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Tesla's Quest for Better Batteries

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17:07   |   Mar 08, 2019

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Tesla's Quest for Better Batteries
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  • This episode of Real Engineering is brought to you by Brilliant.
  • A problem solving website that teaches you to think like an engineer.
  • Tesla has grown rapidly over the past decade, when it became the first American automotive
  • company to go public since Ford in 1956.
  • The attraction towards Tesla is undeniable.
  • Their cars are slick, their acceleration is insane and perhaps most importantly, their
  • brand represents a movement towards renewable energy.
  • Tesla has attracted thousands of well intentioned people who want to play their part in saving
  • the world, but there have been a niggling questions on the minds of many EV owners and
  • EV naysayers.
  • When is that expensive battery going to need to be replaced, and at what cost.
  • As existing Teslas begin to age, and more exotic and demanding models of Teslas come
  • to the fore, like the Tesla Truck and the Roadster 2.
  • These issues are going to become more prominent,
  • These batteries do NOT come cheap, but they are getting cheaper.
  • This chart shows the cost per kilowatt hour for Tesla powerpacks, and the market average.
  • Both dropping dramatically as technology advanced, and manufacturing volumes increased.
  • But that storage capacity slowly creeps away as the battery is used, slowly degrading the
  • range of your electric vehicle.
  • Tesla currently offers a warranty to all Model 3 owners that cover it below 8 years or 160,000
  • kilometres, whichever comes first.
  • Guaranteeing a retention of capacity of at least 70% when used under normal use.
  • If it falls below that, they will replace your battery for free.
  • Finding out what is considered normal use is pretty difficult, but they seem to be reasonable
  • with it going by customer satisfaction reports.
  • From our graph earlier, it’s estimated that Tesla is achieving a cost of 150$ per kwH
  • of battery packs, so the 50 kWh battery pack of the base model would cost around 7,500
  • dollars to replace, so they must be pretty confident on those numbers.
  • As a massive recall of the approximately 193 thousand Model 3s currently shipped would
  • ruin Tesla.
  • [3] Ultimately these batteries are unlikely to drop below the warranties guarantee in
  • those 160,000 kilometres, but even so improving batteries is obviously just a wise business
  • decision to retain those customers in future.
  • This is just one of a myriad of factors that influenced Tesla’s recent landmark acquisition
  • of Maxwell Technologies for $218 million dollars.
  • A rare Tesla acquisition that sets Tesla up for not just cheaper batteries, but better
  • batteries.
  • That will be lighter, have greater range, and live a longer life.
  • It wouldn’t be the first time an automotive company underestimated their battery degradation.
  • When the Nissan Leaf debuted in 2010, the battery production they needed simply did
  • not exist, and neither did the technical expertise required to design battery packs.
  • In those days lithium ion batteries cost about 400 dollars per kWh for laptop grade batteries,
  • and up to 1000 dollars per kWh for ones with the longevity needed for an electric vehicle.
  • To minimise costs Nissan decided to start production of their own batteries, and opted
  • for a small 24 kWh battery, giving it a range of just over 100 kilometres.
  • Suitable for city driving, and that’s about it.
  • But customers soon realised that this paltry range was dwindling quickly.
  • Within just 1-2 years of driving, the Leafs battery capacity was dropping up to 27.5 percent
  • under normal use.
  • [4] Despite careful in-house testing Nissan overlooked some crucial test conditions when
  • developing their battery, and because of this they made some crucial design errors.
  • To learn why this degradation happens, we first need to understand how lithium ion batteries
  • work.
  • A lithium ion battery, like all batteries, contains a positive electrode, the anode,
  • and a negative electrode, the cathode, separated by an electrolyte.
  • Batteries power devices by transporting positively charged ions between the anode and cathode,
  • creating an electric potential between the two sides of the battery and forcing electrons
  • to travel through the device it is powering to equalise the electric potential.
  • Critically, this process is reversible for lithium ion batteries, as the lithium ions
  • are held loosely, sitting into spaces in the anode and cathodes crystal structure.
