A paper recently published in the journal Advanced Materials demonstrated the realization of the anode-free battery concept in dual-ion batteries (DIBs) for the first time.
Study: An Anode-free Zn-graphite Battery. Image Credit: nevodka/Shutterstock.com
DIB and its Limitations
DIBs have recently gained considerable attention for scalable energy storage applications owing to their low cost and high working voltage. Graphitic carbon that operates under the anion intercalation process is typically used as a cathode in DIBs.
Different anions such as bis(trifluoromethanesulfonyl)imide anion (TFSI-), hexafluorophosphate (PF6-), and bis(fluorosulfonyl)imide anion (FSI-) were already investigated as intercalant species for graphite intercalation compounds (GICs).
Although DIBs have witnessed significant advancements, the low energy density of these batteries at the device level remains a major problem that must be addressed. Several efforts were made to increase the DIB energy density by reducing the weight ratio of inactive solvents using concentrated electrolytes.
However, the cathode side anodic corrosion could only be suppressed kinetically at such ultrahigh electrolyte concentration. Additionally, the consumption of a large portion of electrolyte during the DIB charging negatively affects the stability of these batteries. The metal anode plating-stripping efficiency also heavily relies on the passivation interphase that is formed under concentrated electrolytes.
Anode-free Battery Concept as a Potential Solution
Anode-free lithium (Li) metal battery concept was recently developed using an inactive substrate as a current collector, which is more convenient and much safer compared to Li metal during cell assembly.
In this configuration, Li metal can be deposited on the substrate during the charging of the battery, which eliminates the need for the Li+ host graphite in the commercial Li-ion batteries. Thus, a high energy density can be achieved by a considerable reduction in the battery weight.
Similarly, anode-free sodium (Na) battery was fabricated by guiding the Na deposition on the carbon nucleation layer and anode-free zinc (Zn) batteries were realized in water-based electrolytes.
In DIBs, the anode-free configuration has not been achieved due to the narrow potential window of the existing electrolytes and low anode metal plating-stripping efficiency.
Fabrication and Evaluation of Anode-free Zn-graphite DIBs
In this study, researchers constructed the first anode-free DIB in the Zn ion system by fabricating an anode-free Zn-graphite battery (ZGB) containing Zn (TFSI)2/ethyl methyl carbonate (EMC) electrolyte, a graphite cathode, and a silver (Ag)-coated copper foil as the anode substrate.
EMC, graphite powder, alginic acid sodium salt, 250 micrometers Zn foil, Super P, Zn (TFSI)2, and hydrophilic carbon cloth (HCC) were used as the starting materials.
Preparation of Ag-Cu Substrate
Ag strips were patterned on the Cu foil using photolithography. Initially, AZ 5214 E photoresist was coated on the Cu substrate using the spin-coating method, and the coated sample was heated for five minutes at 90 degrees Celsius.
Subsequently, the samples were exposed to 365 nanometers of ultraviolet (UV) light for 10 seconds through a chromium/glass photomask using an SÜSS MJB4 mask aligner and then heated for two minutes at 120 degrees Celsius. An AZ 726 metal ion-free (MIF) developer was utilized for 45 seconds to develop the patterned Cu sample.
Ag with thicknesses of 40, 20, 10, and 5 nanometers was deposited on the patterned CU substrate using the nanoPVD method for 40, 20, 10, and 5 seconds, respectively. Eventually, the redundant metal layer and the photoresist were removed by performing a lift-off process in acetone.
Preparation of Electrolyte
An argon-filled glove box with oxygen and water content below 0.1 parts per million was used to prepare the EMC/Zn(TFSI)2 electrolyte. Zn(TFSI)2 was mixed with EMC and the mixture was stirred for one hour to obtain the EMC/Zn(TFSI)2 electrolyte in a transparent liquid.
Fabrication of Graphite Electrode
Graphite powder was mixed with Alg binder and Super P and the resultant mixture was dissolved into water. Subsequently, the slurry was cast on an HCC/current collector. The obtained graphite electrodes were eventually dried for eight hours at 80 degrees Celsius before their application in the study.
Material Characterization
Field-emission scanning electron microscopy (FE-SEM), NMR spectroscopy, X-ray diffraction (XRD) method, and atomic force microscopy (AFM) were used for material characterization.
Electrochemical Characterization
A homemade Swagelok cell, where the tungsten rod and Zn foil were utilized as working electrode and counter electrode, respectively, was employed to assess the electrochemical stability windows of various electrolytes.
The floating tests and linear sweep voltammetry (LSV) measurements of the Zn//W cell were performed using the CHI 660E electrochemical workstation. The LAND CT2001A battery test system was employed to perform the Zn plating-stripping test in EMC/Zn(TFSI)2 electrolytes.
The electrochemical performance of the graphite cathode and the anode-free” ZGB was evaluated using Swagelok cells and the LAND CT2001A battery test system, respectively.
DFT Calculations
The Vienna ab initio simulation package (VASP) was used to perform all density functional theory (DFT) calculations, while the electronic wavefunctions were defined using the projector-augmented wave (PAW) method. The van der Waals-corrected density functional theory (DFT-D3) was utilized to overcome the limitations of DFT with layered systems.
Significance of the Study
The first anode-free ZGB was fabricated successfully by efficient plating-stripping of Zn on the Ag-Cu substrate. The Ag coating guided the uniform deposition of Zn on the substrate without side reaction or dendrite formation over an extensive range of electrolyte concentrations, facilitating the fabrication of anode-free Zn cells.
The absence of side reaction or dendrite in the Zn plating-stripping cycles led to 99.90 percent plating-stripping Coulombic efficiency (CE)/reversibility within two cycles in both concentrated and dilute electrolytes.
The graphite cathode operated efficiently under reversible TFSI- intercalation without anodic corrosion. An extra high-potential TFSI intercalation plateau was observed at 2.75 volts by intercalating TFSI anions into the graphite cathode, which contributed to a high capacity of 117 milliampere hour per gram to the graphite cathode.
The fabricated anode-free ZG DIB demonstrated impressive cycling stability with 82 percent capacity retention after 1000 cycles and high specific energy of 79-watt hour per kilogram due to the efficient TFSI- intercalation-deintercalation and Zn plating-stripping. The specific energy obtained based on the mass of electrolyte and cathode was substantially higher than conventional ZGBs.
To summarize, the findings of this study demonstrated the successful realization of the anode-free battery concept in DIBs. However, more research is required to develop effective and cheaper substrates to address the interphase issues and realize the concept in newer battery systems.
Reference
Chu, X., Fu, Y., Qu, Z. et al. An anode-free Zn-graphite battery. Advanced Materials 2022. https://onlinelibrary.wiley.com/doi/10.1002/adma.202201957
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