Li-Rich Layered Oxide Microspheres Prepared by the

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Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Li-Rich Layered Oxide Microspheres Prepared by the Biomineralization as High-Rate and Cycling-Stable Cathode for LiIon Batteries Yu-Kun Hou,† Gui-Ling Pan,*,‡ Yan-Yun Sun,† and Xue-Ping Gao*,† †

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Institute of New Energy Material Chemistry, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China ‡ Key Laboratory of Functional Polymer Materials of the Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Biomineralization is a green and skillful approach to prepare Lirich layered oxides with hierarchical micro/nano structure as high-performance cathode for lithium-ion batteries. In this work, to enhance the high-rate capability and cycle stability, Li-rich layered oxide Li1.17Ni0.14Co0.13Mn0.56O2 microspheres are synthesized through a simple biomineralization function, in which yeast cells are used as the nucleating agent and structural template without using any organic complexing agents. These microspheres are constructed by densely aggregated primary nanocrystallites (30−100 nm in size) with abundant mesopores. Furthermore, the in situ surface modification can be induced by the few-layered black phosphorus with good conductivity of electrons, which are derived from the biomass adhering on the primary nanocrystalline structures. Such hierarchical micro/nano assembly structure of the active materials provides more electrochemical reactive sites, facilitates rapid Li-ion diffusion, and suppresses undesirable side reactions caused by the electrolyte corrosion. The Li-rich layered oxide presents good electrochemical performance, including reversible capacity and cycle stability. The large discharge capacity of 318.7 mAh g−1 is obtained for Li-rich layered oxide at 0.1 C rate. At higher rate of 10 C, the large capacity of 134.5 mA h g−1 is maintained. After 270 cycles at varied charge/discharge rates, the high capacity retention of 97.2% is achieved. KEYWORDS: lithium-ion battery, cathode, Li-rich layered oxides, microspheres, biomineralization

1. INTRODUCTION Exploring new electrode materials is vitally significant, and enables lithium ion batteries (LIBs) to meet the urgent demands for hybrid electric vehicles (HEVs), electric vehicles (EVs), and smart grid communications.1−4 As the cathode is the capacity-determining electrode, it is important to investigate cathode materials with large specific capacity and good high-rate performance.5,6 Nowadays, Li-rich layered oxides (LRLO) with high specific capacity (>270 mA h g−1) and low cost are selected as the potential cathode materials for the next-generation high-energy LIBs.7−10 However, LRLO cathode materials suffer from intrinsically poor high-rate performance, fast capacity decline, and potential drop, due to the rearrangements of surface structure originated from the activation of Li2MnO3 above 4.5 V, and the local phase transformation from a hexagonal layered structure to a cubic spinel during long cycling.11,12 Therefore, the LRLO oxides have still faced many basic challenges and are required for the cycle stability and high-rate capability.7,12 As demonstrated previously, reducing active particle sizes to nanoscale level can effectively enhance the high-rate capability © XXXX American Chemical Society

by shortening the Li-ion diffusion and electron transfer pathways in the electrodes.13,14 However, nanoparticles have low packing density and high surface area, usually leading to low volumetric capacity, poor structure stability and undesirable side reactions between electrode/electrolyte interface.15 Therefore, designing electrode materials with hierarchical architecture structure, like microassemblies composed of primary nanoparticles, is demonstrated to be effective method to enhance the cycle life and rate capability of the electrode materials.16−19 For instance, Wu et al.16 have proposed Li1.2Ni0.13Mn0.54Co0.13O2 with 5 μm-sized spherical secondary particles composed of primary nanoplates, which can reduce the specific surface area without sacrificing high-rate performance. Bai et al.17 have used a hydrothermal technology to prepare stable and fusiform Li1.2Ni0.2Mn0.6O2 oxide assembled with primary nanoparticles. Cho et al.19 have developed 0.5Li2MnO3-0.5LiNi0.5Mn0.5O2 cathode with 10 μm-sized Received: August 2, 2018 Accepted: September 24, 2018

A

DOI: 10.1021/acsaem.8b01273 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Schematics of the biomineralization process for the preparation of the biogenic-LRLO sample. (a−c) Illustration of the formation mechanism of the yeast/carbonate bioprecursor. (d) Structural model of the as-prepared biogenic-LRLO oxide templated from yeast cells.

