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Morphology and Dimension Variations of Copper Sul fi de for High- Performance Electrode in Rechargeable Batteries: A Review

Gulnur Kalimuldina, Arailym Nurpeissova, Assyl Adylkhanova, Desmond Adair, Izumi Taniguchi, and Zhumabay Bakenov*

Cite This:ACS Appl. Energy Mater.2020, 3, 11480−11499 Read Online


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Currently, di

erent metal sul

des (NiS, Co




, FeS


, and CuS) have been extensively studied as alternative electrodes for rechargeable batteries that can satisfy the performance requirements for more powerful energy supply and storage technologies for various applications and industries. Among them, copper sul

des have gained signi

cant attention as a promising electrode material in rechargeable metal-ion (Li, Mg, Na, and Al) batteries. A wide range of synthesis routes and methods have been implemented in order to prepare various stoichiometry Cu


S (1

≤ x ≤

2) micro-/nanostructured materials with excellent electrochemical properties. Since the bulk microsized electrode materials have almost reached their performance limits for energy devices, the introduction of nanoscale Cu


S composites is now in high demand. This review focuses on the in

uence of the material morphology and dimensions on their performance in secondary batteries. The structures of Cu


S materials from zero-dimensional (0D) to 3D and their preparation are discussed. The primary purpose of this work is to provide an overview of the unique electrochemical and physical properties of particular structure and dimensionality which can promote these materials

application in the energy storage

eld. Along with this, this work summarizes the information on various synthesis strategies and how they can manage the morphologies of Cu


S micro-/nanocomposites. In the current fast technologically advancing society, the development of the most economically pro

table and e

cient synthesis routes is especially encouraged and required, and this aspect is also commented on in this review.

KEYWORDS: copper sulfide, dimensionality, 0D copper sulfide, 1D copper sulfide, 2D copper sulfide, 3D copper sulfide, lithium-ion battery, metal sulfide


The United Nations Paris Agreement of 2016 declared immediate actions toward reduction of greenhouse gas emissions and mitigation of their negative effects. Environ- mental concerns initiated and motivated the need for the large- scale implementation of renewable energy sources.


However, today the intermittent character of renewable energy sources demands e

cient energy storage systems to mitigate power

fluctuations in order to integrate them into grids.

Among various energy storage technologies, rechargeable lithium-ion batteries (LIBs) have proved to be the most promising and applicable for over the past two decades due to their capability to rapidly store and release electricity and to some extent maintain their storage capacity over cycling. Along with the energy generation, the fossil fuel powered transport causes critical air pollution and this motivated rapid develop- ment of the market of electrical vehicles (EVs), which is also mainly powered by LIBs. LIBs also dominate the market of

rechargeable batteries for other portable electronic devices and applications.

Microsized bulk composites used in LIBs have already reached their limits in electrochemical performance and cannot satisfy the requirements of the next-generation electrochemical energy devices (EEDs). Therefore, the development of new energy storage materials is urgent for enhanced support of the ever rapidly growing practical needs of our society. Novel advancements in nanoscience are opening new prospects to remarkably improve the performance of materials for super- capacitors, fuel cells, and especially the electrodes for LIBs.

Received: July 16, 2020 Accepted: October 28, 2020 Published: November 16, 2020

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It is acknowledged that the electrodes are an essential part of the battery in relation to their design, structure, and dimensions, since these parameters directly affect the battery power, energy densities, and capacity.


According to Mei et al.


an increase of active mass loading of electrodes helps to increase the energy density, while the power density decreases.


Also, the decrease of particle size increases power and energy densities.


Lu et al. investigated di

erent sizes of LiMn




, and concluded that the reduction of the particle size was responsible for the higher Coulombic e

ciency and speci

c capacity.


Since the Li-ion transport in LIBs is rate-limiting in solid materials, i.e., electrode materials, the bigger the particle size, the higher the limiting transport path.


Also, it is worth mentioning that recently single crystalline active materials appeared to show superior perform- ance.


To investigate the impact of the one-dimensional (1D) and 3D electrodes’ thicknesses on the electrochemical and thermal characteristics, Zhao et al. reported that, with an increase of thickness, energy density as well as internal resistance became higher. The investigation stated that in the case of LiMn




material with an increase of cathode material thickness from 35 to 65


m, the energy density of 140.99 Wh kg


rose to 228.15 Wh kg


, while the power density dropped from 313.31 Wh kg


to 258.11 Wh kg


. This decrease was directly connected to the expansion of discharge time at the same current rate.


In addition to these factors, porosity is also one of the important parameters because it a

ects the kinetics of ion di



Meso- and micropores could facilitate access of the electrolyte that improves the transfer of ions between electrodes during charge/discharge processes.

The currently known commercial electrode materials are still in their

rst generation. The researched electrode materials can be divided into three main categories, depending on their electrochemical reaction mechanisms, as intercalation (LiCoO


), alloying (Si, Sn), and conversion (M




, M = transition metal and X = O, S, F, P, N). So far, the di

erent approaches (nanostructuring, surface modi

cations, carbon coating, and others) have been tried in an attempt to improve the electrochemical properties of the conventional cathode (LiCoO


) and anode (graphite) materials. Although such battery systems have been widely used in portable electronics, today they are facing critical power limitations for large-scale applications.


In the case of commercially available cathode LiCoO


, although almost all of the Li can be extracted to give a theoretical capacity of 274 mAh g


, only a little over half of the capacity is practically reversible for charge/discharge (

4.2 V vs Li/Li


). Capacity fading is severe upon extraction of >0.7 Li due to the loss of oxygen, electrolyte decomposition, and the problems of cobalt dissolution in typical electrolytes.


Alternatives to LiCoO


are necessary because of its high cost, toxicity, and poor safety, making it unsuitable for large-scale energy storage applications.

On the other hand, the most used graphite anode allows the intercalation of only one Li ion with six carbon atoms, with a resulting stoichiometry of LiC


and thus an equivalent reversible capacity of 372 mAh g




Hence, there also is an urgency to replace graphite anodes with materials with higher capacity, energy, and power density. Today the path leading to metal-ion batteries with improved energy and power densities has, as a major challenge, the selection of suitable electrode materials, which can provide high capacity and easy di

usion of Li ions into the structure, along with good cycling life and freedom from safety concerns.

