Letter pubs.acs.org/NanoLett
Ultrathin Spinel Membrane-Encapsulated Layered Lithium-Rich Cathode Material for Advanced Li-Ion Batteries Feng Wu,†,‡,§ Ning Li,†,§ Yuefeng Su,*,†,‡ Linjing Zhang,† Liying Bao,†,‡ Jing Wang,†,‡ Lai Chen,† Yu Zheng,† Liqin Dai,† Jingyuan Peng,† and Shi Chen†,‡ †
School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing Key Laboratory of Environmental Science and Engineering, Beijing 100081, P. R. China ‡ National Development Center of High Technology Green Materials, Beijing, 100081, P. R. China S Supporting Information *
ABSTRACT: Lack of high-performance cathode materials has become a technological bottleneck for the commercial development of advanced Li-ion batteries. We have proposed a biomimetic design and versatile synthesis of ultrathin spinel membrane-encapsulated layered lithium-rich cathode, a modification by nanocoating. The ultrathin spinel membrane is attributed to the superior high reversible capacity (over 290 mAh g−1), outstanding rate capability, and excellent cycling ability of this cathode, and even the stubborn illnesses of the layered lithium-rich cathode, such as voltage decay and thermal instability, are found to be relieved as well. This cathode is feasible to construct high-energy and high-power Li-ion batteries. KEYWORDS: Biomimetic design, high rate, layered/spinel cathodes, Li-ion batteries, membrane, nanocoating
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was reported to substantially reduce the heat generation of the layered core.23 We have reported a spinel/layered core−shell heterostructured cathode, making full use of Li+ capacity of layered bulk.24 Recently, ultrathin nanocoating by the atomic layer deposition (ALD) method showed obvious electrochemical improvement to an individual layered or spinel cathode.25−27 To combine the advantages of these strategies, herein, we propose a biomimetic design and versatile synthesis strategy of ultrathin spinel membrane-encapsulated layered lithium-rich cathode material, a modification by ultrathin nanocoating. It is well-known that, in most higher eukaryotes, the thin plasma membrane is not only essential for the cell to be stable in the environment, but the membrane protein can also play as “alkali ion pump” to active transport alkali ions across the membranes by consuming the ATP (Scheme 1).28,29 Hence, with an ultrathin nanolayer of 4 V spinel Li1+xMn2O4 as a “membrane” encapsulating on a layered lithium-rich cathode, the spinel membrane can be expected to maintain the layered bulk stable during high-voltage cycling, and most importantly, this high Li+ conductive membrane can rapidly transport Li+ between the electrolytes and the layered bulk as a “Li+ pump”. To demonstrate this idea, an easy and versatile nanocoating strategy has been adopted as shown in Scheme 1, differing from existing methods such as ALD, chemical vapor deposition, or
ecause of the critical issue of climate change and the massive consumption of fossil energy, Li-ion batteries are being intensively pursued to power vehicles (EVs) and store renewable energy (such as solar and wind energy).1−3 This promotes the need of developing bifunctional (high-energy and high-power) cathode materials. The manganese-based spinel cathodes LiM2O44−9 and layered lithium-rich cathodes Li[LixM1−x]O210−17 (M represents Mn and other transition metal such as Ni, Co, etc.) show great potential to constructing highpower and high-energy batteries. However, the individual spinel and layered cathodes have their own intrinsic drawbacks. For example, spinel cathodes yield a limited capacity of ca. 140 mAh g−1 above 3 V, although their 3D-framework endows them with a high Li+ conductivity and good thermal stability.5,8 Indeed, layered lithium-rich cathode delivers a high capacity of ca. 250 mAh g−1, while it suffers from intrinsic poor rate capability, voltage decay, capacity fade, and thermal instability.15−17 Yet, the layered-spinel composite still shows great potential to yield high-capacity and high-rate performance. Thackeray’s group18−20 has initially reported high-capacity all Mn-based integrated layered-spinel composites, then extended to Li−Ni−Mn−O layered-spinel composites. A series of highenergy Li−Ni−Mn−Co−O layered-spinel composites was also proposed.21,22 Although the optimization of the synthetical condition and compositional ratio has been carried out, the original goal of exploiting a bifunctional cathode has not been really achieved. Hence, the rational design is still essential to obtain high-performance layered-spinel composites. Surface modification from the micro- to nanoscale has been carried out to address these challenges. Spinel-layered core−shell cathode © 2014 American Chemical Society
Received: March 29, 2014 Revised: May 19, 2014 Published: May 20, 2014 3550
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Scheme 1. Schematic Diagram for the Design and Synthesis of Ultrathin Spinel Membrane-Encapsulated Layered Lithium-Rich Cathode Material
Figure 1. FESEM images of the PLLR sample (a), the sample heat-treated at 500 °C (b), 750 °C (c), and 900 °C (d); and their corresponding TEM images from (e) to (h).
