Increasing the specific capacity of Li ion battery cathode is considered as an attractive route to lower the battery weight, volume, and cost. Following the successful development of the olivine-structured LiFePO₄ cathode, lithium transition metal silicates have emerged as materials of considerable potential because of their high specific capacity. Theoretically, these transition metal silicates have the potential to facilitate the insertion/extraction of two lithium ions per formula unit with a capacity of ∼333 mAh/g.

However, to date, reversible two-lithium-ion capacity has not been achieved due to the structural instability of these transition metal silicates...Recent efforts geared at achieving full capacity have also failed to register the two-lithium-ions capacity and stable cyclic performances. The lack of success thus far has left the impression that no further dramatic improvement in the specific capacity can be anticipated with the silicate family despite the attractive ~333 mAh/g theoretical capacity. Reversible two-lithium-ion capacity with stable cyclic performance requires a cathode that is stable toward structural and volume changes during battery operation. Judicious control of the structural stability and volume changes is essential for breakthrough performance and subsequent development of silicate family cathodes for them to take their place in large-scale applications such as electric vehicles.

—Rangappa et al.

The team theorized that two-dimensional (2D) nanosheets (NS) could be used to provide the required stability for Li₂MSiO₄ (M = Fe, Mn, Co, Ni) cathode materials against structural and volume change. For the study published in Nano Letters, they synthesized Li₂MSiO₄ nanosheets by a rapid one-pot supercritical fluid reaction using a mixed solvent of aqueous ethanol.

The electrochemical performances of the samples were evaluated using a beaker-type three-electrode cell with metallic Li as a counter and reference electrode. The Li/Li₂MSiO₄ cells were cycled between 1.5 and 4.8 V at a temperature of 45 ± 5 °C and C/50 rate.

The specific capacities observed above room temperature are higher than the values expected for the extraction of two lithium ions per formula unit (333 mAh/g). These results demonstrate, for the first time, the ability to extract/insert the second lithium ion completely, involving Fe^3+/4+/Mn^3+/4+ couples at high temperatures. The successful extraction/insertion of two lithium ions can be attributed to improved kinetics at the high temperature supplied in conjunction with the NS morphology of the samples. The plateau at 3.1 V observed on the first charging cycle shifts to 2.9 V on second cycle. However, the second cycle discharge curves are similar indicating no major structural change occurring after the first charge.

...The successful two lithium ions reversible capacity is attributed to the short lithium ion diffusion path due to the NS morphology and uniform conductive matrix provided by the conductive polymer−MWCNT coating. The NS morphology mitigated the limitation of structural instability encountered in lithium metal silicate-based cathode materials, allowing for the successful insertion/extraction of two complete lithium ions. Therefore, we believe that electrode materials based on the NS morphology should be ideal candidates for expansion of Li ion battery technology in automobile, aerospace, and power-grid applications that demand the development of lightweight, long-lasting batteries.

—Rangappa et al.

Resources

• Dinesh Rangappa, Kempaiah Devaraju Murukanahally, Takaaki Tomai, Atsushi Unemoto, and Itaru Honma (2012) Ultrathin Nanosheets of Li₂MSiO₄ (M = Fe, Mn) as High-Capacity Li-Ion Battery Electrode. Nano Letters doi: 10.1021/nl202681b