The need for satisfying the required power demands of EVs and large-scale energy grid systems is pushing researchers

boundaries further in the exploration of many other materials with the most prominent electrochemical properties. In the past few decades metal sul

des (NiS, Co




, FeS


, CuS) have started to be considered as economically viable alternatives due to their unique properties of high conductivity and higher speci

c capacities in comparison to metal oxide electrode materials.


Metal sul

des have more valence states and crystal dimensions, making them more attractive due to their electrochemical superiority from the capacity and rate capability points of view.


Additionally, metal sul

des are abundant in the Earth crust (for instance, chalcocite); thus, they and their composites are cheap, which means their application can be utilized widely.


Among metal sul

des, copper sul

des (Cu


S, 1

≤ x ≤

2) have attracted considerable attention due to their low cost and high theoretical speci

c capacity (337

560 mAh g




The main feature that makes Cu


S especially attractive is its high electrical conductivity. It was reported that Cu


S possessed a high electrical conductivity of about 5

140 S cm


and CuS of around



S cm


, respectively.


The general electro- chemical reactions of these compounds can be considered as follows with the

nal products as Cu and Li




+ + → +


2Li 2e CuS Cu Li S2 (1)

+ + → +


2Li 2e Cu S2 2Cu Li S2 (2)



S materials show two discharge potential plateaus at 2.1 and 1.7 V vs Li/Li


for CuS


and a single potential plateau around 1.7 V for Cu




Over the past ten years, more works have been devoted to investigating the complex reaction mechanism of Cu


S with metal anodes (Li, Na, Mg, Al) in half- cells and more detailed charge

discharge processes have been reported.


Depending on the synthesis procedures of Cu


S, its electrochemical performance varies because it is strongly related to its structure, including the crystalline nature, particle size, dimensionality, and interface. Therefore, in this work, Cu


S materials with di

erent morphological structure dimensionalities from 0D to 3D are reviewed, including the synthesis procedures and their structure-related electro- chemical performances, when applied as cathode or anode materials in secondary batteries. Copper sul

de has a wide range of nonstoichiometric compounds such as Cu


S, Cu


S, Cu


S, Cu


S, Cu


S, Cu


S, CuS, and CuS




Among such a large number of Cu


S phases, CuS and Cu


S are the most investigated. In this review, we aim to accentuate the critical importance of dimensionality on the performance of Cu


S materials not only in LIBs but also in magnesium-ion batteries (MIBs), sodium-ion batteries (NIBs), and aluminum- ion batteries (AIBs) along with discussions on the synthesis routes.


Nanostructured materials (NSMs) have attracted great attention due to their unique properties, which were a boon to their application areas in both fundamental and applied sciences because their physical, chemical, and electronic properties show dramatic differences with a change of the dimensionality of the material. Pokropivny and Skorokhod introduced a scheme of classification for NSMs in 2007 such as 0D, 1D, 2D, and 3D, as shown inFigure 1, which was based


on the then recently developed composites.43,44Thisfigure has been taken as a guide for defining morphology classifications of the subject of studies in this review. Such classification is highly dependent on the electron movement along the dimensions in the NSMs, which generate a series of novel physical and chemical properties that differ from those of conventional bulk materials. For example, rather than electrons in 0D NSMs being entrapped in dimensionless composites, 1D NSMs can move electrons along thex-axis, and they are less than 100 nm in size. With the increase of dimensionality, the movement of electrons is enhanced, as in 2D, and 3D NSMs where electrons freely move along thex−y-axes andx,y,z-axes, respectively.31,43−45

Many techniques have been developed to synthesize and fabricate 0D−3D CuxS NSMs with controlled size, shape, dimensionality, and structure. Therefore, the details of the different types of CuxS NSMs are discussed here from 0D to 3D, which are synthesized or fabricated by a variety of both simple and complex methods in order to achieve the most attractive electrochemical properties for the next-generation rechargeable metal-ion batteries.46 Obtained structures of CuxS known as nanoparticles, nanorods, nanowires, nanoneedles,47−49 nanosheets,50,51 nanothin films,52 hollow spheres,53 flower-like structures,54and others were taken as the basis of classification by morphological dimensions as shown inFigure 2.

Figure 1.Dimensionality classification of nanostructures. Reprinted with permission from ref44. Copyright 2007 Elsevier.



Although a vast amount of different synthesis methods have been developed to fabricate 0D materials, only a few 0D NSMs with CuxS have been successful.46 Today, quantum dot synthesized CuxS is widely used in supercapacitors and solar cells, but less in LIBs. This is because for controlled synthesis of 0D and 1D metal sulfide nanostructures, various hard or soft templates such as aluminum oxide pores or surfactant molecules are usually required. These additions to the synthesis procedures increase the reaction complexity and result in potential impurity in the products.55

A simple approach is to use commercially available NSMs as recommended by Wu et al.56to check properties of CuxS in MIBs.

Such a system delivers a high reversible capacity of 175 mAh g−1at 50 mA g−1and a rate capability providing 90 mAh g−1at 1000 mA g−1as well as a stable cyclability over 350 cycles. These promising properties have been linked to the small-sized copper sulfide particles, which facilitate the solid-state diffusion kinetics.

The work of Kravchyk and co-workers57 presented a highly reversible electrochemical reaction between Mg and CuS nano- particles for MIBs as well, with high capacities of 300 mAh g−1 at room temperature and high cyclic stability over 200 cycles. The current density was 0.1 A g−1with a Coulombic efficiency of 99.9%.

TEM, XRD, and EDX analysis results are shown inFigure 3. The

authors used a mixed solution method with a heat treatment procedure to fabricate the desired nanostructured CuS electrodes. On the other hand, Lui et al.58 applied a hydrothermal method and subsequent calcination using glucose as a carbon source to obtain a Cu2S/C composite material for LIBs. After the heat treatment at 800

°C, the authors succeeded in the formation of interconnected spherical particles. The Cu2S/C composites exhibited a discharge capacity of 300 mAh g−1after 100 cycles.


