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​Solid-state Silicon batteries Properties and Applications

Solid-state silicon batteries are one of the subclasses of lithium-ion batteries which are one of the best-known batteries in the market. In these batteries, a silicon-based anode is present at one electrode which is responsible for the carrying of charge. Silicon is an excellent material and has a very high specific capacity because of which markets have started incorporating them in lithium-ion batteries.

Their characteristics and properties are highly efficient owing to the environment in which they are used. This has massively increased their applications and usage in daily life as well mainly the portable electronic devices and technology.

Introduction

Silicon batteries are a subclass of lithium-ion batteries and this happens when silicon is used as an anode and lithium ions work as the charge carriers. It is observed that the silicon materials are capable of having a much larger specific capacity which is equal to 3600 mAh/g in the case of pristine silicon whereas comparatively graphite has occupies a specific capacity range as 372 mAh/g.

Volume change

Silicon has a large volume change which is equivalent to almost 400% relying upon the crystallographic densities. This happens when lithium is used to tackle the obstacles having a highly reactive state so that the anode of a silicon battery can be commercialized.

 

Battery anodes

All the commercial battery anodes possess little range of silicon to affect their performance positively. These ranges are not known or told as they are kept secretive but it is assumed to be 10% of the entire anode. These silicon batteries contain the cell configurations in which Si is present in those compounds which have low voltage and then become capable of storing lithium via a displacement reaction which includes silicon oxycarbide, silicon monoxide, or silicon nitride.

Anode material

Silicon has been proved as an excellent material to be studied and used as an anode material for lithium-ion batteries as it has a high specific capacity which is excellent for these batteries. During this entire process, the anode materials based upon silicon suffer in terms of huge volume changes which go on in the process of charging and discharging. This leads to certain side reactions, electric contact is lost and pulverization of silicon occurs too. These changes may hinder the commercialization process of silicon as they promote poor cycle life.

Lithiation and delithiation

Lithiation and delithiation behaviors and processes are also studied in this regard as these include interphase reaction mechanisms. There are a lot of nanostructured silicon anodes which portray high specific capacity and cycle life, both in comparison to the commercially based carbon anodes. Nonetheless, a few issues do exist in this nanostructured silicon which can never be ignored and for that, a lot of researches have been carried out to come to a conclusion and eradicate all the hurdles which hinder the working processes of commercial lithium-ion batteries.

Performance of silicon anodes

It is evident from all the studies, researches, and experiments that silicon anodes work far better than all the other materials used as anodes due to the compatibility and efficacy that they bring forth and enhance the life and health of batteries. Developments have been made over the past years to maintain the credibility of silicon anodes as it is necessary to have a keen observation for their defaults and maintenance. There are some important factors for the successful commercialization of silicon anodes that need to be followed strictly including the development of silicon electrodes with high potency materials and silicon-based lithium-ion batteries.

Dependence on non-renewable energy

In recent times, it has been highly observed that mankind has become dependent on non-renewable energy which has sparked major concerns regarding the environment, human beings and their health, and the climate which surrounds them. All of this has led to highlighting the prominence of developing and emphasizing clean energy uses. The same efforts are being made in the technological world too so that a better world can be created for everyone to feel safe and healthy. Clean energy uses are being mandatory and their use is being emphasized for all the batteries that are now being worked on or are in the process of recreation. It is a very important step in building up a safe world for upcoming generations while keeping technological safety in mind too.

Energy storage technology

In the technological market, energy storage mechanisms are being adopted for the better working of portable electronic devices and bringing a change in the different technological markets which mainly includes grid-scale energy storage. Lithium-ion batteries have also been on the same ground and have adopted this energy storage technology practice. Due to this, lithium-ion batteries have made their mark in the technological market owing to the specifications that these bring forth. Solid-state silicon batteries come under the same category and are the key ones to bring a change in the market. The applications have massively increased due to the usage of silicon in forming the anodes.

Characteristics of silicon anodes

Silicon anodes are the building blocks of solid-state silicon batteries due to the characteristics that these possess. These include high specific capacity ranges, good battery sizes with efficient life and health, and less costly. All these characteristics add up to the efficient working of solid-state silicon batteries. When all these characteristics are combined for the proper functioning and processing of the battery, they initiate a series of reactions that are highly beneficial for the battery itself and the products that these become a part of, mainly the portable devices.

Energy density

There is a long-term goal set for the better functioning of silicon batteries by USABC. This states that the pack system for lithium-ion battery must have to reach 235 Wh kg-1 or 500 Wh L -1 while having a discharge rate that is equivalent to 1/3 C. This is equivalent to 1000 cycles which possess the requirement of 15 years calendar life.

The potential of silicon

Silicon when used as an anode for rechargeable li-ion batteries, works the best to enhance the energy density of Li-ion batteries as it is a material that contains high theoretical capacity and very low potential of the electrodes in the case of solid-state. However, researches and experiments are an ongoing process to add up to the quality of solid-state silicon batteries which not only enhance the working of batteries but also pave a way for the invention of various other updated things. This is not an easy-going process and requires constant hard work and keen observations, all of which are continued.

