The U.S. Department of Energy’s (DOE) Argonne National Laboratory, in collaboration with Hong Kong University of Science and Technology (HKUST), has developed a new particle-level cathode coating for lithium-ion batteries meant to increase their life and safety.
The idea, three years in the making, was developed at Argonne in collaboration with HKUST. It was funded by the DOE’s Office of Renewable Energy and Energy Efficiency, Vehicle Technologies Office.
“This is an incredibly exciting advancement,” said Khalil Amine, Argonne distinguished fellow and head of the Technology Development group in the Electrochemical Energy Storage department within Argonne’s Chemical Sciences and Engineering division.
“This could significantly improve our experience with the devices we’ve come to rely on.”
The initial experiment was conducted in Hong Kong: HKUST had the ideal set-up and was able to carry out the work under the laboratory’s specifications.
Lithium batteries, used to power everything from electric cars to cell phones and computers, have been using a cathode coating technology for more than 15 years.
But it is not without limitations: It is only a partial coating, one that covers just a small part of the outside of the cathode particle and does not protect the cathode when operating at a high voltage or at high temperature.
The cathodes researchers were studying are metal oxides made of nickel, manganese and cobalt.
A cathode charged at high voltage generates oxygen, oxidizing the electrolyte, creating an unwanted film on the cathode and causing energy loss. High temperatures increase the speed of these reactions, compromising the electro-chemical performance of the battery itself.
The new coating, made with a conducting polymer called poly(3,4-ethylenedioxythiophene) (PEDOT), marks a breakthrough in lithium-ion battery technology since it fully and completely protects each particle of the cathode – inside and out – from reactivity with the electrolyte.
PEDOT is applied using Argonne’s oxidative chemical vapor deposition technique, which uses gas to ensure the coating is applied to every particle of the cathode, forming a robust skin.
The conventional coating slows down lithium diffusion in and out of the cathode particle, decreasing battery efficiency because of poor electronic and ionic conductivity.
By contrast, the new Argonne coating facilitates the transport of lithium ions and electrons in and out of the cathode, boosting battery energy.
Argonne’s Center for Nanoscale Materials (CNM), a DOE Office of Science User Facility, has played a significant role in the experimentation. Researchers used CNM’s Zeiss NVision 40 focused ion beam-scanning electron microscopy dual-beam system and FEI Talos F200X (S)TEM equipped with a SuperX energy-dispersive X-ray spectrometer to confirm the coating of PEDOT on both primary and secondary particles of layered cathodes and their stability after battery cycling.
Argonne assistant chemist Gui-Liang Xu of the Chemical Sciences and Engineering Division (CSE), scientist Yuzi Liu (CNM), postdoctoral appointee Xiang Liu (CSE), postdoctoral research scholar Han Gao (CSE), visiting graduate student Xinwei Zhou (CNM), physicist Yang Ren of the Advanced Photon Source, another DOE Office of Science User Facility at Argonne, and Zonghai Chen (CSE) also contributed to the project, Amine said.
Currently, lithium-ion batteries operate at 4.2 V at the cell level. The new coating can help increase the voltage to 4.6 V. This 15 percent difference can lead to a significant cost reduction of the overall battery pack.
“This would increase the driving range of electric cars and boost the battery life of cell phones and laptops, ultimately changing the way we live,” Amine said.
A paper on the topic was published in Advanced Energy Materials in December 2019, and another published in the journal Nature in May 2019.
Conductive polymers have been significantly attractive for a wide range of electronic applications owing to their key advantages, such as their easy handling, solution- and low-cost processability, chemical diversity and tuneability, and biocompatibility, as well as their unique combination of mechanical and optoelectronic properties [1,2].
Unlike the typical polymers mostly used for insulating and packaging purposes in the plastics industry, the conductive polymers allow electricity to pass through owing to the alternating single σ and double π bonds among the carbon atoms in their structure. These synthetic materials are organic macromolecules with conjugated backbones that contribute delocalized π electrons via sp2 hybridization, leading to the actual electrical conductivity through the molecular backbone [3,4].
Their superior features compared to their inorganic counterparts have made them increasingly interesting for both academic and industrial research and engineering to fully realize the newly emerged field of ultra-thin and plastic electronics. Since the discovery of the first-ever conductive polymer, a wide variety of different synthetic conductive polymers have been developed and extensively explored [5].
Depending on the extent to which they conduct electricity, these low-density organic molecules may be utilized both as semiconductors for wide-ranging electronic device applications and as conductors to replace the metal components of the devices.
Besides their easy processability and flexibility, polymers also have a number of other important advantages over the vast majority of electronic materials, including metals, such as their light weight and non-corrosive nature [4,6–10].
Among the conductive polymers, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) stands out as the most successful and widely studied, which has also been practically and commercially realized [11–13].
This prosperous conductive polymer is a mixture of two polymer ionomers, positively charged PEDOT and negatively charged sulfonated polystyrene (PSS), where PSS acts as a template polymer and surfactant [13].
