Solvent‐Free Manufacturing of Lithium‐Ion Battery Electrodes via Cold Plasma

Slurry casting has been used to fabricate lithium‐ion battery electrodes for decades, which involves toxic and expensive organic solvents followed by high‐cost vacuum drying and electrode calendering. This work presents a new manufacturing method using a nonthermal plasma to create inter‐particle binding without using any polymeric binding materials, enabling solvent‐free manufacturing electrodes with any electrochemistry of choice. The cold‐plasma‐coating technique enables fabricating electrodes with thickness (>200 μm), high mass loading (>30 mg cm−2), high peel strength, and the ability to print lithium‐ion batteries in an arbitrary geometry. This crosscutting, chemistry agnostic, platform technology would increase energy density, eliminate the use of solvents, vacuum drying, and calendering processes during production, and reduce manufacturing cost for current and future cell designs. Here, lithium iron phosphate and lithium cobalt oxide were used as examples to demonstrate the efficacy of the cold‐plasma‐coating technique. It is found that the mechanical peel strength of cold‐plasma‐coating‐manufactured lithium iron phosphate is over an order of magnitude higher than that of slurry‐casted lithium iron phosphate electrodes. Full cells assembled with a graphite anode and the cold‐plasma‐coating‐lithium iron phosphate cathode offer highly reversible cycling performance with a capacity retention of 81.6% over 500 cycles. For the highly conductive cathode material lithium cobalt oxide, an areal capacity of 4.2 mAh cm−2 at 0.2 C is attained. We anticipate that this new, highly scalable manufacturing technique will redefine global lithium‐ion battery manufacturing providing significantly reduced plant footprints and material costs.


Introduction
[3][4] Nonetheless, the increased economic integration of LIBs is hindered by large-scale LIB manufacturing challenges.10] In combination, these components of the slurry casting manufacturing method account for 30% of the cell manufacturing cost while impacting 25% of the overall cell cost ($137 kWh À1 ). [11,12]Furthermore, the slurry casting manufacturing process severely constrains the realization of high areal energy density batteries.In short, the additional process steps, times, and energy expenditures for making thicker electrodes (>100 lm) are untenable for a large-scale manufacturing process. [13]fforts have been made to develop alternative manufacturing techniques such as electron beam curing, spray-printing, 3D printing, pulsed-laser deposition, spray-drying, dry pressed manufacturing, and electrostatic spray deposition to increase the mass loading of active Slurry casting has been used to fabricate lithium-ion battery electrodes for decades, which involves toxic and expensive organic solvents followed by high-cost vacuum drying and electrode calendering.This work presents a new manufacturing method using a nonthermal plasma to create interparticle binding without using any polymeric binding materials, enabling solvent-free manufacturing electrodes with any electrochemistry of choice.The cold-plasma-coating technique enables fabricating electrodes with thickness (>200 lm), high mass loading (>30 mg cm À2 ), high peel strength, and the ability to print lithium-ion batteries in an arbitrary geometry.This crosscutting, chemistry agnostic, platform technology would increase energy density, eliminate the use of solvents, vacuum drying, and calendering processes during production, and reduce manufacturing cost for current and future cell designs.Here, lithium iron phosphate and lithium cobalt oxide were used as examples to demonstrate the efficacy of the cold-plasmacoating technique.It is found that the mechanical peel strength of coldplasma-coating-manufactured lithium iron phosphate is over an order of magnitude higher than that of slurry-casted lithium iron phosphate electrodes.Full cells assembled with a graphite anode and the cold-plasmacoating-lithium iron phosphate cathode offer highly reversible cycling performance with a capacity retention of 81.6% over 500 cycles.For the highly conductive cathode material lithium cobalt oxide, an areal capacity of 4.2 mAh cm À2 at 0.2 C is attained.We anticipate that this new, highly scalable manufacturing technique will redefine global lithium-ion battery manufacturing providing significantly reduced plant footprints and material costs.
[16][17][18][19][20][21][22][23][24][25][26][27][28][29] Du et al. reported electron beam cured acrylated polyurethanes as new binders for LIBs cathode, in which process the cathode can be cured instantly.These new binders have a stable potential window at 2-4.6 V and a similar charge-transfer resistance to the conventional polyvinylidene fluoride (PVDF) binder. [21]Lao et al. used low temperature directly writing 3D printing to fabricate thick lithium iron phosphate (LFP) electrodes, which improved pore volume and porosity and led to ~10% increase in the specific capacity compared to electrodes fabricated with the traditional slurry casting method. [14]Liu et al. [9] demonstrated a scalable drying printing manufacturing method to increase energy density and to reduce cost ~20% cost due to the solvent-free and dry process.Wu et al. reported a scalable architected electrodes with a facile and scalable templated phase inversion method.The mass loading of cathode materials reaches ~100 mg cm À2 with controllable quality. [20]However, these processes require either the use of organic solvents, a special thermal processing to activate binders or a high temperature calcination.
Here, we report a new manufacturing process using cold plasma process-powder coating (CPC) for electrode fabrication with the ability to print LIBs in arbitrary geometries and thicknesses.The basic principle of the CPC manufacturing process is to leverage a nonthermal plasma for creating inter-particle binding network, such as aluminum (Al) particles used in this study and producing an in situ formed 3D metallic network of highly conductive electrodes with any battery chemistry of choice.Replacing the polymeric binder with a conductive (e.g.Al) network in a 3D electrode architecture is a paradigm shift towards improving electronic conductivity which is critically important to high power automotive and energy storage applications. [30,31]By eliminating polymer binders, the NMP solvent, and conductive additives, our technology will reduce electrode processing costs by around 40%.In terms of manufacturing costs, by eliminating the multiple steps, the CPC process can reduce costs associated with capital investment, facilities, and maintenance by 70%; and labor and energy costs by more than 70%.Therefore, our technology paves the path to cell costs of less than $60 kWh À1 of usable energy at the pack level and fast charging capabilities for a wide range of battery markets.Furthermore, with CPC, the bonding between the active material particles and current collector metal is far more tenacious than polymer binders as used in the conventional slurry or dry casting processes, and thereby, the new process extends the cycle life of the fabricated cathode electrode.The porosity of CPC cathode is ~20% which is much lower compared with that of the slurry casting commercial samples (~35%).Furthermore, CPC collapses several traditional manufacturing steps including slurry mixing, casting, drying, and calendering into a single-step process.In this work, the commercially available low-cost LiFePO 4 was selected to evaluate the cycling stability, electrode structure, and mechanical integrity enabled by CPC.In addition, lithium cobalt oxide (LCO), which has much higher intrinsic electrical conductivity (~8.5 9 10 À6 S cm À1 in the discharged state, and ~4.6 9 10 À4 S cm À1 in the charged state) than that of LFP (~10 À9 S cm À1 ), [32,33] was also used to demonstrate the versatile processing capability of the CPC manufacturing technique.The comparison of these two CPCfabricated electrodes allows us to better understand and design the CPC parameters for different battery chemistries with an aim to produce battery electrodes with higher areal capacities and specific energies than electrodes fabricated via other manufacturing techniques.