Figure 2. HRTEM images of the PLLR sample (a) and the USMLLR sample (b). The inset is two times enlarged view in green rectangle of (b). XRD patterns (using Cu Kα radiation) (c) and Raman spectroscopies (d) of the PLLR sample and the USMLLR sample.
“dip and dry” (we proposed in ref 24). The polymer dispersant, polyvinylpyrrolidone (PVP), was initially dispersed on the p r i s t in e l a y e r e d li t h i u m - r i c h m a t e r ia l s ( P L L R ) , Li1.2Mn0.6Ni0.2O2, to assist the subsequent homogeneous decoration of the manganese salt. Eventually, the 4 V spinel Li1+xMn2O4 membrane could be yielded on the surface after heat treatment, due to the ion diffusion from the bulk.24,30 It is
demonstrated that this ultrathin spinel membrane-encapsulated layered lithium-rich cathode yields a superhigh capacity, outstanding rate capability, and excellent capacity retention, and the stubborn illnesses of the layered lithium-rich cathode such as voltage decay and thermal instability, are also found to be relieved. 3551
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coating. Yet, an additional shoulder band emerges around 670 nm−1 in the USMLLR sample as indicated by the arrow, which is possibly attributed to the existence of 4 V spinel Li1+xMn2O4.16,31 This is also verified by the XPS spectra (Figure S2), the EDX analysis (Figure S3), and the later electrochemical tests. Consequently, all of the above evidence have provided convincing proofs that the spinel membrane has constructed the surface of the USMLLR material. We have characterized the PLLR material and the USMLLR material as cathodes in the half-cells between 2.0 and 4.8 V at a C/10 rate (∼25 mA g−1) at 30 °C. The first and second cycle charge−discharge curves of the PLLR sample and the USMLLR sample are illustrated in Figure 3a. It is notable that the
To optimize the synthesis of ultrathin spinel membrane, different temperatures (500, 750, and 900 °C) are applied in the heat-treating process. The as-prepared materials were characterized by both field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The FESEM image of the PLLR sample in Figure 1a shows assembled microstructure of submicrometer crystallites, and the particle-size of the samples presents no obvious change until the heat-treated temperature increases to 900 °C in Figure 1b−d. The TEM image of the PLLR sample in Figure 1e presents a clean surface. However, the surface shows great changes after surface modification. The TEM image of the sample heat-treated at 500 °C in Figure 1f displays a rather uniform distribution of an amorphous coating layer with a width of 5−7 nm. This is supported by the existence of the PVP dispersant during the coating process. For the sample heattreated at 750 °C in Figure 1g, the homogeneous coating layer becomes much thinner, with a width of 1−2 nm, which is the expected ultrathin membrane for the bulk. However, when the temperature increases to 900 °C in Figure 1h, the coating layer disappears, and the primary crystallites are severely agglomerated, resulting in a dramatical increase of particle size, which is obviously not suitable for fast kinetics. Hence, 750 °C is considered as the suitable temperature to obtain the membrane, and the corresponding sample is labeled as USMLLR (ultrathin spinel membrane encapsulated-layered lithium-rich cathode). To identify the surface microstructure, we observed the PLLR sample and the USMLLR sample under high-resolution TEM (HRTEM). The PLLR sample in Figure 2a displays a bare and clean surface and an apparent layered structure with an interplanar spacing of ca. 0.472 nm.11,12,16,17 As to the USMLLR sample in Figure 2b, its bulk layered structure is wellpreserved, presenting layered fringes with interplanar spacing of ca. 0.470 nm. Apparently, a homogeneously ultrathin coating layer can be observed. The enlarged view in the inset shows that this outer layer is well-crystallized, and some lattice fringes are clearly legible, with interplanar spacings of ca. 0.204 nm and ca. 0.147 nm. These fringes are not included in the hexagonal layered structure but quite indexed with the [400] and [440] planes of the cubic spinel (Fd−3m), respectively,6−8 which is consistent with our former reports. 