Exploration Into The Charge Storage Mechanism Of Mn Oxides-based Cathode Materials For Energy Storage Devices

Together,the increasing demand for energy and the threat from Global Warming make electrical energy storage?EES?a worldwide strategic priority.EES is expected to play a key role in the decarbonization of electric power sources.supercapacitor and Sodium ion battery?SIBs?,have emerged as promising candidates in EES field due to their potential to offer an exciting approach to meet the increasing power demands and the grid-scale energy storage respectively.Considering for the commercial application of EES,limitations in the availability of the transition metals for cathode materials are of concern,driving development towards more sustainable elements such as Mn.However,Mn-based supercapacitors face the challenge of limited understanding of its complicated charge storage mechanism,which greatly hinder their practical application;Mn-based layered oxides cathodes for SIBs suffer from fast degradation of structural stability,preventing its large-scale application in the electric grid domain.Thus,in situ characterization methods were utilized in this thesis to study the electrochemical process of Mn-based cathode materials for both supercapacitor and SIBs,so as to clarify their electrochemical behavior and hence provide insights for the design of superior material for commercial applications.Therefore,the main results of this study are as follows:?1?The phase evolution of?-Mn_0.98O₂ based electrode materials for pseudo-capacitors was investigated using in operando Raman spectroscopy.It was found that?-Mn_0.98O₂ tends to exist as a more stable mix-phase of?-MnO₂ and Mn₃O₄ when immersed in aqueous electrolytes.At the very early stage of first slow anodic scan(1 mV s^-1),hydrated H⁺would intercalate into the tunnel sites and replace part of the hydrated cations along with the deintercalation of larger amount of M⁺.When the voltage increased further,the H⁺was removed from the tunnels together with cations.When the voltage continue increase till no ions left in tunnels,Mn²⁺began to de-intercalated from the tetrahedral sites of Mn₃O₄,contributing to the energy storage.These intercalation/deintercalation processes occurred at different voltage ranges are high reversible.?2?The structural evolution of Mn₃O₄ as pseudo-capacitor electrode upon potential cycling was investigated using in operando Raman spectroscopy.It was revealed that the spinel Mn₃O₄ are susceptible to the applied potential and can transform into birnessite-type MnO₂ during potential cycling in a Na₂SO₄ electrolyte via the extraction of Mn²⁺cations,which is the main reason for the much improved electrochemical performance along with cycling.A directly crystal structure change induced by the extraction of Mn²⁺cations was proposed in this phase transition process.The generated birnessite MnO₂ with Na⁺and Mn²⁺filling in between the laminates experienced a reversible structure change corresponding to the intercalation/deintercalation of Na⁺and the oxidation/reduction of Mn²⁺during potential cycling.The absence of involvement of H⁺in the energy storage process implies that the traditional energy storage reactions is not suitable to every Mn-based oxide,a little bit different in different crystal structures.This could be the main reason that our samples have such good cycling life,which is comparable to carbon based electrodes.?3?Then Mn²⁺was pre-adsorbed in the large tunnel through dipping the?-MnO₂electrode in aqueous solution of MnSO₄.The electrochemical performance of the sample before and after Mn²⁺pre-adsorption was well characterized and its energy storage behaviour change was also captured via in-operando Raman analysis.It was found that the capacity of the sample was largely enhanced at the initial several hundred cycles and then dropped quickly after the modification of Mn²⁺.Combination of in-operando Raman spectra analysis and DFT calculations revealed that the proton H⁺is more energetically favorable at the small?1×1?tunnels while Mn²⁺and Na⁺tend to be adsorbed in the larger?2×2?tunnels.And the involvement of H⁺storage in the?-MnO₂ can be tuned by the tunnel structure evolution of MnO_x induced by the adsorption/de-adsorption of Mn²⁺.?4?Highly porous nanostructured?-MnO₂ electrodes have been synthesized and examined in pseudocapacitors,demonstrating a specific capacitance of 1,068 F g^-1 and unprecedented stability in a potential window of 0 to 1.0 V?vs:Ag/AgCl?.The charge storage mechanism of the layered?-MnO₂ nanoflakes was investigated by in operando Raman spectroscopy.It was revealed that the charge storage mechanism for layered?-MnO₂ during cycling is dominated by Na⁺intercalation/deintercalation,accompanied by interlayer spacing expansion/contraction,with little involvement of proton.Moreover,theoretical calculations also confirmed that the insertion of Na⁺is more energetically favorable than H⁺at all sites of the interlayer in?-MnO₂,offering a rational explanation of the proposed mechanism and the observed excellent stability.?5?A group of promising lithium doped sodium manganese oxide cathode materials were synthesized and characterized.Na_0.6Li_0.2Mn_0.8O₂ exhibited exceptionally high initial capacity of²23 mAh g^-1 and excellent capacity retentions,attributed primarily to the absence of phase transformation in a wide potential range of electrochemical cycling,as confirmed by in-operando X-ray diffraction?XRD?,Rietveld refinement,and high-resolution⁷Li solid-state NMR characterizations.The systematic study of structural evolution and the correlation with the electrochemical behavior of the doped cathode materials provides new insights into rational design of high-performance intercalation compounds by tailoring the composition and the crystal structure evolution in electrochemical cycling.