Research On Advanced Cathode Materials For Sodium Ion Batteries

Rechargeable lithium ion batteries have been considered as promising power source for the electric vehicles and the grid energy storage systems.However,lithium resources are limited and will restrict the huge applications of lithium ion batteries.It is of great significance to develop eco-friendly sodium ion batteries which employ abundant sodium resources.In this thesis,we aims to explore excellent cathode materials with high capacity,long cycle life and good rate performances for sodium ion batteries.1.We synthesized the nano P2-Na_2/3[Ni_1/3Mn_2/3]O₂ material by agarose based sol-gel method,and investigated its electrochemical performances when worked as cathode in SIBs between the voltage range of 2.0V⁴.0V and 2.0V⁴.5V.The Na_2/3[Ni_1/3Mn_2/3]O₂ material delivered a high specific capacity of 140 mAh/g in2.0V⁴.5V,however,with inferior cyclic stability.The specific capacity was 86mAh/g when cycled in 2.0V⁴.0V,with excellent cyclic stability.We further prepared a flexible and binder-free reduced graphene oxide（RGO）/Na_2/3[Ni_1/3Mn_2/3]O₂composite electrode combining the Na_2/3[Ni_1/3Mn_2/3]O₂ particles with RGO sheet by theπ-πbonds of RGO.This composite electrode presented high electrical conductivity and showed excellent rate performance with specific capacity of 58.2mAh/g at high current density of 10C.At current density of 1C the composite electrode still delivered specific capacity of 80 mAh/g after 200 cycles in 2.0V⁴.0V,indicating excellent cyclic performance.2.The cathode materials for SIBs should provide appropriate ion channels for sodium ions to pass through effectively.In this view,we synthesized a Na₄Fe（CN）₆/NaCl solid solution with open framework structure.The differences of the lattice parameters of NaCl and Fe（CN）₆^4-/3-are within 15%,suggesting that NaCl crystals could form a solid solution with Fe（CN）₆^4-/3-.The mutual substitutions of NaCl and Fe（CN）₆^4-/3-in solid solution left some vacancies thus forming inter-connected cages as commendable ion channels.When worked as cathode in SIBs,the solid solution demonstrated multiple charge/discharge voltage platforms with a high specific capacity of about 90 mAh/g at 3.9 mA/g.At a current density of90 mA/g,it delivered a specific capacity of 75 mAh/g and maintained capacity retention of 87%after 200 cycles.3.The open framework of prussian blue and its analogous have effective ion channels for insertion/extraction of sodium ions.In this thesis,we investigated the properties of Na₂MnFe（CN）₆（PBM）and Na₂NiFe（CN）₆（PBN）first,and then modulated their crystal structures as Na_1.76Ni_0.12Mn_0.88[Fe（CN）₆]_0.98（PBMN）which combined the advantages of PBM and PBN.During insertion/extraction of sodium ions,the Mn sites in PBMN exhibited the redox reactions of Mn^2+/3+to deliver high capacity,while the Ni sites kept unreactive to balance the structural disturbances.The PBMN delivered a high capacity of 118.2 mAh/g at current density of 10 mA/g,and also exhibited particularly excellent cyclic stability with 83.8%capacity retention after 800 cycles（100 mA/g）which is much better than PBM.The PBMN cathode and hard carbon anode were assembled together and packed in an aluminum plastic film to perform as original sodium ion full cell.The cell delivered an energy density of81.72 W h/kg and a power density of 90 W/kg at current density of 100 mA/g that could compete with the lead-acid batteries.The cell also showed an excellent cycle life of over 200 cycles with a capacity retention of 79%.4.There are two types of redox sites in the Na₂FeFe（CN）₆（PB）structure which are Fe^2+/3+and Fe（CN）₆^4-/3-.Fe（CN）₆^4-/3-with a C end in the low-spin state（Fe1）and Fe^2+/3+with an N end in the high-spin state（Fe2）.We firstly investigated the relationship between the electrochemical activity of the Fe1 sites and the coordinated water in PB,and then removed the coordinated water via electron exchange between the PB and the graphene oxide（GO）at high temperature of 220℃.The oxide group of GO acquired one electron from the Fe2 site of Fe2²⁺-（NC）₅（OH₂）in the PB to deviate from the GO.Then,the Fe2²⁺oxidized to Fe2³⁺and induced the redox reactions of Fe1^2+/3+through the C≡N bridge.Accompanying this process,the coordinated water in the Fe（CN）₆ vacancy was removed to form a cation vacancy to compensate the electric change in the PB framework.The reduced graphene oxide/PB with 6.2 wt%RGO content as RGOPC3,delivered high specific capacity of 163.3mAh/g（30 mA/g）and two voltage plateaus for Fe1 and Fe2,while the pure PB only exhibited one obvious voltage plateau for Fe2 and specific capacity of 131.2 mAh/g that is lower than RGOPC3.RGOPC3 also exhibited excellent cyclic stability and good rate properties,with specific capacity of 112 mAh/g at a current density of 800mA/g.After 500 cycles at 200 mA/g,the capacity retention of the RGOPC3 cathode still reached 91.9%.The excellent electrochemical properties of PB without coordinated water presented a good method for prussian blue analogous to enhance the performances in SIBs.5.The synthesis temperature of prussian blue materials can influence the valence states of Fe ions,crystal defects and cell sizes,thus resulting in the differences of battery performances.In this thesis,we synthesized the prussian blues（Na₂FeFe（CN）₆,PBs）at different temperatures firstly.With increasing synthesis temperature,the Fe（CN）₆^4-and Fe²⁺at reduction state is inclined to change to oxidation state,and then the content of the oxidation state of Fe atoms in PBs crystal structure increase.The cell sizes of PBs are also proportional to the synthesis temperature where becoming larger with higher temperature.All the above facts lead to the different electrochemical properites of PBs.The PB synthesized at low temperature delivered higher capacity,better rate and cyclic performances.Hence,the low temperature environment is beneficial to synthesize high performance prussian blues.