Dai-Huo
Liu‡
a,
Hong-Yan
Lü‡
a,
Xing-Long
Wu
*a,
Jie
Wang
a,
Xin
Yan
a,
Jing-Ping
Zhang
a,
Hongbo
Geng
b,
Yu
Zhang
b and
Qingyu
Yan
*b
aNational & Local United Engineering Laboratory for Power Batteries, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, China
bSchool of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. E-mail: alexyan@ntu.edu.sg
First published on 16th September 2016
In this communication, in order to develop superior electrode materials for advanced energy storage devices, a new strategy is proposed and then verified by the (Si@MnO)@C/RGO anode material for lithium ion batteries. The core idea of this strategy is the use of a positive cycling trend (gradually increasing Li-storage capacities of the MnO-based constituent during cycling) to compensate the negative one (gradually decreasing capacities of the Si anode) to achieve ultralong cycling stability. As demonstrated in both half and full cells, the as-prepared (Si@MnO)@C/RGO nanocomposite exhibits superior Li-storage properties in terms of ultralong cycling stability (no obvious increase or decrease of capacity when cycled at 3 A g−1 after 1500 cycles) and excellent high-rate capabilities (delivering a capacity of ca. 540 mA h g−1 at a high current density of 8 A g−1) as well as a good full-cell performance. In addition, the structure of the electrodes is stable after 200 cycles. Such a strategy provides a new idea to develop superior electrode materials for next-generation energy storage devices with ultralong cycling stabilities.
Conceptual insightsThis work introduces a completely new concept for developing superior electrode materials for advanced batteries. The core idea of this concept is to use a positive cycling trend (the gradually increased capacities during cycling) to compensate the negative one, which can contribute towards the achievement of ultralong cycling stabilities. In this communication, this concept is verified for the first time using a nanoscale (Si@MnO)@C/RGO composite as the anode material for a lithium ion battery. Upon cycling, for the as-prepared (Si@MnO)@C/RGO, the MnO nanoparticles can deliver a positive cycling trend, due to the gradual activation of new a Mn2+/Mn4+ redox couple, to compensate the gradually decreased capacities of the Si nanospheres. Electrochemical tests demonstrate that the (Si@MnO)@C/RGO composite exhibits not only superior Li-storage properties in terms of ultralong cycling stability without an obvious decrease or increase in capacity (e.g., the capacities can stabilize at around 800–820 mA h g−1 within 1500 cycles at 3 A g−1) and excellent high-rate capabilities (for example, it can achieve a Li-storage capacity of 470 mA h g−1 at 10 A g−1) in the half cells, but also good full-cell performances when coupled respectively with commercial LiFePO4 and LiNi0.6Co0.2Mn0.2O2 cathodes. Not only has the present work successfully prepared one outstanding anode nanocomposite for lithium ion batteries, but also more importantly it provides a completely new strategy for developing superior electrode materials for advanced energy storage devices. |
In addition to the vast majority of negative cycling trends, some electrode materials were interestingly reported to show gradually increased Li-storage capacities (conversely defined as a “positive cycling trend”) due to some specific reasons; e.g. carbon nanotube arrays grown on a stainless steel substrate exhibited 250% capacity increment after 1200 cycles due to the change of structure and surface defects.30 Unfortunately, such increased capacities could not be effectively used in the common full cells as the amount of cathode materials is fixed. Hence, it is still a difficult challenge, as well as a very significant and interesting one, to make such a positive cycling trend useful in storing Li in full cell applications.
Herein, we propose for the first time that the positive cycling trend can be employed to compensate the negatively charged ones of other materials working in the same voltage window via an easily scalable preparation processes, which provides a new approach towards developing superior electrode materials for advanced LIBs, or the possibility of expanding this approach for other energy storage devices. This concept was demonstrated by integrating MnO nanoparticles (NPs) and commercially available Si nanospheres (NSs) into one nanohybrid. Upon cycling, MnO NPs can deliver the gradually increased capacities originating from the gradual activation of one new Mn2+/Mn4+ redox couple,31,32,33 to compensate the gradually decreased capacities of the Si NSs due to the pulverization and degradation of the electrode.9,10 A proper combination of these two constituents in conjunction with the reduced graphene oxide (RGO) based network could deliver an ultrastable Li-storage property without any obvious capacity decrease or increase (e.g., the capacities were stabilized at about 800–820 mA h g−1 within 1500 cycles at 3 A g−1) but with an outstanding high-rate performance (for example, it can achieve a Li-storage capacity of about 470 mA h g−1 at a very high current density of 10 A g−1). The practical applications were demonstrated in two full-cell systems with commercial LiFePO4 and LiNi0.6Co0.2Mn0.2O2 as the cathode materials, respectively. It should be noted that in addition to the MnO anode, several other electrode materials including anode (such as carbon nanotubes,30 FeOx@carbon nanowires,34 MnO@ZnMn2O4/N–C nanorods,35 and ZnCo2O4–ZnO–C composites36) and cathode (such as a polyaryltriazine-derived composite37) materials exhibit the similar “positive cycling trend”, implying that our proposed strategy is versatile for developing advanced electrode materials.
