Li-ion storage in morphology tailored porous hollow Cu₂O nanospheres fabricated by Ostwald ripening

Abstract

Uniform Cu₂O nanospheres with tailored hollow structure are successfully synthesized by employing the Ostwald ripening approach, and used as anodes in Li-ion battery. The synthesis involved a facile room-temperature chemical reaction of cupric nitrate with hydrazine. Hydrazine served as an alkali and reducing agent, converting Cu²⁺ to Cu₂O nanoparticles, which self-assembled due to their large interfacial energy and formed 100–400 nm nanospheres. The Cu₂O nanospheres were characterized by SEM, TEM, XRD, UV-vis spectroscopy and BET techniques. These Cu₂O nanospheres were comprised of a mesoporous shell (20–50 nm) and a hollow interior part (50–100 nm). Galvanostatic charge–discharge measurements at different current densities, slow scan cyclic voltammetry (CV) and impedance measurements were used to analyze the electrochemical performance of the Cu₂O nanospheres. The hollow Cu₂O nanospheres exhibited a capacity of ∼650 mA h g⁻¹ at 100 mA g⁻¹ current density, showing greater than 80% capacity retention after 100 cycles. The enhanced electrode performance is attributed to the mesoporous hollow nanostructure that ensured an increased number of electrochemical sites, shorter Li ion diffusion lengths facilitating fast electrochemical kinetics, and sufficient void spaces to buffer the volume expansion.

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Introduction

Lithium-ion batteries (LIBs) due to their high energy density, long operating life, no memory effect features and environment friendliness are being predominantly used in the development of portable electronics and wireless technologies.^1,2 However, the performance of LIBs still falls short in terms of their capability to power electric/hybrid vehicles and grid storage applications. Therefore, significant research efforts have been made towards the development of new electrode materials for next generation LIBs that could deliver even higher energy/power densities and longer cycle lives.^3,4 Recently, transition metal oxides, such as Co₃O₄, MnO₂, Fe₂O₃, SnO₂, Cu₂O, etc. have been considered as alternative anode materials for LIBs due to their high theoretical capacities based on “conversion” reactions.⁵ It has been found that the performance of metal oxides for Li storage depends not only on the composition, but also on their structure, shape, size and crystallinity. Thus, the control of the structure and morphology of metal oxides in enhancing their electrochemical performance is an important aspect that is being investigated by researchers. In this regard, various nanostructured metal oxides such as core–shell, yolk–shell, cubes, hollow spheres, nanowires/nanotubes, etc. have been developed and studied.^6–9

Among transition metal oxides, Cu₂O has been exploited in various research fields, such as catalysis, sensors, solar cells, supercapacitors and LIBs.^10–15 As an anode in Li-ion battery, Cu₂O decomposes into Cu and Li₂O on lithiation, whereas upon delithiation, the oxide is restored back. However, during Li insertion and extraction, the electrode undergoes volume fluctuations which can result in electrode pulverization and hence rapid capacity fading.^16,17 In order to improve the electrochemical performance of Cu₂O, researchers are working on combining Cu₂O with low expansion matrix phases such as carbon/or directly growing nanostructured copper oxides on conductive substrates with shape controlled morphologies.^18–24 As regards, the Cu₂O nanostructures investigated earlier, Paolella et al. fabricated octahedral shaped Cu₂O nanocrystals based on thermal decomposition of organometallic precursors (Cu-acetylacetonate) in surfactants and reported a capacity of 100 mA h g⁻¹ after 40th cycles.²⁵ Shin et al. showed the synthesis of Cu₂O nanorod arrays through an electrodeposition process, and reported an increase in specific capacities from 323 to 1206 mA h g⁻¹ during 500 cycles, attributing it to the phase transformation from Cu₂O to CuO.²⁶ Park et al. showed the synthesis of Cu₂O nanocubes by a one-pot polyol and a sequential dissolution–precipitation process.²⁷

Although Cu₂O crystals with the multiple morphologies have been synthesized and used as anodes as discussed above, there is need for further work on the large scale synthesis and size control of the nanostructures. It is desirable to develop a simple method, where morphology and yield of nanostructure could be controlled in mild operating conditions. Recently, hierarchical and hollow nanostructures have gained tremendous attention in various applications due their unique chemico-physical properties, including high surface area, reasonably good conductivity, low density, void space, permeability and adjustable framework.^28–30

