Screening PrecursorâSolvent Combinations for Li4Ti5O12 Energy

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Screening Precursor-Solvent Combinations for Li4Ti5O12 Energy Storage Material Using Flame Spray Pyrolysis Florian Meierhofer, Haipeng Li, Michael Gockeln, Robert Kun, Tim Grieb, Andreas Rosenauer, Udo Fritsching, Johannes Kiefer, Johannes Birkenstock, Lutz Mädler, and Suman Pokhrel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11435 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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ACS Applied Materials & Interfaces

Screening Precursor-Solvent Combinations for Li4Ti5O12 Energy Storage Material Using Flame Spray Pyrolysis ⊥

⊥

ζ

ζ, §

Florian Meierhofer, Haipeng Li, Michael Gockeln, Robert Kun, er,

φ, β

Udo Fritsching,

⊥, β

Johannes Kiefer,

χ, β

†

⊥, β

Johannes Birkenstock, Lutz Mädler,

Pokhrel

⊥

φ

Tim Grieb, Andreas RosenauSuman

⊥*

Foundation Institute of Materials Science, Department of Production Engineering, Universi-

ty of Bremen, Germany, ζInnovative Sensor and Functional Materials Research Group, Department of Production Engineering, University of Bremen, Germany, §Fraunhofer Institute for Manufacturing Technology and Advanced Materials, IFAM, Germany, φInstitute of Solid State Physics, Electron Microscopy, University of Bremen, Germany, χTechnische Thermodynamik, University of Bremen, Germany, †Central Laboratory for Crystallography and Applied Materials, University of Bremen, Germany, βMAPEX Center for Materials and Processes, University of Bremen, 28359 Bremen, Germany

E-mail contacts and phone number of corresponding authors: [email protected]

++49 421 218-51218

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ABSTRACT: The development and industrial application of advanced lithium-based energy storage materials are directly related to the innovative production techniques and the usage of inexpensive precursor materials. The Flame spray pyrolysis (FSP) is a promising technique that overcomes the challenges in the production processes such as scalability, process control, material versatility and cost. In the present study, phase pure anode material Li4Ti5O12 (LTO) was designed using FSP via extensive systematic screening of lithium and titanium precursors dissolved in five different organic solvents. The effect of precursor and solvent parameters such as chemical reactivity, boiling point and combustion enthalpy on the particle formation either via gas-to-particle (evaporation/nucleation/growth) or via droplet-to-particle (precipitation/incomplete evaporation) is discussed. The presence of carboxylic acid in the precursor solution resulted in pure (>95 mass%) and homogeneous LTO nanoparticles of size 4-9 nm, attributed for two reasons (1) stabilization of water sensitive metal alkoxides precursor and (2) formation of volatile carboxylates from lithium nitrate evidenced by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and single droplet combustion experiments. In contrast, the absence of carboxylic acids resulted in larger inhomogeneous crystalline titanium dioxide (TiO2) particles with significant reduction of LTO content as low as ~34 mass%. In-depth particle characterization was performed using X-ray diffraction (XRD) with Rietveld refinement, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry (TGA-DSC-MS), gas adsorption, and vibrational spectroscopy. High-resolution transmission electron microscopy (HRTEM) of the LTO product revealed excellent quality of the particles obtained at high temperature. In addition, high rate capability and efficient charge reversibility of LTO nanoparticles demonstrate the vast potential of inexpensive gas-phase synthesis for energy storage materials.

Keywords: Precursor-solvent combination, Flame spray pyrolysis, Single droplet combustion, Energy materials, Crystalline nanoparticles, Li4Ti5O12


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1. INTRODUCTION The zero-strain Li4Ti5O12 (LTO) based secondary Li-ion batteries were first commercialized in the 2000’s by SCiBTM (Super Charge ion Battery), a sister company of Toshiba.1 The non-lithiated LTO anode forms isostructural Li7Ti5O12 during electrode cycling via Li+ diffusion.2-4 Such diffusion can reach up to 3 moles of Li+ per unit formula (Li4+xTi5O12, where x ≤ 3) providing a theoretical energy storage capacity of 170 mAh/g. The lithium intercalation proceeds with high electrode integrity as it suppresses the mechanical degradation of the electrode. The high Li+ extraction/insertion voltage plateau at about 1.55 V vs. Li+/Li provides outstanding safety characteristics and good cycle performance.3,

