PNNL team reports growth of dendrite-free lithium films with self-aligned and compact nanorod structure
02 December 2014
Suppressing lithium (Li) dendrite growth is one of the most critical challenges for the development of Li-metal batteries—i.e., high-energy density batteries using a Li-metal anode such as Li-sulfur or Li-air. (Earlier post.) Researchers at Pacific Northwest National Laboratory (PNNL) report for the first time the growth of dendrite-free lithium films with a self-aligned and highly compacted nanorod structure. Their paper appears in the ACS journal Nano Letters.
Lithium metal is a very promising anode material for high-capacity rechargeable batteries due to its theoretical high capacity of 3,860 mAh g−1 (~10x that of the 372 mAh g−1 of graphite anodes in Li-ion batteries), but it fails to meet cycle life and safety requirements due to electrolyte decomposition and dendrite formation on the surfaces of the lithium metal anodes during cycling. Thus, numerous efforts have been and are being made to develop a safe, extended cycling lithium-metal electrode and/or supporting electrolyte (e.g., earlier post, earlier post, earlier post, earlier post.)
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| Credit: ACS, Zhang et al. Click to enlarge. |
Most of the work in suppression of dendrite formation can be divided into three categories, the team noted in their paper:
Improve the stability and uniformity of the solid electrolyte interphase (SEI) on the Li electrode surface by optimization of solvents salts, and electrolyte additives. However, the SEI alone cannot completely eliminate dendrite formation/growth because of its weak mechanical strength.
Form alloys of Li and non-Li metal during electrodeposition, which is achieved by adding inorganic compounds or a second salt to the electrolytes. However, most of these metal cation additives will be consumed by forming alloys during Li deposition, so the suppression of Li dendrite formation may not be sustainable.
Use mechanical barriers to block dendrite growth. However, while such a protective layer acts as an effective physical barrier to block dendrite penetration, it does not alter the growth mechanism of Li dendrites on a fundamental level. As a result, porous Li may still be generated beneath the physical barrier and lead to a rapid increase in the impedance of the Li anode and battery failure even without a dendrite-related short.
The PNNL team itself had earlier developed a cesium hexafluorophosphate (CsPF6) as an electrolyte additive which was shown to suppress Li dendrite formation.
In this work, we further investigated the evolution of the cross-sectional morphologies of such Li films during deposition/stripping cycles. The effects of Cs+ and other components in electrolytes on the voltage profiles of electrochemical deposition and the chemistry of the SEI formed on copper (Cu) substrates prior to Li deposition are systematically investigated. It is revealed for the first time that the apparently dense Li films grown in the presence of CsPF6 additive actually consist of self-aligned, highly compacted nanorods. We also found that, before the reductive decomposition of carbonate solvents at about 0.9−1.2 V, a thin SEI has already formed on the Cu surface at about 2.05 V vs Li/Li+. The quality of this underlying SEI predecessor would essentially dictate the morphologies of the Li to be deposited.
The new understanding on the internal microstructure of the dendrite-free Li films deposited electrochemically, their structural evolution during stripping/deposition processes, and the synergistic effect of Cs+ additive and preformed underlying SEI on Li deposition will help researchers in this field design new approaches to enable a metallic Li anode, the “Holy Grail” of Li-based battery chemistries.
—Zhang et al.
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| Schematic of Li deposition and stripping processes when the Cs additive is used. (a) The initial distribution of cations/anions before Li deposition; (b) redistribution of cations/anions after an electric field is applied and the formation of the initial SEI layer (represented by a blue line) before Li deposition; (c) the initial growth of small Li (represented by gray area) tips; (d) the dendrite−free Li films with self-aligned and highly compact nanorods; (e) Li film after partial stripping; and (f) Li film after redeposition. The blue lines covering at the surface of deposited Li (gray) represent the SEI layer formed during the deposition and stripping processes. Credit: ACS, Zhang et al. Click to enlarge. |
Resources
Yaohui Zhang, Jiangfeng Qian, Wu Xu, Selena M. Russell, Xilin Chen, Eduard Nasybulin, Priyanka Bhattacharya, Mark H. Engelhard, Donghai Mei, Ruiguo Cao, Fei Ding, Arthur V. Cresce, Kang Xu, and Ji-Guang Zhang (2014) “Dendrite-Free Lithium Deposition with Self-Aligned Nanorod Structure” Nano Letters doi: 10.1021/nl5039117
Will that also allow fast than 0.5C discharge rates and >20KW charging of these Lithium metal batteries ?
Nice that it burns less, but if it still charges and discharges far too slowly, it could not be used on long Range EVs to make their so much expected 500Miles # 160KWH battery...
Posted by: Patrick Free | 02 December 2014 at 06:33 AM
Affordability, longer duration and major increased energy density are the basic requirements for future extended range BEVS
Charging/discharging rates (if it is a problem) can be dealt with during normal design evolution. However, it may always be longer than FCs and liquid fuel ICEVs.