24,30 Actually, the interdiffusion of metal ions at high-temperature heat treatment may lead to intact integration of the ultrathin membrane and the layered bulk and thus endows these two parts with good compatibility and Li+ penetrability. The ultrathin membrane structure is further confirmed by structural characterization. The XRD patterns of both samples are shown in Figure 2c. The patterns of the PLLR sample collected for comparison are well indexed with α-NaFeO2 layered structure (R3̅m symmetry) except for weak superlattices reflections around 2θ = 20−25°, corresponding to the C/ 2m.10−12 As to the USMLLR sample, it combines with the patterns of both hexagonal layered lithium-rich structure and cubic spinel structure (Fd−3m symmetry), and the weak shoulder peaks assigned to the spinel membrane are pointed by the asterisks. This spinel phase becomes more evident with the increasing of heat-treated temperature as shown in Figure S1 in the Supporting Information. To further discern these oxygen cubic-close-packed spinel and layered structure, Raman scattering as a powerful tool has been applied to determine the short-range local structure. Both two samples share the typical characterizations of layered lithium-rich materials in Figure 2d, inferring good maintenance of layered structure after
Figure 3. First and second charge−discharge curves (a) and the corresponding dQ/dV plots (b) of both samples; the solid lines are for the first cycle and the dashed lines for second cycle.
discharge capacity and initial Coulombic efficiency shows much increase after spinel membrane decoration. The PLLR sample exhibited representative initial charge/discharge curve of lithium-rich cathode, consisting of a slope region below 4.5 V (oxidation of Ni2+ to Ni4+) and a plateau region at 4.5 V (irreversible oxidation of O2p) on charge and a smooth slope (reduction of Ni4+ to Ni2+, and then Mn4+ to Mn3+) on discharge.11,12 The corresponding cathodic and anodic peaks could be clearly identified in its differential capacity versus voltage (dQ/dV) plots in Figure 3b, except for a shoulder cathodic peak observed around 2.5 V (indicated by black asterisk), which resulted from the SEI decomposition.16,32 In the case of the USMLLR sample in Figure 3a, few differences could be seen from charge/discharge curves when compared with the PLLR sample. However, its dQ/dV plots in Figure 3b displayed different anodic peaks around 3.1 and 4.2 V indicated by the green dashed lines on both the first and the second charges as well as the cathodic peak at 2.8 V indicated by the red asterisk. These redox peaks, including a possibly overlapped cathodic peak around 4.0 V by Ni4+ reduction of the layered content, did not occur in the PLLR sample but are consistent with the Mn4+↔Mn3+ redox couples of the 4 V lithiated spinel Li1+xMn2O4.6,8,18−20 There are no redox peaks assigned to nickel ions of the spinel phase, indicating negligible diffusion of the nickel ions from bulk into the outer spinel membrane. This clearly confirmed the 4 V Li1+xMn2O4 component of the outer spinel membrane. Furthermore, the cathodic peak for Mn4+ below 3.5 V grew much bigger, indicating that oxygen vacancies have been maintained by the ultrathin spinel membrane.24,30,33 Hence, we can conclude that the increasing discharge capacity of the USMLLR sample is yielded by both leveraging the 3552
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Figure 4. Rate discharge capacities of the PLLR sample and the USMLLR sample, under the charge rate of C/10 and discharge rates of C/10, C/5, C/2, 1C, 2C, 5C, and 10C in sequence for each five cycles (a). The cycling performance at C/10 (b). Charge−discharge curves of the PLLR sample (c) and the USMLLR sample (d), and the corresponding average discharge potential curve vs cycle number (e). The 1C rate charge/discharge cycling performance, before activation under a charge rate of C/10 and a discharge rate of 1C (f).
