Electrode/Electrolyte Interfacial Chemistry Modulated by Chelating Effect for High‐Performance Zinc Anode

Although Zn metal has been regarded as the most promising anode for aqueous batteries, its practical application is still restricted by side reactions and dendrite growth. Herein, an in‐situ solid electrolyte interphase (SEI) film formed on the interface of electrode/electrolyte during the plating/stripping of zinc anodes by introducing trace amounts of multidentate ligand sodium diethyldithiocarbamate (DDTC) additive into 1 m ZnSO4. The synergistic effect of in‐situ solid electrolyte interphase forming and chelate effect endows Zn2+ with uniform and rapid interface‐diffusion kinetics against dendrite growth and surface side reactions. As a result, the Zn anode in 1 m ZnSO4 + DDTC electrolytes displays an ultra‐high coulombic efficiency of 99.5% and cycling stability (more than 2000 h), especially at high current density (more than 600 cycles at 40 mA cm−2). Moreover, the Zn//MnO2 full cells in the ZnSO4 + DDTC electrolyte exhibit outstanding cyclic stability (with 98.6% capacity retention after 2000 cycles at 10 C). This electrode/electrolyte interfacial chemistry modulated strategy provides new insight into enhancing zinc anode stability for high‐performance aqueous zinc batteries.


Introduction
Aqueous rechargeable batteries, as a safer and more sustainable alternative to organic lithium ion batteries, present a tremendous competitive advantage in the energy storage system. [1,2][9] Therefore, it is of great significance to regulate the interfacial chemistry of zinc metal anode in the mild acid electrolyte for aqueous zinc ion batteries.
Many efforts have been devoted to the modification of the metal anodes (including Li, Na, K, Zn, and Mg) using a functional protect layer to tune the interface of the electrode/ electrolyte. [10,11]Constructing conductive interfacial layers on the surface of the Zn anode, such as carbon-based materials [12][13][14][15] and organic polymer, [16,17] has been proposed as a cost-effective strategy to protect the zinc anodes.Although such protective layers can uniform the surface electric field and inhibit the dendritic growth of zinc anode in mild acid electrolytes, the higher overpotential and by-product accumulation still threaten the cycling life of zinc anode. [18,19][31][32] However, most additives have little effect on inhibiting side reactions, especially at high current density.Therefore, seeking a highperformance and cost-efficiency aqueous electrolytes additives is an effective way to optimize interfacial chemistry enabling a stable zinc anode.
Herein, we design a hybrid electrolyte by introducing sodium diethyldithiocarbamate (DDTC) as a multidentate ligand additive in 1 M ZnSO 4 electrolyte and reveal its effects on modulating the electrode/ electrolyte interfacial chemistry.Experiments and theoretical calculations demonstrate that the DDTC possesses a strong interaction with zinc ions, which disturbs the hydrated solvation structure of Zn 2+ (Zn H 2 O ð Þ 2þ 6 ) and induce the fast rapid plating/stripping of zinc.The Zn//Zn symmetric cells can stably cycle more than 600 cycles with a lower polarization voltage (<80 mV), even at an ultra-high current density of 40 mA cm −2 .More importantly, a solid electrolyte interphase (SEI) film derived from the decomposition of Zn-DDTC on the interface of electrode/electrolyte during the plating/stripping of zinc anodes, which effectively inhibits the dendrite growth and the occurrence of side reactions.As a result, the full cell of Zn//MnO 2 in ZnSO 4 + DDTC electrolyte exhibits a long-term cycling performance of 2000 cycles (with 98.6% capacity retention).

