Electrochemical Bipolar Behaviors Of Manganese Dioxide Electrode Material And Its-Based High-Performance Symmetric Micro-Supercapacitors

With the development of the Internet of Things and information technology,people’s production and life are gradually entering the era of intelligence.Whether it’s wearable,implantable health detection devices,multi-node sensors for environmental monitoring,and lightweight,miniaturized electronics have boomed in recent years.In order to meet the increasing performance requirements of the above devices,micro-energy storage devices,which are an important part of the above devices,must also develop in an all-round way in the direction of lightweight,miniature,high energy density,long cycle life,and easy system integration.Among many micro-energy storage devices,micro-supercapacitors inherit the advantages of supercapacitors,have the characteristics of high power density,fast charge and discharge rate,and excellent cycle stability,and their power source components for self-powered/wireless microelectronic systems have received more and more attention in recent years.In general,the energy storage performance of micro supercapacitors is determined by the performance of the positive and negative electrodes（1/C full=1/C positive electrode+1/C negative electrode,energy density E=1/2 C all x U²）.Therefore,the asymmetric cell structure（Obtain a wider operating voltage window,i.e.U=V positive electrode-V negative electrode）has become a winning strategy for improving energy density.However,for micro supercapacitors,the synthesis of positive and negative electrode materials under the premise of micro-device structure has become the key point and difficulty in the preparation of such devices,which will greatly increase their production costs and delay the process of large-scale commercial application.In principle,the electrochemical polarity（that is,positive and negative electrodes）of the electrode material is determined by the redox potential,so the regulation based on the electrochemical reaction can theoretically realize the electrochemical polarity control of the specific electrode material.Based on this,the electrochemical polarity of the electrode material in symmetrical micro supercapacitors is expected to greatly improve their electrochemical performance（energy density）.Taking the typical supercapacitor electrode material Mn O₂ as an example,Mn O₂ has the comprehensive advantages of intrinsic pseudocapacitance characteristics,ultra-high theoretical capacitance,large potential window and abundant reserves,and is highly anticipated in high energy/power density energy storage systems.However,there are two challenges in the practical application of Mn O₂ in symmetrical micro supercapacitors:on the one hand,Mn O₂electrodes as negative electrodes have almost no capacity in neutral electrolytes,and the capacity is low even in alkaline electrolytes.Therefore,it is difficult to construct high-performance symmetrical micro-supercapacitor devices with Mn O₂ electrode materials,limited by the low capacity of the negative electrode.On the other hand,the electrochemical reaction of Mn O₂-based electrode is related to the Mn³⁺/Mn⁴⁺redox reaction,which can be described as:a certain proportion of Mn³⁺ions on Mn O₂ are in a charged state;Mn³⁺tends to cause Jahn-Teller distortion of[Mn O₆]polycrystals during discharge,resulting in a disproportionate reaction of 2Mn³⁺→Mn⁴⁺+Mn²⁺.Due to the water solubility of Mn²⁺,it subsequently diffuses from the electrode and dissolves into the electrolyte around the electrode surface,which is one of the important reasons for the capacitance attenuation of Mn O₂-based electrodes.Based on this,starting from the mechanism of electrochemical reaction of Mn O₂,this paper realizes the electrochemical bipolar regulation of Mn O₂ electrode based on the precise control of electrochemical reaction,so as to realize the great improvement of the performance of Mn O₂-based symmetric microsupercapacitors.The main work and research results are as follows:（1）High-performance supercapacitor Mn O₂ anode material based on active site regulation.Using hydrogen plasma to treat Mn O₂ deposited at a constant potential increased its oxygen vacancy content successfully,increased the surface area to make Mn O₂ more fully contact in the water system electrolyte,and increases the active site of electrochemical reactions.The results of electrochemical tests show that after plasma treatment,Mn O₂ has a high specific capacitance of 380 F g^-1 at a current density of 1 A g^-1,which is much higher than 240 F g^-1of pristine Mn O₂,and after 10000 cycles of constant current charging and discharging(20 A g^-1),the capacitance retention rate can reach 83.5%,which has excellent electrochemical performance.（2）Mn O₂ anode material and its electrochemical energy storage mechanism based on ion pre-embedding to realize the regulation of crystal water content.Through the comparison experiment of hydroxide ion concentration and the analysis of the results of quartz crystal microbalance,it is found that the content of crystal water is very important in the reaction process of Mn O₂ as negative electrode in alkaline electrolyte.In the experiment,pre-embedded different ions were prepared and the number of intercalated ions was adjusted to adjust the crystal water content of Mn O₂.The electrochemical test results and thermogravimetric data show that the pre-intercalated ions can increase the crystal water content of Mn O₂,and the different number of embedded ions can also affect the crystal water content.The final prepared Mn O₂（Al-0.01）electrode material has the most excellent performance.The electrode material has a crystal water content of about 20%,and has a specific capacitance of 442 F g^-1 at a current density of 1 A g^-1,which is higher than the original potentiostatic deposited Mn O₂ and plasma-treated Mn O₂.However,the rate performance of Al-0.05 is the best,and it has the best comprehensive electrochemical properties.It can have a capacitance retention rate of 97.3%after 10,000 constant current charge and discharge(5 A g^-1),which provides a method for other metal oxides to improve their crystal water content and electrochemical performance.（3）High-performance symmetrical miniature supercapacitors based on electrochemical bipolar Mn O₂ electrodes.Give the basis for the above Mn O₂ anode performance regulation.In the experiment,treating Mn O₂ with hydrogen plasma to increase its surface contact with water,and pre-intercalating ions into Mn O₂ to increase the content of crystal water,increased water content from the surface and inside respectively,successfully improved the electrochemical performance of Mn O₂ as a negative electrode in alkaline electrolyte,and it is possible to build symmetrical miniature supercapacitors.An electrochemical bipolar Mn O₂ electrode was obtained,and through laser etching assistance,a symmetrical micro supercapacitor with a planar interfinger configuration is obtained.Electrochemical performance tests show that the device can obtain an areal energy density of up to 33.63μWh cm^-2 at a power density of 1 m W cm^-2,and the capacitance retention rate of the device after 10,000 cycles of electrochemical cycles in series and parallel is 94.5%,which is a high-performance planar cross-finger symmetric miniature supercapacitor device with great potential.