PSS was chosen based on its water solubility, to enable the solution processability of PEDOT in polar solvents, including water, as well as to highly increase and stabilize the conductivity of PEDOT via charge balancing [13–16].
PEDOT:PSS has been widely applied to organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), organic photovoltaics (OPVs) including organic solar cells (OSCs) [13], perovskite solar cells (PSCs) [17], cutting-edge technologies like touchscreens, electronic papers, and next-generation energy storage and conversion devices functioning as capacitors, batteries, thermoelectric devices, etc. [12,13,18], due to its easy synthesis, low cost, and other unique features, such as its low-temperature processing compatible with organic devices [19], easily tunable viscosity [20], high ductility, high transparency in the visible-light range, and high electrical conductivity [12,13,16].
PEDOT:PSS empowers the fabrication of highly transparent, flexible, ultra-thin, and even biodegradable functional films [12,21]. In addition to its chemical and physical durability, the sufficiently stretchable [16,21] conductive polymer is also stable against environmental factors [13].
The feasible control over the material properties to adjust the electrical conductivity and transmittance through a simple PEDOT:PSS ratio adjustment, along with its solution processability and good-film-forming ability [13] and its work function (Wf) and film morphology tenability, makes it a ubiquitous substance that can be employed both as an active layer and as an electrode material for reproducible and practical device applications [22–25].
In the following two sections of this review paper, the fundamental techniques reported for the improvement of the electrical conductivity of PEDOT:PSS as well as the preeminent deposition methods for forming PEDOT:PSS films will be summarized.
The potential applications of the polymer will also be briefly presented in the fourth section. In the following fifth section and its subsections, the recent advancements in the field of PEDOT:PSS-based electrodes for organic devices like OLEDs, OTFTs, and OPVs will be presented and discussed in detail. Finally, a brief summary and the paper’s conclusions will present an overview of the main points of the article to the reader.
Techniques for the enhancement of the electrical conductivity of PEDOT:PSS
Despite the outstanding features of PEDOT:PSS, including its relatively high electrical conductivity, and the commercial interest in low-cost, large-area, and flexible electronics [13], the bare PEDOT:PSS is not sufficiently conductive for practical application for meeting the high-level requirements of industrial development.
The pristine PEDOT:PSS usually exhibits less than 1 S cm−1 electrical conductivities [16,26–30].
Therefore, various effective methods have been developed to increase its electrical conductivity, with either doping or de-doping the polymer as the target [15,16]. Very frequently, the desired performance of PEDOT:PSS is achieved through the combination of the two [31–34].
Doping aims to increase the concentration of the mobile charge carriers responsible for the conduction through the polymer backbone, by adding extra charges to the whole structure as well as neutralizing some of the PSS – polyanions.
This mechanism is usually referred to as ‘secondary doping’ [28] because the conductive structure of PEDOT:PSS is achieved primarily through the p-type doping of PEDOT with PSS, leading to a very stable ionic bonding between the two counter ions of PEDOT+ and PSS–.
De-doping intends to remove the excess of the insulating PSS from the whole structure to improve the electrical conductivity, because the main reason for the low electrical conductivity is the confinement of a significant segment of the conductive PEDOT chains by the insulating PSS chains [14]. Figure 1(b) shows the structural model of PEDOT:PSS proposed by Soleimani-Gorgani to illustrate the role of PSS in the reduction of the conductivity of PEDOT [14].
Deposition techniques for PEDOT:PSS films
Conductive thin and uniform PEDOT:PSS polymer films are formed by a wide range of environment-friendly coating techniques developed to produce high-quality solution-processed films at a low cost [16].
The common techniques include spin coating, dip coating, slot die coating, bar coating, spray coating, doctor blading, and drop casting [46,64,68,69]. Some printing technologies, such as roll-to-roll (R2R), screen, and inkjet printing, are also among the wet-film deposition methods highly suitable for PEDOT:PSS [13,70].
These large-scale and high-volume deposition advantages of PEDOT:PSS, combined with its conductivity enhancement opportunities, lead the way towards the replacement of the high-cost and high-energy-vacuum-deposited or vacuum-sputtered ITO [33,37] to take plastic electronics to new extremes.
Among these film deposition techniques, inkjet printing is particularly interesting as it enables the creation of the finest details of high-resolution electronic devices at a low cost, by simply transferring the digital images onto plastic, fabric, and other rigid or flexible substrates using the conventional computer printers.
This deposition method eliminates material waste and completely prevents pollution by precisely and rapidly transferring the necessary amount to the desired regions, leading to considerably reduced fabrication costs [70–72].
It is also the most preferred technique for the patterning of polymer conductors [19]. From a mechanical viewpoint (the surface tension of the used ink), the water-based ink formation makes PEDOT:PSS highly suitable for inkjet printing [14]. In the beginning of this decade, a facile deposition of PEDOT:PSS as anode electrodes for OLEDs by an ordinary desktop inkjet printer was suggested by Ummartyotin et al. [73]. Recently, Bihar et al. demonstrated PEDOT:PSS electrodes inkjet-printed on paper for medical device applications [74].
