Battery Research

Harrick Plasma → Applications → Energy → Energy Storage → Battery Research

Recent advances in battery research have made lithium-ion batteries a mainstay of electric vehicles, smartphones, and grid-scale energy storage. Lithium-ion batteries contain several major components. A positively charged region (cathode) stores lithium (usually in the form LiCoO₂) while a negatively charged region (anode) stores a different solid element or compound. The cathode and anode are kept apart by an ionically porous separator. When the battery charges, an electrolyte transports lithium ions from the cathode to the anode. These ions react with the anode material in a process called lithiation. Alternatively, the electrolyte can transport these Li⁺ ions in the opposite direction, which releases electrons that power an external circuit.

Below you will learn how plasma treatment has advanced battery research. Plasma treatment improves the adhesion of battery components, enhances electrochemical reactions, and can change the ionic porosity of battery separators.

Lithium-ion Batteries with Modified Separators

Despite their many advantages, Li-ion batteries require further safety improvements. A common point of failure is the battery separator, which is usually made of polyethylene (PE) or polypropylene (PP). In batteries with high energy density, the separator often tears or shrinks, causing contact between the cathode and anode. The resulting short-circuit is a serious safety risk.  

To improve the mechanical and thermal stability of these separators, many researchers apply additional coatings using layer-by-layer (LbL) assembly or grafting. In a study by Moon et. al., plasma treatment was an essential part of this process. Moon plasma treated a porous polyethylene (PE) separator to activate its surface prior to atomic layer deposition (ALD) of aluminum oxide (Al2O3). The separator was then dip-coated in polydopamine (PDA).

Compared to bare PE separators, the coated separators experienced an average of 13x less shrinkage during high-temperature testing, indicating improved thermal stability. The PE/Al₂O₃/PDA separators also had a 450% higher electrolyte uptake than the bare PE separators. Increased electrolyte uptake has been associated with lower interfacial resistance and improved battery life.

Modified separators can also improve Li-ion battery performance. For example, silicon dioxide (SiO₂­) is typically deposited inside a porous polyethylene (PE) separator to increase the volumetric energy density of Li-ion batteries. Choi et. al. used plasma treatment to assist with this process. The separator was plasma treated to improve subsequent adsorption of water vapor to the surface. After introducing the sample to silicon tetrachloride (SiCl₄) vapor, the water and SiCl₄ vapors reacted to form SiO_2­.

This workflow was also used by Kim et. al. for studies of Na-ion batteries. Kim noted that longer plasma treatment times increased the separator’s hydrophilicity. During Kim’s study, batteries containing the SiO₂-coated separators had 18 times the capacity of batteries containing the bare separators after 50 battery cycles.

Lithium-ion Batteries with Solid Electrolytes

Although most lithium-ion batteries contain liquid electrolytes, alternatives such as solid composite electrolytes (SCEs) are attracting attention. Liquid electrolytes are hazardous due to their flammability and Li dendrite growth, which can short-circuit the battery. In contrast, solid composite electrolytes avoid both of those problems. SCEs consist of ceramic pieces in a polymer matrix. Ideally, these solid composite electrolytes (SCEs) can combine the high ionic conductivity and durability of ceramic electrolytes with the electrolyte-electrode interfacial stability of polymers. Unfortunately, due to separate fabrication steps of the electrolyte and electrodes, many voids form at the electrolyte-electrode interface. These voids increase the interfacial resistance, which decreases battery performance.

To combat this issue, Kim et. al. developed a continuous fabrication process for Li-ion batteries containing solid composite electrolytes. The SCE was formed from a solution of LLZO and PVDF-HFP in n-methyl-2-pyrrolidone (NMP). After sputtering a vanadium oxide (V₂O_5-x) layer onto a current collector to form the battery cathode, the SCE was drop-casted onto the cathode. The device was then hot pressed to remove most of the voids formed during drop-casting. Kim then plasma-treated the device prior to depositing a lithium film for the anode. This plasma treatment reduced the SCE/anode interfacial resistance by up to 3 times compared to the interfacial resistance between the hot-pressed SCE and the lithium anode.

