Research On Low Temperature Performance Of Electrochemical Energy Storage Devices Based On Functional Electrolytes

With the development of new energy sources and new energy storage technologies,electrochemical energy storage devices have received more and more attention and research.Currently,the largest commercial-scale electrochemical energy storage devices are lithium-ion batteries,however,due to the low storage capacity of lithium,expensive,poor safety and insufficient power density,lithium-ion batteries are unable to meet people’s energy storage needs for a wider range of application scenarios.In pursuit of lower cost and inherently safer electrochemical energy storage technologies,new electrochemical energy storage devices such as zinc ion batteries and supercapacitors/hybrid capacitors have been developed.As the application range of these new energy storage devices gradually increases,the environment they face will also become more complex.For example,high latitude cold regions,highland areas have lower temperatures,and about half of the territory in China in winter has an average temperature below zero degrees.This requires electrochemical energy storage devices need to have better low-temperature performance.At present,there have been some research reports on this issue,but most of the research only broadens the lower temperature limit of use,lack of consideration for the cycle performance and electrolyte,electrode stability and other factors resulting in the low temperature performance of electrochemical energy storage devices is still not ideal,especially the aqueous electrochemical energy storage devices.Based on the above problems,this thesis focuses on the development and design of aqueous functionalized electrolytes to solve the shortcomings of aqueous electrolytes such as freezing,salt precipitation,low ionic conductivity,and susceptibility to side reactions at low temperatures and to assemble three electrochemical energy storage devices including zinc ion hybrid capacitors,zinc ion batteries,and supercapacitors.The details and conclusions of the study are as follows:（1）A novel antifreeze electrolyte（0.4 m Zn SO₄+2.7 m Mg（Cl O₄）₂）with an ultra-low freezing point（<-80℃）and excellent electrical conductivity（7.50 m S at-60℃）was developed using a low-cost 1 mol kg^-1（m）Zn SO₄ electrolyte as the subject of study by adding inorganic Mg（Cl O₄）₂ as a low-temperature additive.cm^-1).As a result,this organic-free hybrid electrolyte allows the assembled aqueous zinc ion hybrid capacitor（AZHCs）to operate even at-60℃with a high potential window of 2.4 V.In addition,at-30℃,this AZHCs device exhibits the highest specific capacity(123 m A h g^-1),the highest energy density(126 Wh kg^-1 at 1050W kg^-1),excellent rate performance(75.8 Wh kg^-1 at 10495.4 W kg^-1),and a stable operating life of more than 32 000 cycles.This study provides an inspiration for the preparation of AZHCs with excellent low-temperature properties and high energy and power densities at low temperatures.（2）A hybrid Zn（Cl O₄）₂/Li₂SO₄/DMSO electrolyte was prepared using dimethyl sulfoxide（DMSO）as the electrolyte additive and zinc perchlorate（Zn（Cl O₄）₂）as the zinc salt.It was found that the addition of DMSO could well form hydrogen bonding with the water molecules in the electrolyte and thus replace the hydrogen bonding between water and water,which could well inhibit the water activity and reduce the proportion of free water thus reducing the occurrence of electrolyte-interface side reactions and the growth of dendrites.As a result,the assembled Zn||Zn（Cl O₄）₂/Li₂SO₄/DMSO||Zn symmetric cell can be deposited/exfoliated stably for more than 5600 h（about 236 days）at a current density of 0.5 m A cm^-2,which is more than50 times than that of the group without additives.It also has a stable cycle time of over 40,00hours at a low temperature of-25℃（about 134 days）.Meanwhile,the assembled Zn||Zn（Cl O₄）₂/Li₂SO₄/DMSO||Li Fe PO₄ full cell with dual cation energy storage mechanism exhibits excellent stability of 400 cycles at 6 C current density at room temperature and 103m A h g-1（0.6 C）at-20℃low temperature with a capacity retention rate of 83.2%after 600cycles.The capacity retention rate was 83.2%after 600 cycles.（3）A low-concentration cryogenic electrolyte(1 mol kg^-1 Mg（Cl O₄）₂+30%EG aqueous solution)was developed using ethylene glycol（EG）as a low-temperature additive.The supercapacitor prepared with this electrolyte has a specific capacity of 33.6 F g^-1 and a high capacity retention rate of 85.2%after 40,000 cycles.More importantly,the ESW of the SCs can be widened from 1.8 V to 2.1 V at a low temperature of-30℃.The widened ESW delivers a specific capacity of 26.4 F g^-1 at 1 A g^-1,which is 21.5%higher than that at 1.8 V.The device had a better cycling stability with a capacity retention rate of 75.5%after 40 000 cycles.