Results and Discussion
Figure 1a schematically depicts the Zn growth behaviors and interfacial chemistry of the Zn anodes in 1 M aqueous ZnSO 4 electrolytes with/ without the addition of DDTC.In the absence of DDTC, the direct Although Zn metal has been regarded as the most promising anode for aqueous batteries, its practical application is still restricted by side reactions and dendrite growth.Herein, an in-situ solid electrolyte interphase (SEI) film formed on the interface of electrode/electrolyte during the plating/stripping of zinc anodes by introducing trace amounts of multidentate ligand sodium diethyldithiocarbamate (DDTC) additive into 1 M ZnSO 4 .The synergistic effect of in-situ solid electrolyte interphase forming and chelate effect endows Zn 2+ with uniform and rapid interface-diffusion kinetics against dendrite growth and surface side reactions.As a result, the Zn anode in 1 M ZnSO 4 + DDTC electrolytes displays an ultra-high coulombic efficiency of 99.5% and cycling stability (more than 2000 h), especially at high current density (more than 600 cycles at 40 mA cm −2 ).Moreover, the Zn//MnO 2 full cells in the ZnSO 4 + DDTC electrolyte exhibit outstanding cyclic stability (with 98.6% capacity retention after 2000 cycles at 10 C).This electrode/ electrolyte interfacial chemistry modulated strategy provides new insight into enhancing zinc anode stability for high-performance aqueous zinc batteries.contact of the electrolyte and thermodynamically unstable Zn anode induces corrosion and HER parasitic reactions, forming the by-product of the electronic insulation Zn 4 SO 4 (OH) 6 .This unstable Zn/electrolyte interface results in uneven Zn plating/stripping, dendrites formation, and lower Zn utilization as well.In contrast, the DDTC-Zn chelates adsorbed onto the Zn surface provide a protective layer to reduce interface electron capture of H 2 O, thus suppressing H 2 evolution and maintaining local pH.Moreover, the decomposition of a small amount of chelates will generate stable sulfur-rich SEI, further promoting the zinc ion de-solvation and inhibiting dendrite growth.
The cyclic reversibility and durability of the zinc anode in electrolytes with and without DDTC additive are evaluated by galvanostatic charge-discharge for Zn//Zn symmetric cells.The cycling performance of Zn//Zn symmetric cell in different concentrations of additives electrolyte is shown in Figure 1b and Figure S1, Supporting Information.Especially in 1 M ZnSO 4 + 20 ppm DDTC electrolyte, it exhibits an ultra-long lifespan (2000 h) of more than 30 folds that of the electrolyte without DDTC additive.Besides, the overpotential of zinc in the 1 M ZnSO 4 + DDTC electrolyte is significantly lower than that in the 1 M ZnSO 4 electrolyte (Figure S2, Supporting Information), which indicates that the DDTC is beneficial to the plating/stripping of zinc.As shown in Figure S3, Supporting Information, it exhibits an ultra-long lifespan in the 1 M ZnSO 4 + DDTC electrolyte for both 10 mA cm −2 , 1 mAh cm −2 and 20 mA cm −2 , 1 mAh cm −2 , which is both much longer than the pure 1 M ZnSO 4 electrolyte.In addition, the Zn//Zn cell could stably have cycled in ZnSO 4 + DDTC electrolyte more than 600 cycles at a high current density of 40 mA cm −2 and a high areal capacity of 10 mAh cm −2 , corresponding to a total cumulative capacity of 6000 mAh (Figure 1c).The zinc anode could be stably cycled in the ZnSO 4 + DDTC electrolyte under the discharge depth of 36.6%, which is far longer than in the pure ZnSO 4 electrolyte (Figure S4, Supporting Information).And the electrode/electrolyte interfacial chemistry could be modulated by the chelating effect of the Zn 2+ and DDTC, effectively reducing the hysteresis potential.In contrast, the Zn//Zn cell exhibits a high overpotential in pure ZnSO 4 electrolyte that is mainly due to the accumulation of dead zinc and by-products (the inset of Figure 1c).Moreover, as shown in Figure 1d, the rate performance of Zn//Zn cell in ZnSO 4 + DDTC electrolyte is superior to in the pure 1 M ZnSO 4 electrolyte.The Zn//Zn cell in pure ZnSO 4 electrolyte suffers from a much higher overpotential and a sudden death with the short circuit as well.The coulombic efficiency (CE) of Zn//Cu cells in different electrolytes is displayed in Figure 1e.The CE and cycling performance of Zn//Cu cells in ZnSO 4 + DDTC electrolytes (over 99.3% and 700 cycles) is much higher than that in the pure ZnSO 4 electrolyte.The corresponding capacity-voltage profiles of the Zn//Cu cells at different cycles further confirm the results, as shown in Figure 1f and Figure S5, Supporting Information.Obviously, the enhancing CE in the ZnSO 4 + DDTC electrolytes is due to the great inhibition of the dendrite growth and side reactions.
