17 Jun 2017
Could a new material involving a carbon nanotube and graphene hybrid put an end to the dendrite problem in lithium batteries?
The high energy capacity of lithium-ion batteries has led to them powering everything from tiny mobile devices to huge trucks. But current lithium-ion battery technology is nearing its limits and the search is on for a better lithium battery. But one thing stands in the way: dendrites. If a new technology by Rice University scientists lives up to its potential, it could solve this problem and enable lithium-metal batteries that can hold three times the energy of lithium-ion ones.
Dendrites are microscopic lithium fibers that form on the anodes during the charging process, spreading like a rash till they reach the other electrode and causing the battery to short circuit. As companies such as Samsung know only too well, this can cause the battery to catch fire or even explode.
“Lithium-ion batteries have changed the world, no doubt,” says chemist Dr. James Tour, who led the study. “But they’re about as good as they’re going to get. Your cellphone’s battery won’t last any longer until new technology comes along.”
So until scientists can figure out a way to solve the problem of dendrites, we’ll have to put our hopes for a higher capacity, faster-charging battery that can quell range anxiety on hold. This explains why there’s been no shortage of attempts to solve this problem, from using Kevlar to slow down dendrite growth to creating a new electrolyte that could lead to the development of an anode-free cell. So how does this new technology from Rice University compare?
For a start, it’s able to stop dendrite growth in its tracks. Key to it is a unique anode made from a material that was first created at the university five years ago. By using a covalent bond structure, it combines a two-dimensional graphene sheet and carbon nanotubes to form a seamless three-dimensional structure. As Tour explained back when the material was first unveiled:
“By growing graphene on metal (in this case copper) and then growing nanotubes from the graphene, the electrical contact between the nanotubes and the metal electrode is ohmic. That means electrons see no difference, because it’s all one seamless material.”
Close-up of the lithium metal coating the graphene-nanotube anode (Credit: Tour Group/Rice University)
Envisioned for use in energy storage and electronics applications such as supercapacitors, it wasn’t until 2014, when co-lead author Abdul-Rahman Raji was experimenting with lithium metal and the graphene-nanotube hybrid, that the researchers discovered its potential as a dendrite inhibitor.
“I reasoned that lithium metal must have plated on the electrode while analyzing results of experiments carried out to store lithium ions in the anode material combined with a lithium cobalt oxide cathode in a full cell,” says Raji. “We were excited because the voltage profile of the full cell was very flat. At that moment, we knew we had found something special.”
Closer analysis revealed no dendrites had grown when the lithium metal was deposited into a standalone hybrid anode – but would it work in a proper battery?
To test the anode, the researchers built full battery prototypes with sulfur-based cathodes that retained 80 percent capacity after more than 500 charge-discharge cycles (i.e. the rough equivalent of what a cellphone goes through in a two-year period). No signs of dendrites were observed on the anodes.
How it works
The low density and high surface area of the nanotube forest allow the lithium metal to coat the carbon hybrid material evenly when the battery is charged. And since there is plenty of space for the particles to slip in and out during the charge and discharge cycle, they end up being evenly distributed and this stops the growth of dendrites altogether.
According to the study, the anode material is capable of a lithium storage capacity of 3,351 milliamp hours per gram, which is close to pure lithium’s theoretical maximum of 3,860 milliamp hours per gram, and 10 times that of lithium-ion batteries. And since the nanotube carpet has a low density, this means it’s able to coat all the way down to substrate and maximize use of the available volume.
“Many people doing battery research only make the anode, because to do the whole package is much harder,” says Tour. “We had to develop a commensurate cathode technology based upon sulfur to accommodate these ultrahigh-capacity lithium anodes in first-generation systems. We’re producing these full batteries, cathode plus anode, on a pilot scale, and they’re being tested.”
The study was published in ACS Nano.