One-dimensional structures are attractive for researchers because of their enhanced optical, magnetic, and mechanical properties and electrochemical performance.59 They find applications in nano- thermometers, solar cells, and light-emitting diodes as well as electrodes for LIBs. One-dimensional NSMs vary in diameter from 1 to 100 nm and can be in tube-, rod-, wire- or belt-shaped forms.60 Plenty of methods to fabricate CuxS nanowires/nanorods, namely, the hydrothermal approach, template-assisted route, high-pressure autoclave process, electrodeposition, and thermal evaporation, have been used.61−66The 1D composites as nanowires and nanorods are suitable for application in LIBs because their performance can satisfy the requirements for advanced batteries. The advantages of long cyclability and enhancedflexibility lead to the improvement of the contact area between electrolyte and electrode. In the result, increased charge/discharge capacities as well as enhancement of both electrons and Li-ions conductivity were observed.67−71Low-dimensional NSMs with large surface areas exhibit superior mechanical, thermal, chemical, and electrical properties to those of bulk materials. The application of 1D NSMs can be possible in a wide range of applications that are not possible with bulk dimensional materials.59 The reported works dedicated to the preparation of CuxS with such beneficial 1D morphologies have delivered excellent electrochemical properties such as cycling stability and higher capacity. The recently reported 1D CuxS materials can be grouped into two categories depending on the morphologies: nanowire and nanotube/rod/fiber structures.

Nanowire Structures. The advantages of nanowires for energy storage systems are that they have a high contact area between the electrode and electrolyte. Also, it is essential to mention the shortened pathway for Li-ions and electrons transport. These features can improve the electrochemical performance and stability of the electrodes at higher charge−discharge rates.69,72 The advantage of these properties were demonstrated by Lai et al.73who applied a facile solution method to fabricate 1D Cu2S nanowire arrays. Due to the pure metal sulfide phase, which was provided via this route, it has generated much interest among researchers. Additionally, the material obtained by this method has shown a good cyclability delivering 250 mAh g−1for 100 cycles as well as a high Coulombic efficiency (Figure 4). In another work, Feng et al.49 obtained new copper sulfide

nanowire bundles via a template method without any surfactant use, by mixing them in dimethylsulfoxide−ethyl glycol solvents. Because of a specific structure of copper sulfide nanowires, the capacity and cycling stability of LIBs were increased.

Nanotube/Rod/Fiber Structures.Similarly to nanofibers, nano- tubes/nanorods have also attracted the attention of researchers. They also own a high surface area; and in these materials this advantageous property for enhanced electrochemical activity is strengthened further by the advantages of an interconnected porous network structure with Figure 2.Different architectures of CuxS developed for rechargeable


Figure 3. TEM image (a) and XRD pattern (b) of CuS NPs. (c) Elemental maps of a single CuS NP using EDX measurements in the HAADF-STEM mode for Cu + S, Cu, and S/ (d) Line scans representing the intensity distribution of Cu (red) and S (green) within the CuS NPs. Reprinted with permission from ref 57.

Copyright 2019 The Authors under Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/

4.0/), publilished by Springer Nature.

Figure 4.SEM images of the as-grown Cu2S nanostructures. (a) Low- magnification view of a large-area Cu2S nanowire arrays grown on a copper substrate. (b) Cycle performance of a Cu2S nanowire array/Li cell operated at a high rate of 2 C. The inset shows the corresponding Coulombic efficiency (CE) of the Cu2S nanowire array/Li cell.

Reprinted with permission from ref73. Copyright 2010 Royal Society of Chemistry.


well-directed 1D conductive paths. Carbon networks have been investigated where nanoscale copper sulfide (n-Cu2S) and microscale copper sulfide (μ-Cu2S) were deposited over networks of single- walled carbon nanotubes (SWCNTs) by atomic layer deposition (ALD).74The ALD Cu2S was deposited on networked 1D SWCNTs at 135°C using an ALD reactor with the morphology shown in SEM images inFigure 5. When the authors compared the effect of different

scales of deposited Cu2S on their electrochemical outcomes, the SWCNT-μ-Cu2S showed a capacity of 200 mAh g−1with fast decay upon cycling. The lack of capacity recovery compared to theμ-Cu2S suggests that perhaps the material was too dense to allow a rearrangement in the electrode composition. In contrast, the SWCNT-n-Cu2S demonstrated a capacity of about 260 mAh g−1 for 200 cycles. The capacity recovery after a long cycle number in SWCNT-n-Cu2S was linked to increased extensive SWCNT lithiation on the accessible surface.

Zhou et al.75published research on 10 nm nanorods by a simple sol−gel method without further thermal operations. According to this work, besides great Coulombic efficiency as well as reversible capacity, the utilized anodes based on these nanorods indicated good cycling up to 250 cycles.

Cai et al.76obtained CuxS/Cu nanotubes in poly(ethylene glycol) (PEG). The presynthesized Cu nanowires were converted into nanotubes via a mass diffusion. The work demonstrated that CuxS/Cu nanocomposites show a stable cyclability with afifth-cycle discharge capacity of 282 mAh g−1. It was concluded that the PEG capping helped to reduce the dissolution of polysulfides.

A tubular carbon matrix was used to impregnate sulfur through heat treatment in a sealed vessel.77The electrode was coated on a Cu foil, and a Cu2S/tubular mesoporous carbon composite was produced via electrochemical charge−discharge-assisted processes. Cu2S with in situ preparation showed the highly reversible and very stable capacity of 270 mAh g−1for 300 cycles at 1 C rate. Li et al.45synthesized 1D CuxS nanorods via the hydrothermal route without using any surfactants. Both of the electrodes based on CuS and Cu2S composites demonstrated high cyclability as well as rate capability, with initial capacities found as 370 and 260 mAh g−1, respectively.

Due to a facile synthesis and excellent electrochemical performance, these nanorods are promising anodes for LIBs. Yang et al.78presented a CuS cathode material via a surfactant-assisted hydrothermal route. It was highlighted that participation of sulfurating reagents and synthesis temperature directly impacted the morphology of copper sulfide nanorods.

Chen and coauthors79prepared a new combination of Cu2S with an N and S dual-doped carbon matrix (Cu2S@NSCm) by a simple in situ polymerization process and subsequent carbonization for both LIB and NIB. The morphological study showed that short nanorods

have the length of∼200 nm and a diameter of∼40 nm. The first charge/discharge capacities of Cu2S@NSCm in LIB were 558.4 mAh g−1and 894.8 mAh g−1, respectively, in the 0.01−3.0 V range vs Li/

Li+. As for the NIBs, the first discharge capacity of 949.8 mAh g−1 dropped to 182.3 mAh g−1 after 50 cycles in a potential range of 0.01−3.0 V vs Na/Na+at 100 mA g−1.


