There are few technologies, in today’s cutting-edge technological environment, that have a difficult time finding new levels of performance. Battery technology is one of them. With the exception of a few experimental offerings, batteries and their performance metrics remain much the same as they have been for decades.
That isn’t to say that there hasn’t been progress, there has. But, when compared to other technologies, progress in battery technology is slow and only incremental. Take, for example, semiconductors. Innovations such a FinFETs, 3D transistors, MuG/MiGFETs, tri-gates, etc., and fast-track evolutionary improvements in wafer geometries (300 nm to 10 nm, for example) improve performance significantly, sometimes by orders’ of magnitude; not so with batteries. And there haven’t even been any new (such as when rechargeable lithium were developed) tangential battery technologies in a long time.
However, that hasn’t stopped battery research. In fact, research within the battery field is expanding. With the IoE looming over the horizon, the need for high-density, micro-sized portable power is going to skyrocket. The cry for high density, small form-factor rechargeable batteries will be one of the biggest challenges for many of the new IoE devices (such a motes and Internet dust, remote miniature sensors, etc.).
But progress is slow, simply due to the physical metrics of rechargeable batteries. Because they are ionic, and rely on a chemical reaction, there is a limit to both what materials can be used and what processes can be employed to recharge them.
Since we really only know the metrics around a few of the broad scope of devices that will be part of the IoE, at this stage, trying to cross-tabulate battery technologies with devices, would be more of an exercise in prognostication than a expectation of reality. Therefore, the article will focus on the lithium battery technologies that will likely show up as the IoE unfolds. What technology subset will find its way into what devices is reserved future articles.
While batteries transverse a lot of different technologies, from alkaline, to zinc to mercury to lead-acid to NiCad, to NiMh, to the various flavors of Lithium, even biochemical, the technology of most interest for the IoE will ion-based.
Since battery technology is well understood and volumes have been written about it, it is left to the reader to investigate specific chemistries if they desire. This article will be bounded to the most promising; lithium-ion, and introduce some derivative such as lithium-sulfur and magnesium-Ion, and their technology. It will also discuss magnesium-sulfur batteries. For reference, appendix one is a chart that lists the characteristics of the more common technologies.
Battery Technology 101
All battery chemistries and technologies, primary or secondary are chemical in nature and that is how the battery produces electricity. Simply stated, the electrolyte is the medium that unites the anode and cathode, which completes the electric current. The anode experiences an oxidation reaction when two or more ions in the electrolyte combine with it. The reaction releases one or more electrons.
Simultaneously, the cathode goes through a reduction reaction when the ions, the cathode substance, and the free electrons combine. Basically, the reaction in the anode creates electrons, which are absorbed by the cathode, yielding electricity (see Fig. 1).
The generic chemical reactions’ formulas for lithium-ion (Li-ION) are as follows:
These reaction will be similar with any ion process. Of course for tangential materials the formula modifiers will change.
These formulas are the mathematical representation of the process. In short, they state that the lithium ion is the cation that travels from the anode to the cathode where it is ionized to form Li, and picks up one electron (Li+). The electrolyte can vary but is typically made up of lithium salts, (LiPF6, LiBF4, LiClO4, for example) in an organic solvent carrier, typically ether. The anode is generally a carbon such as graphite. The cathode is typically lithium cobalt oxide (LiCoO2). This configuration has an intrinsic voltage of 3.6 V, which is the reference voltage for lithium cells.
The Edge of Lithium Technology
Because of the extreme envelope of some of the IoE devices (ultra-lightweight, small and mobile,) battery technology for them will have to take a new vector. High energy density, ultra-small size, and long life are three of the primary requirements of these devices.
Fortunately. The last few years has seen a fair amount of resources being thrown at battery development. And results are showing up. While most of them are still on the drawing board, some have been prototyped, and the next five years promises to put some real advancements in battery technology on the table.
Raising the Li-ION Bar
The Li-ION camp has some interesting developments on the drawing boards, since Li-ION batteries aren’t like to go away any time soon. The biggest challenge is in the critical factors of this battery technology – capacity and charge rate, and how to improve the.
One novel approach promises to improve Li-ION charge life by an order of magnitude and increase capacity and cycle time by addressing charge density and charge rate. If successful, improving these parameters has the potential to, increase, significantly cell longevity and shorten the charge cycle. That means that the cell can last longer and charge faster. Another benefit is that the cell footprint can be reduced, bringing them closer to the realm of the miniature IoE devices that are also on the drawing board.
In today’s rechargeable lithium cells, the carbon-based graphene sheets, of which the anode is made, can bind six carbon atoms to one lithium atom. This has been the standard anode material for years.
One approach is to tighten up that ratio with alternative materials. As well, there are anomalies in the travel of the lithium ions along the graphene sheets to the rest area between them. These two parameters are the primary limiting factor in capacity and charge rates.
