Electrostatic Spray Deposition Derived Anode materials for Lithium Ion Battery Application
Electrostatic spray deposition (ESD) technique is based on using an electrostatic field by applying a high DC voltage to form and accelerate atomic level low viscosity liquid droplets from the tip of a nozzle as a result of electrostatic force. The ESD technique provides a simple and versatile method for generating a rich variety of morphologies, such as dense, porous and fractal, and single or multi-component materials. By using ESD method, various materials, such as: SnO2, Fe2O3, V2O5 and Li4Ti5O12, were prepared for the application of lithium ion batteries. In this review, the fabrication procedure, material properties, and electrochemical performance of the ESD derived anode materials will be reviewed and discussed.
Electrostactic spray deposition; anode; lithium ion batteries; SnO2; Fe2O3; V2O5; Li4Ti5O12
These days, the requirement of advanced energy storage device increases year by year []. In comparison with other rechargeable batteries, such as lead-acid battery, Ni-Cd battery, Ni-MH battery, the currently commercialized lithium ion batteries have the higher voltage (3.6 V of the nominal voltage), higher energy density and specific energy (125 Wh kg-1), and longer cycle life (more than 1000 cycles) [,]. Nowadays, lithium ion batteries are considered the most promising energy storage technologies for mobile electronics, electric vehicles and renewable energy systems operating on intermittent energy sources such as wind and solar [, ]. A typical lithium ion battery consists of active cathode and anode materials, a porous polypropylene membrane separator acting as electrically insulation, and an intervening electrolyte of LiPF6 in a mixture of free-H2O solvent. In a commercialized lithium ion battery, the layer LiCoO2 and the layer graphite are used as cathode and anode materials, respectively. As it is depicted in 1, during charge process, the electrochemical potential difference between the anode and cathode drives the Li+ ions to move from layer LiCoO2 and simultaneously intercalate into graphite anode internally through the electrolyte, coupling with negatively charged electrons to keep overall charge neutrality. On the other hand, Li+ reverse direction on discharge, leaving the anode, and intercalate the crystal structure of cathode [,]. The whole electrochemical insertion/extraction process is a solid-state redox reaction involving electrochemical charge transfer coupled with Li+ insertion/extraction into/from the structure of an electronic and ionic conductive cathode or anode. During the whole charge/discharge process, only Li+ move between the cathode and the anode again and again. Therefore, lithium ion battery has also been called a “rocking-chair battery” [,].
As the rapid development of electronic devices, especially electric vehicles (EV) and hybrid electric vehicles (HEV) expected to partially replace conventional vehicles and help to solve the problems of air pollution and climate change, high energy density and high power density lithium ion batteries are urgently needed [,,]. The creation of high performance electrode materials is critical for creating high performance lithium ion batteries, for example, the total capacity of lithium ion batteries is expressed as follows []:
(1)where CA and CC are the theoretical specific capacities of the cathode and anode materials, respectively, and 1/QM is the specific mass of other cell components (electrolyte, separator, current collectors, case, etc.) in g mAh-1. In term of various cathode materials, such as LiCoO2, LiNiO2, LiMn2O4, LiFePO4, LiCo1/3Ni1/3Mn1/3O2, their specific capacities are too limited (less than 200 mAh g-1), and no way was performed to increase their theoretical capacities. Metallic lithium as anode material has high specific capacity and energy density. Lithium can be electroplated onto the anode during the charge process, leading to in the growth of dendrites due to the non-uniform current density on the surface of lithium anode. These growing dendrites contact the cathode electrode, an internal short occurs, which causes the combustion of the lithium. Therefore, lithium anode shows severe safety problems . For this reason, carbon based anode was currently commercialized. But it has the limited theoretical capacity of 372 mAh g-1 to form LiC6 intercalation compound. Provided that the specific capacities of anode materials are increased up to 600 mAh g-1, the total battery capacity can be distinctly enhanced (see 2) . Therefore, recently many attentions have been attempted to obtain anode materials providing higher discharge capacity for carbon replacement, such as Sn [,], SnO2 [,], transition metal oxides (such as: Fe2O3 [,], CoO [,], CuO [,]), etc.