Recently, 2D materials are gaining even more interest due to their unique structure and surface area resulting in enhanced and pronounced physical and chemical properties. Furthermore, 2D materials exhibit special shape-dependent features; therefore, they can be used as the major components for building blocks in nano- devices.80,81Another benefit of utilizing 2D materials is that this type of composite can easily interact and combine with 0D, 1D, and 2D materials.80,82,83Thus, the mentioned features of these materials can be utilized for secondary batteries in allfields (anodes, cathodes, and separators). Due to the large surface area, porous structure, and good chemical stability, 2D materials can be employed as host materials for copper sulfide cathodes.84−86Besides being used as host materials, 2D materials promote the reaction kinetics to lethargic kinetics and eventually increase the electrode output. Over the past years, numerous types of 2D materials such as graphene, borophenes, transition metal oxides/sulfides/nitrides/carbides/phosphides, metal−organic frameworks (MOFs), polymers, and 2D hybrid materials have been applied for LIB compartment improvements.85 Furthermore, 2D nanomaterials can be hybridized to the van der Waals solids of hetero-nanosheets which have properties (synergistic or compensating) that cannot be easily achieved by general 2D nanomaterials.46In this section, various kinds of 2D CuxS materials known as nanoplates, nanobranches, and nanosheets prepared for LIBs and NIBs have been demonstrated.

Graphene-Based CuxS Structures. It has been reported that most of the metal sulfides suffer from fast reversible capacity fading due to the large volume variation during the lithiation and delithiation processes and dissolution of lithium polysulfides in the electrolyte.87 The use of a carbon matrix is considered as one of the effective approaches to solving these issues, as it acts as a buffer to mitigate the volume expansion and an electrically conductive media.88,89So far, the investigation papers on the improvement of CuxS properties through adopting graphene, graphene oxide (GO), or reduced graphene oxide (rGO) matrix have been reported on using CuxS as the anode material for LIBs.

Hydrothermal treatment is one of the most available and simple methods to obtain 2D NSMs. For example, Tao et al.90fabricated CuS/graphene composite by the one-pot hydrothermal method using thiourea both as the sulfur source and reducing agent. To see the effect of the addition of graphene, the authors compared charge− discharge profiles of CuS and CuS/graphene. For the CuS electrode, thefirst discharge and charge capacities were 525 and 311 mAh g−1, respectively. However, thefirst discharge and charge capacities of the CuS/graphene composite were significantly improved up to 827 and 484 mAh g−1, respectively. Another work with rGO as the framework for CuxS was reported by Ren et al.50 CuS nanoparticles were homogeneously dispersed on the surfaces of rGO nanosheets via a hydrothermal method as well. The obtained“double-sandwich-like” structure led to improvement in the electrochemical performance of electrodes. Authors linked the reported achievements to the reduced transport path length for both Li+ions and electrons due to the rGO double-sandwich-like structure, as it improves lithium insertion/

desertion from the liquid electrolyte or anode structure. The discharge capacity showed 648.1 mAh g−1 at the second cycle and later increased at the 100th cycle to 710.7 mAh g−1.

Ding et al.91reported the combination of hydrothermal and freeze- drying steps to prepare hierarchical CuS. The freeze-drying technique gave uniform mixing of the CuS microparticles withflexible graphene layers as can be observed inFigure 6. The BET analysis of specific surface area of CuS/graphene composites showed 177.34 m2 g−1, Figure 5. SEM images of the ALD-coated SWCNTs following (a)

100, (b) 200, (c) 400, and (d) 600 Cu2S ALD cycles. Reprinted with permission from ref74. Copyright 2015 Elsevier.


which was much higher than that of bare CuS (32.29 m2 g−1).

Accordingly, the CuS/graphene electrode delivered higher initial discharge and charge capacities of 1318.9 and 854.6 mAh g−1, respectively. More complex composite materials as graphene/

polyaniline/CuS nanocomposite (GR/PANI/CuS NC) was fabri- cated by a combination of hydrothermal and in situ polymerization methods.92The cycle performance showed outstanding properties of GR/PANI/CuS NC with a reversible specific capacity of 1255 mAh g−1after 250 cycles.

Microwave-assisted hydrothermal synthesis is a fast route to fabricate desirable morphologies of NSMs with stable stoichiometric compositions. For instance, Yuan et al.93 synthesized a CuS/rGO nanoflower composite by an ultrafast microwave-assisted hydro- thermal method using Cu(NO3)2, thiourea, and GO powders as the precursor. The rate capability tests of the CuS/rGO electrodes exhibited capacities of 543, 484, 473, 458, and 453 mAh g−1at the current densities of 100, 200, 500, 800, and 1000 mA g−1, respectively.

One-pot microwave irradiation was also used to prepare CuS/

graphene electrodes.94 CuS spherical particles with a diameter of about 800 nm were formed on the graphene sheets. The electrochemical tests showed that pristine CuS had a capacity of 379 mAh g−1 after 100 cycles at 0.2 A g−1, while CuS-G exhibited improvements delivering the capacity of 497 mAh g−1after 100 cycles.

Implementation of CuxS electrodes in NIBs has also been gaining in application over the past few years. For instance, microwave treatments have been used by Li and coauthors95to obtain CuS with a different mass percentage of rGO (20.8%, 26.7%, and 34.0%).

Through the modification of a cutoffpotential, there was an attempt to improve the electrochemical properties of CuS/rGO with diethylene glycol dimethyl ether (NaFS/DGM) electrolyte. At the potential range of 0.4−2.6 V vs Na/Na+a discharge capacity of 392.9 mAh g−1after 50 cycles was delivered for 34.0% of the rGO in the CuS.

CuS nanowires were successfully obtained on the rGO by a simple one-pot general solution method in a DMSO-EG solvent.49 Compared to pure CuS nanowires, this type of composite showed excellent energy storage performance and long-term stability. For

instance, its reversible capacity was 620 mAh g−1at 0.5 C after 100 cycles. In another work, Zhang et al.96have similarly synthesized CuxS microspheres wrapped in rGO with a two-step method. Initially, Cu/

rGO was prepared by mixing the solution, and then CuxS/rGO was obtained by adding sulfur. XRD showed the mix of the phases as CuS and Cu2S in their electrodes with a capacity of 320 mAh g−1for 100 cycles at a constant current of 200 mA g−1. Contrary, wet chemical methods could be considered a much simpler way to prepare hierarchical CuS/rGO composites.97 The SEM image of the hierarchical CuS-rGO demonstrated that the diameter of the samples was about 2.5μm with the hierarchical CuS microballs confined by graphene. The initial specific discharge capacity of the hierarchical CuS-rGO was 810 mAh g−1. However, after 50 cycles, the capacity value of the hierarchical CuS-rGO dropped and remained at 450 mAh g−1. Such excellent results of application of the graphene-based media in CuxS electrodes could be attributed to the synergistic effects between CuxS and graphene in the composite electrode.