Low-Cost and Dry Manufacturing via CPC
Figure 1a,b display the schematic and real images of the CPC manufacturing of battery electrodes where N 2 was used along with dry cathode powders and electrically conductive binding materials (herein Al particles).The major compositions of cold plasma are neutral N 2 gas atoms with a low degree of ionization (plasma density) (<50 g m À3 ).The dominant kinetic energy borne of cold plasma is electron, and the electron temperature is usually much higher than the ion or neutral gas temperature (non-equilibrium plasma).It leads to a lower thermal impact on the electrode materials owing to low ion temperature and no adverse chemical or structural changes to the starting materials. [34,35][38] Most importantly, the CPC manufacturing can be completed in open-air environment at room temperature and atmospheric pressure, but also allows to produce a thick electrode sheet (100 cm 2 and 150 lm in thickness) in about 3 minute.When applying CPC for LIB electrode materials, not only can thick and dense electrode layers be deposited in a single step as shown in Figure 1c, but the very nature of plasma processing modifies the surface energy of the electrode particles for improved wettability. [39]Figure S1, Supporting Information compares the top view images of the CPC-fabricated and the slurry-casted LFP cathodes.Both electrodes have a thickness of ~200 lm.However, cracks appear in the LFP cathode fabricated via the slurry casting method, due to the weak binding strength provided by the polymer binder.The laminate fabricated via CPC shows no evidence of cracking primarily owning to the in situ formed 3D metallic inter-particle binding network.

