Design And Electrochemical Properties Of Novel Magnesium-based Secondary Batteries

Lithium–ion batteries are the most mature secondary batteries which are widely used in our life.However,the limited available Li resource on the earth plagues the application of Li–ion batteries in electric vehicles and large–scale energy storages.Magnesium–ion batteries become the focus of researches on novel energy storage batteries because Mg is abundant on the earth,and it can provide higher energy density and security when used as negative electrodes of secondary batteries.Compared with the rapid development of Li–ion batteries technology,the progress of Mg–ion batteries research is very slow.The difficulty of Mg²⁺ ions diffusion in the lattice of the materials which cased by the strong Coulombic interaction between the bivalent Mg²⁺ ion with high charge density and the electrode materials leads the poor specific capacity and rate performance of the batteries.Recent years,the Li⁺/Mg²⁺(Na⁺/Mg²⁺)dual–salt batteries are constructed by combining the advantages of Li–ion?Na–ion?batteries and Mg–ion batteries.On the one hand,Mg metal with high volume specific energy and security is used as negative electrode.On the other hand,Li⁺?Na⁺?ions intercalate/deintercalate on the cathode materials rapidly.The batteries can obtain high energy density,excellent rate performance and cycling stability.However,the electrochemical reaction mechanism of Mg–based dual–salt batteries remains to be further understood.Considering the potential applications of layered oxides and sulfides with the fast ions transport channels in Li–ion?Na–ion?batteries and Mg–ion battery,we constructed Mg–ion batteries,Li⁺/Mg²⁺ dual–salt batteries and Na⁺/Mg²⁺ dual–salt batteries by using the above materials,then characterized the electrochemical properties,and studied the electrochemical reaction mechanism in details by combining the first principle calculation and experimental methods.First,we synthesized the one–dimensional Ti O₂–B nanowires with a naked?–110?surface by the solvothermal method,and characterized their electrochemical properties as electrode materials for Mg–ion batteries.The materials showed the discharge specific capacities of 121,64,40 and 34 m Ah g–1 at the current density of 5,100,1000 and 5000 m A g–1,respectively.Besides,the materials had a small capacity–fading rate of 0.08% per cycle after 500 cycles at the current density of 200 m A g–1,and the Coulombic efficienicy was close to 100%.The results of CV,XRD and Raman showed the capacity was mainly derived from the adsorption of Mg²⁺ on Ti O₂-B nanowires surface.The first principle calculation showed that it is difficult to intercalate into Ti O₂-B lattice for Mg²⁺ ions because of the high Mg²⁺ ion diffusion barrier.Mg²⁺ ions adsorbed onto the 4–coordinated sites on the naked?–110?surface and bonded with unpaired surface oxygen atoms.This storage mechanism can avoide the strong interactions between the Mg²⁺ ions and the electrode materials,and it is beneficial to obtain excellent cycling stability and rate capability.But constrained by the limited specific surface area of the materials and the strong surface/interface reaction of the nanomaterials,it is hard to obtain a high Mg²⁺ storage capacity in Mg–ion batteries of Ti O₂–B nanowires.The low electrochemical reaction potential leading a low specific discharge energy cannot meet the needs of people.It has been reported that Ti O₂–B can obtain a high specific capacity of 250 m Ah g–1 and excellent rate performance in Li–ion batteries.In order to make use of the excellent Li⁺ ions storage properties of the materials,we fabricated Li⁺/Mg²⁺ dual–salt batteries using Ti O₂–B nanowires.And the batteries showed a high discharge specific capacity of 242 m Ah g–1 at the current density of 20 m A g–1.When the current density increase to 1 A g–1,the discharge specific capacity retained 114 m Ah g–1.After 200 cycles at the current density of 200 m A g–1,the discharge specific capacity had no obvious decay.The first principle calculation,CV and EIS showed that the Ti O₂–B nanowires had small charge transfer resistance and low Li⁺ ion diffusion barrier in Li⁺/Mg²⁺ dual–salt electrolyte,and Li⁺ ions intercalation was happened in the Li⁺/Mg²⁺ dual–salt batteries with a pseudocapacitance of 74%.The transition metal sulfides with two-dimensional layered structure have a weak van der Waals effect,so Li⁺?Na⁺?ions can be transported rapidly between layers.As a common two-dimensional transition metal sulfides,MoS₂ exhibits excellent Li⁺ ions storage performance.We synthesized self-assembly MoS₂ nanoflowers by solvothermal method,and characterized their electrochemical properties in Li⁺/Mg²⁺ dual–salt batteries.Through optimizing the concentration of Li⁺/Mg²⁺ dual–salt electrolyte we got a high