Recharging The Battery

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 are relatively flat.

There has been some progress, of course. But when compared to other technologies such as transistors, memories, I/O and even power management, progress in battery technology is slow and only incremental. Moreover, there have not even been any new battery technologies in years.

Battery research is hardly standing still. On the contrary, research within the battery field is expanding. With the Internet of Everything looming over the horizon, the need for high-density, micro-sized portable power is going to skyrocket. The need for high density, small form-factor rechargeable batteries will be one of the biggest challenges for many of the new IoE devices, such as motes, Internet dust, and remote miniature sensors.

Yet progress remains slow 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, this article will focus on the battery technologies that will likely show up as the IoE unfolds, including lithium ion, lithium-sulphur, magnesium-ion and magnesium sulphur. 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. (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 Figure 1).

Figure 1: Li-ON cell cross section.

The generic chemical reactions’ formulas for lithium-ion (Li-ON) are as follows:

The first half of the reaction at the positive electrode is:

The second half of the reaction at the negative plate is:

These reactions 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, (LiPF^6, LiBF^4, LiClO^4, 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, over the last few years a fair amount of resources have been thrown at battery development. And results are showing up. While most of them are still on the drawing board, some have been prototyped. At this point it appears as if some real advancements will show up over the next five years.

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 those.

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 significantly increase cell longevity and shorten the charge cycle. That means 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.

A new approach is to tighten up that ratio with alternative materials. In addition, there are anomalies in the travel of the lithium ions along 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 more than 10 times the energy of graphite (which contains multiple layers of graphene). 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, which is 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.

Li-ion derivatives

Lithium-sulfur. 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 more than 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. And the compounds 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.

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 diisopropenylbenzene (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 DIB, the result was the cracking can be prevented, 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. 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—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) than 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, even though it is only theoretical. No working models have yet been developed. However, the promise of what it can deliver, as much as 12X the energy density vs. lithium-ion, and a 5X improvement in charge-discharge efficiency. That makes this a technology to watch.

But, of course, it 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 being that it is very difficult to plate and strip for battery construction. Moreover, 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. That’s true for all ion-based battery technologies, not just Mg.

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.

Conclusion
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 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 energy storage using a small form factor. It must be safe easy to manufacture, and with an eye on the evolving IoE. Because 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.

Appendix A

Source: batteryuniversity.com

Other pertinent metrics

• 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.

• Cycle life also is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges. 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.

• Internal protection circuits typically consume 3% of the stored energy per month.

• 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.

• Being capable of high current pulses applies to discharge only; charge temperature range is more confined. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.

• Cost of battery for commercially available portable devices is derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.