Strengthening the Electrodes for Li-Ion Batteries with a Porous

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25081−25089

Strengthening the Electrodes for Li-Ion Batteries with a Porous Adhesive Interlayer through Dry-Spraying Manufacturing Jin Liu,†,‡ Brandon Ludwig,†,§ Yangtao Liu,‡ Heng Pan,*,§ and Yan Wang*,‡ ‡

Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, United States

§

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ABSTRACT: The manufacturing technologies for electrodes have a great influence on the performance of Li-ion batteries. Manufacturing procedures largely determine the microstructure of electrodes, and thus affect how active materials are involved in the electrochemical reactions. However, the usage of solvent in the dominant slurry-casting method weakens its competence on obtaining desired microstructures and properties. In this study, an improved adhesion strength is achieved during the fabricaion of graphite anodes with our solvent-free manufacturing method. Through dry-spraying an interfacial “adhesion enhancer” layer between the current collector and the electrode coating, the mechanical strength (from 0.5 kPa to over 83.0 kPa) and electrochemical performance (from 24.2% to 92.4% as the capacity retention in 100 cycles) are significantly improved. Results here demonstrate a simple and economical route to practically control the microstructure of electrodes during manufacturing and potentiate the strategy enabled by dry-spraying to design and manufacture advanced batteries. KEYWORDS: graphite anode, lithium-ion battery, solvent-free, additive manufacturing, mechanical property

1. INTRODUCTION

For slurry-casting manufacturing (slot-die coating), the usage of solvents (mostly as N-methyl-2-pyrolidone (NMP) and water) has been the bottleneck to further control the microstructure of electrodes.18,19 Within the past decades, this dominant manufacturing method has been studied in depth to produce Li-ion batteries at an industrial-grade high efficiency.20−23 However, the mixing and recycling procedures for solvents add some uncertainty to the quality of electrodes during manufacturing.24,25 A model has been raised to show how AM particles and binder/carbon particles behave diversely during the drying procedure.26 Light and nanosized binder and carbon particles are driven by the heat/mass flux to move toward the heating/evaporation interface, whereas AM particles separately sediment to the current collectors.26−28 This brings in the dilemma for manufacturers to make a difficult choice: by keeping a high production efficiency, the high drying rate always sacrifices part of the “serviceable” binder/carbon materials. These binder/carbon materials accumulate at the top/bottom interfaces of the coating and wasted.24,25,29 Because of this inhomogeneous distribution of materials, excess binders are always required to protect the electrodes from defects caused by the insufficient bonding strength.20,30 In contrast, our dryspraying manufacturing method avoids this issue as it is a

The optimization of manufacturing technologies is necessary to fulfill the growing demands for boosting this electrical-powered world.1,2 For the industry of Li-ion batteries, the manufacturing mostly refers to the filming processing of electrodes that coats the current collector with a composite layer, which consists of active materials (AMs), binder materials, and carbon additives. Within these raw materials, AMs are the carriers of electrochemical reactions, whereas the rest of the materials form a “microstructure framework” to mechanically support the coating layer and provide the ion/electron transfer pathways for AMs. A proper microstructure, or rather a high mechanical stability, is crucial and necessary for batteries to survive during the handling, packaging, and using procedures.3,4 Particularly when accommodating some new materials, such as core−shell structured cathodes5−7 and silicon/graphite anodes,8−12 a well-established microstructure is essential to strongly host AMs during the repeated lithiation/delithiation procedures. Because of their unique material characteristics, electrodes based on these materials would suffer a larger amount of volume change during cycling (up to 400% for silicon).13 A few in situ and in operando experiments have been conducted to monitor and highlight this challenging circumstance for batteries.14−17 Due to the dynamic changes of microstructure during the service life of batteries, some structure conditions in this way restrict advanced materials from realizing their optimum properties. © 2019 American Chemical Society

