The improved cycling stability of nanostructured NiCo2O4 anodes for lithium and sodium ion batteries

Developing the high-capacity anode materials such as conversion-type metal oxides which possess both Li and Na storage activity is very practical for the high-energy Li-ion battery (LIB) and Na-ion battery (LIB). Herein, we use NiCo2O4 anodes as a model to investigate the morphology evolution which accounts for the poor cycling performance and understand the effect of structure optimization on the electrochemical performance. Three NiCo2O4 samples with different morphologies of microspheres, nanospheres and nanosheets are synthesized. Firstly, the serious structural degradation of NiCo2O4 microspheres is observed whether it works as a LIB or SIB anode. In addition, a significant difference between the lithiation and sodiation capacity of NiCo2O4 materials reveals Na+ ions only partially intercalated in NiCo2O4 and the conversion reaction limited by the strain. Next, NiCo2O4 nanosheets on Ni foam as a binder-free anode for the LIB are investigated which suggest the positive effect of 3D nanostructures on the morphology stability. As a result, NiCo2O4 nanosheets deliver a high lithiation capacity of 1092 mAh g−1 after 100 cycles at 0.5 A g−1 and an excellent rate capacity of 643 mAh g−1 at 4 A g−1. Finally, NiCo2O4 nanospheres are evaluted as a SIB anode which indicate the smaller particle size of active materials is beneficial to the release of stress and structure stability during discharge–charge processes. Relatively speaking, the nano sheet structure is with the best electrochemical performance based on the capacity retention. A rational design of the electrode’ architecture is very important for the conversion-type 3d transition metal oxide anodes for the advanced LIB and SIB.


Introduction
The Li-ion battery (LIB) are the most successful rechargeable battery technology in commercial use.The Na-ion battery (SIB), as the attractive next generation, are promising in some cost-sensitive fields to replace the LIB [1][2][3][4].However, the existing low-energy LIB/SIB products are not suitable for the rapid economic growth.The urgent need for higher energy is driving the development of high-capacity active materials according to that the energy is proportional to the capacity.The research on cathode materials for the LIB/SIB has yielded good results [5][6][7][8].In comparison, anode materials have got much less attention.Thus, it is very significant to improve the electrochemical performance of anode materials.
Though Na shares similar properties with Li, some anode materials such as graphite with good electrochemical activity for the LIB cannot tolerate Na intercalation, due to larger radius of Na + ions and other factors.Hence, the development of high-capacity anode materials which possess both Li and Na storage activity is very practical, especially at cost reduction [9,10].The 3d metal oxides such as NiCo 2 O 4 are one of the most promising candidates which achieve high theoretical capacities by the multielectron-transferbased conversion reaction [11][12][13][14][15]. Zhang et al. prepared mesoporous Co 3 O 4 materials which possess a theoretical lithiation capacity of 890 mAh g −1 according to 8 Li + reaction (NiCo 2 O 4 + 8Li + + 8e − → Ni + 2Co + 4Li 2 O).It is common for the conversion-type anode materials that the initial capacity is higher than the theoretical value, due to the electrolyte-related side reactions, the formation of SEI film and the interfacial storage of Li + .This phenomenon is more obvious for the nano-structure materials with the greater specific surface area.Interestingly, the mesoporous Co 3 O 4 anode delivered an initial lithiation capacity of 1448 mAh g −1 at a current density of 0.1 A g −1 , much more than the theoretical value.The additional capacity was thought to be related to the electrolyte-derived solid electrolyte interphase (SEI) film [16].Yu et al. designed a NiO/Co 3 O 4 / NiCo 2 O 4 heterostructure with a lithiation capacity of 1081 mAh g −1 at 0.1 A g −1 [17].Qiao et al. constructed the RGO/ NiCo 2 O 4 @C material with an initial lithiation capacity of 2048 mAh g −1 at 0.3 A g −1 [18].
However, the capacity retention of these conversiontype 3d transition metal oxide anodes is not satisfactory, associated with the collapse of material structures during the continuous lithiation-delithiation processes.There are no sites for the conversion-type anodes to storage lithium.When being lithiated, the active materials are wrapped by generated products and the volume is inevitably changed.It impedes Li + embedding.The utilization of such materials is seriously reduced by the pulverization of active materials which leads to the loss of electrical contact [19,20].The nanostructure is a promising strategy to address the above issue through releasing the stress [21][22][23][24].A nano-octahedron Ni-Co-Mn oxide anode was synthesized by Ling et al. and retained 78.9% of its initial lithiation capacity after 500 cycles at 1 A g −1 , much better than its counterpart (24.1%) [25].Yet, the systematic research on the loss of the original morphology of 3d transition metal oxides as LIB/SIB anodes and the effect of nanostructuring on their structural stability during electrochemical processes is lacking, which is meaningful to the application.
For the conversion-type anodes, the structural stability is usually evaluated by the morphology change after the electrochemical process and used to understand the cycling stability.Herein, we use NiCo 2 O 4 as a model to investigate the morphology evolution of the conversion-type LIB/SIB anodes and the effect of structure optimization on the electrochemical performance.Three NiCo 2 O 4 samples with different morphologies of microspheres, nanospheres and nanosheets are synthesized.Firstly, the Li and Na storage properties of NiCo 2 O 4 microspheres are understood.The serious structural degradation is observed whether the microsphere works as a LIB or SIB anode which accounts for the poor cycling performance.Next, NiCo 2 O 4 nanosheets on Ni foam as a binder-free anode for the LIB are investigated which suggest the positive effect of 3D nanostructures on the morphology stability of NiCo 2 O 4 materials.As a result, NiCo 2 O 4 nanosheets deliver a high lithiation capacity of 1092 mAh g −1 after 100 cycles at 0.5 A g −1 and a rate capacity of 643 mAh g −1 at 4 A g −1 .Finally, NiCo 2 O 4 nanospheres are evaluated as a SIB anode which indicate the smaller particle size of active materials is beneficial to the release of stress and structure stability during electrochemical processes.A rational design of the electrode' architecture is very important for the conversion-type 3d transition metal oxide anodes for the LIB and SIB.