The dynamic and static evolution of surface morphologies of plating/stripping processes of Zn in 1 M ZnSO 4 and 1 M ZnSO 4 + DDTC electrolytes were investigated by microscopy and SEM images.As shown in Figure 2a,b, the surfaces of zinc electrodes are covered by porous and uneven notorious dendrites and by-products after cycling 100 h.And the zinc foil suffers from severe corrosion after cycling in pure ZnSO 4 electrolyte from the cross-section SEM image in Figure 2c.Furthermore, the AFM image in Figure 2g reveals that the sizes of the dendrite reach to micrometer after cycling in the pure ZnSO 4 electrolyte, which is the typical reason that the zinc anode undergoes a sudden short circuit to death.The high current density will cause an aggravation of these conditions, as displayed in Figures S6 and S7a,b, Supporting Information, where the zinc anode is seriously damaged after cell failure.In contrast, the surface of zinc foil after cycling is smooth, uniform, and dense without clear dendrites in the 1 M ZnSO 4 + DDTC electrolyte (Figure 2d-f), in which the altitude difference of the zinc surfaces is far smaller than in ZnSO 4 electrolyte (Figure 2h).This distinction is more clear at high current densities.As shown in Figures S7c,d and S8, Supporting Information the surface of zinc electrodes is more smooth and dense in the 1 M ZnSO 4 + DDTC electrolyte after cycling without clear dendrites.The SEM images of Cu foil after cycling for both 1 M ZnSO 4 and 1 M ZnSO 4 + DDTC samples are added in Figure S9, Supporting Information.The SEM image of 1 M ZnSO 4 + DDTC electrolyte is more uniform and dense without dendrite growth during zinc deposition.Compared to it, the scalelike loose structure of 1 M ZnSO 4 electrolyte is more likely growing to dendrite after continuous cycling.The results further confirm that the addition of DDTC effectively induced the deposition of zinc and inhibited dendrite growth.Besides, these results are further revealed by the XRD patterns after cycling 100 h at these current densities and capacities (Figure S7e, Supporting Information).The diffraction peaks of Zn 4 SO 4 (OH) 6 Á5H 2 O are detected after cycling in the pure 1 M ZnSO 4 without the addition of DDTC.However, no peaks could be indexed to the by-products after cycling with the addition of DDTC in the electrolytes, proving that the DDTC effectively inhibits the formation of dendrites and by-products.The in-situ optical visualization observations are also used to detect dynamic behaviors of zinc plating in different electrolytes in Figure 2i. [33]At a high current density of 30 mA cm −2 , the uneven and loose layer is forming rapidly after 5 min in pure 1 M ZnSO 4 electrolyte.Subsequently, the rampant Zn dendrites rapidly grow on the Zn anode surface after 15 min.On contrary, almost no Zn dendrites could be observed on the surface of the zinc anode in the 1 M ZnSO 4 + DDTC electrolyte, and the zinc deposition layer is relatively dense without large volume change.These results manifest that the DDTC is effective in inducing zinc deposition behavior, leading to uniform plating without Zn dendrites.
