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Aerosol-assisted Synthesis of Colloidal Aggregates with Different Morphology: Toward the Electrochemical Optimization of Li3VO4 Battery Anodes Using Scalable Routes Pedro Tartaj, José M. Amarilla, and María B. Vázquez Santos Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b05018 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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Chemistry of Materials

Aerosol-assisted Synthesis of Colloidal Aggregates with Different Morphology: Toward the Electrochemical Optimization of Li3VO4 Battery Anodes Using Scalable Routes. Pedro Tartaj,* Jose M. Amarilla, Maria B. Vazquez-Santos Instituto de Ciencia de Materiales de Madrid, CSIC, Campus Universitario de Cantoblanco, 28049, Madrid, Spain. Email:[email protected] ABSTRACT: The improvement of properties through strict morphology control often requires the use of difficult to scale up synthesis routes. Thus, a compromise between scalability and morphology control is required to partially exploit the advantages of this control in materials functionality. Here we show that a scalable and continuous route (aerosol route) is able to produce Li3VO4 colloidal aggregates with different morphology (spherical and platelet-like) using easy to handle economic precursors (V2O5, LiOH and LiNO3 in stoichiometric amounts). The key for these differences in morphology resides on controlling the nature of the intermediate stages that can occur during particle formation in aerosol synthesis. We also show that the electrochemical response of Li3VO4 is strongly dependent on morphology. Thus, optimization of morphology allows building anodes that to our knowledge outperform other reported Li3VO4 anodes, and even compete with most of the reported Li3VO4/C composites at adequate high rates (2-8 A/g). Finally, we have developed a simple and scalable coating protocol (suspensions with solid concentrations of 100 g/L are used) that additionally improves the long-term stability of the optimized anodes. Combination of the two scalable methods leads to Li3VO4 anodes that operating at a safe cutoff voltage of 0.2V can retain a high capacity (280 mAh/g) with excellent coulumbic efficiency (> 99.9%), even after 500 cycles at a competitive rate (2A/g discharge-charge).

INTRODUCTION A fundamental drawback common to many synthesis routes that aim to produce powder-like solids with a highly controlled configuration is the lack of scalability.1 Both nucleation and growth mechanisms are sensitive, and scaling up can lead to results that are different to those obtained under controlled lab conditions.2 This lack of scalability severely restricts to the lab scale some of the advances observed in terms of functionality that are the result of controlled configurations. Perhaps finding the right balance between control and scalability is the most practical approach to develop synthesis routes. Developing scalable routes that ultimately could partially mimic the advantages of controlled configurations is thus of general interest.3 This is especially true for materials used in electrochemical devices that aim to efficiently handle clean energies. Among these technologies, Li-ion secondary batteries (LIBs) are currently the leading electrochemical energy storage technology, dominating the market for use in applications as portable consumer electronics or hybrid and electric vehicles.4 In these batteries there is interest in the replacement of graphite anodes for applications in which high capacity or safety are main concerns. Graphite with a theoretical capacity of 370 mAh/g inserts Li+ ions at potentials below about 0.2 V vs Li/Li+. Li electroplating is thus a significant safety concern, mainly at high rates, where accumulation of Li ions takes place on the solid electrolyte interface. In terms of capacity significant advances have been made in LixSi and LixSn alloys to accommodate the significant changes in volume (ca. 300%).5 Hovewer, cracking of the solid electrolyte interface (SEI) due to mechanical strains still represents an issue to solve to preserve the initial capacity from fading.6 Furthemore, in LixSi alloys insertion takes place at voltages around 0.2V. Thereby, Li electroplating represents an additional safety

