Ultrathin LiMn2O4 Nanowire Cathode Materials for Higher Li-ion Power Densities for HEV and EV Applications

Discharge curves at different rates after charging at 1C. Credit: ACS, Lee et al. Click to enlarge.

A team of researchers from the Korea Advanced Institute of Science and Technology, Università degli Studi di Milano-Bicocca (Italy), and Stanford University have synthesized ultrathin LiMn₂O₄ nanowires for use as a Li-ion cathode material offering high power densities.

Galvanostatic battery testing showed that the ultrathin LiMn₂O₄ nanowires deliver 100 and 78 mAh/g at very high rate (60C and 150C, respectively) in a larger potential window with very good capacity retention and outstanding structural stability. Such performances are due to both the favorable morphology and the high crystallinity of nanowires, the researchers said in a paper published online 26 August in the ACS journal NanoLetters.

Although lithium ion batteries can provide higher energy density (W h/kg) than other secondary systems, they have limited power density (W/kg) compared to double layer and pseudocapacitors. Hence the improvement of the specific power density in lithium ion batteries is a fundamental issue to develop better HEVs and EVs. Spinel LiMn₂O₄ is a promising candidate to replace layered Ni or Co oxide materials as cathode in lithium ion batteries because of its intrinsic low-cost, environmental friendliness, high abundance, and better safety.

However, the application of LiMn₂O₄ in high power systems requires the development of fast kinetic electrodes which appears nowadays possible thanks to the use of nanostructured morphologies...we believe ultrathin nanowire LiMn₂O₄ is a promising cathode material for lithium ion batteries for HEV and EV applications thanks to its high rate capability and superior structural stability.

—Lee et al.

The researchers had earlier shown that one-dimensional nanosized materials have faster kinetics and higher rate capability than micrometer-sized materials due to the large surface-to-volume ratio that enhances the contact between active material grains and electrolyte. LiMn₂O₄ nanorods with 150 nm diameter showed good capacity retention (around 60%) up to 5C.

In the current paper, they synthesized the ultrathin spinel nanowires using a two-step process: a solvothermal reaction to prepare α-MnO₂ nanowires followed by solid state reaction with LiOH.

The team used a coin-type cell configuration to evaluate the electrochemical properties of the nanowire materials as cathode electrodes. When charged at 1C the electrode shows a discharge specific capacity of about 90 mAh/g at 20C between 3.1 and 4.3 V vs Li. When the cycling potential range is enlarged to overcome electrode kinetic limitations, nanowires are able to deliver “relevant discharge capacities” (around 80 mAh/g) even at an extremely high current density (22.2 A/g, 150C rate), with high reversibility and good capacity retention, they found.

As produced LiMn₂O₄ nanowires have around 10 nm diameter, and they are several micrometers in length. Such morphology improves the kinetic properties at very high current rate and was capable of the facile structural transformation of the cubic and tetragonal phase in the large compositional range.

—Lee et al.

Source: Green Car Congress