In addition, the XRD patterns were performed to investigate the component characteristics of depositing Zn that gather from Zn//Zn cells after cycling 50 h at 1 mA cm −2 for 1 mAh cm −2 .As depicted in Figure 2j,k, a set of diffraction peaks at a range of 5 and 25 degrees could be indexed to the side products of Zn 4 SO 4 (OH) 6 Á5H 2 O (JCPDS card no.39-0688) after cycling in 1 M ZnSO 4 electrolyte.The formation process of the species follows the following reactions: [34,35] ZnÀ2e À !Zn 2þ (1) On the contrary, no diffraction signals of these by-products are detected in the case of the addition of DDTC, confirming that the DDTC effectively suppressed HER and corrosion on the Zn electrode.Besides, the Zn//Zn cells did not undergo a significant volume expansion after long cycling in the ZnSO 4 + DDTC electrolyte (Figure S10, Supporting Information).Tafel profiles and LSV curves are also used to determine the inhibiting corrosion and HER properties of the zinc anode in the ZnSO 4 + DDTC electrolyte.As shown in Figure S11, Supporting Information, compared with the 1 M ZnSO 4 electrolyte, the Zn anode in the ZnSO 4 + DDTC electrolyte exhibit a more positive corrosion potential, and a more negative corrosion current as well, demonstrating that the DDTC could mitigate the corrosion reaction.LSV curves in Figure S12, Supporting Information further verify that the introduction of DDTC would significantly inhibit the HER of the ZnSO 4 electrolyte.As shown in Figure S13, Supporting Information, the current densities of pure 1 M ZnSO 4 electrolytes progressively increase in 900 s, suggesting the long-time 2D diffusion of the absorbed ions.The sharp edge or protrusion on the surface of the zinc anode has a stronger electrical field, thus the Zn tends to deposit around the tips instead of on smooth regions of the anode, which is responsible for the "tip effect" and dendrite growth. [36]In contrast, the Zn electrodes tend to have a stable current density with a 3D diffusion behavior after a transient 2D plane diffusion process (≈30 s) with the addition of DDTC.
The effect of the DDTC on modulating the zinc electrode interface was further studied.As shown in Figure S14a, Supporting Information, the 1 H NMR spectra of the different electrolytes display that the 1 H peak shifts from 4.7046 to 4.6998 ppm after the addition of DDTC.It is demonstrated that the surrounding electron density decreases and the proton shielding in water is weakened due to the interaction between DDTC and D 2 O. [37,38] Thus, the solvation structure of Zn H 2 O ð Þ 2þ 6 is changed by adding DDTC.The results of FTIR and Raman also confirm it.The wavenumber of ν (SO 2À 4 ) goes through an obvious blue shift after the addition of DDTC (Figure S14b, Supporting Information), indicating that the 6 is partially replaced by DDTC. [39,40]eanwhile, the Raman spectra in Figure S14c, Supporting Information further demonstrate that the presence of DDTC disrupts the electrostatic coupling between Zn 2+ and SO 2À 4 . [40,41]The chelates of the Zn-DDTC molecular diagram are depicted in Figure S15, Supporting Information, in which two S atoms of -NCS 2 groups anchor a Zn ion in the center of the bidentate configuration.The adsorption energies of DDTC anions and H 2 O on (002) and (101) crystal planes were calculated by using first-principles calculations (Figure 3a). Figure 3b,c and Figure S16, Supporting Information show the charge density difference and 2D contour map of electron density statistics at the interface between Zn and solvent molecules (DDTC and H 2 O), which reveal the larger tendency of electron transfer from DDTC to the surface of Zn than that of H 2 O, confirming the stronger interactions between DDTC and Zn.The ratio of the I (002) /I (101) crystal plane shown in Table S1, Supporting Information, which is calculated from Figure 2j and Figure S17, Supporting Information, illustrates that the (002) crystal plane is indeed enhanced slightly.Therefore, we further calculated the adsorption energies of the (002) crystal plane for H 2 O and DDTC − .From Figure 3a and Figure S18, Supporting Information, the adsorption energy of DDTC − on (101) is −1.62 eV, slightly stronger than that on the (002) crystal plane, so that the DDTC − is preferentially adsorbed onto the (101) crystal plane.Thus, the DDTC chelate with Zn 2+ and take precedence decomposition into the SEI layer on the (101) crystal plane, prompting the Zn 2+ deposition along the (002) crystal plane.This is the reason that the (002) crystal