LiNi0.5Mn1.5O4 Porous Nanorods as High-Rate and Long-Life

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LiNi0.5Mn1.5O4 porous nanorods as highrate and long-life cathode for Li-ion batteries Xiaolong Zhang, Fangyi Cheng, Jingang Yang, and Jun Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl401072x • Publication Date (Web): 16 May 2013 Downloaded from http://pubs.acs.org on May 17, 2013

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The remarkable high-rate and long-life performance of spinel-type LiNi0.5Mn1.5O4 porous nanorods assembled with nanoparticles as cathode materials for rechargeable lithium-ion batteries was attributed to the porous 1D nanostructures that can accommodate strain relaxation by slippage at the subunits wall boundaries and provide short Li-ion diffusion distance along the confined dimension.

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LiNi0.5Mn1.5O4 porous nanorods as high-rate and long-life cathode for Li-ion batteries Xiaolong Zhang, Fangyi Cheng, Jingang Yang, Jun Chen* Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry; The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin, Nankai University, Tianjin 300071, China. KEYWORDS: LiNi0.5Mn1.5O4, Spinel, Porous structure, Nanomaterials, Li-ion battery ABSTRACT: Spinel-type LiNi0.5Mn1.5O4 porous nanorods assembled with nanoparticles have been prepared and investigated as high-rate and long-life cathode materials for rechargeable lithium-ion batteries. One dimensional porous nanostructures of LiNi0.5Mn1.5O4 with ordered P4332 phase were obtained through solid-state Li and Ni implantation of porous Mn2O3 nanorods that resulted from thermal decomposition of the chain-like MnC2O4 precursor. The fabricated LiNi0.5Mn1.5O4 delivered specific capacities of 140 and 109 mAh g-1 at 1 C and 20 C rate, respectively. At 5 C cycling rate, a capacity retention of 91% was sustained after 500 cycles, with extremely low capacity fade (< 1%) during the initial 300 cycles. The remarkable performance was attributed to the porous 1D nanostructures that can accommodate strain relaxation by slippage at the subunits wall boundaries and provide short Li-ion diffusion distance along the confined dimension.

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Nanomaterials chemistry has recently been the main impetus for electrochemical devices with advanced energy conversion and storage such as rechargeable lithium-ion batteries (LIBs).1–4 Nanostructured active materials with confined dimensions benefit fast Li+ transport due to dramatically decreased distance over which Li+ must diffuse in the solid state.2 Designing onedimensional (1D) nanoporous structure is one of the most favourable strategies to improve the electrode performance. 1D nanostructures provide short transport path along the confined radial dimension, while the connected porous framework could not only allow for efficient active mass-electrolyte contact but also accommodate better the strains related to the structural transformation upon repeated Li+ insertion/extraction.5–7 Spinel LiNi0.5Mn1.5O4 (LNMO), which possesses fast three-dimensional Li+ diffusion channels and high operational voltage (~4.7 V versus Li+/Li0), is a promising high-power cathode material with low cost and less environmental impact.8–18 However, it remains challenging to achieve simultaneously remarkable rate capacity and cyclability for LNMO due to complex performance-influencing factors and electrolyte/structure instability under high potentials.10,11 Cation doping, surface modification, and creating nanostructures could stabilize electrode structure and enhance the formation of a passivating solid-electrolyte interphase (SEI) layer.12–17 Nevertheless, only limited types of LNMO nanostructures (e.g., nanoparticles,16 hollow spheres17) have been reported. Thus, it is not easy to achieve size and shape control of LNMO with high purity because of undesirable phase separation and grain sintering in its synthesis that generally involves prolonged calcination at high temperature. Particularly, the combination of 1D nanostructure and porosity in LNMO is elusive as the cubic crystallographic structure unfavors anisotropic crystal growth. Herein, we present a morphology-inheritance route to prepare 1D nanoporous LNMO, which, to the best of our knowledge, has never been reported and electrochemically investigated up to

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date. Figure 1 schematically illustrates the synthesis process of 1D nanoporous LNMO. Manganese oxalate, which acts as a kind of coordination polymer and tends to form 1D chainlike molecular structure,19,20 was selected as the starting material. A simple precipation of Mn2+ and C2O42- in microemulsion system yielded 1D hydrated MnC2O4 with confined diameter (Details of the preparation is described in Supporting Information). Subsequent calcination of MnC2O4 (Figure S1) led to porous Mn2O3 (Figure S2) as a result of CO2 release from oxalate decomposition. Then, Li and Ni were implanted into the Mn2O3 1D porous structure by impregnation and solid state heat treatment, resulting in the final 1D nanoporous LNMO product.

