EES Beijing Institute of Technology Li Li/Chen Renjie Team: Differential Capacitance Screening of Metal Cation Additives for Zinc Ion Batteries
In June 2024, the?Li Li/?Chen Renjie team of Beijing Institute of Technology?published an online paper "?Screening metal cation additives driven by differential capacitance for Zn batteries " in?the journal Energy & Environmental Science (impact factor >?30 )?. The study "screened metal cation additives driven by differential capacitance to compress the double layer, increase the zinc ion nucleation overpotential, and promote uniform deposition of zinc ions.
Research Summary
In electrochemical devices, the electric double layer (EDL) theory states that the potential difference of the Helmholtz layer significantly affects the activation energy and reaction rate of the electrochemical reaction, thereby determining the uniformity of metal ion deposition. In this study, we compared the differential capacitance of several metal cation sulfates and selected Ce(SO?4?)?2?to adjust the EDL at the Zn anode/electrolyte interface, taking into account factors such as cost and safety. Molecular dynamics simulations showed that under the action of Ce?4+?, a more compact diffusion layer was formed, resulting in a decrease in the potential difference of the Helmholtz layer,?an increase in the overpotential of Zn?2+ deposition, and a decrease in the corrosion rate of the Zn electrode. In situ XRD and Raman spectroscopy analysis showed that the deposition of Zn?2+?on the (002) crystal plane was promoted, while the evolution of hydrogen was inhibited. The?Ce 4+?-modified electrolyte enabled the Zn anode to withstand 600 cycles at a current density of 10 mA/cm2, with a Coulombic efficiency of 99.6%. The compressed EDL extended the life of Zn-Zn batteries to 2500 h at 5 mA/cm2 current density and achieved 300 stable cycles at 20 mA/cm2 and 10 mAh/cm2. The assembled Zn full cells with high-loaded cathodes exhibited good cycling performance. Our results reveal the potential of utilizing high-valent metal cations such as Ce?4+?to tune EDL and achieve uniform Zn?2+?deposition, providing a promising direction for developing practical Zn-ion battery electrolytes.
?Background
Zinc-ion batteries (ZIBs) have attracted much attention as promising candidates for next-generation sustainable large-scale energy storage systems due to their abundant resources, high cost-effectiveness, and high volumetric capacity of the Zn anode. However, the main challenges facing their commercialization are the uncontrolled growth of Zn dendrites and side reactions at the electrode/electrolyte interface. Addressing these issues is crucial to fully realize the potential of ZIBs and achieve their successful implementation in practical applications.
In electrochemical devices, the electrical double layer (EDL) is the region where electrochemical reactions occur, playing a key role in energy conversion and storage. The EDL consists of two main regions: the Helmholtz layer and the diffusion layer. The Helmholtz layer restricts the entry of ionic species, while there is an uneven distribution of ions in the diffusion layer. The particles involved in the reaction are located in the Helmholtz layer, and the reaction rate and activation energy are affected by the potential difference across the Helmholtz layer.
In order to improve the uniform deposition of zinc, researchers have explored a variety of methods, including increasing the concentration of metal salts, increasing the electrode charge, and increasing the valence of metal ions. Among them, the use of high-valent metal cations is considered to be an effective and convenient strategy, especially for large-scale applications.?In previous studies, a variety of metal cations including?Na?+?, Ce3?+?, La3?+?, Y3?+?, etc. have been used as additives. However, the research usually focuses on the electrostatic shielding effect caused by metal cations with lower reduction potentials, while ignoring their effects on the kinetic behavior of?Zn2?+?deposition. In addition, given the wide variety of metal cations, current research lacks systematicity in their screening and selection as additives.???
Key Highlights
Highlight 1
For the first time, a method for screening metal cation additives using differential capacitance based on the electric double layer (EDL) theory was proposed.
Highlight 2
Through molecular dynamics simulations, the electrical double layer structure near the zinc anode can be visualized, revealing that?a more compressed diffusion layer is formed with the help of Ce4+.
Highlight 3
In situ optical microscopy, X-ray diffraction, and Raman spectroscopy confirmed the deposition of Zn2+ on the preferential (002) crystal plane, achieving dendrite-free growth and significantly inhibiting the formation of by-products. In addition, in situ electrochemical mass spectrometry quantified the inhibitory effect of hydrogen evolution side reactions. The cycle life of the symmetric cell is close to 2500 hours, which is almost ten times longer than that of the blank electrolyte. All-zinc batteries with high-loaded positive electrodes also exhibit enhanced long-term cycling performance.
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?Graphical analysis
Figure 5. (a) Zn-Zn battery experiments at 5 mA cm?2 current density and 2.5 mAh cm?2 capacity. (b) Zn-Zn battery experiments at 20 mA cm?2 current density and 10 mAh cm?2 capacity. (c) Zn-Zn battery experiments at 10 mA cm?2 current density and 20 mAh cm?2 capacity. (?d) The zinc plating/stripping performance in this study is compared with the reported data based on capacity and current density. (e) Coulombic efficiency (CE) of Zn-Cu batteries under different electrolytes is measured. (f) Hydrogen production of Zn-Zn batteries under different electrolytes is tested by in situ differential electrochemical mass spectroscopy.
Summary and Outlook
In this study, we proposed the use of high-valent metal cation Ce?4+?as an additive to compress the electrical double layer (EDL). Through molecular dynamics simulations, we demonstrated that the introduction of Ce?4+?can effectively reduce the thickness of the diffusion layer. This compression of the EDL significantly improved the performance of zinc-ion batteries (ZIBs). It was found that Zn?2+?was deposited uniformly on the (002) crystal plane, thereby reducing the growth of zinc dendrites. In addition, the generation of hydrogen evolution reactions and byproducts was significantly reduced, improving the overall stability and safety of Zn-Zn batteries. The results showed that the electrolyte containing Ce?4+?additives exhibited excellent cycling stability in Zn-Zn batteries with a service life of 2500 h, which is almost 10 times that of ZS electrolyte without this additive. We also applied Ce?4+?additives in Zn-ion full cells with a cathode composed of LFP and ZVO. These cells showed high specific capacity and good capacity retention even after long-term cycling. This success in zinc-ion batteries demonstrates the effectiveness and potential of using high-valent metal cations such as Ce?4+?to suppress zinc dendrite growth and improve the overall performance of metal batteries. This study not only provides a practical solution to reduce zinc dendrite growth in zinc-ion batteries, but also provides valuable insights into manipulating EDL. The knowledge gained from this study can be extended to other metal anodes, paving the way for the next generation of high-capacity and long-life battery systems.