Experimental section
Chemicals (analytical grade) were purchased from Sinopharm chemical reagent Co., Ltd.

The preparation of NiCo 2 O 4 microspheres
Firstly, 4 mmol of nickel sulfate and 8 mmol of cobalt Sulphate were dissolved in 50 ml of deionized water to form solution A. 12 mmol of sodium carbonate and 5 mmol of ammonium bicarbonate were dissolved in 50 ml of distilled water to form solution B. Then, solution A was quickly poured into B.After stirring for 120 min, the carbonate sediment was obtained.Finally, NiCo 2 O 4 microspheres were synthesized after drying the carbonate precursor at 50 °C and followed by sintering at 450 °C for 120 min in a muffle furnace.

The preparation of NiCo 2 O 4 nanospheres
Firstly, 0.1940 g of Ni(NO 3 ) 2 •6H 2 O and 0.3877 g of Co(NO 3 ) 2 •6H 2 O were dissolved in 80 ml of isopropanol and then transferred to an autoclave.After being heated at 160 °C for 150 min, the nanosphere precursor was obtained.At last, NiCo 2 O 4 nanospheres were prepared by sintering the precursor at 350 °C for 180 min in air.

The preparation of NiCo 2 O 4 nanosheets on Ni foam
1.9386 g of urea and 0.4933 g of ammonium fluoride were dissolved in 80 ml of distilled water and then transferred to an autoclave with Ni foam.After being heated at 120 °C for 150 min, the nanosheet precursor was obtained.At last, NiCo 2 O 4 nanosheets were prepared by sintering the precursor at 350 °C for 180 min in air.