In this work, we have tailored the structure of Cu₂O nanospheres by employing the Ostwald ripening approach creating a hollow core structure. Although different hollow structures have been synthesized earlier using the soft/hard template techniques, Ostwald ripening presents an easy approach for fabricating various symmetric/asymmetric hollow nanostructures.^31–34 Here, we have used hydrazine as a reducing agent for converting Cu²⁺ to Cu₂O nanoparticles, which self-assemble due to their large interfacial energy and form 100–400 nm nanospheres with a hollow core. Compared to solid nanospheres (Cu₂O–S), the hollow nanospheres (Cu₂O–H) exhibited a superior electrochemical performance, exhibiting reversible capacity of ∼500 mA h g⁻¹ after 100 cycles with more than 80% capacity retention. The nanometer-sized primary particles on the shell surface not only increase the feasibility of conversion reaction but also enable the reversible formation/decomposition of the organic layer formed at the surface of the active material, contributing to a high specific capacity.

Experimental

Material

Copper nitrate (Cu(NO₃)₂·3H₂O), polyvinylpyrrolidone (PVP), hydrazine hydrate (N₂H₄·H₂O) were purchased from Loba Chemie and used as such without purification.

Methods

The synthesis of Cu₂O nanospheres was carried out by similar way as reported elsewhere with substantial modification.³⁵ PVP (4 g) was dissolved in the 200 mL of 0.01 M Cu(NO₃)₂·3H₂O aqueous solution under constant magnetic stirring. Finally, 100 μL of hydrazine solution (80 wt%, Sigma-Aldrich) was added into the reaction mixture. The colour of solution was blue initially which immediately turned to orange after addition of hydrazine. The solution was centrifuged and the precipitate was washed with water and anhydrous ethanol several times to remove the excess PVP and other impurities, such as unreacted copper and nitrates. The precipitate was re-dispersed in ethanol and stored for further characterization. Hollow Cu₂O nanospheres were also fabricated by similar method as described above. Here, colloidal solution was stirred for 1 h at room temperature for Ostwald ripening, followed by addition of another 100 μL of N₂H₄ solution to initiate a shell growth process. The solution was centrifuged and precipitate was washed with water and anhydrous ethanol. Finally, precipitate was re-dispersed in ethanol and stored for further characterization.

Material characterization

The structural and morphological characterization of Cu₂O nanospheres was performed by field emission scanning electron microscopy (FESEM, ZEISS Supra 40VP, Germany) and Transmission Electron Microscopy (TEM, FEI Titan G2 60-300 TEM). High-resolution TEM (HRTEM) and selected area electron diffraction pattern (SAED) was analyzed to determine the amorphous and crystalline nature. The crystal structure of Cu₂O nanospheres was determined by X-ray diffraction (XRD) (PANalytical, Germany) of Cu Kα radiation (λ = 1.5406 Å) between 20° to 80° at a scanning speed of 2° per minute. Surface area analysis was carried out using the Brunauer–Emmett–Teller (BET) nitrogen adsorption method (Quanta Chrome). UV-visible analysis was carried out on Barian, Cary 50 Bio UV-visible spectrophotometer.

Electrochemical characterization

The electrochemical performance of the Cu₂O nanospheres was studied by using them as anodes in a Li-ion half cell. The electrodes were prepared by coating a homogenous slurry composed of the active material (Cu₂O), acetylene black and polyvinylidene fluoride (PVDF) in the weight ratio 70 : 15 : 15 on to a copper foil. The coating was vacuum dried in oven at 120 °C for 6 h, after which the electrodes were cut into a circular shape with diameter appropriate for 2032 coin cells. The Li-ion half-cell was assembled inside an argon filled glove box. A metallic lithium foil ∼750 μm was used as the counter electrode whereas a microporous polypropylene membrane (Celgard 2320) was employed as separator and a 1 M LiPF₆ solution in a solvent mixture of ethylene carbonate and diethyl carbonate (EC/DEC, 1 : 1 (v/v), Merck) as the electrolyte. Galvanostatic charge/discharge measurements were conducted in the potential range between 0.05 to 3.0 V at current densities: 100, 200, 300, 500 and 1000 mA g⁻¹ using a battery analyzer (MTI Corporation, USA). Cyclic voltammetry was conducted at a slow scan rate of 0.1 mV s⁻¹ whereas impedance spectra was recorded in the frequency range 100 kHz to 10 mHz with a perturbation a. c. amplitude of 10 mV using a PGSTAT 302N electrochemical workstation (Metrohm Autolab, Netherlands).