respectively. More surprisingly, it delivered a high capacity of 124.8 mA h g−1 even at the 10C rate, over 40% of the capacity at the C/10 rate (inset of Figure 4a). It even maintained ca. 275 mA h g−1 when returned to the C/10 rate. An obvious IR drop was observed from the curves in the Figure S4. To the best of our knowledge, these high capacities are among the highest values obtained for the power performance of similar materials reported in the literature.11−14,23,24,33 Furthermore, thicker spinel membranes (3−4 nm) were also decorated on the lithium-rich cathode (Figure S5), yielding considerable rate discharging capacities, but slightly inferior to the USMLLR sample. Thus, we can believe that the ultrathin spinel membrane is capable of enabling the layered lithium-rich cathode with an outstanding rate performance. Considering that the layered lithium-rich cathode suffers from inevitable phase transformation, dissolution of transitionmetal ions, and the erosion from the electrolytes,14−16,24,33 whereas the 4 V spinel cathode may suffer from Jahn−Teller distortion when deeply reducing Mn4+,6−8,18,19 we checked the cycling performance of the PLLR material and the USMLLR
electrochemical activity of the outer 4 V spinel Li1+xMn2O4 and maintaining lithium vacancies. With a high Li+ conductive spinel membrane on the surface, the USMLLR material is expected to have good Li+ inserting/ extracting kinetics. We have characterized the different rate discharge performance of this material at C/10, C/4, C/2, 1C, 2C, and 5C rates during 2.0−4.8 V with 5 cycles per step, while the C/10 rate constant current was applied during the charging process. As shown in Figure 4a, the USMLLR material combined advantages of high capacity from the bulk layered lithium-rich cathode and high rate capability from the spinel membrane. The PLLR sample tested for comparison showed a poor rate capability, a dramatic capacity drop with the increasing of C-rate, and nearly no capacity was obtained at the 10C rate. When cycling back to C/10 rate, a capacity of only 190 mA h g−1 was maintained. The reason was that the fast Li+ insertion/extraction damaged the fragile surface of the layered structure at high rates. On the contrary, the USMLLR sample yielded maximal discharge capacities of 247.9 mAh g−1, 223.8 mA h g−1, and 200.1 mA h g−1 at 1C, 2C, and 5C rates, 3553
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material at a C/10 rate. In Figure 4b and c, the PLLR sample delivered a maximal capacity of 283.4 mA h g−1, and only 229.0 mA h g−1 remained after 50 cycles, with a modest capacity retention of 80.8%. In contrast, for the USMLLR sample in Figure 4b and d, it displayed great enhancement both in specific capacity and cycling ability. It yielded a maximal capacity of 295.6 mA h g−1, and the capacity retained 280.0 mA h g−1 at the 50th cycle, with an excellent capacity retention of 94.7%. The capacity, however, displayed an abnormal variation during cycling, initial decrease, and then increase. Note that this variation was accompanied by voltage decay and phase transformation during the discharge in Figure 4c and d. The average discharge potential of the USMLLR in Figure 4e showed an aggressive drop in former 30 cycles, and it then remained steady in prolonged cycles. Nevertheless, the PLLR presented a sloping drop of average discharge potential due to the continuous phase transformation (Figure S6). At the same time, a layered-to-spinel phase transformation was promoted by the ultrathin spinel membrane of the USMLLR sample in Figure S7 and S8; yet this transformation ceased after 30 cycles. Instead, the spinel mass derived from phase transformation was gradually activated, resulting in an output of higher capacity. As this activation was not observed in the PLLR sample, it should be attributed by the existence of the ultrathin spinel membrane. Meanwhile, the coexistent spinel phase of the ultrathin spinel membrane and the transformed spinel possibly yields good structural compatibility and stability with the layered bulk, thus alleviating voltage decay of the USMLLR sample. We also tested the higher rate charge/discharge