Lithium-ion Batteries with Novel Anode Materials

Due to its low cost and widespread availability, graphite is the most common anode material for lithium-ion batteries. Unfortunately, graphite anodes are prone to unwanted solid electrolyte interphase (SEI) formation. At certain operating voltages, the battery’s electrolyte decomposes and reacts with the anode, forming a passivation layer. This layer slows the diffusion of lithium ions toward the anode and traps some of these ions. The drop in available ions reduces the battery’s capacity over time, causing premature failure.

To limit electrolyte decomposition, researchers are examining alternative anode materials for lithium-ion batteries. One such material is titanium dioxide (TiO₂). Charlton et. al. studied electrochemical interactions between TiO₂ anodes and various electrolytes. This process required TiO₂ films with excellent mechanical stability. As part of the TiO₂ deposition process, Charlton plasma treated carbon films to introduce oxygen-rich species. This step enhanced the subsequent atomic layer deposition (ALD) of a TiO₂ film. Charlton observed that the titanium dioxide reacted with a lithium hexafluorophosphate (LiPF₆) electrolyte to form hydrofluoric acid (HF), which can decrease battery performance. This effect was reduced when a hydrofluoric acid scavenger (tributylamine) was added to the electrolyte.

Lithium – Sulfur Batteries

Despite the popularity of lithium-ion batteries for high-capacitance energy storage, alternative battery types are gaining attention. One example is lithium-sulfur (LiS) batteries, which have 3-5 times higher energy density and specific capacity than lithium-ion batteries. However, lithium-sulfur batteries experience a detrimental “shuttle effect”, in which intermediate polysulfides (LiPS) dissolve and diffuse toward the electrodes via the separator. This process reduces the battery’s capacity and Coulombic efficiency.

To combat the shuttle effect, researchers can modify the electrochemical properties of lithium-sulfur battery separators. For example, Gu et. al. altered the ionic permeability of a battery separator using plasma treatment and layer-by-layer (LbL) assembly. Gu plasma treated a polyethylene (PE) separator to make it hydrophilic before dip-coating it in alternating solutions of polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA). This layer-by-layer (LbL) assembly produced ultrathin films of ion-permselective polyelectrolytes. During cyclic voltammetry measurements, negative ions (like the polysulfides responsible for the shuttle effect) were repelled from the dip-coated separator. Depending on the number of dip-coated layers, a resulting current density decrease of 72% – 98% was observed. Additionally, the Coulombic efficiency nearly doubled to almost 100%.

The shuttle effect can also be minimized by adjusting the physical properties of lithium-sulfur battery separators. Ou et. al. coated a polypropylene/polyethylene/polypropylene (PP/PE/PP) separator with graphene. Ou then plasma treated the separator to introduce pores in the graphene. Graphene’s small pores allow Li⁺_­ ions to pass through the separator but block large polysulfides. The pore size can be tailored by adjusting the plasma treatment time. After 95 battery cycles, the battery containing a plasma treated separator retained 74% of its capacity. In contrast, the battery containing a non-plasma treated separator retained only 66% of its capacity over that same period.

Despite recent progress in reducing the shuttle effect, this improvement often comes at a cost. Ion-selective separator coatings often repel not only polysulfides, but also Li⁺ ions. This reduced Li⁺ _­conductivity limits battery performance and prevents LiS battery usage in applications requiring fast charging. Paniagua-Vasquez et. al. modified a separator to improve its ion conductivity, which can be quantified by the separator’s electrolyte uptake. Paniagua-Vasquez et. al. plasma treated a polypropylene (PP) separator to functionalize it before adding either a carbon black/PVDF or carbon black/chitosan coating. The separator’s water contact angle (WCA) decreased from 101° before plasma treatment to 82.1° after plasma treatment, showing increased hydrophilicity. This surface functionalization promoted the subsequent adhesion of the carbon black coatings to the separator. Electrolyte uptake in batteries containing the coated separators was up to 1.8 times higher than in batteries containing non-coated separators.