According to their electrochemical mechanism with lithium during cycles, anode materials can be divided into three groups: intercalation/de-intercalation, alloying/de-alloying, and conversion reaction. The first one is mainly related with carbon based anode [,]. The second one includes SnO2, Si and Sn anodes etc [,]. The last one is referring to transition-metal oxides or nitrides or phosphides or sulfides (MxNy, M=Ni, Co, Cu etc., N=O, N, P, S etc.) [,,,]. The first group anode has already been commercialized. Due to the low capacity of graphite, the last two group anodes with high specific capacities have been intensively investigated. However, their poor cycle performances severely hinder their application in lithium ion batteries. For example, SnO2 anode reacts with lithium as following []:
SnO2 + 4Li+⇒Sn + 2Li2O
Sn + xLi+ + xe−⇔LixSn (0≤ x≤ 4.4)
Equation (2) is irreversible. Followed by the irreversible reaction, metallic tin reacts with Li to form various LixSn (x≤4.4) alloy, such as Li0.4Sn, LiSn, Li2.3Sn, Li3.5Sn, and Li4.4Sn []. In order to achieve the high cycling efficiency and the good cycle life, the movement of Li ions in SnO2 anode should not change or damage its crystal structure. Lithium atomic radius (2.05 Å) is far higher than Sn (1.72 Å) []. When the maximum 4.4 Li react with Sn to form Li4.4Sn, such large atomic uptakes (440% rise in the number of atoms) can induce large volume change (359%) []. The repeated huge expansion/contraction causes the great stress in Sn lattice, which results in the cracking, crumbling and pulverization among Sn particles and the consequent loss of electrical contact between Sn anode and current collector. Therefore, SnO2 anode has poor cycle performance. Tens of years ago, people thought that the transition-metal oxides have no intercalation/deintercalation sites in their crystal structures, furthermore, they cannot form some alloys with lithium during charge and discharge, and so these metal oxides have long been disregarded as possible reversible anode materials for lithium ion batteries. Till 2000, Tarascon and co-workers were the first to propose the rock salt structured transition-metal oxides as anode materials for lithium ion batteries []. Transition metal oxides, such as Fe2O3, NiO, CoO, Co3O4, and CuO, exhibit reversible capacities about three times larger than those of commercialized graphite. They work in a mechanism of the conversion reaction, i.e.
which is different from the intercalation/de-intercalation or alloying/dealloying mechanism. During cycles, nanoscaled transition metals are formed due to the electrochemical grinding, for example, obtained Cu particle size is only 2 nm when Cu2O anode was discharged [], therefore, these nanosized transition metal are easy to aggregate and degrade their electrochemical performance. On the other hand, during the charge-discharge process a large volume expansion always occurs due to the generation of electrochemical non-active Li2O so that the active materials may break off from the current collector, leading to obvious capacity fade [].
Electrostatic spray deposition (ESD) initially developed by Schoonman and coworkers at Delft University of Technology is a unique preparation method of various films []. Because of its simple and low-cost setup, non-vacuum, low temperature deposition condition, and good control of the film composition and morphology, ESD technique is superior to other film formation methods such as sputtering, CVD and sol-gel. Just as its many advantages, ESD has already been employed to create anode films with different morphologies including dense, fractal-like porous or sponge-like three-dimensional cross-linked porous structure []. Particularly the 3D sponge-like films with high porosity and a narrow pore size distribution are mechanically strong, beneficial to suffer the large volume change of anode materials during charge/discharge process. It is also expected that Li+ transport property of 3D sponge-like porous anode films is faster than in the films of other morphologies due probably to their high surface area []. ESD also makes possible the preparation of an anode film without the use of extra binder and conductive materials, improving the energy density of lithium ion batteries. Clearly, ESD-derived anode films are favorable for the application in lithium-ion batteries. In this review, we review ESD technique was employed to prepare various anode materials with different morphology. We focus on the review on the electrochemical properties of different morphology anodes. The relationships between film morphologies and deposition conditions such as solvent composition of the precursor solutions and substrate temperature were investigated.