secondary microspheres composed of submicron flake-like primary particles yielding excellent cycle stability and good high-rate capability. Such multilevel assembly nanoarchitecture cathode materials show improved electrochemical performances without jeopardizing the advantages conferred by the nanotechnology.18 Bionanotechnology is one of the most powerful routes to program a controllable fabrication of hierarchical micro/nano architecture materials.20,21 Many inorganic energy storage materials with well-organized micro/nano structures are obtained by the design and preparation via the biomineralization method.20,22−25 More importantly, biomineralization is a typical self-organized formation of extraordinary architecture structures under ambient conditions.26 As a typical biotemplate, yeast cells have spherical shape, plentiful surface charge, high surface area, and adsorption capability of transition metal cations.27,28 In our previous work,29 the biomineralization of yeast cells was demonstrated to be effective to regulate the hierarchical structure of iron/ manganese phosphate microspheres. Moreover, many works have shown that the surface high-conductive modification and the formation of nanocrystallites are mostly in favor of enhancing the electrochemical activity of LRLO cathode materials.30,31 If both nanocrystallization and high-conductive surface coating by the biomineralization method are simultaneously combined into LRLO to further manipulate hierarchically micron-sized nanoarchitectures, it would be satisfied to highly optimize the electrochemical properties of the LRLO oxides. Herein, to enhance the high rate capability and cycle stability of Li-rich layered oxides (LRLO, Li1.17Ni0.14Co0.13Mn0.56O2), a simple way is used to prepare LRLO microspheres aggregated by dense primary nanocrystallites, with yeast cells as the growth template and nucleating agent. The hierarchical micro/ nano structure of the LRLO material combines the high electrochemical activity and fast Li-ion diffusion, and reasonable packing density. Meanwhile, in situ surface modification can be induced in the biomineralization process, which is favorable to enhance the high-rate performance and cycle life. The as-prepared biogenic LRLO sample exhibits optimized electrochemical performance.

2. EXPERIMENTAL SECTION 2.1. Preparation of the LRLO Samples. In the experiments, chemical reagents with analytical grade are used. The detailed experimental procedure about biomineralization process was described previously.29 The biogenic-LRLO oxide was prepared with the initial biomineralization and subsequent solid-phase reaction process. In details, a certain amount of active dry yeasts (5 g) was mixed into the fresh prepared glucose culture media (100 mL, 0.1 g mL−1) to conduct the cultivation of the yeast bacteria. The bacteria solution was cultivated at 38 °C and kept for 40 min. After that, the purified active yeasts were obtained by centrifugation and rinsed with distilled water. The mixed solution (20 mL) of MnSO4·H2O (0.56 mol L−1), NiSO4·6H2O (0.14 mol L−1), and CoSO4·7H2O (0.13 mol L−1) was slowly dripped into the above yeast suspension solution (100 mL). After stirring for 5 h under the room temperature, good bioadsorption was obtained. Later, Na2CO3 solution (20 mL, 0.83 mol L−1) was dropped slowly into the mixed solution of Mn2+/Ni2+/ Co2+/yeast, which was further stirred for 30 min. After that, the above mixture biosolution was stilled for overnight to carry out the biomineralization function. The yeast/carbonate bioprecursor was collected by centrifugation and rinsing with distilled water, and followed by freeze-drying to constant weight. The biogenic-LRLO sample was finally obtained after mixing the dried carbonate precursor with LiOH·H2O in the molar ratio of 0.83:1.176 (CO32−/Li+), and calcining in a resistance furnace at 480 °C for 3 h, and further at 850 °C for 10 h in air, respectively. The rate of heating was set at 2 °C min−1. The blank-LRLO oxide was also obtained without participation of yeast cells. 2.2. Materials Characterization. The X-ray diffraction (XRD) patterns of samples were obtained from characterized by diffractometer (Rigaku Smart Lab). The morphology and microstructure of all the samples were conducted by a scanning electron microscope (SEM, JEOL-JSM7800F), and a transmission electron microscope (TEM, JEOL-JEM2800). The Raman spectra of the samples were measured by a RTS-HiR-AM Raman spectrometer (532 nm in wavelength). The X-ray photoelectron spectra (XPS) were detected on X-ray photoelectron spectrometer (Thermo Scientific ESCALAB 250Xi). The Brunauer−Emmett−Teller measurement using N2 absorption at 77 K was performed on JW-BK112 system. 2.3. Electrochemical Measurements. The cathodes were consisted of active materials (80 wt %), Super P (10 wt %), and polyvinylidene fluoride (PVDF, 10 wt %), which were dispersed in Nmethyl-2-pyrrolidone (NMP) to obtain an uniform slurry. The obtained slurry was coated on the current collector (Al foil, 0.02 mm in thickness) and dried in a vacuum oven at 100 °C for 12 h. The cathodes were punched with a diameter of 10 mm, and the mass B