discharge specific capacity of 239.2 m Ah g–1 at the current density of 100 m A g–1.MoS₂ nanoflowers also had perfect rate performance and cycling stability.We got a discharge specific capacity of 109 m Ah g–1 at the current density of 1 A g–1 and a capacity retention ratio of 87.2% after 2300 cycles.We studied the changes of material structure and chemical composition during charging and discharging processes by XRD and ICP–OES.The results showed that MoS₂ nanoflowers kept a stable layered crystal structure in Li⁺/Mg²⁺ dual–salt batteries,and the electrochemical reaction was mainly due to the reversible intercalation and deintercalation of Li⁺ ions with a small amount of Mg²⁺ ions adsorption and desorption on the surface of materials.Although MoS₂ nanoflowers exhibited good electrochemical performance in Li⁺/Mg²⁺ dual–salt batteries,the low intrinsic electronic conductivity of MoS₂ still limited the performance of the materials at large rate.VS2 is a transition metal sulfide with metal conductivity.As a cathode material for Li–ion batteries,it has excellent rate performance and cyclic stability.In order to make full use of the excellent Li–ion storage properties of VS2 materials,we synthesized VS2 nanosheets by solvothermal method then constructed Li⁺/Mg²⁺ dual–salt batteries.Compared with Ti O₂-B nanowires and MoS₂ nanoflowers,VS2 nanosheets exhibited better electrochemical performance in Li⁺/Mg²⁺ dual–salt batteries.The VS2 nanosheets showed a reversible Li⁺ ions storage capacity of 168,160,147,135,121 and 93 m Ah g–1 at the current density of 50,100,200,500,1000,2000 and 5000 m A g–1,respectively.After 500 cycles the VS2 nanosheets retained a discharge specific capacity of 102 m Ah g–1 at the current density of 1 A g–1 with a capacity–fading rate of 0.04% per cycle.The first principle calculation,XRD and XPS all showed a weak van der Waals effect between the two-dimensional layers of VS2 nanosheets,so the materials would react with the Li⁺/Mg²⁺ dual–salt electrolyte.The Ph2 Mg and THF organic molecules inserted into the layers and the d–space expended from 5.78? to 8.76?.As a result,the electrostatic interreaction between Li⁺ ions and the materials was reduced,and the Li⁺ ion diffusion barrier in the materials was greatly reduced,which made VS2 nanosheets exhibit excellent electrochemical performance in Li⁺/Mg²⁺ dual–salt batteries.A large number of Li⁺ ions are needed in Li⁺/Mg²⁺ dual–salt batteries to support electrochemical reactions in order to maintain the efficient intercalation/deintercala tion of Li⁺ ions on the cathode.Therefore,Li⁺/Mg²⁺ dual–salt batteries can not solve the problem of limited Li resource faced by large-scale energy storage batteries.Sodium is abundant in nature,widely distributed and convenient to purify.If we construct Na⁺/Mg²⁺ dual–salt batteries as the same as Li⁺/Mg²⁺ dual–salt batteries,it will play an important role in solving the low cost manufacturing problem of energy storage batteries.Considering the excellent electrochemical performance of Ti S2 in Na–ion batteries,we constructed Na⁺/Mg²⁺ dual–salt batteries using commercial Ti S2.The Ti S2 Na⁺/Mg²⁺ dual–salt batteries showed excellent rate performance,which could obtain the discharge specific capacity of 200,160,138,108 and 75 m Ah g–1 at the current density of 200,500,1000,2000 and 4000 m A g–1,respectively.The ultra-long cycle life showed capacity retention ratio of 90% after 3000 cycles at the current density of 2 A g–1,and 100% after 20000 cycles at the current density of 4 A g–1.Similar to VS2 nanosheets in Li⁺/Mg²⁺ dual–salt batteries,the weak van der Waals effect between Ti S2 layers led the materials react with the Na⁺/Mg²⁺ dual–salt electrolyte,and the organic molecules inserted into the layers causing the d–space expended and reduced the ion diffusion barrier.Na⁺ and Mg²⁺ ions intercalated into the expened Ti S2 layers together,but due to the sluggish diffusion kinetics of Mg²⁺ ions,the electrochemical process was mainly composed of Na⁺ ion intercalation.Above all,we using layered oxides and sulfides constructed Mg–ion batteries,Li⁺/Mg²⁺ dual–salt batteries and Na⁺/Mg²⁺ dual–salt batteries.Then we revealed the Mg²⁺ ions storage on Ti O₂–B nanowires surface in Mg–ion batteries,clarified the Li⁺ions pseudocapacitance storage in Li⁺/Mg²⁺ dual–salt batteries,found transition metal sulfides Mg–based dual–salt batteries materials based on in-situ layers expended,and constructed high performance Na⁺/Mg²⁺ dual–salt batteries using two-dimensional transition metal sulfide.The above works not only provide a reliable theoretical basis for revealing the electrochemical reaction mechanism of different Mg-based secondary batteries,but also provide a feasible technical scheme for improving the electrochemical performance of Mg-based secondary batteries.