Robust Biomass-Derived Carbon Frameworks as High-Performance Anodes in Potassium-Ion Batteries

Potassium-ion batteries (PIBs) have become one of the promising candidates for electrochemical energy storage that can provide low-cost and high-performance advantages. The poor cyclability and rate capability of PIBs are due to the intensive structural change of electrode materials during battery operation. Carbon-based materials as anodes have been successfully commercialized in lithium- and sodium-ion batteries but is still struggling in potassium-ion battery field. This work conducts structural engineering strategy to induce anionic defects within the carbon structures to boost the kinetics of PIBs anodes. The carbon framework provides a strong and stable structure to accommodate the volume variation of materials during cycling, and the further phosphorus doping modification is shown to enhance the rate capability. This is found due to the change of the pore size distribution, electronic structures, and hence charge storage mechanism. The optimized electrode in this work shows a high capacity of 175 mAh g^{-1} at a current density of 0.2 A g^{-1} and the enhancement of rate performance as the PIB anode (60% capacity retention with the current density increase of 50 times). This work, therefore provides a rational design for guiding future research on carbon-based anodes for PIBs

Developing Synchrotron-based Techniques for Characterising Anode/Electrolyte Interface in Li/Na Batteries

With the increasing demand for lower carbon emissions for the mitigation of global warming, Li/Na battery systems are becoming increasingly ubiquitous in both industries and our daily life. The development of electric vehicles (EV), hybrid electric vehicles (HEV) and mobile devices has put forward the requirements for Li/Na batteries with higher energy density, power density, safer and longer operation time. These factors are significantly affected by the interfacial behaviour at electrolyte/anode interface. Therefore, understanding the interface is very important in improving the performance of Li/Na battery systems. However, the direct characterization and investigation of the anode/electrolyte interface are hard due to the buried and heterogeneous nature of the interface. Owing to the wide energy range, high brightness and flux of the beam provided by synchrotron radiation, an unprecedented opportunity was present to obtain new insights into the material chemistry of the interface. The synchrotron-based techniques were used across the entire thesis, which could provide a deeper understanding of the interfacial behaviour between electrolyte and anode in Li/Na batteries. In the thesis, the main area of focus will include the application of ex situ and in situ synchrotron radiation-based techniques in the study of the anode/electrolyte interface in Li/Na batteries and the broader design of in situ cells. Based on these studies, the interface behaviour between anodes and different electrolyte systems in Li/Na batteries was investigated, especially the chemical evolution. The developed synchrotron radiation-based techniques and designed specialized in situ cells can be further utilized in understanding other battery systems, not limited to electrode/electrolyte interface but also changes in other components of cell configuration during the operation of batteries. In Chapter 4, two types of sp2 carbon-based polymer anode were synthesised which show similar chemical structures and different topological structures. The influence of the topological structures on the Li+ storage at the liquid electrolyte/anode interface was investigated by the Near-edge X-ray absorption fine structure (XANES) of C k edge. The potential of such polymer anode was proved by functionating with the -SO3H group, delivering a higher storage capacity. To further boost the energy density of Li-ion batteries by directing using Li metal anode, PEO-based composite polymer electrolytes with the addition of Lithium Bis(trifluoromethanesulphonyl)imide (LiTFSI) and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) fillers were proposed in Chapter 5. The addition of fillers improves the ionic conductivity, electrochemical stability, and contact with Li metal. Combing with the electrochemical tests and in situ X-ray imaging techniques, the role of fillers on the interfacial behaviour between composite polymer electrolytes and Li anode was investigated. In Chapter 6, TiO2 incorporated Na3Zr2Si2PO12 (NZSP-TiO2) solid electrolyte was synthesised to inhibit the metal filament formation during plating at ceramic/metal anode interfaces. The NZSP-TiO2 electrolyte shows an improved density and better cycling stability. An in situ X-ray imaging experiment was designed to investigate the role of TiO2 on the electrochemical behaviour between solid electrolyte and Na metal anode during the cycles

