Features of small li-ion rechargeable batteries

2. Features of small li-ion rechargeable batteries
2-1 Key features of small li-ion
rechargeable batteries* The following data are not guaranteed values.

Small li-ion rechargeable batteries have higher
capacity and less leakage current than electric doublelayer capacitors. Compared with standard lithium-ion
rechargeable batteries, they have higher-speed charging
and discharging, longer life, and better safety. The SLBbatteries are capable of high-power discharge, whichis extremely difficult to achieve with standard lithium-ion
rechargeable batteries. They can be discharged overlonger periods of time, which is not possible with electric
double-layer capacitors.

2-2 Comparison with electric doublelayer capacitors
Small li-ion rechargeable batteries contain nearly 50 times
as much energy as its electric double-layer capacitors of
the same size. Our small li-ion rechargeable batteries canbe used in place of electric double-layer capacitors topower devices for longer periods of time.
* The following data are not guaranteed values.

Figure 2-2. Changes in voltage on the φ3x7L; 0.35mAh
when stored fully charged at different temperatures

days, NICHICON’s electric double-layer capacitors drop
in voltage to 1.97V. By contrast, after being charged to
2.8V, Small li-ion rechargeable batteries (φ3x7L) drop involtage to 2.6V, indicating the leakage current is small.
Because this product demonstrates little selfdischarge, they can be used for a long period oftime without charging. This allows them to be used inconjunction with energy-harvesting technology, improving
efficiency as they can store energy for a long period oftime after being charged.

2-3 Comparison with general lithiumion rechargeable batteries

Figure 2-1. Comparison of volume energy densities of
NICHICON’s electric double-layer capacitors
and its small li-ion rechargeable batteries

Materials in small li-ion rechargeable batteries” store
and release electrical energy through the electrochemical
reactions of lithium ions in the electrode active materials
and the electrolyte. In the charged state, the selfdischarge of the battery is low, as the electrode active
materials are chemically stable by the charged voltage.
The anode active material, lithium titanate, has excellent
thermal stability, and resists self-discharge at hightemperature. Figure 2-2 shows the self-discharge
behavior of a fully charged φ3x7L; 0.35mAh products at
different ambient temperatures.
After being charged to 2.7V and stored at 60°C for 30

2-3-1 Ultra-fast charge/discharge function
Small li-ion rechargeable batteries are superior tostandard lithium-ion rechargeable batteries in theircharge/discharge characteristics. Standard lithium-ion
rechargeable batteries typically take about 1 hour tocharge, while product can be charged and discharged atup to 20C (20 times the current value needed forcharging in one hour; for the φ3x7L; 1C=0.35mA,
20C=7mA). Figure 2-3 shows the charging time andcharging rate for different C rates on the φ3x7L. If
charged at 20C, the battery could be charged to 80% of afull charge in around three minutes.
The ability to charge the batteries rapidly optimizes
their use in devices that require short charging times andwhen charging is interrupted or forgotten.

Small Li-Ion Rechargeable Batteries Technical Notes CAT. 8601

Features of small li-ion rechargeable batteries* The following data are not guaranteed values.

Figure 2-3. Relationship between charging time andcharging rate of φ3x7L at different charging current rates

Figure 2-4 shows discharging time versus remaining
capacity for the φ3x7L at different C rates. If discharged
at a current value of 20C, the maximum guaranteeddischarging current value, the battery could bedischarged completely in approximately 3 minutes. Dueto the extremely large discharge current given theproduct’s size, the battery is well suited for use in devices
that require high power and a small size.

followed by the cathode and electrolyte reacting. Sparks
from the short circuit and the release of oxygen due to thecollapse of the cathode’s crystal structure can result in anoxygen combustion reaction. Various exothermic
reactions occur one after another resulting in thermal
runaway and fire. The SLB use of lithium titanate for theanode, does not suffer thermal runaway due to its low
resistance and can be charged and discharged at
extremely low temperatures (-30°C).
Figure 2-5 shows charging curves for the φ3x7L being
charged at 1C for various ambient temperatures. Figure
2-6 shows the discharging curves. At low temperatures,the reaction resistance increases for the electrolyte within
the device and between the electrode and electrolyte,
necessitating a higher voltage to initiate the charge(discharge), and capacity decreases. At -30°C, charge
capacity decreases to around 52% of the level at room
temperature, and discharge capacity falls to around 46%.
Standard lithium-ion rechargeable batteries have higher
resistances, so operation at extremely low temperatures
can result in overcharge/over-discharge voltages.
Thermal runaway can result in ruptures or fires, so inmany cases temperature management in cells that
contain thermistors within battery packs is designed toprevent low-temperature operation. The SLB, however,can be used in cold climates and remain safe to use.