5

Moreover, nanoscale LTO results in short diffusion

lengths for Li+ migration counteracting the drawback of low apparent chemical diffusion coefficients (~10-10-10-12 cm2/s).6 Highly porous and surface structured electrodes made of crystalline nanoparticles also provide a high solid/liquid interfacial area for efficient charge transfer and mechanical stability.7-8 There have been tremendous efforts in designing appropriate LTO particles for enhanced energy storage using different synthetic techniques including solid-state, hydro/solvothermal and sol-gel.9 However, these routes require several complex chemical processes, long reaction times, and expensive thermal post processing to obtain crystalline LTO. The flame spray pyrolysis (FSP) is an attractive method for large-scale production of ultrafine, pure, and highly crystalline oxide materials.10-16 Ernst et al. synthesized LTO particles from lithium tert-butoxide dissolved in tetrahydrofuran (THF) and titanium(IV) isopropoxide (TTIP) dissolved in xylene (Li/Ti ratios in the range of 0.5 to 1.0, [M]total = 0.8-1.6 M).17 The highest LTO purity of 90 mass% was obtained for a Li/Ti ratio of 0.8 with a total metal concentration of 0.8 M. Bresser et al. synthesized LTO nanoparticles with multiple phase impurities such as TiO2 (anatase and rutile) and Li3Ti3O7 using FSP.18 Waser et al. demonstrated size-controlled production of 3 ACS Paragon Plus Environment


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pure nano-crystalline LTO by controlling the quench gas entrainment of the flame spray reactor.19 The major drawback was the usage of expensive liquid lithium acetylacetonate and solvents. In another report, Kim et al. used inexpensive lithium nitrate (LNT) and TTIP dissolved in a mixture of ethyl alcohol and distilled water (volumetric ratio 3/7) with a total metal concentration of 0.5 M to prepare amorphous LTO particles.20 The amorphous powder transformed into the crystalline phase (23 to 300 nm) only after sintering at 600-800 °C. Similarly, Du et al. synthesized LTO particles in a pyrolysis furnace at 800 °C using lithium acetate and titanium butoxide dissolved in ethanol at stoichiometric ratio.21 The post-annealed LTO in air or N2 atmosphere resulted into pure or carbon mixed particles showing reasonable discharge capacities of 96 and ~70 mAh/g at room temperature, respectively. In another report, Birrozzi et al. produced LTO nanoparticles using various Li/Ti ratios from different precursor-solvent combinations.22 However, the purity of LTO did not exceed 86 mass%. The material contained multiple impurities such as TiO2, Li0.57Ti0.86O2, and Li2CO3 exhibiting a discharge capacity of 115 mAh/g at 50C. While flame spray pyrolysis is a fast and economic technique to produce LTO nanomaterials with good dis-/charge capacities compared to micro-sized particles,18 the formation of secondary phases along with the major LTO phase is not yet understood. It is hypothesized that the selection of precursors and solvents is a key to design phase pure and ultrafine LTO particles without applying any expensive post-treatments. Here, we aim at (1) designing LTO particles via systematic screening of precursorsolvent combinations, (2) understanding the effect of precursor chemistry on the nanoparticle formation, and (3) demonstrating the electrochemical performance of asprepared LTO nanoparticles. For this purpose, three different lithium precursors (lithium nitrate (LNT), lithium tert-butoxide (LTB) and lithium acetylacetonate (LAA)) and titanium isopropoxide (TTIP) were dissolved in five organic solvents (ethanol (EtOH), 4 ACS Paragon Plus Environment

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benzyl alcohol (BnOH), tetrahydrofuaran (THF), xylene and ethylhexanoic acid (EHA)). The precursor-solvent combinations were analyzed using infrared (IR) spectroscopy and the resulting nanomaterials were comprehensively characterized via highresolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) with Rietveld refinement, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry (TGA-DSC-MS), gas adsorption, and vibrational spectroscopic analysis. We show that this approach allows insights into the chemical mechanisms underlying the particle formation during FSP unravelling the direct relationships between the precursor-solvent chemistry and the electrochemical performance.

2. EXPERIMENTAL SECTION 2.1. Precursor-solvent preparation and nanoparticle production. Lithium nitrate (LNT; 99 %, Alfa Aesar), lithium tert-butoxide (LTB, 99 %, Alfa Aesar), lithium acetylacetonate (LAA, 99.9 %, Sigma Aldrich) and titanium isopropoxide (TTIP, 99.9 %, Sigma Aldrich) were used as lithium and titanium precursors, respectively. Similarly, five organic solvents namely ethanol (EtOH, 99.8 %, VWR Chemicals), benzyl alcohol (BnOH, 99 %, Sigma Aldrich), tetrahydrofuran (THF, 99 %, VWR Chemicals), xylene (99.5 %, VWR Chemicals) and 2-ethylhexanoic acid (EHA, 99 % Sigma Aldrich) were considered for dissolving the precursors. In the first step, the lithium and titanium precursors were dissolved separately in different solvents (Table 1). TTIP dissolved completely with all the five solvents. The resulting single metal combinations (TTIP-EtOH, TTIP-BnOH, TTIP-THF, TTIP-Xylene and TTIPEHA) were mixed with the lithium precursor solutions (LNT-EtOH, LNT-BnOH, LNT-THF, LTB-EHA and LAA-EHA). It must be noted that the LAA and LTB precursors separately mixed with four solvents (EtOH, BnOH, THF and Xylene). LNT with xylene and EHA gave rise to precipitation or colloidal particles. Hence, these precipitating and/or colloidal solutions 5 ACS Paragon Plus Environment


