A somewhat past prime lemon was sitting around making me wonder.. Let's say you're a Florida citrus baron and your car's battery is run down. You remembered the lemon battery you helped your kid make for a sixth grade science project and wonder how many lemons would it take to crank the car?
A lemon battery is pure simplicity. Push a galvanized nail and a copper wire into the unsuspecting piece of fruit and connect it to something like a light emitting diode current flows through the completed circuit and the bulb glows. The flow of electrons from the zinc to the copper only happens when the circuit completes. In theory the battery could provide power as long as there is zinc and an acidic enough environment to keep the reaction going. In reality there are many factors that shut things down before the zinc is completely dissolved.
I had never made the measurement. A minute playing around and the Zn-Cu battery measured 0.82 volts at 1.2 milliamps - just under a milliwatt. A average car starter needs about 3 kilowatts of power to crank an engine. Assuming you have some way to connect your fruit without loss, you'll need about three million lemons. Calling AAA is a bit more practical. Lemon batteries score high as science education demos, but low on almost every other battery metric you can imagine.
You can almost go to the periodic table, pick a couple of elements, and make a battery. Most will be even worse than even our lemon Zn-Cu battery, but a few will stand out in one way or another. Depending on the application you are interested in six major criteria: how much energy can be stored (energy density), how much power can be delivered (power density), lifetime (how many charge/discharge cycles), charging rate, cost, and safety. Unfortunately no single chemistry is optimal across more than one or two criteria. But then good engineering has always been about finding a good-enough optimization.
Smartphones and cars make us think of lithium ion batteries. If you're building a space probe nickel-hydrogen is your friend. It doesn't match lithium-ion in most criteria - it costs several hundred times as much per kilowatt-hour - but it can go through tens of thousands of charge-discharge cycles. Not having to replace a battery after a few hundred cycles when you're in deep space makes it seem cheap. Flow batteries are relatively inexpensive and can go through a large number of charge/discharge cycles, but have a low energy density that relegates them to stationary use. Some lithium-ion chemistries have low costs, but tend to be highly flammable. Power densities - how quickly energy can be transferred from the battery - vary widely ... a very important automotive consideration. Energy and power densities are stronger functions of temperature in some chemistries. This may be ok if your battery is in an indoor power farm, but a serious problem in cars and smartphones. Dozens of other battery chemistries have their niches, but a lithium-ion comes closer to satisfying more of the criteria than most competitors for many consumer applications.
Lithium-ion is a catchall term describing dozens of chemistries. It only tells you the charge is carried by a lithium ion. The important bits that define a battery - its anode, cathode and electrolyte - are not specified. The graphite-cobalt oxide battery in your smartphone has very different characteristics from the graphite-lithium manganese oxide battery in your portable drill. The battery in your smartphone is not ideal for automotive use, but the fact billions are manufactured made them cheap enough to balance some of their other problems. Tesla felt the wide availability of consumer electric class batteries was enough of an advantage that they could accept some of its issues. So far that seems to be working well.
Although smartphones are the current major user of lithium-ion batteries we'll see a transition to automotive use rather soon. A plug-in EV requires about 5,000 times as much power storage as a smartphone. Two hundred thousand EVs is about a gigaphone of batteries. For automobiles it is far from clear which chemistry will win in the short or medium term. The hope is that entirely different chemistries will become practical in the long term. I'm also hoping for improvements in smartphone class batteries. We may see a factor of two in power density in the next decade, but I'm getting ahead of myself.
In principle a lithium ion rechargeable battery is simple.2 Generically they consist of two electrodes separated by a thin membrane and an electrolyte that allows ions to freely pass from electrode to electrode. When the battery is charges lithium ions are released from the positive electrode, the cathode, which is a lithium alloy such as lithium cobalt oxide. The ions move towards the negatively charged electrode, the anode. Anodes are usually made from graphite and the ions find spots between the carbon atoms to settle in. This process requires power electric power to drive it.
To use the power just complete the circuit by connecting the electrodes to something like a motor. The lithium atoms embedded in the graphite give up electrons which travel through the circuit running car's electric motor. At the same time lithium ions leave the graphite and travel through the electrolyte and membrane to the cathode where they meet up with electrons that have made the trip. That's it - a nice reversible system.
Graphite is used as an anode largely because its a good conductor, but it isn't efficient at storing lithium ions during charging - it takes six carbon atoms to hold one lithium ion. Silicon seems more attractive - in theory one silicon atom can hold four lithium ions. A silicon anode can, in theory, store much more energy. Unfortunately material science hasn't come up with a good way to make a stable anode that can take many charges and the rigors of movement. Well - perhaps that is temporary. There have been a few exciting laboratory developments that might yield at least a factor of two better energy density and perhaps at a lower cost. You could double the range of the Bolt or Model 3 to four hundred miles. Assuming one of the developments work out a viable automobile battery is probably a decade out, but there are signs of progress.3 Currently lithium-ion batteries have about 1/80th the energy density of gasoline. In automotive use it isn't quite as bad as it sounds as electric drive trains are more than three times as efficient as internal combustion drive trains and you can get away with some extra weight. A factor of two would be an enormous improvement that would answer most range anxiety worries. Do it at a low enough cost and you have to worry about ramping manufacturing and charging infrastructure quickly enough to meet consumer demand.
The long term may move beyond lithium-ion chemistries entirely. Metal air batteries use metals like aluminum, lithium, magnesium, silicon and the oxygen in air along with aqueous electrolyte. In theory they have energy densities approaching gasoline when you factor in the efficiency of the electric drive train. Non-rechargeable primary batteries have been built, but a practical rechargeable is only a dream at this point.
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1 If you've done it right citric acid causes zinc on the nail into the lemony pulp as charged ions leaving electrons behind in the metal. ( Zn → Zn2+ + 2e- ) Positively charged hydrogen ions in the pulp, technically the electrolyte, combine with a couple of electrons at the copper surface forming hydrogen ( 2H+ + 2E- → H2 ). The oxidized state of Zn has lower energy and the energy released provides the power that flows through the circuit lighting the light emitting diode.
2 Rechargeable batteries are called secondary batteries, non-recharcables are primaries. Although primaries often have much better energy densities than secondaries, replacing them ranges from very expensive to impractical. An iPhone 6 Plus battery stores about 15 watt-hours - that would be nine AAA alkaline batteries. The iPhone's battery should be good for about three years of charges. If you need to recharge your phone every day you'd pay about eight dollars a day for three years - $2,800 - just for batteries! Imagine the landfill and production issues.
3 I'm partial to a novel silicon approach that uses nanoparticles. It should be stressed there are several very serious approaches underway with a dizzying press release schedule. Most of them will not amount to anything commercial, but are adding to the knowledge base - at least those that are published. There is no guarantee solutions exist. An area where software is not eating the world...
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Recipe Corner
Nothing to report foodwise other than my seasonal shift to my pressure cooker. They're safe and much faster than normal cooking - just the trick for keeping the kitchen cool.
The reason they work is not their higher temperature (250°F is not that much above boiling). Steaming is efficient as a huge amount of energy is released in the form of latest heat. But the mechanism is often confused ... It isn't forcing liquid into the food - that turns out to be a very slow process. It turns out to be the density of the steam. Density increases by a factor of two so the steam contains twice as many water molecules to deposit their latent heat.
You don't need to know how a pressure cooker works - just know modern cookers do a fabulous job. I even have hiked with a lightweight model. When you're in an area where you have to pack your fuel, being able to speed up cooking dramatically quickly reduces your fuel need and the total weight of your pack.
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