Xiamen University ACS Energy Lett.: The effect of dangling bond defects and zinc-philic site engineering on stable zinc anode
Original Electrochemical energy Electrochemical energy 
022-11-16 08:16 Published in Chongqing
First author: Zheng Jiaxian (Xiamen University)
Corresponding author: Yizhou Zhang* (Nanjing University of Information Engineering), Zhoucheng Wang* (Xiamen University), and Hanfeng Liang* (Xiamen University)
【Background】
Safe, economical and ecologically friendly water-based zinc ion batteries (ZIBs) are considered to be a feasible supplement to lithium-ion batteries . Unfortunately, the "tip effect" of the zinc anode caused by uneven zinc deposition promotes the formation of zinc branches, thereby shortening the lifespan of the ZIB. At the same time, the production of by-products related to water decomposition in acid electrolytes (such as zinc hydroxide sulfate, or ZHS) reduces the utilization efficiency of metal zinc. Recently, various strategies have been proposed, including support structure design, electrolyte formulation and protective layer engineering, to solve the above problems. Compared to the former two, building protective layers for zinc anodes is attractive because they not only protect metal zinc from direct contact with bulk electrolytes, but also evenly distribute Zn2+. Importantly, the diffusion behavior of Zn2+ ions and free water molecules on the Zn anode is greatly influenced by the inherent properties of the protective layer, which can be adjusted through surface functionalization, crystal surface adjustment and defect engineering. The defects of
material can provide additional diffusion and storage space for metal ions. Defect engineering has been widely used in various cathode or anode materials to improve ion storage capacity. Compared to commonly reported vacancy defects, dangling bonds (DBs) are often overlooked despite being present in most materials. DBs usually have a great influence on the electronic properties of materials. DBs with unsaturated coordination tend to have high energy and therefore can form strong bonds with specific atoms . In this way, the regulation of DBs can optimize the binding strength of the catalytic reaction intermediates, thereby improving the performance of catalyst .
Although impressive progress has been made in the preparation of materials with vacancy defects, the synthesis of materials with DB defects is often more difficult and generally cannot be achieved through commonly used synthesis schemes. Fortunately, magnetron sputtering technology provides a perfect solution for the manufacturing of non-density coatings. This work proposes that non-equiaxed silicon nitride (SiNx) with both Si DB defect and zinc-philic N site can improve the overall performance of the zinc anode. SiNx The Si dangling bonds (Si DBs) and N sites in the film have rich space charges. Under the action of the external electric field, these space charges will be rearranged in a specific direction to form negatively charged Si DBs, thereby changing the ion diffusion behavior of the anode surface. On the one hand, these defects can be closely bound to Zn atoms, avoiding the aggregation of Zn sediments, thereby eliminating harsh dendrimers. On the other hand, the SiNx film can effectively inhibit the decomposition of H2O.
Due to the novel interface modification strategy, SiNx @Zn anode provides excellent life (at 1.0 mA cm–2, 4600 hours). In addition, the full battery containing SiNx @Zn anode has excellent cycle capabilities of more than 6000 cycles. Therefore, this work not only examines the functions of the N-sites of Si DBs and SiNx films, but also provides new insights into the design of artificial coatings to stabilize the Zn anode through defect engineering.
Figure 1. Structural characterization. (a) Si and SiNx XRD patterns of films, (b) Raman, and (c) XPS spectra.(d) Silicon and (e) SiNx The crystal structure of the film . (f)Si and (g)SiNx films. (h) Si and (i) SiNx side view SEM images and elemental images of the film. The Si and SiNx protection layers are developed by DC magnetron sputtering method.
Figure 2. Electrochemical properties. Long-term cycles of symmetrical cells at (a) 1 mA cm–2 and 1 mAh cm–2 , (b) 2 mA cm–2 and 2 mAh cm–2 , and (c) 5 mA cm–2 and 1 mAh cm–2 . (d) CE of asymmetric batteries with bare Ti, Si@Ti and SiNx@Ti electrodes measured at 5 mA cm–2 and 1 mAh cm–2.
Figure 3. Structural evolution. (a) Linear polarization curve of Zn anode. Various zinc anodes (b) after 2 h of cycle at 1 mA cm–2 and (c) XRD patterns after soaking in 3 M ZnSO4 electrolyte for 3 days. SEM images of Si@Zn after plating in (d) and (e) peeling at 1 mAh cm–2. SiNx @Zn SEM image after peeling in (f) plating and (g)1 mAh cm–2. (h) Top view of SiNx@Zn after electroplating at 1 mAh cm–2.
density functional theory (DFT) calculation was used to evaluate the interaction between Zn atoms and Zn metal, Si and SiNx. Figure 4a shows the Zn atomic model attached to the Si DBs/N sites of Zn(101), Si films, and SiNx films. The adsorption energy of Zn atoms on Si DBs (-2.26 eV) and N site (-2.17 eV) of SiN x films is more negative than that of Zn (101) (-1.22 eV) and Si (010) (-1.03 eV). The N sites of Si DBs and SiNx have strong zinc-philicity, which indicates that their ability to capture zinc atoms has been improved, thereby effectively inhibiting the aggregation of zinc atoms on the Zn surface ( Figure 4b). It should be noted that the adsorption energy of Zn atoms on Zn (101) is more negative than that of Zn atoms on Si (010). Therefore, the Si film cannot provide preferential adsorption points for reduced Zn atoms ( Fig. 4b). However, the Si film can protect the Zn anode from direct contact with the electrolyte, thereby reducing corrosion of the Zn anode to a certain extent. Therefore, compared with the exposed Zn symmetrical battery, the lifespan of Si@Zn symmetrical battery is slightly improved ( Fig. 2). Benefiting from the defect site of the SiNx film, the ion conductivity of SiNx calculated by EIS is 3.04 × 10–4 S cm–1, which is greater than the ion conductivity of the Si film (1.41 × 10–4 S cm–1), which shows that the SiNx layer can simultaneously accelerate and uniformly transmit the interface of Zn2+ ions.
