Motoaki Nishijima

Accelerated discovery of cathode materials with prolonged cycle life for lithium-ion battery

Large-scale battery systems are essential for efficiently utilizing renewable energy power sources from solar and wind, which can generate electricity only intermittently. The use of lithium-ion batteries to store the generated energy is one solution. A long cycle life is critical for lithium-ion battery when used in these applications; this is different from portable devices which require 1,000 cycles at most. Here we demonstrate a novel co-substituted lithium iron phosphate cathode with estimated 70%-capacity retention of 25,000 cycles. This is found by exploring a wide chemical compositional space using density functional theory calculations. Relative volume change of a compound between fully lithiated and delithiated conditions is used as the descriptor for the cycle life. On the basis of the results of the screening, synthesis of selected materials is targeted. Single-phase samples with the required chemical composition are successfully made by an epoxide-mediated sol-gel method. The optimized materials show excellent cycle-life performance as lithium-ion battery cathodes.

Effect of heat treatment time on cycle performance of LiMn2O4 with “Nano Inclusion” for lithium ion batteries

A LiMn2O4 cathode material with “Nano Inclusions” has been prepared by employing various heat treatment times. The effect of the heat treatment time on the electrochemical properties has been investigated. XRD powder pattern analysis and SEM observation indicated that LiMn2O4 formed at low temperature sintering collapsed at the initial stage of the heat treatment and LiMn2O4 particles with ZnMn2O4 “Nano Inclusions” grew in the subsequent heat treatment. HAADF-STEM images revealed that both the particle size of LiMn2O4 and that of the “Nano Inclusions” increased as the heat treatment time increased. While all the LiMn2O4 samples with “Nano Inclusions” exhibit a decreased initial discharge capacity in comparison with LiMn2O4 without “Nano Inclusions”, they show an improved discharge capacity retention rate. The sample heat-treated for 4 hours surpasses LiMn2O4 without “Nano Inclusions” in discharge capacity over the 31st cycle. The larger the size of the “Nano Inclusion”, the more the crack propagation was considered to be suppressed, but on the other hand, the larger the dead region increased. It is considered that an appropriate size of “Nano Inclusions” was formed by the 4 hour heat treatment.

Crystal chemical investigation of nano inclusion in LiMn2O4 cathode material of lithium ion battery

In this study, LiMn2O4 was fabricated with “Nano Inclusions” and the detailed crystal structure of the sample was studied by HAADF-STEM observation with an electron beam tilted to the specimen, electron diffraction, HRTEM observation. HAADF-STEM observation and electron diffraction revealed that the “Nano Inclusions” were located within single LiMn2O4 crystals. HRTEM observation clarified that the (100) plane of LiMn2O4 and the (110) plane of ZnMn2O4 have a common cubic close-packed oxygen arrangement and connect to each other without forming grain boundaries, due to similar atomic arrangements of LiMn2O4 along the 〈100〉 direction and of ZnMn2O4 along the [110] and [10] direction.