Carbon-Coated/-Added CuxS Structures.Coating is one of the most effective approaches used to boost the electrochemical performance of different materials. For instance, carbon coating is widely used to enhance Li−S battery properties.98The improvements have been ascribed to the physical and chemical stabilities of the coating layer, which is beneficial in maintaining the morphology of well-designed nanostructures and reduces the loss of active mass during long-term cycling.88 As for the CuxS electrodes, carbon coating/addition not only enhances the electronic conductivity but also reduces the lithium polysulfides dissolution in the electrolyte and their diffusion/shuttle.99

Zhang et al.100fabricated carbon-coated C@Cu1.96S nanosheets by annealing MOF and commercially available sulfur powder. MOF is known as an effective precursor for fabrication of carbon materials and metal oxides with novel nanoarchitectures or enhanced porosities.

The electrochemical performance of the obtained C/Cu1.96S was improved, showing a high reversible capacity of 240 mAh g−1during the 50th cycle with capacity retention of 94%. Liu et al.89also applied a one-pot microwave-assisted solvothermal approach highly used in preparation of 1D NSMs to fabricate copper sulfide and carbon hybrid nanotubes. The obtained hybrid material as CuS-CNTs exhibited 339 mAh g−1 at 0.5 A g−1. It was reported that unique hybrid nanotubes were one of the main facilitators of high reversible capacity, cycling stability, and rate capability.

Nanoplates and nanodisks are known for their enhanced access by electrolyte and accordingly improved Li+diffusion. The method of chemical dealloying was used to prepare CuS nanowire-on-nanoplate by Wang and co-workers.48According to this work, the nanoplate matrix was uniformly covered with nanowires with 4−7 nm width and 40−60 nm lengths. The CuS nanowire-on-nanoplate structured electrode exhibited a capacity of 425 mAh g−1after 100 cycles.

In another approach, copper sulfide nanodisks (CuS-NDs) were fabricated using a simple low-temperature reaction and applied as the anode materials for NIBs with acid-treated single-walled carbon nanotubes (a-SWCNTs), which acted as a paper-like nanohybrid. The nanohybrids had a high reversible capacity of 610 mAh g−1and high rate current rates of 0.1−3 A g−1during the conversion reaction which formed Na2S and Cu metal in reverse.101 CuS nanoplates were also prepared for NIBs by using a solvothermal method from a transparent microemulsion consisting of CTAB,n-pentanol, and copper nitrate trihydrate. Carbon disulfide was added to the microemulsion before transferring it into the Teflon-sealed autoclave. During the electro- chemical tests of CuS with Na, such nanoplates experienced severe capacity deterioration of up to 80 mAh g−1at 0.2 C, and CuS after the first 13 cycles. However, with the increase of the cycle number, the capacity gradually recovered to 570 mAh g−1. The work tried to understand the reasons for the capacity recovery using a series of analysis of ex situ TEM of the cycled CuS nanoplates. According to the studies, sodium insertion−extraction stress changes the morphology of the nanoplates into small grainy parts with 1−20 nm sizes (Figure 7).102

Thin Film CuxS Structures.The advantage of thinfilm is that it does not need additional carbon sources to enhance conductivity or Figure 6. Typical morphologies and structures of CuS and CuS/

graphene composite: (a, b) SEM images and (c) TEM image of CuS.

The inset of panel c is the SAED pattern of CuS. (d) SEM, (e) TEM, and (f) HRTEM images of the CuS/graphene composite. The inset of panel f is a typical SAED pattern of the CuS/graphene composite.

Reprinted with permission from ref91. Copyright 2017 Elsevier.


polymer binders due to the extreme thinness of the electrode.

However, only in a few works on the synthesis of CuS thinfilms the electrodes application in secondary batteries can be found. One of them is by the Tarascon group103on the preparation of CuS thinfilms by electrodeposition by cyclic voltammetry (CV). Here the electrolytic bath consisted of Cu(TFSI)2 and sulfur powder in the ionic liquid [EMIm]TFSI. InFigure 8, SEM images of the resulting CuS flakes with 50 nm thickness are shown. This CuS structure exhibited thefirst discharge capacity of 545 mAh g−1at C/20, which is close to 97.4% of the theoretical capacity.

A widely used microwave-assisted method mentioned above was also implemented to fabricate CuS thinfilms by Xiao et al.104Initially, circular Cu metal foil with a diameter of 12 mm was dispersed in sulfur dissolvedN-methyl-2-pyrrolidinone (NMP) solution and then microwave irradiated. The morphology of the reaction product changed from CuS nanobuds to nanosheets with the increase of the microwave irradiation temperature. The binder-free Cu-supported CuS nanosheets demonstrated a reversible capacity of 490 mAh g−1 for 100 cycles at 1 C rate. On the other hand, a less stable cyclability was shown by nanobuds-shaped CuS with capacity of about 271 mAh g−1after 100 cycles at the same current density.

Another approach as hydrothermal treatment allowed direct growth of Cu2S film on a Cu foil as demonstrated by Ni and coauthors.52The obtained 2μm thickness Cu2S thinfilm revealed a capacity of 0.32 mAh cm−2in the 200th cycle at a current density of 0.1 mA cm−2. Mazor et al.105 also investigated CuS thin film application in microbatteries. The electrodeposition from an electro- lytic bath of 1,2-propanediol propylene glycol, ethylenediaminetetra- acetic acid−disodium−copper (CuNa2EDTA), and elemental sulfur was used in this case. Planar Li/CuSx cells showed reversible capacities of 120−150μAh cm−2and a peak power of 18.5 mW cm−2.



Due to the unique high surface area, large surface-to-volume ratio, and more favorable structural stability in comparison with their 0D, 1D, Figure 7.Ex situ observation of CuS nanoplates disintegration. (a) Schematic model demonstrating the disintegration in CuS nanoplates. Low- magnification TEM images and corresponding SAED patterns of (b) pristine CuS (scale bar, 200 nm) and desodiated CuS nanoplates (scale bar, 100 nm) after (c) 20 cycles, (d) 50 cycles, and (e) 240 cycles at 0.2 C. Inset graph in panel d shows the size distribution of CuS nanograins. TEM images of the SEI layers on the surface of NaxCuS after (f) 20 cycles (scale bar, 20 nm) and (g) 240 cycles (scale bar, 10 nm). Reprinted with permission from ref102. Copyright 2019 The Authors under CC BY-ND 2.0 (https://creativecommons.org/licenses/by-nd/2.0/), published by Wiley-VCH.

