New Lithium Ion Chemistry Delivers High Capacity and Excellent Cycling Stability

(a) SEM image and (b) cross-sectional images of Li[Ni_0.67Co_0.15Mn_0.18]O₂ with a concentration gradient outer layer. Kim et al. (2009) Click to enlarge.

A team from Hanyang University (Korea), Iwate University (Japan) and Argonne National Laboratory in the US synthesized a novel Li[Ni_0.67Co_0.15Mn_0.18]O₂ cathode material encapsulated completely within a concentration-gradient shell and investigated its electrochemical and thermal properties.

The discharge capacity of the concentration-gradient Li[Ni_0.67Co_0.15Mn_0.18]O₂ electrode increased with increasing upper cutoff voltage to 4.5 V, and cells with this cathode material delivered a very high capacity of 213 mAh/g, with excellent cycling stability even at 55 °C. A paper on the work was published online 3 August in the journal Electrochimica Acta.

In an earlier paper (Sun et al. 2009) published in the journal Nature, the team suggested that this type of material could enable production of batteries that meet the demanding performance and safety requirements of plug-in hybrid electric vehicles.

Ni-rich metal oxides are attractive as lithium storage materials for rechargeable lithium batteries because of their lower cost and higher reversible capacity than commercialized LiCoO₂. Of these compounds, Li[Ni_1-xCo_x]O₂ is one of the most promising; however, the material suffers from low thermal stability and multiple phase transition resulting from structural instability during cycling.

The researchers had earlier reported that two novel core-shell cathode materials, Li[(Ni_0.8Co_0.1Mn_0.1)_0.8 (Ni_0.5Mn_0.5)0_.2]O₂ and Li[(Ni_0.8Co_0.2)_0.8(Ni_0.5Mn_0.5)_0.2]O₂, exhibit excellent cyclability and thermal stability [2, 3]. The core delivered high capacity, while the shell provided high thermal stability.

However, in these core-shell materials, the composition of the transition metals at the interface between the core and the shell changes rather sharply, even though a small amount of metal diffusion is observed in the interface, they reported. If the abrupt composition change occurs at the interface between the core and the shell, the interface could act as a barrier to Li⁺ diffusion, thereby decreasing the electrochemical properties of the material, they noted. The concentration-gradient approach is intended to address that issue.

The Li[Ni_0.67Co_0.15Mn_0.18]O₂ has a core of Li[Ni_0.8Co_0.15Mn_0.05]O₂ that is rich in Ni, a concentration-gradient shell having decreasing Ni concentration and increasing Mn concentration toward the particle surface, and a stable outer layer of Li[Ni_0.57Co_0.15Mn_0.28]O₂.

In their paper, they investigated and compared the electrochemical and thermal properties of the new material to those of the core Li[Ni_0.8Co_0.15Mn_0.05]O₂ material alone.

The researchers attributed the enhanced thermal and lithium intercalation stability of the Li[Ni_0.67Co_0.15Mn_0.18]O₂ to the gradual increase in tetravalent Mn concentration and decrease in Ni concentration in the concentration-gradient shell layer.

Source: Green Car Congress