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were no longer considered for screening the spray solution. In the second phase of the precursor-solvent screening, TTIP-EtOH, TTIP-BnOH, TTIP-THF, TTIP-Xylene, and TTIP-EHA were separately mixed with LNT-EtOH, LNT-BnOH, LNT-THF, LTB-EHA, and LAA-EHA to obtain a stable and well dissolved solution. From 25 permutations and combinations, TTIPEtOH+LNT-EtOH, TTIP-EtOH+LNT-BnOH and TTIP-Xylene+LNT-THF mixtures gave rise to precipitation and/or colloidal solution 1-3 days after preparation [Figure S1 (a-c)]. Nanoparticle production was carried out in a custom made FSP reactor. The total metal concentration was kept constant at 0.5 M for all the precursor-solvent combinations. The liquids were fed with 5 mL/min into the reactor and dispersed into a spray using 5 L/min oxygen (99.95 Vol%, Westfalen) at a constant pressure drop of 1.5 bar corresponding to critical flow conditions.12 The spray was instantaneously ignited by a premixed methane-oxygen flame (1.5 and 3.2 L/min, respectively) supplied through the annular gap surrounding the spray (inner and outer diameters 10 and 10.3 mm, respectively). All gases were controlled with calibrated mass flow controllers (Bronkhorst, El-Flow). The powders were collected on glass fiber filters (Pall A/E 25.7 cm in diameter) at 60 cm above the nozzle using a vacuum pump (Busch SV 1025).23-26 2.2. ATR-FTIR measurements. A qualitative composition infrared analysis of all aspurchased chemicals utilized for precursor preparation was accomplished by using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy (Agilent Cary 630, wavenumber range 650 to 4000 cm-1, nominal resolution 2 cm-1, diamond (1 reflection) and ZnSe (5 reflections) internal reflection elements for solid and liquid chemicals, respectively). All single and bi-metal precursors and as-prepared flame-synthesized powders were investigated to derive possible reaction mechanisms taking place during precursor solvation. 2.3. Single droplet combustion. The precursor solution was pumped from a stack reservoir through the Teflon tube and passed to the single droplet generator.27 The generator was adjusted to a frequency of 4 Hz, with the velocity ranging from 0.5 to 1 m/s. The oxygen co6 ACS Paragon Plus Environment

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flow (0.4 L/min, 99.95 volume %) was used to provide sufficient oxidizing atmosphere for droplet combustion. While these droplets were ignited via spark electrodes, the spontaneous combustion followed by µ-explosion of the droplets was in situ recorded using a high-speed camera (Photron SA4, Type 500K-M1) at an imaging rate of 40,000 Hz. Residues/particles produced from the droplet combustion were collected on a copper grid located at 10 mm above the ignition electrodes for further analysis.28 2.4. X-ray diffraction (XRD) measurements. The mixed Li-Ti oxides nanoparticles were loaded on top of background-free Si-holders in D8 Advance diffractometer, equipped with a primary Johansson monochromator producing pure Cu-Kα1 (λ = 0.15406 nm) radiation, fixed divergence of 0.4°, primary and secondary Soller slit with 2.5° aperture, circular sample holder with 16 mm diameter, a slit of 0.2 mm in the position of the primary focus and a LynxEye position sensitive detector with 3° total aperture and 0.015625° channel width, applying a continuous scan in the range of 3-90° 2θ and an integration step width of 0.0118613° 2θ. The crystallite size and content of the LTO nanoparticles were determined from the Rietveld refinements of the XRD patterns using the BRASS program (Table S2).29 The refined parameters included scale factors, lattice parameters, crystallite size and microstrain parameters as well as texture using the (111) reference direction with the March-Dollase approach, and the atomic parameters of LTO in cases where it was the predominant or even sole phase. The structural models used were: Li4Ti5O12 (ICSD collection code 160655) with cubic cell and space group 3, anatase TiO2 (ICSD collection code 202242) with tetragonal space group 4 ⁄ , rutile TiO2 (ICSD collection code 159915) with 4 ⁄ , Li2CO3 (ICSD collection code 66941) and Li0.57Ti0.86O2 (ICSD collection code 8262) with space group , respectively. The quality of the Rietveld refinement was evaluated in terms of the R factors, i.e. Rwp, RBragg and the background corrected R’p (Table S2). A volume weighted average crystallite size (dXRD) and the root-mean-square (rms) lattice micro strain for each of the 7 ACS Paragon Plus Environment