Figure 1. Schematic diagram of the preparation of 1D nanoporous LiNi0.5M1.5O4. In a typical synthesis, LiNi0.5Mn1.5O4 porous nanorods (LNMO PNR) were obtained by firing LiCH3COO, Ni(CH3COO)2 and Mn2O3 (500 oC-decomposed MnC2O4) in the molar ratio of 1.04: 0.50: 1.50. Figures 2a,b, and c display scanning electron microscope (SEM) images of MnC2O4, Mn2O3, and LNMO, respectively. All samples exhibit 1D wire or rod shape, with diameters of 100–400 nm and lengths of more than 10 μm. Apparently, the material morphology is essentially preserved during the calcination and solid-state reaction process. Transmission electron microscope (TEM) imaging (Figure 2d) shows that the prepared LNMO rods are 3


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composed of interconnected nanosized subunits with highly porous structure. High-resolution TEM (HRTEM) image and fast Fourier transform (FFT) pattern (Figure 2e and inset) further reveals single crystalline character of the nanoparticulate subunits, which present (111) crystal planes of the spinel phase. Energy dispersive spectrometry (EDS) mapping analysis (Figure 2f,g) indicates that Ni and Mn are homogeneously in LNMO rods. The Ni/Mn ratio detected from the EDS spectra (Figure 2h) is close to 1:3, in agreement with compositional results determined by atomic adsorption spectrometry. Furthermore, the BET specific surface area of LNMO PNR is 6.9 m2 g-1 (Figure S3), which is larger than that of microparticles but smaller than that of nanoparticles.16,21

Figure 2. SEM images of (a) manganese oxalate, (b) Mn2O3, and (c) LNMO. (d) TEM and (e) HRTEM images of LNMO (the inset is the corresponding FFT pattern). EDS mapping of (f) Mn and (g) Ni, and (h) EDS spectra of LNMO. To identify the crystallographic phase, we performed powder X-ray diffraction (XRD) analysis on the as-prepared LNMO PNR. Figure 3a shows the Rietveld refinement of the XRD data. The diffraction peaks can be readily assigned to well-crystallized cubic spinel LNMO (JCPDS Card No. 80-2184). The refinement provides acceptable fitting, indicative of high phase purity. A

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large mismatch of the (111) line is possibly attributed to preferred crystal orientation, as can be also deduced from the single FFT spot line in HRTEM image (Figure 2e). Calculated by using Scherrer equation, the average grain size of LNMO PNR is 50 nm, which coincides with TEM observation. The nominal spinel LNMO adopts two types of crystal structures with space groups of P4332 and Fd3m, corresponding to ordered and disordered Ni/Mn location in octahedral sites, respectively.22,23 Raman spectroscopy is a useful tool to discern the two phases. Figure 3b shows Raman mapping on a selected area of 10 μm × 10 μm while Figure 3c displays three representative spectra. All spectra present fingerprints of a typical cation-ordered P4332 phase, with a distinguishable peak split around 595 cm-1 arising from the lowered symmetry of cation ordering.24,25 The crystal structure of P4332-type LNMO is schematically shown in Figure 3d. Characteristically, Ni2+ and Mn4+ ions exhibit a cation site ordering over two distinct octahedral positions at Wyckoff 4a and 12d sites, respectively.23

Figure 3. (a) Rietveld refined XRD patterns with experimental data (red dots), calculated profile (cyan lines), allowed positions of Bragg reflection (green vertical bars) and difference curves (blue lines), (b) Raman mapping within a selected 10 μm × 10 μm area, and (c) representative 5


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Raman spectra (as marked points) of LNMO PNR. (d) Crystal structure of the P4332 phase LNMO. The electrode performance of 1D nanoporous LNMO was investigated by using coin-type Li cells. For comparison, bulk LNMO sample (denoted as LNMO bulk) with average particle size of ~2 μm was also prepared (Figure S4) and electrochemically tested. The LNMO PNR and LNMO bulk electrodes were cycled at various charge/discharge rates ranging from 1 to 20 C (1 C equals to a current of 147 mA g-1) over a potential window of 3.5–4.95 V. Typical galvanostatic profiles of LNMO PNR and LNMO bulk are shown in Figure 4a and Figure S5, respectively. The discharge curve at 1 C has a dominant plateau at about 4.65 V, which is attributed to Ni2+/Ni4+ redox.8 No minor plateau at 4.0 V associated with the Mn3+/Mn4+ redox couple can be observed, further evidencing the presence of P4332 phase.18 The narrow charge/discharge plateau separation implies a small electrode polarization. Compared to LNMO bulk, LNMO PNR exhibits a longer plateau and a slightly reduced charge/discharge voltage difference. Figure 4b further compares the rate capabilities of the two samples. At the rates of 1, 2, 5, 10 and 20 C, the corresponding discharge capacities are 140 to 136, 128, 120, and 109 mAh g-1 for LNMO PNR, versus 120, 112, 102, 93, and 80 mAh g-1 for LNMO bulk. Obviously, the 1D nanoporous sample exhibits much better rate capability than the bulk counterpart. Both samples regain the 1 C capacity after rate testing. Figure 4c further illustrates the cycling performance conducted at 5 C rate for 500 cycles. Compared to the bulk and state-of-art LNMO materials reported in literatures,15–17 LNMO PNR shows better cyclability. Notably, the discharge capacity of LNMO PNR varies from 124 to 123 mAh g-1 during the first 300 cycles, corresponding to a high capacity retention of 99.2%. After 500 extended cycles, the total capacity decay is 9%, significantly lower than that of bulk LNMO. Despite having a lower tap density, LNMO PNR renders much larger volumetric capacity and higher energy density relative

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to the bulk LNMO at the high rate of 20 C (Table S1). These results reveal remarkable energy density, high-rate capability and long-term cycling stability of the prepared 1D nanoporous LNMO.

