•Self-doping is effectively to improve the electrochemical performance of TiNb2O7.•The lattice parameter of self-doping Ti1-xNb2+xO7 change little with x.•The narrowest band gap was 0.768 eV for Ti0.98Nb2.02O7 by the First-principle.Titanium niobium oxide (TiNb2O7) has been known as a potential candidate for the “zero-strain” material Li4Ti5O12 for power lithium ion battery, but its wide band gap energy (Eg ∼ 2.0 eV), which representing low electronic conductivity, limits its application. Here, by adjusting the ratio of raw material TiO2 and Nb2O5, a series of Ti1-xNb2+xO7 (x = 0, 0.01, 0.02, 0.04, 0.06) were prepared via a high temperature solid-state reaction, and both the morphology and crystal structure of Ti1-xNb2+xO7 remained unchanged. The band gap calculated by the first-principle was 0.768 eV for Ti0.98Nb2.02O7, which proved that the electronic conductivity was improved, and the electrochemical performance was also enhanced. The reversible capacities Ti0.98Nb2.02O7 was higher than that of pure TiNb2O7 at different current density, and also its cycle performance was improved. All these results demonstrated that self-doping was an effective method to improve the electrochemical performance of TiNb2O7.Download high-res image (429KB)Download full-size image

•The external short circuited battery is cycled 1000 times at 0.6C with 30% DOD.•Aging mechanism during long-term cycling of short circuited batteries is studied.•The polarization of LiCoO2 electrode is one of major reason of capacity fading.•The structure degradation of LiCoO2 can also cause the capacity loss of battery.Commercial LiCoO2/mesocarbon microbeads (MCMB) batteries (CP475148AR) are short circuited by different contact resistances (0.6 mΩ and 5.0 mΩ) for short times. The short circuited battery is cycled for 1000 times, and the effect of the short-time external short circuiting on the capacity fading mechanism during long-term cycling of LiCoO2/MCMB battery is studied by analyzing the morphology, structure, and electrochemical performance. The results of SEM indicates that the morphology of LiCoO2 material is almost unchanged, except that the particle surface becomes smooth, and the solid electrolyte interphase (SEI) film on the surface of MCMB electrode becomes nonuniform due to the high temperature caused by short circuiting. The lithium ions are more difficult to de-intercalate from the anode and the lattice structure of LiCoO2 degrades according to the results of X-ray diffraction (XRD). The high discharge current caused by short circuiting can damage electrodes, leaving vacancies in structure. The damage of electrode structure can lead to a decrease of diffusion coefficient of lithium (D), so polarization increases and mainly caused by the LiCoO2 electrode. The capacity deterioration of short circuited battery during long-term cycling is mainly caused by the increase of polarization and capacity loss of electrodes.

•The over-discharged battery is cycled for 1000 times at 0.6 C with 30% DOD.•Aging mechanism of the over-discharged battery is studied.•The effect of over-discharge on MCMB material is much serious than LiCoO2.•The increase of SEI film is the reason of capacity loss of MCMB electrode.•The copper deposited on the surface of anode decays the performance of battery.The LiCoO2/mesocarbon microbeads (MCMB) batteries are over-discharged to 102% DOD, 105% DOD and 115% DOD, respectively, then are fully charged and cycled 1000 times at 0.6 C with 30% DOD. The capacity fading mechanism during long-term cycling of over-discharged batteries is analyzed by electrochemical and physical characterization. No remarkable difference is found on the morphology of LiCoO2 material from SEM. However, copper is detected on anode over-discharged to 115% DOD by EDS and XRD. The structures of LiCoO2 and MCMB materials have almost no change according to the result of XRD test. The performance of MCMB material can be degraded by over-discharge, however the LiCoO2 material still keeps good performance. The capacity of MCMB electrode is improved after being washed with water to remove the SEI film, indicating that the capacity loss of MCMB electrode is mainly attributed to the increase of SEI film. The capacity deterioration of over-discharged battery is mainly caused by the dissolution of copper current collector and the deposition of Cu on the surface of anode during the following charging process. The copper deposited on the surface of anode can hinder the lithium intercalation into/de-intercalation from the anode and promote the increase of SEI film.

Lithium compound deposition on mesocarbon microbead (MCMB) anode after long-term cycling was studied in LiCoO2/MCMB battery. Lithium compound deposition did not generate on the activated MCMB anode, but it generated unevenly on the long-term cycled anode. Gray deposition composed of dendrites and particles was formed on the lower surface of the MCMB layer first, then on the upper surface. The deposition and MCMB layer peeled off from the current collector, and a bump was formed in the cycled anode. The exfoliation and thick deposition increased the ohmic resistance, film resistance, and charge transfer resistance of the cell and decreased the capacity significantly. Metallic lithium did not exist in either the upper or the lower deposition layer according to the results of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), the discharge curve, and anode potential. The outer region of both the lower and the upper deposition layers consisted of Li2CO3, LiOH, ROCO2Li, and ROLi. The inner region of the etched lower deposition layer mainly consisted of Li2O, LiF, and Li2CO3, and that of the etched upper deposition layer mainly consisted of Li2CO3, ROCO2Li, ROLi, and LiF. Solid electrolyte interphase (SEI) film hindering the intercalation of lithium ions into carbon layers and LiCoO2 cathode providing lithium source for the deposition were the two reasons leading to the formation of lithium compound deposition during long-term cycles. Because SEI film on the lower surface of MCMB layer was thicker than that on the upper surface, lithium compound deposition generated on the lower surface first.Keywords: lithium compound deposition; lithium ion batteries; long-term cycling; mesocarbon microbead; solid electrolyte interphase film

Lithium deposition on the surface of a graphite anode during long-term cycles was evaluated using a LiCoO2/graphite battery. The batteries were charged/discharged at 1 C and 25 °C within the voltage range of 2.75–4.2 V for 600, 700, 800, 900 and 1000 cycles. Scanning electron microscopy (SEM) results indicated that both solid electrolyte interphase (SEI) film and lithium deposition appeared on the surface of the cycled graphite anode. Dendritic and granular lithium deposits grew on the anode non-uniformly. Metallic lithium existed in the deposition according to differential scanning calorimetry (DSC) results. Capacity declined distinctly from the 800th cycle, corresponding with the growth of lithium deposits. An SEI film was formed on the surface of the lithium deposits. Results of X-ray photoelectron spectroscopy (XPS) test indicated that the composition of SEI film on the surface of the lithium deposits was the same as that of the SEI film on the surface of cycled graphite. Capacity loss from the electrolyte consumed by the formation of the SEI film was 23.61%, while the loss from other battery components was 76.39%. Formation of lithium deposits consumed active lithium in the battery and led to capacity loss. According to test results of the three-electrode cell, the average anode potential at the end of constant-current charging for full battery became more negative with the cycling, and this phenomenon was related to the generation of lithium deposits.