DOI: 10.1021/acsaem.8b01273 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials loading of the cathode was set to be about 2.20−2.25 mg cm−2. In a glovebox with high-purity argon, CR2032 coin cells were assembled to characterize the performance of the cathodes. The electrolyte was composed of LiPF6 (1 M) in ethylene carbonate (EC) and dimethyl carbonate (DMC) mixture solvents (3:7 v/v). Metallic lithium was adopted as the reference and counter electrodes. The Celgard 2300 porous membrane was served as the separator. The coin cells were charged/discharged in a potential range of 2.0−4.8 V on a LANDCT2001A battery tester at various rates (1 C = 300 mA g−1). Cyclic voltammetry (CV) tests were conducted on a CHI600C electrochemical workstation (Shanghai Chenhua). Electrochemical impedance spectra (EIS) were performed on electrochemical workstation (Zahner IM6e) with 5 mV amplitude of perturbation in the frequency range of 10 kHz to 10 MHz.

3. RESULTS AND DISCUSSION In the biologically directed mineralization, genes (DNA) and biomolecules are mostly involved in the nucleation and growth of the inorganics on the biomatrix.29 The sufficient surface charge on the cell membranes can improve the mineralization ability of yeast cells with the aid of biosurface active macromolecules.28 Figure 1 displays the formation mechanism of the yeast/carbonate bioprecursor (Figure 1a−c). The asprepared biogenic Li1.17Ni0.14Co0.13Mn0.56O2 sample is hereafter marked as biogenic-LRLO. During the cultivation of yeast cells, the macromolecule metabolites with hydrophilic negatively charged anion groups, including extracellular proteins and polysaccharose, are generated on the cell surface.29 The mixed NiSO4, CoSO4, and MnSO4 solution is dropped in the cultured biosuspension, cations (Ni2+, Co2+, and Mn2+) are quickly incorporated with anion groups on the outer surface or inside the inner wall of the biomolecule cells by electrostatic interactions (Figure 1a). When Na2CO3 precipitator is dripped into the Ni2+/Co2+/Mn2+/yeast biosolution, the CO32− anions are combined with Ni2+/ Co2+/Mn2+ cations to further generate transition-metal carbonate bioprecipitates on the biotemplate of yeast cells (Figure 1b). In the biomineralization process, the active biomacromolecules serve as initial bionucleation hosts and regulate the growing of transition-metal carbonate nanocrystallites on the wall of yeast cells (Figure 1c). Such selfassembly of yeast cells induces the formation of nanocomposite microspheres by the structural template of the cells. The X-ray diffraction demonstrates that the bioprecursor has a phase structure matched well with MnCO3 (Figure S1). Finally, biogenic-LRLO is prepared by the subsequent calcining treatment of the transition-metal carbonate bioprecursor with LiOH. The biogenic-LRLO sample possesses with micron-sized spherical morphology, which is composed of aggregated mesoporous primary nanocrystallites, as illustrated in Figure 1d. To verify the advantage of the biofabrication on manipulating the hierarchically biogenic-LRLO microspheres, a blank sample (signed hereafter as blank-LRLO) was prepared without biotemplate of yeast cells under the same synthetic condition. XRD patterns of the as-prepared LRLO samples are indicated in Figure 2. Clearly, the diffraction peaks of both the samples are in good match with the typical structure of hexagonal layered α-NaFeO2 with space group R3̅m. The additional weak short-ranged superstructure reflections at 2θ = 20−25° are attributed to the presence of Li2MnO3 and the ordering of cations (Li, Ni, Co, and Mn) in the transitionmetal layer.32−34 In the meantime, the distinct splitting of (018)/(110) and (006)/(102) indicates that the oxides have a