2. ESD process
3a schematically shows the ESD experimental set-up. Its set up consists of a nozzle connected with syringe as means of liquid supply, a heated substrate, and a DC high voltage power supply. During the process of films deposition, it includes five steps as following : (1) spray formation: When a DC high voltage is applied to the metallic nozzle, an electrostatic field is immediately set up across the nozzle and the grounded substrate. A precursor solution is atomized into charged droplets by the electrohydrodynamic force. (2) droplet transport, evaporation, disruption; under the coulombic force, droplets move towards the heated substrate, and solvent evaporate due to the heated substrate. Evaporation of the solvent results in shrinkage of the droplet. The total charge on the surface of droplet does not change, so the shrinkage of droplet causes the increase of its charge density. A charged droplet may be disrupted into a few smaller droplets after reaching a maximum attainable charge density. (3) preferential landing: the induced charge distribution on the surface of the grounded substrate generally is not uniform. Therefore, the electric field on the local curvature of the surface is stronger than at other places. When a charged droplet approaches the surface, it will be attracted more towards these more curved areas. (4) discharge, droplet spreading: When a charged droplet contacts with the surface of the substrate or the earlier formed layer, it starts to discharge by transferring the charge to the grounded substrate either immediately or through the layer to the substrate. The relative value of interfacial tensions between the substrate and ambient gas, between the substrate and the drop liquid, and between the drop liquid and ambient gas determine the spreading rate of droplets: S = γsv-γsl-γlv. If S < 0, only partial wetting occurs with equilibrium reached at a finite contact area. If S≥ 0, the drop spreads until it completely covers the surface. The value of S is intimately related to the spreading rate. Therefore, the choice of substrate will affect the spreading rate of the liquid droplet, and may finally affect the morphology of the layer. (5) decomposition and drying of the precursor salt: the decomposition and reaction (either partial or complete) of the solute may have occurred before the droplets reach the substrate, which is expected if the surrounding temperature is high enough and dried droplets have been formed. Rearrangement of these dry particles on the substrate surface by surface diffusion is not expected at moderate deposition temperatures less than 500 oC used in this ESD experiment. In this case, a grain-like structure is expected to be formed instead of a very dense morphology.
3. (a) Schematic of the ESD experimental set-up. (b) Processes involved in ESD: (1) spray formation; (2) droplet transport, evaporation, disruption; (3) preferential landing; (4) discharge, droplet spreading; (5) decomposition and drying of the precursor salt.
(a) (b) []
In the ESD technique, a parent solution is sprayed onto a substrate by means of electrostatic force. The surface of the conducing substrate is controlled at a certain temperature where evaporation and chemical reactions occur simultaneously to form the deposited film. In this technique, many parameters can have some influence on the morphology of film, such as: a high voltage, the distance between nozzle and substrate, the flow rate and viscosity of precursor solution, substrate temperature. At low deposition temperatures or using a high boiling point solvent, the solution chemistry including the precipitation process and the pyrolysis or reaction of the solutes are also important parameters. Therefore, we can positively modify the parameters to tailor the morphology of deposited films, for example, Gao et al. investigated the influence deposition temperature, substrate materials, and solvent composition on the morphology of La0.7Ca0.3CrO3-δ []. They prepared the La0.7Ca0.3CrO3-δ films with porous reticulated models, cagelike particles, and interconnect nanowire structure. With increasing substrate temperature, the porosity of La0.7Ca0.3CrO3-δ film remarkably decreases; the pore and pore-wall sizes both became smaller. When changing the substrate material from nickel, aluminum, and alumina substrates to copper substrate, the microstructure of La0.7Ca0.3CrO3-δ film converted from porous reticulated model to cagelike particle model. They thought it's due to the bigger contact angle on the copper substrate than that on other substrates. Moreover, by changing the solvent composition, the layer morphology may also be significantly changed from a porous reticulated model to interconnect nanowire structure.