Figure 8.SEM images of CuSfilms prepared with different reaction times. Reprinted with permission from ref103. Copyright 2012 Royal Society of Chemistry.


and 2D counterparts, 3D architectures have been investigated with much interest. As was mentioned before, the behavior of nanoma- terials depends on their shape, size, morphology, and dimensions.

Therefore 3D materials, as for 2D materials, due to their controllable structure, high surface area, great absorption abilities, and better electron conductivity (better transportation of electrons and ions) are very promising as electrodes for energy storage devices. Because the main criteria for selection of the electrode materials for rechargeable batteries are their energy and power densities, advanced 3D nanoarchitecture materials have been developed using two core principles: increasing the electrochemically active surface area and shortening the ionic transport pathway. To do so, the surface area should be increased by constructing hierarchical structures or hollow structures while the thickness should stay as thin as possible to provide a short ion diffusion length. Different types of 3D CuxS nanomaterials were developed utilizing different approaches from hard-template to template-free synthesis methods and investigated in rechargeable metal-ion batteries.

Hierarchical Structures. Hierarchical architectures can be defined as assemblages of molecular units, their aggregates of primary nanoparticles embedded within other particles or agglomerates that may, in turn, be part of even larger units of increasing levels of the organization.106 The controllable synthesis of 3D hierarchical CuxS nanostructures with highly active building blocks generates high specific surface areas and facilitates fast migration of ions, enhancing performances of energy storage devices. The growth of hierarchical CuxS architectures can be achieved by utilizing simple solution chemical route methods, including solvothermal, hydrothermal, and so on. Chen et al.107 obtained novel stick-like copper sulfide hierarchical structures via a hydrothermal approach by using β- cyclodextrin as the ligand which generated different morphologies.

The 3μm in diameter sticks were composed of tens to hundreds of well-arranged and self-assembled hexagonal-phase covellite CuS nanoplates with a thickness of about 25 nm, as seen inFigure 9b.

Electrochemical measurements vs Li+/Li0 demonstrated good cycle stability with a capacity of 93 mAh g−1after 30 cycles.

Using the same method but different Cu and S sources and a surfactant, nanoflakes, microspheres, microflowers, and nanowires of CuS were prepared and investigated as cathode materials for LIBs.

Phases of all four products examined by XRD were indexed as an orthorhombic CuS. When tested in coin cells, the initial specific discharge capacities for CuS nanoflakes, microspheres, microflowers, and nanowires were 200, 258, 225, and 262 mAh g−1, respectively.

Because of their nano-/microparticle size and compact structure, microspheres showed the best capacity retention whereas the nanowire electrode showed the best rate capability (58 mAh g−1at 1 C). This is due to the high accessibility of the electrolyte, short lithium diffusion length, and porous nanowire structure.108Although the stick architectures have been reported, they are not as common as hierarchical flower-like architectures for copper sulfides. So far, a solvothermal method was adopted to design sphere-like CuS hierarchical structures without any template and surfactants. In a typical synthesis procedure, Cu and S sources were introduced to different solutions with subsequent heat treatment.109,110 Micro- spheres with the morphology consisting of tens to hundreds of well- arranged and self-assembled 20 nm thin cubic phase CuS nanoplates were obtained from a mixture of sulfur and ethanol. They delivered an initial capacity of around 600 mAh g−1.109 Wang and co-workers38 also described very similar structures assembled from hexagonal CuS nanoplates with high crystallinity. The microspheres as the cathode material for AIBs, built from nanoplates with a mean edge length of about 1μm and an average thickness of about 13 nm (Figure 10c,d), exhibited a capacity of about 90 mAh g−1 with nearly 100%

Coulombic efficiency after 100 cycles at a current density of 20 mA g−1.

A one-step solvothermal method was practiced to synthesize unique CuS-CTAB (cetyltrimethylammonium bromide) micro- spheres, with tunable interlayer space and micropores, by embedding CTAB directly into hexagonal CuS layers. The 3 μm CuS-CTAB microspheres consist of CuS-CTAB nanowalls with an increasing interlayer spacing from 0.8 to 1.2 nm. The main benefit of using CTAB surfactant is that it can absorb polysulfide anions to prevent the dissolution of polysulfide as well as to buffer the volume change.

As a result, the CuS-CTAB electrodes had a reversible capacity of 684.6 mAh g−1and possessed a long-term cyclability of 1000 cycles.

The excellent rate and ultralong cycling stability performance were attributed to the inserted cationic CTAB molecules which overcome the main problems of CuS electrodes in conversion reaction and limiting polysulfide shuttling.111

Once again, the one-step solvothermal method, assisted by a polymer template was used to synthesize the Cu9S5-AHP (amino- ended hyperbranched polyamide) structure. Here AHP was used as a template and an additive agent. AHP and salt of copper were dissolved in a methanol solution, after which copper cations connected with AHP amino groups, forming specific hierarchical Cu9S5 microspheres under thermal conditions. To investigate the influence of AHP on the formation of microspheres, the mole ratio of AHP was varied (Cu9S5-AHP-x,x= 0.5, 1.5, 2.0). A 3D microflower- like architecture with a uniform size of∼500 nm was obtained with Cu9S5-AHP = 1.5, which consisted of assembled rhombohedral Cu9S5 nanoplates with uniform thickness. The Na-storage properties of as- prepared Cu9S5-AHP-1.5 were tested. As a result, the cell delivers an outstanding cycling stability, achieving reversible capacity of 386.0 mAh g−1 after 200 cycles with a capacity retention rate of 90%. A polymer framework, where Cu9S5 nanograins were uniformly dispersed, enhances the diffusion of Na ions and alleviates the nanoparticle aggregation, hence improving the electrochemical performance.112

In another work, 3D CuS nanoplates for NIBs were synthesized by Park et al.113The obtained CuS nanoplates showed a unique structure with two thin interweaving plates having an average diameter of∼300 nm and thickness of∼30 nm as shown inFigure 11. Such morphology is assumed to provide a large surface area and access to Na ions during the charge−discharge processes. The cycling was performed in a potential range of 0.05−2.6 V, and after 80 cycles the capacity and Coulombic efficiency were maintained close to ∼560 mAh g−1 and

∼100%, respectively for 50 cycles. However, the initial capacity of 680 mAh g−1deteriorated significantly to 80 mAh g−1.

