of ZnSe to Se n facilitate the whole reaction in Zn||Se batteries.
To further understand the origin of the improved catalytic activity and kinetics, the LSV and electrochemical impedance spectra (EIS) measurements are conducted following the protocols well developed by oxygen reduction reaction (ORR) community 22  The rate capability tests further certify that the Se-in-Cu[Co(CN) 6 ] cathode delivers the highest speci c capacity among the four samples under all testing rates (0.2 A×g -1 to 10 A×g -1 ) (Figure 4c). Moreover, the Zn battery with Se-in-Cu[Co(CN) 6 ] cathode shows obvious discharge plateau and high speci c capacity of 430.6 mAh×g -1 even at a high current density of 10 A×g -1 , which also con rms the fast Se 4+ /Se 2redox reaction in Cu[Co(CN) 6 ] framework (Figure 4d).
Meanwhile, the Se-in-Cu[Co(CN) 6 ] cathode shows a higher capacity retention rate (98.6%) and Coulombic e ciency (~100%) after cycling at 0.2 A×g -1 for 100 cycles, compared with Se-in-Ni[Co(CN) 6 ] (75.3 % and 96 %, respectively), Se-in-Co[Co(CN) 6 ] (60.3 % and 94 %) and Se-in-porous carbon (25.9 % and 88 %) ( Figure 4e). These ndings demonstrate that the (Cu/Ni/Co)[Co(CN) 6 ] framework can effectively immobilize the dissolved Se n eliminate the capacity decay caused by the shuttle effect. The Se-in-Cu[Co(CN) 6 ] cathode also shows the outstanding long-term cyclability with 90.6 % initial capacity retained after 6000 cycles at current density of 5 A×g -1 (Figure 4f). The above results reveals that the enhanced performance is not only a physical increment of polar (Cu/Ni/Co)[Co(CN) 6 ], but more of an optimal 'Se n con nement-catalysis' process by the double polar sites.

Phase transformation and interfacial chemistry
To securitize the Zn 2+ storage mechanism in the Se-in-Cu[Co(CN) 6 ] cathode, we analyzed the Se Meanwhile, the ZnSe phase is also detected at discharge process. While, the intensity of characteristic peaks representing SeO 2 at 23.4 o gradually decreases with discharging process and almost completely disappear at full discharge state. Similarly, the vibrational mode with frequency of 231.5 cm -1 corresponding to Se n , appears at discharge process in Raman spectra, which presents the formation of Se n phase. In comparison, the intensity of vibrational mode representing SeO 2 at 250.4 cm -1 gradually disappears during discharge process (Figure 5c) 23 . The weak vibrational modes with frequency of 251 cm -1 is assigned to the ZnSe, indicating the formation of ZnSe during the discharging process.
We also employed ex-situ X-ray photoelectron spectroscopy (XPS) to study the redox reactions of the Se cathode at different discharged states of 1.0 V, 0.1 V, and following charge states of 1.5V, 2.1 V. When the battery is discharged to 0.1 V, a pair of peaks of Se located at 54.6 eV (3d 5/2 ) and 55.5 eV (3d 3/2 ) con rms the existence of Se 2- (Figure 5d). According to above XRD and Raman analysis, the present of Se 2detected is formation of ZnSe. Subsequently, the peaks moved back to 55.3 eV (3d 5/2 ) and 56.1 eV (3d 3/2 ) for Se 3d when charging to 1.5 V, which is de ned as Se 0 (Figure 5e). Upon further charge to 2.1 V, a sharp peak located at 59.2 eV appears in the Se 3d spectrum, manifesting Se 0 is oxidized to Se 4+ (Figure 5f). When discharged to 1.0 V, a pair of peaks located at 55.4 eV (3d 5/2 ) and 56.3 eV (3d 3/2 ) are derived from Se 0 , which indicates Se 4+ is reduced to Se 0 (Figure 5g). Based on the above-mentioned results, this Zn||Se battery achieves a Se-based reversible 6-electron transfer reaction of Se 2-Se 0 Se 4+ .
The time-of-ight secondary ion mass spectrometry (ToF-SIMS) on the cycled Zn metal are performed to directly characterize Zn metal corrosion. The results show that, with Se-in-porous carbon cathode, many Se agglomerates are identi ed on the surface of cycled Zn metal anodes in the cells (Figure 5h), demonstrating a severe shuttle effect of Se n and Zn metal corrosion. The intensities of the Se element on the surface of the cycled Zn metal anodes became weaker in the case of the Se-in-Ni[Co(CN) 6 ] and Se-in-Co[Co(CN) 6 ] cathode (Figure 5i, j), but were still visible, demonstrating that the Ni[Co(CN) 6 ] and Co[Co(CN) 6 ] frameworks cannot completely eliminate the shuttle effect and Zn metal corrosion. By sharp contrast, only a very weak Se signal that is assigned to the formation of a Se-containing solid-electrolyte interphase layer can be observed on the surface of the cycled Zn metal anode employing the Cu[Co(CN) 6 ] host (Figure 5k). The suppressed shuttle effect is attributed to the highly effective Se conversion and strong chemical a nity to pristine Se n , reactive Se n and ZnSe (Table S1).

