#phosphate-based lithium-ion

A battery is an electrochemical device that produces a voltage potential when placing different metals in acid solutions.The open circuit voltage (OCV) attained varies according to the metals and acid solutions (electrolyte) used.Applying a charge or discharge places the battery in the closed circuit voltage (CCV) condition;charging raises the voltage and discharging lowers it.This voltage behavior is governed by the internal battery resistance;a low resistance produces less fluctuation under load or charge than a high one.

Charging and discharging agitates the battery and a full stabilization takes up to 24 hours.Temperature also has a role;cold rises the voltage and heat lowers it.Manufacturers rate a battery by assigning a nominal voltage and with a few exceptions,these voltages follow an agreed convention.Rating some lithium battery higher than the standard 3.6V/cell may help in product marketing but for the user,a chemistry-specific voltage counts.Here are the nominal voltages of the most common batteries in brief.

Lithium-ion

The nominal voltage of lithium-ion had been 3.60V/cell.This is a practical figure because it represents three nickel-based batteries connected in series (3 x 1.2V = 3.6V). Some cell manufacturers mark their Li-ion products as 3.70V/cell or higher.This poses a marketing advantage because of higher watt-hours on paper (multiplying voltage times current equals W).It also creates unfamiliar references of 11.1V and 14.8V when connecting three and four cells in series.Let this higher voltage not cause confusion; equipment manufacturers will always adhere to the nominal cell voltage of 3.60V for most Li-ion systems,and the standard designation of 10.8V and 14.4V will always work.

The phosphate-based lithium-ion deviates from others in the Li-ion family and the nominal cell voltages are specified at between 3.20 and 3.30V.Because of the voltage difference,the two lithium-ion families are not interchangeable.New lithium 18650 battery will have other voltages and specialty chargers may be needed.

How did this higher voltage creep in?To calculate the nominal voltage,we take a fully charged battery that measures 4.20V and then fully discharge it to 3.00V at a rate of 0.5C while plotting the average voltage.For Li-cobalt,the average voltage comes to 3.6V/cell.Performing the same discharge on a fully charged Li-manganese with a lower internal resistance will result in a higher average voltage.Pure spinel has one of the lowest internal resistances,and the plotted voltage on a load moves up to between 3.70 and 3.80V/cell.This higher midpoint voltage does not change the full-charge and end-of-discharge voltage threshold.

Nickel-based

In consumer applications,NiCd and NiMH are rated at 1.2V/cell,industrial,aviation and military batteries adhere to the original 1.25V.There is no difference between the 1.2V and 1.25V cell;the marking is simply preference.

Primary Batteries

The alkaline delivers 1.5V,silver-oxide 1.6V and primary lithium 3V.The 9-volt battery has six cells in series.Do not charge primary battery because overcharge can produce explosive gases.

Lead Acid

The nominal voltage of lead acid is 2.00 volts per cell,however when measuring the open circuit voltage (OCV),the voltage of a fully charged battery should be 2.10V/cell.Keeping lead acid below 2.10V/cell will cause the buildup of sulfation.

Lithium-ion has not yet reached full maturity and the technology is continually improving.The anode in today's cells is made up of a graphite mixture and the cathode is a combination of lithium and other choice metals.It should be noted that all materials in a battery have a theoretical energy density.With lithium battery,the anode is well optimized and little improvements can be gained in terms of design changes.The cathode,however,shows promise for further enhancements.Battery research is therefore focusing on the cathode material.Another part that has potential is the electrolyte.The electrolyte serves as a reaction medium between the anode and the cathode.

The battery industry is making incremental capacity gains of 8-10% per year.This trend is expected to continue.This,however,is a far cry from Moore's Law that specifies a doubling of transistors on a chip every 18 to 24 months.Translating this increase to a battery would mean a doubling of capacity every two years.Instead of two years,lithium-ion has doubled its energy capacity in 10 years.Today's lithium-ion comes in many "flavours" and the differences in the composition are mostly related to the cathode material.Table 1 below summarizes the most commonly used lithium-ion on the market today.For simplicity,we summarize the chemistries into four groupings,which are Cobalt,Manganese,NCM and Phosphate.

The cobalt-based lithium-ion appeared first in 1991,introduced by Sony.This battery chemistry gained quick acceptance because of its high energy density.Possibly due to lower energy density,spinel-based lithium-ion had a slower start.When introduced in 1996,the world demanded longer runtime above anything else.With the need for high current rate on many portable devices,spinel has now moved to the frontline and is in hot demand.The requirements are so great that manufacturers producing these batteries are unable to meet the demand.This is one of the reasons why so little advertising is done to promote this product.E-One Moli Energy (Canada) is a leading manufacturer of the spinel lithium-ion in cylindrical form.They are specializing in the 18650 battery and 26700 cell formats.Other major players of spinel-based lithium-ion are Sanyo,Panasonic and Sony.

The newest addition to the lithium-ion family is the A123 System in which nano-phosphate materials are added in the cathode.It claims to have the highest power density in W/kg of a commercially available lithium-ion battery.The cell can be continuously discharged to 100% depth-of-discharge at 35C and can endure discharge pulses as high as 100C.The phosphate-based system has a nominal voltage of about 3.3V/cell and peak charge voltage is 3.60V.This is lower than the cobalt-based lithium-ion and the battery will require a designated charger.Valance Technology was the first to commercialize the phosphate-based lithium-ion and their cells are sold under the Saphiona name.

Sony is focusing on the nickel-cobalt manganese (NCM) version.The cathode incorporates cobalt,nickel and manganese in the crystal structure that forms a multi-metal oxide material to which lithium is added.The manufacturer offers a range of different products within this battery family,catering to users that either needs high energy density or high load capability.It should be noted that these two attributes could not be combined in one and the same package;there is a compromise between the two.Note that the NCM charges to 4.10V/cell,100mV lower than cobalt and spinel.Charging this ni-mh battery chemistry to 4.20V/cell would provide higher capacities but the cycle life would be cut short.

Instead of the customary 800 cycles achieved in a laboratory environment,the cycle count would be reduced to about 300.In Figure 4 we compare the energy density (Wh/kg) of the three lithium-ion chemistries and place them against the traditional lead acid,nickel-cadmium,nickel-metal-hydride.One can see the incremental improvement of Manganese and Phosphate over older technologies.Cobalt offers the highest energy density but is thermally less stable and cannot deliver high load currents.