Received: February 17, 2019 Accepted: May 31, 2019 Published: May 31, 2019 25081

DOI: 10.1021/acsami.9b03020 ACS Appl. Mater. Interfaces 2019, 11, 25081−25089

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Fabrication Scheme of Graphite Anodes through Dry Spraying: (a) Bare Copper Foil; (b) Spraying the Adhesion Enhancer Layer onto the Copper Foil; (c) Spraying the Coating Layer onto the Copper Foil; (d) Calendering

solvent-free method.31 All materials are fully dry mixed and deposited onto the current collector via an electrostatic sprayer, and simultaneously calendered to products with required parameters. There is no waiting time for stirring and drying, and therefore limits the material redistribution during these procedures. As expected, dry-sprayed electrodes have shown better microstructure conditions and physical properties in our previous works (such as adhesion/cohesion strength, structure integrity, materials homogeneity, etc.).32,33 In addition, it has been demonstrated to be widely viable for state-of-art materials (LiCoO2 (LCO), LiNiCoMnO2 (NCM), LiMn2O4 (LMO), nano-LMO, graphite, carbon additives, binders, etc.), and some advanced formulations (ultra-low binder (less than 1 wt %), thick coatings (300 μm), etc.).34,35 Most importantly, this method is originally designed as a scalable system to manufacture electrodes/components for batteries at a low-cost basis.31 Within this method, the transition from conceptualization to commercialization would be accomplished readily for any progresses in coming. Along with the investigations on microstructure control, the dry-spraying method is expected to facilitate the development of advanced electrode designs with specific microstructure preference, such as the hierarchically sandwich structure1,36,37 and the self-stand microstructures.38,39 Here we start to discuss the feasibility of dry-spraying on the microstructure control through improving the adhesion strength for the widely used graphite anodes. As a common concern for graphite anodes, the adhesion strength between the current collectors and coating layers has been thoroughly studied in conventional manufacturing.40−42 This problem mainly comes from the chemical instability of copper in the atmosphere, which impacts the surface roughness and surface energy, and in turn affects the wettability of the current collector surface.43,44 In industry, prior-casting treatments for the current collectors sometimes are quite indispensable, such as adding corrosive additives into the slurry recipe and treating the copper foil with lasers.45,46 Otherwise, electrodes may face a severe delamination and batteries demonstrate rapid quality degradation.47 Consequently, improving the adhesion strength for graphite anodes would be a good example to discuss the application of dryspraying method on modifying the microstructure and manufacturing advanced designs for batteries. The dry-sprayed anodes treated with the adhesion enhancer (procedures as shown in Scheme 1) remarkably outperformed untreated anodes electrochemically and mechanically. The bonding strength and the morphology conditions of the dry-sprayed

graphite anodes with various loading amounts of the interlayer are revealed through the scanning electron microscopy (SEM) and bonding tests. The performance comparison between drysprayed anodes and slurry-cast anodes is also provided in this report.

2. EXPERIMENTAL SECTION 2.1. Electrode Preparation. Anode electrodes were prepared with 92 wt % MCMB powder (MTI), 2 wt % Super-C65 carbon black powder (Timcal), and 6 wt % PVDF powder (MTI). Cathode electrodes were prepared with 90 wt % NCM powder (MTI), 5 wt % Super-C65 carbon black powder (Timcal), and 5 wt % PVDF powder (MTI). The porosity of all thin dry-sprayed electrodes was maintained at the range of 35% for cathodes, and 40% for anodes. The areal loading of cathode electrodes was designed as 6.5 mg cm−2 at the thicknesses of 45 μm (including the aluminum foil at the thickness of 16 μm). The areal loading of anode electrodes was designed as 4 mg cm−2 at the thicknesses of 45 μm (including the copper foil at the thickness of 10 μm). Anode samples at the areal loading of 6 and 8 mg cm−2 were investigated as well. After casting/spraying, all samples were dried in the atmosphere for 12 h at 60 °C and then transferred to dry inside the vacuumed oven for 12 h at 80 °C. The porosity of the sprayed (or cast) electrode was determined by considering the theoretical density of the mix (active material, carbon black, and binder) according to the following equation. Porosity = [T − S ((W1/D1) + (W2/D2) + (W3/D3))]/T, where T is the thickness of the electrode laminate (without Al foil current collector); S is the weight of the laminate per area; W1, W2, and W3 are the weight percentage of active material, PVDF binder, and C65 within the electrode laminate; and D1, D2, and D3 are the true density for Li[Ni1/3Co1/3Mn1/3]O2, PVDF, and C65, respectively. The theoretical densities for Li[Ni1/3Co1/3Mn1/3]O2 active material, PVDF, and C65 are 4.68, 1.78, and 2.25 g cm−3, respectively. All the porosities were calculated by assuming that the weight fractions and density of each material were not changed by the fabrication process. Dry powders were premixed with zirconia beads in a BeadBug microtube homogenizer (Benchmark Scientific) for 60 min at 2800 rpm. After mixing, powders were added to the fluidized bed spraying chamber. The fluidized bed chamber was fed into the spraying system with the electrostatic voltage set to 25 kV and the carrier gas inlet pressure was set to 1 psi. Distance from the deposition head to the grounded aluminum current collector was kept constant at 1.5 in. and the coating time was kept constant at 10 s. Surface morphology of the deposited material was investigated using a Helios NanoLab DualBeam operating with an emission current of 11 pA and 5 kV accelerating voltage. The details in spraying setup configuration, thickness control and material composition on spraying behaviors can be found in the previous publication.1 A hot roller was used for thermal activation and increasing the density of the electrode material. The bottom roller 25082


