Study On The Modification Of Layered Cathode Material NaCrO₂ For Sodium-ion Batteries

Due to the scarcity and uneven distribution of lithium resources,its application in the field of large-scale energy storage was limited.Sodium and lithium are in the same main group of the periodic table and have similar chemical properties.With abundant resources,low price,and environmental friendliness,sodium-ion batteries were expected to be a potential replacement for lithium-ion batteries in the field of energy storage.Among them,the cathode materials are the key factor restricting the development of sodium-ion batteries.And layered transition metal oxides are a class of cathode materials that can easy to synthesize and have high capacity.In this paper,Na Cr O₂,an O3-phase layered oxide cathode material,was studied as the object of research.A systematic research work was carried out to study the rational modification of the material’s surface sensitivity and poor circulation,and the specific research contents and main conclusions are as follows:（1）Using PVDF and La₂（CO₃）₃·8H₂O as raw materials,the stable amorphous carbon-La F₃ dual-phase modified layer was formed in situ on the surface of Na Cr O₂ by two-stage heat treatment.The method greatly improved the electrochemical performance of the material while sacrificing less discharge capacity.The good electrochemical performance was attributed to the in-situ generation of the dual-phase modified layer on the surface of Na Cr O₂,which reduced the side reactions of active material and electrolyte,improved the surface/interface stability,air stability and electrical conductivity of Na Cr O₂.The important mechanism of the surface modification of the dual-phase modified layer in improving the structural stability of Na Cr O₂ and inhibiting the phase transition was revealed by various characterization methods.The electrochemical performance of the material after modification has been significantly improved.Under the voltage window of 2.3-3.5 V,Which has a specific discharge capacity of90.4 m Ah g^-1 at 2 C after 900 cycles.The capacity retention is 80.86%.However,after 400 cycles of Na Cr O₂,the discharge specific capacity is only 56.5 m Ah g^-1,and the capacity retention is 47.24%.（2）The polyanionic cathode material Na₂Fe PO₄F with strong inductive effect and high voltage resistance and amorphous conductive carbon were compositely modified and applied on the surface of Na Cr O₂.A high-voltage-resistant island-like conductive network modification layer was formed.Providing a high-voltage-tolerant cathode to mitigate the adverse effects of the main material at high voltage.While improving the interfacial stability of the cathode material,the interfacial electron and sodium ion transport characteristics were improved to alleviate the problems of the existing Na Cr O₂,which are prone to irreversible phase transformation and the rapid capacity decay caused by structural instability under high voltage.Research shows,the co-grid at the interface between the two-dimensional structure of the phosphate cathode material and the layered Na Cr O₂ provides a channel for sodium ion transport,which greatly enhances the kinetic performance of sodium ion transport.The comprehensive optimization of specific capacity,cycle stability and rate performance could be achieved by adjusting the compounding amount ratio.The electrochemical stability and sodium ion transport kinetics of the composite materials have been significantly improved,especially the NCO@5%NFPF/C of 5 wt.%Na₂Fe PO₄F/C composite Na Cr O₂,which was cycled at 2.3-3.7 V for 400 cycles under 5 C.The discharge specific capacity is 102.6 m Ah g^-1,the capacity retention is 85.93%,and the discharge specific capacity is as high as 105.3 m Ah g^-1 at 40C.The discharge capacity of Na Cr O₂ after 400 cycles is only 45.8 m Ah g^-1,the capacity retention is 34.88%,and the discharge capacity at 40 C is 44.5m Ah g^-1.This paper contains 42 figures,13 tables and 115 references.