Preparation Of Cathode Materials For Lithium-ion Battery And Its Performance

The lithium-ion battery (LIB), as one of the most extensive applied secondary battery,and most promising secondary battery for electric vehicle application, has been intensivelyresearched due to its superiors advantages, e.g. high voltage flat, high specific capacity, highenergy density, no memory effect, longer cycle and calendar life, low cost, environmentfriendly and so on. Cathode materials are the key materials for LIBs, they not only play thecrucial role for the performance of LIB, but also do a significant effort to reduce the cost ofLIBs, and realizing the commercialization of electric vehicles. In this thesis, a series ofcathode materials, including LiFePO₄, Li₃V₂(PO₄)₃and Li₂MnSiO₄, has been prepared by aspray drying method with organic additive as assistant reagent and carbon source. Theelectrochemical properties and battery performances of these materials have been investigatedintensively, and the materials were characterized by SEM, XRD, CV, EIS etc. Moreover, westudied their performance in aqueous electrolyte solution and explored the possibility ofconstructing a rechargeable LIB based on aqueous electrolyte solution.To avoid the using of inert gas in large amount, we tried to calcine the cathode materialsin a carbon bath, instead of protected with inert gas. Combined with solid preparation process,a decent performance LiFePO₄/C composite was successfully prepared with characters ofuniform and smaller grain size and high specific capacity of the initial discharge. The capacityis140.4mAh/g, and coulomb's efficiency of first time is98.5%, the decay of capacity ofsample for40cycles is only3.3%, confirmed the feasibility of calcining sample in carbonbath without protection of inert gas.A novel "wet chemical method-spray drying"(WCM-SD) process was invented andinvestigated. By adding different types of organics, a series of high performance LiFePO₄/Ccomposites with various morphology were prepared by the process, the effects of the type andadded amount of organic materials, the composition and concentration of the slurry, theparameters of spray drying process, the calcining temperature and time, on the structure andperformance of material were investigated. It was found that the organic additive influenceboth the morphology and micro-structure of material significantly, a uniform LiFePO₄/Cmicrospheres with micro-nano structure were prepared when polyvinyl alcohol (PVA) wasused as organic additive; the more the organic materials was added, the more the carbonformed in final materials, and appropriate amount of carbon will results to high performanceof the materials. The optimum calcination temperature is ca.750℃, and the optimum addedamount of PVA is ca.200g/mol, the sample prepared with the new process and with optimum conditions could reach the initial capacity of166.5mAh/g at0.1C.To minimize the organic additive, we substituted part of carbon source by carbon black,denoted as organic/inorganic hybrid carbon source materials. A good result has achieved forthe sample by using this hybrid carbon source materials, such as uniform nanoscale primaryparticles, high specific surface area, mesoporous structure, excellent conductivity, etc., whichmay provide fast ion (Li+) and electron (e-) channels during charge/discharge process. Thesamples prepared with hybrid carbon source showed a theoretical upmost capacity at0.1Cand excellent cycling performance at various rates. A pilot mass production up to kg scale hasbeen carried out with this hybrid carbon source materials process, and the performance of thesample was almost the same as that of LiFePO₄sample prepared at experimental scale,andthe decay for the capacity is only12.4%after200cycles at0.5C charge/discharge rate.A series of Fe-site doped LiFe_0.95M_0.05PO₄/C (M=V, Mn, Ni, Co or Cu) composites weresuccessfully prepared by adding soluble salt of transition elements into precursor slurry. Andthe effects of doped elements on the performances of LiFePO₄were investigated, it was foundthat doping with nickel or vanadium could greatly enhance the capacity of material at highrate, and doping with manganese or cobalt were observed no obvious positive effects, whiledoping with copper would deteriorate the performance of material. The optimal dopingamounts of Ni and V is ca.3%, the LiFe0.97M0.03PO₄/C (M=V or Ni) samples exhibited thebest performance; The specific capacity of LiFe_0.97Ni_0.03PO₄/C composite was about80%higher than that of non-doping sample at discharge rate of10.0C, and the performance decayfor LiFe_0.97Ni_0.03PO₄/C is only8.8%after200charge/discharge cycles, much lower than thatof LiFePO₄/C (25.8%) at same discharge rate. The lattice parameters of Ni-doping sample,calculated by Rietveld refinement, showed that the doping with appropriate amount of Nicould change the lattice parameters, which may lead to the fast insertion/extraction of Li+ions;EIS and CV tests showed that doping with3%Ni atoms could sharply decrease chargetransfer resistance and enhance the electrochemical reversibility of material.Spherical Li₃V₂(PO₄)₃/C and Li₂MnSiO₄/C composite were successfully prepared byspray drying method, and the effects of calcination temperature on the structure andperformance were investigated. The results showed that the optimal calcination temperaturefor the preparation of monocline Li₃V₂(PO₄)₃was750℃, and the LVP-750sample preparedat this temperature had the highest capacity, with the initial discharge capacity of191.4mAh/g at0.2C. what's more, it exhibited excellent cycling performance at various rates. Theorthorhombic Li₂MnSiO₄could be prepared by calcining at temperature in rang of700to850℃, but trace of Li₂SiO₃and MnO impurities were co-existed. The sample prepared at800℃ had the highest initial specific capacity of143.1mAh/g, and it was about40%higher thanthat of Li₂MnSiO₄/C sample prepared by a traditional high temperature solid-state method,the cycling stability of sample was obviously superior to the later.For the safety consideration, we investigated the performance of LiFePO₄in aqueoussolution, and explored the feasibility of design an aqueous rechargeable lithium-ion battery. In1.0M lithium ion aqueous solution, the LiFePO₄showed excellent electrochemicalperformance and stability; The diffusion coefficient of Li+insertion/extraction of LiFePO₄(1.22×10^-14cm²/s and9.97×10^-15cm²/s, respectively) can be calculated derived by linearfitting the square root of the scanning rates and oxidation/reduction peaks current; Byintegrating the curve of CV below the rate of5mV/s, the capacity of LiFePO₄, which wascalculated by CV curves test below the rate of5mV/s, is agreed well with the capacityderived from charge/discharge test in LIB; Furthermore, a LiMn₂O₄/LiFePO₄battery withaqueous electrolyte solution was designed and prepared, it is interesting that this batteryworked very well, after activated with20charge/discharge cycles, the capacity of LiFePO₄could reach the theoretical upmost value, and can retained over70%after1000long-termcycles at2.0C. Consequently, LiFePO₄can work in aqueous solution vey well, and it ispossible to design and fabricate an aqueous rechargeable lithium-ion battery, which will besafer than that with organic solvent.