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Figure 1. (a) Rate performance comparison for dry-sprayed samples with/without the adhesion enhancer. The rate performance is based on the same rate constant charging/discharging from 0.1C to 10C after formation procedures. b) The cycling performance comparison at the rate of 0.5C for drysprayed samples with/without the adhesion enhancer. The 10th, 30th, 50th, and 100th cycle potential vs specific capacity for each are provided. Multiple samples were tested in a half-cell configuration. temperature was set at 190 °C and the top roller temperature was set at 100 °C. Slurry casting electrodes at the same recipe were fabricated in the lab. Slurries were manually stirred at first. Then the automatic thick film coater with doctor blade (MSK-AFA-III) from MTI was used to automatically cast prepared slurry onto the foils. Samples were then calendered to the same porosity and thickness as dry-sprayed ones. 2.2. Electrochemical Measurements. All the samples were tested with an Arbin BT2043 tester. All tests were built into 2032-coin cells. Cathode and anode electrodes were punched into disks at the diameter of 11 mm and 12 mm. One piece of Celgard 2500 microporous separator was used for all tests. The electrolyte is 15.2 wt % LiPF6, 25.4 wt % ethylene Carbonate, 59.4 wt % ethyl methyl carbonate from Gotion. Half-cell tests for graphite anodes were against Li foil from MTI as reference and counter electrode. All the full cells were built with drysprayed NCM cathodes and graphite anodes. Two cycles at 0.1C as the formation procedures were conducted for all the cells before tests. All the half cells were cycled between the working window of 0.01 V to 2 V. Cycling performance comparison between dry-sprayed anodes with the adhesion enhancer and without the adhesion enhancer were tested with a constant current charge/ discharge procedure at the rate of 0.5C. Cycling performance comparison between dry-sprayed anodes and slurry-cast anodes were tested for 15 cycles at 1C, 15 cycles at 3C, and 15 cycles at 5C, with a constant current−constant voltage (CC−CV) charge/discharge procedure. In this procedure, cells were maintained at the cutoff voltage (0.01 V) constantly during lithiation, until the current was dropped below 0.05C. All the full cells were cycled between 2.8 V to 4.3 V with a constant current charge/discharge procedure at the rate of 0.5C. There was a rest time for 15 min between charge and discharge procedures. Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were tested on the Biologic potentiostat/galvanostat VMP-3 using EC-Lab at room temperature. A lithium metal foil with 15.7 mm diameter from MTI was chosen as the counter and reference electrode. The CV measurement was performed over a voltage range of 0.0−1.5 V vs Li/Li+ at a scan rate of 0.1 mV s−1. The EIS measurement was performed in the frequency range of 10 mHz to 100 kHz, and the applied bias voltage was set at open circuit voltage (OCV) with 10 mV AC amplitude between the counter and working electrodes. 2.3. Morphology Characterization. The cross-section and surface morphology was observed by scanning electron microscopy

(SEM) (JEOL JSM-7000F electron microscope). EDS mapping was applied. 2.4. Mechanical Bonding Measurements. A Mark-10 Series 4 force gauge was paired with a Mark-10 ES10 manual hand wheel test stand to determine the bonding strength of the coated electrode material. To test the strength, the coated current collector was mounted onto the test stand base with the center of the coated region directly below the force gauge. A 0.5 in. diameter flat head (Mark-10) was attached to the force gauge with a piece of double-sided tape (7 mm by 12 mm) attached to the flat head. The force gauge was lowered until the flat head touched the current collector and compressed to 50 N. After compression, the force gauge was raised at a rate of 1 rotation over 20 s until the tape attached to the flat head decoupled from the coated area. The maximum tensile force was recorded and converted to the maximum strength by incorporating the known contact area of the tape.