3. ESD derived anodes
2.1. SnO2 anode
SnO2 as a high capacity anode material has been extensively investigated since 1997 . However, its high capacity is undermined by poor cyclability in applications that continues to beset the material even up to today. Many significant research efforts have been devoted to improve the cycle performance of SnO2 anode . In our group, we used ESD technique to create SnO2 films with different morphologies as anodes of lithium ion batteries. By using glycol based precursors, as-deposited SnO2 has 3D reticular-like porous structure on nickel foam ( 4a). The cross-section image shown in the inset reveals the film thickness is about 20 um. After an ethanol based precursor was employed, dense SnO2 films were deposited on nickel foam ( 4b) and nickel foil ( 4c). Clearly, the precursor solution has an important influence on the morphology of films. The different precursor solution leads to the different evaporation rate of solvent during the flight of droplets towards the substrate, causing the different morphology of deposited films.
4. SEM images of different SnO2 thin films. (a) A 3D porous film from glycol precursors on nickel foam; (b) a dense film from ethanol precursors on nickel foam; (c) a dense film from ethanol precursors on nickel foil. The insets are the corresponding cross-sectional views respectively.
We investigated the cycle performance of SnO2 anodes with different morphology. In the first cycle, compared with the two dense SnO2 films, the porous SnO2 film shows a higher initial capacity loss ( 5a,b,c). It is because the large specific surface area of 3D porous SnO2 expenses more lithium, and more irreversible SEI layers are formed in this process []. After 100 charge/discharge cycles, the specific capacity of porous SnO2 is about 689 mAh g-1, corresponding to 94.8% of the second capacity. However only 452 and 390 mAh g-1 were delivered for dense SnO2 on nickel foam and nickel foil, corresponding to only 63.9% and 55.5% of initial capacities, respectively. 5e presents rate capabilities of three SnO2 anodes. It shows that discharge capacities decreased with the increase of rates for all three SnO2 anodes. The 3D porous SnO2 film displays the best capacity retention and the highest discharge capacity. Therefore, 3D porous SnO2 film improves its rate capability as well as the cycle performance. We thought the high porosity and large surface area of porous SnO2 positively contribute to Li+ diffusion, leading to an improved rate capability []. In the AC impedance measurements ( 5f), the charge transfer resistance of the 3D porous SnO2 is significantly less than that of the dense SnO2 on nickel foil. The 3D porous structure leads to a smaller charge transfer resistance and thus reduces the total battery resistance. More importantly, the porous structure offers a “buffer-zone” to accommodate the large volume change upon cycles, leading to more tolerance to stress cracking. This is well supported by SEM pictures of SnO2 anodes after 10 cycles at 0.5 C ( 6). Clearly, stress cracking is observed for dense SnO2 films on both nickel foam ( 6b) and nickel foil ( 6c) substrates. After 10 cycles, porous SnO2 film still maintains its previous morphology. In comparison to the dense SnO2 films, the highly interconnected pores also facilitate most of the surface area to be readily accessible for the liquid electrolyte. Therefore, the porous SnO2 anode can improve its electrochemical performance.
5. Voltage profiles of 3D porous SnO2 on nickel foam (a), dense SnO2 on nickel foam (b) and dense SnO2 on nickel foil (c) electrodes cycled between 0.01 and 3V (vs.Li+/Li) at 0.5 C. Capacity retention (d) and rate capability (e) of three SnO2 thin-film electrodes cycled between 0.01 and 3V (vs. Li+/Li). (f) AC impedance spectra of three fully discharged (0.01V) SnO2 electrodes cycled at 0.5 C after 50 cycles. (—△—) 3D porous SnO2 on nickel foam, (—○—) dense SnO2 on nickel foam and (—□—) dense SnO2 on nickel foil.
6. SEM images of the fully charged (3.0 V) thin-film electrodes after 10 cycles at 0.5 C: (a) 3D porous SnO2 film on nickel foam; (b) dense SnO2 film on nickel foam; (c) dense SnO2 on nickel foil.
