Tuesday, October 27, 2015

The Hero with four faces: Part 2

In this post we continue looking at the best-known battery chemistry for each of four metrics of importance:  energy density, charge time, life, and cost, aka, a “Hero” battery.  In Part 1, we looked at energy density and charge time.  Today, we will look at cycle/calendar life and cost.

And oh… there really is another metric that is also important (arguably more important!): safety.  I talk about this metric at the end.    

The cycle/calendar-life Hero:  Many battery people will argue that the Edison battery (yes, that Edison) is the Hero for cycle and calendar (or shelf) life.  And rightfully so.  I have a different Hero in mind, but, I can see why others think of the Edison battery.  I have heard that there are 30-year old batteries that still work!  And they cycle thousands of times, even tens of thousands of times.    

Family heirloom, anyone?

The Edison battery has a nickel hydroxide positive electrode (or, as battery people have been calling of late, cathode) and an iron negative electrode.  The positive electrode undergoes intercalation of protons, which I like (If you want to know why, read “A Brief History of batteries-Part 1 and while you are at it, read Part 2 also); it undergoes no disruptive structural change, although there is lattice expansion that occurs due to the co-intercalation of the water and potassium ions.  There is a side reaction wherein oxygen is evolved throughout the charge/discharge cycle, but, for reasons that I will get into soon, this is not an issue.  This electrode is virtually indestructible. 

The iron electrode undergoes a reaction by which the metal anode reacts with the electrolyte to form an hydroxide layer.  Lithium batteries have a similar reaction, by the way.  The Li metal reacts with the solvent and forms a layer.   Except that the layer is not reversible.  Luckily, the iron hydroxide is very reversible.  So much so, that we can cycle it thousands of times. 

So why are we all not taking to our lawyers about changing our living will to bequeath a nickel-iron battery to our children?  For one, the energy density is kind of low.  Very low!  At 25 Wh/kg, capacitors start looking interesting!  

 There is a second problem.  The battery requires regular topping off with water.  Like.  Every. Week.

Similar to the old lead-acid batteries.  The positive electrode of the lead-acid battery evolves oxygen during operation (so does the nickel-metal hydride battery).  The oxygen comes from water electrolysis.  Any oxygen that is released is removed from the battery via the vents.  A few weeks of this and the water level decreases.  Hence we fill in deionized water. 

Obviously, everyone hated the tedium of opening the hood, opening the vent, checking the density, and adding water.  When I could be using those precious 3 minutes to watch Gotham instead.   The humanity!  (well.. ok, there are some corrosive chemicals to deal with, so don’t try this at home). 

In lead-acid (and in the nickel-metal hydride battery) we solved this issue by “sealing” the battery.  This forces the evolved oxygen to migrate to the negative electrode and recombine to form water again.  Ergo, oxygen comes out, but nothing is lost.  Except electrons.  The battery self-discharges, but no capacity fade is observed. 

Why does this not apply to the Ni-Fe battery?  Because the iron electrode also breaks down the water and evolves hydrogen.  Hydrogen, unfortunately, does not recombine (at least not in any appreciable amount).  So any hydrogen coming out is lost through a vent.  So the water will dry up and you will be missing 3 vital minutes of insert-favorite-activity-here. 

So what do you do with a low-energy density battery that requires constant babying?  You keep it in the basement and bequeath it to you children and hope they forget that you really have nothing of value left.

Or you use it to learn some lessons.  Like, intercalation is good.  Or side reactions (like oxygen evolution) are ok, if they can be reversed.  Or making layers (like hydroxide) is perfectly fine, if said layers are reversible.  Or, water-based systems are very forgiving, and they can be maintained very easily. 

Are the problems in the Ni-Fe un-solvable?  I actually think they are not.  Other systems, like the zinc electrode, have had similar gassing issues and folks found that mercury was effective in poisoning the hydrogen evolution reaction (in addition to poisoning a few other things).  

Or maybe we can automate the watering system (a battery-drip-irrigation system anyone?).

Despite this, I don’t consider Ni-Fe to be the Hero because this system is not that practical, atleast as we know it today. My Hero is a chemistry that is close enough to the Ni-Fe system.

It is the Ni-hydrogen battery. 

This is the battery in the Hubble Space Telescope.  Launched in 1990, the battery operated in space for 19 years after which it was changed during service mission 4.  These batteries cycle 20,000 times when cycled at 50-60% of its full capacity and have an energy density of 75 Wh/kg; close to other nickel-based batteries.  Now that sounds like a Hero to me. 

So why does it cycle so well?  The positive electrode is the same nickel electrode that I like so much.   The negative is an electrode that evolves hydrogen on charge and recombines to form water on discharge. 

Wait… did we not just conclude that making hydrogen is bad?  Like seven paragraphs ago.  Contradiction much?

In my defense, I only contradict myself between posts, not within posts. 

In the Ni-hydrogen cell someone really made lemonade out of that lemon.  They decide to use the gassing reaction as the reaction to store energy.  But then, the question is: How do we store the gas? 

Therein lies the genius of the design: The battery sits inside a pressure vessel.  When the battery charges, the pressure builds inside the pressure vessel, but it stays inside.  There are no side reactions.  When you discharge it reacts right back.  The nickel electrode still has the oxygen side reaction, but this stays within the pressure vessel and recombines.  Nothing is lost.  Like I said, genius.  Of all the innovations in batteries I have seen and read about, this is the one that I’m still impressed by. 

If only we could get our Li-ion batteries to cycle the way of these older systems.

All seven readers of this blog know that there must be a catch with this chemistry.  In this case, it is cost.  Adding the pressure vessel does not help.  I don’t have any hard numbers, but I’m told it is several thousands of dollars a kWh.