3. RESULTS AND DISCUSSION 3.1. Dry Sprayed Graphite Anodes and the Performance. PVDF is widely used in electrode manufacturing as the binder materials, which is also chosen for all electrodes mentioned in our work of dry-sprayed electrodes to make a fair comparison. Intentionally adding extra amount PVDF provides the room for improving the adhesion strength between the current collector and coating layer. Moreover, this simple strategy does not bring any other materials into the mixture or devices into the system. This simple strategy would also help us focus the investigation on the manufacturing procedures itself without involving any more variations on the material part. A layer of pure PVDF powder, defined as the adhesion enhancer, was first sprayed to the prepared copper current collector (shown in the spraying scheme in Scheme 1a, b). The anode coating layer was subsequently laminated onto the PVDF layer (Scheme 1c) and calendered (Scheme 1d) together to reach the desired thickness and porosity. This new design requires no modifications to our existing dry-spraying system but simply using an updated two-step spraying strategy (Scheme 1b, c). As a result, a significant improvement was observed on the performance of dry-sprayed electrodes. 25083


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is reasonable that the performance difference due to the adhesion strength could be further revealed with thicker electrodes. It would be harder for thicker coatings to sustain on the current collector without enough adhesion strength. 3.2. Morphology of Dry-Sprayed Anodes and the Adhesion Enhancer. Figure 2a shows the SEM images of the

In Figure 1a, b, the effects of adhesion enhancer can be represented through the electrochemical performance comparison for samples with/without the adhesion enhancer, including the rate and cycling performances. All samples were fabricated with the same procedures and design specifications, other than adding the adhesive interlayer. At the beginning of rate testing, there is no discernible difference them at the rate of 0.1C, which mostly shows the selected active materials’ capability (MesoCarbon MicroBeads (MCMB) at 330−340 mAh g−1) under this working circumstance. When cycling the cells at higher rates, samples treated with the adhesion enhancer showed a 10−20% higher specific discharge capacity compared with untreated samples (from 331.5 mAh g−1 to 333.2 mAh g−1 at 0.1C, from 295.6 mAh g−1 to 317.6 mAh g−1 at 0.2C, from 200.8 mAh g−1 to 234.7 mAh g−1 at 0.5C, from 92.2 mAh g−1 to 126.8 mAh g−1 at 1C, from 29.1 mAh g−1 to 50.2 mAh g−1 at 2C, from 13.0 mAh g−1 to 22.9 mAh g−1 at 3C, from 4.0 mAh g−1 to 11.4 mAh g−1 at 5C, from 0.2 mAh g−1 to 4.5 mAh g−1 at 10C). After the rate tests for less than 10 cycles, all untreated samples showed a capacity loss for more than 20% at the rate of 0.1C (from 270.6 mAh g−1 to 334.7 mAh g−1). Meanwhile, treated samples can still reach to its original discharge capacity without any noticeable capacity loss. This difference has been marked inside the figure with the red dotted circle. The cycling performance comparison at 0.5C for samples with/without the adhesion enhancer further revealed this difference in a half-cell configuration. The capacity vs potential profiles for the 10th, 30th, 50th, 100th cycles (Figure 1b), clearly show that the untreated dry-sprayed samples demonstrate a rapid capacity decline from the very beginning of the cycle testing. After cycling for 100 cycles, the specific discharge capacity has dropped from 260.09 mAh g−1 to 62.87 mAh g−1, which refer to a 24.2% capacity retention ratio of its original value. Comparably, treated samples showed a very stable result that maintained 92.4% after 100 cycles (252.63 mAh g−1 out of 273.51 mAh g−1). In addition, the untreated samples clearly showed a higher overpotential in the profile, whereas the treated sample showed a clear working window during cycles. All drysprayed anodes mentioned within the rest of this work were fabricated with an adhesive interlayer unless specifically stated otherwise. In the rate performance comparisons for dry-sprayed anodes with/without the treatment, it was not anticipated that there would exist a large difference between them. Even though the bonding strength were poor for those samples without the enhancer layer, it should not cause the delamination of materials and affect the performance immediately. However, these electrodes started to show much larger differences after being charged/discharged for 100 cycles. The lack of adhesion strength without enhancer layer would possibly cause part of the AM particles to lose the contact with the current collector and gradually lead to a fast capacity fade. Furthermore, the drysprayed anodes showed competitive performance compared with slurry-cast electrodes, and the dry-sprayed ones tend to perform better at high cycling rates. This phenomenon was specifically recognized during the cycling tests when increasing the rate from 1C to 3C. This can be attributed to that drysprayed samples with higher mechanical strength should be theoretically better bonded when samples suffer more volume change and relevant internal stress at a high rate. Note also that most of samples related to this work were manufactured at a thin thickness around 45 μm. In terms of the proven capability on dry-sprayed thick electrodes (up to 300 μm after calendering), it