In addition to the shuttle effect, another common issue in Li-S batteries is lithium dendrite growth. These dendrites can short circuit the battery, deposit lithium onto the anode, and reduce the battery’s capacity. Yang et. al. studied a method to simultaneously reduce both Li dendrite growth and the shuttle effect in Li-S batteries. Yang used Harrick Plasma’s Expanded Plasma Cleaner to activate the surface of a Celgard battery separator. The separator was then coated with silicone nanofilaments (SNFs) and polydopamine (PDA). This coating significantly reduced the number of dendrites which formed in the batteries. Additionally, the SNF/PDA layer significantly reduced the shuttle effect. XPS measurements showed that polysulfides adsorbed to the nanofilaments via oxygen- and nitrogen-containing functional groups.  Batteries containing the coated separators also had about a 20x longer life than those containing the bare Celgard separators.

Redux Flow Batteries

For applications requiring huge (100s of kW) energy storage capabilities, redox flow batteries (RFBs) are becoming increasingly popular. Unlike Li-ion batteries, RFBs contain two electrolytes (positive and negative) which flow through their respective electrodes. Oxidation and reduction reactions occur between each electrolyte and its electrode, producing electrons to power the circuit. The energy storage capacity of an RFB can be increased by increasing the volume of electrolytes used.

Most RFBs contain porous electrodes made from carbon felt, carbon paper, or graphite felt. These materials exhibit excellent chemical stability, ionic conductivity, and porosity. Unfortunately, they have a low surface area for electrochemical reactions and are hydrophobic, which prevents liquid electrolyte from flowing through the electrode. As a result, the RFB produces a lower current density than desired.

To solve this problem, researchers often functionalize carbon-based electrodes using plasma treatment. As reported by Estevez et. al., oxygen plasma treatment introduces C-O, C=O, and O-C=O functional groups without changing the electrodes’ surface morphology. These functional groups increase the number of active sites on the surface and make the electrodes hydrophilic. Pratt et. al. and Hudak et. al. plasma treated their RFB electrodes to achieve these goals.

An alternative way to make RFB electrodes hydrophilic was studied by Mizrak et al. The authors plasma treated carbon paper electrodes before depositing hydrophilic MXene (Ti₃C₂T_x) nanoparticles on the surface. Plasma treatment ensured a homogeneous distribution of nanoparticles on the electrodes, which has been shown to improve electrode performance [Estevez et. al.]. These MXene nanoparticles improved the system’s reversibility under cyclic voltammetry testing, as shown in Figure 1.

Figure 1: Cyclic voltammetry measurements of vanadium-based RFBs containing electrodes coated with varying concentrations of MXene nanoparticles. Batteries with higher MXene concentrations show smaller potential differences between their positive and negative peaks, revealing improved reversibility. Data reproduced from Mizrak et. al.

Unique Battery Research

Plasma treatment has also advanced the development of many novel batteries. Adams et. al. developed a potassium-ion battery containing a potassium cathode and a carbon nanofiber (CNF) anode. Before battery lifecycle studies, the carbon nanofibers were either treated with oxygen plasma to functionalize them or left pristine. Oxygen plasma treatment increased the batteries’ Coulombic efficiency after 5 cycles by 7.5% compared to batteries containing non-plasma treated CNF. Adams proposed that oxygen plasma treatment encouraged the formation of K₂CO₃ during battery charging and discharging, which may improve battery stability.

In another example, Gao et. al. plasma treated cathodes for a wearable biobattery. Polyethylene terephthalate (PET) yarn was plasma treated to make it hydrophilic before adding a PEDOT:PSS conductive coating. When incorporated into the biobattery, this electrode harvested biochemical energy from bacterial respiration.