The structure and morphology of the (Si@MnO)@C/RGO nanohybrid were firstly characterized using transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and scanning electron microscopy (SEM). Fig. 2a and b show its typical TEM and HRTEM images, clearly disclosing that the MnO NPs of 10–20 nm adhere homogenously on the surface of ∼100 nm Si NSs. Furthermore, the Si@MnO subunits are coated by the partially graphitized carbon layers with a thickness of 3–5 nm. The SEM images (Fig. 2c and its inset) show that the (Si@MnO)@C building blocks have been embedded in the RGO-based conductive networks, forming secondary particles of tens of micrometers. The Barrett–Joyner–Halenda (BJH) fitting from the N2 adsorption–desorption isotherm curves discloses the presence of mesopores in the (Si@MnO)@C/RGO nanohybrid with a wide size distribution centered at 10–40 nm (Fig. S2, ESI†), which may be originated from the voids in the 3D RGO networks and will be very beneficial in accommodating the successive volume changes of the active electrode materials. In addition, X-ray diffraction (XRD, Fig. 2d) and Raman spectroscopy (Fig. 2e) were further employed to check the phases of the (Si@MnO)@C/RGO nanohybrid. All diffraction peaks in the XRD pattern can be well indexed to Si and MnO. All the Raman peaks also originate from the Si and carbonaceous materials,38 confirming that the as-prepared (Si@MnO)@C/RGO is composed of Si, MnO and carbon without any detectable impurities. Furthermore, the XRD peaks of MnO are broader than those of Si, indicating the smaller grain size of the MnO NPs in comparison to the Si NSs. Based on the Debye–Scherrer equation,39 the grain sizes of the Si NSs and MnO NPs are about 50–100 nm and 10.5–12.5 nm, respectively. Both are consistent with the (HR)TEM observations.
The Li storage properties were firstly investigated in half cells and compared with two controls, MnO@C/RGO and Si@RGO. The phases of the two control samples were characterized as shown in Fig. S3 and S4 in the ESI.†Fig. 3a compares the cycling stability of all three samples cycled at 0.5 A g−1. For Si@RGO derived from the commercial Si NSs, the specific capacities decrease rapidly to only about 420 mA h g−1 from the initial capacity of 1345 mA h g−1 after 150 cycles at 0.5 A g−1. In contrast, for MnO@C/RGO, it shows gradually increased specific capacities from 665 to 1410 mA h g−1 within 150 cycles, which should originate from the gradual activation of Mn2+/Mn4+ redox31–33 and the strong interphase interaction between the MnO NPs and RGO nanosheets.40 When such two constituents are integrated into one composite, the as-formed (Si@MnO)@C/RGO exhibits superior Li-storage stability without an obvious decrease or increase in specific capacity under the same test conditions. Fig. 3b shows the corresponding galvanostatic curves of the three samples at the 1st, 30th, 60th, 90th, 120th and 150th cycles. The curve profiles for (Si@MnO)@C/RGO overlap clearly, while those for the other two controls show obvious shifts. This confirms the outstanding cycling stability of (Si@MnO)@C/RGO, which can be also demonstrated at other current densities (e.g., 0.2 A g−1, 2 A g−1 and 5 A g−1), as shown in Fig. S6 in the ESI.† In addition, (Si@MnO)@C/RGO exhibits a higher reversible voltage plateau (or mean voltage) in comparison to the Si@RGO control. A higher voltage plateau of anode materials can inhibit the Li-dendrite formation during the successive Li-insertion/extraction processes. Fig. S7 in the ESI† further shows the cyclic voltammetry (CV) curves of (Si@MnO)@C/RGO for the initial 5 cycles between 0.005 and 3 V vs. Li+/Li at a scan rate of 0.1 mV s−1. In the profiles, the peak couples at 0.17 V/0.34 V and 0.38 V/0.53 V correspond to the alloying/dealloying reactions of the Si NSs, while the peak couple at 0.82 V/1.35 V should be attributed to the redox of Mn0/Mn2+.33 From the inset of Fig. S7 in the ESI,† the oxidation process from the Mn2+ to Mn4+ species can also be clearly observed at about 1.89 and 2.1 V.33 Furthermore, the as-prepared (Si@MnO)@C/RGO has an ultralong cycling life time. As shown in Fig. 3c, the specific capacities delivered by (Si@MnO)@C/RGO at 3 A g−1 are at about 800–820 mA h g−1 with Coulombic efficiencies of close to 100% for 1500 cycles.
