Regulate Of Key Materials And Reaction Mechanism Of High-performance Zn-iodine Batteries

High-performance secondary batteries with low cost,high safety and excellent cycle stability can build reliable electric energy storage technology,which plays a very key role in developing high efficiency and clean energy.Aqueous batteries with the advantages of intrinsic safety,abundant raw materials and potential low cost,which are the current research focus.Aqueous zinc-iodine battery uses electrochemical conversion reaction to store electric energy,which has faster reaction kinetics and higher energy efficiency than aqueous zinc ion battery based on ion insertion-extraction electrode.Based on the multivalent characteristics of halogen,the multi-electron transfer iodine electrode reaction can be realized through regulating the microenvironment of electrode and electrolyte,which can further improve the energy density of aqueous zinc-iodine battery.The iodine electrode reaction with multi-electron transfer has not been systematically studied.Besides the interaction between electrode components and electrolyte or electrode carrier and its influence on the electrode reaction mechanism are still unclear.In this paper,the strong correlation between intermediate I₃^-and high valence I⁺ion in iodine conversion reaction and electrolyte or electrode environment was systematically studied.Furthermore,the proposed catalytic strategy can realize efficient iodine electrode reaction.Combined with the regulation of solvation structure of electrolyte and the design of functional catalytic materials,long term and stable two-electron and four-electron zinc-iodine batteries are constructed.The main research contents and results are as follows:（1）The relationship between I₃^-ion and electrolyte solvent in iodine conversion reaction.In two-electron iodine batteries,the electrode reaction based on organic electrolyte generally involves the multi-step conversion reaction of I₃^-ions,such as Li-I₂and Na-I₂batteries,and their voltage curves generally have two voltage platforms.The formation of I₃^-in aqueous electrolyte is not clear.Generally,the voltage curve of aqueous iodine battery has only one voltage platform,but most literatures think that I₃^-ions are generated in aqueous electrolyte,which can be inhibited by porous materials.The electrochemical reaction process of iodine in different solvents was studied by rotating disk electrode and ultraviolet spectrum.It was found that iodine was transformed in one step（I（?）I₂）in proton solvent containing hydroxyl group and in two steps（I（?）I₃^-（?）I₂）in aprotic solvent.By studying the interaction between solvent and iodine species,the thermodynamic essence of these electrochemical processes was clarified.Based on this discovery,hybrid EG-H₂O electrolyte was selected to improve the utilization rate of active materials,realize one-step conversion of I（?）I₂,inhibit the production of intermediates I₃^-,accelerate the conversion reaction kinetics,and then improve the coulombic efficiency of the battery and strengthen the cycle stability of the battery.This work further suggests that the formation of I₃^-in aqueous electrolyte is controlled by its reaction kinetics,which provides a thermodynamic basis for the catalytic strategy of iodine conversion reaction.（2）The two-electron iodine electrode catalytic reaction and high performance two-electron zinc-iodine battery.The intermediate I₃^-ion is highly soluble in electrolyte,which is the main reason for the capacity attenuation and self-discharge of aqueous iodine batteries.We deeply studied the electrochemical reaction process of iodine,and found that the formation of I₃^-intermediate was controlled by the kinetic process of electrode reaction.We designed a variety of N-doped carbons as catalysts to enhance the kinetics of iodine electrode reaction,and studied the catalytic kinetics of different N sites on iodine electrode reaction.The experimental results show that the efficient catalytic action of graphite N can effectively inhibit the production of I₃^-intermediate,which makes the electrode reaction of iodine change from two-step reaction to one-step reaction,and improves the utilization rate and stability of iodine electrode.Furthermore,PNC-1000 material with high graphite nitrogen content was prepared as iodine carrier,and the constructed zinc-iodine battery showed excellent rate performance(the specific capacity is 175 mAh g^-1at current density of 4 A g^-1)and long cycle life(the specific capacity is 200 mAh g^-1after 10,000 cycles at current density of 1.0 A g^-1).（3）The four-electron iodine electrode catalytic reaction and high performance four-electron zinc-iodine battery.The four-electron iodine electrochemical reaction can improve the energy density of zinc-iodine battery,but the high-valence I⁺ion is easy to hydrolyze,so it is generally necessary to use high-concentration electrolyte to inhibit hydrolysis.The long-term stability of four-electron zinc-iodine battery is mainly controlled by the chemical stability of I⁺ion.Our research found that the electron transfer phenomenon between I⁺and materials can improve the chemical stability of I⁺,and the functional Fe _SA-NC cathode carrier can realize efficient four-electron reaction.The experimental results and theoretical calculations show that the d orbital of Fe atom and the p orbital of iodine system（I_n^-/I⁰/I⁺）could form d-p conjugate structure,in which Fe atom as the active site can improve the stability of I⁺and accelerate four-electron iodine kinetics conversion reaction.The highly reversible four-electron iodine conversion reaction can still be obtained even if the electrolyte concentration is reduced.Thanks to the catalytic effect of Fe _SA-NC and the enhancement of I⁺stability,the constructed zinc-iodine battery can effectively improve the utilization rate of active materials(the specific capacity can reach 358.5 mAh g^-1at 4 A g^-1),achieve a high energy density(762 Wh kg^-1)and long cycle life(the capacity remains about 380 mAh g^-1after 7000 cycles at 2 A g^-1).（4）The optimizated electrolyte for four-electron zinc-iodine battery.Although the high-concentration electrolyte can realize the reversible I⁺/I₂/I^-electrode reaction,the viscosity of the electrolyte is high,which affects the mass transfer process of the electrode reaction and made electrode reaction kinetics slow.In addition,the high-concentration aqueous electrolyte is easy to freeze at low temperature,and the low-temperature performance of the battery is poor.We used the"molecular crowding effect"to construct PEG-H₂O mixed electrolyte,and realized the efficient conversion of reversible I⁺/I₂/I^-electrode reaction at the conventional electrolyte concentration（～2 mol）.The study of PEG-H₂O mixed electrolyte shows that:（1）PEG can break the hydrogen bond of water,inhibit the activity of water and change the solvation structure of ions in electrolyte,thus improving the stability of I⁺in conventional electrolyte;（2）PEG can promote the deposition of（002）zinc in the electrolyte,and own positive effect on inhibiting the growth of dendrites,which can prolong the life of the anode.When the electrolyte is hybrid electrolyte 2m 14-6,the ultra-high rate performance of the four-electron zinc-iodine battery can be achieved(it can run stably for more than 9000 cycles at 2 A g^-1,and the capacity is 413.2 mAh g^-1).At the same time,the PEG mixed electrolyte has good electrochemical performance at low temperature,and the four-electron zinc-iodine battery can still maintain the specific capacity of 205.8 mAh g^-1after running for 1500 cycles at-25℃.