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New tin-seeded germanium nanowire array anodes for Li-ion batteries show high capacity and lifetime

10 February 2014

Master.img-000
The chart shows the discharge capacities of the Sn-seeded Ge NW electrode over 1,100 cycles. The active material was charged and discharged at a C/2 rate in the potential range of 0.01−1.5 V. The illustration shows the formation of the porous network over time. Credit: ACS, Kennedy et al. Click to enlarge.

Researchers at University of Limerick and University College Cork (Ireland) have developed high-performance and high-capacity lithium-ion battery anodes from high-density tin-seeded germanium nanowire arrays grown directly from the current collector. The anodes retain a reversible capacity of 888 mAh/g after 1,100 cycles at a C/2 rate. The material exhibits good high-rate performance characteristics, even at very high discharge rates of 20–100C; the NW electrode achieved a discharge capacity of 435 mAh/g after 80 cycles at a discharge rate of 100C.

In a paper in the ACS journal Nano Letters, the researchers show, using ex situ high-resolution transmission electron microscopy and high-resolution scanning electron microscopy, that this high performance can be attributed to the complete restructuring of the nanowires that occurs within the first 100 cycles to form a continuous porous network that is mechanically robust.

Si and Ge nanowire (NW) based materials have emerged as viable candidates for next generation rechargeable lithium- ion battery anodes with energy and power densities that are multiples of current graphitic based electrodes. The key advance is the capability of NWs to overcome the well-known pulverization problem that is detrimental to the cycle life and hence viability of their bulk counterparts. NWs also provide good electrical conductivity along their length, have a high interfacial area in contact with the electrolyte, have an optimal short diffusion distance for Li-ion transport, and can be grown directly from current collectors, eliminating the need for binders and conductive additives.

Ge (max. theoretical capacity of 1384 mAh/g) has received less attention than Si (3579 mAh/g), although it has a higher rate of diffusivity of Li at room temperature (400×) and a greater electrical conductivity (10,000×) making it suitable for high power applications. Gold is the most common catalyst for Ge NW synthesis, however as it is expensive and does not reversibly alloy with lithium, alternative more cost-effective catalyst materials that can contribute to the specific capacity of the electrode are desirable.

Improving the cycle life of simple Ge NW arrangements as Li-ion anodes would be very interesting as their ease of processability and scalability, particularly if solution grown, can offer viable alternatives to graphitic materials.

—Kennedy et al.

The low energy, wet-chemical synthesis process begins with a stainless steel substrate with an evaporated layer of Sn on its surface being placed in the vapor zone of a high boiling point solvent. Diphenylgermane is injected into the flask at 430 °C. The Sn (tin) nanoparticle seeds form in situ at this temperature and act as sinks for the Ge that decomposes from the precursor, facilitating high-density NW growth by the vapor−liquid−solid (VLS) mechanism.

Tin also has a high maximum theoretical capacity (994 mAh/g); the researchers found that the Sn seeds at the ends of the NWs reversibly alloy with lithium and contribute to the electrodes’ overall specific capacity.

Taking into account the mass of both the Sn seed and the Ge NW, the team calculated a maximum theoretical specific capacity for the composite anode of 1320 mAh/g. The NWs exhibited an initial discharge capacity of 1103 mAh/g and an average Coulombic efficiency (CE) of 97.0%. The bulk of the capacity fade was during the first 100 cycles; capacity loss beyond 100 cycles dropped by only 0.01% per cycle.

Master.img-007
Rate capability, high rate capability and high discharge rate performance of the Sn-seeded Ge NW electrodes cycled in the voltage range of 0.01−1.5 V.

(a) The charge and discharge capacities and the C.E. values are shown for an electrode that was charged and discharged at rates of C/10, C/5, C/2, C, 2C, and then back to C/10.

(b) The high rate capability of the electrodes is shown. The capacities and the CE values of an electrode charged and discharged at rates of C/2, C, 5C, 10C, 20C, 40C, 60C, and then back to C/2 is presented.

(c) The discharge capacities measured for 5 cycles at 6 different discharge rates are shown. The charge rate was kept constant at C/2 for all cycles.

(d) Capacity data showing discharge capacities of the material at 20 and 100C discharge rates. The electrodes were charged at a constant rate of 2C.

Credit: ACS, Kennedy et al. Click to enlarge.

The team found that the electrolyte additive, vinylene carbonate (VC), plays an important role, facilitating the formation of the stable porous network of nanowires.