Figure 2. (a) Comparison of current collectors for samples with different surface treatments: bare copper foil, copper foil treated with dry-sprayed adhesion enhancer (loading amounts of 0.01, 0.03, 0.08, and 0.16), and copper foil with slurry-cast PVDF layer. b) EDS mapping of dry-sprayed adhesion enhancer (0.16 mg cm−2 loading amount). (c) EDS mapping of slurry-cast PVDF layer (slurry-cast PVDF layer). Scale bars and the information on recipes and elements are inserted into the figures. (d) Top surface SEM photos of dry-sprayed anodes at different scales, and a sample picture of dry-sprayed anodes.

copper current collector with different loading amount of PVDF adhesion enhancer. These samples were characterized after spraying and no calendering procedure was performed. Bare copper and copper with slurry-cast PVDF layer are also provided as references. Within these figures, PVDF aggregates grows in the size from the scale of nm to μm when the loading amount increases. These aggregates were coated onto the copper current collector very well during the handling and testing even though there exist some defects on the current collectors that we used (dark linear patterns in Figure 2a, as the oxidations or the indentations). No apparent changes on color or weight were found after spraying the PVDF layer. Furthermore, the energydispersive X-ray spectroscopy (EDS) mappings were also provided here for the dry-sprayed PVDF layer (Figure 2b) and slurry-cast PVDF layer (Figure 2c). Scale bars are inserted as shown in the figures. At the same magnifications, the drysprayed samples in Figure 2b showed a clear scattered arrangement of PVDF aggregates on the copper current collectors, whereas the slurry-cast samples in Figure 2c showed a dense insulating thin film. The SEM images of a dry-sprayed anode surface are shown in Figure 2d. Similar to our previous findings in dry-sprayed cathodes, these dry-spray anodes look flattened and free of obvious defects at any magnification. These samples did not show any delamination behavior during the bonding tests. More SEM images can be found in Figure S1, including dry-sprayed 25084


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materials dissolved in the liquid phase to diffuse between the PVDF layer and the coating layer, which causes a random material redistribution. In contrast, the dry-sprayed PVDF enhancer layer is light and thin so as to add a very limited amount of PVDF to the sprayed location. The scattered morphology of dry-sprayed PVDF layers creates extra specific surface area and leaves the route for the electron/ion transfer within the coating and current collector. Through precisely planning the surface roughness and the local concentration of binder materials, the study of this solvent-free method could be expanded to customizing designs for advanced materials and formulations. 3.3. Effect of Adhesion Enhancer on Adhesion Strength. To control the bonding strength between the current collector and the coating layer (as known as the adhesion strength), the relationship between the material loading of adhesive interlayer and the resultant bonding strength were studied for the dry-spraying method. The bonding (adhesion) strength were collected at the value when fractures happened. As shown in Figure 3, the adhesion strength improved from 0.5 kPa (without adhesion enhancer) to 82.7 kPa (with adhesion enhancer loading of 0.03 mg cm−2 and above), which clearly shows how the adhesion enhancer efficiently strengthens the dry-sprayed anodes. Adhesion