Validation in Zn||Se pouch cells with ultra-high mass loading
In light of the excellent coin-cell performance of the Se-in-Cu[Co(CN) 6 ] cathode, we fabricated a pouchtype cell with a loading mass of 4.45 mg·cm -2 and total 100 mg Se loading in a single-piece cathode with size of 5×4.5 cm. The as-prepared pouch cell is cycled at high current density of 5 A·g -1 . The cell exhibited a speci c capacity of over 500 mA h·g -1 with a capacity retention rate of 90.4 % and high Coulombic e ciency of ~100 % for 450 cycles (Figure 6a). Pouch cells with loading mass up to 8.9 mg·cm -2 and total 200 mg Se cathode using Se-in-Cu[Co(CN) 6 ], cathode and Se-in-porous carbon compartment coupled with Zn metal anode, are thus developed and tested for comparison. As shown in Figure 6b, the Se-in-Cu[Co(CN) 6 ], cathode-based pouch cell still demonstrates very stable cycle life with 91.8 % initial capacity retained and Coulombic e ciency close to 100 % within 500 cycles. By contrast, the Se-inporous carbon-based pouch-type cell experiences continuous capacity fading (with only 18.7 % capacity retained after 480 cycles) and uctuant Coulombic e ciency. We also assembled an A-h-level Zn||Se-in-Cu[Co(CN) 6 ] pouch-type battery consisting of four whole sets of anode-electrolyte-cathode stacks, to validate our concept in high Se loading operation (the inset of Figure 6c). It is mentioned that the Zn metal is only 200% excess (corresponding to a theoretically negative/positive capacity ratio of ~2.4), and the electrolyte/Se ratio is controlled at 5 ml×mg -1 . We increase the total Se loading of the pouch cell to 2.4 g with a double-side coating (corresponding to loading mass of ~12.3 mg×cm -2 ). As shown in Figure   6c, the as-assembled cell delivers ~1350 mA·h g -1 at 0. We further compare the electrochemical performance of the Zn batteries based on different cathode materials to reveal the advantages of the Zn||Se system. Remarkably, the Zn||Se batteries deliver a highest discharge capacity among reported Zn batteries (Figure 6e), which bene ts from the effective conversion reaction mechanism of Se 0 and Se 2-, and high Se utilization. Meanwhile, our delivered Zn||Se batteries delivered an ultra-high energy density, 728.9 Wh·kg -1 (Se) and maximum power density of 7078.8 W·kg -1 (Se) , which is greatly higher than that of other reported cathodes in rechargeable Zn batteries ( Figure. 6f) [24][25][26][27][28][29][30][31][32][33][34] .

Conclusion
The Prussian blue analogies with tunable electrocatalytic functionalities are employed as Se hosts for high-energy-density and long-lifespan Zn||Se batteries with enhanced kinetics and high mass loading. The Cu[Co(CN) 6 ] hosts give full access to ion diffusion and meanwhile the metallic Cu and Co species in the framework act as electrocatalytic sites to promote electrochemical kinetics of Se redox reaction, which enables high energy and long-cyclic lifespan Zn||Se batteries even with high-loading mass up tõ