Results and discussion

Structural and morphological analysis of as prepared Cu₂O nanostructures was carried out by SEM and TEM as displayed in Fig. 1. SEM image reveals the formation of 100–400 nm spheres (Fig. 1a). These nanospheres were well dispersed with narrow size distribution. The surface of nanospheres is rough, which suggests that they are composed of primary nanoparticles. The detailed structural analysis was carried out by TEM, HRTEM and SAED as displayed in Fig. 1b–d. TEM images also confirm the formation of submicrometer spheres, which are composed of 10 nm primary nanoparticles (Fig. 1b and c). HRTEM image shows clear lattice fringes, where the interplaner distance is 0.246 nm, which suggest that individual particles grew along [111] direction (Fig. 1d). Furthermore, presence of ring pattern in SAED pattern confirms the polycrystalline nature of nanospheres, which also suggest that nanospheres were formed due to self-assembly of single crystalline primary particles (inset Fig. 1d).

Fig. 1 (a) SEM, (b and c) TEM and (d) HRTEM with SAED pattern (inset) of solid Cu₂O nanospheres.

The Ostwald ripening of these Cu₂O nanospheres resulted in the formation of hollow nanospheres as evident in Fig. 2. SEM image confirms that there is no change in size and shape after Ostwald ripening and 100–400 nm particles are formed (Fig. 2a). However, these particles seem to be hollow in nature as displayed in SEM image of individual particles (inset of Fig. 2a). The formation of hollow nanospheres is confirmed by TEM images as shown in Fig. 2b and c. These nanospheres show the presence of 100–50 nm hollow space surrounded with 20–50 nm shell. The shell is composed of 15 nm nanoparticles, which is slightly larger than the primary particles of solid nanospheres. HRTEM image clearly confirms that there is no change in growth direction after Ostwald ripening and particles grew along [111] direction. SAED pattern confirms that these hollow nanospheres are also polycrystalline in nature, composed of single crystalline primary particles.

Fig. 2 (a) SEM, (b and c) TEM and (d) HRTEM with SAED pattern (inset) of hollow Cu₂O nanospheres.

The formation mechanism suggests that hydrazine is the reducing agent, which converts copper nitrate solution to Cu₂O precipitates as described below:

2Cu²⁺_(aq) + N₂H_4(aq) + H₂O → Cu₂O_(s) + 2NH_3(g) (1)

Initially, Cu₂O gets nucleated and grows to form nanosized particles. The polar imide group of PVP contain N and O atoms, which probably have a strong affinity for copper ions. Thus, PVP accelerates the nucleation steps. These nanocrystals coalescences to form larger particles by the attachment of their high-index planes due to their high interfacial energy. Furthermore, the adsorption of PVP on the surface of nanoparticles may have reduced the particle growth and also facilitate the nanospheres formation to minimize their surface energies by coalescence. This coalescence of nanoparticles to form larger structures is thermodynamically feasible. This is mainly because, the reduction in the number of particles decreases the excess energy associated with their interfaces. Ostwald ripening resulted into the formation of central cavity due to the growth of larger outer crystallites at the expense of smaller inner ones due to their higher solubility and undergo mass transport through the dissolving and recrystallization processes.³⁵ Therefore, interior cavities were formed within the solid spheres during Ostwald ripening, which resulted in hollow Cu₂O nanospheres formation (Fig. 3).

Fig. 3 Schematic illustration of hollow Cu₂O nanospheres formation.

Structural and phase analysis of as prepared solid and hollow Cu₂O nanospheres was carried out by XRD and shown in Fig. 4a. Diffraction pattern confirms the formation of single phase Cu₂O crystal structure (JCPDS file no. 05-0667) in both specimens. The peaks identified at the 2θ angle of 29.7, 36.5°, 42.5°, 61.7° and 73.9° correspond to the (110) (111), (200), (220) and (311) planes of cubic Cu₂O, respectively. The diffraction pattern also reveals the absence of secondary phase or other impurity related peaks, which confirms the formation of high purity specimen. There is no difference in diffraction pattern of two specimens except a higher peak intensity for hollow nanospheres as compared to solid Cu₂O nanospheres, which is related to their larger crystallite size as evident from TEM.

Fig. 4 (a) XRD pattern and (b) UV-visible spectrum of solid and hollow Cu₂O nanospheres.