Decrepitation

In electron conductions and Li-ion pathways, breaks are made because of the decrepitation or fracture, thus causing capacity fading and increasing the electrode resistance during cycling. Relevant differential capacity curves were used to identify rapid and gradual increases during cycling in the electrode resistance for the 3- and 1- µm-thick non-porous films. The positions of the discharging and charging peaks continue towards more positive and negative potentials as the cycling continues. As the cycling continues, the peak currents correspondingly decrease. Remarkable cycling stabilities were displayed by the porous films as compared to the non-porous films. 3000 mAh g-1 of high capacities were delivered by both of the films and after 100 cycles, they exhibited more than 93 percent of their 10th cycle’s capacities and high Coulombic efficiencies going more than 99.8%.

Peaks Position

There was no virtual change in the positions of all peaks in the differential capacity plots for the porous films, which proves that during cycling there is structural stability. For the thick and non-porous film, the Increase in the electrode impedance was excellent according to the electrochemical impedance spectra that were measured after the cycling performance tests, showing the fast capacity fading. When 0.6 mAh cm-2 was the areal capacity, there were not many distinguishing differences shown by the porous and nonporous films after the cycling between their impedance spectra.

Differences in impedance spectra

There were proper differences in the impedance spectra between the porous and non-porous films with 2 mAh cm−2 of areal capacity. As compared to the porous film, they are more in the non-porous film, suggesting that both increasing the film’s interfacial charge transfer resistance and lowering the Li-ion diffusivity causes the capacity to fade in the thicker non-porous film.

Cycled films

An electron microscope was used to observe the cycled films that were taken out from the cells for identifying the morphological changes in the cycled anodes which are the reasons for the major differences in the cycling stability in between the thicker films. After 100 cycles, the pores stayed in the porous film with a little expansion in their diameters and the width and density of the cycled non-porous film’s through-thickness cracks were broader and higher as compared to that of the cycled porous film. It is suggested by the narrower cracks with lower density for the porous film that the changes in the outer shape are suppressed by the pores via accommodation of volume expansion for lessening the increase in the resistance and maintaining the contact to the solid electrolyte.

High-rate characteristics

At last, there should be a discussion on the impact of the pore on the rate capability. In porous materials, the rate capability is normally improved when mixed with the liquid electrolytes due to the pores being filled with the liquid electrolyte as it shortens the Li diffusion length and enlarges the surface of the electrode. Solid electrolytes don’t enter the pores and the solid system’s rate capability could be worsened by the porous structure as for the electrode reactions, Li diffuses through extremely thin pore walls which in the delithiated state are of 10nm in thickness.

Repetitive large volume

Upon delithiation/lithiation, repetitive large volume change acts to pulverize or decrepitate Si particles. Recently, it was revealed by an in situ TEM (transmission electron microscopy) study that the 870 nm of the diameter of amorphous Si nanospheres, which is more than 150 nm of critical diameter, do not fracture. Thus, in solid electrolytes, anode film’s remarkable cycling performance is because of its amorphous nature with submicrometre dimensions, and morphology confinement, and suppression of production of SEI via the solid electrolyte. The thickness of the films should be more than 3 micrometers for attaining 2 mAh cm-2 of practical areal capacity.

Critical fracture diameter

There has been a recent increase in the liquid electrolytes critical fracture diameter for porous Si particles to 1.5 micrometers. There have been extensive studies on the usage of porous materials in Li-ion batteries as electrodes with liquid electrolytes. The electrode surface area is increased by the pore walls in such systems by functioning as the reaction surfaces when the pores are infiltrated by the electrolyte. The length of the Li diffusion is lessened to the thickness of the pore wall. But due to the solid-electrolyte particles not being capable of entering the pores, these benefits are not present in those solid-state batteries. Also, the anode’s structural integrity is strengthened by the porous structure and it also improves Si films cycling stability in the solid electrolytes with enough thickness for practical applications.

Cycling characteristics

There are notes and observations of the capacities of porous and non-porous film anodes. Porous Si films profiles are approximately the same as the profiles of the non-porous films and it’s also the same as the profiles of the 0.3-µm-thick amorphous Si films that were reported previously in a thiophosphate-based solid electrolyte. It is indicated by these results that the amorphous Si phase, making the pore walls is the reason for the electrode reaction. There were major differences in the porous and non-porous films’ cycling performance, the performance was dramatically enhanced by the porous structure.

According to situ

The amorphous spheres of 870 nm of diameter do not fracture according to the in situ TEM observation of lithiation of amorphous Si nanospheres. The critical value of film thickness was exceeded by the films whose thickness was examined in this study (3 and 1 micrometer for non-porous films), and therefore capacity fading was shown by the non-porous films, which the increasing thickness accelerated. Only 47% of capacity was showed by the 3-µmthick film at the 10th cycle after 100 cycles, whereas for the 1-micrometer thick film, 82% was the capacity retained. According to these results, it can be seen that while the fading capacity in the thin films with thickness a little above the limit is moderate when the thickness is increased, it also increases significantly together.

