Studies On Fe-Mn Based Layered Oxide Cathode Materials For Sodium-Ion Batteries

In recent years,the development of new energy technologies has become an important issue related to the fate of mankind.As a promising alternative for lithium-ion batteries（LIBs）,sodium-ion batteries（SIBs）have been attracting renewed attentions due to the infinite sodium sources and great potential for large-scale energy storage.Considering the theoretical gap between the energy density of SIBs and LIBs,it is widely suggested to develop high-performance and low-cost SIBs systems by equipping suitable electrode materials so as to improve the competitiveness.Notably,Fe-Mn based layered oxide come to be one of the best choices of cathode materials owing to their high capacity and earth-abundant ingradients.However,problems such as low practical capacity,poor cycling performance,slow kinetics,and sensitivity to humidity still restrict the application development of Fe-Mn-based layered cathodes.In the first part of this thesis,taking O3-Na_xFe_1/2Mn_1/2O₂ as a typical material,the capacity fading mechanism of Fe-Mn based layered oxide cathode materials is systematically investigated.Based on ex-situ XAS,M?ssbauer spectroscopy,in-situ XRD and DEMS measurements,it shows that Fe ion migration is majorly triggered by the accumulation of Jahn-Teller active Fe⁴⁺and the induced structural distortion of the local environment of Fe.The irreversible migration of Fe ions suppress O3-P3phase transition during cycling,causing capacity loss.More importantly,within the voltage window where Fe ion migration does not occur,the severe capacity decay is mainly caused by the Fe⁴⁺activated and Mn-dissolution aggravated surface passivation.Replacing Fe with low-valent ions is one of the potential strateges to solve the problems of Fe-Mn based layered cathode materials.Among various choices of cation,Cu²⁺is eye-cathing owing to its low cost and electrochemically activity.Hence,in the second part,the impacts of Cu substitution on the electrochemical behaviors of Fe-Mn based layered oxides are studied in details.Firstly,the solid solubility of Cu in Cu-Fe-Mn system can be limited by the increasing proportion of Mn⁴⁺due to the large difference of ionic diameter.Secondly,the introduction of Cu can improve the cycling stability of Fe-Mn based layered oxides by mitigating the formation of Mn³⁺as well as the dissolution of Mn.Further research on the Cu-Mn binary system shows that the limited reduction of Mn may be due to the strong Jahn-Teller effects of Cu²⁺which aggravate P2-P’2 phase transition and thereby limit the depth of sodium intercalation.In addition,it is proposed to directly prepare Cu-Fe-Mn based oxides by utilizing natural chalcopyrite as precursor so as to further reduce the cost of materials production.The as-obtained material show a specific energy exceeding 340 Wh/kg and a capacity retention of 94%after 100 cycles at1C,demonstrating the feasibility of this new strategies.In the third part,the formation mechanism of P2/O3 biphasic structure is studied in great details.It is elucidated that,the P2/O3 biphasic structure is originated from the compositional non-uniformity and the induced inhomogeneity of local cation potential,which is essentially caused by the thermodynamic/kinetic factors during solid-state reactions.Then,a universal strategy for preparing P2/O3 biphasic materials is proposed,namely by designing a“critical cation potential”and regulating the“composition entropy”.Notably,the successfully obtained P2/O3intergrown Na_0.7Ni_0.2Cu_0.1Fe_0.2Mn_0.5O_2-δexhibits a highly reversible capacity of over 110 m Ah/g within 4.05-2.0 V,a capacity retention of 54%at 20 C,and a capacity retention of 84%after 500 cycles at 5 C,which are much better than P2 and O3 single-phase analogues.Coupled with soft carbon anode,the full battery give a specific energy of 190 Wh/kg,showing good potential for practical applications.Moreover,the"synergistic effect"of P2/O3 biphasic structure is further attributed to the competitive reactions between P2,O3,and their intermedia phases.