Which bring me to…

The Cost hero:  This is a hard one.  But let me start with a few irrefutable facts:
  • All battery companies will drive cost to less than $100/kWh sometime in the future, irrespective of the chemistry.  But, what if the battery is made purely of gold, you ask?  Trust me, it will still cost less than $100/kWh. 

  • This price will drop to whatever-the-number-needs-to-be, to be less than or equal to whatever-is-claimed-by-competing-companies.  

  •  The actual math behind the claims will be guarded in a fashion that would make the CIA proud and will consist of various confusing assumptions related to cost of materials, cost vs. price, claims about cycle life, assumptions about Indians dumping the gold hidden in their mattresses etc.  

Now that we have settled on that, let us talk about cost. 

One can define cost a few different ways: Either the capital cost of the battery or as a cost per cycle (or charge passed) over the life of the battery.  Cost drops with scale of manufacturing (think, Tesla’s Gigafactory) and as this scale increases the cost of materials starts to dominate the overall cost of the battery.  Packs cost more than the cell, but as the chemistry gets more complicated (read, Li-ion) the pack costs are significantly higher than cell costs compared to simpler chemistries (read, lead-acid).  And then there is cost and price.  How is one to define a Hero, far less identify one?

The cheapest battery one can buy is the lead-acid battery which costs anywhere from $80/kWh to $150/KWh (and more, depending on the complexity of the system).  Unfortunately, the cost and the cycling go hand-in-hand.  So deep-cycle batteries cost more than backup-power batteries. 

And how long would a deep-cycle lead-acid battery last?  The 8 years and one thousand cycles that some companies promise, or the 5 years and a few hundred cycles that most seem to get?  Too many variables to make a judgment.

Then there is the Li-ion battery where the cost is expected to drop and we are not sure where it will end up.   Three different cost estimates suggest that the costs will hover somewhere between $170-220/kWh at the system level.  So twice the cost of the lead-acid, but with twice the life?  Assuming that the predicted cost reduction pans out.  And will lead-acid also drop in the same time?  Probably.    

Considering where Li-ion battery costs are today (at $350/kWh or more), one could call the lead-acid battery the Hero. But let us wait for 5 more years and revisit this.  We are living in a very interesting time indeed! 

So let us forget about identifying Hero’s and let us get down to things we can learn. 

It helps to have cheap materials.  Things like zinc, carbon and manganese are good.  Lanthanum-nickel alloy and cobalt oxide, not so much. 

But we cannot get too carried away with this either.  One of the cheapest batteries, from a materials perspective, should be the sodium-sulfur chemistry (the cost of these two materials would in the cents/kWh range).  But to actually make the sodium and sulfur work in a battery setting requires solid conductors and high temperature operation.  The final cost: $400/kWh or more! 

It helps to have aqueous systems: the manufacturing is simple.  No need for dry rooms.  And there is a certain amount of system-level advantages to be gained when complex pack designs and battery management systems can be avoided.  So lead-acid-like design and manufacturing would help.

But cost is really cost per unit of energy ($/kWh).  So low energy systems don’t help.  Li-ion stands a gain because of this.  But the advantage of Li-ion (the high voltage compared to aqueous systems) is also the reason we need equipment with high cap. ex. 

If you did not know this already, there is no free lunch. 

These are the four Hero’s.  Each metric has a different Hero (lithium thionyl chloride, lithium titanate, nickel-hydrogen, and lead-acid).  High energy requires high voltage.  This tends to kill the charging capability.  If you want life, it is best to have reversible side reactions.  And cost… what can I say about that one! 

Hold on a second!  Clearly I’m not addressing the elephant in the room.   The one problem that I really ought to be providing insights about and identifying a Hero for: safety. 

A Safety Hero? Well… If you thought cost was hard, safety is even worse.  Batteries are energy storage devices where the oxidizing and reducing chemicals are stored right next to each other.  So asking me to identify a safety Hero is like asking which one of the unsavory characters in Pulp Fiction is the good one (The Wolf?  Haven’t you told your best buddies that you would help them move a dead body?)

I would have said that any system that does not have a flammable electrolyte would help.  But the lead-acid fire at the wind farm at Kahuku, HI would make that sentence a bit hollow.  Clearly, if you work hard at it, any battery can go up in flames!

Having said that, we have been using batteries day in and day out for a long time without anything bad happening, so one can learn some lessons from it.   

Clearly flammable electrolytes don’t help.  Materials that can result in exothermic reaction don’t help.  Bigger the amount of chemicals next to each other, bigger the risk.   So flow batteries will be inherently safer than contained batteries. 

If you ask me for a ranking, Li-ion would be the worst; aqueous chemistries are MUCH better; flow batteries would be the safest.

But safety is not just about fires.  Many flow devices have toxic and corrosive chemicals in them.  These devices have to be contained.

What this all means is that a lot depends on the battery company.  If care is taken to make the battery safe, they can be made safe.  But that costs money.  Companies that pay attention to this and spend the money to do it will find ways to keep the battery safe.

Summary:  So what do our four (or five) Heroes teach us?   That buying a battery is like buying a house; you have to compromise. 

Remember the Third law of batteries from my blog post The Three Laws of Batteries (and a Bonus Zeroth Law):  “Of the four metrics that batteries are graded on for a given application (i.e. performance, cost, life, and safety), typically, only two can be simultaneously achieved. If the battery is designed to also perform satisfactorily on a third metric, it will fail spectacularly on the fourth.”

That about sums up the dilemma. 


1 comment:

  1. How about a tabular data with metrics on columns and chemistry in rows? I want to see where NiMH sits compared to LiOn.