In addition to the outstanding cycling stability, the as-prepared (Si@MnO)@C/RGO nanohybrid also exhibits excellent rate capabilities (Fig. S8, ESI†). For example, when the current densities increase from 0.1 A g−1 to 8 A g−1, a high capacity retention of 46.9% can be achieved. At a very high current density of 10 A g−1, it can still deliver a specific capacity of ca. 470 mA h g−1. In comparison, MnO@C/RGO delivers much lower specific capacities at the same testing current densities in the range from 0.1 to 10 A g−1, implying that the incorporation of Si NSs into (Si@MnO)@C/RGO can increase the Li-storage capacities of the final composite. Additionally, (Si@MnO)@C/RGO also exhibits much higher capacity retentions in comparison to the Si@RGO control (e.g., 50.2% vs. 25.1% at 6 A g−1).
There are other structural merits in addition to the compensation effects for the enhancement of electrochemical properties in the as-prepared (Si@MnO)@C/RGO. Firstly, the complete carbon coating and large number of voids in the 3D RGO networks have the ability to accommodate the volume change of the (Si@MnO)@C subunits during the successive Li-insertion/extraction. Fig. 4 shows the TEM images of the (Si@MnO)@C/RGO material after 200 cycles at 3 A g−1. As disclosed, the Si@MnO particles are still coated by the carbon layers and wrapped in the RGO-based networks, although the Si NSs and MnO NPs have already merged together due to the successive lithiation/delithiation processes. Secondly, the flexible and highly conductive 3D scaffold composed of the RGO nanosheets and carbon coating can effectively enhance the charge transport. Such a 3D scaffold is also robust enough to maintain the structural integrity of the material during the successive Li-insertion/extraction processes.16,29,31 This is also verified by the TEM image (Fig. 4a) of the (Si@MnO)@C/RGO material after 200 cycles at 3 A g−1, in which the cycled secondary particles are still wrapped in the RGO networks, as well as the elemental mappings of the (Si@MnO)@C/RGO electrode after the same cycling tests (Fig. S9, ESI†), in which carbon as well as other elements are uniformly distributed. In addition, the RGO nanosheets may also contribute to the positive cycling trend32 and promote the Mn2+/Mn4+ redox reaction. Thirdly, the utilized Si material is a commercially available product, which indicates a scalable material preparation process and the possibility of practical applications as anode materials for lithium full cells.15
To demonstrate the potential practical applications, we further evaluated the full-cell performance of the as-prepared (Si@MnO)@C/RGO via coupling with commercial LiFePO4 and LiNi0.6Co0.2Mn0.2O2 cathode materials, respectively. Fig. S10 in the ESI† shows the galvanostatic curve of the LiFePO4 material in the half cells cycled at 34 mA g−1, which shows the typical charge/discharge profiles for the LiFePO4 cathode material as reported previously.39 After it was fabricated into (Si@MnO)@C/RGO//LiFePO4 full cells, Fig. 5a shows the CV patterns of the initial three cycles recorded at 1 mV s−1 between 1 and 3.6 V. It is disclosed that all curves are well overlapping, implying the excellent reversible charge/discharge processes of the as-fabricated (Si@MnO)@C/RGO//LiFePO4 full cells. As shown in Fig. 5c, such full cells can deliver stable specific capacities without an obvious decay after 150 cycles at 5C (charging/discharging the battery for 1/5 hour), except for a slight decrease of about 7% during the initial 10 cycles. The capacity decay in the initial cycles suggests that further optimization is very essential for the full cell assembly. In addition to the good cycling performance, the as-fabricated (Si@MnO)@C/RGO//LiFePO4 full cells also exhibit a superior rate performance as shown in Fig. 5b. At a high rate of 10C, the cells show a high capacity retention of ca. 74% in comparison to the capacity at 0.2C. Note that it is the coin-type full cells with a capacity of 3–4 mA h, which can be further increased via fabricating larger cells in future practices. The inset of Fig. 5c further shows that one flexible pouch full cell fabricated using (Si@MnO)@C/RGO and LiFePO4 can power a light-emitting diode (LED) bulb. In addition, the battery performances of the (Si@MnO)@C/RGO//LiNi0.6Co0.2Mn0.2O2 full cells are illustrated in Fig. S11 in the ESI.† Such full cells exhibit a charging/discharging ability at 1.0–4.0 V with an excellent rate performance (e.g., a high capacity retention of ca. 69% when tested at 10C in comparison to 0.2C), which demonstrates the successful fabrication and preliminary evaluation of the (Si@MnO)@C/RGO//LiNi0.6Co0.2Mn0.2O2 full cells.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nh00150e |
‡ Equal contribution. |
This journal is © The Royal Society of Chemistry 2016 |