Significance of current collectors in the energy storage systems
By: Rajesh Kuppusamy
Tags: Electrodes, Battery, Current Collectors, Energy Storage

Introduction

The battery market is in constant search for an energy storage technology for several new applications and consumer electronics, such as cell phones and laptops. In battery technology, although the core material chemistry is well developed in terms of electrode active materials, the actual energy density of the Li-ion battery is 2.5 to 3 times lower than the material level because of additional essential components like current collectors, separators, electrolyte, metal tabs, and casing layers which are required for the proper functioning of the battery system.

Among them, the current collector accounts for 15 to 20 % of the weight and plays a vital role in carrying the charge from the electrode material to the external circuit. Energy storage devices like supercapacitors, lithium-ion batteries, and other metal ion batteries have been chosen to showcase viable current collectors in each respective system.

1. Current collector for supercapacitors: The current collector in supercapacitors is usually made of aluminum foil which plays the main function of conducting current from the activated carbon electrodes to power sources or electrical appliances and supporting the activated carbon electrode.

Therefore, an ideal current collector should possess high electrical conductivity, low contact resistance with the carbon electrode, and strong and stable bonding with the carbon electrode.

In the commercial supercapacitors, the current collector mainly depends on the kind of electrolyte that is being used in them, the most common electrolyte composition being quaternary ammonium salt in acetonitrile solvent. These systems can utilize aluminum foil as neither the salt nor the solvent react or corrode aluminum. Figure 1 shows the commercial supercapacitor assembled with Al current collector.

Figure 1. The commercial Aluminum current collector used in supercapacitor

The research in the EDLC capacitor reports that after a certain number of cycles the carbon layer is often peeled from the Al current collector in activated carbon-based supercapacitors with organic electrolytes resulting in declined electrochemical performances. For this reason, more often than not, etched aluminum foil is used instead of smooth foils. This increases the bonding strength between the activated carbon layer and the foil and maintains a stable performance across millions of cycles.

Recently, a scalable approach to modifying Al current collectors using sol-gel deposition of conducting layers, graphene-modified Al, laser-treatment of Al has been explored in the context of further improving the current collector. These coatings have the advantage of reducing charge-transfer resistance, protecting the Al current collectors from corrosion, improving the bonding strength between the current collector and carbonaceous electrode to prevent delamination.

2.Current collector research for Lithium-ion Battery: Lithium-ion battery (LIB) has been studied extensively for the last twenty years as it can deliver a far higher energy density compared with other energy storage devices such as lead-acid batteries and traditional supercapacitors. The conventional LIB comprises active electrode materials, separator, current collectors and electrolyte which may be either a non-aqueous liquid or a solid organic polymer.

A LIB electrode contains the electrode active material layers coated on the current collector in the thickness range of 10 to 100 µm. The current collector is often an aluminum (Al) foil used for the cathode and copper (Cu) foil used for the anode. The thickness of the current collectors varies between 10 to 20µm. An advantage of metal foils being used as current collectors is their high electrical conductivity. However, the weight of a metallic current collector is considerable and a 12-μm thick current collector makes up around 10% of the total weight of the battery.

The commercially available Lithium-ion battery uses Cu as anode current collectors because of its electrochemical stability from 0 to 3 V vs. Li/Li+ in LiPF6 electrolyte. Similarly, the cathode Al is stable at high potential, but it can react with Li at the lower potential to form an inorganic alloy of lithium-aluminum. In electrolyte, the lithium-ion therein being supplied by dissociation of a lithium salt dissolved in the electrolyte which can electrochemically form an alloy with other metal mainly Al at the low potential. This shows that the physical and chemical properties of the current collector can directly influence the performance of LIBs. Figure 2 shows the commercially available current collector assembly in LIB.

Figure 2. The commercial Cu anode and Al cathode current collector used in LIB

The upcoming new current collector should be in the form of a free-standing film for use in the flexible LIB. The film can be attained by electrically conducting layer coated on Lightweight and thin, mechanically durable and chemically stable materials which are critical characteristics necessary for a current collector. In cathode current collectors, instead of aluminum, graphene paper, flexible metallic mesh type current collectors, graphene nanosheets and 3D foam type material can be fabricated as a flexible current collector due to their very low thickness around 2 μm and their electrical conductivity.

The Al current collector at cathode corrodes in the presence of LiPF6 –carbonate solvents at a high operating voltage of the battery, also the passive film formation on Al surface by the electrolyte undergoes decomposition and dissolution at those high operating voltages.

In the future, research in current collectors should be focused on more electrically conductive, lightweight, electrochemically stable, thin, flexible, and free-standing materials. Issues such as weldability and structural integrity also need to be accounted for when non-metallic materials are being considered.

3.Current collector research in NIB and KIB: LIBs have been great contenders owing to their high energy density and high operating voltage. On the other hand, sodium and potassium are widely available and can be extracted from seawater indicating that its resources are effectively infinite, and they also share a similar mechanism with LIB. When the anode material, cathode material, and the current collector are based on earth-abundant elements eventually the entire proposition could be better than LIBs. But yet, there are a lot of caveats in these systems that need to be overcome.

Just like in LIBs, other alkali ion batteries can use aluminum as their cathode current collectors. But unlike LIBs, neither sodium nor potassium undergo alloying reactions with aluminum and thus, they allow the use of this metal foil even on the anode side. The earth-abundant Al has an advantage of less corrosive nature, high electrical conductivity, more mechanical strength and ability to form very thin foils which are inspiring the researchers to move forward towards the NIB/KIB.

The NIB has been commercialized by companies such as Faradion, Novasis, and natron energy by using Al as current collectors for both anode and cathode. Faradion limited reports that the only 13% Al is contained in the pouch and 3.6 % in the 18650 format which is substantially more lightweight than the LIB current collector net weight. The commercial Al as the anode and cathode current collector used in NIB and KIB system are shown in Figure 3.

Figure 3. The commercial Al anode and cathode current collector used in NIB and KIB.

The NIB and KIB by using aluminum current collectors can discharge down to 0 V by utilization of complete energy which may effectively improve the energy density and also improve the safety of these batteries.

The surface-modified Al current collector like conducting polymer-coated, graphite layer coated, and carbonous film deposition may improve the electrical contact and electrochemical performance of both the electrodes effectively.

Conclusion and Future research:

In this article, we have described the important component in the various battery systems, which is the current collector and their vital role in the energy storage system like supercapacitors, lithium-ion batteries, sodium-ion batteries, and potassium ion batteries.

The current collector varies from system to system due to their specific electrochemical reactions between the electrolyte solution / ionic salt. Each system has to be carefully studied in order to identify the best-suited current collectors.

The emerging NIB and KIB have similar chemistry with LIB. But unlike the Li-ion battery, here the commonly available aluminum can be used as a current collector due to its high potential stability and non-alloying reaction with Na and K, which has inspired the researchers to pay more attention to develop the earth-abundant metal ion battery for future energy storage.