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by Eva Fox November 28, 2019

Rechargeable batteries are an integral component of energy-storage systems for electric vehicles and for grid storage (for example, for backup power during a power outage, as part of a microgrid, etc.). Depending on the application, the energy-storage systems require different properties. Tradeoffs in the chemistry of a battery system may need to be made to create a suitable system for a particular application. For example, in automobile applications—particularly those in an electric vehicle—the ability to charge and discharge quickly is an important property of the system. An electric vehicle owner may need to quickly accelerate in traffic, which requires the ability to quickly discharge the system. Further, fast charging and discharging places demands on the system, so the components of the system may also need to be chosen to provide sufficient lifetime under such operation conditions.

The average lifespan of a car is 150 000 miles. An average driver drives 13 500 miles every year meaning a car last about 11 years. Now imagine you had a car that could last 400,000 miles, 800,000 miles. How about 1 million miles? Your car could last an entire lifetime.

In April, Elon Musk announced Teslas would soon be powered by a battery with a lifespan of more than 1 million miles. "The current battery pack is about maybe 300,000 to 500,000 miles. The new battery pack that'll probably go into production next year is designed explicitly for a million miles of operation," said Elon Musk.

In September, a team of battery researchers at Dalhousie University, with support from Tesla, published a paper that describes a very special kind of battery - a battery that it says "should be able to power an electric vehicle for over 1 million miles."

Image from patent

Soon after, Tesla filed a patent for a battery with similar cell composition to the one in the paper.

Tesla filed a patent application on August 7, 2018, and published it on September 12, 2019.
"The present disclosure relates to rechargeable battery systems, and more specifically to the chemistry of such systems, including operative, electrolyte additives and electrodes, for improving the properties of the rechargeable lithium-ion-battery systems.
Improved battery systems with two-additive mixtures including in an electrolyte solvent that is a carbonate solvent, an organic solvent, a non-aqueous solvent, methyl acetate, or a combination of them. The positive electrode of the improved battery systems may be formed from lithium nickel manganese cobalt compounds, and the negative electrode of the improved battery system may be formed from natural or artificial graphite."

Many of the Dalhousie researchers including Jeff Dahn, Xiaowei Ma, and Stephen Glaxier, are listed as inventors.


So, is this the battery Musk needs? The paper presents the results of years of testing on a new battery cell formula and chemistry. And the team says the results from tests on a battery are “far superior” that other lithium –ion batteries.

So, what’s so good about it? Battery science is an exercise in experimentation. The right tweak in the combination and efficiency of the elements commonly used for batteries could yield big results. In addition to a winning combination, the million-mile battery uses large crystals instead of many small crystals. This single-crystal nanostructure is less likely to develop cracks when the battery is charging. Cracks cause a decrease in the lifetime and performance of the battery. 

How’d the new design do? Well, the life of a battery is measured in discharge cycles. Using an amount equal to 100% of the battery’s charge is one cycle. Where a typical lithium-ion battery could give you only 1,000 to 2,000 discharge cycles, tests showed the million miles battery had 95% of its life left after 1,000 discharge cycles and about 90% after 4,000.

Image: Business Insider

You’re probably thinking : “That’s awesome”, “We’ve got a great battery here.” And you’re right. This is an awesome battery. But we’re not going to see this version of the battery in a Tesla.
There is one major thing Tesla would have to sort out before it can use a battery like this in its cars. This cell chemistry uses a large amount of cobalt. Cobalt, a popular element in battery development carries inherent challenges. One, cobalt is finite running out. So it’s very expensive. Two, mining cobalt is hazardous and some cobalt mines have exploited children for labor.
Tesla is trying to eliminate cobalt from its batteries entirely.

So if we don’t have viable battery here, what the significance of the battery in the paper?
Well, it shows that we’re close to obtaining a battery that lasts a million miles and is compatible for use in Teslas, one that is cheaper and probably contains less cobalt. A major announcement is on the horizon. Specifically, we’re waiting on Tesla’s battery and Drivetrain investor day, which has been pushed back to early 2020. The paper and patent represented a big advancement in battery tech and are undoubtedly a preface to the actual million-mile battery composition, that will land itself in Teslas in the coming years.

Image from patent

Long-term cycling data plotted as percent initial capacity versus equivalent full cycles for NMC/graphite cells as described in the legend. The data from this work for 100% DOD cycling was collected to an upper cutoff potential of 4.3 V. The data from Ecker et al.,2 used 4.2 V as 100% state of charge. The purple and green data (this work) should be compared to the black data (Ecker et al.). Data for restricted range cycling (i.e. 25 – 75% SOC and 40 -60% SOC) for the cells in this work is not available but is expected to be far better than the data shown for 0 – 100% DOD cycling by analogy with the cells tested by Ecker et al.

Capacity remaining versus storage time for NMC/graphite cells as determined by reference performance testing every several months. The data from Ecker et al.2 and Schmitt et al.6 are for Sanyo UR18650E and Sony US18650V3 cells, respectively. The voltages and temperatures at which the cells were stored are given in the legends.

a) Measured properties of the NMC532/graphite 402035 (40 mm x 20 mm x 3.5 mm thick) pouch cells used here. The positive electrode was 94% active material, the loading was 21.1 mg/cm2 (target was 21.3) and the electrode density was 3.5 g/cm3. The negative electrode was 95.4% active material, the loading was 12.2 mg/cm2 (target was 11.8) and the electrode density was 1.55 g/cm3. b) Stack energy density of the NMC532/graphite couple for several electrode thicknesses. b) Stack energy density calculations – gives values for the electrode stack (negative coating/copper/negative coating/separator/positive coating/aluminum/positive coating/separator). Assumptions – copper foil = 8 μm, aluminum foil = 15 μm, separator = 16 μm, N/P capacity ratio = 1.1 at 4.3 V, average cell voltage = 3.75 V. The highlighted row represents the design used in this work.

 

Featured Image Credit: chargedevs


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