Voltage profiles and differential capacity plots revealed that the NWs behave as a composite anode material as both the Ge NWs and the Sn seed reversibly alloy with Li. We believe that the fabrication of Sn seeded Ge NW electrodes via the SVG system is a scalable method and their application as anodes for Li-ion batteries offers a viable alternative to conventional graphite electrodes, as they exhibit comparable stability and higher capacities over extended cycles. Furthermore the excellent high-rate capabilities while discharging suggest that the NWs may also be suited for high power applications that require very high discharge rates such as battery electric vehicles and power tools.

—Kennedy et al.

The research was supported by Science Foundation Ireland (SFI) under the Principal Investigator Program to Dr Kevin Ryan and also by EU funding through the GREENLION Project. The GREENLION project is a large scale collaborative project within the FP7 framework with the goal of manufacturing greener and cheaper lithium-ion batteries for electric vehicle applications.

Resources

  • Tadhg Kennedy, Emma Mullane, Hugh Geaney, Michal Osiak, Colm O’Dwyer, and Kevin M. Ryan (2014) “High-Performance Germanium Nanowire-Based Lithium-Ion Battery Anodes Extending over 1000 Cycles Through in Situ Formation of a Continuous Porous Network,” Nano Letters doi: 10.1021/nl403979s

February 10, 2014 in Batteries | Permalink | Comments (14) | TrackBack (0)

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Comments

Nice results, but GE is way too rare to be used for mass production of batteries for EV, but they made it clear. If they can repeat it with Si would be great

Now I KNOW we're all getting numb to battery announcements when even Harvey won't chime in with:

This could lead to the batteries we need by 2020 with 400 mile range....

:-)

If 5-5-5 is to succeed, battery capacity should roughly doubled, while costs halved - after 15 months.

@Dave D:
Yep, I've become a new battery chicken!

A lot of words and numbers, yep no tangible results. Till then I keep driving my gasser and wait for an economical hydrogen method of producing the gas for el cheapo price.

Keep driving slow, this is the only science available today.

Totally cracked me up, DaveD. We love you Harvey.

I don't need 5-5-5. I think 2-2-2 would be a game changer, but I haven't seen that either.

"Now I KNOW we're all getting numb to battery announcements when even Harvey won't chime in with:

This could lead to the batteries we need by 2020 with 400 mile range...."

DaveD, that was funny! I love it. Might I even suggest adding a '?'... like this...

This could lead to the batteries we need by 2020 with 400 mile range....?

Lol!

Come on guys Harvey is our resident optimist, without that we would all be cynics. The materials are expensive, but like research findings in general they could point to something when scientists ask why it works.

SJC,
Oh, believe me, I admire Harvey's optimism and hope and wish I could have more of it myself. I was just giving him a little ribbing in a light-hearted and friendly way...hence the smiley face :-)

Dave

When it comes to innovation, optimism is not about making overly rosy prediction or wishes, but just looking on the positive evolution of things. But having unreasonable expectation won't move things faster, I don't think that 300miles range EV quoted below 30K$ will be that popular before 2020, I just don't see it. Technology is moving too slowly

@ All Y'all,

Many of us have the 1,000-prediction-stare, and the only thing we've seen produced is Panasonic cranking out incrementally better 18650's for Tesla.

That said, when batteries finally achieve the 300-mile range and 1,000 cycles for a lot less money we're all hoping for, do you think it will be a series of incremental improvements, or do you think a single breakthrough will make practical a home-run technology like Sulfur/zinc/LiAir/xxx(your ad here)?

Let's not forget that 5-5-5 batteries are and will be preceded and followed by:

a. 1-1-1 batteries since about 2010 in Nissan Leaf and others.

b. 2-2-2 batteries in current Tesla Model S since 2012.

c. 3-3-3 batteries in Tesla Model E by 2015/16 or so.

d. 4-4-4 batteries by 2018/19 or so.

e. 5-5-5 batteries by 2020/21 or so.

f. 10-10-10 batteries sometimes between 2025 and 2030.

I get your drift, Harvey. In very broad brush, yes.

But the 5x battery goal was set by DOE in November 2012. So we could see that in the Model E, which is expected at the end of 2017, or another make.

Some observers have said debut in 2015 but that is clearly optimistic. A prototype buck might be shown in 2015 but it won't be on the road until 2017 soonest.

Model X won't even be in volume production until Q1 2015, and that's if there are no delays, as there were with the Roadster and Model S (this stuff is tough, there are lots of dependencies on suppliers, and Musk is known to be a perfectionist, rightly so).

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