Sodium-storage Mechanistic Study And Structural Design Of Organic Electrode Materials In Sodium-ion Battery

The growing demand for lithium-ion batteries?LIBs?by the electric vehicles will inevitably lead to the shortage of lithium resources,which will further affect the price and application scale of these devices.Sodium shares similar physicochemical properties with lithium,but is quite cheap and abundant.Therefore,a strong desire emerged to replace LIBs with sodium ion batteries?SIBs?.Among electrode materials of SIBs,organic materials have obvious advantages of high theoretical specific capacity,rich resource of raw materials,and flexible ability of structural design.Up to date,a variety of organic SIBs electrode materials have been developed.However,the exact sodium storage mechanism in these electrode materials is still quite elusive.For example,the well-established enolization or single-double bond reformation mechanisms cannot explain the sodium-storage ability of pure carbocyclic rings and non-conjugated organic molecules.Therefore,exploring the sodium storage mechanism of organic electrode materials by using theoretical methods is urgently needed,especially for the rapid screening/identification of organic electrode materials with high sodium-storage activity.From using the first-principles calculations,we study the crystal structure and electronic structure of organic electrode materials,in order to explore the potential organic electrode materials with high electrochemical properties of sodium-ions storage in sodium-ions battery.Also,their possible sodium-storage mechanism is proposed in details based on these calculations,such as the diffusion path of sodium-ions,the discharge voltage,structural phase transition and physical property?e.g.,electrical conductivity?changes of the organic materials during the process of discharging.Meanwhile,these data are contributed to analyze the relationship between the material structure and the sodium-storage mechanism/material properties.These works will establish some fundamental theoretical approaches for designing the new organic electrode materials of SIBs.This article is mainly committed to solve the problem of how to activate the ability of carbons store sodium-ions and stabilize the carbons in process of electrochemical reactions,to come up with the new mechanism of sodium-storage,so the special research contents based on the above issues as follows:1.Activating the sodium-ions storage sites and analyzing the structural stability of the aromatic carbon rings in conjugated system Zn-PTCDA:The structural framework of the conjugated metal organic can be controlled by the coordination environment of different transition metals.In this article,we design a wavy-layered 3D structure of metal-organic compound zinc perylenetetracarboxylates?Zn-PTCDA?with stretched space between adjacent perylene planes by replacing sodium ions with zinc ions in Na-PTCDA.According to both computational and experimental results,it is found that Zn-PTCDA with wavy-layered 3D structure can enable aromatic rings activated as Na⁺-storage sites and there are two reductions in Zn-PTCDA.The redox center of the first step reaction is the unsaturated functional group?C=O?,while the second reaction takes place on the sp² hybridized carbon in the aromatic carbon ring,which should not occur in the case of Na-PTCDA.That is to say,the three-dimensional open frame can activate the sodium-ions storage ability of aromatic carbon ring in organic electrode materials,and thereby increasing the capacity of organic electrode materials in SIBs.We believe that these understandings are helpful for the development of a big family of 3D open framework structured organic materials for application in high-capacity Na-ion batteries.2?Activating the sodium-ions storage sites and analyzing the structural stability of O=C-OH in non-conjugated material CHDA:The functional groups acid?O=C-OH?between adjacent molecules in the crystal structure of 1,4-cyclohexanedicarboxylic acid?C8H12O4,CHDA?are diagonally constituted,which are found to be able to react with two Na⁺to provide a high theoretical specific capacity of 284 mA h/g.Based on both calculations and experiments,a new mechanism for the sodium-ions storage in CHDA is proposed and named as hydrogen-bond transfer.This mechanism verify that the hydrogen-bond transfer is accompanied with the orbital evolution of?*bond to?bond.This new mechanism can explain the stability of the two embedded electrons.At the same time,the activity of intermediates with unpaired electrons can be inhibited by the reformation of?bonds and the structural steric hindrance.The other non-conjugated materials with diagonally aligned carboxylic acids have not only the analogous structures to take place hydrogen-bond transfer,but also the similar electrochemical performances with good reversibility and relatively constant voltage as CHDA.The work sheds light on the development of novel organic anode materials in Na-ion batteries.3?High-capacity organic electrode materials are designed based on the superposition stability of carbon:From the first-principles calculation,it is found that the carbon rings in the series of cyclooctatetraene-based?C₈H₈?organic molecules have electrochemical properties of high-capacity and high-voltage for sodium-ions storage.Fused molecules of C₈-C₄-C₈(C₁₆H₁₂)and C₈-C₄-C₈-C₄-C₈(C₂₄H₁₆)respectively have four and eight electron-deficient carbon,where multiple redox reactions can take place.Our calculations predict C₁₆H₁₂ and C₂₄H₁₆as sodium-storage materials exhibit discharge capacity of 525.3 and 357.2mA h g^-1,with the voltage change from 3.5-1.0 V and 3.7-1.3 V vs Na⁺/Na,respectively.Electronic structure analyses reveal that the high discharge voltages are attributed to superposed electrochemical reactions including double bond reformation and aromatization from carbon rings.High thermodynamic stability of these C₂₄H₁₆-based systems means a strong feasibility for experimental realization.The present work provides evidences that cyclooctatetraene-based organic molecules fused with C₄ring are promising candidate materials of high-capacity and high-voltage organic rechargeable cathode materials.