cycling performance of the PLLR material and the USMLLR material by charging and discharging at a 1C rate, before activation under a charge rate of C/10 and discharge rate of 1C in Figure 4d. The PLLR material displayed a rapid capacity fade from 204.8 to 112.2 mA h g−1 after 100 cycles, whereas the USMLLR material yielded a superhigh capacity of 244.8 mA h g−1 with capacity retention of 89.9% after 100 cycles. Furthermore, the voltage decay for the USMLLR sample was found to be relieved at this high rate cycling as well, although serious phase transformations occurred (Figure S9). Consequently, it is believed that the ultrathin spinel membrane can promote the bulk layered-to-spinel transformation and activate the derived spinel mass, and the bulk structure and outer spinel membrane can maintain and stabilize each other well during different rate cycling. Apart from the rate capability and cycling ability, the thermal stability of these materials is of significant importance for their applications in commercial electrical vehicles and energy storage systems. Figure 5 shows the DSC curves of the PLLR and the USMLLR electrodes charged to 4.8 V. The PLLR cathode exhibited an initial exothermal peak at 197 °C with the overall heat generation of 1173 J g−1. Conversely, the initial exothermic reaction temperature for the USMLLR electrode reached a much higher value of 225 °C with greatly reduced heat generation (292 J g−1). This good thermal stability of the USMLLR materials can be ascribed to the highly thermal stability of the spinel membrane. Therefore, the overall extremely excellent performance of the USMLLR material can result from its rational biomimetic design and synthesis. The ultrathin 4 V spinel-membrane as a Li+ pump provides a Li+ diffusion highway between the electrolytes and the bulk layered mass, making full use of high Li+ storage capacity of the layered structure. The oxygen cubicclose-packed spinel membrane and layered bulk show good
Figure 5. DSC profiles of the delithiated PLLR material and the delithiated USMLLR material.
integration and compatibility, in terms of cycling ability and thermal stability. Although the spinel membrane has promoted the bulk phase transformation, the voltage decay of the USMLLR can be relieved, and the derived spinel mass can be activated during cycling. The thermally stable spinel membrane is attributed to the thermal safety of the USMLLR as well. In summary, we have developed a novel ultrathin spinel membrane encapsulated layered cathode material for highenergy and high-power Li-ion batteries. The ulthathin spinel membrane is confirmed to be responsible for the high capacity, excellent cycling ability, outstanding rate capability, and thermal stability as well as the relieved voltage decay. The biomimetic design and versatile nanocoating strategy adopted in this work should inspire the development of advanced intercalation materials. This bifunctional cathode material may provide a bridge to the future advanced Li-ion batteries for application in electric vehicles and renewable energy storage, although much research still needs to be completed to finally enable this material.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Detail of experimental and additional figures depicting experiment results. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Author Contributions §
F.W. and N.L. contributed equally to this work. N.L. conceived the idea and discussed with F.W. and Y.S.; N.L. designed and carried out the experiments. N.L., F.W., and Y.S. analyzed the data and wrote the manuscript. L.Z. helped with the DSC experiments, and all authors participated in discussions of the research.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the Key National Basic Research and Development Program of China (2009CB220100), National Natural Science Foundation of China (51102018, 21103011), Program for New Century Excellent Talents in University (NCET-13-0044), and BIT Scientific and Technological Innovation Project (2013CX01003). 3554
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REFERENCES
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