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mixed Li-Ti-oxide phases was determined from the line-broadening analysis.10 The instrumental contribution to the peak broadening was taken into account during the full profile fitting by instrumental parameters derived from a fit of standard crystalline LaB6. 2.5. BET surface area measurements. BET measurements were carried out using a Quantachrome NOVA 4000e Autosorb gas sorption system. The powders were placed in test cells and allowed to degas for 2 hours at 200 °C in flowing nitrogen. The BET isotherm measurements were performed using nitrogen as adsorbent at 77 K and relative pressure P/Po over the range of 0.01-0.99. From the plot of [(P/Po)/(1- P/Po)] versus [P/Po] ranging between 0.05 and 0.3, straight lines were obtained with the correlation coefficient being greater than 0.999. The BET surface area is related to an average equivalent primary particle size given by the equation dBET = 6/(ρp·SSA), where dBET is the average diameter of a monodispersed particle calculated from the measured specific surface area of the powder and the theoretical density ρp. 2.6. TEM measurements. For the particle imaging using transmission electron microscopy (TEM), samples were prepared directly from the flame reactor via thermophoresis. The carbon and graphene coated copper grids (Plano) were exposed for 913 K). Reaction with carbon dioxide is conceivable and would explain the larger amounts of Li2CO3 detected by the powder XRD of flame spray samples, e.g., LNT-EtOH+TTIP-EtOH (17.5 mass%, cf. Figure 3 and 5). The morphology of the LTO nanoparticles obtained from different precursor-solvent combinations was studied using low and high-resolution TEM (HRTEM). A series of representative images of particles obtained from both single droplet combustion and FSP using LNT-ETOH+TTIP-EHA and LNTETOH+TTIP-EtOH are shown in Figures 8. The particle size distributions were measured on agglomerated ensembles of primary particles along their axes (bottom line of Figure 8). The 30 ACS Paragon Plus Environment

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LNT-ETOH+TTIP-EHA combination (1st and 3rd column of Figure 8) results in a mean primary particle diameter (dmean) of 17.5 nm and 9.5 nm for the single droplet and FSP (1st and 3rd column of Figure 8), respectively.

Figure 8. TEM images of particles obtained from (a) single droplet (SD) and (b) flame spray pyrolysis (FSP) for LNT-EtOH+TTIP-EHA and LNT-EtOH+TTIP-EtOH reveal homogeneous and inhomogeneous particle formation, respectively. In particular, a lognormal size distribution (bottom row: green histograms with solid lines) and a small geometric standard deviation (σg) are indicative for the gas-to-particle growth. Incomplete vaporization and combustion results in larger mean diameters (dmean) and σg (red histograms).



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The geometric standard deviations (σg) of 1.48 and 1.37, with narrow particle size distributions are the consequences of the gas-to-particle mechanism initiated by a fast precursor release due to droplet µ-explosions. In contrast, the combustion of LNT-EtOH+TTIP-EtOH resulted in multimodal particle size distributions with larger dmean (172 and 44.7 nm) and σg (3.32 and 4.81) for both single droplet and spray combustion, respectively (2nd and 4th column of Figure 8). However, tedious ex situ microscopic evaluation can be avoided by convenient online aerosol characterization to derive almost instant information on the particle size distribution and presence of larger particles from incomplete vaporization.60 In conclusion, the particle products obtained from single droplet and spray combustion are very similar with respect to size, crystallinity and phase purity. This clearly suggests the need for spray solutions with a high combustion enthalpy and metal carboxylic complexes in order to obtain ultrafine single crystalline particles.

3.9. Organic impurities of as-obtained Li4Ti5O12 nanopowders. Before electrochemical cell manufacturing, the mass loss of LTO nanoparticles from three different precursor-solvent combinations were analyzed and found to be very similar during heat treatment up to ~200 °C [Figure 9 (a)]. This is attributed to the loss of physically adsorbed combustion gases such as H2O, -OH and CO2 as indicated by the in-situ MS measurements in Figure 9 (b). Further heating of the nanoparticles obtained from LTB-EHA+TTIP-EHA and LAA-EHA+TTIP-EHA combinations reveal organic impurities oxidized at ~282 °C. This is evident from a sharp exothermic DSC-signal along with the formation of gaseous H2O, OH, CO2, and CO. In contrast, nanoparticles produced by the LNT precursor show weaker DSC-signals indicating almost no oxidation. The TGA-DSC-MS analyses showed that the particles obtained from LNTEtOH+TTIP-EHA combination have a higher degree in gravimetric purity (93.1 mass%) than those obtained from LTB-EHA+TTIP-EHA and LAA-EHA+TTIP-EHA (88.4 and 87.9 mass%, respectively). The TGA-DSC-MS results are in good agreement 32 ACS Paragon Plus Environment

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with the powder FTIR [Figure 9 (c)]. The IR bands occurring at ~2962, 2933, and 2879 cm-1 for LTB-EHA+TTIP-EHA and LAA-EHA+TTIP-EHA are due to -CH groups from unreacted solvent and/or precursors. The flame synthesized LTO powders have a high specific surface area and are therefore sensitive to surface reactions.