  • This is called intercalation.
  • So, when the opposite electric potential is applied to the battery it will force the lithium
  • ions to transport back across the electrolyte bridge and lodge themselves in the anode once
  • again.
  • This process determines a huge amount of the energy storage capabilities of the battery.
  • Lithium is a fantastic material for batteries, with an atomic number of 3, it is the 3rd
  • lightest element and the lightest of the metals.
  • Allowing it’s ions to provide fantastic energy to weight characteristics for any battery.
  • But, the energy capacity of the battery is not determined by this, it is determined by
  • how many lithium ions can fit into these spaces in the anode and cathode.
  • For example, the graphite anode requires 6 carbon atoms to store a single lithium ion,
  • to form this molecule (LiC6).
  • This gives a theoretical maximum battery capacity of 372 mAh per gram.
  • Silicon however can do better.
  • A single silicon atom can bind 4.4 lithium ions, giving it a theoretical maximum battery
  • capacity 4200mAh per gram.
  • This seems great, and can provide increases in battery capacity, but it also comes with
  • drawbacks.
  • As those 4.4 lithium ions lodging themselves into the silicon crystal lattice causes a
  • volume expansion of 400% when charging from empty to full.
  • This expansion creates stress within the battery that damages the anode material, that will
  • eventually destroy it’s battery capacity over repeated cycles.
  • Battery designers are constantly looking for ways to maximise this energy density of their
  • batteries while not sacrificing longevity of the battery.
  • So what exactly is being damaged in the batteries that causes them to slowly wither away?
  • When researchers began investigating what caused the Nissan Leaf’s rapid battery degradation,
  • they began by opening the battery and unrolling the batteries contents.
  • They found that the electrode coatings had become coarse over their life, clearly a non-reversible
  • reaction was occurring within the cell, the change was expected.
  • In fact the chemical process that caused it is vital to the operation of the battery.
  • When a battery is charged for the very first time a chemical reaction occurs at the electrolyte
  • electrode interface, where electrons and ions combine.
  • This causes the formation of a new layer between the electrode and electrolyte called the solid
  • electrolyte interphase.
  • The name is exactly what it suggests, it’s a layer formed by the liquid electrolyte reacting
  • with electrons to form a solid layer.
  • Thankfully, this layer is permeable to ions, but not electrons.
  • So it initially forms a protective layer over the electrode that allows ions to enter and
  • insert themselves via intercalation, but it is impermeable to electrons.
  • [10] Preventing further reaction with the electrolyte.
  • At least that’s the idea under normal conditions.
  • [11]
  • The problem is, under certain conditions this layer can grow beyond just a thin layer of
  • protective coating, and result in the permanent lodgement of the lithium that provides the
  • battery with its energy storage.
  • This process is not entirely well understood and is outside the scope of this video, but
  • we can identify some factors that increase the rate of this formation.
  • The expansion of the silicon electrode battery we mentioned earlier causes the fracture of
  • the SEI layer, exposing fresh layers of electrode to react with the electrolyte.
  • Charging rate and temperature can also accelerate the thickening of this layer.
  • NASA performed their own in depth study of this effect, and released a report in 2008
  • titled “Guidelines on Lithium-ion Battery Use in Space Applications” sharing their
  • findings.
  • [12]
  • The temperature that the battery is charged and discharged at plays a massive role in
  • the batteries performance.
  • Lowering the temperature lowers chemical activity, but this is a double edged sword.
  • Lowering the chemical activity negatively affects the batteries ability to store energy.
  • Which is why batteries have lower ranges in cold countries, but lowering the chemical
  • activity also decreases the formation rate of that SEI layer.
  • This is on of reason the Nissan Leaf’s battery lost a huge amount of capacity over just 2
  • years in many countries.
  • Nissan performed most of its testing in stable laboratory conditions, not over a range of
  • possible temperatures.
  • Because of this they failed to realise the disastrous effect temperature would have on
  • the life of the battery, and failed to include a thermal management system, which is common
  • place in any Tesla.
  • This of course reduces the energy density of the battery.