Amorphous Si films

The extremely high-rate capability was displayed by the amorphous Si films, for instance, extremely small activation and diffusion overvoltages, and therefore for solid-state electrochemical cells in this study, the dominant factor that’s observed in the polarization is the resistance overvoltage in the electrolyte layer. Thus polarization arising from the electrolyte layer resistance was excluded to obtain the discharge curves for comparing the rate capabilities inherent in the films. According to the results, the non-porous and porous films have almost identical discharge curves which show that rate capability is not detrimentally affected by the introduction of the pores.

Maintenance

At 2 mA cm-2 of current density, >2 mAh cm-2 of a remarkable high areal capacity was delivered by the thicker films, and even at 10 mA cm−2 (17 C), greater than 3000 mAh g−1 of discharge capacities were maintained by the thinner films. Discharge capacities of more than 1.2 mAh cm-2 and 1700 mAh g-1 were shown by the films even at 10 mA cm-2 of higher current density (3C discharge rate). According to these results, in the porous form, the amorphous Si’s high rate capability in the solid electrolytes is retained.

Lithium-ion Battery

Characterization and Synthesis of amorphous Si films

Radio-frequency magnetron sputtering (Advanced Optics Vacuum Co., Ltd, SPAD-2240UM) was used to deposit amorphous Si films at glancing angle geometry from a 5N-pure Si target (Kojundo Chemical Laboratory Co., Ltd) with a 30° angle between the substrate normal and the target, maintaining the temperature of the substrate at 100 °C and the substrate bias at 5 V. 15 cm was the distance between the target and the substrate.

Substrates

4000-grit sandpaper was used to polish the substrates of 10-mm-diameter stainless-steel disks. Ultrasonic treatment in acetone was used to clean them (99.5% purity, Wako Pure Chemical Industries, Ltd), and they were annealed before deposition at 800 °C in a vacuum. Si films were deposited through a mask with 8.5 mm of opening diameter for eliminating the loss of the active material that’s deposited on the substrate disks’ side.

Thickness

A surface profiler was used for measuring the thickness of the attained films whereas an electric microbalance was used for measuring the weight of the films. A scanning transmission electron microscope and a field-emission scanning electron microscope were used for observing the obtained film’s cross-sectional and surface morphologies. After the deposition of a protective carbon layer, a multi-beam processing system (JIB-4501, JEOL Ltd) was used to prepare the specimens and that system incorporated a focused ion beam (FIB) milling and a thermionic scanning electron microscope.

Characterization of obtained film’s crystallinity

The GIXRD measurements were done on a diffractometer by SmartLab, Rigaku Corp. for characterizing the obtained film’s crystallinity, and in it, Cu Kα was utilized at 45 kV of operating voltage as the radiation source in parallel beam mode. The incident X-rays’ grazing angle was 0.25 as compared to the sample surface’s grazing angle, There was the collection of the diffraction data in the two thetas (2θ) range from 90° to 3°.

Laser beam

A cylindrical lens was used to shape the laser beam into a line. Then an objective lens with 0.70 of numerical aperture and 50× magnification 8: used to focus that line onto the surface of the sample. At the sample, 100 µW was the power of the laser. The same objective lens was also used to collect the backward Raman scattering signals from the illuminated line and send it through a 70-µm-wide entrance slit into a high-resolution spectrometer. With 15 seconds of integration time, all Raman scattering spectra were attained as eight spectra’s averages. There was the calibration of wave numbers by reference to Ne’s emission lines.

Electrochemical measurements and Cell fabrication

They utilized a thiophosphate-based solid electrolyte, 80Li2S20P2S5 glass for the electrochemical measurements. According to the reports in the literature, mechanical milling was done to synthesize it with some little modifications. A pestle and an agate mortar were used to manually mix the P2S5 crystalline powders (99% purity, Sigma-Aldrich Co. LLC) and Li2S crystalline powders (99% purity, Furukawa Co., Ltd) in a 4:1 molar ratio. At 1:16 of powder-to-ball ratio, they placed the mixture (2g) with 45 ml inner volume into a zirconium oxide pot along with zirconium oxide balls that are 5 mm in diameter. They sealed the pot under an Ar atmosphere and a planetary micro mill was used to perform high-energy ball-milling at 25 °C (Pulverisette 7 classic line, Fritsch GmbH).

Attachment of amorphous Si film

To make the working of solid-state silicon batteries better, the use of amorphous Si film has been introduced in the market. In this case, the amorphous Si film is attached to the electrolyte so a working electrode can come into existence. This initiates the counter electrode at the opposite side which is done by attaching a 10mm diameter disk of Li metal foil which is formed after a hole is punched through a Li metal foil. These disks are pressed together at the rate of ~120 MPa which is done through a battery cell that gets prepared in the lab as its bolts are tightened so it can act as a two-electrode cell. It is important to keep a check on the porous structures that are formed that can either tolerate the pressure which is applied while assembling the cell.

Conclusion

Solid-state silicon batteries are considered as one of the best categories of lithium-ion batteries as their working has massively impacted the technology market and still going strong. Silicon is an excellent material and therefore has made its mark in this regard owing to the excellent characteristics that these batteries possess.

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