Highly Reversible Cycling Performance in CPC Fabricated Electrodes
The LFP cathodes with the thicknesses of 60 and 230 lm were prepared via CPC.24 wt.%Al particles with an average size of 500 nm were used as the binder and electrically conductive fillers for LFP particles, without carbon and polymer additives.The electrodes had ~9 and ~33 mg cm À2 mass loading of active materials, respectively.Electrochemical cycling behavior was examined by using 2032 coin cells with both half-cell and full-cell configurations as shown in Figure 2. The half-cell cycling results for these two electrodes are shown in Figure 2a.Details regarding the electrode manufacturing and cell assembling are included in the experimental section.Cathodes were cycling at a constant current of 28.13 mA g À1 (60 lm) and 25.95 mA g À1 (230 lm) versus lithium.The average discharge areal capacity is 0.45 AE 0.002 mAh cm À2 and 1.36 AE 0.08 mAh cm À2 for the thin and thick cathodes, respectively.The gravimetric and volumetric energy density is ~245 Wh kg À1 and ~288 Wh L À1 .The capacity retention is 91.3% and 84.4% for the thin and thick electrodes after 50 cycles.As the thickness of the active layer has a threefold increase, the areal capacity is tripled.In parallel, two slurry casting LFP cathodes with the thicknesses of 45 lm (mass loading: ~6 mg cm À2 ) and 80 lm (mass loading: ~13 mg cm À2 ) were cycled in the same protocols.We found that the average discharge areal capacity and capacity retention is 0.48 AE 0.01 mAh cm À2 and 94.9% for the LFP cathode with a thickness of 45 lm.The LFP cathodes with a thickness of 80 lm deliver an areal capacity of ~1.5 mAh cm À2 ; however, they exhibited rapid capacity degradation (Figure S2, Supporting Information).The comparison confirms that the LFP cathodes fabricated via CPC enable highly reversible cycling even for thicker electrodes.In addition to the half-cell cycling, Figure 2b presents the capacity retention of the CPC-LFP cathode in a full-cell configuration at cycling rate 28.13 mA g À1 , where a lab-made graphite anode was used to pair with the CPC-LFP cathode.Cycling data indicate the highly reversible cycling performance from the CPC-LFP cathodes with average coulombic efficiency 99.95% and a well-reserved capacity retention during cycling (81.6% after 500 cycles).

Enhanced Mechanical Integrity via CPC
Mechanical integrity of battery electrodes determines the long-term cycling stability, since any mechanical failure occurred during cycling will deteriorate cycling performance.The electrodes fabricated with CPC would be more robust than the electrodes fabricated with the polymeric binders due to the charge transfer interaction between particles and between particles and current collectors.It is well known that debonding of electrodes will seriously affect the capacity and life of batteries and cause a short circuit inside the battery, resulting in spontaneous combustion.For electrodes fabricated with polymeric binding materials, the adhesion strength of the polymer binder determines the debonding behavior of electrodes. [40]In the thin electrodes (≤50 lm), the strength of the polymer binder can accommodate the mechanical demands, such as compression and tension when undergoing cutting, rolling during cell manufacturing, and the repeated contraction and expansion during cell cycling.However, for high-energy-density electrodes with much higher thickness (>100 lm), the adhesion strength of the binder, which is usually based on hydrogen bonding, cannot withstand the mechanical challenges, leading to cracking and debonding during cell fabrication and cell cycling. [41,42] peeling test is used to investigate the mechanical strength (bonding) between the laminate and current collector.The peel strength is defined as the load per unit width, that is, the peel force divided by the width of the sample.Here, we performed the peel testing with an adhesive tape (Figure S3, Supporting Information) and a 180°peel tester (Figure S4, Supporting Information) to evaluate the adhesion strength of the electrodes fabricated via slurry casting with a PVDF binder and the electrodes fabricated via the CPC with Al particles.When using scotch tapes, the LFP electrode fabricated via the slurry casting was easily removed.In sharp contrast, the CPC-LFP cathode remained intact (Figure S3, Supporting Information).Results from the 180°peel tester are shown in Figure S4, Supporting Information and Table 1.The peel strength of the CPC-LFP cathode with a thickness of 70 lm and a mass loading of 12 mg cm À2 is 1.69 N cm À1 , while the peel strength of the slurry casting LFP cathode is <0.2 N cm À1 .Compared to the electrodes fabricated by the wet slurry casting, the peel strength of the CPC-LFP electrode has an ten-fold increase by, indicating that the adhesion of the CPC electrodes is significantly improved as compared to the electrodes made by the conventional slurry casting method using the PVDF as a binder.
We believe that the improved adhesion enhances the loading density of the CPC electrodes.As shown in the cross-sectional scanning electron  microscopy images, the LiFePO 4 particles were tightly packed without any noticeable cracks.The material distribution was examined using energy dispersive spectroscopy mapping of CPC samples before and after 500 cycles.Note that there are no carbon additives in the CPC electrodes, as confirmed in Figure 3a.After the 500 charge/discharge cycles, there are increased carbon-based precipitates caused by the leftover of carbonate electrolyte used in the coin cells.No cracks were observed in the CPC-LFP electrodes even after 500 charge/discharge cycles.The enhanced electrode integrity results in the stable cycling performance as displayed in Figure 2b.The distribution of Fe was uniform; however, the Al particles agglomeration particles were observed in both pristine and cycled samples.The formation of Al agglomeration would impair the electronic conduction through the electrodes.