Chen et al. used ESD method to create the Li2O-CuO-SnO2 composites as anodes of lithium ion batteries. These composites consist of hollow porous spheres that are randomly arranged. The mean diameter of the spheres is about 5 um. The magnified images reveal that the porous spheres consist of a multideck-cage structure, where the thickness of the “grids” ranges from 60 nm to 100 nm. The diameters of pores are not uniform, ranging from 200 nm to 1 um. These morphologies are totally different from that of 4. They stated it's due to the different precursor solution. Sn acetate based precursor can form the three-dimensional reticular porous structure. But Sn nitrate based precursor can cause hollow porous sphere. Exactly, the precursor solution has an important influence on the morphology of films. In 7e, after 100 cycles the reversible discharge capacity is 1158.5 mAh g-1 (98.7% of the first discharge capacity) for the Li2O-CuO-SnO2 electrode, but only 560.8, 718.1, and 823 mAh g-1 for the SnO2, CuO-SnO2 and Li2O-SnO2 electrodes. The latter three capacities correspond to 48 %, 72 %, and 74% of their first discharge capacity. It is noted that the Li2O-free SnO2 displays the worst capacity retention, while CuO-SnO2 is better than Li2O-free SnO2 but worse than Li2O-SnO2. Among the four samples, the ternary Li2O-CuO-SnO2 electrode displays the best capacity retention and highest discharge capacity. This ternary system also shows the excellent rate capability ( 7f); it exhibits 875 mAh g-1, when first cycled at 1C, and the following capacity values at other C-rates: 700 mAh g-1 at 3C, 650 mAh g-1 at 5C, 575 mAh g-1 at 7C, 525 mAh g-1 at 8C, and back to 775 mAh g-1 at 1C again. They thought the outstanding high capacity, capacity retention, and rate capability of the Li2O-CuO-SnO2 are related with its special multideck-cage morphology and the ternary composition. The nanostructured particles shorten the transport lengths for both electrons and lithium ions, while the unique porous structure ensures a high electrode-electrolyte contact area and confers the ability to accommodate the volume change during charge/discharge processes.
7. SEM images of as-deposited thin films on Cu foil substrate: (a) SnO2, (b) Li2O-SnO2, (c) CuO-SnO2, and (d) Li2O-CuO-SnO2. The inset pictures are FESEM images. Cyclability and rate capability of tin-based composite oxide films. (e) Capacity retention of the four thin-film electrodes cycled between 0.01 and 3 V versus Li+/Li at a cycling rate of 0.5C and (f) rate capabilities of a Li2O-CuO-SnO2 thin film.
Wang et al. also employed ESD technique to prepare the amorphous SnO2-SiO2 composite anodes []. They stated the formation of the reticular porous structure consists of different physical and chemical processes. Atomized droplets reach the substrate surface and gradually spread out. The substrate temperature at the sides of the droplet is higher than that at the center, making the solvent at the edge of the droplet occur evaporate more quickly than at the center []. Moreover, the addition of Si(OMe)4 in the precursor can increase the viscosity of the droplet on the substrate, hence hindering the aforementioned spreading process and resulting in more reticular porous morphology in corresponding as-deposited films.
8. SEM images of SnO2 films with 0%-15% of SiO2 content. (a) 0% of SiO2 content. (b) 5% of SiO2 content. (c) 10% of SiO2 content. (d) 15% of SiO2 content.
They tested the cycle performance of SnO2-SiO2 anodes. In the curve of discharge capacity versus cycle number ( 9a), the pure SnO2, SnO2-SiO2 (5%), SnO2-SiO2 (10%), SnO2-SiO2 (15%) exhibit the retentive capacities of 174, 242, 356, and 501 mAh g-1 after 100 cycles, respectively. During cycle, the formed Sn metal reacts with lithium to form LixSn alloy, causing large volume change ( 9b). The inactive matrix of SiO2 can provide space in which the reactants are dispersed, which hinder the aggregation of nanosized Sn particles. For amorphous SiO2, the Si-O bond is so strong that it cannot be easily broken by Li, so the SiO2 matrix facilitates the diffusion of Li. The porous SnO2-SiO2 composite can effectively accommodates volume changes during charge/discharge ( 9b). Therefore, SnO2 films with 15% of SiO2 showed an initial reversible capacity of 879 mAh g-1 and the best cycle performance.
9. (a) Cyclability of tin-based composite oxide films (capacity retention of the four thin film electrodes cycled between 0 and 3 V). (b) Schematic illustration of the role of inert SiO2 matrix.