concern in these alloys. Conversion electrodes have very pronounced charge/discharge hysteresis and lack of stability. Thereby, these conversion electrodes are still not suitable for replacing graphite as anodes in commercial batteries.5 TiO2 anatase and Li4Ti5O12 perhaps represent the safer and more established materials to replace graphite anodes.7 However, with a specific capacity around 200 mAh/g and an insertion potential around 1.5V, these anodes have very low specific energy (we should keep in mind that specific energy also depends on the potential difference between anode and cathode). Recently, Li3VO4 anodes based on the β polymorph have emerged as a possible alternative to graphite anodes.8–27 These anodes can reversible insert up to 2 Li ions (corresponding to a theoretical capacity of 395 mAh/g) at a safe but still low voltage window between 0.2 and 1V. Even recent studies suggest the possibility of a reversible insertion of a third Li ion but at potentials close to Li electroplating (0.02V).16,27 Furthermore, Li3VO4 as an intercalation anode undergoes small volume changes, thereby SEI cracking due to mechanical stresses is very limited.10 More specifically, undoped Li3VO4 anodes have been prepared by conventional solid-state reactions but stable capacities not higher than 280 mAh/g at very low rates (20 mA/g) were obtained.8,10 Solution precipitation, sol-gel and freeze-drying routes followed by thermal treatments led to poor performances at high rates (100 mAh/g at a rate of 2 A/g for the best electrodes).10,18–21 Finally, hydrothermal routes that are difficult to scale up have led to a significant improvement in performance (215 mAh/g at a rate of 1.6 A/g for the best electrodes).11,12 Addition of carbon or nitrogen-carbon in the form of composites (either as coatings or forming more or less interconnected networks) has been so far proven necessary to improve the performance of Li3VO4 anodes.13,14,16–20,22–27 Thus,

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in terms of performance and up to our understanding, the best electrochemical response has been reported for hollow Li3VO4 particles combined with reduced graphene oxide.17 However, hollow particles have intrinsically poor volumetric capacity, and besides the synthesis route involved partial etching of Li3VO4 (losses are not satisfactory for scalability). C/N combined doping has been also proved as a good combination to obtain high performance anodes but at cutoff voltages of 0.02V (close to Li electroplating).25,26 Perhaps, and to our understanding, the method that so far better matches scalability and performance, is the one that recently reported the preparation of carbon coated Li3VO4 anodes from the thermal decomposition of the organic precursor VO(C5H7O2)2 in combination with LiOH.20 In terms of scalability and continuity, aerosol spray methods represent a viable alternative to solution routes for the preparation of processable powder-like materials.25 These methods are relatively simple, continuous and can make use of easy to handle and economic reactants. The generated droplets constitute the reactors while there is the possibility to reach acceptable configuration control in the resulting powder-like products. Basically, this control can be reached by variations in solubility of reactants, temperature changes, presence of sacrificial agents or melting intermediates, and mixed sol/solution reactants.3,28–36 Furthermore, aerosol routes cannot only produce processable powder-like solids but also are routinely used as an efficient and rapid method to obtain thin-films.37–39 In fact, aerosol routes have been already used to prepare Li3VO4 powder-like materials.15 However, the results were discouraged as anodes prepared from these powders displayed poor performance even at a low cutoff voltage of 0.1V (at 0.2 C ca. 250 mA/g after the 2nd charge/discharge cycle and at 1C ca. 200 mA/g after the 2nd discharge/charge cycle). Here we show that aerosol routes can produce Li3VO4 colloidal aggregates with different morphology (spheres and platelet-like) starting from easy to handle and economic reactants (V2O5, LiOH or LiNO3). This morphology control allows building Li3VO4 anodes that clearly outperform the electrochemical capabilities of Li3VO4 anodes (either less scalable o with similar scalability) and previous aerosol studies, and even compete with Li3VO4/C composites. Furthermore, we show that a simple and efficient protocol involving concentrated solutions of active materials (100 g/L) can be applied to deposit carbonbased amorphous coatings on the Li3VO4 aggregates. Combination of the two scalable methods leads to anodes that operating at a safe cutoff voltage of 0.2V can retain a high capacity (280 mAh/g) with excellent coulumbic efficiency (> 99.9%) even after 500 cycles at a competitive rate (2A/g dischargecharge). EXPERIMENTAL SECTION Materials. All products were purchased from SigmaAldrich. V2O5 (99%), LiNO3 (Reagent plus), LiOH•H2O (ACS reagent, 98%), H2O2 (30 wt%), EtOH (96%), Poly(vinyl pyrrolidone (PVP, 25KDa). Distilled water was used in all experiments. CAUTION, despite the high purity of the V2O5, we found that some batches came with impurities (Na and Li bronzes that imparted a green coloration). Aerosol synthesis. The aerosol device used for the preparation of powders is schematically displayed in Figure S1 (see supporting information). The aerosol device basically consists of two furnaces set at different temperature to assure a better drying of the droplets before thermal transformations. The