Range of applications of PEDOT:PSS
The formulation of PEDOT:PSS generally yields an ink dispersion, which is widely used for numerous purposes, ranging from specific protective coatings and planarization layers to the hole transport layers (HTLs) and non-metallic electrodes of various electronic devices [1,15–17].
PEDOT:PSS has the most efficient structure among all organic thermoelectric materials [17,18,43]. As this discussion focuses on identifying the achievements in the sphere of PEDOT:PSS-based electrodes as well as the development directions therein, the alternative applications of PEDOT:PSS will be briefly mentioned.
These applications mainly depend on the transparency and flexibility of this polymer as well as on its compatibility with diverse and large-area fabrication methods like R2R manufacturing, inkjet printing, and spin coating.
The impressive development directions in this field include antistatic, protective, or even shielding film coatings for several appliances [14,69], stable polymeric electrolytes for capacitors, effective interfacial and buffer layers [13,33], thermally stable surface coating and binder layers for lithium batteries [75,76], and common solution-processed hole injection layers (HILs) and HTLs for OLEDs, OTFTs, OPVs, organic electrochemical transistors (OECTs), etc. [13,15,16,37].
In the recent past, D. H. Yoon et al. proposed that PEDOT:PSS be used as a multifunctional material for composite battery electrodes [77]. The application of PEDOT:PSS as an electrode material for large-area, flexible, and wearable organic electronics has been considerably interesting due to its excellent optomechanical properties, relatively high electrical conductivity, and advantageous processing methods arising from its polymer nature [11,16].
PEDOT:PSS-based polymer electrodes
Optoelectronic devices, including OLEDs, OPVs, and liquid-crystal displays (LCDs), require thin films of conducting materials for use as transparent electrodes to emit or absorb light at least from one side.
These materials should have higher than 80% optical transmittance in the visible-light region for efficient light emission or absorption, as well as electrical conductivities greater than or equal to 103 S cm−1 to provide the necessary charge carrier conduction for low operational voltages [26,28,31,33,38,78–80].
A conventional electrode material that combines the above-mentioned two essential parameters is the widely applied ITO with higher than 90% transmittance and conductivity values reaching 105 S cm−1 [33,66].
Besides the fact, however, that the film thicknesses of the present-day and forthcoming generations of electronics are approaching nanometer levels, such electronics also demand high flexibility and a light weight from the device components. Sadly, the high-performance ITO belongs to the family of ceramic materials, which are naturally too brittle for flexible device application [9,33].
In addition, intrinsic brittleness is not the only demerit of ITO. This transparent conductive oxide (TCO) is usually obtained through high-temperature [37,81,82] and expensive deposition methods using the rare metal indium, which makes its synthesis increasingly expensive [37,66].
Moreover, the patterning of ITO on glass substrates requires wet chemical etching using acids that are highly toxic. There is also a possible diffusion of the tin (Sn) and indium (In) metals as well as of the oxygen (O2) atoms into the organic layers, leading to the device failure and degradation [66,81,83].
Therefore, finding an alternative electrode material to replace ITO is an actual challenge that the optoelectronics field is facing today. Different kinds of materials involving CNTs, graphene, Ag NWs, and conducting polymers have been tested for this purpose [9,33,34].
Although, each one of these materials has merits for replacing ITO, they all have drawbacks incompatible with practical application. In addition to the main issues, such as the critical surface roughness of the electrodes that lead to high leakage currents, thereby limiting the device performance [17,78,79], and undesirable electrical parameters like a low Wf and a high Rsh, which increase the power consumption and crack down on the benefits of the material [78,79], the synthesis and processing of CNTs, graphene, and Ag NWs are not as cheap and easy as, for instance, those of conductive polymers [13].
These polymers are inherently highly flexible, and their fabrication is environment-friendly and low-cost. Thus, conductive polymers are regarded as among the most promising candidates for replacing ITO, despite the fact that their electrical conductivity is considerably lower.
The conductive polymer PEDOT:PSS has been broadly studied as one of the most suitable materials for producing transparent, colorless, and flexible electrodes for capacitors, batteries, thermoelectric devices, liquid-crystal devices, photodiodes, OLEDs, OTFTs, OECTs, and OPVs as is or as a part of a composite [12,18,20,31,46,84].
More information: Bing‐Qing Xiong et al. Boosting Superior Lithium Storage Performance of Alloy‐Based Anode Materials via Ultraconformal Sb Coating–Derived Favorable Solid‐Electrolyte Interphase, Advanced Energy Materials (2019). DOI: 10.1002/aenm.201903186
Gui-Liang Xu et al. Building ultraconformal protective layers on both secondary and primary particles of layered lithium transition metal oxide cathodes, Nature Energy (2019). DOI: 10.1038/s41560-019-0387-1