3.2 Transition metal oxide anodes
Since Tarascon et al. for the first time proposed transition-metal oxides as anode materials for lithium ion batteries , many investigations have been focused on these anode materials . During charge/discharge, nanosized transition metals formed due to the electrochemical grinding are easy to aggregate. A large volume expansion also occurs due to the generation of electrochemical non-active Li2O so that the active materials may break off from the current collector. They cause the obvious capacity fading. To improve the cycle performance of transition metal oxides, some studies have proposed the synthesis of materials with a three-dimensional network structure that have high surface area and good ionic conductivity [, ].
Chen et al. prepared porous Fe2O3 films at different temperatures (170 oC, 200 oC and 240 oC) by electrostatic spray deposition []. They found all three films exhibit a reticular three dimensionally porous structure ( 10). They stated that the porous structure is related to the solvent evaporation and decomposition of inorganic salts. Furthermore, the films contain two types of micropores with different pore sizes, the larger one ranging from 5 to 10um, and the smaller one about 1um on the wall of the cross-linking structure. The thickness of the film can be clearly seen as about 30 um from the cross section image of 200 oC-deposited thin films (see 10d). Compared with the bulk Fe2O3, all of these films exhibit relatively better capacity retention. The 200 oC-deposited film exhibits excellent electrochemical behaviors. It has both high reversible capacity (1080 mAh g-1), and it still retains 896 mAh g-1 after 40 cycles. Thus, there is about a capacity loss of 0.45% per cycle. Most importantly, it shows the highest energy efficiency with the lowest initial capacity loss ever reported for Fe2O3 anode. In comparison with the SEM image of 200 oC-deposited thin film before and after 20 cycles, the porous structure seems in good condition and there is even no visible crack on the surface of thin film. They conclude that the cycling performance of Fe2O3 electrode is enhanced by the porous structure because it can effectively buffer the volume expansion and shrinkage during cell cycling.
11. (a) Voltage profiles of the Fe2O3 films in the first discharge-charge cycle. (b) Specific capacity and Coulombic efficiency vs. cycle numbers for the Fe2O3 thin films deposited at different temperatures: 170 oC (square), 200 oC (cycle) and 230 oC (triangle). (c) Ex situ SEM image of 200 oC-deposited film cycled for 20 cycles. All of the cells were cycled between 0 and 3V at a current density of 270 mA g-1.
Chen et al. also investigated butyl carbitol based solvent to create the reticular CoO-Li2O composite anodes []. Interestingly, the film morphology strongly depends on the precursor solution. The acetate based solution can produce the reticular structure ( 12b), but only dense for nitrate based solution ( 12a). The thickness of the reticular film is about 30 mm. They employed XRD to verify the nanocrystalline CoO and amorphous Li2O in the reticular film. They mentioned the reticular CoO-Li2O film showed a good cycle performance. From the second cycle to the 100th cycle, a gradual increase capacity was observed. The specific capacity is 781 (1st cycle), 679 (2nd), 673 (10th), 695 (20th), 779 (80th), and 788 mAh g-1 (100th). They stated the rise in capacity is also partly attributed to the increase in Co valence. In comprising with dense CoO-Li2O, the reticular film showed excellent rate capability due to a smaller impedance, reaching a specific capacity of 650 mAh g-1 when cycled at a 5C rate. They also prepared the nanoporous cuprous oxide/lithia composite anode []. This composite also has good cycle performance and rate capability. Clearly, the porous transition metal oxide can effectively enhance its cycle performance.