Optical property of solid and hollow Cu₂O nanospheres was analyzed by UV-visible spectroscopy, as shown in Fig. 4b. As Cu₂O is a p-type semiconductor with bulk band gap value of ∼2.17 eV, the absorption band must fall in the range of 500–600 nm depending on particle sizes.³⁵ However, a broad absorption band in the range of 450–600 nm is observed for both specimens and no characteristic peak related to the band gap is observed. There is no change in absorption spectra of both sample except the intensity of absorption band in hollow nanosphere is slightly higher as compared to solid Cu₂O nanospheres. The high intensity of hollow nanospheres over solid nanospheres is possibly related to increase in crystallite size. It implies that the band gap related characteristic peak of Cu₂O nanosphere has disappeared due to the presence of broad absorption band in same region. The presence of broad absorption band in visible region is possibly related to the scattering of light from colloidal dispersion of Cu₂O nanospheres, as the size of the nanospheres was around 100–400 nm.³⁶ These nanospheres possibly causes light extinction through diffuse reflection/transmission.

The specific surface area of the synthesised solid and hollow Cu₂O nanospheres was analysed by N₂ adsorption/desorption measurements and shown in Fig. 5. The adsorption/desorption curve can be classified as a type IV isotherm. The BET surface area of solid and hollow Cu₂O nanospheres was calculated as 10.67 and 37.78 m² g⁻¹, respectively. The hysteresis loop in the curve confirms that mesopores are present in the sample. The Barrett–Joyner–Halenda (BJH) desorption method was used to calculate the pore size distribution of solid and hollow Cu₂O nanospheres as shown in the inset of Fig. 5a and b, respectively. The average pore size was 3.8 nm and total pore volume was 0.1016 ccg⁻¹ for hollow Cu₂O nanospheres. These results indicate that hollow Cu₂O nanospheres could be very effective Li-ion battery due to the presence of high surface area, mesopores and pore size volume.

Fig. 5 Nitrogen adsorption and desorption isotherms and corresponding pore size distribution curves (insets) of (a) solid (b) hollow Cu₂O nanospheres.

The hollow core geometry is quite useful for Li-ion battery application as it provides copious electrochemical reaction sites as well as the requisite structural stability to overcome the mechanical deformations resulting from volume expansions, thus leading to excellent capacity retention. Further, the interstitial spaces between the primary nanoparticles on the shell surface generate high mesoporosity promoting facile mass transfer and electrolyte penetration.³⁷ Therefore, further analysis was carried out on hollow nanospheres. In order to understand the role of morphology on Li ion storage, the electrochemical properties of solid and hollow Cu₂O nanospheres have been compared. Fig. 6 shows the CV of as-fabricated half-cells with Cu₂O–H and Cu₂O–S electrodes as anodes, at a slow scan rate of 0.1 mV s⁻¹ conducted between 0.01 to 3 V. During the first discharge cycle, two cathodic peaks appear at ∼0.7 V and 1.1 V, corresponding to the formation of SEI layer at the electrode surface and conversion of Cu₂O into Cu–Li₂O composites (eqn (2)).^38,39

Cu₂O + 2Li⁺ + 2e⁻ → Cu + Li₂O (2)

Fig. 6 CV of (a) Cu₂O–H, and (b) Cu₂O–S electrodes performed at a slow scan rate of 0.1 mV s⁻¹.

In the oxidation scan, two broad shoulder peaks can be observed at about 1.5 V and 2.6 V, which are attributed to the decomposition of SEI layer, and re-oxidation of metallic Cu into Cu₂O. During charging, lithium carbonate (Li₂CO₃) and the alkyl carbonate lithium salts in SEI are known to decompose reversibly.³⁸

It can be observed that the oxidation peaks are less pronounced in the CV of Cu₂O–S electrode (Fig. 6b). This is mainly due to the solid structure of Cu₂O–S nanospheres which constrain the charge transport kinetics within the material. The hollow core–shell structure of Cu₂O–H nanospheres, on the other hand, enables fast movement of charge carriers, resulting in large peak currents as evident in Fig. 6a.

