Study Of Lithium Anode Stability

Lithium metal secondary batteries have been drawing intensive attention in recent years due to their high specific energy.However,the growth of Li metal dendrites takes place in the cathodic deposition of Li⁺.This not only decreases Coulombic efficiencies of batteries,but also brings safety issues.Suppressing Li metal dendrite growth is critical for the development Lithium metal secondary batteries.The study of this dissertation is about Li metal electrodes,specifically focusing on electrochemical noise method for characterizing the stability of Li metal electrodes,and Nafion-based artificial SEI film for suppressing Li metal dendrites.At first,the study focuses on optimizing the internal architecture of coin cells to eliminate the disturbance from cell assembling on electrochemical tests.The results demonstrate that the use of O-shape elastic spacers in Li||Cu half cells leads to preferred Li deposition on the near-edge region of Cu foils,consequently decreasing Coulombic efficiencies of Li||Cu half cells.Moreover,O-shape spacers bring unstable voltage responses in galvanostatic charge/discharge cycling of Li||Li symmetric cells.Replacing O-shape elastic spacers with nickel foam plates changes electrical contact from line-to-plane style to plane-to-plane style.This ensures the uniform distribution of current in electrode surface,thus eliminating the disturbance caused by O-shape elastic spacers.Then,through carefully checking if Li metal dendrites pierce separators,and precisely measuring local potential of Li electrodes near a Li metal probe reference electrode,this dissertation deeply investigate the limitation of voltage-time profiles of Li||Li symmetric cells in characterizing Li metal dendrite growth.When a Li||Li symmetric cell was charged/discharged with a relative high capacity of 8 mAh cm^-2,even though its voltage-time profile displays a stable voltage response,metallic Li piercing separator had been observed,and the potential-time profile measured by the Li probe reference electrode shifted negatively with the presence of random potential oscillation.These results demonstrate that the voltage-time profiles fail to indicate the level of Li metal dendrite growth under high-capacity charge/discharge conditions.To address this issue,electrochemical noise as a compensation method was induced in electrochemical characterizations.Experimental data show that if Li metal dendrites become more serious,the potential noise of Li metal electrode at open circuit state becomes stronger.The strength of open-circuit potential noise has positive correlation with the amount of Li metal dendrites.Meanwhile,with cycling span prolonged and the SEI film of Li anode thickened accordingly,the strength of current noise decreases and noise resistance increases.This result indicates that noise resistance can characterize SEI film impedance.In the study,it was found that the potential noise of a cycled Li metal electrode weakens much after aged at open circuit potential for long time.Based on this phenomenon,the source for the potential noise is speculated as follows:By Ostwald ripening,dead Li metal dendrites with small radius of curvature tend to dissolve,and dissolved Li is subsequently deposited on the flat region of Li electrode.The dissolving and depositing processes disturb electrode/electrolyte interface,resulting in the potential oscillation of Li electrode.In the end,on the basis of above works that optimize the internal architecture of test coin cells and propose electrochemical noise as a reasonable characterization method,this dissertation explores Li⁺-Nafion-based artificial SEI films for suppressing Li metal dendrite growth.Li⁺-Nafion-based artificial SEI films were prepared by ultrasonic spray technology.The relationship between the structure of the artificial SEI films and its performance on suppressing Li metal dendrite growth,and associated mechanisms,were investigated.Li metal electrodes were characterized by Coulombic efficiency of Li||Cu half cells,voltage-time profiles of Li||Li symmetrical cells,electrochemical noise at open circuit and SEM observation.The addition of N,N-Dimethylformamide in sprayed aqueous Nafion solution improves its wettability on propathene separators.This is crucial for achieving highly uniform and nonporous Li⁺-Nafion thin layer with a thickness of 0.8μm.The Li⁺-Nafion layer was pressed on Li metal electrode surface to act as an artificial SEI film.Such preparation method and implantation process are convenient to be carried out,and are superior to reported spray and spin coating methods that use Nafion organic solutions.Because the Li⁺-Nafion layer shortens Sand’s time,the dendrite growth is suppressed.By adding sodium carboxymethylcellulose（CMC）in the sprayed Nafion solution and fixing the mass ratio of Nafion with CMC at 10/1,a Li⁺-Nafion/CMC（10/1）layer with a unique grid-like structure is obtained.The framework and interior of the grid-like structure are mainly composed of CMC and Li⁺-Nafion,respectively.The mechanical properties of selective micro-regions in Li⁺-Nafion/CMC（10/1）was measured by atomic force microscopy.The result shows that the CMC-rich framework has higher Young modulus compared to the interior Li⁺-Nafion.Due to the reinforcing effect of CMC,the Li⁺-Nafion/CMC（10/1）layer is superior to the Li⁺-Nafion layer for suppressing Li metal dendrites.Implanted Li⁺-Nafion and Li⁺-Nafion/CMC artificial SEI films in Li-S cells all improve the cycle performance.Li-S cells using Li⁺-Nafion/CMC layer works longer than cells using Li⁺-Nafion layer because the suppressing-dendrite function of the former layer is more pronounced than the latter layer.Using Li⁺-Nafion/CMC（10/1）artificial SEI film improves the discharge capability of sulfur cathodes in Li-S batteries from 286.4 mAh g^-1 to 423.7 mAh g^-1 after 200 cycles at 0.2 C.This reached capacity is superior to 380 mAh g^-1 delivered by the battery using Li⁺-Nafion（HU）artificial SEI film.