Source: Rice University
10 Jun 2015
|Many of us would be hard-pressed to spend a day without using a lithium-ion battery, the technology that powers our portable electronics. And with electric vehicles (EVs) and energy storage for the power grid around the corner, their future appears pretty bright.|
|So bright that the iconic California-based upstart Tesla Motors stated that their newly announced residential Powerwall battery is sold out until mid-2016 and that the strong market demand could meet the capacity of their upcoming battery “gigafactory” of 35 gigawatt-hours per year – the daily electrical energy needs of 1.2 million US households.|
|When released by Sony in the early 1990s, many considered lithium-ion batteries to be a breakthrough in rechargeable batteries: with their high operating voltage and their large energy density, they outclassed the then state-of-the-art nickel metal hydride batteries (NiMH). The adoption of the lithium-ion technology fueled the portable electronic revolution: without lithium-ion, the battery in the latest Samsung Galaxy smartphones would weigh close to four ounces, as opposed to 1.5 ounces, and occupy twice as much volume.|
|Yet, in recent years lithium-ion batteries have gathered bad press. They offer disappointing battery life for modern portable devices and limited driving range of electric cars, compared to gasoline-powered vehicles. Lithium-ion batteries also have safety concerns, notably the danger of fire.|
|This situation raises legitimate questions: What is coming next? Will there be breakthroughs that will solve these problems?|
|Better lithium chemistries|
|Before we attempt to answer these questions, let’s briefly discuss the inner mechanics of a battery. A battery cell consists of two distinct electrodes separated by an insulating layer, conveniently called a separator, which is soaked in an electrolyte. The two electrodes must have different potentials, or a different electromotive force, and the resulting potential difference defines the cell’s voltage. The electrode with the largest potential is referred to as the positive electrode, the one with the lowest potential as the negative electrode.|
|Next-generation batteries could improve on energy density, allowing for longer run-time on electronics and driving range on EVs. (Image: Author and Wikipedia, Author provided)|
|During discharge, electrons flow through an external wire from the negative electrode to the positive electrode, while charged atoms, or ions, flow internally to maintain a neutral electrical charge. With rechargeable batteries, the process is reversed during charging.|
|Lithium-ion batteries’ energy density, or the amount of energy stored per weight, has increased steadily by about 5% every year, from 90 watt-hours/kilogram (Wh/kg) to 240 Wh/kg over 20 years, and this trend is forecast to continue. It’s due to incremental refinements in electrodes and electrolyte compositions and architectures, as well as increases in the maximum charge voltage, from 4.2 volts conventionally to 4.4 volts in the latest portable devices.|
|Picking up the pace of energy density improvements would require breakthroughs on both the electrodes’ materials and the electrolyte fronts. The biggest awaited leap would be to introduce elemental sulfur or air as a positive electrode and use metallic lithium as a negative electrode.|
|In the labs|
|Lithium-sulfur batteries could potentially bring a twofold improvement over the energy density of current lithium-ion batteries to about 400 Wh/kg. Lithium-air batteries could bring a tenfold improvement to approximately 3,000 Wh/kg, mainly because using air as an off-board reactant – that is, oxygen in the air rather than an element on a battery electrode – would greatly reduce weight.|
|A lithium air battery uses oxygen from the air to drive an electrochemical reaction – if it would work outside the lab. (Image: Na9234/wikimedia, CC BY)|
|Both systems are intensively studied by the research community, but commercial availability has been elusive as labs struggle to develop viable prototypes. During the discharge of the sulfur electrodes, the sulfur can be dissolved in the electrolyte, disconnecting it from the electronic circuit. This reduces the amount of lithium that could be removed from the sulfur during the charge and hurts the overall reversibility of the system.|
|To make this technology viable, critical milestones must be reached: improve the positive electrode architecture to better retain the active material or develop new electrolytes in which the active material is not soluble.|
|The lithium-air battery, too, suffers from this difficulty of being repeatedly recharged as a result of problems caused by reactions between the electrolyte and air. Also, with both technologies, protection of the lithium electrode is an issue that needs to be solved.|
|Savior in sodium?|
|For all of the aforementioned batteries, lithium is an essential component of the battery. Lithium is a fairly abundant element around the world but unfortunately only at trace levels, which prevents its worldwide commercial extraction. Although it is found in harvestable conditions in a few ores that could be mined, most of the production of lithium comes from brines of high-altitude salt lakes, mostly in the Andes in South America.|
|Despite this relatively difficult extraction, lithium carbonate can be found at around US$6 per kilogram, and since an electric vehicle battery pack requires only about three kilograms of lithium carbonate, its cost is not a major concern to date.|
|The concern here is more about geopolitics: every country seeks energy independence, and replacing oil with lithium batteries as a transportation fuel simply shifts the dependence from the Middle East to South America.|
|One possible solution would be to replace lithium with the element sodium, which is 2,000 times more abundant.|
|Electrochemically speaking, sodium is almost comparable to lithium, which makes it an extremely good candidate for batteries. Sodium-ion batteries research has exploded in recent years, and their performance, once commercialized, could be on par with their lithium-ion counterparts.|
|While sodium-ion batteries might not bring any significant cost or performance advantage over lithium-ion technology, it could offer a path for every country to manufacture their own batteries with readily available resources.|
|No matter what, all of these emerging technologies are likely to suffer from the same safety concerns as the current lithium-ion cells. The threat comes from the flammable solvent-based electrolyte which makes it possible to operate at voltages above two volts.|
|Indeed, because water splits into oxygen and hydrogen above two volts, it cannot be used in three volt-class lithium or sodium batteries and has been replaced by expensive flammable carbonate solvents. Alternatives such as solvent-free electrolytes do not provide a good enough conductivity for ions at room temperature to handle high-power applications, such as powering a car, and are therefore not used in commercial cells.|
|Fortunately, with the current lithium-ion technology, it has been estimated that only one in 40 million cells undergoes dramatic failure, of a fire. Although the risk cannot be fully suppressed, engineering controls and conservative designs can keep it in check.|
|In sum, the current lithium-ion batteries offer fairly good performances. Emerging chemistries such as lithium-sulfur or lithium-air have the potential to revolutionize portable energy storage applications, but they are still at the lab research stage with no guarantee of becoming a viable product.|
|For stationary energy storage applications such as storing wind and solar energy, other types of batteries, including high-temperature sodium-sulfur batteries or the redox flow batteries, might prove more sustainable and cost-effective candidates than lithium-ion batteries, but that could be a story for another article.|
|Source: By Matthieu Dubarry, Assistant Researcher in Electrochemistry and Solid State Science at University of Hawaii, and Arnaud Devie, Postdoctoral Research Fellow at University of Hawaii, via The Conversation|