plane becomes the preferential growth crystal plane.Nevertheless, the adsorption energies of the (002) crystal plane and (101) crystal plane are very close, which causes the enhancement of (002) to be less obvious.As DDTC has the lowest adsorption energy on the zinc surface, DDTC will preferentially form a local enrichment layer on the surface of the zinc anode, in which the diffusion is constrained (Figure S19, Supporting Information).With the deposition of zinc ions after the dechelation effect, DDTC forms a water-poor electric double layer at the interface to inhibit the interfacial side reactions.This multidentate coordination holds a much higher relative binding energy than that of coordination between zinc ions and water molecules through solvation (Figure 3d).Therefore, in the local electrolyte environment, Zn ions exist in the form of Zn-DDTC chelates.Additionally, the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are calculated based on frontier molecular orbital theory.As presented in Figure 3e, the LUMO of free H 2 O, Zn 2+ -H 2 O, DDTC − , and Zn-DDTC are 2.06, −0.75, 0.01, and −1.88 eV, respectively, revealing that DDTC chelated with Zn 2+ will be reduced preferentially.Besides, the chelating effect from the DDTC additive can interrupt the solvation structure of Zn 2+ typically dominated by H 2 O, which is conducted to the preferential reduction and decomposition of the additive on the surface of the zinc anode to derive SEI films.
To investigate the possible SEI layer, transmission electron microscopy (TEM) was carried out on a 10 μm zinc foil section, which has cycled 20 cycles in the 1 M ZnSO 4 + DDTC electrolyte at 1 mA cm −2 and 1 mAh cm −2 .A distinct SEI layer about 10-15 nm thick on the surface of the Zn anode is observed (Figure 3f), confirming the SEI formation.The surface components of SEI are also investigated by X-ray photoelectron spectroscopy (XPS) (Figures S20 and S21, Supporting Information).In addition to the adsorption of DDTC and sulfate group (Figure 3g) species, a small amount of metal carbonate (288.5 eV), [42] Energy Environ.Mater.2024, 7, e12608 sulfide (163.0 eV), [43] and sulfite (168.7 eV) [44] signals are detected on the surface of the zinc anode after cycling, confirming the reduction decomposition of DDTC additive.After an Ar + etching process, the peak intensity of ZnS gradually decreases.And the signal peak that belongs to ZnSO 3 gradually strengthens, indicating that the SEI derived from DDTC is mainly composed of sulfur-containing inorganic salts (Figure 3h).The Zn 2+ transference number is measured to investigate the ion conduction in the SEI layer.Generally, a low Zn 2+ transference number is prone to yield a large ion concentration gradient near the zinc anode interface, resulting in an enhanced interfacial electric field to motivate dendrite growth.According to the detailed data analysis in Figure S23 and Table S2, Supporting Information, the Zn 2+ transference number at the zinc anode interface after cycling in the DDTC electrolytes significantly increased (Figure 3i).The DDTC additive adsorbed on the interface, and the SEI layer derived from it can effectively hinder the formation of detrimental products and the growth of zinc dendrites.
Full batteries were assembled to evaluate the practicability of DDTC additive via treating zinc metal as an anode (Figure 4a).The MnO 2 is loaded onto carbon cloths (CC) as cathode and ZnSO 4 + DDTC as electrolytes.The MnO 2 was synthesized according to the previous method, [45] and its successful synthesis was proved by XRD (Figure S24a, Supporting Information) and SEM (Figure S24b 4b.This implies that the existence of DDTC additives leads to a higher specific capacity.Moreover, the rate performances of the Zn//MnO 2 cells in the ZnSO 4 electrolytes with/without DDTC additives were also investigated.This displays favorable rate performance with the specific capacity remaining 42.1% of the initial at the current density of 50 C (Figure 4c).The Zn//MnO 2 cells in ZnSO 4 + DDTC electrolyte exhibit high capacities of 284.2, 277.7, 243.9, 217.4,194.6, 164.4,and 119.7 mAh g −1 at 2, 5, 10, 15, 20, 30, and 50 C, respectively (Figure 4d).Meanwhile, the cyclic stabilities of Zn//MnO 2 cells in the ZnSO 4 electrolytes with/without DDTC additives were measured by continuous charging and discharging at a current density of 10 C. After 2000 cycles, the Zn//MnO 2 cell can still deliver a high specific capacity of 229.73 mAh g −1 (98.6% of the initial capacity) (Figure 4e).