Figure 2. XRD patterns of the as-prepared LRLO samples.

well-defined layered structure of LRLO materials,33,35 which are further confirmed by the HRTEM observation as shown in Figure S2. On the basis of the calculated data from XRD patterns, the intensity ratio (K) of the I(003)/I(104) for the biogenic-LRLO sample (K = 1.088) is higher than that (K = 0.962) of blank-LRLO, implying the lower degree of cation mixing in the biogenic-LRLO sample via the biomineralization,32 which is in favor of improving the electrochemical properties. In addition, X-ray fluorescence (XRF) spectra are performed to evaluate the molar ratio of transition-metals in the samples (Table S1). The molar ratio of transition-metals (Ni/Co/Mn) in two samples is almost identical to the target composition (0.14/0.13/0.56). Therefore, the well-crystallized LRLO (Li1.17Ni0.14Co0.13Mn0.56O2) with a typical layered structure can be obtained via the biomineralization of yeast cells. SEM images of the carbonate bioprecursor and biogenicLRLO sample via the biomineralization process are displayed in Figures S3 and 3, respectively. As shown in Figure S3, the transition-metal carbonate bioprecursor appears as an uniform spherical particle morphology, similar to yeast cells with particle size of about 1−2 μm. This microsphere secondary particle is composed of more fine aggregated nanoparticles. Here, Ni, Co, Mn, O, C, and P elements are evenly dispersed in the microsphere of the carbonate bioprecursor. The P and C elements in the bioprecursor are mostly detected from the biomass of yeast cells. After lithiation and calcination, the biogenic-LRLO sample still maintains a well-defined spherical particle morphology (Figure 3). Apparently, the secondary microspheres of the biogenic-LRLO are constructed by small primary nanocrystallites with particle size of 30−100 nm (Figure 3c). In addition, inner mesopores can be clearly observed within microspheres, and the average pore size is about 20 nm (Figure S4). The nanocrystallites can provide more electrochemical reactive sites, and mesopores can guarantee good electrolyte penetration into the active surface area. Obviously, after the calcination, the Ni, Co, Mn, and O elements show an uniform distribution in the single microsphere. However, C element are burned off, and P element still exist on the particle. As shown in Figure 4, the biogenic-LRLO sample presents hierarchically spherical particles, which are aggregated by dense and small primary nanocrystallites. The crystallite size of the primary nanocrystallites is about 100 nm, in consistent with the SEM observation. In Figure 4c, the biogenic-LRLO sample C

DOI: 10.1021/acsaem.8b01273 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 3. SEM (a−c) and STEM (d) images of the biogenic-LRLO oxide with elemental mappings (e−j) on the selected region.

Figure 4. TEM (a−d) and STEM (e) images of the biogenic-LRLO sample with P/O line-scanning (f) in a selected region.

biogenic-LRLO sample is 1.45 wt % as measured by XRF spectroscopy. Therefore, the few-layered black phosphorus can be formed on the surface of biogenic-LRLO sample, which is derived from the biomass of cellulate membranes,27,43−45 such as phospholipid bilayers (Figure S8). Accordingly, the fewlayered black phosphorus has high conductivity of electrons,46 and Li ions can be reversibly diffused without destroying the layered structure (Figure S9).36−38 Such in situ surface modification plays an key role for promoting the electrochemical reactivity and high-rate performance of the biogenicLRLO sample. The XPS spectra are also presented to determine the chemical state of transition metal on the surface for both the samples (Figure 5). It is demonstrated that the Mn 2p3/2 peak of the biogenic-LRLO sample is in 641.8 eV, a bit lower than that (642.1 eV) of the normal Mn4+ oxidation state of the blank-LRLO sample, showing the mixed valence state of Mn3+/4+ in the surface layer of the particle.47 Meanwhile, the peak of Co 2p3/2 is slightly moved to a lower energy (779.9 eV) in the biogenic-LRLO sample as compared to that (780.1 eV) of the blank-LRLO sample. The lower binding energy of Mn 2p3/2 and Co 2p3/2 in the biogenic-LRLO sample is associated with the local chemical environment change because of the modification of the black phosphorus on the particle

