| Alkali metal-ion batteries have been playing a critical role for the electrification of our economy Among them,lithium-ion batteries(LIB s)as ubiquitous power sources,dominate the portable energy storage market and are used in many electronic devices,including laptop,mobile phones,cameras,and electric vehicles.However,diversified requirements have rendered LIBs alone fall short of satisfying all the societal needs.Recently,to provide more sustainable energy storage solutions,progresses have been made on sodium-ion batteries(SIBs)and potassium-ion batteries(PIBs)as sodium and potassium as charge carriers are much more abundant than lithium,which offers promising prospects for large-scale grid energy storage.Electrode materials are one of the most important components for alkaline metal ion batteries.Designing electrode material structures with controllable nanostructures remains a major challenge to obtain high performance alkali metal ion batteries.LIB s,SIB s,and PIB s employ the rocking-chair mechanism,where charge carriers,Li+,Na+,and K+inserted/deinserted into/out of the electrodes.However,the related electrochemical reaction models or theories in LIB s cannot be simply used to explain the electrochemical behaviors in sodium and potassium ion batteries.Therefore,it is urgent to investigate the microstructural and phase evolutions in alkali metal ion batteries,and deeply understand the electrochemical reaction mechanism,ultimately providing theoretical guidance for designing high performance metal-ion batteriesIn this dissertation,todorokite manganese oxide nanorods with tunnel structure and the graphene-titanium oxide nanotubes with hollow structure can be controlled through the design and optimization of the synthesis method,and then applied to alkali metal ion batteries.The advanced in situ transmission electron microscopy(TEM)technology has been utilized to dynamically study the microstructure and phase evolutions of manganese oxide and graphene-titanium oxide nanotubes during the intercalation/extraction of alkali metal ions.The ion transport characteristics and energy storage mechanisms in alkali metal ions are investigated.The main innovative results are as follows(1)Controllable synthesis of todorokite manganese oxide nanorods with tunnel structure andthe graphene-titanium oxide nanotubes with hollow structure.Controllable synthesis oftunnel size-specific phases of todorokite-MnO2 has been preliminarily achieved withmanganese nitrate,sodium hydroxide and magnesium chloride as raw materials.Single-crystaltunnel-structured MnO2 nanorods with 4×3 tunnels(～9.2 × 6.9 A)have been corroborated via XRD,SEM,TEM and atomically resolved STEM imaging.In addition,TiO2 nanotubes were synthesized by hydrothermal method using commercial TiO2 powder and NaOH as raw materials.Afterward,the as-obtained Graphene-TiO2 composite was fabricated via the direct growth of few-layered graphene on TiO2 nanotubes with the aid of plasma-enhanced chemical vapor deposition(PECVD)in a facile and scalable fashion.(2)In situ TEM identifying the intercalation-conversion mechanism of todorokite manganese oxide for LIBs.By utilizing the ensemble of in situ HRTEM image,ED,EELS,DFT calculations coupled with C V test,we identified two distinct regions of intra-nanorod lithiation and a limited inter-nanorod lateral transfer of lithium.Upon initial lithiation,we observed the one-way motion of double lithiation reaction fronts that highlighted intercalation reaction and conversion reaction interfaces,respectively.The intercalation process led to the tunnel distortion and the radial(a-c plane)expansion,while the conversion reaction resulted in the formation of Mn metal and Li2O phases via an intermediate phase Mn2O3.During the first delithiation,an unusual reciprocating-motion delithiation RF was observed.The driven dynamics were revealed by employing a phase field model,indicating that the delithiated product could undergo a pre-nucleation stage directly associated with different degrees of lithium removal.After that,a reversible and symmetric phase transformation between Mn and Mn2O3 is established during subsequent lithiation-delithiation cycles.(3)In situ TEM identifying the structure and phase transition of todorokite manganese oxide for SIBs and PIBs.Upon the first sodiation,a two-step structure evolution has been revealed by fully utilizing the in situ HRTEM and DFT calculations:lattice a and c first unexpectedly exhibit synchronous contraction and then occur asymmetric expansion.By virtue of EELS and ED observations,todorokite MnO2 nanorods are found to undergo a unique multi-step phase transformation:MnO2→Na0.25MnO2→NaMnO2→MnO.Multiple sodium-ion extraction pathways were observed during the first desodiation,resulting that sodiation products of MnO and Na2O transformed to NaMnO2 phase.Moreover,potassium transport pathways in todorokite MnO2 have been dynamically tracked.However,ED and EELS indicate that the phase structure evolutions during the potassiation are different from the sodiation mechanism and similar to the lithiation mechanism.In the depotassiation,phase evolutions of todorokite MnO2 have been successfully revealed with atomic spatial resolution:Mn→Mn2O3→MnO.In addition,by tracking accurate morphology evolution during depotassiation,the fast growing nanosized pores embedded in the nanowire are identified to be the key factor that weakens the mechanical strength of the material and thus cause a rapid capacity decrease.(4)In situ TEM identifying the reaction mechanisms of TiO2 and Graphene-TiO2 nanotubes for PIBsDuring the first potassiation,Graphene-TiO2 nanotubes have an expansion of about 5%,which is much smaller than that of pure TiO2 nanotubes.In situ HRTEM reveales that the intercalation of potassium ions into the Graphene-TiO2 nanotubes would cause a slight expansion of the lattice fringes.ED indicates that the original phase of the Graphene-TiO2 nanotubes still remains,but a new phase of KxTiO2 formed.In addition,by applying a reverse voltage,the intercalated potassium ions can be gradually extracted,causing the shrink of nanotube.The potassiation/depotassiation cycle indicates that the Graphene-TiO2 nanotubes have good reversibility.Thus-derived PIBs exhibit a high reversible capacity of 332 mAh/g at 0.05 A/g and an unprecedented high-rate cyclic stability at 5A/g for 3000 cycles with a capacity fading of 0.008%per cycle. |
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