Evolution and expansion of Li concentration gradient during charge-discharge cycling.

To improve the performance of Li-ion batteries (LIBs), it is essential to understand the behaviour of Li ions during charge-discharge cycling. However, the analytical techniques for observing the Li ions are limited. Here, we present the complementary use of scanning transmission electron microscopy and atom probe tomography at identical locations to demonstrate that the evolution of the local Li composition and the corresponding structural changes at the atomic scale cause the capacity degradation of Li(Ni0.80Co0.15Mn0.05)O2 (NCM), an LIB cathode. Using these two techniques, we show that a Li concentration gradient evolves during cycling, and the depth of the gradient expands proportionally with the number of cycles. We further suggest that the capacity to accommodate Li ions is determined by the degree of structural disordering. Our findings provide direct evidence of the behaviour of Li ions during cycling and thus the origin of the capacity decay in LIBs.

Improved Thermal Stability of Lithium-Rich Layered Oxide by Fluorine Doping.

The thermal stability of lithium-rich layered oxide with the composition Li(Li1/6 Ni1/6 Co1/6 Mn1/2 )O2-x Fx (x=0.00 and 0.05) is evaluated for use as a cathode material in lithium-ion batteries. Thermogravimetric analysis, evolved gas analysis, and differential scanning calorimetry show that, upon fluorine doping, degradation of the lithium-rich layered oxides commences at higher temperatures and the exothermic reaction is suppressed. Hot box tests also reveal that the prismatic cell with the fluorine-doped powder does not explode, whereas that with the undoped one explodes at about 135 °C with a sudden temperature increase. XRD analysis indicates that fluorine doping imparts the lithium-rich layered oxide with better thermal stability by mitigating oxygen release at elevated temperatures that cause an exothermic reaction with the electrolyte. The origin of the reduced oxygen release from the fluorinated lithium-rich layered oxide is also discussed.

Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17-x Ni0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.