Experiments have replaced the graphene with silicon. Silicon can bind four carbon atoms to one lithium atom. While this may seem counterintuitive, it isn’t because silicon atoms are larger than carbon atoms. The math proves the principle, and it means silicon anodes, theoretically, would be able to store over 10 times the energy of graphite. However, silicon has one small problem; it is too malleable and expands and contract during charging. That process fragments and destroys the silicon in short order, and renders the battery useless. So while silicon addresses the first issue, capacity, that is negated by the instability of the silicon.
To combat this, a novel approach has been developed where silicon is placed between the graphene sheets. The combination of the two materials allows more lithium ions to accumulate at the electrode and also stabilizes the silicon.
A second trick is to create tiny in-plane defects (holes) in the graphene so the ions can move through the graphene instead of along it. That way, more ions get to the anode, faster, reducing charge time. The combination of these techniques increases the energy density, reduces the negative effects of silicon fragmentation and reduces charge time.
This is one example of research that is going on to improve Li-ION performance. There are other areas of research that focus on similar improvements in cathode technology as well as the electrolyte. Moreover, further progress is promised using nanotube technology but this is still very experimental.
Lithium-sulfur – Perhaps one of the more exciting areas of development in rechargeable lithium is in the lithium-sulfur (Li-S) technology.
Lithium-sulfur batteries have the potential to leave lithium-ion technology in the dust. There has been a heavy focus on the metal oxide component of the cell. The direction has been to use sulfur. Sulfur is cheap and plentiful, and weighs less than half as much as cobalt, atom for atom. It also packs over twice the lithium ions into a given volume vs. cobalt oxide.
However, there are some challenges. LI-S compounds are very difficult to manage. The sulfur has a tendency to combine with lithium. When it does, it forms a compound that crystallizes and gums up the cell’s components. It also has a tendency to crack under repeated cycling. The compounds also tend to leak from their place within the cell. So far, these issues end up rendering the battery useless after only a few dozen cycles, maximum.
To address the first issue, the answer has been to try to stabilize the cathode. One approach some researchers tried was to heat the sulfur to 185 degrees C. This will cause the element’s eight-atom rings to melt into long chains. Next they added a DIB, which is a carbon-based plastic precursor. That process will link the sulfur chains together. The result is what is called a co-polymer.
By Adding diisopropenylbenzene (DIB), the result was that the cathode can be prevented cracking, to some degree, which, in turn helps keep the Li-S compounds from crystallizing. While this approach has its merits, the overall success is marginal, since in tests, after 500 cycles, the battery retained only half of its original capacity. That may suffice in some applications, where float is the primary requirement but, for IoE devices, especially remote or autonomous ones, that regularly cycle, that is too few cycles. There is also similar research going on in cathode stabilization, using other materials.
To address the second issue, one vector has researchers developing microscopic, hollow carbon shells (which are conductive), coated with a polymer that is designed to contain the Li-S compound. The experiments seem to work. Under test, these structures were able to retain a much higher energy storage capacity (630 mAh/g) that the typical storage capacity of Li-ION (200 mAh/g). That energy density remained consistent through 600 fast charge and discharge cycles.
There is work being done at other facilities across the globe in various peripheral trajectories.
Magnesium-ion – Another very promising battery technology is magnesium-ion (Mg-ION). There is a lot of excitement for this technology, although it is still in its highly theoretical infancy. No working models have yet been developed. However, the promise of what it can deliver, as much as 12 times the energy density vs. lithium-ion, and a five times improvement in charge-discharge efficiency. That makes this a technology one to watch.
But, of course, it also is not without challenges. On the plus side, Mg is quite abundant, and generally cheap. It is also easier to handle than lithium. And, unlike Li, which is a one to one ion-electron transfer, with Mg the ratio is two electrons per ion. Theoretically, out of the gate, the capacity is doubled.
On the minus side, it has a higher number of issues than Li-based technologies; the most significant is that it is very difficult to plate and strip for battery construction. And, the double backpack of electrons slows the speed of the molecule through the electrolyte and electrodes to a crawl. Therefore, there is a flurry of research going on to find suitable electrolytes, and edge-of-the-envelope developments such as liquid electrodes, as well. And, not just for Mg, but for all ion-based battery technologies.
Expect to see a lot of progress in battery technology in the next few years. Much of it will focus on the big energy applications like vehicles, but as the IoE unfolds, there will be pressure to scale these new technologies down to the micro level.
Much of what has been discussed throughout this article is focused in applications such as Batteries for EVs and application that demand much higher energy sources than miniature IoE devices. With battery technology, it is just easier to work on a large scale, have success, then try to scale it down. There is also more money in the automotive and industrial segment, today.
There is a pressing need for high-density, low cost, small form factor, safe, and easy to manufacture batteries with an eye on the evolving IoE. Since the IoE is still more of a concept than a reality, the development is in areas that are here, now, but expect progress to ramp up significantly at the micro end in the next few years.
1. Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
2. Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
3. The discharge is highest immediately after charge, then tapers off. The NiCd capacity decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
4. Internal protection circuits typically consume 3% of the stored energy per month.
5. 1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference between the cells; it is simply a method of rating.
6. Capable of high current pulses.
7. Applies to discharge only; charge temperature range is more confined.
8. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
9. Cost of battery for commercially available portable devices.
10. Derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.