12. SEM images of the substrate and the CoO-Li2O thin films. (a) Surface morphology of a dense film from nitrate precursors. (b) Surface morphology of a reticular film from acetate precursors. The electrochemical performance of the CoO-Li2O thin-film electrodes cycled between 0.01 V and 3 V (vs. Li+/Li). (c) The capacity-cycle number curves of the CoO-Li2O films at a cycling rate of 1C. (d) The discharge capacity of the CoO-Li2O films as a function of the cycling rate (0.1C-5C), (■=reticular film, ○=dense film; C=capacity)
On the basic of Li2O-CuO-SnO2 composites proposed by Chen et al., Guo et al. also used ESD to synthesize the three-dimensional Li2O-NiO-CoO composite anode. They employed acetate based precursor solution, and sponge like porous (not hollow porous spheres) was obtained for this ternary composite (see 13). It is clear that Li2O-NiO-CoO film from glycerol/butyl carbitol solution show a three dimensional network structure with uniformly monodispersed round pores. The size of the pores is in the range of 2-3 um. The “grids” consist of sub-walls, which are approximately 200 nm in thickness, and sub-holes, which are around one micrometer in diameter. In term of the Li2O-NiO-CoO film from ethanol/butyl carbitol solution, the film texture is significantly different. A three-dimensional network structure with belt-like crossovers was formed. The holes are not uniform, and the belts are less than one micrometer in thickness. After 25 cycles, they found that the three-dimensional network structures were maintained for both thin-film electrodes. Therefore, the porous structure can effectively improve their electrochemical performance. During charge/discharge, the existence of the Li2O component in the composite anode may prohibit the growth of both CoO and NiO . It is supported by the curves in 14. Both Li2O-NiO-CoO films from glycerol/butyl carbitol and ethanol/butyl carbitol solution have high specific capacities, good cycle performance and rate capabilities.
15. (a) Cycling behavior of the Li2O-NiO-CoO composite thin-film electrodes cycled between 0.01 and 3V versus Li+/Li at a cycling rate of 0.5 C: (○) using glycerol/butyl carbitol as solvent; (●) using ethanol/butyl carbitol as solvent. (b) The reversible capacity of the Li2O-NiO-CoO composite thin-film electrodes as a function of the cycling rate (0.1-5 C): (○) thin-film using glycerol/butyl carbitol as solvent; (●) thin-film using ethanol/butyl carbitol as solvent.
3.3 V2O5 anode
Vanadium pentoxide (V2O5) as anode of lithium ion batteries has poor cycle performance during cycling []. Many efforts are underway to stabilize the structure for better cycleability, for example, low-temperature synthesis and the incorporation of transition and non-transition metals into the V2O5 structure [,]. By means of ESD, the porous V2O5 films were prepared in air in lieu of oxygen as in the case of other expensive vacuum techniques such as sputtering, PLD, and electron beam evaporation []. The deposited V2O5 film at the substrate temperature of 200 oC has a very porous nature ( 16A). Moreover, annealing of V2O5 film at 275 oC did not significantly change the nature of porous, which indicates the good thermal stability of porous V2O5 films. The X-ray diffraction pattern of the deposited V2O5 film exhibits an amorphous nature without any V2O5 diffraction peaks. With annealing, diffraction peaks start emerging annealing. The growth of the diffraction peaks can be ascribed to an increase in crystallinity. Further, this also indicates that the increase in annealing temperature of the deposited films results in an enhancement of the in-plane orientation of the V-O-V chains []. The synthesized V2O5 film exhibits a high stable charge-discharge capacity of 270 mAh g-1 at C/5, and delivers 260 mAh g-1 even when the current rate was increased to the 1C rate (see 16D). The delivered capacities show no fading even after 25 cycles. The good cycle performance is due to the porous nature of synthesized V2O5 films, which is beneficial for the lithium intercalation/de-intercalation during charge/discharge.
16. (A) SEM photographs of as-deposited V2O5 films at 200 oC. (B) X-ray diffraction patterns of V2O5 films: (a) as-deposited; (b) annealed at 220 oC; (c) annealed at 225 oC; (d) annealed at 250 oC; (e) annealed at 275 oC. (C) Capacity (mAh g-1) vs. cycle number for deposited V2O5 films: (a) deposited; (b) annealed at 200 oC; (c) annealed at 225 oC; (d) annealed at 250 oC; (e) annealed at 275 oC. (D) Galvanostatic cycling of V2O5 films annealed at 275 oC with current rate of: (a) C/5; (b) 1C.
