How long a lithium-ion battery will last depends on many factors
Lithium-ion battery life is defined in studies as beyond its useful life when its capacity falls by 20 percent or more.
Lithium-ion batteries start to degrade as soon as they’re made, and even when you aren’t using them, so drive as many miles as you can in the next 8 years to make the most if it, since not driving reduces the overall mileage you can expect to get.
Even the same model car will vary tremendously depending on how it’s driven
Temperature. This can change battery life by 5 years or more. Ideally a lithium-ion battery should be kept between 14-86 degrees Fahrenheit – above 86 F and the battery can be permanently degraded, so cooling technology is used in Tesla’;s Model S and the Chevy Volt, but not in the Nissan Leaf for protection. Below 14 the battery can’t provide full power.
Driving range (depth of discharge). If you drive long distances before recharging, you may shorten the lifetime to just 300-500 cycles and the battery capacity will drop to 70%. It will last much longer if you drive half or less the maximum range and then recharge, extending cycle life as high as 1,200-1,500 cycles. Fully charging isn’t good either, so the Tesla Roadster and other EV dno’t allow you to recharge more than 95% of the original power or drain the power to less than 2%.
To compensate for capacity loss, EV manufacturers increase the size of the batteries to allow for some degradation within the guaranteed service life, but that increases vehicle weight, battery cost, and lowers the driving range and efficiency.
Be skeptical of “breakthroughs” such as the Oak Ridge National Laboratory battery that retains 90% of capacity after 10,000 cycles but doesn’t mention energy density in “Solid electrolyte: the key for high-voltage lithium batteries,” Advanced Energy Materials (2014). Any advancement in one area almost always results in a loss in other area(s), as explained in Who Killed the Electric Car.
According to Popp et. al. in their 2014 “Lifetime analysis of four different lithium ion batteries for plug-in electric vehicle” for Transport Research Arena, Paris, the commercial Nickel-cobalt oxide version is superior to all other experimental cells in their capacity, energy content, energy density, and series resistance, but have the worst environmental impacts.
We won’t know until 2020 how long EV batteries actually last, when they start to decline in significant numbers. Newer chemistries implemented meanwhile will keep everyone guessing.