* The following data are not guaranteed values.
* The following data are not guaranteed values.

Figure 2-4. Relationship between discharging time andremaining capacity of φ3x7L

Figure 2-5. Charging curves for φ3x7 at a current value ofat different discharging current rates

1C for different ambient temperatures

2-3-2 Low-temperature characteristics
Small li-ion rechargeable batteries have excellent lowtemperature characteristics. Standard lithium-ion
rechargeable batteries charging at temperatures
significantly below 0°C becomes difficult, because lithium
ions are less likely to be adsorbed by the anode(graphite) and tend to condense as lithium metal. Also, if
the temperature is extremely low, dendrites form whenlithium metal condenses, which can penetrate through theseparator between the cathode and anode, causing aninternal short circuit. If an internal short circuit occurs, anextremely high current passes through the short-circuit
area, generating heat. Next the anode and electrolyte
react causing decomposition within the electrolyte

Figure 2-6. Discharging curves for φ3x7L at a current
value of 1C for different ambient temperatures
Small Li-Ion Rechargeable Batteries Technical Notes CAT. 8601

Features of small li-ion rechargeable batteries
2-3-3 Overcharge resistance
Small li-ion rechargeable batteries are more resistant toovercharging than standard lithium-ion rechargeable
batteries. Product characteristics for this battery indicate
its maximum rated voltage at 2.8V, but no significant
capacity degradation was observed in charge/discharge
cycle tests involving repeated charging to more than
2.8V. Figure 2-7 shows the results of a charge/discharge
cycle test conducted on the φ3x7L, where the battery isrepeatedly charged to 3.3V at a current value of 10C anddischarged to 1.8V, indicating the retention rate for 1C
discharge capacity based on the number of cycles. Even
when repeatedly overcharged to 3.3V for 1,500 cycles,
the retention rate was 98% of initial capacity.
These results suggest that even when operating inconditions where the charging voltage is not controlled nosudden degradation in the product’s characteristics
occurs.
* The following data are not guaranteed values.

* The following data are not guaranteed values.

Figure 2-8. Rate of change in discharge capacity whenstored fully charged at different ambient temperatures

Figure 2-9 shows the rate of change in discharge
capacity for the φ3x7L following charge/discharge cycles
between 2.8V to 0V at different ambient temperatures.
Although the decrease in capacity was more significant
than in a test where the battery was stored fullydischarged, after 1,800 cycles the battery’s capacity
decreased by approximately 30% at 60°C and by 22% at
40°C, indicating the product would not suddenly fail.
* The following data are not guaranteed values.

Figure 2-7. Capacity retention rate versus number ofcharge/discharge cycles when φ3x7L charged to 3.3V

2-3-4 Over-discharge resistance
Small li-ion rechargeable batteries are more resistant toover-discharging than standard lithium-ion rechargeable
batteries. Product characteristics for this battery indicate
its rated minimum voltage at 1.8V, but when subjected toa fully discharged cycle test where the battery is shortcircuited for a long period of time, it could again becharged and discharged.
For example, the SLB could be used in temperature
sensors for clean-energy applications. If used in energyharvesting of solar power, it could remain uncharged for along period of time. Figure 2-8 indicates changes indischarge capacity if the φ3x7L is attached to a resistor(15Ω) and stored at different ambient temperatures. No
significant decline in capacity was observed at different
temperatures even when stored fully charged for 1,000hours.

Figure 2-9. Rate of change in discharge capacity at
different ambient temperatures
following a full discharge cycle to 0V

Small Li-Ion Rechargeable Batteries Technical Notes CAT. 8601