  • Adding tubing, the glycol needed to exchange heat, along with the heat pumps and valves
  • needed to make a thermal management system, not only adds weight, but it draws energy
  • away from the battery to operate.
  • But it plays a vital part in maintaining the performance of the battery.
  • Nissan’s choice to not include a thermal management system, even in the 2019 version,
  • makes it a poor choice for anyone living in anything but a temperate climate.
  • Ofcourse, just cycling the battery though it’s charged and discharged states is one
  • of the biggest factor in degrading the battery.
  • Every time you cycle the battery you are giving the SEI layer opportunities to grow.
  • Minimising the number of times a cell is cycled will increase it’s life, and maintaining
  • an ideal charge and discharge voltage of about 4 volts minimises any resistive heating that
  • may cause an increase in chemical activity.
  • This is where Maxwell technologies comes into play.
  • Maxwell has two primary technologies that Tesla will be taking advantage of.
  • The first is what Maxwell are known for, their ultracapacitors.
  • Ultracapacitors serve the save fundamental job as batteries, to store energy, but they
  • function in an entirely different way and are used for entirely different purposes.
  • The fundamental difference between a capacitor and a battery is that a battery stores energy
  • through chemical reactions, as we saw for lithium ion batteries earlier this is done
  • through insertion into the crystal lattice.
  • Capacitors instead store their energy by ions clinging onto the surface of the electrode.
  • This is a standard ultracapacitor schematic.
  • On each side we have an aluminium current collector with thin graphite electrodes on
  • each, separated by an electrolyte and an insulating separator to prevent the passage of electrons.
  • In an uncharged state ions float in the electrolyte.
  • When a voltage is applied during charging, ions drift towards their opposite charge and
  • cling to the surface, holding the charge in place.
  • When a device is then connected to the capacitor this charge can quickly leave while the ions
  • drift back into the electrolyte.
  • The key limiting factor for ultracapacitors is the surface area available for this to
  • happen, and nanotechnology has allowed for amazing advances in the field.
  • This is what the inside of a ultracapacitor looks like, it contains hundreds of layers
  • of these electrode pairs.
  • But even with this enormous surface area, ultracapacitors simply cannot compete with
  • batteries when it comes to energy density.
  • Even Maxwell’s best ultracapacitors have an energy density of just 7.4 Wh/kg [13] while
  • the best guess for Tesla’s current energy density is about 250 Wh/kg.
  • Counter to what corporate owned tech channels may tell you, ultracapacitors are not intended
  • to be a replacement for batteries.
  • They are intended to work in conjunction with batteries.
  • Ultracapacitors strength is their ability to quickly charge and discharge without being
  • worn down.
  • This makes them a great buffer to place between the motors and the battery.
  • Their high discharge rate will allow them to give surges of electricity to the motors
  • when rapid acceleration is needed, and allow them to charge quickly when breaking.
  • Saving the battery from unnecessary cycles and boosting its ability to quickly provide
  • current when needed for acceleration.
  • This is going to be a massively important technology for two upcoming Tesla vehicles.
  • The Tesla Roadster, which will boast an acceleration of 0-60 in just 1.9 seconds, which a normal
  • battery would struggle to achieve the discharge rate needed without damaging itself.
  • The second vehicle is the Tesla Truck.
  • I have made a video in the past noting that the Tesla Truck is going to be limited in
  • its range and cargo hauling ability as a result of the heavy batteries it will need, as trucks
  • are limited in weight to about 40 metric tonnes in most countries.
  • This ultracapacitor technology will boost its ability to regain energy from breaking
  • significantly, and thus allow its battery capacity to decrease, in turn allowing the
  • truck to swap batteries for cargo.
  • The second technology Maxwell has been toting as their next big breakthrough is dry coated
  • batteries.
  • [9] This is a manufacturing advancement that Maxwell claims will reduce the cost of manufacturing.
  • A factor Tesla has been working fervently to minimize with the growth of the gigafactory.
  • So what are dry coated batteries.
