Studies On O3-type Layered Oxide Cathode Materials For Sodium-ion Batteries

Over the past three decades,commercialization of the lithium-ion batteries（LIBs）has revolutionized the world.These batteries offer high energy density and reliable cycling performance which has enabled the development of a variety of portable electronic devices which are critical for the functioning of modern society.Further advances in battery technology have the potential to help address the global energy crisis and mitigate climate change.To achieve a sustainable yet high quality of life,our consumption of fossil fuels must be replaced by energy from renewable sources such as solar,wind and waves.Due to the scarcity of resources used to produce lithium-ion batteries,coupled with increasing demand for their utilization in electric vehicles,alternative battery technologies are required for large-scale energy storage systems.As such,Na-ion batteries,which exhibit similar chemistry to that of LIBs,are promising alternatives.Sodium is widely available and is the sixth most abundant element in the Earth’s crust,which presents an opportunity to significantly reduce the cost of cathode materials.Recently,layered transition metal oxides,Prussian blue analogues and polyanionic materials have been prominently reported as potential cathode materials for sodium-ion batteries.Among these materials,layered oxides（Na_xTMO₂,TM=transition metal ion（s）,such as Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Li,and a mixture of multiple of these elements）are currently dominating the field of research due to their high theoretical specific capacity and unique structural advantages.In this thesis,several rational design strategies are proposed for the synthesis of functional Na-ion layered oxide cathode materials,to achieve desired and enhanced electrochemical properties for specific applications.O3-type layered oxide materials are being considered as one of the most promising cathodes for Na-ion batteries owing to their higher capacity,however,they usually suffer from structural damage at the highly desodiated state.In the first part,to achieve the stable/high-capacity O3-type Na-ion cathodes,a series of Ni-rich O3-Na[Ni_xFe_yMn_1-x-y]O₂（x=0.6,0.7 and 0.8）oxide cathodes were successfully prepared and the phase transitions at high voltage were systematically investigated.Combined with the electrochemical measurements and structural characterizations,the structural transitions from O3 to O′3,P3,O3′′phases during the Na⁺（de）intercalation process were demonstrated in the voltage range of 2.0-4.2 V.Moreover,several reasons for the high-voltage capacity decay are revealed:1)the thermodynamic instability of high-voltage phase due to less Na⁺in the crystal structure;2)large volume change during the high-voltage phase evolution with inferior Na⁺diffusion kinetics;3)formation of microcracks and cathode-electrolyte interphase on the surface of cathode particles.To address the above issues,a reasonable upper cut-off voltage of 4.0 V was set to prevent the formation of O3′′phase and reduce electrolyte decomposition,which leads to a high reversible capacity of～152 m Ah g^-1(～467 Wh kg^-1)with a superior capacity retention of～84% after 200 cycles at 0.5C,showing great Na storage performance.This work provides insights on the relationship of the structure-property for the further development of high-performance Ni-rich O3-type Na-ion cathodes.Material innovation on high-performance Na-ion cathodes and the corresponding understanding of structural chemistry still remain a challenge.In the second part,we report a new concept of high-entropy configuration to design layered oxide cathodes for Na-ion batteries.An example of layered O3-type Na Ni_0.12Cu_0.12Mg_0.12Fe_0.15Co_0.15Mn_0.1Ti_0.1Sn_0.1Sb_0.04O₂ has been demonstrated,which consists of nine-component ions in TM site having various oxidation states from bivalent to pentavalent,was prepared initially as a proof of concept.The high-entropy structural stabilization enables this cathode material super cycling stability and much enhanced Na-storage capacity.This material showed better capacity retention at different current rates,in which after 500 cycles,about 83%of the capacity is still maintained,much higher than those of the reported small-component cathodes.Moreover,the highly reversible phase-transition behavior was observed between O3and P3 phase transition during the charge-discharge process,and most importantly,this behavior is delayed effectively with more than 60%of the total capacity being stored in O3-type region,directly against the representative O3-type Na-ion cathodes in literatures.Possible origin can be owing to the multicomponent elements in this high-entropy matrix which is able to accommodate the changes of local interactions during Na+（de）intercalation process.The results from the high-entropy strategy give more insights for developing new layered cathode materials.The irreversible consumption of sodium at the anode side during the first cycle prominently reduces the energy density of Na-ion batteries.Various different sacrificial cathode additives have been recently reported to address this problem,but critical issues such as by-products（e.g.,CO₂）release during cycling and incompatibility with current battery fabrication procedures potentially deteriorate the full-cell performance and prevent the practical application.In the third part,we propose an additive-free self-presodiation strategy to create lattice-coherent but component-dependent O3-Na_xTMMn O₂ cathodes by a quenching treatment rather the general natural cooling.The quenching material preserves higher Mn³⁺and Na⁺content,which is able to release Na⁺via Mn³⁺oxidation to compensate for sodium consumption during the initial charge while adopting other TM to provide the capacity in the following cycles.Full cells fabricated with hard carbon anode and this material as both cathode and sodium supplement reagent have a nearly 9.4%cathode mass reduction,around 9.9%energy density improvement(from 233 to 256 Wh kg^-1)and 8%capacity retention enhancement（from 76%to 84%）after 300 cycles.This work presents the route to rational design cathode materials with sodium reservoir property to simplify the presodiation process as well as improve the full-cell performance.