Design Of Several Oxide Electrode Materials With Enhanced Ionic And Electronic Transport Properties

The excessive consumption of fossil fuels in vehicles and the corresponding exhausts bring serious environmental issues,pushing forward the development of clean and renewable energies.Electrochemical energy storage devices,which store and release energies via electrochemical reactions,merit much attention as power sources due to their high performances and low pollution.Among them,Li-and Na-ion batteries exhibit high energy densities and relatively low power densities.As a result,they have achieved rapid development in portable electronic devices,while their use in large scale equipment such as electric vehicles is restricted.In contrast,supercapacitors deliver extraordinary high power densities but with low energy densities,which also has difficulty in powering huge devices.These performances are highly subject to the kinetics of ion and electron transports in electrodes.Accordingly,improving the ionic and electronic transports by tunning electrode materials are key in achieving high energy and power densities simultaneously.Metal oxides are commonly used as active materials in Li-and Na-ion batteries as well as supercapacitors.Energies are stored via reversible redox reactions occuring at their surface or throughout the bulk,which makes metal oxides possess attractive virtues such as high theoretical capacity,low cost,and low toxicity.However,compact atomic structures in metal oxides usually hinder the rapid,effective,and reversible flows of ions,especially for large sized ions like Na-ion.Meanwhile,most metal oxides are semiconductors or insulators with poor intrinsic conductivities,which also depresses their capacity and rate performance.To address these issues,nanostructured hybrid electrodes have been widely investigated,which present high surface area and thus shorten transport distances of ions.Moreover,conductive media are combined with metal oxides to improve their electronic conductivities,while nanostructured particles need binders to ensure the stabilities.As a consequence,large areas of interfaces induced by these units result in the high contact resistance and the block of superior electrode performances.In this work,we propose methods of alloying,tuning allotropic structures,and manipulating oxygen vacancies to enlarge the diffusion channels of vanadium and manganese oxides by using first-principles calculations,which is beneficial to the Na-ion transport.Meanwhile,their electronic structures are modified to improve intrinsic conductivities,and semi-coherent or coherent interfaces between active materials and conductive media are formed to reduce contact resistance.The main contents are divided into three parts:1.Although α-V₂O₅ exhibits a high theoretical capacity,confined sizes of diffusion channels lead to high diffusion energy barriers for Na-ion.Moreover,it is a semiconductor with a large band gap.In order to improve its poor rate performance as electrodes for Na-ion batteries or pseudocapacitors,we demonstrate that hydrogen incorporation in α-V₂O₅ effectively modifies the channel size and electronic structures by forming O-H bonds.Based on first-principles studies,transformations of H_xV₂O₅ at x = 0.5 5 from α-V₂O₅ are all exothermic processes.Among various structures of hydrogen-incorporated α-V₂O₅,H₂V₂O₅ presents enlarged diffusion channels along both [010] and [001] directions,where the corresponding diffusion energy barriers decrease 34.93% and 41.81% respectively.As incorporated H atoms bring defective energy levels in the band gap,H₂V₂O₅ retains metallic properties throughout the Na-intercalation process.Moreover,due to the larger space for Na-intercalation in H₂V₂O₅,the volume expansion decreases to 4.63%.Therefore,H₂V₂O₅ shows enhanced rate performance and cyclability.2.Among the various allotropic structures of MnO₂,δ-phase is a layered structure while α-phase consists of one dimensional channels.Both of them have interstitial sites large enough for accommodating and transporting Na-ion.By using first-principles calculations,diffusion energy barriers for Na-ion in δ-MnO₂ are 0.053 and 0.052 eV,while 2 × 2 and 1 × 1 channels of α-MnO₂ shows much higher values（0.099 and 0.549 eV respectively）.Aiming at enhancing the electronic transport property,the model of Au/MnO₂ hybrid electrode is constructed by considering nanoporous Au as conductive media.It is found that a semi coherent interface can be formed between Au and δ-MnO₂ where the orientation of δ-MnO₂ is favorable for Na-ion transport.In contrast,the semi coherent interface model of Au/α-MnO₂ is not conductive to the Na-ion transport direction.Even if the Au/α-MnO₂ heterostructure is formed with the most ideal orientation for Na-ion transport,the mass of Na-ion available for the transport is only the half of that in Au/δ-MnO₂.By adjusting the electrodeposition method,nanoporous Au/δ-MnO₂ and Au/α-MnO₂ electrodes were fabricated respectively.The experimental results are consistent with theoretical predictions,confirming the enhanced Na-ion transport by tuning allotropic structures.3.c-V₂O₃ with the corundum structure exhibits a metallic property,which belongs to electron-correlated oxides.As the relatively small proportion of volume and mass,as well as the additional contribution of capacity,it can be used as a low-cost conductive medium material.First,we investigate the electronic transport property of the c-V₂O₃/λ-MnO₂ interface structure.Based on first-principles calculations,V-O-Mn chemical bonds are formed at the c-V₂O₃（001）//λ-MnO₂（111）interface,improving the stability of the heterostructure.Moreover,electrons transfer from c-V₂O₃ to λ-MnO₂ which make the electronic structure of λ-MnO₂ near the interface transforms into the semi-metallic state,suggesting the reduction of contact resistances.In experiments,the fabricated three dimensional bicontinuous nanoporous c-V₂O₃/λ-MnO₂ electrode shows the excellent conductivity,according well with theoretical results.Then,we investigate c-V₂O₃/λ-MnO₂ heterostructure formed by the oxidation of c-V₂O₃,which is constructed as a seamless conductive media/active materials electrode system for further improving Na-ion and electron transports.The orientation relationship of c-V₂O₃（012）//r-VO₂-x（011）conforms to the phase transition from c-V₂O₃ to r-VO₂.However,the mismatch of 8.08% along [100] directions of both structures leads to the formation of oxygen vacancies with an ordered planner arrangement in r-VO₂ to maintain the interface coherency.According to the period of planner oxygen vacancy sites,the new phase is defined as V2n+2O4n+3.Within first-principles studies,only when n is 1 and 2,the mismatches with c-V₂O₃ are low enough for the coherent interface.Furthermore,as neighboring rutile slabs separated by the oxygen vacancy plane slightly slip,the original 1 × 1 rectangle channels transform into enlarged ones with the hexagonal shape.Consequently,intercalation energies of Na-ion are reduced and extremely low diffusion energy barrier are obtained,which are 0.024 eV（n = 1）and 0.019 eV（n = 2）respectively.In addition,V2n+2O4n+3 achieves superior electronic conductivities at both interface and bulk regions.The fabricated pseudocapacitor based on nanoporous c-V₂O₃/r-VO₂-x electrodes realizes exceptionally high power and energy densities.The corresponding x is 0.22,suggesting the hybrid phase of V2n+2O4n+3 with n = 1 and 2 indeed improves electrode performances.