Figure 4. (a) Typical charge-discharge curves of LNMO PNR. (b) Rate capability and (c) cycling performance of LNMO PNR and bulk samples at rate of 5 C.

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Since no surface coating and cation doping is introduced to LNMO PNR, its prominent durable high-rate performance could be mainly ascribed to the unique 1D porous nanostructure. Compared to micrometer-sized bulk samples, the rods are built up with nanosized grains, which have short solid-state diffusion length for rapid Li insertion from surface to core of active materials, as demonstrated in a variety of 1D nanostructured electrodes.5–7,26–29 This, along with the intrinsically fast Li+ diffusion within the 3D spinel lattice,30 leads to enhanced electrode kinetics and thereby high rate capability. Generally, the capacity fade of LNMO is believed to originate from the instability of electrode and electrolyte under high operational voltage.10–15 The present LNMO PNR crystallizes in the ordered P4332 structure featuring the absence of highspin Mn3+,18 which minimizes Mn disproportionative dissolution and Jahn-Teller structural distortion. On the other hand, the presence of a passivating SEI layer on the electrode surface, which was mainly composed of Li2CO3 and LiF as evidenced by FTIR (Figure S6) and XPS (Figure S7), is of great importance in preventing electrolyte decomposition. As expected, the formation of SEI layer results in the relatively low coulombic efficiency during the initial cycle under low charge-discharge rate (Figure S8). However, the SEI layer is usually torn up by particle deformation during repeated charge-discharge process, as there is a transition among three cubic phases with different lattice parameters.14,31 Figure 5 schematically illustrates morphological change and electron transportation in bulk and 1D nanoporous LNMO. The bulk particles may break or pulverize, leading to damage of the protective SEI layer and loss of electrical contact between active mass and current collector. In 1D porous nanostructure, the strain caused by lattice variation during Li insertion/extraction could be accommodated by the slippage at the subunits wall boundaries, which has been proven in both spinel cathode and Si anode.1,2,5,32,33 The facile strain relaxation allows for the maintenance of structural integrity. Additionally, it has been reported that nano-sized LNMO has 8


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a modified surface composition, which correlates with its superior electrochemical behaviour over micron-sized sample.16 Furthermore, on deep electrode discharging at high rate, Li+ ions are prone to segregate on particle surfaces due to limited transportation, which may not only block diffusion passage but also result in accumulation of Mn3+, causing severe Jahn-Teller distortion and detrimental side reactions. Nanosized particle favors fast electrode kinetics and helps to mitigate ion pile-up on LNMO surface.27 Consequently, all these factors contribute to the remarkable cyclability of the 1D porous nanostructures. A combination of XRD, SEM, TEM, and Raman study (Figures S9-S12) clearly indicates essential morphological and structural preservation of LNMO PNR after extended cycles. However, it should be mentioned that the capacity fade tends to accelerate on long-term cycling, which is feasibly associated with other instable factors such as deterioration of electrode paste and metallic Li anode. In-depth understanding the degradation mechanism requires further investigation by in situ microscopic and spectroscopic techniques.

Figure 5. Schematics of morphology change and electron transportation in bulk and 1D nanoporous LNMO electrodes on electrochemical cycling. (a) The deformation of bulk particle during repeated Li+ insertion/removal process would destroy the SEI layer formed on the electrode surface and lead to poor contact between particles and current collector. (b) Due to

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slippage at the subunits wall boundaries, 1D nanoporous structure could facilely accommodate the strain caused by the variation of lattice parameters. In summary, ordered P4332-type LNMO porous nanorods with high purity were fabricated by morphology inheritance strategy. Furthermore, when applied as cathode materials for rechargeable LIB, the resultant LNMO porous nanorods exhibited excellent long-term cyclability and high-rate capability, delivering a 20 C discharge capacity of 109 mAh g-1 and 5 C capacity retention of 99% up to 300 cycles. The results would shed light on developing manganese oxides derived electrode materials with novel nanostructures and superior electrochemical performance. Supporting Information. Detailed experimental procedures, and additional materials characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Programs of National 973 (2011CB935900), NSFC (21231005), 111 Project (B12015), and the Fundamental Research Funds for the Central Universities. REFERENCES

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Graphic TOC

The remarkable high-rate and long-life performance of spinel-type LiNi0.5Mn1.5O4 porous nanorods assembled with nanoparticles as cathode materials for rechargeable lithium-ion batteries was attributed to the porous 1D nanostructures that can accommodate strain relaxation by slippage at the subunits wall boundaries and provide short Li-ion diffusion distance along the confined dimension.

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LiNi0.5Mn1.5O4 Porous Nanorods as High-Rate and Long-Life

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