Engineers are familiar with non-experts who quickly extrapolate from recent technical advances to future results, casually stating “at this rate, such-and-such will reach ‘x’ in five years” or something along those lines. Engineers are also familiar with the real world of extending advances, which generally has complex nonlinearities in design, development, test, and manufacturing.
Let’s look at production first. It’s been demonstrated that once the initial issues are resolved, volume increases and costs drop, often dramatically, as production gains more experience on the sourcing, assembly, test, and costing There are hundreds of examples including ICs, smartphones, microwave ovens, and more. The slope and depth of improvement depends on many factors, of course, but eventually a lower limit is reached, but the shape of the curve is clear, Figure 1.
Figure 1 Once manufacturing gets going, works through the various impediments, and masters the process, progress is rapid until it reaches a plateau. Source: Bill Schweber
Ironically, the situation in the product design and development phase which precedes manufacturing is often the opposite. Instead, there’s the informal 80/20 (or sometimes shown as 90/10) rule, which states that completing the last 20% of a project often takes 80% of the time. To put it another way, that last 20% takes as much time and effort as the first 80%, Figure 2.
Figure 2 For design and development, progress is usually rapid until the final-stage issues need to be resolved, when it slows down considerably. Source: Bill Schweber
This is not surprising, since it’s those final details of design validation, prototype, test, tracking down and eliminating the last bugs, and similar that consume time and energy. There’s an inflection point in the rate of progress where the challenges and effort required increase drastically while progress is modest or marginal.
For example, trying to trim system dissipation by a small amount to get the run time past a marketing-defined objective can take as long as the bulk of the design/debug effort. This is a situation which affects leading-edge products such as smart phones as one obvious example.
That’s why it’s important to recognize that there are many times when performance which is “good enough” and appropriately tailored to the application and user needs, budget, and availability is the better design alternative.
Consider this: one area getting marginal gains at great cost R&D time and effort is batteries. It seems like every university is deep into battery research; among the many reasons for this is that’s where the grant money is these days. Of course, many commercial organizations and companies are also doing battery research, again using similar grants as well as their own or investor money.
Everyone is hoping to improve battery attributes such as capacity, charge time, number of operating cycles, safety, cost, manufacturability, and more. The harsh reality is that despite all this effort, actual progress over the last few years has been slow (except perhaps on cost, due to manufacturing progress), and is measured in a few percent or fractions of a percent.
Still, the aspects which researchers are trying to improve are not necessary priorities for everyone. Some end-users can accept reduced capacity (run time) if the tradeoff is a much lower cost, simplified materials and manufacturing, or improved safety, or environmental impact.
That’s why I was intrigued by the research paper “Water-in-Polymer Salt Electrolyte for Long-Life Rechargeable Aqueous Zinc-Lignin Battery” by a team based at Linköping University (Sweden) and published in the Wiley journal Energy and Environmental Materials. Their battery is not the best by any measure, but it is pretty good, low cost, and easy to fabricate, Figure 3. It’s a good fit with low-cost solar panels and can bring a modest amount of electrical power to areas where it is not available.
Figure 3 The battery developed by the researchers is small and the technology appears to be scalable. Source: Linköping University
They worked with zinc-metal batteries (ZnBs) which use widely available zinc and a non-flammable aqueous electrolyte. The primary difficulty with zinc batteries has been poor durability due to zinc reacting with the water in the battery’s electrolyte solution, which leads to the generation of hydrogen gas and dendritic growth of the zinc, thus rendering the battery essentially unusable.
To stabilize the zinc, they used a substance called a potassium polyacrylate-based water-in-polymer salt electrolyte (WiPSE). The researchers have demonstrated that when WiPSE is used in a battery containing zinc and lignin, stability is very high. Lignin is a tough, woody biopolymer that binds cellulose and hemicellulose fibers and provides stiffness to plants, and it is the second most abundant polymer after cellulose, Figure 4. It is a widely available byproduct of the manufacture of paper products. Both zinc and lignin are inexpensive, and the battery is easily recyclable.
Figure 4 Lignin is a waste product from the paper industry. Source: Linköping University
I won’t review the chemistry details of their project; you can read it in their paper if you are interested. They claim their battery is stable and can deliver over 8000 cycles at a high current rate of 1 amp/gram while maintaining about 80% of its performance, with 75% capacity retention up to 2000 cycles at a lower current rate (0.2 A/gram), Figure 5. In addition, the battery retains its charge for approximately one week, significantly longer than other similar zinc-based batteries that discharge in just a few hours.
Figure 5 a) Schematic illustration of Zn-lignin battery with WiPSE. b) Cyclic voltammetry (CV) of both Zn and L-C electrode in WiPSE showing that the device exhibits 1.3 V of cell potential. c) CV at different sweeps and d) GCD at different current rates of Zn-lignin battery in WiPSE. e) Presents capacity retention and coulombic efficiency estimation as cyclic stability analysis of Zn-lignin battery in WiPSE up to 8000 cycles at 1 A/gm current rate. f) Voltage vs time plot for 48 hours to analyze the self-discharge of Zn-lignin battery in WiPSE. Source: Linköping University
Whether this zinc-lignin battery will actually be deployed successfully in another story. Will all advances, and especially with batteries, there is usually a difficult journey with many problems when scaling from lab prototype to pilot production, and even more when transitioning from pilot phase to full-scale production.
Another virtue of these zinc-lignin batteries may be that they can be fabricated successfully on a smaller scale; that would be a major plus as well. Currently, the batteries developed in the lab are small. But they believe they are scalable—the weakness of so many battery advances.
Whatever the outcome, I like that these batteries do not attempt to outperform or even perform comparably to more expensive, complicated, and often hazardous lithium-based chemistries. Instead, they adapt to the fact that less stringent requirements mean that the R&D can possibly shrink the time-versus-hassle path before the curve’s inflection point is reached.
If only this were the case for many other projects which keep engineers working long hours, trying to get that last percent of performance increase or cost decrease—well, that would be nice but unlikely to happen.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
Related content
- Is a Concrete Rechargeable Battery in Your Future?
- What’s that?…A fuel cell that harvests energy from…dirt?
- Don’t ignore the need for small, low-capacity batteries
- Advanced tech gives insight into battery’s behavior
The post When is a “good-enough” battery good enough? appeared first on EDN.