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nozzle atomizer was specially designed with a moving needle to control the size of the droplets in order to obtain fine and relatively uniform powders. For Li3VO4 formation, the temperature of the first furnace was 300 °C while the temperature of the second furnace was 700 °C. Two different aqueous solutions were used for spraying. One aqueous solution was prepared by dissolving V2O5 and LiOH in the presence of some H2O2 with concentrations of V 0.3M, LiOH 0.9M and H2O2 0.3M. The other aqueous solution was prepared by dissolving V2O5, LiNO3 and LiOH in the presence of H2O2 with concentrations of V 0.3M, LiNO3 0.6M, LiOH 0.3M and H2O2 0.3M. In order to prepare these solutions, we firstly dissolved at 70 °C/2h in a closed vessel V2O5 and LiOH (V 0.3M, LiOH 0.3M, H2O2 0.3M). After cooling the solution, the necessary amounts of LiOH or LiNO3 salts (0.6M) were added to give the total concentration of 0.9 M in Li ions mentioned above for the different solutions. Importantly, H2O2 was added to increase solubility, and therefore efficiency, while the presence of LiOH in the solution containing LiNO3 was needed to assure dissolution of the vanadium oxide. These solutions were nebulized at 1.6 mL/min with an air pressure of 1.7 kg/cm2. Finally, the resulting particles were collected with a metal filter. Importantly, in all cases the reaction was quantitative. In terms of reaction yield referred to a stoichiometric amount of reactants, we obtained around 50% of efficiency operating under a continuous mode. This value could be further improved as we are severely limited by space constraints. For example, our device is configured in a vertical mode and furnaces are relatively short. Coating protocol. The carbon-based coating protocol involving the different Li3VO4 powders obtained by aerosols was as follow: 1 g of Li3VO4 was suspended in 7.5 ml of EtOH, after stirring for around 15 minutes, we added 2.5 mL of a poly(vinyl pyrrolidone)/EtOH solution (50 mg/mL). This slurry was then stirred overnight in a closed vessel set at a temperature of 80 °C, followed by drying under continuous magnetic stirring. The resulting solid was heated at 400 °C/2h in N2 atmosphere to develop the carbon-based amorphous coating. Importantly, we used poly(vinyl pyrrolidone) (PVP) as carbon-based source because is a polymer that has a very good solubility in EtOH, and Li3VO4 powders were soluble in water. Electrodes processing. Conventional composite film electrodes were fabricated by dispersing the different Li3VO4 samples (70 wt.%), carbon black (20 wt. %) (TIMREX SuperP) and the polyvinylidene fluoride binder (10 wt. %) (Mw ∼534000, Aldrich) in 1-methyl-2-pyrrolidinone (Aldrich). The resulting slurry was stirred during 24 h and casted onto the current collector (a cooper foil) using a doctor Blade (specifically the blade was set at a 200 µm height). Electrodes with ~1.5 mg/cm2 of active material were then dried at 80 ºC/2h and later at 120 ºC under vacuum overnight. Finally, the dried electrodes were transferred to an argon globe box (H2O content < 1 ppm) for cells assembly. Structural, textural and electrochemical characterization. Phase identification was performed by X-ray diffraction analysis using a Bruker D8 Advance instrument (CuKα radiation , 40 kV, 30 mA). The morphology of the nanomaterials was examined by transmission electron microscopy (TEM, 2000 FX2, JEOL) and field emission scanning electron microscopy (FE-SEM, Hitachi, SU 8000, Japan). High-resolution transmission electron microscopy (HR-TEM) studies were