anodes and slurry-cast anodes with comparable design specifications. The surface morphology of dry-sprayed anodes was the same as the previous dry-sprayed cathodes. We suspect that the flattened texture of dry-sprayed samples come from its one-step shaping procedure that the coatings undergo during calendering. Slurry-cast coatings need to undergo a two-step shaping procedure to optimize the microstructure and physical parameters. In the slurry-casting method, all materials are first dried and sedimented to the direction of gravity. In the first step of deformation, binder and carbon particles at the nano size fall into the void area between AM particles at the micro size,resulting in a rugged surface morphology of slurry-cast electrodes. In the second step of deformation when calendering, AM particles is mainly exposed, contacted, and pressed by the rollers, whereas binder/carbon particles still stay untouched below the top surface. Considering the swelling behaviors of materials during the electrolyte injection, this flattened surface morphology of dry-sprayed samples is expected to maintain a more uniform force distribution, and relatively better electrochemical conditions (distributions of ion, electron, and materials) that limit the growth of lithium dendrite to some extent. To study the effects of an “adhesion enhancer” layer with the slurry-casting method, we also prepared slurry-cast anodes laminated with a slurry-cast PVDF interlayer (in Figure S2). These anodes were prepared using the same specifications as the dry-sprayed samples. However, two-layer lamination through slurry-casting does not work well for improving the bonding and electrochemical performance for tested samples. In Figurs S2a− e, we show the slurry-cast films of PVDF with different height setting of the doctor blade, from 10 to 100 μm. During the casting, the high viscosity of PVDF solution makes it hard to be pasted as a thin film of PVDF solution onto the current collector. Below the blade height setting of 10 μm at this concentration, the solution can no longer be evenly pasted onto the copper surface, which left noticeable bare area as shown in Figure S2a. When being casted at the setting from 30 to 100 μm, we can only obtain an electron-insulating thick layer of PVDF, which is not conductive enough to test the electrochemical performance. Therefore, a slurry-cast PVDF layer is not a suitable method for strengthening electrodes at the current collector interface. Moreover, the slurry-cast PVDF layer added a much larger weight of PVDF to the current collector as compared with the dry-spraying method. Through SEM and bonding tests, the dry-sprayed “enhancer” layer is recognized as a scattered thin film of PVDF to improve the adhesion strength. In comparison, our results showed that slurry-casting method can produce only a thick and uneven film of PVDF material. Even though it is possible to lower the concentration of PVDF solution to achieve a smaller loading amount with the slurry-casting method, the PVDF materials tend to form PVDF agglomerate chunks via slurry-casting instead of the evenly dispersed PVDF aggregates via dryspraying. The surface tension of copper foil will cause the PVDF to precipitate together when the solvents are gradually evaporated. In addition, slurry-cast lamination layers (copper current collector, PVDF layer, electrode coating layer) would be dried and deformed diversely considering the different thermal expansion of involved materials into the consideration. This may create interfacial stress and deformation between each layer which may cause surface wrinkles or even cause the coating to faile/fracture. In addition, the solvent would also assist

Figure 3. (a) Mechanical bonding characterization of samples, including slurry-cast anodes, dry-spraying anodes with different loading amount of adhesion enhancer (no adhesive interlayer, 0.01, 0.03, 0.08, and 0.16 mg cm−2). SEM micrographs of dry-sprayed anodes with (b) no adhesion enhancer, (c) 0.01 mg cm−2 loading amount of adhesion enhancer. (d) Loading amount of enhancer over 0.01 mg cm−2 (0.03, 0.08, and 0.16 mg cm−2). 25085


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Figure 4. Performance comparison of dry-sprayed graphite anodes and slurry-cast anodes. (a) Rate performance at same rate charging/discharging from 0.1C to 10C. (b) Rate performance at constant same rate charging at 0.1C and changing rate discharging from 0.1C to 10C. (c) Rate performance at changing rate charging from 0.1C to 10C and constant same rate discharging at 0.1C. (d) Cycling performance at same rate charging/discharging at 1C, 3C, and 5C for 15 cycles. At least four cells were tested for each.

strength higher than 82.7 kPa should be expected when the adhesion enhancer loading is increased, but cohesive failure within the electrode layer becomes the limiting factor. Bonding tests of anodes with higher areal loading of the electrode material were also performed to observe any changes. It was found that the bonding strength of dry anodes was still governed by the loading of the adhesive interlayer. Among these samples, there exist three fracture scenarios:

