What the physic limits to lithium ion battery costs?
There are six factors critical that will determine the widespread usage of batteries: energy density, i.e. how much electricity they're able to store per cubic cm or per kg; charge-cycle durability, i.e. the number of charge cycles a battery can go through before it wears out; charging time; robustness in the cases of temperature extremes; availability of fast-recharging stations; and price. These factors ultimately decide the cost of using a particular type of battery.
While regular lithium batteries have an energy density of 140 to 210 watt-hours per kilograms, there is also a special type called the 'lithium polymer' battery that can attain energy densities up to 260 watt-hours per kilogram. These special lithium batteries have a gel-like plastic as the electrolyte.
* Cycle life: This is the number of times the battery can be charged and discharged before it operates at less than 80% of its original capacity due to temperature and the depth of discharge. A "deep cycle" is usually defined as at least 80% depth of discharge.
* Depth of discharge: This is the amount of a full charge used during a cycle.
* Thermal runaway: This is the temperature at which the battery is irreversibly damaged.
Lithium battery limitations
Typical lithium batteries are limited to about 500 deep cycles, which is the number of times a battery can go from full charge to about 20 percent, before their performance deteriorates significantly. Although this may be acceptable in use cases like in smartphones and laptops, for use cases like in solar panels, these lithium batteries only last for a maximum of 16 months. Also, lithium batteries have a thermal runaway of 302 degrees Fahrenheit.
The lithium battery capacity is limited by the capacity of cathode rather than the capacity of the anode. Commonly used cathodes such as lithium cobalt oxide (LCO), lithium nickel manganese cobalt oxide (NMC), and lithium nickel cobalt aluminum oxide (NCA) have usable capacities of 140 mAh/g, 170 mAh/g, and 185 mAh/g, respectively, which are lower than the widely used graphite anode (experimental capacity: ~330 mAh/g). Lithium is a geographically concentrated and comparatively expensive material. Alternatives for better anodes include silicon but only a maximum of ~41 percent improvement can be achieved in gravimetric energy density on a cell level using silicon anode and existing cathodes. The problem is silicon anodes have a tendency to expand volumetrically up to 400 percent which makes it impractical for practical applications such as automotive and mobile computing. This expansion will damage the electrodes, delaminate the electrodes from the current collectors and separator, and reduce cycle life of lithium batteries.
Wolfgang Mack, vice president of business development at Menlo Park-based Capacitor Sciences, states that, based on the electrochemical potential, lithium-ion chemistries used in standard 18650-size cylindrical cells could theoretically reach an energy density of around 400 watt-hours per kilogram. But the commercially achievable limit is more likely to be around 260 watt-hours per kilo. According to a “first-principles analysis,” lithium-ion batteries have already reached 87 percent of their commercially achievable cell limit for energy density.
In conclusion, lithium ion battery have a physical limit of energy density of around 400 watt-hours per kilogram.