displays the typical layered structure with good crystallinity. In addition, a coated layer with a thickness of about 10 nm is distinguished on the nanocrystallite surface (Figure 4d). The lattice spacing of the well-defined layer is 0.52 nm (Figure 4d), which is in accordance with the few-layered black phosphorus materials.36−38 Accordingly, the presence of phosphorus on the nanocrystallite surface of biogenic-LRLO sample is also revealed by the regional chemical scanning in STEM-EDS model (Figure 4e and f). The alternating surface phosphorus and oxygen composition change is obtained in a selected region of the biogenic-LRLO. It means that phosphorus exists on the particle surface of the sample.32 To further identify the surface phosphorus and Li-rich layered oxide structure, Raman scattering is used here to measure the short-range microstructure.39,40 In Raman spectra (Figure S6), the peaks at 485 and 610 cm−1 are assigned to the Eg and A1g bands of the Li-rich layered oxides, respectively, demonstrating good layered structure in both samples.39−41 In addition, the biogenic-LRLO sample shows three vibration modes at 366, 435, and 468 cm−1, which are assigned to the characteristic bands of the A1g, B2g, and A2g of the black phosphorus,38,42 respectively. Moreover, the existence of elemental phosphorus is further verified by XPS spectra (Figure S7). Clearly, the black phosphorus content in the D

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Co4+), accompanying simultaneously with the extraction of Liions from the layered framework of the LiMO2 (M= Mn, Ni, and Co) component.50 The obvious anodic peak at 4.70 V is related mainly to the activation of the Li2 MnO3-like domain.51,52 The cathodic peak at 3.70 V is related to the subsequent reduction processes of Co4+ to Co3+ and Ni4+ to Ni2+, and the cathodic potential at 3.25 V corresponds to the reduction of Mn4+ to Mn3+.50−52 The weak reduction peak at 2.7 V in the initial cathodic process of the blank-LRLO cathode is caused by the residual spinel phase existing in the LRLO sample,40,51 which is disappeared in the following cycles. In the second and third CV curves, the anodic peak shift slightly to a lower potential of 4.05 V after the activation of Li2MnO3 component in the first CV cycle, while the obvious anodic peak around 4.70 V in the first CV cycle is almost vanished. Clearly, the current density of the reversible anodic/ cathodic peaks in CVs are larger for the biogenic-LRLO sample as compared with the blank-LRLO oxide, demonstrating an improved activity for the electrochemical insertion/ extraction processes in the biogenic-LRLO sample. In the charge−discharge curves (Figure 6c and d) tested between 2.0 and 4.8 V (vs Li/Li+), the biogenic-LRLO sample delivers the maximum discharge capacity of 318.7 mA h g−1 at 0.1 C rate (30 mA g−1) after the activation. By comparison, the maximum reversible capacity of the blank-LRLO sample is lower (267.3 mA h g−1) in the second cycle. The enhanced electrochemical activity of the biogenic-LRLO sample can be ascribed to the more active sites in nanocrystallites, and in situ surface modification by high-conductive black phosphorus on the primary nanocrystallites. Furthermore, the biogenic-LRLO sample shows an outstanding high-rate capability with discharge specific capacities of 286.3, 250.6, 227.1, 182.5, 163.3, and 134.5 mA h g−1 at various rates of 0.5, 1, 2, 5, 7, and 10 C rate, respectively (Figure 7). Especially, after 80 cycles at different current densities, the biogenic-LRLO cathode can still deliver a large discharge capacity of 310.7 mA h g−1 at low current density (0.1 C) with a high capacity retention of 97.6%, suggesting the good tolerance of various abuse uses to maintain good cycle stability. On the contrary, the discharge capacities at various

Figure 5. XPS spectra of the samples: (a) Mn 2p, (b) Co 2p, (c) Ni 2p, and (d) O 1s core level.