with the current collector surface after the bonding tests were performed. It should be noted that this type of adhesive failure at the current collector interface was also observed in slurry-cast-anodes where the bonding strength was observed to be 22.0 kPa. 3 When the loading amount of adhesion enhancer is 0.03 mg cm−2 and above, the fracture location was noticeably transferred to within the electrode layer itself. This indicated that the limiting strength area of the anodes had switched from adhesive failure at the current collector interface to cohesive failure within the electrode layer. However, the bonding strength of these anodes had increased to 82.7 kPa, which is noticeably higher than the slurry-cast anodes and is nearly the same strength of slurry-cast cathodes.31 Cohesive failure was further observed with SEM micrographs (Figure 3d) which clearly show failure at the top layer of the electrode surface. A clear transition from the flat, smooth surface after calendering to the rough failure surface can also observed. Because of the cohesive fracture within the electrode layer, it was not possible to measure the adhesive strength at the current collector interface when the loading amount of adhesion enhancer is more than 0.03 mg cm−2.

1 Without the adhesion enhancer, most of the dry-sprayed anodes suffered delamination during the calendering procedure (as shown Figure 3a) and so the bonding strength is 0 kPa. When delamination was not observed, the anodes demonstrated poor adhesion (0.5 kPa) and readily failed at the current collector interface. SEM micrographs (Figure 3b) show a tested sample where the electrode layer is beginning to separate from the current collector. 2 When a minimal amount of adhesion enhancer is applied to the current collector the bonding strength of the drysprayed anodes increased to 14.6 kPa. Failure still occurs at the current collector interface as shown in Figure 3c, but a much higher degree of contact between the electrode layer and current collector was observed. Further evidence of increased contact can be seen by some residual anode particles that were still in contact 25086


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In Figure S6, EIS was carried out to probe the electrical properties of the slurry-cast and dry-sprayed electrodes. These battery impedance spectra were obtained before cycling and after 20 cycles. The two partially formed semicircles were fitted through the equivalent circuitry in the inset of Figure S6. Among them, Rb is the bulk resistance of the battery that represents the electric conductivity of the electrolyte, separator and electrodes. The semicircle at high frequencies is related to RSEI and CSEI, which are the resistance and capacitance of the solid electrolyte interface (SEI) on electrodes, respectively. Rct and Cct are faradic charge-transfer resistance and its relative double-layer capacitance, which correspond to the semicircle at the medium frequencies. For the fresh cells before cycling, the SEI have not been formed on the electrode surface, therefore the typical two semicircle curves can hardly be reached at this stage. However, the initial bulk resistance of the battery can still be addressed in them. For the fresh cell with a slurry-cast anode, the initial bulk resistance was 25 Ω. In the meantime, a comparable smaller initial bulk resistance at 12.5 Ω was obtained by the cell with a dry-sprayed anode. This is consistent with what we have claimed above that a lower contact resistance could be initiated though our advanced dry-sprayed coating. After 20 cycles, similar RSEI values around 10.5 Ω, were obtained from these two batteries with different anodes. With the same active material, electrolyte and cycling protocol, the SEI formed on their surface should be similar to each other. In addition, the cell with dry-sprayed anode showed a smaller Rct at 15.5 Ω than the cell with slurrycast anode at 19 Ω. This should also refer to another advantage of using the adhesion layer though our dry manufacturing method that charge can be transferred easier from the electrode to current collector, which lead to a better electrochemistry performance as we have shown above. In Figure S7, similar CV curves of MCMB electrodes manufactured through different methods were obtained at the beginning of cell cycling. With the same setting of electrode formula and all other components, these cells showed a typical profile of MCMB electrodes during the CV test. These redox peaks in voltammogram are correspond to the Li+ insertion and extraction. The anodic peaks in Figure S7 were around 0.280 V for both anodes, and the corresponding cathodic peaks were around 0.166 and 0.051 V. These similar CV profiles indicate that the contact network between active materials and binder/carbon materials in both kinds of electrodes were efficient and sufficient to provide a good conduction and intercalation/deintercalation reversibility.