An and colleagues114 synthesized a copper sulfide microflower composite by a straightforward dealloying method. The investigation showed that this type of anode could enhance the diffusion of the Na ions and also provided more space to accommodate volume changes.

For the electrochemical properties, high discharge capacity (325.6 Figure 9.XRD pattern (a), SEM images (b, c), and HRTEM image

(d) of the products prepared at 120°C for 12 h. The inset in panel d shows the SAED pattern. Reprinted with permission from ref 107.

Copyright 2013 Elsevier.


mAh g−1 at 0.1 A g−1), outstanding rate capability, and excellent cycling stability even after 5000 cycles with negligible capacity degradation were exhibited. Flower-like 10μm diameter microspheres

of copper sulfides (CuSs) were synthesized via a facile hydrothermal method by Shi and co-workers.115The unique hierarchical structure of CuS was composed of approximately 50 nm thickness nanosheets.

The constructed Na/CuS cell delivered an initial discharge capacity of 348.6 mAh g−1which decreased to 41.8 mAh g−1after 100 cycles. On the other hand, Li116 fabricated copper sulfide microspheres via a simple microwave method without any additional additives/

surfactants. It should be noted that the discharge capacity remained at 162 mAh g−1even after 200 cycles, showing capacity retention of 95.8%, demonstrating a high cycling stability.

Spray Pyrolysis is another widely used method for the preparation of spherical nanoparticles. Kalimuldina and Taniguchi34,117−119 demonstrated a simple synthesis method of Cu2S and spherical particles with a geometric mean diameter of 0.45 μm with the precursors being Cu(NO3)2and thiourea. Cu2S on a Cu foil current collector exhibited a stable capacity of 250 mAh g−1at 1 C and 220 mAh g−1at 30 C, respectively.34Another work of these authors on CuS showed promising results with 440 mAh g−1for 200 cycles at 1 C and an excellent rate capability with a high capacity of 340 mAh g−1at 10 C.87

Hybrid CuxS Three-Dimensional Structures. Along with the above hierarchical CuxS architectures, modified CuxS structures have been reported. Decorating the structure with carbon and GO sheets suppresses severe volume change and prevents the escape of polysulfides from the active material, thus enhancing the electro- chemical performance. Qin et al.120 prepared a CuS@Sisal fiber carbon (SFC) composite as anode material for lithium-ion batteries via a facile hydrothermal approach, which consisted of homogeneous dispersion of copper sulfide nanoparticles on a Sisal fiber carbon surface. In comparison with general SFC, CuS@SFC exhibited better electrochemical properties; for instance, a discharge capacity was 903 mAh g−1, while the reversible capacity after 30 cycles was 303 mAh g−1.

In another work, Jing and colleagues121 reported an easy and straightforward synthesis of a new anode material based on copper sulfide and S, N, and C sources. The anode was fabricated via a facile one-step calcination of copper pyrithione (C5H4NOS)2Cu. The obtained black powder consisted of 29 nm nanoparticles well-wrapped by a carbon layer with non-uniform thickness, which could clearly be observed via transmission electron microscopy. In terms of electro- Figure 10. Morphologies and compositions of the as-prepared 3D hierarchical nanostructured CuS microspheres. (a−c) FE-SEM images at different magnifications, (d, e) elemental mapping images of Cu and S, (f, g) TEM images, (h−j) HRTEM images and corresponding SAED pattern, and (k) representative EDS spectrum of the as-prepared CuS. Reprinted with permission from ref38. Copyright 2017 American Chemical Society.

Figure 11. Three-dimensional structure of as-synthesized CuS nanoplates. (a) XRD patterns of nanoplates, (b) SEM image and the corresponding schematics of three-dimensional CuS nanoplates (scale bar, 200 nm). HR-TEM images of (c) side plane (scale bar, 5 nm) and (d) basal plane with [001] zone axis (ZA) showing that each plane corresponds to {100} and {001}, respectively (scale bar, 5 nm).

Reprinted with permission from ref113. Copyright 2018 The Authors under Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), published by Springer Nature.


chemical performance, the Cu9S5/NSC electrode delivered a high reversible capacity of 412.0 mAh g−1 at 100 mAh g−1as well as an excellent cycling stability even after 200 cycles.

Zhang and co-workers96 fabricated novel copper sulfide micro- spheres wrapped with rGO for LIBs via a two-step solvothermal reaction. The electrochemical performance of this anode was promising with the high cycling stability and rate capability.

Moreover, the synergistic effect between CuxS and rGO nanosheets was responsible for accommodating volume changes as well as for preventing the dissolution of polysulfides. In comparison with the previous works, their copper sulfide composite maintained a reversible capacity of 320 mA g−1even after 100 cycles at a 1 C rate.

A new method of obtaining a CuxS cathode is considered as environmentally friendly and economically feasible.64It is based on controllable thermal sulfurization, which leads to the formation of CuxS different phases. Desired phases of copper sulfide can be prepared at higher temperatures via using MOF of Cu-BTC {[Cu3(C9H3O6)2(H2O)3]}n(HKUST-1) and sulfur placed in a quartz box. Moreover, the electrochemical properties of the composite prepared by this method were found to be excellent, where the specific capacity reached 220 mAh g−1even after 200 cycles.

Hollow Structures. In comparison with all the structures of copper sulfides mentioned above, 3D hollow architectures attracted much interest due to their superior energy storage abilities. It was suggested that this is due to the large surface area (outer and inner surfaces), low material density, and capability to cope with volume expansion during cycling in comparison to the nonhollow counter- parts. Additionally, the thin walls of hollow structures efficiently facilitate the diffusion of electrons and ions, thus enhancing the electrochemical kinetics. Design and fabrication of hollow architec- tures have been mainly conducted by the hard and soft template, and one-pot template-free synthesis methods.122However, these methods are costly and complex and require toxic reagents.123Despite these challenges, a significant amount of a variety of hollow structures were extensively studied in order to be used in rechargeable batteries.

Hard-template synthesis is the most effective strategy available for fabricating the shape-controlled hollow structures, which can maximally inherit the shapes and configurations of the templates.

