How much better? Let’s take a look.
There are four main rechargeable chemistries used today, lead acid, nickel cadmium, nickel metal hydride, and lithium ion, and conditions vary for how each is characterized in terms of capacity. Lead acid are tested using a C/20 discharge rate, while NiCd, NiMH, and lithium ion are all tested using a C/5 discharge rate. NiCd cells typically exceed their rated capacity by up to 10%, while NiMH often miss their rated capacity by up to 10%, and if we compress the discharge rate of lead acid testing from C/20 to C/5 making all things equal, we get a reduction in capacity of about 20%. So accounting for all these variations and adjustments, Table 1 roughly compares the chemistries on an even basis.
The lithium ion values represent a large format type manganese based cathode system similar to what are currently available from LG Chem and other manufacturers for electric vehicle applications.
|Chemistry||Nominal Cell Voltage||Capacity Ahr/kg C/5 discharge rate||Energy Whr/kg C/5 discharge rate||Life Cycles 100% DoD fade to 80%||Energy Life-Cycle Product kWhr/kg|
|Sealed Lead Acid||2.0||12.5||25||500||13|
|Nickel Metal Hydride||1.2||53.3||64||500||32|
At present, lithium ion offers nearly an order of magnitude more, or 10 times the energy return over their lifetime than the next best chemistry NiMH, and nearly 25 times more than lead acid. Development work around the older chemistries is mainly complete now and stagnant, while development work around lithium ion technology is still young and fast paced, currently attracting thousands of scientist and researchers from around the world working toward yet to be discovered improvements and unlocking tremendous potential that still exists.
Practically, lithium ion energy density could be improved by a factor of 2-3 over the next 5-10 years, while cycle life improvement could also be doubled or quadrupled during the same period, resulting in an overall improvement by another factor of 5-10 from today.
As well as having superior “Energy Life-Cycle Product”, lithium ion cells have many other superior features that are both electrically and physically attractive in areas concerning discharge profile, charge-discharge efficiency, cost, manufacturing, environmental, and recycling.