The preparation of NiCo 2 O 4 anodes and coin cells
NiCo 2 O 4 nanosheets on Ni foam were used as the binder-free electrodes.NiCo 2 O 4 nanospheres (or microspheres) were mixed with Super P and PVDF (7:2:1, mass ratio) in NMP to form a slurry and then coating it on Cu foil.The loading mass of nanosheets, nanospheres and microspheres are around 1.5 mg cm −2 , 2 mg cm −2 and 2 mg cm −2 , respectively.The load of 1.5 mg/cm 2 is calculated by measuring the mass of a circular Ni foam with the diameter of 12 mm before and after loading the NiCo 2 O 4 nanosheets.The fabricated electrodes were assembled with a separator (Celgard 2500) and Li (or Na) metal to obtain coin cells (CR-2032).The electrolyte for Li-ion batteries is including of LiPF 6 , EC and DMC.The electrolyte for Na-ion batteries is including of NaClO 4 , EC and DEC.
TGA with air atmosphere operation for the precursor is described in Fig. 2a.The mass loss of about 32% is calculated in thermal decomposition processes.Based on the DTG diagram, NiCO 3 in Ni-Co carbonate precursors is decomposed at about 245 °C with the mass loss of 10%.CoCO 3 in Ni-Co carbonate precursors is decomposed at about 350 °C with the mass loss of 22%.The value agrees with the feed ratio of Ni and Co elements.For improving the crystallinity of NiCo 2 O 4 materials, the higher temperature of 450 °C is used to synthesize samples.N 2 adsorption isotherms and pore size (Fig. 2b and c) suggest the mesoporous characteristics of NiCo 2 O 4 microspheres.The specific  220) and (311) lattice planes are identified by the interlayer distance of 0.46, 0.29 and 0.24 nm, respectively.In addition, the diffraction rings in SAED patterns of NiCo 2 O 4 microspheres reveal the polycrystalline structure (Fig. 2h). Figure 2i depicts the crystal structure of spinel NiCo 2 O 4 .Ni occupies octahedral sites and Co occupies tetrahedral and octahedral sites.Obviously, there is no space for Li + or Na + ions embedding.Thus, the volume change of NiCo 2 O 4 materials during charge-discharge processes is predicted.
The NiCo 2 O 4 microsphere electrodes are evaluated as the anode for Li-ion batteries.Figure 3a exhibits CV curves of the first and second cycles, in which the obvious difference of reduction peaks at about 0.85 V is observed, associated with the formation and growth of SEI layer in the initial Figure 3c shows the rate capacities of 1018, 1035, 996, 918, 829 and 654 mAh g −1 based on the galvanostatic discharge/charge test at 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0 A g −1 , respectively.The discharge capacities decrease as the currents increased, attributed to the transport kinetics of charge carriers.A rise in the discharge capacity when the current recovers to 0.1 A g −1 is due to the activation of NiCo 2 O 4 microsphere electrodes.Further, the cycling performance based on the galvanostatic discharge/charge test is described in Fig. 3d.The capacities increase slightly in the initial 50 cycles and then decay significantly in the subsequent cycles.After 100 cycles at 0.5 A g −1 , the capacity is of 566 mAh g −1 .Nyquist plots of the microsphere electrodes after being activated at 0.1 A g −1 for three cycles are plotted in Fig. 3e.R sei and R ct of the equivalent circuit are assigned to the ion transport resistance in SEI layer and active materials, respectively [32].The fitting impedance data is 31 Ω for R sei and 9 Ω for R ct .Further, Fig. 3f-h describe SEM images of the first lithiated (f) and delithiated (g) and the 100-cycled (h) electrodes.The surface of electrodes at fully discharge state is covered by the products of electrolyte-related reactions.After being fully charged, the surface layer is partially decomposed and the microsphere-like morphology is identifiable, although the size of primary particles is larger than the pristine.As well known, the lithium intercalation leads to volume changes of NiCo 2 O 4 materials [33].However, an irreversible damage of material structures is observed at the 100-cycled electrode, due to the accumulated stress in continuous electrochemical processes.The loss of electric contact between active materials results in the capacity fading.
In addition, the sodium storage performance of NiCo 2 O 4 microspheres is investigated.The sodiation and desodiation processes are suggested by CV curves (Fig. 4a) and galvanostatic discharge-charge curves (Fig. 4b), similar to the electrochemical processes of NiCo 2 O 4 microspheres as LIB anodes.Due to the larger radius of Na + ions, the sodiation suffers from poorer electrochemical activity and more sluggish kinetics [34].The microspheres show an initial sodiation capacity of 697 mAh g −1 with the coulombic efficiency of 62%.The difference in reduction potentials of the electrolyte between the LIB and SIB is observed.Figure 4c shows the rate capacities of 390, 285, 205, 151 and 133 mAh g-1 at 0.05, 0.1, 0.2, 0.4, and 0.5 A g −1 , respectively.Figure 4d describes the cycling performance.After 50 cycles at 0.5 A g −1 , the sodiation capacity of 170 mAh g −1 is retained.Further, SEM images of the microsphere electrodes after the first sodiation (Fig. 4e) and desodiation (Fig. 4f) are exhibited.The microsphere-like morphology is destroyed, suggesting the large strain when sodium intercalates NiCo 2 O 4 microspheres.Thus, a stable structure is necessary for the conversion-type materials as LIB/SIB anodes.