Figure 9. (a) Thermogravimetric and differential scanning calorimetric analyses (TGA-DSC) in synthetic air with (b) online mass spectrometer (MS) of gases evolved from Li4Ti5O12 particles. Desorption of physically adsorbed impurities, e.g., H2O, OH and CO2 results up to 200 °C. Particles based on LTB-EHA+TTIP-EHA (blue line) and LAA-EHA+TTIP-EHA (red line) show additional mass loss in the range between ~250-400 °C due to exothermic oxidation of organic contaminations which is in line with -CH stretch vibrations around 2962 cm-1 from powder FTIR (c). Powders prepared from LNT-EtOH+TTIP-EHA (green line) do not show exothermic DSC-signals and are free from organic contaminant.



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The major surface impurities in all the samples are water and OH groups indicated by the broad characteristic OH stretching and bending signatures at ~3400 and 1637 cm-1, respectively. Moreover, comparison of the stoichiometric equivalent ratio of the flame reactor with the integrated -CH signal in Figure S1 (g), S9 and 10 (b) suggests the degree of organic impurities in the LTO powders due to lack of oxygen, which could easily be optimized by lowering the equivalent ratio of the reaction (e.g., increasing dispersion oxygen or lowering liquid feed). It must be noted, that the three selected LTO nanoparticles have very similar primary particle diameters (8.9-9.5 nm), specific surface areas (162-169 m²/g), and LTO content (94.5-98.3 mass%) as illustrated in Figure S8, S10 (a) and 5 (a), respectively. Moreover, clear differences did result in the degree of organic purity [Figure 8, S9 and S10 (b)] which must be taken into account for analysis of the electrochemical performance in the following section. 3.10. Crystal structure of non lithiated, partially lithiated and fully lithiated Li4Ti5O12. The crystal structure of IVLi3VI(LiTi5)O12, first reported by Deschanvres et al. shows 3 out of 4 Li+ ions fully occupy Wyckoff site 8a coordinated tetrahedrally to four oxygen atoms.61 All the 8a sites are concurrently coordinated octahedrally to the fully occupied neighboring 16d site in the ratio of 1 Li to 5 Ti ions on average. From Figure Figure 10 (a)-(e), it is obvious that the unoccupied site 16c is just in between the two neighboring 8a sites and two neighboring 16d sites offering a pathway for Li migration during charging or discharging. Figure 10 (e) shows the fully lithiated structure of VILi6VI(LiTi5)O12 with a full depletion of the site 8a with concurrent full occupation of the octahedrally coordinated 16c site. Experimental and computational studies show intermediate solid solutions with simultaneous partial occupation of 8a and 16c would not exist.62-64


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Figure 10. The non-lithiated, partially lithiated and fully lithiated conditions of the LTO nanoparticles. The structure plots were derived from the XRD patterns of the pure LTO obtained from LNT-EtOH+TTIP-EHA precursor solvent combination. The XRD patterns were refined using standard ICSD collection code 160655. The structure plots were drawn using our own University of Bremen developed crystallographic BRASS program. (a) Projection of the nonlithiated structural state of LTO showing layers including the Li1 positions on site 8a. (b) Single layer selected from (a) being used for clarity in the [111] projections. (c)-(e) which are rotated by 90° from (b) about the horizontal axis. (c) layer b in the non-lithiated structural state, 8a-Li1 positions are tetrahedrally coordinated by oxygen in the corners (not shown explicitly). (d) partially lithiated state as described in the literature with randomly selected partial Li-occupations of 16c (33% orange = Li, 67% white = not occupied) and 8a (77% blue = Li, 23 % light blue = not occupied).53, 65 Arrows in (d) denote possible hopping pathways for Li in the depicted layer. However, further, symmetrically equivalent pathways are available in various directions inclined to this layer. (e) Fully lithiated structural state. 35 ACS Paragon Plus Environment


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In addition, a single crystal structure on partially lithiated Li5.35Ti5O12 showed both crystallographic sites are partially occupied (8a by 77 % and 16c by 33 %). However, it should be noted that this is an average picture on a single crystal with ~0.1 mm length; it could be a matter of discussion, whether or not this might be an average picture on a two-phase system in the single crystal consisting possibly two end-members. It is clear, that this cannot be evidenced from our data. However, the result of the single-crystal study are useful to visualize a possible partial, random occupation of sites 8a and 16c with nominal composition

