Weiping Tang

Synthesis of Li1.33Mn1.67O4 spinels with different morphologies and their ion adsorptivities after delithiation

From different precursors, including γ-, β-MnO2, hollandite, and H+-form birnessite manganese oxides, well-crystallized Li1.33Mn1.67O4 spinels were synthesized at 400°C by using LiNO3 as a flux. Alkali metal ion adsorptivities of the spinels after delithiation were investigated. SEM observation showed that each spinel obtained preserved the morphology of its precursor. The lack of morphology change, and the time-dependence of the lithium insertion into the precursors, illustrate that the formation of the spinel is a topotactic template reaction. None of the delithiated spinels preserved the original ion-exchange sites in their precursors but all showed nearly the same high affinity for Li+, while the amount of uniform acidic sites increased and the steric hindrance during lithiation decreased in the order of spinels obtained from birnessite, hollandite, and γ-, β-MnO2. This is probably because birnessite has more (buffer) space for the accommodation of ions in its layered structure than other precursors with a tunnel structure and some part of the space remains after structural transformation to the spinel.

Synthesis of lithium manganese oxide in different lithium-containing fluxes

By using a needle-like raw material γ-MnOOH as a Mn-source, single crystals of lithium manganese oxides were synthesized in a variety of lithium-containing fluxes including LiNO 3 , Li 2 CO 3 , LiOH, LiCl and Li 2 SO 4 . Needle-like Li 1.33 Mn 1.67 O 4 and Li 2 MnO 3 crystals and plate-like polyhedral Li 2 MnO 3 crystals were obtained in an LiNO 3 flux heated within different temperature ranges (300–400, 500–750 and 900–1000 °C, respectively). Not only polyhedral and film Li 2 MnO 3 but also octahedral LiMn 2 O 4 single crystals were obtained in an LiCl flux by use of different shaped containers to control the exposure to the atmosphere. Orthorhombic LiMnO 2 crystals with tube- and rod-like shapes could be obtained at the bottom of the LiCl melt at 1000 °C. The lithiation reaction (at low temperature) and dissolution–precipitation controlled growth (at high temperature) of Li 2 MnO 3 were also observed in Li 2 CO 3 and LiOH fluxes. According to the reaction mechanism, the Li-containing fluxes investigated can be classified into four types: (1) LiNO 3 , oxidizing flux; (2) LiCl, non-oxide flux; (3) LiOH and Li 2 CO 3 , oxidic, but non-oxidizing fluxes; and (4) Li 2 SO 4 , no reaction.

Preparation of β-MnO2 nanocrystal/acetylene black composites for lithium batteries

Composites consisting of β-MnO2 nanocrystals and acetylene black were synthesized by thermal decomposition of Mn(NO3)2 with acetylene black. The effects of heating temperature and specific surface area (SBET) of the starting acetylene black on the formation and properties of the composite were investigated. The decomposition of Mn(NO3)2 mixed with acetylene black progressed at a lower temperature than that of Mn(NO3)2 alone. The formation and the size of β-MnO2 nanocrystals depended strongly on the heating temperature (T) and SBET of the starting acetylene black. The β-MnO2/acetylene black composite could be produced for acetylene black of SBET = 60 m2 g−1 at 160 °C ≤ T ≤ 320 °C and SBET = 133 m2 g−1 at 160 °C ≤ T ≤ 300 °C. However, the composite could not be obtained for acetylene black with larger specific surface area (SBET = 300 m2 g−1). The size of β-MnO2 nanocrystals decreases and their dispersion in the composites increases with an increase in the SBET or the heating temperature. The lithium insertion behavior and voltage feature in the first discharge process depend strongly on the size of the β-MnO2 nanocrystals as well as their dispersion. The electrochemical lithium insertion progressed topotactically for well-dispersed β-MnO2 nanocrystals, retaining the framework of their rutile structure to permit a large amount of lithium insertion (Li/Mn = 1.15 in the solid). The charge/discharge curves showed a flat voltage plateau around 2.8 V and a stable cycling feature up to the 20th cycle. It is likely that the easy lattice expansion of β-MnO2 nanocrystals along the a crystal axis plays an important role in the topotactic lithium insertion/extraction reactions.

Preparation of fine single crystals of spinel-type lithium manganese oxide by LiCl flux method for rechargeable lithium batteries. Part 1. LiMn2O4

Uniform fine single crystals of LiMn2O4 spinel were synthesized by a LiCl–Mn(NO3)2 flux method. The effects of reaction temperature and time on the composition, size and morphology of lithium manganese oxide were investigated by chemical, X-ray diffraction and selected area electron diffraction analyses, and SEM observation. The composition and size of the single crystals depended strongly on the heating temperature. Stoichiometric LiMn2O4 single crystals were yielded at 850 °C, whereas lithium-rich and lithium-poor crystals were obtained at 650–750 °C and 950 °C, respectively. Plate-like Li1.18Mn2O4.28 single crystals with a thickness of less than 0.1 µm, showed a high first discharge capacity of 112 mAh g−1 with a good cycle feature. The increase in the size of single crystals resulted in a decrease of the capacity at the first cycle, but did not accelerate the capacity fade over the following cycles.

Synthesis of lithium-rich LixMn2O4 spinels by lithiation and heat-treatment of defective spinels

Single-phase lithium manganese oxide spinels were synthesized in two steps: (1) the preparation of defective spinel precursors with different Li ∶ Mn ratios through lithium insertion into Li-birnessite manganese oxide in a LiNO3 flux at 400 °C, followed by (2) heat-treatment of the precursors at 750 °C in air. Depending upon the Li ∶ Mn ratio, the precursors yielded three types of single-phase spinels: a lithium-deficient type with lithium and oxygen deficiencies, stoichiometric LiMn2O4, and a lithium-rich type with lithium in the 16d-site of space group Fdm but with oxygen deficiency. The electrochemical measurements showed that good rechargeability on cycling between 3–4.3 V could be obtained in all the lithium-rich spinels Li[LixMn2 − x]O4 − δ, where 0 < x < 0.33 and δ < 0.2.

Co-Precipitation Synthesis of Acetylene Black/Li-Birnessite Composite Suitable for a Li-Rechargeable Battery

Binary composites of acetylene black/Li-birnessite manganese oxide were synthesized through the co-precipitation of Li-bimessite with acetylene black. The galvanostatic charge/discharge tests showed that the composites had a higher discharge capacity than their corresponding physical mixture.