3.4 Li4Ti5O12 anode
During charge/discharge process, spinel lithium titanate Li4Ti5O12 involves two phases (spinel Li4Ti5O12 and rock salt-type Li7Ti5O12) having the same symmetry, leading to the near zero change in the unit cell volume [,]. This anode has also other advantages, such as: flat Li insertion voltage, excellent reversibility during charge-discharge cycles, and good safety characteristics [,]. Therefore, spinel lithium titanate Li4Ti5O12 has attracted great attention as an anode material of lithium ion batteries [,,]. The main obstacle of Li4Ti5O12 is, however, its poor rate capability. One effective way for the rate-performance enhancement is to reduce its particle size. Smaller particle size means shorter diffusion length and higher surface reaction sites, which would lead to improved lithium intercalation kinetics [,,,]. ESD spurs the formation of porous nano-Li4Ti5O12, improving its rate capability. Porous Li4Ti5O12 was prepared by ESD technique with lithium acetate and titanium butoxide as the precursors []. Before and after the 700 oC annealing, both Li4Ti5O12 films have a very porous fractal-like morphology ( 17a and b). The ‘branches' in the annealed film are thinner than those in the film before the annealing. No change was observed after annealing, indicating ESD derived Li4Ti5O12 has good thermal stability. It is obvious that the optimal annealing temperature is 700 oC, which leads to the best cycling performance with a coulombic efficiency of nearly 100%. The initial capacity of ESD derived Li4Ti5O12 anode is close to this theoretical value. After a few cycles, a very stable capacity of about 150 mA g-1 is achieved during the subsequent cycles ( 17c and d). Its coulombic efficiency can keep at nearly 100%.
17. SEM pictures of Li4Ti5O12 thin films before (a) and after (b) 700 oC annealing. (c) Coulombic efficiency of electrochemical cycling of the Li4Ti5O12/Li cells in which the Li4Ti5O12 thin film electrodes were deposited at 300 oC but annealed at different temperatures, i.e. 650, 700, 750, 800 and 850 oC. (d) charge (curve a) and discharge (curve b) capacity versus cycle number of a Li4Ti5O12 thin film/1M LiPF6 in (EC-DEC, 1:1 v/v)/Li cell at a cycling rate of C/18. The coulombic efficiency is also shown in the graph (curve c).
3.5 Non-metal oxide anodes
ESD was usually employed to prepare metal oxide anodes for lithium ion batteries. Some metallic anode can be synthesized by using this technique. Valvo et al. investigated Sn and Sn-Co alloy as anodes for lithium ion batteries by using ESD technique []. Sn particles prepared by solution based precipitation method are strongly agglomerated and not homogeneous in size. The primary particles are spherical and have sizes in the range of 30-300 nm. ESD derived Sn powder exhibits a large surface area, and that the particle size distribution is quite narrow (See 18a). All the particles are spherically-shaped and loosely agglomerated. The typical particle size is estimated around 40-50 nm. In 18b, the primary particle size of ESD derived Co-Sn alloy is below 10 nm and is merely amorphous this is agreement with XRD result ( 18c). However, Co-Sn alloy showed a poor cycle performance ( 18d). Authors thought it is due to a more or less pronounced oxidation of the nanoparticles during the washing and the final collection procedures.
(a) SEM image of Sn powders obtained by ESRP at 2 kV. (b) TEM micrographs of “as-produced” Co-Sn particles by ESRP (9 kV, 1.5 ml h-1) in DMSO. (c) XRD pattern of Co-Sn powders. (d) Cycle performance of Co-Sn powders.
ESD technique opens the opportunity to precisely control the morphology of anode materials of lithium ion batteries. Their morphologies are determined by a high voltage, the distance between nozzle and substrate, the flow rate and viscosity of precursor solution, substrate temperature. In this paper we have reviewed various anode materials synthesized by ESD technique. Obviously, the morphologies play a great role in improving the cycle performance of anode materials. Especially, the three dimensional sponge-like anode materials with a high porosity and a narrow pore size distribution are mechanically strong, beneficial to suffer the large volume change of anode materials during charge/discharge process. On the other hand, Li+ transport properties of 3D sponge-like porous anode materials are faster than those of other morphologies due probably to their high surface area. ESD also makes possible the preparation of an anode material without binder and conductive agent, which enhances the energy density of lithium ion batteries. Therefore, ESD-derived anode materials are favorable for the application in lithium-ion batteries.
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