Surface Modification And Electrochemical Investigation Of The Lithium-rich Manganese-Based Cathode Materials

Rechargeable lithium-ion batteries have been widely used in portable devices due to their high energy density and long cycle life. Recently, they are considered as crucial power sources for electric vehicles(EVs) and green grid. These potential applications in the new energy fields put forward higher requirements for lithium ion batteries, summarized as "three high and two low" : high energy density, high power density, high safety, low cost and low(no) pollution. Traditional cathode materials, represented by lithium cobalt oxide, usually show reversible capacities of less than 200 mAh g-1, which cannot fully meet the demands of high-performance lithium ion batteries. Therefore, it is very important to develop high capacity and high rate cathode materials to improve the energy density and power density of lithium ion battery.Lithium-rich manganese-based materials have been currently investigated as promising cathode candidates for lithium ion batteries due to their high capacity(> 250 mAh g-1) and low cost. However, some notable drawbacks of these materials, such as the initial large irrevisible capacity loss, poor cyclic stability and rate capability, and severe voltage decay during cycling, hinder its practical applications. To overcome these shortcomings, several approaches including surface coating, film-forming electrolyte additive and modification of surface layer structure, were proposed in this thesis to improve the electrochemical performance of the Li1.2Ni0.13Mn0.54Co0.13O2(LNMCO) material. Combined with scanning electron microscope(SEM), transmission electron microscope(TEM), X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS), Inductively Coupled Plasma Optical Emission Spectrometer(ICP-OES), infrared spectroscopy(IR), electrochemical impedance spectroscopy(EIS) and galvanostatic intermittent titration(GITT) methods, the relationship between surface structure, composition and electrochemical performance was carefully studied. The detailed contents are summarized as follows:1. A polyimide(PI) protective layer was coated on the surface of LNMCO material. The polyamic acid(PAA) was used to coat on the surface of lithium rich material, then the PAA coating layer was transferred to PI by themal treatment. By 1.0wt.% PAA solution, a ~ 3 nm coating layer is evenly coated on the surface, while higher concentration solutions(2.5 wt.% and 5.0 wt.%) results in the existence of polymer membrane between composite particles. Three new peaks appear in the infrared spectrum of PI-LNMCO-450 material and the binding energy of Mn in PI-LNMCO-450 material is slight lower than that in LNMCO material, indicating the existence of charge transfer between LNMCO material and coating layer, Mn(IV) of LNMCO material absorbing electrons from C=O or phenyl of the PI coating layer and formation of the Mn(III)…O and Mn(III)…benzene interactions, during the thermal imidization process. Electrochemical performances including cycling stability and rate capability are evidently improved by the PI coating layer. PI coating layer separates cathode material from electrolyte and effectively stabilizes the electrode/electrolyte interface at high voltage. Moreover, the reduction of partial tetravalent manganese to trivalent manganese is benefical to lithium ion migration in the material surface.2. Tris(trimethylsilyl)phosphate(TMSP) was investigated as a electrolyte additive. Linear sweep voltammetry results show that the decomposition voltage of TMSP additive is lower than that of reference electrolyte. And the high voltage stability of reference electrolyte is greatly improved by the addition of TMSP. TEM and XPS results clearly illustrate the partcipation of TMSP additive in the formation of a stable solid electrolyte interphase(SEI) layer on the LNMCO material surface. The LNMCO/Li cell cycled in 1.0 wt.% TMSP-containing electrolyte demonstrates a stable cycling performance with a capacity retention of 90.8% after 50 cycles.3. The LNMCO material was treated by hydrazine vapor to modify the surface structure of the LNMCO particle. Differing to the treament by high concentration hydrazine hydrate solution which causes serious structure damage, the hydrazine vapor treatment is relatively mild and the thickness of treatment layer is controllable. The ICP and XRD results indicate that lithium ions are leached out from the Li2MnO3 component of LNMCO through Li+/H+ exchange process by the hydrazine vapor. TEM images of LNMCO-HV material indicate that the inner particle maintains the layered structure, while a ~3 nm Li-deficient layer forms on the surface. Compared with LNMCO material, the discharge capacity and coulombic efficiency of LNMCO-HV material are enhanced with a shorter plateau at ~4.5 V plateau in the initial charge process. After the thermal treament at 300 oC, the removal of the incorporated protons and migration of transition metal ions into the lithium vacancy result in the formation of spinel Li1-xM2O4 phase on the surface of LNMCO-HV-300. The LNMCO-HV-300 sample exhibits high initial discharge specific capacity(295.6 mAh g-1) and coulombic efficiency(89.5 %) with an obvious 2.8 V cathodic peak in the corresponding dQ/d V curve, which is a characteristic of spinel-like phase corresponding to the insertion of Li+ into the empty 16 c octahedral sites.4. The ionic intercalation reaction of LNMCO electrode in Na-ion electrolyte was initially incestigated. Linear sweep voltammetric curve shows that the sodium ion electrolyte(1 M NaClO4/PC) starts to slightly decompose at 4.0V, and violently over 4.75 V. The specific capacity of LNMCO decreases significantly when cycling in the voltage range of 1.7~4.5 V, due to the decomposition of electrolyte at high voltage. From the GITT results, lithium ions are leached out from the LNMCO material in the initial charge process of LNMCO/Na cell, and both sodium and lithium ions are inserted during the following discharge process. The XRD results indicate that a Na-excess phase is formed after the initial discharge of NMCO/Na cell, while in the LNMCO/Na cell, the Na-excess phase is gradually formed during cycling. Adding 5.0 vol.% FEC, the stability of 1 M NaClO4/PC electrolyte is greatly improved. Thus, LNMCO/Na cell exhibits a stable cycling performance.