  • Currently in order to coat their current collectors with the electrode material Tesla, in partnership
  • with Panasonic’s patented technology, must use first dissolve the electrode material
  • in a solvent which is then spread over current collector, both are then passed through an
  • oven for drying, where the solvent evaporates leaving just the electrode material behind.
  • This adds cost of the manufacturing procedure as the solvent is lost in the process, and
  • the baking process takes energy.
  • On top of this the solvent is toxic, so removing it from the process would benefit the environment.
  • Maxwell instead uses a binding agent and conductive agent, which I assume will work similarly
  • to electrostatic painting.
  • Where a metal being painted will be given a negative charge, while the paint will be
  • given a positive charge as it is sprayed attracting it to the metal where it will cling to it.
  • This painting process also eliminates the solvents needed in paint.
  • In this paper, published by Maxwell technologies, they detail how their dry coating manufacturing
  • techniques could result in a high energy storage capacity of the electrodes, due to a denser
  • and thicker coating.
  • Resulting a potential increase in battery capacity to 300 Watt hours per kilogram, 20%
  • up from our best estimates of Tesla’s current specs.
  • Only time will tell if this claim can be realised at an industrial scale.
  • Perhaps, more importantly to Tesla, they now own this manufacturing technique.
  • Currently Panasonic owns the manufacturing process for Tesla, there is a literally a
  • line of demarcation in the gigafactory separating Panasonic and Tesla, denoting the point at
  • which the ownership of batteries transfers hands.
  • Having to buy their batteries from Panasonic adds cost, that Tesla will want to avoid in
  • future and this step could allow for full vertical integration of their battery manufacturing.
  • Thereby making electronic vehicles more affordable to the everyday consumer.
  • All of this technology is powered by incredibly smart engineers working to solve really interesting
  • problems, and with so much focus on battery technology across the entire tech industry
  • there’s a high demand for qualified engineers.
  • For anyone looking to build or advance their engineering career I’d highly recommend
  • Brilliant.
  • Brilliant recently introduced a new feature, called “Daily Problems”, which will present
  • with you with interesting scientific and mathematical problems to test your brain.
  • Like this one, that teaches you about rolling resistance.
  • One of the ways vehicles lose energy.
  • It takes you through a short explanation of rolling resistance, giving you the framework
  • you need to rationalise a question they pose to you.
  • Here the answer is pretty simple.
  • Rolling resistance occurs from the loss of energy to the ground and wheels, so by driving
  • from gravel to concrete we lose less energy to the ground.
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References:
[1] https://www.bloomberg.com/news/articles/2017-12-05/latest-bull-case-for-electric-cars-the-cheapest-batteries-ever
[2]https://www.tesla.com/sites/default/files/downloads/tesla-new-vehicle-limited-warranty-en-us.pdf
[3]https://www.bloomberg.com/graphics/2018-tesla-tracker/
[4] https://batteryuniversity.com/learn/article/bu_808b_what_causes_li_ion_to_die
[5] https://batteryuniversity.com/index.php/learn/article/what_is_the_c_rate
[6] https://www.energy.gov/eere/articles/how-does-lithium-ion-battery-work
[7] Study of SEI. How it happens. http://www.upsbatterycenter.com/blog/new-research-real-time-view-battery-process/
[8] intercalation reference https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Book%3A_Inorganic_Chemistry_(Wikibook)/Chapter_08%3A_Ionic_and_Covalent_Solids_-_Structures/8.4%3A_Layered_structures_and_intercalation_reactions
[9] Dry battery explaination http://www.powersourcesconference.com/Power%20Sources%202018%20Digest/docs/3-1.pdf
[10] https://www.nature.com/articles/s41524-018-0064-0 best SEI resoruce
[11] https://arxiv.org/ftp/arxiv/papers/1210/1210.3672.pdf
[12]https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090023862.pdf
[13] http://www.maxwell.com/images/documents/3V_Flyer_3001369_EN_1.pdf
[14] https://cleantechnica.com/2018/06/09/100-kwh-tesla-battery-cells-this-year-100-kwh-tesla-battery-packs-in-2020/

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