Study On The Energy Storage Properties Of Titanium-Based Polyanionic Anode Materials

With the forced shift of the global attention towards clean energy sources such as wind and solar,it is of great significance to develop advanced energy storage technologies.In the past few decades,lithium-ion batteries?LIBs?,as the leading energy storage device,has been witnessed great success regarding fundamental research as well as practical usefulness.However,lithium is a scarce element in Earth's crust,which makes it hard to support the ever-increasing demand of LIBs for grid-scale energy storage.Indeed,several researchers have predicted that lithium may be even more expensive than gold before petroleum gets exhausted.In contrast,sharing the similar physical and chemical properties with lithium,sodium and potassium elements present the natural abundance in Earth's crust and ocean,which are also easy to extract.Therefore,sodium-ion batteries?NIBs?and potassium-ion batteries?KIBs?are considered to be the ideal alternatives as beyond-lithium energy storage systems.Note that,for NIBs and KIBs,the operation principle is also“rocking-chair”mechanism,which means that the experience in exploring LIBs can be adopted,making the development of new energy storage devices more efficient.However,the much larger ionic radii of sodium ion and potassium ion make it much harder to find suitable host materials.Therefore,the exploration of high-performance electrode materials is the main scientific issue that needs to be addressed for the future application.Among the well-developed electrode materials,NAtrium Super Ionic CONductor?NASICON?-type polyanionic materials,with a general formula of AMM'?XO₄?₃,possess open framework and large ion-diffusing channels,which are considered to be promising anode materials for rechargeable batteries.Nevertheless,their insulation property and the large molecular weight penalty are the main bottlenecks that confine their rate capability and theoretical capacity,respectively.In the scenarios of KIBs,the situation is much more severe since the larger ionic radius make the K⁺very“picky”for the selection of electrode materials.To solve these issues,in this thesis,we designed and prepared several Ti-based polyanionic materials in the following levels:functional design;decoration of the surface/interface properties;nanomanufacturing.As the anode materials for NIBs and KIBs,their electrochemical properties have been comprehensively studied,as well as the energy storage mechanism.The main conclusions are summarized as follows:Firstly,we synthesized Ca_0.5Ti₂?PO₄?₃@C composites as anode material for NIBs by a sol-gel method.By introducing vacancies in alkaline earth metal sites,we can manually increase the intercalation sites in the crystal structure and thus enhance the specific capacity of the material.Between 3.0 and 0.01 V,the electrode delivered a reversible capacity of 264 mA h g^-1,corresponding to 4 mol sodium ions inserting into/extracting from the structure,which is higher than the well-known NASICON-type material,NaTi₂?PO₄?₃(208 mA h g^-1).After 30 cycles,the capacity retention is 99%.Meanwhile,the conductive carbon matrix in the composite can facilitate fast electron transfer during the electrochemical process.As a result,the composite exhibited superior rate capability:under a current rate of 2 A g^-1,a capacity of 92 mA h g^-1 can be de delivered;as the current rate increase from 6 A g^-1 to 20 A g^-1,the capacity maintains nearly unchanged.From the kinetics analysis,we can draw the conclusion that the excellent rate performance can be attributed to the pseudocapacitive effect of the composite.Secondly,we designed and prepared AgTi₂?PO₄?₃ as anode material for NIBs,and comprehensively studied the sodium storage properties as well as the reaction mechanism between 3.0 and 0.01 V.During the galvanostatic charge&discharge process,a reversible capacity of 193 mA h g^-1 can be achieved,with the decent cycling stability.From the in situ XRD results,the replacement reaction has been evidenced in the initial discharge process,forming Ag and NaTi₂?PO₄?₃.During the following cycles,the electrochemical reaction is dominant by the?de?intercalation of sodium ions in NaTi₂?PO₄?₃,while Ag does not participate in the reversible redox reactions.The electrochemical“in situ”Ag nanoparticle coating can effectively enhance the electronic conductivity of the electrode.Besides,the ex situ TEM images at different state of charge demonstrate the similar results.The electrochemically“in situ”coating phenomenon can provide a new avenue to enhance the conductivity of NASICON-type materials.Thirdly,we synthesized hierarchical KTi₂?PO₄?₃@C nanocomposites through electrospray method,which are secondary micro-shperes assembled by primary nanoparticles.The hierarchical structure can provide short ion-diffusion channel,large specific area and porous structure,and thus provide superior sodium and potassium storage properties.Between 3.0 and 0.01 V,the reversible capacities in NIBs and KIBs are 284,292 mA h g^-1,respectively.Furthermore,the material demonstrated excellent rate and long cycling performance:At current rates of 10 A g^-1 and 1 A g^-1,the electrode delivered reversible capacities of 136 mA h g^-1 and 131 mA h g^-1 for sodium and potassium storage,respectively.According to different scan rate CV tests and GITT analyses,sodium and potassium ions share the similar diffusion process in KTi₂?PO₄?₃lattice.However,the diffusion coefficient of sodium ions are 10 to 100 times higher than that of potassium ions,which results in the difference in the two systems.Lastly,we extended the application of KTi₂?PO₄?₃@C nanocomposites from organic electrolyte to aqueous electrolyte.For the first time,we used the potassium acetate,which is low-cost and nontoxic,as the solvent to make a“water-in-salt”aqueous electrolyte.The electrolyte exhibited a long stable voltage window,3.2 V,which is much wider than that of the general aqueous electrolytes.Facilitated by this expanded electrochemical window,KTi₂?PO₄?₃@C nanocomposites exhibited highly reversible redox behavior in this electrolyte,which otherwise only works in non-aqueous electrolytes.Besides,the electrode shows a 0.2 V lower voltage hysteresis than in non-aqueous electrolytes,suggesting a better energy efficiency.Under a current rate of 200 mA g^-1,the electrode demonstrated a capacity of 62 mA h g^-1,which is almost the theoretical value.Besides,the electrode showed superior rate and long-term cycling performance:at 1 A g^-1,the capacity retention reached 69%after 11000 cycles.In sum,in this thesis,we designed and prepared several Ti-based polyanionic materials,thanks to the modifiability of the molecule of NASICON materials.We then used several tools to comprehensively investigate the energy storage properties as well as mechanism of these materials.To solve the low electronic conductivity and low theoretical capacity issues of these materials,we improved them by structural designing,surface/interface modifying and nano-manufacturing.We also extended the application of these materials to multiple types of energy storage devices.We hope that the presented work can provide strong insight into the design and optimization of NASICON-structured Ti-based polyanionic anodes.