TiO₂ as second phase in Na₃Zr₂Si₂PO₁₂ to suppress dendrite growth in sodium metal solid-state batteries

Solid-state sodium–metal batteries will not achieve reasonable power density without electrolytes that solve the dendrite (filamentation) problem. Metal-filament formation during plating at ceramic/metal interfaces can cause electrical failure by internal short-circuit or mechanical failure by electrolyte fracture. Herein, an Na3Zr2Si2PO12 (NZSP) sodium-ion-conducting NASICON electrolyte in which TiO2 is incorporated as an additive is presented, leading to a two-phase composite NZSP(TiO2) with improved density, Young's modulus, hardness, grain structure, and bulk permittivity. These features of NZSP(TiO2) suppress dendrite growth along grain boundaries, microcracks, and micropores. As well as demonstrating ultralow ceramic/Na kinetic resistance with electrochemical measurements, X-ray photoelectron spectroscopy is performed to probe interfacial reaction mechanisms. The TiO2 phase forms within grain boundaries and along NZSP surfaces. This modifies the two-phase material's microstructure and improves its electrochemical performance, while also increasing the critical current density for dendrite formation. Design guidelines are discussed to mitigate microscopic defects and dendrites in two-phase ceramic electrolytes

The decisive role of CuI-framework O binding in oxidation half cycle of selective catalytic reduction

Cu-exchanged zeolite is an efficient catalyst to remove harmful nitrogen oxides from diesel exhaust gas through the selective catalytic reduction (SCR) reaction. The SCR performance is structure dependent, in which a Cu with one adjacent framework Al (1AlCu) has lower activation energy in oxidative half-cycle than Cu with two adjacent framework Al (2AlCu). Using a combination of operando X-ray absorption spectroscopy, valence to core - X-ray emission spectroscopy and density functional theory calculations, here we showed that 1AlCu proceeds with nitrate mechanism, in which side-on coordination of O2 at a CuI(NH3)xOfw (fw = framework) is the rate-limiting step in the oxidation half-cycle. As a result, the CuI(NH3)xOfw at 1AlCu can easily yield a transient CuIINOx intermediate upon breaking of Cu-Ofw after interaction with NO. In the meantime, 2AlCu has high barriers for Cu-Ofw bond breaking and proceeds with dimer mechanism. Our results show the coexisting of both dimer and nitrate mechanism, in particular at high Cu loadings, in which controlling the strength of the Cu-Ofw coordination is key for the O-O split in the nitrate pathway

Effect of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ Fillers on the Interfacial Properties between Composite PEO-LiTFSI Electrolytes with Li Metal during Cycling

PEO-LiX solid polymer electrolyte (SPE) with the addition of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) fillers is considered as a promising solid-state electrolyte for solid-state Li-ion batteries. However, the developments of the SPE have caused additional challenges, such as poor contact interface and SPE/Li interface stability during cycling, which always lead to potentially catastrophic battery failure. The main problem is that the real impact of LLZTO fillers on the interfacial properties between SPE and Li metal is still unclear. Herein, we combined the electrochemical measurement and in situ synchrotron-based X-ray absorption near-edge structure (XANES) imaging technology to study the role of LLZTO fillers in directing SPE/Li interface electrochemical performance. In situ XRF-XANES mapping during cycling showed that addition of an appropriate amount of LLZTO fillers (50 wt %) can improve the interfacial contact and stability between SPE and Li metal without reacting with the PEO and Li salts. Additionally, it also demonstrated the beneficial effect of LLZTO particles for suppressing the interface reactions between the Li metal and PEO-LiTFSI SPE and further inhibiting Li-metal dendrite growth. The Li|LiFePO4 batteries deliver long cycling for over 700 cycles with a low-capacity fade rate of 0.08% per cycle at a rate of 0.3C, revealing tremendous potential in promoting the large-scale application of future solid-state Li-ion batteries