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Figure 1: (A) X-Ray diffraction patterns for samples obtained from spraying solutions containing V2O5 and only LiOH or mixtures of LiOH/LiNO3. The pattern also shows a simulated diagram corresponding to the electrochemically active orthorrombic Li3VO4 β polymorph (JCPDS No. 38-1247). (B) SEM picture of samples obtained from spraying solutions containing V2O5 and only LiOH. (C) SEM picture of samples obtained from spraying solutions containing V2O5 and mixtures of LiOH/LiNO3. The scale bar for (B) and (C) is the same. (D) High-resolution SEM picture of the particles obtained from spraying solutions containing V2O5 and mixtures of LiOH/LiNO3. The picture clearly shows that the largest particles consist of aggregates of platelets. Importantly, the presence of small spots/grains in (D) is due to the Au coating added to obtain the high-resolution pictures.

carried out in a JEOL 3000F. The electrochemical properties of samples were evaluated against a lithium-metal foil (it also acts as reference electrode) using galvanostatic cycling at different discharge rates (Arbin-BT4 battery system) in a cell voltage window between 3 and 0.2 V vs Li/Li+. All the tests were carried out using coin-type cell configurations (size 28032). A Whatman BSF-80 glass fibber was used as separator and a 1M LiPF6 solution in anhydrous mixtures of ethylene carbonate and dimethyl carbonate (1:1 weight ratio) as electrolyte. Cells were assembled in an argon glove box (H2O content < 1 ppm). The electrochemical measurements were conducted under thermostatic conditions at 30 °C. Experiments were carried out at 30 °C instead room temperature because our temperature controller operates better at this slightly higher temperature. RESULTS AND DISCUSSION Aerosol synthesis and mechanism of particle formation. Details of the aerosol device and the experimental protocol used for the preparation of powder-like solids are given in the experimental section and the supporting information. Basically, two different solutions with Li3VO4 stoichiometry (Li/V = 3) were prepared for aerosol nebulization. V2O5 was the source of V ions for both solutions. The difference between these two solutions came from the source of Li ions. LiOH was used in one of the solutions while mixtures of LiNO3/LiOH (2:1 molar ratio) were used in the other solution. For this second solution, our ideal scenario involved having solutions with Li ions coming only from LiNO3 salts. However, LiOH in these proportions was required to dissolve the V2O5. The X-ray diffrac-

tion (XRD) patterns of the samples obtained from spraying these two different solutions correspond to the electrochemically active orthorhombic Li3VO4 β polymorph (JCPDS No. 38-1247, Figure 1). SEM pictures show morphology differences for samples obtained from LiOH or LiNO3 precursors (Figure 1). Thus, the use of LiOH precursors led to samples that consist of spherical particles with sizes within the colloidal regime (< 1 µm). According to HR-TEM, these spherical colloids consist of aggregates of crystalline subunits oriented in different directions (Figure 2). Thus, they can be considered as polycrystalline spheres. The addition of LiNO3 led to a significant portion of aggregates formed by, what appear to be, platelet-like crystalline subunits (Figure 1C and D). According to HR-TEM, these platelet-like crystals have similar orientation (Figure 2). Interestingly, even the largest platelet aggregates have sizes below about 1 µm (colloidal size), thereby suggesting that Li3VO4 formation also occurs at droplet level. Furthermore, the HR-TEM also shows that both platelets and spherical particles have (100) exposed facets. As better explained below this result is important when explaining the differences in electrochemical performance of the aggregates. Recently, aerosol-assisted molten salt synthesis has emerged as a suitable route to produce particles with an acceptable controlled in size, and a shape different to that of the typical spherical morphology observed in aerosol routes.36,40,41 Specifically, the presence of a molten phase in this aerosol route results in particles often with a shape similar to the equilibrium form found in nature.36 Interestingly, the morphology of