surface.48 The peak of Ni 2p3/2 is located in 854.7 eV for two samples, demonstrating the existence of Ni2+.51 As for the O 1s core level, the binding energy at 529.8 eV for the blank-LRLO is related to the lattice oxygen (M−O−M, M = Mn, Co, Ni),50 which is shifted slightly to a lower location at 529.6 eV for the biogenic-LRLO. Besides, a shoulder peak can be found at 531.7 eV in the biogenic-LRLO, which is a typical characteristics of the surface oxygen vacancies due to the oxidation of the lattice O2−, as well as the oxidation of chemically adsorbed species on the surface.29,50,51 These above XPS results suggest that the oxidation state of Mn and Co cations is slightly lower after in situ surface modification by black phosphorus, while Ni cations maintain unchanged in both the LRLO samples. The electrochemical performance of the LRLO cathodes are investigated by cyclic voltammograms (CVs) and galvanostatic charge−discharge tests, as shown in Figure 6. In the initial CV curve (Figure 6a and b), the anodic peak at about 4.17 V is ascribed to the oxidation of cations (Ni2+ to Ni4+, Co3+ to

Figure 7. (a) High-rate performance of the cathodes at various rates (0.1, 0.5, 1, 2, 5, 7, and 10 C). (b) Cycle performance of the cathodes at 2 C rate. (c) Variation in charge−discharge midpoint potential vs the cycle number at 2 C rate. (d) High-rate capability of the biogenicLRLO cathode.

Figure 6. (a and b) Cyclic voltammograms of the LRLO electrodes (0.1 mV s−1). (c and d) Initial three charge/discharge profiles of the LRLO electrodes at 0.1 C rate. E

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ACS Applied Energy Materials rates are much lower, and the capacity retention is only 85.1% after 80 cycles at various rates for the blank-LRLO oxide. When increasing the current density to 2 C rate for cycling, the biogenic-LRLO sample delivers a relatively large reversible capacity of 226.2 mA h g−1, and a high capacity retention of 94.5% after 200 cycles. However, the blank-LRLO cathode displays lower initial discharge capacity (176.4 mA h g−1) and a poor capacity retention of 76.0% at 2 C rate. During longterm cycling, the reversible capacity fading of the cathode is unavoidable because of an irreversible structure evolution from the hexagonal layered structure to a rock−salt or spinel phases.52 Compared to the reversible capacity decay, the charge/discharge potentials are more alert to the structure evolution in LRLO oxides during cycling.49 Here, the midpoint potential in the discharge curve is used to judge potential change for the electrode.45,49 As displayed in Figure 7c, for the blank-LRLO cathode, the midpoint potential in the discharge curves is dramatically decreased from 3.21 to 2.74 V with a fast drop of 14.64% over 200 cycles at 2 C rate. In comparison, the midpoint potential in the discharge curves of the biogenicLRLO cathode is slightly decreased from 3.24 to 3.19 V with a low drop of 1.54% over 200 cycles. In the meantime, the midpoint potential in the charge process increases smoothly for the biogenic-LRLO cathode because of the low polarization in the electrochemical reaction during cycles.17,19 In the charge curves of the blank-LRLO cathode, the midpoint potential is increased from 3.92 to 4.11 V with an obvious increase of 0.19 V after 200 cycles. In surprise, the increase of the charge midpoint potential for the biogenic-LRLO cathode is as low as 0.02 V over 200 cycles. It means that the charge/discharge potential difference for the biogenic-LRLO cathode is lower as compared with the blank-LRLO cathode over 200 cycles. The improved potential stability and slow capacity decay of the biogenic-LRLO cathode can be further confirmed by the typical discharge curves in different cycles, as demonstrated in Figure S11. On the contrary, the large potential/capacity drop is observed for the blank-LRLO cathode in 200th cycle. Apparently, the nanocrystallites, mesoporous structure and in situ surface modification of the biogenic-LRLO sample is feasible to suppress the fast decay of both capacity and potential during long-term cycling.51,52 The high-rate capability is desired for LRLO cathode materials.7,53 Figure 7d presents the cycling performance of the biogenic-LRLO electrode at high current density. Clearly, the biogenic-LRLO cathode delivers high and stable discharge capacities at various rates. It should be noticeable that after activation at different rates (from 0.1 to 5 C rate), the biogenic-LRLO cathode exhibits the high Coulombic efficiencies close to 100% within the following cycles, especially cycled at ultrahigh 10 C rate. After 270 cycles at varied rates, the cathode can still deliver a discharge capacity as high as 309.8 mA h g−1 at 0.1 C with a superior capacity retention of 97.2% (vs 318.7 mA h g−1 of the second cycle), enduring the various current densities to maintain good stability during the long-term cycling. The remarkable high-rate performance and long-term cycling feature of the biogenic-LRLO sample can be credited to the spherical particle morphology, hierarchical micro/nano assembly structure, and in situ surface modification by high-conductive black phosphorus. SEM images of the biogenic-LRLO cathode after 200 cycles at 2 C rate are provided in Figure 8. After long-term cycling, the spherical morphology of the LRLO secondary particle can be retained without obvious pulverization or cracks to