The effect of SiNx on the distribution of electric field and Zn2+ ion field was further studied using COMSOL Multiphysics software. As shown in Figure 4c,d, for the exposed Zn anode, a significant intensity gradient local electric field and Zn2+ ion concentration were observed. During the electroplating process, the electric field was sharply strengthened at the protruding tip. The above-mentioned enhanced electric field and irregular Zn2+ ion distribution can accelerate the aggregation of Zn2+ ions. Due to the "tip effect", these small "tips" gradually gather onto large, sharp Zn branches ( Figure 4b). However, for SiNx @Zn anode, the electric field and Zn2+ ion distribution become significantly uniform, which ensures uniform deposition of zinc, thereby increasing the lifetime of SiNx @Zn anode.In order to avoid the possible effect of pre-existing Zn dendritics on the accuracy phenomenon, the electric field and Zn2+ ion distribution of bare Zn and SiNx@Zn anode with a flat surface were further simulated, and similar results were obtained.
Figure 4. Theoretical calculation. (a) Adsorption energy of Zn atoms on the substrates of Zn, Si and SiNx substrates. (b) Schematic diagram of Zn deposition on exposed Zn, Si@Zn and SiNx @Zn anode. Simulated (c) electric field and (d) bare Zn and SiNx@Zn2+ ion field on the surface of Zn.
Promising commercial applicability of SiNx@Zn anode was evaluated using a full cell with Mn1.4V10O24 -nH2O (MVO) cathode. Both the CV plots of Zn||MVO and SiNx @Zn||MVO show similar redox peaks at ∼0.6 and ∼1V ( Figure 5a), which is related to the redox behavior of V4+ /V3+ and V5+ /V4+, indicating that the protective layer will not affect the intercalation chemistry of Zn2+. In addition, the CV curve shows that the current density of SiNx @Zn||MVO batteries is greater than that of bare Zn||MVO, which indicates that capacity and electrochemical activity are enhanced. Rate capacity tests show that compared with bare Zn||MVO batteries, SiNx @Zn||MVO batteries can provide higher capacity at various current densities ( Figure 5b). Such an improvement can be attributed to SiNx @Zn||MVO batteries have lower charge transfer resistance ( Figure 5c). Importantly, the curve slope of SiNx @Zn||MVO batteries in the low frequency region is small, indicating that the Zn2+ ion diffusion kinetics are faster ( Fig. 5d). SiNx @Zn||MVO battery can run 6000 times, and the capacity retention rate is as high as 74%（ Figure 5e). However, the exposed Zn||MVO battery will have battery failure within 2400 cycles. In order to further evaluate the advantages of SiNx @Zn anode in different systems, we also assembled the SiNx @Zn||MnO2 battery and tested it in 3 M ZnSO4 electrolyte. SiNx @Zn||MnO2 battery again shows enhanced performance compared to bare Zn||MnO2 batteries. These results show that the SiNx film can effectively alleviate the harsh zinc dendrites and prolong the cyclic performance of ZIB.
Figure 5. Performance of the whole battery. (a) CV diagram of the full battery. (b) Rate performance. (c) EIS curve before the cycle and (d) impedance and the slope of low frequency. (e) Cycle stability of SiNx @Zn||MVO and Zn||MVO batteries at 5 A g–1.
In summary, a SiN , with a large number of Si DB sites was developed through reactive DC magnetron sputtering technology, which solved the Zn dendrites and side reaction problems of Zn anode. Both theoretical calculations and experimental analysis have confirmed that the SiNx layer can not only effectively and uniformly distribute Zn2+ ions, but also inhibit side reaction , thereby inhibiting severe Zn dendrites growth and by-product (ZHS) accumulation. Benefiting from the above advantages, the SiNx film with defect points greatly extends the life of the modified zinc anode, so that the symmetrical battery can operate at least 4600 hours at 1 mA cm–2. In addition, SiNx @Zn||MVO full battery achieves excellent cyclicity of 6000 cycles (5 A g–1). This work opens a new way to improve the overall performance of zinc anode through defect engineering of protective coatings.
Simultaneous Dangling Bond and Zincophilic Site Engineering of SiNx Protective Coatings toward Stable Zinc AnodesACS Energy Letters (IF 3.991 ) Pub Date: 2022-11-14, DOI: 10.1021/acsenergylett.2c02282
Jiaxian Zheng, Guoyin Zhu, Xin Liu, Hongxing Xie, Yangduo Lin, Ye Zeng, Yizhou Zhang, Appala Naidu Gandi, Zhengbing Qi, Zhoucheng Wang, Hanfeng Liang