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Figure 2: Importantly, for a better display of all of the features in this Figure, parts (A), (B), (C) and (D) are shown again at high magnification in the supporting information (Figures S6, S7, S8 and S9). (A) HR-TEM images along with the corresponding low-resolution TEM images of uncoated spherical particles. The polycrystalline nature of the spheres is clearly manifested by the presence of different crystals oriented along different directions. Thus, the spherical particles can be considered as polycrystalline spheres. (B) HR-TEM images and the corresponding low-resolution TEM images of the uncoated platelet aggregates. Importantly, the different contrast observed in the HRTEM image for this particle reflects the present of different platelet-like crystals equally oriented but at different levels. This result confirms that some particles seen in high-resolution SEM (Figure 1D) were aggregates of platelet-like particles. Also importantly, the HRTEM also shows that both platelets and spherical particles have (100) exposed facets. As better explained below this result is important to explain the differences in electrochemical performance of the aggregates. (C) and (D) clearly show (especially by contrast with the uncoated samples) the presence of an amorphous carbon-based coating surrounding both spherical and platelet particles. Interestingly, the morphology is preserved after the coating.

the platelet aggregates here obtained resembles those of Li3VO4 particles prepared from solid-state or solution routes followed by thermal treatment (so in essence solid-state), but in our case with a more satisfactory control in size and shape, and a better scalability.8,10,16,18,2,23 Thus, on the basis of the mechanisms involved in aerosol molten studies, the plateletlike morphology observed in our studies when LiNO3 was present, seems to point to some melting occurring during the aerosol process. In order to obtain the Li3VO4 samples, the temperature in the second furnace of our aerosol device was set to 700 °C (see experimental section and supporting information for details). Importantly, when the temperature of the second furnace was set to a lower value (500 °C), we observed V2O5 and LiNO3 (Figure S2 in supporting). LiNO3 melts at a temperature of 255 °C, so the platelet morphology and the fact that LiNO3 was detected during the early stages of the aerosol process, suggest that the melting of LiNO3 can be responsible for the change in morphology. V2O5 melting seems to be excluded, as its melting temperature (700 °C) is much higher than the one corresponding to LiNO3 (255 °C). Besides, the Li3VO4 samples derived from LiOH led to spherical particles and V2O5 was present in both reactions. Thermal and XRD studies carried out on solid precursors obtained by simply drying the solution used for spraying support the presence of LiNO3 (endothermic peak at 255 °C, Figure S3 in supporting). This result confirms LiNO3 as the melted phase and probably the formation of LiVO3 as an intermediate prior to Li3VO4 formation. When the aerosol synthesis involved spraying

solutions containing only LiOH, the typical spherical morphology associated with drying followed by phase formation and densification was observed. Li2O melts at a temperature (1440 °C) well above that of our working temperature. Therefore, the formation of polycrystalline spheres is expected when only LiOH is used as Li3VO4 precursor. The mechanism involved in the formation of these two different morphologies is schematically represented in Figure 3. Importantly, in the aerosol molten studies, the melted phase coexists with the final crystalline phase during all stages of the aerosol process during heating. Here, Li salts are not added in excess (Li/V molar ratio = 3). The result as displayed in Figure 3 is that the last stages of the heating process can be similar to those of the classic mechanism of particle formation in aerosol synthesis. The control reached in previous studies involving the formation of particles of other compounds by aerosol-assisted molten salt synthesis (Fe2O3, NaInS2 and CoFe2O4) was better than that here obtained for Li3VO4.36,40,41 However, we are of the opinion that our route is easier to scale up. The aerosolassisted molten salt route used for the preparation of these other compounds involved a previous stage to the spraying process (the synthesis in solution of colloidal suspensions). Long-term stability of colloidal suspensions as required for scalability imposes very strict conditions. Furthermore, this method also used sacrificial salts that though feasible to recover add more steps to the process. Using, as we report here, solutions of commercial V2O5 and Li ion salts (LiNO3 or


