Preparation And Research Of Sodium Iron Pyrophosphate Cathode Material

Energy storage technology has received significant attention for portable electronic devices,electric vehicle propulsion,bulk electricity storage and the renewable sources,such as solar energy and wind power.This has led to the development of efficient,clean,and sustainable sources of energy as well as new technologies for energy conversion and storage.As an important class of rechargeable batteries,sodium-ion batteries?SIBs?have attracted great interest because of the large abundance and low cost of Na resources?Polyanionic cathode materials are being widely used in the SIBs owing to their low cost,stable structure,good thermodynamic performance and high safety.The 3D open frameworks,facile ion transport pathways and high operating voltage and electrochemical active of pyrophosphate make them promising candidates for energy storage and conversion.We synthesized Na_3.12Fe_2.44?P₂O₇?₂ cathode material by sol-gel method based on the progress of sodium iron pyrophosphate.Because of using iron powder as the iron source,the synthesis of materials cost is lower.And the sample particles synthesized by sol-gel method have good microstructure and electrochemical properties.The three-dimensional porous structure not only promotes the penetration of the electrolyte,but also provides a good electron/ion migration pathway during the charging and discharging.As a consequence,its electrochemical performance obtains a great enhancement.The sodium iron pyrophosphate Na_3.12Fe_2.44?P₂O₇?₂ presents a promising alternative for use as the cathode of sodium ion batteries.In this thesis,for the first time,we report the design of an ultrafast“flash-combustion synthesis”strategy to prepare coral-like Na_3.12Fe_2.44?P₂O₇?₂/C with good surface properties.The single-phase Na_3.12Fe_2.44?P₂O₇?₂ is successfully prepared in as short a time as three minutes.Each ultrafine Na_3.12Fe_2.44?P₂O₇?₂ particle with a nanoscale carbon coating is enwrapped in a microscale carbon matrix,forming a coral-like architecture.Benefiting from the hierarchical carbon decoration,the coral-like composite attains good surface properties,the coral-like composite delivers 107 and 88 mAh·g^-1at the 0.05 C and 2 C rates,and achieves capacity retentions of 96.0%and 95.3%at 0.2 C and 2 C.The low-cost sodium iron pyrophosphate Na_3.12Fe_2.44?P₂O₇?₂,which enables facile ion transport,used as the cathode of sodium ion batteries.However,high moisture sensitivity and severe surface oxidation lead to a large irreversible capacity and inferior electrochemical kinetics.The sodium intercalation chemistry and surface properties of coral-like Na_3.12Fe_2.44?P₂O₇?₂/C are investigated.The dynamic mechanism of the surface oxidation reaction is disclosed.Benefiting from the hierarchical carbon decoration,the coral-like composite attains good surface properties that enable low moisture sensitivity and fast sodium intercalation.It retains 95%of the initial capacity after 200 cycles alternating between 20 and 5 C rates.The clarification of the correlation between the surface properties and the sodium intercalation chemistry provides clues to the design and construction of high-performance Na_3.12Fe_2.44?P₂O₇?₂for large-scale applications.