IV

Li2.31VILi2VI(LiTi5)O12 in an intermediate

state of the structure during lithiation and/or de-lithiation [Figure 10 (d)]. As the sites are directly neighbored, the hopping of Li+ ions along pathways with consecutive 8a and 16c positions might be a sensible mechanism to understand Li migration into or out of the structure during charging or discharging. 3.11. Electrochemical characterization. The electrochemical performance of FSPsynthesized LTO nanoparticles obtained from three precursor-solvent combinations based on LNT, LTB and LAA was measured. The voltage profiles as a function of the specific capacity (1C rate with a potential of 1.0 - 2.0 V for the first, the second and the 450th discharge/charge cycle) show a curved character and a shortened plateau region for all the three LTO samples, typical for nanosized LTO [Figure 11 (a)]. The smooth potential drop at the beginning of the cell discharge in the range of 2.00-1.55 V is attributed to a single-phase reaction of the LTO. The small primary particle size and large specific surface area result in the expansion of the solid-solution region before the two-phase transition region, similar to the results obtained by Bresser et al. for the larger LTO (20-30 nm).18 In the two-phase reaction regime, the flat voltage plateau is established at a cell potential of 1.55 V, where Li-deficient (Li4Ti5O12) and Lirich (Li4+δTi5O12) phases coexists. The elongation of single-phase solid-solution region is due to the high surface-to-volume ratio of the ultrafine LTO particles. The progressive homogeneous coverage of each pristine LTO surface with Li4+δTi5O12 during lithiation gives rise to 36 ACS Paragon Plus Environment

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increased thickness of Li4Ti5O12/Li7Ti5O12 interface. The mismatch of the Fermi levels and/or band gap drives the Li+ penetration more and more into the particle for the formation of such interface66 and creates a flat plateau region which is observed in the voltage profiles.62 To understand this process, the crystal structure of LTO needs to be considered (cf. Figure 10). The insertion of Li+ ions into the (8a) sites is possible at the higher potentials and explains (1) the higher discharge capacity observed in the first cycles, and (2) dramatic drop in the cell potential above 60-65% SOC (state-of-charge) [Figure 11 (a)]. Further Li+ ions insertion into the partially lithiated structure above ~60 % SOC needs much higher activation energy resulting in higher over potential giving rise to fast voltage drop after the two-phase plateau region up to 100% SOC.67 The long-term cycling behavior of the three LTO samples synthesized from different precursor-solvent combinations was investigated via galvanostatic discharge/charge measurements. The LTO loading on the respective electrodes was corrected based on the mass loss data acquired from the TGA-DSC-MS measurement. Specific discharge capacities (DC) collected over 450 cycles at 1C, are presented in Figure 11 (b). The coulombic efficiencies22 are initially low for all the cells but approached >99.5 %, indicating a highly reversible discharge/charge reaction. After 450 dis-/charge cycles, the LTO obtained from LNTEtOH+TTIP-EHA, LTB-EHA+TTIP-EHA, and LAA-EHA+TTIP-EHA demonstrated reversible discharge capacities of 146.5, 141.8 and 144.8 mAh/g, respectively. The specific discharge capacity vs. number of cycles and the observed irreversible capacity losses for all the three materials during excessive cycling is divided into two characteristic regimes. The apparent initial cell formation phase (up to ca. 10-12 cycles) showed discharge capacity drop for all three particles followed by a gradually descending plateau region during excessive cycling. This effect might be due to the domination of irreversible chemical reaction between lithiated LTO particles and the constituents of the organic electrolyte (i.e. ethylene carbonate/diethylene carbonate and LiPF6).68 37 ACS Paragon Plus Environment


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Figure 11. (a) Dis-/charge curves at 1C-rate for the 1st, 2nd and 450th cycle for LTO samples prepared from LNT (green), LTB (blue) and LAA precursor solutions, each combined with TTIP-EHA. (b) Long-term cycling performance at 1C reveals slight reduction in the specific discharge capacity (DC) for the first cycles due to irreversible chemical reactions between LTO particles and electrolyte. (c) Voltage profiles vs. specific capacity for LTO containing electrode prepared from LNT-EtOH+TTIP-EHA precursor solution combination using different C-rates. (d) Rate performance with varying C-rates ranging from C/5 to 25C with LTO nanoparticles prepared by LNT (green square), LTB (blue circle) and LAA (red triangle) precursor solutions (each combined with TTIP-EHA). Error bars refer to the variation of three different electrodes. Only every 12th data point is shown.

