The Difference Between a Lithium-Ion Battery and a Lithium-Ion Capacitor

A lithium-ion capacitor (LIC) is a type of supercapacitor that combines elements of both Li-ion batteries and electric double-layer capacitors (ELDCs). The cathode in an LIC is made of activated carbon, similar to that in an ELDC, while the anode is composed of carbon material pre-doped with lithium ions, akin to those found in Li-ion batteries. LICs are asymmetric devices, unlike the symmetrical structures of double-layer capacitors and pseudocapacitors. This asymmetric structure leads to a series of performance trade-offs when compared with ELDCs, pseudocapacitors, and Li-ion batteries.

Li-ion Capacitor Construction

Like many other energy storage technologies, Li-ion capacitors (LICs) consist of four main components: an anode, a cathode, an electrolyte, and a separator. The anode of the LIC, which is the negative side, incorporates the Li-ion battery element. It operates based on an electrochemical reaction and charges using the reversible intercalation of lithium ions. However, it is not strictly a Li-ion anode; rather, it is a hybrid structure within the hybrid supercapacitor.

Most of today’s LICs use a modified version of LTO (Li4Ti5O12) due to its high coulombic efficiency, stable operating voltage, and minimal volume change during lithium intercalation and deintercalation. Modifications are necessary because basic LTO has poor lithium-ion diffusivity and low electrical conductivity. The LIC anode is fabricated using nanostructured metal oxides with a high specific surface area to increase capacity. Depending on the design and manufacturer, enhancements such as nanotubes (single- and multi-walled), nanoparticles, nanowires, and nanobeads are used to boost the power density of the LIC anode.

The anode of LICs is often pre-lithiated (doped) to prevent significant potential drops during charge and discharge cycles. Doping lowers the anode potential, resulting in a higher output voltage for the capacitor. Typically, LICs have output voltages ranging from 3.8V to 4.0V, with minimum voltages between 1.8V and 2.2V. Unlike conventional ELDCs and pseudocapacitors, which can be discharged to zero volts without damage, LICs experience degradation of the electrolyte and electrodes when approaching their maximum or minimum voltage, leading to irreversible damage.

Pre-lithiation compensates for the irreversible loss of capacity in the anode after the initial charge/discharge cycles. This capacity loss results from the formation of a solid electrolyte interface (SEI) film on the anode. Pre-lithiation mitigates the negative effects of SEI formation, ensuring better performance and longevity of the LIC.

In Li-ion capacitors (LICs), an electric double layer is used to store energy in the cathode. The cathode must exhibit good conductivity and a high specific surface area. Although activated carbon has traditionally been used as a cathode material in LICs, newer designs have incorporated various materials to enhance performance. Depending on the product and manufacturer, four types of cathode materials are used in LICs: heteroatom-doped carbon, graphene-based materials, porous carbon, and bifunctional cathodes.

The combination of an LTO-like anode with the electrical conductivity and ionic diffusivity of various carbon-based cathode materials enables the unique performance capabilities of LICs. Degradation is more of a concern for the anode because it employs an electrochemical process, whereas the cathode operates strictly on electrostatic principles.

In addition to the anode and cathode, nearly all energy storage devices include an electrolyte and a separator, and LICs are no exception. LICs require high ionic conductivity, often provided by non-aqueous electrolytes that do not react with lithium. A lithium-based salt solution, similar or identical to that used in Li-ion batteries, can be used.

LIC separators are similar to those used in Li-ion batteries. They are chemically inert and provide electrical insulation between the anode and cathode while allowing ions to pass through, supporting the operation of the LIC.

Li-ion Capacitor Performance

Li-ion capacitors (LICs) offer higher operating voltages (up to 3.8 V maximum), significantly greater capacitance and energy density (up to 10 times higher), and lower self-discharge rates compared to symmetric supercapacitors. While symmetrical supercapacitors feature lower equivalent series resistance (ESR) and can deliver higher power levels, they can also be fully discharged to zero volts, potentially offering higher energy storage capabilities in certain scenarios. LICs, on the other hand, exhibit much lower leakage currents and can maintain a charge for extended periods.

Unlike Li-ion batteries, the voltage of LICs and other supercapacitors varies linearly with the state of charge. To provide stable voltages for system operation, electronic power converters are required. Due to their asymmetric construction, the anode capacitance in LICs is several orders of magnitude larger than that of the cathode. Consequently, during charging and discharging, the potential change at the anode is much more significant than the potential change at the cathode.

Applications for LICs

The use of Li-ion capacitors (LICs) necessitates a cell management system (CMS), which is simpler than the battery management system used with Li-ion batteries. A CMS is essential for maximizing the lifespan of LICs, as it prevents discharging below approximately 2.2 V and ensures equal cell voltages in designs where multiple cells are connected in series. Battery management systems, in contrast, primarily ensure safe operation.

LICs are ideal for applications that benefit from high energy densities, high power densities, environmental ruggedness, and durability. The adoption of LICs can often eliminate the need for secondary energy storage devices, thereby reducing the overall cost. Examples of LIC applications include wind power generators, uninterruptible power systems (UPSs), voltage sag compensation, photovoltaic power generation, and regenerative energy recovery in electric vehicles and industrial machinery, including robots.

For instance, power outages, lasting from a few seconds to several minutes, can cause significant damage and production loss in industrial processes sensitive to short-duration downtimes, such as chemical or semiconductor processing. LICs can be used to design high-power, compact, and rugged backup power systems suitable for industrial environments.

An example of this is an 80-kW industrial UPS built with LIC modules, providing a backup time of two minutes in a compact footprint. Compared to traditional lead-acid battery systems, the LIC UPS offers several advantages:

- The LIC design generates less heat than a standard long-term backup UPS system using valve-regulated lead-acid (VRLA) batteries.

- LICs are essentially maintenance-free, while VRLA batteries require yearly maintenance and cannot withstand short power interruptions.

- The LIC UPS delivers ultra-high power density in a reduced footprint, making it easier to integrate into existing operations.

- The system’s ultra-fast recharge of the LICs ensures maximum availability and total protection from short power drops.

- The LIC UPS can be safely installed in a standard room without special temperature, humidity, or dust control, whereas VRLA batteries require special ventilation to avoid hydrogen generation.

- The embedded cell-to-cell monitoring system provides continuous monitoring to prevent unexpected failures and reduce maintenance operations.

- Partial and frequent discharges do not compromise system reliability or operation, as LICs are not sensitive to regular or unexpected supply failures.

These features make LICs a versatile and reliable choice for a wide range of demanding applications.

Summary

Li-ion capacitors (LICs) represent a hybrid between Li-ion batteries and electric double-layer capacitors (ELDCs), offering a distinct energy storage alternative with superior power density compared to Li-ion batteries and higher energy densities than traditional supercapacitors. LICs are more robust than both Li-ion and lead-acid batteries, free from the safety and environmental concerns associated with rechargeable batteries. Moreover, implementing cell management systems for LICs is simpler than for Li-ion and lead-acid batteries. These attributes make LICs highly suitable for diverse applications, including wind power generators, uninterruptible power systems (UPS), voltage sag compensation, photovoltaic power generation, and regenerative energy recovery in electric vehicles and industrial machinery, including robots.