Li-Ion batteries have a well-rounded balance of cost, weight, and capacity. This has led to their widespread adoption in high-tech and lightweight applications. Their success over other rechargeable batteries, in nearly every consumer goods market, has led to a renewed expectation of better battery performance: faster charging and discharging while lasting longer. Along with raised expectations, the rapid growth of Li-Ion technology has opened new visions for what can be made possible through having more efficient energy storage—one of which is the widespread adoption of electric vehicles.
Global electric vehicle sales have grown at a 32% compound annual growth rate over the last 4 years1. While the total market for EVs is still a small percentage of the overall vehicle sales, Bernenberg Bank predicted that EV sales will gain a solid foothold by eventually breaking 5% of total sales by 20202. This rapid growth does not include a large number of commercial fleet vehicles adopting hybrid and EV technology, for example, UPS’s adoption of hybrid technology into some of its fleet3, Telsa’s commercial electric semi-truck4, and Workhorse Group’s electric fleet pickup.
This adoption of electric and hybrid vehicles relies heavily on the progress of rechargeable technology—demanding longer vehicle range, better performance, and lower costs. Lithium-ion batteries are particularly suited for this type of application with properties including:
- Highest energy density of any mass-produced battery
- Low maintenance requirements
- High degree of design flexibility
- Relatively negligible memory effect
- Low self-discharge rate
- Nearly 3x the voltage capacity of the next level batteries at 3.6 V5
While Li-Ion batteries are the leading edge in rechargeable technology, the increased demands from electric vehicles have pushed the industry to increase energy density, charge/discharge capacity, and storage efficiency in these batteries. Unfortunately, as seen in the consumer goods market, these pressures have accentuated some of performance and safety limitations with Li-Ion batteries—particularly with the battery packs used in EVs. These packs have three main areas of both performance and safety limitations: regular use degradation, overheating and packing inconsistencies.
Regular Use Degradation
During thermal cycling, the charging or discharging causes internal resistance and thermal expansion which causes stress on the materials in the batteries, shortening their useful life. This puts a small amount of strain on the mechanical and material systems in the battery—which is relieved when the battery cools down to its original state. However, this cycle generates a cumulative effect of degrading the materials and putting expansion and contraction strain on mechanical system of the battery.
Pouch cells are particularly susceptible to this type of stress, whereas cylindrical cells tend to mitigate this type of life-shortening. Additionally, this thermal cycling accelerates material degradation. When the materials no longer perform with optimal materials, there is a significant loss of recoverable power and capacity.