Understanding Lithium-ion Capacitors

Provided by JTEKT Corporation, this guide explores the nature, mechanics, and advantages of Lithium-ion capacitors.

Overview of Power Storage

Capacitors are specialized energy storage devices categorized as secondary batteries (rechargeable). While various materials have been utilized to develop different types, the market for high-capacity storage is primarily split between two technologies:

1. Electric Double Layer Capacitors (EDLC)
2. Lithium-ion Capacitors

Key Characteristics

The defining trait of a capacitor is its ability to facilitate extremely rapid charging and discharging cycles.

While they cannot match the total energy storage of a standard battery, they excel in other areas. Compared to lead-acid or standard lithium-ion batteries:

• Power Density: Significantly superior.
• Durability: Minimal performance degradation over repeated cycles.
• Longevity: Much longer operational lifespan.

Performance Comparison Matrix

The following table compares standard storage devices. (Legend: $\text{◎}$ = Excellent, $\text{◯}$ = Good, $\text{△}$ = Average)

Feature	Li-ion Battery	EDLC	Conventional Li-ion Capacitor	Libuddy (JTEKT)
Energy	$\text{◎}$	$\text{△}$	$\text{◯}$	$\text{◯}$
Power Density	$\text{△}$	$\text{◯}$	$\text{◯}$	$\text{◎}$
Service Life	$\text{△}$	$\text{◯}$	$\text{◯}$	$\text{◎}$
Temp Range ($\text{°C}$)	$-20 \sim 60$	$-40 \sim 70$	$-30 \sim 70$	$-40 \sim 85$
Operating Voltage ($\text{V}$)	$\sim 4.3$	$0 \sim 2.7$	$2.2 \sim 3.8$	$2.2 \sim 3.8$

What is Libuddy? Libuddy is JTEKT's advanced Lithium-ion capacitor. By enhancing heat resistance, it has achieved a wider operating temperature range, which in turn significantly boosts both its service life and power density.

Principles and Architecture

A Lithium-ion capacitor is essentially a hybrid device. It merges the positive electrode of an EDLC with the negative electrode of a Lithium-ion secondary battery.

This hybrid design creates a "best of both worlds" scenario:

• Higher energy density than standard capacitors.
• Increased safety compared to standard Lithium-ion batteries.

Charge and Discharge Mechanisms

Device Type	Positive Electrode (+)	Negative Electrode (-)
Li-ion Battery	Lithium chemical reaction	Lithium insertion/removal
EDLC	Electrolyte ion adsorption/desorption	Electrolyte ion adsorption/desorption
Li-ion Capacitor	Electrolyte ion adsorption/desorption	Lithium insertion/removal

Technical Specifications

The following data highlights the quantitative differences between these technologies:

Metric	Li-ion Battery	EDLC	Conv. Li-ion Cap	Libuddy
Energy (Wh/L)	$200 \sim 400$	$3 \sim 6$	$10 \sim 20$	$13$
Power Density (W/kg)	$500 \sim 2,000$	$1,000 \sim 4,500$	$1,000 \sim 3,500$	$7,000$
Cycle Life (cycles)	$1,000 \sim 3,000$	$> 100,000$	$> 100,000$	$> 1,000,000$
Temp Limit ($\text{°C}$)	$-20 \sim 60$	$-30 \sim 70$	$-30 \sim 60$	$-40 \sim 85$

Form Factors and Production

There are two primary physical designs for these capacitors:

• Can type
• Laminated type $\leftarrow$ Used by JTEKT

The manufacturing process for Lithium-ion capacitors closely mirrors the production steps used for standard Lithium-ion batteries.

# Conceptual representation of Capacitor Selection
def select_storage(priority):
    if priority == "high_energy":
        return "Lithium-ion Battery"
    elif priority == "ultra_fast_cycle":
        return "EDLC"
    elif priority == "balanced_performance_and_life":
        return "Lithium-ion Capacitor (Libuddy)"

Electrical Glossary

To better understand the metrics above, refer to these definitions:

• Current (A): The volume of electrons flowing per second.
• Voltage (V): The potential difference between two specific points.
• Watt (W): The amount of electrical energy output per second.
  • Formula: $P = V \times I$
• Watt-hour (Wh): The total accumulated electrical energy.
  • Formula: $E = P \times t$
• Internal Resistance (mΩ): The inherent resistance within the storage device.
• C-rate (C): A measure of how quickly a battery is charged or discharged relative to its capacity.
• Joule heat (J): Thermal energy produced when current passes through a conductor.
• Primary battery: A non-rechargeable battery (discharge only).
• Secondary battery: A rechargeable battery.
• Float charging: Charging a battery at a constant voltage to maintain full charge.
• Power density (W/L, W/kg): Electrical power relative to volume or mass.
• Energy density (Wh/L, Wh/kg): Electrical energy relative to volume or mass.