Study On The Effect Of Ion Pre-embedding On The Performance Of Aqueous Zn/δ-MnO₂ Ion Batteries

At present,environmental pollution and energy shortage are becoming more and more serious,and it is urgent to develop new and effective energy storage devices.Aqueous zinc ion batteries use aqueous solution as electrolyte solution,which is safe,environmentally friendly and has high ionic conductivity,and its encapsulation process can be carried out in air,which is convenient and less costly.The anode material of aqueous zinc ion batteries generally uses zinc metal with high theoretical capacity;therefore,it is the cathode material that is the key factor affecting its electrochemical performance.Compared with many cathode materials,MnO₂,with its low cost,high operating voltage,high theoretical capacity,environmental friendliness and non-pollution,has been considered as an ideal cathode material,especially the layered structure ofδ-MnO₂.However,δ-MnO₂as a cathode material has problems such as manganese dissolution,structural instability,capacity not reaching the theoretical capacity,and controversial charge storage mechanism.Currently,there are modification measures such as elemental doping,material compounding,defect engineering,and pre-embedding.Among them,ion pre-embedding is a well-established modification method.Studies have shown that pre-embedding ions betweenδ-MnO₂layers can enhance the interlayer structure and improve the electrochemical performance of the battery,and multiple ion pre-embedding is better than single ion pre-embedding to optimize the electrochemical performance of the battery.However,most of the current studies are on the pre-embedding of individual ions inδ-MnO₂,and not many studies have been conducted on the pre-embedding of multiple ions inδ-MnO₂,and the mechanism of which is not yet clear.Based on this,this thesis selectsδ-MnO₂as the cathode material for aqueous zinc ion batteries and adopts the strategy of ion pre-embedding to enhance its electrochemical performance.The details of the study are as follows:（1）Study of the effect of Al³⁺pre-embedding on the performance of aqueous Zn/δ-MnO₂ion batteries.In view of the fact that the actual capacity ofδ-MnO₂is much lower than the theoretical capacity,Al³⁺with three charges was chosen as the pre-embedded ion in this thesis,because multiple charges can form a stronger electric field and carry more water to enter the material interlayer in a solventized manner,thus expanding the layer spacing as much as possible.Therefore,Al-δ-MnO₂was prepared by pre-embedding Al³⁺into the interlayer ofδ-MnO₂by hydrothermal method.The pre-embedding of Al³⁺enlarged the crystalline spacing of its（001）crystalline surface to 0.74 nm,which facilitated the ion embedding/deembedding,improved the ion transport rate,and enhanced the electrical conductivity.Electrochemical test results show that Al-δ-MnO₂has a high capacity of 215.7 m Ah g^-1at a current density of 0.1A g^-1,which is 38.6%higher than that ofδ-MnO₂,and possesses a better multiplicative performance.（2）Study on the effect of Al³⁺and Mg²⁺co-embedding on the performance of aqueous Zn/δ-MnO₂ion batteries.In view of the fact that the Al³⁺pre-embedding strategy improves the capacity ofδ-MnO₂,but the capacity retention is not high and the cycling stability is not good,the effect of two ion pre-embedding on the battery performance is further investigated in this thesis.First,considering that the pre-embedding of Al³⁺may make the layer spacing too large and the layer structure unstable,and thus the capacity retention is not high,Mg²⁺with two charges was chosen as the pre-embedding ion,and it was shown that Mg²⁺can improve the structural stability of vanadium pentoxide.Mg-δ-MnO₂was synthesized by the same method,and the pre-embedding of Mg²⁺resulted in a crystalline spacing of 0.67 nm on the（001）crystal plane,and Mg²⁺acted as an interlayer"cationic column",which enhanced the stability of the layered structure.The electrochemical test results showed that the capacity retention of Mg-δ-MnO₂was as high as 93.7%after 100 cycles at a current density of 0.1 A g^-1,but the initial capacity was not greatly enhanced.Finally,it was investigated whether Al³⁺and Mg²⁺would have a synergistic effect.Al-Mg-δ-MnO₂was synthesized by the same method,and the co-embedding of Al³⁺and Mg²⁺resulted in a crystalline spacing of 0.7 nm on the（001）crystal plane.The electrochemical test results show that after 100 cycles at a low current density of 0.1 A g^-1,the remaining capacity of Al-Mg-δ-MnO₂is still 180 m Ah g^-1,with a capacity retention rate as high as 89%;after 1000 cycles at a high current density of 1 A g^-1,the capacity retention rate is as high as 90%.Al³⁺and Mg²⁺are co-embedded in theδ-MnO₂interlayer,which can not only increase the capacity by expanding the layer spacing of Al³⁺,but also stabilize the structure by injecting Mg²⁺into the interlayer"pillar",and the two play a synergistic role,showing better specific capacity,multiplicity performance and cyclic stability performance.The synthesis method is simple and can be synthesized in large quantities.The proposed multi-ion co-embedding strategy can provide a new way and idea to improve the performance of aqueous Zn-ion batteries.