Electrodeposition Of Zn And Sn Based Anode Materials From Ionic Liquids And Their Properties

With the use of electronic devices and the demand for energy storage in the transportation and energy sectors,lithium-ion batteries?LIBs?have become a major study and focus.The energy and power densities of the commercially LIBs are still severely limited by the electrode materials.The current generation of LIBs uses graphite as the anode material.In comparison,Zn and Sn have been regarded as the promising anodes.However,overcoming the rapid storage capacity degradation of zinc and tin still remains a major challenge,due to the large volumetric changes.We designed different structured Zn and Sn based materials that can buffer the volume change.We prepared Zn and Sn based materials by ionic liquid electrodeposition.The electrodeposition in ionic liquids is a low energy processing technology.All the materials gain from this technique are electrochemically active and binder free.The electrochemical behaviors of Zn and Sn based materials are systematically investigated,the morphology and electrochemical performance are also discussed.The choline chloride/urea?ChCl/urea?ionic liquids are synthesized and used for the electrodeposition of Zn based anode materials directly from ZnO and CuO.In addition,the 1-ethyl-3-methylimidazolium tetrafluoroborate?[EMIm]BF₄?,1-ethyl-3-methylimidazolium trifluoromethylsulfonate?[EMIm]TfO?and 1-ethyl-3-methylimid-azolium dicyanamide?[EMIm]DCA?ionic liquids with wide electrochemical window are selected for the electrodeposition of Sn based anode materials from SnCl₂ and CoCl₂.The electrochemical behaviors of the Zn???,Cu???/Zn???species in ChCl/urea ionic liquid and the Sn???,Sn???/Co???species in[EMIm]BF₄,[EMIm]TfO,[EMIm]DCA ionic liquids have been systematically investigated.The nucleation-growth process model is established.The corresponding relationship and regulation mechanism between the morphologies and the electrodeposition conditions are determined by controlling experimental parameters.The electrochemical perform-ances of the as-prepared Zn and Sn based electrodeposits as anodes for LIBs are studied by button-type half-cell tests.The main researchs are listed as follows:?1?The concentrations of the dissolved zinc and copper species?mainly in forms of[ZnO·Cl·urea]^-and[CuO·Cl·urea]^-?in the ZnO/CuO-saturated ChCl/urea ionic liquid at 70°C are determined to be about 0.016 M and 0.342 M,respectively.The electrodeposition of Zn and Cu-Zn alloy follow the instantaneous nucleation-growth process.It is indicated that the metal Zn involves instantaneous nucleation-growth mechanism irrespective of substrate material in ChCl/urea.Low temperature and low cathodic potential contribute to the formation of dense,compact Zn on the substrate.The micro/nano Cu-Zn alloys can be obtained by altering the cathode potential,and the Zn content in the Cu-Zn electrodeposits increases with the increasing cathodic potential,the morphologies of the films gradually change from the initial spherical particles to regular polygonal shapes.After 50 cycles,the discharge and charge specific capacities of the Zn and Cu₅Zn₈ electrodes reach at 260 mAh g^–1,254 mAh g^–1;312 mAh g^–1,305 mAh g^–1,respectively.It is suggested that the Cu₅Zn₈ electrode exhibits relatively higher reversible specific capacity and better cycle stability,which related to its composition and structure.?2?The electrodeposition of Sn in[EMIm]BF₄,[EMIm]TfO and[EMIm]DCA ionic liquids takes place via one step transfer process.The reduction of Sn???in these systems involves a diffusion-controlled irreversible process.Compared with the[EMIm]BF₄ and[EMIm]TfO systems,the Sn???species formed in[EMIm]DCA ionic liquid are more easily to be reduced.It is suggested that the relatively better mobility of Sn???species is associated with the low viscosity of[EMIm]DCA and the good coordination of the solution with the Sn???species.The microstructure of the Sn electrodeposits can be significantly influenced by altering the cathodic potential and the anion of the ionic liquids.With the increase of the potential,the Sn electrodeposits seem to grow in a cluster of sponge clusters in[EMIm]BF₄-SnCl₂,and the Sn electrodeposits seem to grow in a defined cubic shape in[EMIm]TfO-SnCl₂,while the Sn electrodeposits seem to grow in a nanoplatelet structure in[EMIm]DCA-SnCl₂.The Sn electrodeposits produced in[EMIm]BF₄ and[EMIm]TfO ionic liquids are preferentially oriented through the?312?plane.However,the Sn electrodeposits obtained in[EMIm]DCA ionic liquid grow preferentially through the?200?plane.Moreover,after 50 cycles,the discharge and charge specific capacities of the Sn electrodes reach at 292 mAh g^–1,277 mAh g^–1;377 mAh g^–1,363 mAh g^–1;515 mAh g^–1,502 mAh g^–1,respectively,indicating that the Sn electrode obtained in[EMIm]DCA exhibits the highest reversible capacity,which is mainly due to the nanostructure.?3?The electrodeposition of Sn on a Ni foam substrate in[EMIm]DCA ionic liquid and its corresponding electrochemical performance are investigated.Compared with the Ni foil,the three-dimensional?3D?porous structured Ni foam can effectively relieve the volume expansion of Sn active material in the process of insertion and removal of lithium,decrease the pulverization.The discharge and charge specific capacities of 3D Ni foam-Sn material are 645 mAh g^–11 and 629 mAh g^–1,respectively,and the coulombic efficiency is 97.5%.Compared with Sn electrodes,the specific capacity and cycle stability of the 3D Ni foam-Sn material are significantly improved.?4?The discharge and charge specific capacities of 3D Ni foam-SnCo nanowire materials obtained at different potentials?–1.3 V,–1.4 V,–1.5 V?in[EMIm]DCA ionic liquid are 709 mAh g^–1,700 mAh g^–1;735 mAh g^–1,724 mAh g^–1;769 mAh g^–1,758 mAh g^–1,respectively.The relative coulombic efficiencies are 98.7%,98.5%,98.6%,respectively.Compared with the 3D Ni foam-Sn material,3D Ni foam-SnCo nanowire materials exhibit more excellent cycle performances.The Li⁺migration path in the electrodeposited SnCo nanowires is short,the electron transmission efficiency is high.A large number of electrochemically active sites can accelerate the rate of Li⁺insertion and extraction reactions.In addition,the introduction of the inactive component Co as the medium not only relieves the internal stress generated by the volume expansion of the Sn active component,but also provides more reactive sites for the Sn active component.