Figure 8. SEM images of the biogenic-LRLO cathode at (a) low magnification and (b) high magnification after 200 cycles at 2 C rate. (c) Concentrations of transition-metal cations in the electrolyte for the samples after 200 cycles at 2 C rate.

individual nanocrystallites. The maintaining of the integrated spherical morphology is in favor of stabilizing the SEI layer on the microsphere surface, and restraining subsequent severe deterioration of cathode on structure and composition.54 The concentrations of transition-metal cations in the electrolyte after 200 cycles at 2 C are measured (Figure 8c and Table S2). Obviously, the dissolved concentration of Mn/Co/Ni ions in the electrolyte for the blank-LRLO sample are serious, leading to the structure instability in the blank-LRLO sample. The surface of the blank-LRLO cathode undergoes serious adverse reactions with the electrolyte because of the large wetting area between the electrolyte and active nanocrystallites in the loose aggregates (Figure S5).15−17 As for the biogenic-LRLO sample, the microspheres with abundant mesopores are beneficial to accommodate the volume expansion in the charge and discharge processes, and in situ surface modification is good to slow down the dissolution of metal constituents in the electrolyte during electrochemical reaction.34 Figure 9 presents impedance spectra of the LRLO cathodes tested at completely discharged state in different cycles at 2 C rate. In the first cycle (solid square/black line), both the plots in the high-frequency region show a semicircle profile, which

Figure 9. (a and b) Impedance spectra of the LRLO cathodes in different cycles at 2 C rate. (c) The equivalent circuit used to fit the impedance data. F

