Studies On Assembly And Electrochemical Properties Of Layered Lithium Manganese Oxides With Supramolecular Structure

Doped and pillared layered lithium manganese oxides were synthesized by ion-exchange, layered double hydroxides (LDHs) precursor and hydrothermal method, respectively. These materials were characterized by XRD, HT-XRD, ICP, XPS, TG-DTA, TG-MS, EXAFS, TEM and SEM, and the electrochemical properties were studied by charge-discharge cycling and cyclic voltammogram methods. The factors which affect the structural stability and electrochemical properties were investigated.The material LiMnO₂ has a layered structure with the monoclinic space group C2/m. SEM micrograph shows that LiMnO₂ possesses a needle-like morphology. The structure transforms to spinel during charge-discharge cycle, as a result the electrochemical behavior were not satisfying.A series of layered O₂ structure Li[Mn(1-x)M_x]O₂ (M=Li, Cr, Mg, Al, Fe) and 03 strucuture Li[Co_xNi_yMn_1-x-y]O₂ were obtained by varying the composition of the host layers. 02 structure materials Li[Mn_1-xM_x]O₂ have an irregular hexagonal morphology. The XRD patterns can be indexed as a structure hexagonal space group P3ml. The electrochemical properties were influenced by the doped elements. Manganese oxides substituted by Al are prime candidates for the cathode of lithium ion batteries, for the charge-discharge capacity and cycling life were improved simultaneously. Layered 03structure materials Li[Co^NiyMn\.x-y]O2 have the a-NaFeO2 structure. It has been found that the content of lithium was improved with the increase of Co content and decrease of Ni content. The local environment of the transitional metals are rather similar in Li[CoxMni..v]O2. The Co/Ni/Mn molar ratio has a remarkable influence on the electrochemical properties. Among these materials, Li[Coi/3Nii/3Mni/3]O2 possesses high reversible capacity (118.1 mAh-g^'), and the most stable cycling life. Moreover, the capacity increases as more Co doped in the material.Supramolecular pillared oxides M-MnC?2 (M=BaN Sr^ ZrO) were prepared by intercalating the guest cations into MnO2 host matrix. The materials M-MnO2 have the same structure as the precursor KJvlnO2. For ZrO-MnO2, EXAFS indicates that Zr atom locates between the MnC>2 layers forming a stable structure. The strong interaction between M2+ ions and MnC?2 layers may restrain structure distortion in charge-discharge cycling, and the cycling properties were improved distinctly. Among these pillared materials, (ZrO)nMnO2 has an advanced capacity and long life. Mn^V2Os?H2O containing double sheets of V2O5 layers has been synthesized by a new method - oxidation of VOSO4 with layered MnC>2. XPS indicates that the oxidation state of vanadium and manganese are +5 and +4, respectively. The molar ratio of Mn/V near surface was almost the same as that in the bulk. The electrochemical properties were improved with the decrease of the Mn content. Mno.i4V205.27"l-53H20 has a specific capacity of 200 mAhg ' between 2.0 and 4.0 V, with the discharge capacity at the 18th cycle remaining as high as 208.3 mAhg"1. It is therefore a potential practical candidate as the cathode material for rechargeable lithium batteries.Four lithium manganese oxide phases have been synthesized for the first time by mild hydrothermal reactions of layered manganese oxide (8-KxMnO2) with different lithium compounds. The four materials obtained are rock salt structure I^MnCb, hollandite structure a-MnC?2, spinel structure LiM^C^, and birnessite structure LiJvlnCh. Of the four lithium manganese oxides, birnessite structure LiJvlnCh demonstrated the most stable cycling behavior with high coulombic efficiency. Its reversible capacity reaches 162.8 mAhg^', indicating that it is a viable cathode material...