Xiaoguang Hao

Oxygen Vacancies Lead to Loss of Domain Order, Particle Fracture, and Rapid Capacity Fade in Lithium Manganospinel (LiMn2O4) Batteries

Spinel-structured lithium manganese oxide (LiMn2O4) has attracted much attention because of its high energy density, low cost, and environmental impact. In this article, structural analysis methods such as powder neutron diffraction (PND), X-ray diffraction (XRD), and high-resolution transmission and scanning electron microscopies (TEM & SEM) reveal the capacity fading mechanism of LiMn2O4 as it relates to the mechanical degradation of the material. Micro-fractures form after the first charge (to 4.45 V vs. Li(+/0)) of a commercial lithium manganese oxide phase, best represented by the formula LiMn2O3.88. Diffraction methods show that the grain size decreases and multiple phases form after 850 electrochemical cycles at 0.2 C current. The microfractures are directly observed through microscopy studies as particle cracks propagate along the (1 1 1) planes, with clear lattice twisting observed along this direction. Long-term galvanostatic cycling results in increased charge-transfer resistance and capacity loss. Upon preparing samples with controlled oxygen contents, LiMn2O4.03 and LiMn2O3.87, the mechanical failure of the lithium manganese oxide can be correlated to the oxygen vacancies in the materials, providing guidance for better synthesis methods.

Water-Free Titania-Bronze Thin Films With Superfast Lithium Ion Transport

Using pulsed laser deposition, TiO2 (-) B and its recently discovered variant Ca:TiO2 (-) B (CaTi5O11) are synthesized as highly crystalline thin films for the first time by a completely water-free process. Significant enhancement in the Li-ion battery performance is achieved by manipulating the crystal orientation of the films, used as anodes, with a demonstration of extraordinary structural stability under extreme conditions.

Improved electrode kinetics in lithium manganospinel nanoparticles synthesized by hydrothermal methods: identifying and eliminating oxygen vacancies

Lithium-rich manganospinel (Li1+xMn2–xO4–δ, lithium manganese oxide) has been synthesized by hydrothermal methods employing potassium permanganate, lithium hydroxide, and acetone as synthons. The solid product crystallizes as 30–50 nm particles with some larger 100–300 nm particles also occurring. Materials prepared by this low-temperature route contain oxygen vacancies which can be demonstrated by combining thermogravimetric analysis, differential scanning calorimetry, and cyclic voltammetry. Oxygen vacancies can be minimized beyond the limits of detection for these experiments by annealing the compound in air at 500 °C for 4 h. At room temperature, Rietveld refinement of the powder neutron diffraction pattern shows an orthorhombic Fddd(α00) superlattice of the Fdm space group for hydrothermally synthesized lithium manganospinel. After annealing, oxygen vacancies are eliminated and the superlattice features disappear. Furthermore, the hydrothermal synthesis of lithium manganospinel performed under a pure oxygen atmosphere followed by annealing at 500 °C for 4 h in air gives superior electrochemical properties. This compound shows a reversible capacity of 115 mAh/g when cycled at a rate C/3 and retains 93.6% of this capacity after 100 cycles. This same capacity is observed at the faster rate of 3C. At 5C, the capacity drops to 99 mAh/g, but capacity retention remains greater than 95% after 100 cycles. Finally, when cycled at 5C at an elevated temperature of 55 °C, the O2 annealed sample shows an initial capacity of 99 mAh/g with 89% capacity retention after 100 cycles. The high rate capability of this material is ascribed to fast lithium-ion diffusion, estimated to be 10−7 to 10−9 cm2 s−1 by electrochemical impedance spectroscopy.

Two-step hydrothermal synthesis of submicron Li1+xNi0.5Mn1.5O4−δ for lithium-ion battery cathodes (x = 0.02, δ = 0.12)

A facile two-step hydrothermal method is developed for the large-scale preparation of lithium nickel manganese oxide spinel as a cathode material for lithium ion batteries. In the reaction, nickel is introduced in a first step at neutral pH, followed by lithium insertion under base to form a product having composition Li(1.02)Ni(0.5)Mn(1.5)O(3.88). The X-ray diffraction pattern and Raman spectroscopy of the synthesized material support a cubic Fd3m structure in which Ni and Mn are disordered on the 16d Wyckoff site, necessary for good cycling characteristics. XP spectroscopy and elemental analysis confirms that Mn remains reduced in the final product (Z(Mn) = 3.82) and that two different chemical environments for Ni exist on the surface. SEM imaging shows a primary particle size of ~200 nm, and galvanostatic cycling of the material vs. Li(+/0) gives a reversible gravimetric capacity of ~120 mA h g(-1) at 1 C rate (147 mA g(-1)) with reversible cycling up to 1470 mA g(-1), supported by rapid Li(+) diffusion. The capacity fade at 1 C is substantial, 17.3% over the first 100 cycles between 3.4 and 5.0 V. However, when the voltage limits are altered, the capacity retention is excellent: nearly 100% when cycled either between 3.4 and 4.4 V (where oxygen vacancies are not electrochemically active) or 89% when cycled between 4.4 and 5.0 V (where the Jahn-Teller active Mn(4+/3+) couple is not accessed).