Extended range (1000+ Km) BEVs would only need slower overnight charging for normal use.
Posted by: HarveyD | 02 December 2014 at 07:54 AM
Charging and discharging rates become moot when the battery capacity is very large. With compact, lightweight, energy-dense batteries at a reasonable cost, the large capacity is a given (consumer demand for several hundred miles range, regardless of in-town range requirements).
Cruising speed requirements are generally only about 15kW = 20hp. No on-street application requires full accelerator application for more than a few seconds.
Once range gets above 500 miles, the number of people who will drive further without an overnight charge gets very small.
Posted by: electric-car-insider.com | 02 December 2014 at 10:38 AM
Catalonia,Spain,Europe. Es desesperante ver muchas pruebas y más pruebas y ver que absolutamente nada avanza....Lo más real sera ver millones de coches a hidrogeno para finales de la decada...La batalla de coches electricos a bateria esta más que perdida.
Posted by: Centurion | 02 December 2014 at 01:59 PM
This could be great for Lithium-Sulphur, and a quick jump to 500 Wh/kg, if the chemistry is compatible.
Posted by: clett | 03 December 2014 at 02:21 AM
Magnesium has a theoretical energy density double that of Lithium. Additionally, Magnesium is an abundant element which is available worldwide (no OPEC-type monopoly) and its safer than Lithium. I'm placing my bets on Magnesium-Sulphur.
Posted by: yoatmon | 03 December 2014 at 02:49 AM
@ HarveyD : You are missing a point here. Long Range EVs are NOT required for users who only do local commutes (2nd car EVs). These ones already buy far cheaper Leaf like little EVs and are Happy with 25KWH, only expecting a little more, and I agree they only need to recharge every night what they consume every day, say 15KWH per day for my 65KM/42M local commutes, that can be charged on very slow chargers, for sure.
But Long Range EV customers are those who want "all purposes" family car replacement, that in addition to local commutes, can also take them to far away vacation places, a few times per year, with all family and their lugguage on board, typically doing up to 1000KM # 625 Miles per day, on the motorway. This is where 500M range # 160KWH battery pack could really be justified tomorrow.And there is no way you can ask such families to wait more than 1H per charge. Hence why Tesla rightly sized their SuperChargers at 135KWH in Europe, as this is EXACTLY what is required to 80% charge 160KWH batteries in 1H. The batteries that could not hold that type of charging rates will not be used on Long Range EVs, full stop. I typically do > 625M/1000KM per day on vacations trips down to south of Europe in the summer, or east of Europe the rest of the year. I expect that to require # 200 KWH per day. On 20KW chargers that would require 10 Hours per day to charge ! Just forget it. My family could wait maxi 2 x 1H for charging during such long vacation trips days. So 20KW charging not an option for that usage. If limited to that Metal Air batteries will only be OK for local commutes fast charging, or for stationary batteries for the Grid or Solar/Wind farms for ex, but not for Long Range EVs.
Posted by: Patrick Free | 03 December 2014 at 08:54 AM
@PF:
An interim solution would be to leave home with a full charge, pick up some electrons during lunch time, slow recharge overnight (10+ hours) while you have supper, rest and sleep.
Another solution would be to rent extra plug in battery modules for the occasional-infrequent long trips.
A third option would be to have tow BEVs, one for short daily trips and one for extended range trips?
Posted by: HarveyD | 03 December 2014 at 09:39 AM
Every EV advocate knows that huge ranges are not necessary. Yet, some wealthy people may choose to have huge range despite the lack of economy. The lack of range is one of the false negatives that has been used by the oil and ICE industry to denigrate EVs. The average person cannot think deeply enough to understand how this is false. Another false negative is safety. They say that these "batteries could blow up". Of course, they could, but they haven't and statistically speaking we know now that EVs are less likely to blow up than ICE cars with a tank of gasoline. Gasoline cars do blow up and people have died in fires and explosions in ICE cars. This has not happened for EVs, and if it were likely it would have happened by now. Except that we could know they are not likely to be as severe as an ICE car from a basic understanding of physics. The EV batteries have less volatile vapor organics and less energy. What can be released explosively or through fire is far smaller than a tank of gasoline. Everyone knows this, but some people make their livings telling everyone how dangerous the world is and that they are here to protect us.
Posted by: Brotherkenny4 | 03 December 2014 at 10:01 AM
B4:
In our very cold snowy area, a 200+ miles range EV would be a bare minimum for city trips. On a Snowy day, you can expect to be stucked in traffic for 2+ hours.
For long trips, a 400+ miles range would currently be a bare minimum due to the very few super quick charge facilities available.
With future lower cost (per kWh) much higher energy density batteries (up to 10X todays) there are no reasons why future post 2020) extended range BEVs could not cover a full day drive or about 500+ miles with a partial short lunch time recharge.
Posted by: HarveyD | 03 December 2014 at 02:44 PM