Fig. 7 shows the galvanostatic discharge–charge curves of Cu₂O–H and Cu₂O–S electrodes, for 1st, 2nd and 100th cycles at current density of 100 mA g⁻¹. As can be observed, the process of Li-ion consumption occurs in steps which manifests as voltage plateaus. The voltage plateau observed between 1.3 and 0.8 V is attributed to the decomposition of Cu₂O into a Cu–Li₂O composite as evident in CV curve also. The voltage slope below 0.8 V corresponds to the decomposition of electrolyte solution and formation of an organic SEI, composed of LiF, Li₂CO₃ and ROCO₂Li salts.^40–42 During the charging process, the main reaction in the voltage range between 0.8 to 1.9 V is the decomposition of SEI, whereas the voltage plateau in the range of 1.9–3.0 V is due to the reaction between Cu and Li₂O to form Cu₂O and Li ions (eqn (3)).³⁹

2Cu + Li₂O → xLi⁺ + xe⁻ + Cu₂O + unreacted Cu (3)

Fig. 7 Galvanostatic charge–discharge profiles of (a) Cu₂O–H, and (b) Cu₂O–S electrodes for the 1st, 2nd and 100th cycles at 100 mA g⁻¹ current density.

During the first discharge cycle, Cu₂O decomposes, resulting in formation of an amorphous Li₂O matrix containing metallic Cu clusters. The interconnected network between Li₂O and Cu results in an improved reversibility of Li₂O back to Li on subsequent charging step. This leads to an increase in the potential of Li ion reactivity towards Cu₂O after the first cycle. Thus, compared to the first cycle, an increase in the discharge voltage plateau is observed in the 2nd cycle. This higher shift in the voltage plateau after 1st cycle is evident in the CV curves also, where the cathodic peaks appearing at 0.7 and 1.2 in the first cycle have shifted to ∼0.8 and 1.5 V in the subsequent discharge cycles.

As can be seen in the galvanostatic charge/discharge profiles, the initial discharge/charge capacities are 1050/640 and 900/560 mA h g⁻¹ for the Cu₂O–H and Cu₂O–S electrodes, respectively. The irreversible capacity loss (ICL) in the first cycle is typically attributed to the formation of a SEI layer.

Fig. 8a compares the cycling performance of Cu₂O–H and Cu₂O–S electrodes for the first 100 cycles. As shown in the figure, Cu₂O–H electrode retains a capacity of ∼500 mA h g⁻¹ at the end of 100 cycles showing less than 20% capacity fade. The Cu₂O–S electrode, on the other hand exhibits a poor cycling stability undergoing rapid capacity fade of almost 40% in 100 cycles. Improved Li storage and electrochemical stability exhibited by Cu₂O–H compared to the Cu₂O–S electrode can be attributed to its morphology, which facilitates faster charge transport kinetics.

Fig. 8 (a) Cycling performance of Cu₂O–H and Cu₂O–S electrodes at 100 mA g⁻¹ current density, and (b) rate performance of the Cu₂O–H electrode at current densities: 100, 200, 300, 500 and 1000 mA g⁻¹.

The Cu₂O–H nanospheres have a hollow core structure with a mesoporous shell, that enables better electrolyte access into the pores and remote pockets on the electrode surface, enabling higher reversible capacities. Whereas, in the case of Cu₂O–S spheres, the solid interior structure restricts the mobility of Li ions. The electrode/electrolyte interfacial contact area is low, which results in lower utilization of the active material. The charge reaction (3), therefore, proceeds to a lesser extent, resulting in lower reversible capacity for Cu₂O–S electrode. Further, the presence of nanometer sized sub-units in the shell suppresses the side reactions with the electrolyte, and ensures good electrical conductivity in material. It makes the conversion reaction more feasible as well as facilitates the reversible formation/decomposition of the organic gel layer at the electrode surface, both of which contribute towards enhancing the specific capacity. Further, the hollow core structure provides sufficient void space which effectively buffers the mechanical stresses generated during charge/discharge cycles, preventing electrode pulverization and capacity fading on continued cycling. The Cu₂O–H nanospheres therefore exhibit a better cycling stability compared to Cu₂O–S (Fig. 8a).

It can be noted that the capacities exhibited by these electrodes are higher than the theoretical capacity of Cu₂O, which can be attributed to the reversible formation/decomposition of a gel like polymeric film at the grain boundaries between the nanoparticles.⁴³ Hu et al. studied a prototype conversion material Ru₂O to understand this origin of additional capacity and showed with experiments and theoretical calculations that the extra capacity in the system is mainly contributed due to the generation of LiOH which subsequently undergoes reversible reaction with Li to form Li₂O and LiH.⁴⁴ The reduction of Cu₂O on lithiation involves the formation of Cu nanoparticles dispersed in a Li₂O matrix, followed by the growth of a polymer gel like film, which partially decomposes during the subsequent charge cycle, while Cu is converted back to Cu₂O. The reversible reactivity of Cu₂O towards Li is mainly attributed to the electrochemically driven formation of highly reactive Cu nanoparticles during the first discharge process, enabling the formation and decomposition of Li₂O in subsequent cycles.³⁹