Physicochemical properties and lithium storage performance of NiCo 2 O 4 nanosheets
The nanosheets are employed to explore the effect of a nanostructure design on NiCo 2 O 4 anodes for the LIB. Figure 5a shows the XRD patterns of NiCo 2 O 4 nanosheets, in which the (220), (311), (511) and (440) planes of spinel NiCo 2 O 4 are identified (JCPDS No. 20-0781).Figure 5b exhibits SEM images and the corresponding EDS element mapping, in which the uniform distribution of Ni, Co and O elements is observed.The  222) and (422) planes are identified.Figure 5f-h depict the XPS spectra.Ni 2p3/2 peaks (Fig. 5g) and Co 2p3/2 peaks (Fig. 5h) at the binding energy of 856.5 eV and 781.5 eV are identified.
The lithium storage performance of NiCo 2 O 4 nanosheets is shown in Fig. 6.The second and third CV curves (Fig. 6a) are almost overlapping, suggesting no obvious side reactions after the second cycle.The diffusion coefficient of Li + ions (D Li ) based on the third CV curve is calculated according to the equation: D Li = 0.5(RT/Az 2 F 2 C Li ) 2 , where A stands for the electrode's area and z (Li + ) = 1 [35].As shown in Fig. 6a, the reduction peak at about 1.05 V and the oxidation peaks at about 1.45 V and 2.30 V are marked as peak i), ii) and iii), respectively.The corresponding D Li is calculated and exhibited in Fig. 6b.It reveals the faster kinetics of cathodic processes in comparison with the anodic processes.Figure 6c shows discharge-charge profiles of the initial three cycles which agree with the CV curves.Figure 6d depicts the rate capacities of 1522, 1545, 1286, 943 and 643 mAh g −1 at currents of 0.1, 0.2, 1, 2 and 4 A g −1 , respectively.The rate retention of NiCo 2 O 4 anodes is significantly improved by the optimal nanosheet architecture.As described in Fig. 6e,    6g), which imply that the diffusion coefficient slows down as cycling [36].It is common for the metal oxide anodes which go through a phase transition and SEI layer evolution during the discharge-charge process [37,38].It's cycling performance is investigated (Fig. 8d).After 50 cycles at 0.5 A g −1 , the sodiation capacity of 230 mAh g −1 is retained.Further, The EIS data of the microsphere and nanosphere electrodes after the first sodiation at 0.1 A g −1 is exhibited in Fig. 8e and f.The fitting impedance data is 75.1 Ω (the microsphere electrode) and 24.4 Ω (the nanosphere electrode) for R sei and 535.9 Ω (the microsphere electrode) and 410.9 Ω for R ct (the nanosphere electrode).What's more, SEM images of the nanosphere electrodes after the first sodiation and desodiation (Fig. 8g and h) and 50 cycles (Fig. 8i) are exhibited.Compared with the microsphere as a SIB anode (Fig. 4e and f), the cycled nanospheres show a higher stability in the sphere-like morphology.

Conclusions
In this work, the different morphology stability of conversiontype NiCo 2 O 4 anodes for the LIB and SIB is studied.NiCo 2 O 4 microspheres show the serious morphology loss during discharge-charge processes against Li or Na, due to the accumulation of stress.To maintain the electrode' architecture, the nanostructures such as nanosheets and nanospheres are employed.NiCo 2 O 4 nanosheets on Ni form are prepared as the LIB anodes and exhibit the effectively enhanced cycling performance with the capacity of 1092 mAh g −1 after 100 cycles at 0.5 A g −1 in comparison with 566 mAh g −1 of NiCo 2 O 4 microspheres.NiCo 2 O 4 nanospheres are synthesized as the SIB anodes and show the higher structure stability during cycling while NiCo 2 O 4 microspheres occur the pulverization.In addition, a significant difference between the lithiation and sodiation capacity of NiCo 2 O 4 materials reveals Na + ions only partially intercalated in NiCo 2 O 4 and the conversion reaction limited by the strain.The structure optimization is an effective strategy for an enhancement of the conversion-type anodes for the LIB and SIB.

Figure
Figure 7a-f present SEM (a, b, d and e) and TEM (c and f) images of the precursors (a-c) and NiCo 2 O 4 nanospheres (d-f).Figure 7g-i show the SEM-EDS elemental mapping

Table 1
Cycling performance of metal oxide anodes for LIBs Materials Cycling performance Reference