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Figure 3: Schematic representation of the mechanisms proposed for the formation of spherical and platelet aggregates. (A) The formation of the Li3VO4 spherical particles occurs through the classic drying followed by phase formation and sintering stages. (B) For platelet aggregates and based on mechanistic studies of aerosol melting,36,40,41 melting seems to occur previous to the nucleation of Li3VO4. As better explained in the main text LiNO3 seems to be the melted phase. As also better explained in the main text, for the platelet particles here prepared there are some differences in mechanism with respect to other compounds obtained through aerosol in the presence of molten phases.36,40,41 Basically, in those studies the melted phase coexisted with the final crystalline phase during all stages of the aerosol heating process. Here in this study Li salts are not added in excess (Li/V molar ratio = 3). Thus, the melted phase is no longer present once Li3VO4 is fully formed. The result is that the last stages of the heating process can be similar to those of the classic mechanism of particle formation shown in (A), that is, densification can take place.

LiOH) in stoichiometric amounts directly for spraying, seem to lead to an aerosol method easier to scale up. Taking into consideration all of these arguments, we understand that the control reached in the sample derived from LiNO3 is acceptable, specially because the differences in morphology with respect to the sample obtained only using LiOH are clear (Figure 1). As we describe below, these differences in morphology led to different electrochemical capabilities. As mentioned eaarlier carbon-based additives usually lead to an enhancement in the electrochemical performance of Li3VO4. We have here developed a simple and effective protocol to deposit carbon-based amorphous coatings using highly concentrated suspensions of the active material obtained from the aerosol route (100 g/L, see experimental section for details). The HR-TEM pictures shown in Figure 2 clearly confirm that the use of this protocol resulted in the development of amorphous carbon-based coatings for both morphologies (corresponding to only 5 wt% of the total content). Figure 2 and Figure S4 in supporting information confirmed the stability of the samples after the coating treatment (morphology and crystallinity are preserved in both samples). Electrochemical characterization. Hereafter we describe the electrochemical performance for the uncoated and coated spherical and platelet aggregates. Galvanostatic discharge/charge curves at 0.08 A/g for the uncoated Li3VO4 anodes between 0.2-3 V vs. Li/Li+ are displayed in Figure 4. Both samples show the typical irreversibility (difference between the initial discharge and charge capacities) that is associated with anodes working at potential below about 1.0 V. This initial irreversibility is understood in terms of Li ions consumption from SEI formation and carbon black passivation. However, the irreversibility for the spherical aggregates is lower than that of the platelet aggregates (20 vs. 30%). As both samples have been prepared under similar conditions,

the higher irreversibility of the platelet aggregates suggests a more extensive SEI formation for this morphology (the platelet-like morphology exposes more surface than the spherical morphology). Importantly, as better explained below, after analyzing the electrochemical response in coated particles, differences in morphology seem not play a significant role. This higher irreversibility seems to anticipate a lower rate performance for the platelet-like morphology. Interestingly, the irreversibility for the spherical morphology is in the line to the best reported for Li3VO4/C composites (15-20%).14,17,20 Figure 4 also shows in discharge the presence of two more or less defined plateau regions (0.8-1 and 0.6-0.7 V) for spherical particles. The position of these plateaus are consistent with those reported for reversible insertion of Li ions in Li3VO4.8,11,16,20 For the platelet-like morphology, after the first cycle, the voltage plateaus are less defined, likely reflecting strong polarization processes associated with the irreversibility mentioned above. Finally, the slight differences on voltage profiles between the first and subsequent cycles are also similar to those reported for insertion of Li ions in Li3VO4.8,11,16,20 Rate performances for uncoated Li3VO4 samples are displayed in Figure 5. Clearly, the spherical morphology outperforms the platelet-like morphology in terms of rate capability. This result and the lower irreversibility mentioned above for the spherical particles suggest that the spherical morphology is less sensitive to SEI formation. In fact, the spherical samples clearly outperform the values reported for pure Li3VO4. Table S1 in supporting information shows a summary of the different synthetic routes and electrochemical capabilities for reported Li3VO4 and Li3VO4/C composites. For scalable solidstate reactions,8,10 stable capacities not higher than 265 mAh/g



