Structural Modulations And Electrochemical Investigations Of Polyanion-type Lithium/Sodium Ion Batteries Cathode Materials

The rechargeable lithium/sodium-ion batteries have been intensively applied in portable electronic devices and now expanded to the applications in new energy vehicles due to the high energy density, high specific capacity and long cycle life. In fact, cathode materials are one of the most important bottlenecks for rechargeable ion batteries. Therefore, to explore new cathode materials, modulate structure, optimize their electrochemical performance, understand the correlation between structure and performance are highly desirable for both fundamental researches and specific applications. In this thesis, we mainly focused on the structural modulation and electrochemical performance of polyanion-type cathode materials LiFePO4 and Na3V2CPO4)3 for lithium-ion batteries and sodium-ion batteries, respectively. Some useful results and understanding were obtained, listed as following:1. A new olivine LiFePO4(LFP)@C/reduced graphene oxide(RGO) nanocomposite was synthesized by a solvothermal method. In this hybrid nanocomposite, the cooperation of the LiFePO4 with a carbon coating and the reduction of the graphene oxide exhibited to produce an effective three-dimensional (3D) conductive network. Compared with conventional LFP@C nanocomposites, the experimental results of LFP@C/RGO nanocomposites posed an outstanding electrochemical performance with higher rate capability and better cycability. In particularly, no obvious capacity fading after 200 cycles at a current of 10 C was observed and a discharge capacity of 119 mAh/g could be remained at 20 C, indicating that RGO-enhanced electronic conductivity of electrode particlesmay play a key role in large power sources when applied in industrial productions, such as electric vehicles.2. Olivine-type LiFe1-xMnxPO4 compounds with different Mn doping ratios were systematically synthesized by a solvothermal method. Synchrotron radiation X-ray absorption spectroscopy combined with first-principles calculations and energy-dispersive Xray spectroscopy measurements revealed that the as-prepared LiFe1-xMnxPO4 samples contained two different phases:LiFePO4 and LiMnPO4. Actually, due to the crystal field effect, the two structures wererandomly stacked and characterized by a pronounced structural distortion of the MO6 (M:Fe orMn) octahedra. Moreover, increasing the Mn doping concentration, the distortion of the MO6 octahedra increased. Considering of the size of LiMnPO4 stacks and the distortion of MO6 octahedra, the best performance were obtained viatuning the optimal Mn doping concentration. Among the different Fe/Mn ratios,the electrochemical tests showed that the as-prepared LiFeo.75Mno.25P04 sample exhibited the best electrochemical performance.The study provide solid experimental view for designing high energy density storage system.3. A simple sol-gel method was developed to prepare Na3V2(PO4)3@C porous microspheres network for high performance SIBs, which can realize NVP nanoparticles successfully embedding into acetylene carbon (SP) and multiple walled carbon nanotubes (MCNTs) matrices. The electrochemical characterizations demonstrated that both Na3V2(PO4)3/SP and Na3V2(PO4)3/MCNTs coulde show excellent cell performance with high rate ability and stable reversibility, especially for the NVP/MCNTs sample. In particular, as high as 112.8 mAh/g and 70 mAh/g of the discharge capacity for NVP/MCNTs system were delivered at 0.2C and 10C, respectively. Furthermore, a good capacity retention of 90% can be maintained after 400 cycles at 1C rate. Moreover, the enhanced electrode performance is ascribed to the contribution from both of the porous microspheres morphology and functional carbon matrices, which can favor the migration of both electrons and ions.4. A thin-layers graphene-encapsulated Na3V2(PO4)3 composite (Na3V2(PO4)3 was synthesized by self-assembly of surface modified NVP and graphene oxide and then followed by reduction to compensate the intrinsic low electronic conductivity of NVP and strengthen its structurestability. The as-synthesized hybrid composite as cathode for Sodium-ion batteries(SIBs) exhibited excellent high specific capacity and superior rate performance with discharge capacities of 115.2 mAh/g at 0.2 C and 70.1 mAh/g at 30 C. An excellent cycling stability was noted with about 86.0% capacity retention at 5 C after 300 cycles. Ex-situ X-ray absorption spectroscopy (XAS) characterization further confirmed the local geometrical environment around vanadium highly conserved during the sodiation/desodiation process, associated with an electrochemical active V3+/V4+ redox couple. Hence the as-prepared hybrid composite can may act promising cathode materials for high-rate SIBs, thanks to the effect interface interaction between NVP nanoparticle sand grephene films.