In the bonding tests, we recognized a difference of adhesion strength between untreated dry-sprayed anodes (0.5 kPa) and slurry-cast anodes (22.9 kPa). This may come from the drying procedure of slurry-casting, during which the binder materials tend to solidify at the outer shell of the slurry with the highest heating rate. The top and bottom of electrode layer of slurry-cast samples should be thus benefited from this behavior. However, our SEM study of the slurry-cast thick cathode electrodes in Figure S3 is a good example to show that this behavior is very limited to improve the overall mechanical strength of the electrode. In these figures, bare NCM particles on the top and bottom surface of electrodes clearly exhibit a tendency to lose contact from the coating. Cracks have occurred severely on the top surface along the entire surface and have happened to the bottom surface as well. Considering the swelling behavior of electrodes with electrolyte, these poorly bonded particles would easily lose the contact within the electrode layer and cause either capacity fade or even short-circuit of the battery. These phenomena indicate that the uncontrolled migration behavior of materials would be an ineffective way to enhance the electrode microstructure. To obtain a higher initial adhesion bonding strength after drying, the electrodes sacrifice their homogeneity and overall mechanical strength, which makes it harder for them to survive during the following handling and lifespan of the battery product. Moreover, this behavior of slurry-cast samples can hardly further improve the adhesion strength of electrodes, which would be an important feature for developing advanced materials as discussed above. 3.4. Electrochemical Performance Comparison between Dry-Sprayed and Slurry-Cast Anodes. Figure 4 shows the detailed electrochemical performance comparison for dry-sprayed anodes and slurry-cast anodes. Figure 4a−c shows the rate performance and the comparison with slurry-cast anodes under different testing schedules. These electrodes fabricated through two methods showed comparable rate performance at low rates. The dry-sprayed samples always showed a slightly better performance than the slurry-cast ones at high rates. When being charged at 0.1C and discharged at 10C, the largest difference was found as around 25% (318.9 mAh g−1 vs 244.5 mAh g−1), which is shown in Figure 4b. In Figure S4, here we also tested the dry-sprayed anodes with higher loading (6 mg/cm2). Anodes with higher loading showed a difference when the charging/discharging rate was higher than 1C, which should due to its the higher thickness. Considering the mechanical/electrochemistry results of dry-sprayed anodes with higher loading, the improvement of loading will not compromise the benefits of dry-spraying on adhesion strength. However, more studies on optimizing the electrode layer would be necessary to further improve the performance of samples with higher loading, which is mainly dominated by the materials property and electrode microstructure conditions. Figure 4d shows the cycling performance comparison between slurry-cast samples and dry-sprayed samples. We can see the growing trend on the performance difference when cycling rate is increased from 1C to 5C. The dry-sprayed samples showed a better capacity retention especially at a higher rate (300 vs 230 mAh g−1 after cycling for 15 cycles at 3C, 21% difference of capacity retention). After 5C, both kinds of samples started to show fluctuant profiles. In Figure S5, we also showed the first fully dry-sprayed full-cell with dry-sprayed cathodes and dry-sprayed anodes, which has ∼75% of capacity retention after 100 cycles at 0.5C charge and discharge.

4. CONCLUSION In this study, we demonstrated the dry-sprayed graphite anode with an ultrahigh adhesion strength for lithium-ion batteries. The bonding strength of dry-sprayed graphite electrodes with enhancer layer is 3.6 times that of slurry-cast anodes and 166 times of dry-sprayed graphite without enhancer layer. The usage of the PVDF “adhesion enhancer” layer significantly improved the electrochemical performance (both rate and cycling) of drysprayed anodes. This improvement was directly related to the adhesion strength between the current collector and the coating layer. Through controlling the loading amount of the PVDF interlayer, the effect of adhesion enhancer was investigated, which showed a flexibility to produce electrodes with high performance. In summary, fulfilling the basic requirement as low-cost fabrication, this new method seems to be a promising manufacturing platform for producing advanced batteries and commercializing materials with superior properties. 25087


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03020.

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SEM images of samples, slurry-casting patterns of enhancer layer through slurry-casting method, full cell test data with all dry-sprayed electrodes (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.W.). *Email: [email protected] (H.P.). ORCID

Yan Wang: 0000-0003-1060-2956 Author Contributions †

J.L. and B.L contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.L. and B.L. designed and investigated the dry-spraying technique on fabricating graphite anodes for Li-ion batteries. J.L. contributed to this work in the battery design, electrochemical testing, and manuscript preparation. B.L. contributed to this work in the design of the manufacturing system, mechanical testing, and manuscript revision. Y.L. assisted in conducting tests for batteries. Y.W. and H.P. supervised the entire project and revised the manuscript. Funding

This work is financially supported by NSF CMMI-1462343, CMMI-1462321, and IIP-1640647. Notes

The authors declare no competing financial interest.

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Strengthening the Electrodes for Li-Ion Batteries with a Porous

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