The electrochemical reaction between lithiated LTO and the electrolyte displays a slight capacity loss in the initial phase of the long-term cycling [Figure 11 (b)]. While the surfaces of the nano-sized LTO particle possess -OH termination, the possible H+ substitution by Li+ in surface -OH further contribute to the irreversibility of the electrochemical reaction.68 The galvanostatic cycling showed 6-8% capacity reduction between 20th and 450th cycle. Similar to 38 ACS Paragon Plus Environment

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this observation, voltage profile curves for the 1st, 2nd, and 450th cycle showed slight capacity reduction in the plateau region [Figure 11 (a) and (c)]. The electrochemical side reactions and/or solid-electrolyte interphase69 formation are not related to capacity reduction in the plateau region since they occur at < 1.00 V (vs. Li/Li+). Such capacity reduction is related to (1) cell polarization via increased potential difference during cycling (2) different crystallographic site occupancies for the nano-sized LTO. Borghols et al. reported the composition at near-surface region of ultrafine LTO could reach or even exceed high Li+ levels driving the material for significant structural rearrangement followed by material degradation and mechanical failure of the electrodes, resulting in capacity reduction.70 However, this was not observed in our study. Figure 11 (c) presents the voltage profiles versus specific capacity for LNT-EtOH+TTIP-EHA based particles, derived from fresh electrodes at current densities of C/5, C/2, 1C, 2C, 5C, 10C and 25C for the 4th cycle of the corresponding C-rate. The discharge capacities (169.8, 164.1, and 169.0 mAh/g) at C/5 for LNT-EtOH+TTIP-EHA, LTBEHA+TTIP-EHA, and LAA-EHA+TTIP-EHA, respectively, are close to the theoretical capacity of LTO (175 mAh/g).4 Increase in the C-rate caused increased cell polarization for all samples, leading to slightly lower discharge capacities from the theoretical value. Such reduction in discharge capacity was moderate up to 10C, but significant at 25C for all the samples [Figure 11 (d)]. However, the relative comparison of dis-/charge rates at 10 and 25C for LNT-EtOH+TTIP-EHA revealed much higher discharge capacity than the other two samples. The TGA-DSC-MS data of the particles obtained from LNT-EtOH+TTIP-EHA showed 93.1 mass% residue compared to 88.4 and 87.9 mass% residues due to adsorbed H2O, OH, CO2 and CO for LTB-EHA+TTIP-EHA and LAA-EHA+TTIP-EHA combinations, respectively (Figure 9). The larger amounts of organic impurities from incomplete precursor combustion on the particle surfaces are in line with the lower discharge capacities at higher C-rates. The discharge capacity at 25C obtained from LNT-EtOH+TTIP-EHA (~100 mAh/g) is higher compared to the other two combinations (~69 and ~69 mAh/g, respectively) due to the obvi39 ACS Paragon Plus Environment


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ous reason of a cleaner LTO surface enabling efficient and reversible Li+ diffusion into the crystal structure. The slight variations in the electrode design, particle loading, and/or electrode layer thickness may have also partially influenced the discharge capacity loss at 25C (Table S3).

4. CONCLUSIONS The flame aerosol synthesis of customized LTO nanoparticles requires long-term stable precursor-solvent mixtures avoiding unwanted chemical reactions including precipitation and hydrolysis, which drives colloidal particle formation prior to the actual flame spray process. To identify suitable precursor-solvent combinations for the production of an optimal singlephase material (Li4Ti5O12 in the present work) and to understand their chemistry, five different solvents, three different lithium precursors and a single titanium precursor were chosen. Only 10 out of the resulting 20 single metal-solvent combinations were sufficiently stable and free from any chemical reactions. In the next step, out of 25 double metal combinations, 23 stable solutions were observed. The flame sprayed products of these combinations were comprehensively characterized. 13 materials had a purity of ≥ 93% LTO. The use of 2ethylhexanoic acid as a solvent for these 13 combinations produced an optimal mass% of LTO via a strong chelating property during the screening process. Moreover, it was discovered that the presence of carboxylic acid transforms inexpensive low-volatile lithium nitrate into a more volatile organometallic lithium complex which enabled gas-to-particle formation during flame synthesis and turned out to be an important criterion to reach high quality LTO nanoparticles. Three candidates (LNT-EtOH+TTIP-EHA, LTB-EHA+TTIP-EHA, and LAAEHA+TTIP-EHA) were chosen for an electrochemical performance analysis to evaluate their potential as energy-storage materials. The results revealed similar discharge/charge reaction kinetics during cycling and rate tests. The assembled half-cells were stable for 450 full cycles at 1C. A discharge capacity of 146.5 mAh/g for the LNT-EtOH+TTIP-EHA-based LTO was 40 ACS Paragon Plus Environment