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Figure 4: Galvanostatic discharge/charge curves (0.2-3 V vs. Li/Li+) corresponding to the first 4 cycles at 0.08 A/g for the uncoated Li3VO4 spherical and platelet-like morphologies. From black to yellow, the lines correspond to the first and 4th discharge cycles. From red to dark green, the lines correspond to the first and 4th charge cycles.

at low rates (0.04 mA/g) were obtained, while the spherical colloids here prepared show at 0.08 mA/g capacities around the theoretical value (400 mAh/g). For less scalable routes such as solution precipitation, sol-gel and freeze-drying routes that require additional thermal treatments, the best samples show capacities of 100 mAh/g at a rate of 2 A/g.10,18–21 For the spherical samples, the capacity at this rate is about 310 mAh/g. When compared to the less scalable routes (hydrothermal methods),11,12 the spherical colloids also outperform hydrothermal routes (310 mAh/g at 2 A/g vs. 215 mAh/g at 1.6 A/g for the best of the hydrothermal samples). Finally, the anodes here prepared also outperform previous studies that reported the aerosol synthesis of Li3VO4.15 At 0.08 and 0.4 A/g these studies reach capacities of 250 and 200 mA/g, respectively after the 2nd charge/discharge cycle at a cutoff voltage of 0.1V. In the samples here prepared the capacities at these rates are 400 and 360 mAh/g, respectively at a higher cutoff voltage (0.2V). Probably, this increase in performance could be partially due to the fact that in those studies, crystallinity was developed by thermal annealing of samples in a conventional furnace.15 Interestingly, the uncoated spherical aggregates also show good rate capabilities when compared with Li3VO4/C composites at competitive rates (2, 4 and 8 A/g). The uncoated spherical samples prepared in this study have capacity values derived from the rate experiments of 310, 275 and 200 mAh/g at 2, 4 and 8 A/g, respectively (Figure 5). These values are superior to those obtained for some Li3VO4/C composites (Table S1).13,16,22 The values are comparable to some solid and hollow (intrinsic low-density) Li3VO4/C composites (Table S1).9,14,18,19,23 However, the values of the uncoated Li3VO4 spherical samples are lower to those recently reported for carbon coated Li3VO4 anodes prepared from the thermal decomposition of the organic precursor VO(C5H7O2)2 in combination with LiOH (Table S1).20 The values are also lower than

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Figure 5: Rate performance at full discharge of uncoated and coated spherical and platelet-like morphologies. For comparison with literature data and assuming 1C (400 mAh/g or ≈ 2 Li) the rates go from 0.2C (0.08 A/g) to 20C (8 A/g).

those obtained by a C/N combined doping. 25,26 However, in these last cases the cutoff voltages were 0.02V (close to Li electroplating) instead 0.2V as in our study. Finally, the values are also lower than those obtained for hollow Li3VO4/C composites (intrinsic low-density) prepared from low-scalable routes involving selective etching of the interior (Table S1).14,17 Interestingly, in some of these studies involving hollow composites, capacities of 205 mAh/g at 50C are obtained.17 These values confirm the good intrinsic Li ion insertion properties of the orthorhombic Li3VO4 β polymorph. Prior to analyze the stability of the uncoated Li3VO4 samples, it is interesting to analyze the rate performance of spherical and platelet-like morphologies after coating. This analysis, among others helps us to better illustrate their different performance. After the coating, the rate performance for the platelets significantly improves (Figure 5). Figure S5 in supporting information showed that even the initial irreversibility lowers (20% vs. 30% for uncoated platelets). In fact, after coating the voltage profiles at a rate of 0.08 A/g were similar to the samples with spherical morphology (Figure S5). All of these data suggest that in the platelet aggregates, the carbon-based coating prevents degradability favoring Li ion diffusion. Likely, the coating also improves the electronic contact, as surface exposition in platelet-like morphologies is significant. The fact that after the carbon-based coating the electrochemical response for the platelet morphology is significantly improved (approaching that of spherical particles), seems to discard effects associated with the specific index of the exposed facets. This result is not surprising as the Li3VO4 structure displays channels for Li ion diffusion in several directions (Figure 6), and more specifically along the [100] direction. According to the HR-TEM shown in Figure 2 both spherical and platelet aggregates have exposed facets (100) perpendicular to this direction. On contrast to the rate performance for platelets, the rate performance for the spherical morphology after coating remains similar (Figure 5). This result is likely associated with the fact that in the uncoated spherical particles degradability was already limited. As mentioned above the initial irreversibility for the uncoated spheres was similar to the best Li3VO4/C composites that have been reported, which suggests