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ACS Applied Energy Materials are assigned to the surface charge-transfer process.54 In the low-frequency region, the Nyquist plot of the blank-LRLO cathode presents as a sloped line, suggesting a semi-infinite Warburg diffusion process in the bulk.55 This is a typical characteristics of oxide cathode materials without any surface modification. Whereas, the Nyquist plot of the biogenic-LRLO cathode shows an arc-like profile in the low-frequency domain, which is ascribed to a finite Nernst diffusion process in a film.50,55 It implies that the semi-infinite diffusion and finite diffusion exist simultaneously in the electrochemical reaction of the biogenic-LRLO cathode.31,50 The finite diffusion process is attributed to the surface coating of few-layered black phosphorus on the primary nanocrystallites of the biogenicLRLO sample, which was also demonstrated in AlF3-coated LRLO sample.56 It means that the surface coating can stimulate the electrochemical reaction process and drive the process conversion from the semi-infinite diffusion (Warburg process) to the finite diffusion (Nernst process),56 which has a vital influence on the subsequent charge/discharge characteristics, especially the high-rate capability. As calculated from the EIS data, the surface charge-transfer resistances (Rct) of the biogenic-LRLO and blank-LRLO cathodes are 90.3 and 295.6 Ω, respectively. Here, the biogenic-LRLO cathode shows the lower charge-transfer resistances, implying an improved electrochemical activity on the microsphere surface. Furthermore, the biogenic-LRLO electrode presents a lower impedance (190.7 Ω) of Warburg diffusion (Wo), suggesting the rapid diffusion kinetics of Li ions across the primary nanocrystallites in dense microspheres.52 The finite Nernst impedance (Ws) in the biogenic-LRLO electrode is small (9.4 Ω), implying a lower diffusion barrier of Li ions through the surface phosphorene layer of the biogenic-LRLO sample. Hence, it makes sense that the high surface electrochemical activity in the primary nanocrystallites, and fast diffusion of Li ions in the thin layer and in the bulk are highly important to contribute the high-rate capability in the biogenic-LRLO electrode. After the first cycle for the initial activation, the electrochemical process is almost identical for the different cathodes based on individual EIS profiles. The charge-transfer and Warburg diffusion processes play the main part in the blankLRLO cathode. In the biogenic-LRLO cathode, the dimension of the semicircle at the high frequency domain decreases, and the arc-like profile at the low frequency is still maintained in different cycles, implying the superior structural stability of the surface phosphorus coating on nanocrystallites in the biogenicLRLO cathode upon the long-term cycling. It is noticeable that, the charge-transfer resistance of the biogenic-LRLO cathode is gradually decreased from first cycle to 100th cycle, and then increased slowly from 100th cycle to 200th cycle (Figure 9a). On the contrary, the charge-transfer resistance is dramatically increased for the blank-LRLO cathode (Figure 9b) from 50th cycle to 200th cycle. The high electrochemical activity of the biogenic-LRLO electrode insures the good capacity/potential stability during cycling. The nanocrystallization, high-conductive surface modification, and hierarchically micro/nano assembly of the LRLO cathode are critical to contribute the improvement in the electrochemical performance. In the biogenic-LRLO electrode, the close-packed secondary microspheres are favorable to obtain the higher tap density (1.94 g cm−3), and larger volumetric specific capacity for high-energy LIBs.19,57 The hierarchical micro/nano structure takes advantages of both

nanocrystallites and microparticles: (1) nanocrystallites can provide more electrochemical reactive sites to achieve large specific capacity. In the meantime, nanocrystallines can shorten Li-ion diffusion pathways for improving the high rate performance; (2) microspheres with abundant mesopores are beneficial to accommodate the volume expansion in the charge/discharge processes, whereupon to avoid the local structural degradation; (3) in situ surface modification by the few-layered black phosphorus with the good conductivity of electrons is in favor of the high-rate capability, and structure/ composition stability of the biogenic-LRLO during long cycling. Hence, to obtain excellent electrochemical performance, the fine structure design of hierarchical LRLO electrode materials can be enabled by the facile biomineralization technology.

4. CONCLUSION In conclusion, the biomineralization technology is an easy and effective method to manipulate the hierarchical micro/nano LRLO microspheres by employing yeast cells as both nucleation, growth and self-assembly template without using any organic agents. The uniform LRLO microspheres with a size range of 1−2 μm are aggregated by dense primary nanocrystallites with abundant mesopores. Such hierarchical micro/nano structure is helpful to improve the diffusion kinetics of Li-ions, as well as the good wetting/penetration of electrolyte. Synergistically, the primary nanocrystallites could afford more electrochemical reactive sites, and shorten Li ion diffusion pathway to improve the high-rate performance. The secondary microspheres with abundant mesopores could accommodate the volume expansion in the charge/discharge processes. Meanwhile, in situ surface modification by the fewlayered black phosphorus is in favor of the high rate capability and structure/composition stability during long cycling. As a result, the biofabricated hierarchical micro/nano LRLO cathode can meet the requirement of the large capacity, good high-rate performance, and stable cycle for desired cathode of LIBs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01273. XRD pattern and SEM images of the carbonate bioprecursor, SEM images of the blank-LRLO sample, pore size distribution and charge/discharge profiles of the biogenic-LRLO cathode, and TEM images and Raman spectra of all the samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel/Fax: +86-22-23500876. *E-mail:[email protected]. ORCID

Xue-Ping Gao: 0000-0001-7305-7567 Notes

The authors declare no competing financial interest. G

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ACS Applied Energy Materials

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ACKNOWLEDGMENTS This work is supported by the 973 Program (2015CB251100), NFSC (21421001) and Fundamental Research Funds for the Central Universities of China are gratefully acknowledged.

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