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achieved. A test of the high rate capability showed comparable performances of the three samples. During long term cycling, a certain material degradation was observed giving rise to a slight capacity loss. The nitrate carboxylation found during our screening process, distinctly reduces the expenses of precursor materials during high temperature particle synthesis. The process is economically favorable to cover the upcoming demands of lithium based energy storage materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website: http://pubs.acs.org. Table S1 and Figure S1 detailing solution preparation, long-term stability of precursor-solvent combinations, photographs of Li4Ti5O12 particles obtained after flame spray pyrolysis, combustion enthalpy and boiling points of single and mixed solvents, equivalent molar ratio and [O2(fuel)stoich/ O2(fuel)real] data for different solvent combinations; Figure S2-S5 high- and low-frequency range FTIR spectra analysis of the pure precursors, solvents, single, multi-metal precursors-solvents combinations and as-prepared flame-made nanoparticles and precipitated LiNO3; Figure S6 (a) and (b), Table S2 XRD refinement data, mass% of the phase components, crystallite sizes. Flame sprayed LTO based particles, XRD refinements of all the patterns not shown in the main text, mass content of Li2CO3 impurity obtained from 25 precursor-solvent combinations; Figure S7 ATR-FTIR of LiNO3 in presence and absence of TTIP before/after heating; Figure S8 TEM overview and particle size distribution in the TEM grid; Figure S9 FTIR investigation of all the 25 LTO powders obtained after flame spraying; Figure S10 specific surface area and FTIR peak integration in the range of 2830-3000 cm-1; Figure S11 and Table S3 battery fabrication procedure and electrode specification after active material layering; Comparing the cost of expensive and inexpensive precursors for fabrication of pure phase 41 ACS Paragon Plus Environment


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Li4Ti5O12 particles; Figure S12 cyclic voltammetry at different scan rates showing redox profiles. Supporting “Single Droplet Combustion Videos” for in situ single droplet combustion of different precursor-solvent combinations as observed from high speed image recording.

ACKNOWLEDGMENTS We would like to acknowledge the support from Institutional Strategy of the University of Bremen “Ambitious and Agile”, German Excellence Initiative (DFG ZUK66/1). FM and UF acknowledge the German Research Foundation (DFG, Project ID: FR 912/33) and the Brazilian-German Collaborative Research Initiative on Manufacturing Technology (BRAGECRIM). HL would like to acknowledge the support from Chinese Scholarship Council (CSC). J.K., U.F. und L.M. thank the Deutsche Forschungsgemeinschaft (DFG) for initiating and partly supporting this research within the priority program SPP 1980 SPRAYSYN under the following grants KI 1396 / 6-1, FR 912 / 42-1 and MA 3333 / 14-1, respectively.” REFERENCES (1) Shimada, N.; Umehara, T.; Otsuka, M. 50 Kw Storage Battery System Applying Scibtm Batteries for Photovoltaic Power Generation Systems. Toshiba Rev. 2010, 65, 15-18. (2) Amine, K.; Belharouak, I.; Chen, Z.; Tran, T.; Yumoto, H.; Ota, N.; Myung, S.-T.; Sun, Y.-K. Nanostructured Anode Material for High-Power Battery System in Electric Vehicles. Adv. Mater. 2010, 22, 3052-3057. (3) Feng, X.; Zou, H.; Xiang, H.; Guo, X.; Zhou, T.; Wu, Y.; Xu, W.; Yan, P.; Wang, C.; Zhang, J.G.; Yu, Y. Ultrathin Li4Ti5O12 Nanosheets as Anode Materials for Lithium and Sodium Storage. ACS Appl. Mater. Interfaces 2016, 8, 16718-16726. (4) Kavan, L.; Grätzel, M. Facile Synthesis of Nanocrystalline Li4Ti5O12 (Spinel) Exhibiting Fast Li Insertion. Electrochem. Solid-State Lett. 2002, 5, A39-A42. (5) Han, C.; He, Y.-B.; Wang, S.; Wang, C.; Du, H.; Qin, X.; Lin, Z.; Li, B.; Kang, F. Large Polarization of Li4Ti5O12 Lithiated to 0 V at Large Charge/Discharge Rates. ACS Appl. Mater. Interfaces 2016, 8, 18788-18796. (6) Takami, N.; Hoshina, K.; Inagaki, H. Lithium Diffusion in Li4/3Ti5/3O4 Particles During Insertion and Extraction. J. Electrochem. Soc. 2011, 158, A725-A730. (7) Li, X.; Lin, H.-C.; Cui, W.-J.; Xiao, Q.; Zhao, J.-B. Fast Solution-Combustion Synthesis of Nitrogen-Modified Li4Ti5O12 Nanomaterials with Improved Electrochemical Performance. ACS Appl. Mater. Interfaces 2014, 6, 7895-7901. (8) Wang, Y.; Zhao, J.; Qu, J.; Wei, F.; Song, W.; Guo, Y.-G.; Xu, B. Investigation into the Surface Chemistry of Li4Ti5O12 Nanoparticles for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 26008-26012.


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Screening PrecursorâSolvent Combinations for Li4Ti5O12 Energy

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Screening PrecursorâSolvent Combinations for Li4Ti5O12 Energy