Thus, hollow CuS nanocubes were fabricated using the most common hard-template Cu2O, as illustrated in Figure 12. Hollow nanocube structures with different side lengths, wall thickness and BET surface areas were obtained by varying the amount of Na2S solution and investigated as MIB cathodes.39 The 20 μm structure with a wall thickness of 18 n,m produced using a dilute solution of Na2S, exhibits high capacity (200 mAh g−1), remarkable rate capability, and superior long-term cyclability. Such hollow architectures greatly enhance the diffusion of Mg-ions and electrons transport; thus, the electrodes exhibit an improved conductivity.

The same procedure was adopted to synthesize CuS nanoboxes of about 500 nm edge length with approximately 10−20 nm wall thickness. CuS/Li cell demonstrates superior cycling stability for 1300 cycles at 2 C without noticeable capacity fading, and even at the high rate of 20 C, a discharge capacity of 371 mAh g−1and 86% capacity retention are obtained. This makes the CuS nanoboxes a promising electrode material for Li metal battery applications. Fast Li+diffusion was found to be responsible for the superior cycle stability and rate capability of the CuS nanobox electrode.124

Along with this, digenite Cu1.8S with a hollow octahedral structure was synthesized and its Na storage properties were investigated. The top-down approach by a combination of a post calcination and a phase transition from the Cu1.6S phase to Cu1.8S was adopted. TEM analysis shows the individual Cu1.8S particle with dimensions of about

∼800 nm in length and∼650 nm in width, and a wall thickness of

∼100 nm. The contrast difference between the core and the shell clearly confirms the hollow characteristic. When employed as an anode in NIBs, Cu1.8S hollow octahedra exhibited good reversibility and a capacity of 403 mAh g−1(93% of the theoretical one) in 1.0 M NaCF3SO3/dyglomera. The Cu1.8S hollow octahedra electrode shows a reversible charge capacity of∼250 mAh g−1 and high Coulombic efficiency of ∼100% over 1000 cycles at the high rate of 2 C in a Figure 12.(a) Schematic illustration for the fabrication process of hollow CuS nanocubes. (b) XRD pattern, (c) XPS spectra, (d, e) SEM images, and (f, g) TEM images of CuS-I. Reprinted with permission from ref39. Copyright 2019 Royal Society of Chemistry.


potential range of 0.5−2.2 V vs Na/Na+without irreversible phase transformation and structural collapse.125

The same hard-template method was adapted to the elegant multistep templating strategy to rationally synthesize hierarchical double-shelled nanoboxes with the CoS2nanosheet-constructed outer shell supported on the CuS inner shell. With these structural and compositional advantages, these hierarchical CuS@CoS2 nanoboxes manifest boosted electrochemical properties of high reversible capacity (625 mAh g−1), outstanding rate performance (304 mAh g−1at 5 A g−1), and excellent cycling stability (79% capacity retention after 500 cycles).126

As mentioned before, hard-template synthesis is very complex and requires costly reagents. Therefore Zhao et al.53 have presented a facile bubble template (soft template) synthesis method to obtain copper sulfide with sub-micrometer hollow and porous spheres (Figure 13). The work briefly compared the initial discharge output of

porous and hollow spherical sub-micrometer particles. During thefirst discharge process, the hollow sphere samples show an initial discharge capacity of 480 mAh g−1, while porous sphere samples exhibit a specific capacity of 276 mAh g−1.

An alternative strategy was adopted by Nagarathinam and co- workers54 to obtain 3D hollow spheres and flower-like spheres of copper sulfide via a hydrothermal reaction by utilizing a novel 2D polymer as a precursor. Because of the cavities that could provide extra electrochemically active sites and a large electrolyte−electrode interface for fast metal-ion diffusion, the hollow sphere particles showed outstanding performances.

Yu et al.37demonstrated the fabrication of a novel hierarchically structured CuS with a pine-needle-like morphology self-assembled (Figure 14) from hollow nanotubes with a simple reflux template free cosolvent assisted method. When employed as an anode for NIBs, the unique architecture was able to deliver a reversible capacity of 522 mA h g−1at 0.1 A g−1, superior rate capability with a discharge capacity of 317 mAh g−1 at 20 A g−1, and a long-term cycling stability with a capacity retention of 90% at 2 A g−1over 600 cycles. Such a unique interconnected hollow structure of CuS is favorable for penetration of the electrolyte and charge transportation, contributing to achieving excellent electrochemical performance.

Similarly, CuS dandelion-like clusters were synthesized by a simple colloidal approach.127 The authors were able to obtain such a structure by varying the solvent. When employed as an electrode in LIBs, electrochemical data indicated improved performance delivering 420 mAh g−1at 1.12 A g−1, and at 0.56 A g−1, the capacity was as high as 500 mAh g−1with good capacity retention.

Hybridized CuxS Hollow 3D Structures. In addition to the development of unique 3D architectures, considerable efforts have been invested in the rational design of nanohybrid CuxS with various carbonaceous materials, transition metal oxides, and metal sulfides, to improve the electrochemical performance by taking advantage of their high electrical conductivity and reliability.36 This strategy is the common approach to effectively increasing the electronic conductivity and accommodating the volume expansion caused by CuS during cycling. A template-engaged strategy was adopted to synthesize nitrogen-doped carbon-coated Cu9S5 bullet-like hollow particles starting from bullet-like ZnO particles. An initial capacity of 385 mAh g−1with a high initial Coulombic efficiency of about 94% was observed when the electrode was tested electrochemically. The unique structure and composition of bullet hollow structured particles pictured in Figure 15 manifest excellent sodium storage properties with a superior rate capability and ultrastable cycling performance over 500 cycles at 0.3 A g−1.128

Additionally, Wang et al.99presented a report on a self-templating thermolysis method to obtain uniform and monodisperse copper Figure 13.(A) X-ray diffraction patterns (a) CuS hollow spheres and

(b) porous spheres. (B) TEM image of the as-prepared CuS hollow spheres. The inset is the corresponding HRTEM image. (C) SEM image of the as-prepared CuS hollow spheres. The higher magnification image is also inserted. (D) TEM image of the obtained CuS porous spheres. Reprinted with permission from ref 53.

Copyright 2012 Elsevier.

Figure 14.Morphology and microstructure of PNL-CuS: (a, b) FESEM images. (c) TEM image. (d) HRTEM image. (e) SAED pattern of PNL- CuS and (f) elemental mapping of Cu and S. The inset of panel a is the image of pine needles. Reprinted with permission from ref37. Copyright 2019 Royal Society of Chemistry.

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