Fig. 8b displays the rate capability of the Cu₂O–H electrode cycled at different current densities. The Cu₂O–H electrode showed specific capacities of 640, 413, 291, 245 and 165 at 100, 200, 300 and 500 and 1000 mA g⁻¹ current densities, respectively. Again, when the current is lowered back to 100 mA g⁻¹, the composite is able to resume a capacity of 515 mA h g⁻¹ demonstrating excellent rate capability. The improved rate capability can be attributed to its porous shell structure that led to reduced Li ion diffusion lengths which in turn promoted faster Li⁺ insertion/extraction, and thus a higher specific capacity and rate capability.

At high current rates, the factor that limits the reversible capacity achieved is the rate of transport of Li ions within the electrode. A hollow-porous structure results in reduced Li ion diffusion lengths which in turn promote faster Li⁺ insertion/extraction, and thus a higher specific capacity and improved rate capability.

In order to further understand the lithiation/delithiation processes in the Cu₂O–H electrode, electrochemical impedance spectroscopy (EIS) was performed in the frequency range 100 kHz to 10 mHz with an a. c. voltage amplitude of 5 mV. Before taking each measurement, the cell was first discharged to 0.05 V at a constant current of 100 mA g⁻¹ and was allowed a relaxation time of 10 minute. Fig. 9 shows the impedance curves of the Cu₂O–H electrode in the as-fabricated state and at the end of 20th, 50th, and 100th discharge cycles. The impedance curve represents the different interfacial processes that occur at the electrode/electrolyte interface, in the form of semicircles in the high and middle frequency zones and a sloping beeline in the low frequency domain.⁴⁵ In the high frequency region, the intercept on the real axis (Z′) corresponds to the electrolyte resistance (R_s).

Fig. 9 Impedance spectra of Cu₂O–H electrode measured in the as fabricated state, and at the end of 5th, 20th, 50th and 100th discharge cycles, in the frequency range between 100 kHz to 10 mHz, and the equivalent electrode circuit (inset).

The semicircle at high frequency represents the resistance to Li ion migration through the SEI layer (R_SEI), whereas the mid-frequency semicircle is assigned to alloying process that Cu undergoes with lithium and the charge transfer impedance encountered at the electrode–electrolyte interface (R_CT). The straight line at a slope of ∼45° observed in the low frequency region (resistive component of Warburg impedance, Z_w) corresponds to the Li⁺ ions diffusion into the electrode bulk. The onset of electrochemical process (conversion and alloying) associated with Li-ion insertion can be observed by the initial enlargement of the semicircles on cycling. The increase in the size of semicircles could be the result of fragmentation of Cu₂O hollow spheres in the successive conversion reactions that changes the electronically conducting pathways within the electrode. Upon cycling, there is a gradual thickening of SEI which is reflected by the increase in charge transfer resistance in the form of enlarged semicircles.⁴⁶ However, as can be observed, there is not much increase in size of semicircles for the 20th and 100th cycle, implying the formation of a stable SEI. Based on the impedance spectra obtained, an equivalent circuit model is constructed, which contains two resistors and constant phase elements (CPE1 and CPE2) in parallel with the resistive Warburg element (as shown in the inset of Fig. 9).

Conclusions

Hollow Cu₂O nanospheres with porous shell were synthesized using a facile template-free Ostwald ripening approach. When tested as anode in Li-ion battery, Cu₂O hollow spheres displayed excellent rate capability, good cycling stability and a high specific capacity (almost twice compared to the commercial graphite based anodes). The superior electrochemical performance can be attributed to the hollow nanostructure which combined with a mesoporous shell guaranteed more lithium storage sites, shortened Li-ion diffusion path lengths and sufficient void spaces to buffer the local volume changes that occur during charge/discharge process. The facile synthesis of the Cu₂O nanospheres and its enhanced electrochemical performance, establishes Cu₂O to be a potential anode material for building next generation LIBs, with higher capacity, better capacity retention and rate capability.

Acknowledgements

This work was supported by Department of Science and Technology (DST), New Delhi, India vide DST-INSPIRE Faculty Scheme (DST/INSPIRE/04/2014/001318; IFA14/MS-20). We acknowledge the use of characterization facilities at Thematic Unit of Excellence on Soft Nanofabrication with Application in Energy, Environment and Bioplatform and Advanced Imaging Centre at IIT Kanpur.

Notes and references

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