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Figure 6: Projections of the Li3VO4 structure along the directions of the principal axes. We can clearly see that the structure shows channels for Li-ion insertion along different directions associated with the principal axes. More specifically, it shows channels along the [100] direction. According to the HR-TEM shown in Figure 2 both spherical and platelet aggregates have exposed facets (100) perpendicular to this direction.

very limited degradability. Furthermore, the contact with the carbon-based coating is probably restricted to the outer regions of the sphere. The overall result is that in spherical particles the improvement with coating at least in short-time studies is not observed. Some support for this hypothesis comes from studies on composites Li3VO4/carbon nanotubes synthesized from hydrothermal methods and subsequent partial etching of the interior.14 Theses studies clearly show a better performance for hollow than for solid Li3VO4 particles. Basically, a large part of the Li3VO4 in solid particles is not in contact with the reduced graphene oxide. If degradability plays also some role in the spherical morphology, the effects must be seen after long-time cyclability experiments. Thus, we carried out long-term cyclability studies at 2 A/g (discharge/charge) for the uncoated and coated spherical morphologies and only the coated platelet-like morphology (uncoated platelet-like samples were excluded as their

Figure 7: (A) Normalized discharge cyclability studies at a rate of 2 A/g (discharge/charge) for uncoated and coated spherical samples, and coated platelet samples. (B) Discharge/charge cyclability

data and Coulombic efficiencies at a rate of 2 A/g (discharge/charge) for the coated spherical particles

capacity at 2 A/g was much lower, 80 mAh/g). The results of these long-time experiments are shown in Figure 7A. The best stability is clearly observed for the carbon-based coated spheres, thereby confirming that the coating indeed protects against degradability even in samples with slow degradation. Finally, Figure 7B shows the cyclability performance and coulombic efficiencies for the best of the samples (carbonbased coated spherical particles) at a competitive high rate and at a safe cutoff voltage of 0.2V. These samples that were prepared from scalable routes show excellent stability. After 500 cycles at 2 A/g (discharge/charge) the samples retain a capacity of 280 mAh/g with coulombic efficiencies > 99.9%. These values are comparable to Li3VO4/C composites prepared from less scalable routes.14,17–20 CONCLUSIONS In summary, we have found that a scalable route (aerosol route) can be used to prepare Li3VO4 colloidal aggregates with an acceptable morphology control starting from easy to handle and economic precursors (V2O5, LiOH and LiNO3). This control allows obtaining spherical and platelet-like morphologies that show different electrochemical capabilities associated with degradability. We have found that the spherical morphology not only electrochemically outperforms reported Li3VO4 anodes obtained from less scalable synthesis routes, but also compete with most of the reported Li3VO4/C composites at high rates (2-8 A/g). Finally, we have developed a simple yet scalable coating protocol (suspensions with solid concentration of 100 g/L are used) that additionally improves the longterm stability of the Li3VO4 spherical colloids. Thus, we have been able to build anodes that operating at a safe cutoff voltage of 0.2V can retain a high capacity (280 mAh/g) with ex-



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cellent coulumbic efficiency (> 99.9%), even after 500 cycles at a competitive rate (2A/g discharge-charge).

ASSOCIATED CONTENT Supporting Information. Table S1 summary of previous studies on Li3VO4 anodes. Figures S1 aerosol device. Figure S2 to S5 Xray diffraction, thermal analysis and electrochemical curves. Figures S6 to S9 high magnification images of Figure 2.

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT Financial support from Ministerio de Economia y Competitividad (SPAIN) under project MAT2014-54994-R is acknowledged.

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