|Schematic drawing of a molten hydrocarbon-assisted solid-state approach for making LiMnPO4 nanoplates and their crystallographic orientation. Credit: ACS, Choi et al. Click to enlarge.|
Researchers at the Pacific Northwest National Laboratory have synthesized electrochemically active LiMnPO4 nanoplates via a novel, single-step, solid-state reaction in molten paraffin. The resulting olivine-structured LiMnPO4 nanoplates (50 nm thick) appear porous and were formed as nanocrystals were assembled and grew into nanorods.
After carbon coating, the prepared LiMnPO4 cathode demonstrated a flat potential at 4.1 V versus Li with a specific capacity reaching as high as 168 mAh/g under a galvanostatic charging/discharging mode, along with an excellent cyclability. A paper on the work was published online 19 July in the ACS journal Nano Letters.
Encouraged by the success of LiFePO4, much research is now focused on the more challenging olivine LiMPO4 (M=Mn, Co, and Ni) structures, especially LiMnPO4 with a higher theoretical energy density (701 Wh/kg ) 171 mAh/g × 4.1 V) due to higher potential than that of LiFePO4 (586 Wh/kg ) 170 mAh/g × 3.45 V), which is considered as the maximum energy density practically achievable within the stability window of well-known carbonate ester-based electrolytes. The other members of the olivine family, LiCoPO4 and LiNiPO4, are more challenging in the effort to develop stable electrolytes because of their higher voltage (4.8 and 5.1 V) vs Li/Li+.7
—Choi et al.
However, LiMnPO4 materials also have a number of limitations, including lower electronic conductivity than LiFePO4, and possible key rate limiting factors, the authors note. While “tremendous effort” has been made to overcome these limitations through minimizing particle sizes, substitutional doping, and enhancement of electronic contact, to date only a few groups have been able to attain more than 120 mAh/g from LiMnPO4, the authors said.
A solid-state reaction in molten surfactant-paraffin media using low-cost and green reagents that mimic both a solid-state and a self-assembly approach has been devised for LiMnPO4 synthesis. Advantages of both routes have been combined into a single step to obtain well-dispersed uniform LiMnPO4 nanoplates with good crystallinity and excellent electrochemical activity as a Li-ion battery cathode.
—Choi et al.
Oleic acid is used as a surfactant, and paraffin acts as a nonpolar solvent that facilitates thermodynamically preferred crystal growth of LiMnPO4 without agglomeration. As a surfactant and solvent, oleic acid and paraffin are cheap, environmentally friendly, and provide a stable environment for moisture-sensitive precursors because of their hydrophobic nature, the authors note.
Precursors included LiCOOCH3·2H2O (reagent grade, Sigma), MnCO3 (99%, Aldrich), NH4H2PO4 (99.999%, Sigma-Aldrich), oleic acid (FCC, FG, Aldrich), and paraffin wax (ASTM D 87, mp. 53~57 °C, Aldrich). After ball milling, the resulting viscous liquid slurry mixture was poured into a glass beaker, dried in an oven at >100 °C for 30 min, and subjected to heat-treatment at 550 °C for 8 h under an ultra-high purity (UHP) atmosphere with a heating rate of 5 °C/min.
Our LiMnPO4 nanoplates obtained by a molten hydrocarbon- assisted, solid-state reaction can be easily scaled up for commercialization. This process shows potential for further improvement using a simplified synthesis route to obtain fully electrochemically active LiMnPO4, which appears to be a promising cathode material for Li-ion batteries.
—Choi et al.
Daiwon Choi, Donghai Wang, In-Tae Bae, Jie Xiao, Zimin Nie, Wei Wang, Vilayanur V. Viswanathan, Yun Jung Lee, Ji-Guang Zhang, Gordon L. Graff, Zhenguo Yang and Jun Liu (2010) LiMnPO4 Nanoplate Grown via Solid-State Reaction in Molten Hydrocarbon for Li-Ion Battery Cathode. Nano Lett., Article ASAP doi: 10.1021/nl1007085