Conclusion
In conclusion, DDTC is investigated to stabilize the Zn anode originating from its chelating effect modulated by the electrode/electrolyte interfacial chemistry.The experimental characterizations and theoretical calculations have proved that the DDTC possesses a strong interaction with zinc ions to form in-situ SEI, which could be inhibited dendrite growth and side reactions.The Zn//Zn symmetric cells can stably cycle more than 600 cycles with a lower polarization voltage (<80 mV), even at an ultra-high current density of 40 mA cm −2 .As a result, the corresponding Zn//MnO 2 full cell exhibits a high specific capacity of 229.73 mAh g −1 after 2000 cycles (98.6% of initial capacity).

Figure 1 .
Figure 1.a) The Zn growth behaviors and interfacial chemistry of the Zn anodes in 1 M ZnSO 4 electrolytes with/without DDTC.Cycling performance of symmetric Zn//Zn cells using 1 M ZnSO 4 electrolytes with/without DDTC additives at b) 1 mA cm −2 for 1 mAh cm −2 and c) 40 mA cm −2 for 10 mAh cm −2 .d) Rate performance at various current densities in 1 M ZnSO 4 with/without DDTC additives.e) CE of Zn//Cu cells in 1 M ZnSO 4 with/without DDTC at 1 mA cm −2 .f) Corresponding voltage profiles of the Zn//Cu cells in the ZnSO 4 with DDTC electrolyte at different cycles.

Figure 2 .
Figure 2. SEM images after 100 h plating/stripping: a-c) In ZnSO 4 electrolytes; d-f) In ZnSO 4 with DDTC electrolytes.g, h) AFM images and i) In situ optical microscopy images of the cross-sectional Zn deposition morphology on Zn electrode with cycling times.j) XRD patterns of Zn electrodes after specific 50 cycles in ZnSO 4 electrolyte with/without DDTC additives.k) 3-25 degrees of the XRD magnification patterns.
Meanwhile, the SEM images and corresponding EDS maps of the Zn anode in 1 M ZnSO 4 + DDTC electrolyte after cycling 100 h are shown in Figure S22, Supporting Information.The clear signals of C, N, and S on the surface of the Zn anode are consistent well with the composition of SEI.

Figure 3 .
Figure 3. a) The adsorption energy of H 2 O and DDTC on the Zn (101) crystal plane.b) The charge density difference and c) 2D contour map of electron density statistics between DDTC − and Zn (101).d) Binding energy of Zn 2+ with different compounds (DDTC and H 2 O) under DFT calculation.e) LUMO, HOMO energies of H 2 O, Zn(H 2 O) 6 , DDTC − and Zn 2+ -DDTC − .f) TEM and g) XPS of SEI layer on Zinc foil.h) The atomic percentages of etching different thicknesses.i) Comparison of the Zn-ion transference number of a bare Zn electrode and an SEI-Zn electrode.

Figure 4 .
Figure 4. a) The schematic illustration of the coin cells.b) CV curves of the Zn//MnO 2 cell with 20 ppm DDTC at different current densities and c) Rate performance of the Zn//MnO 2 cell at 2-50 C in ZnSO 4 with DDTC electrolyte.d) The corresponding discharge/charge profiles of the Zn//MnO 2 coin cell with 20 ppm DDTC at various current densities.e) Cycling performance of the Zn//MnO 2 cell at 10 C (theoretical capacity: 308 mAh g −1 ).