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PNNL Researchers “Grow” LiMnPO4 Cathode Materials via Solid-State Reaction in Molten Paraffin

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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.

Resources

  • 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

Comments

Davemart

What total energy density for the battery is implied by the cathode being 701Wh/Kg?

DaveD

Good question, Davemart. The total capacity depends on both the cathode and the anode. However, the anode has been way ahead of cathodes so far (about 3 times the energy density in the best cases) so this could have a huge effect on the overall battery density as it improves the "weak link" in the chain.

From their description, it sounds like it could have a low power density and the manufacturing steps don't sound like they would lend themselves well to low cost manufacturing. But they can work on those issues now that they have something to chase.
It's a great step, we just have to be realistic about the hurdles still needed if they want to go to market with an affordable battery.

HarveyD

DaveD...This may not be enough for the next generation 600 Wh/Kg battery but it may be one more step in that direction. Since some technology favor higher energy density while others favor high power density, could a combo of 2 different batteries (Hi-energy + High power) give better overall performances?

SJC

That is the idea, since one battery does not do it all you combine two. One for power on acceleration and one for energy for range.

DaveD

HarveyD, you're correct, it is a step in the right direction..and a very good one possibly.

As SJC points out, it is best to combine two different types of batteries or battery/capacitor.

I'm so disappointed that the patent office granted Trinity that patent for a concept that has been applied to devices for decades. This is exactly how they extend battery life in cameras by letting a capacitor handle the burst needed for a flash.

Now everyone who has been working on this type of system for years, or even decades will have to deal with Trinity trying to sue them for something obvious.

HarveyD

Patents' rights and law suits have gone wild for decades. It may be time to regulate against frivolous claims or is that what one should expect with too many lawyers everywhere.

SJC

When national energy security is at stake, cornering the market with a monopoly license like a patent seems a bit much. I understand that the investors want to protect their investment, but lots of innovation is a variation on a theme and a patent land mine does not help.

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