CPC Effects on the Electrode Structure and Electrochemical Properties
X-ray diffraction spectroscopy (XRD) was conducted to identify the CPC effects on the CPC fabricated LFP cathodes.Figure S5, Supporting Information confirmed that the CPC fabricated LFP cathodes reserved their original crystalline structure compared with pristine LFP powder.Furthermore, operando XRD characterization was performed in the CPC-LFP electrodes (Figure S6, Supporting Information).Figure 4a shows the charge/discharge voltage profile of the CPC-LFP electrode at a current density of 14 mA g À1 .The charging process involves the delithiation of LFP electrode (A ?C), while the lithiation occurs during the discharging process (C ?E).Operando XRD data were analyzed at chosen delithiated and lithiated, as denoted by A, B, C, D, and E. As displayed in Figure 4b,c, the diffraction peaks from the LFP remain during the whole charge/discharge process.In addition to the XRD peaks from the crystalline LFP, there are new peaks that appear during charging, which can be indexed to the FePO 4 phase (FP) (( 200) and ( 020)).The XRD peaks from the FePO 4 phase diminish during the subsequent discharging (i.e.lithiation), confirming the reversible lithiation/delithiation reaction between LFP and FP.However, the LFP phase can still be observed after delithiation, indicating incomplete delithiation of LFP materials during the electrochemical lithiation/delithiation process.

Phase Distribution in Charged LiFePO 4 Electrodes
Radially accessible tubular in situ X-ray (RATIX) cell has been used to study the phase distribution in fully charged CPC-LFP electrodes (Figure S7, Supporting Information).As shown in Figure 4d, the XRD data were collected from the top to the bottom of the charged CPC-LFP.The peak areal intensity was  The composition of the charged CPC-LFP electrode is plotted in blue in Figure 4d.It confirms that the LFP and FP phases co-exist along with the cathode thickness (~75 lm).There is slight less FP phase at the top area of the electrode.However, if the LFP was fully charged, the LFP phase should be fully converted to FP phase with a negligible amount of LFP phase.As plotted in Figure 4d, charged CPC-LFP electrode (at the cutoff voltage of 4 V) includes both LFP and FP phases.There is around 45-55% of LFP phase remained in the charged electrodes.Because only half of the LFP phase participates in the electrochemical charging reactions, the CPC-LFP electrodes cannot deliver their theoretical charging capacity.As shown in Figure S8, Supporting Information, the specific gravimetric capacity of the CPC-LFP electrodes is ~70 mAh g À1 at 14 mA g À1 as compared to the theoretical capacity of 170 mAh g À1 .
The low specific capacity can be attributed to the low electronic conductivity in the CPC-LFP electrodes.It is well known that LFP particles are usually surface coated/doped with carbon-based or other electronic conductive materials to increase the electrical conductivity.However, the conductive coating layers are vulnerable against cold plasma and can be removed during the CPC process.To confirm it, we have used the cold plasma to surface treat the LFP electrodes made with the carbon black, PVDF additive.As shown in Figure S9, Supporting Information, the color of the electrodes has changed from black to gray after the cold plasma treatment, suggesting some carbon black particles were removed.
We believe that the conductive carbon-based coating layers which were extensively used in LFP manufacturing process can be ablated during the CPC process.Considering the intrinsic electrical conductivity of the LFP is only ~10 À9 S cm À1 , a continuous electronic network has is necessary to realize the full charging capacity for the CPC-LFP electrode.Although a better understanding of the electrochemical participation of the CPC-LFP electrodes necessitates a systematic investigation, we expect that an improved intrinsic electrical conductivity would significantly enhance the electrochemical material participation and gravimetric capacity.

Potential of the CPC Technique
When applying our developed CPC technique to the LiCoO 2 (LCO) material, the CPC-LCO electrodes can deliver their full reversible capacity.The specific capacity of the 40 lm CPC-LCO is ~150 mAh g À1 at 14.68 mA g À1 with a cutoff voltage of 4.3 V.Note that the intrinsic electrical conductivity of the LCO material is about 8.5 9 10 À6 S cm À1 and increases to 4.6 9 10 À4 S cm À1 for its charged state. [32,33]The electrical conductivity of the LCO material is three magnitudes higher than that of the LFP material (~10 À9 S cm À1 ).Without adding carbon, polymer, and solvent, the dry CPC technique enables fabricating the LCO electrodes with various mass loading and areal capacities.
Figure 5a compares the rate performance of the CPC-LCO and CPC-LFP electrodes.Both cells were fabricated with the CPC cathodes and the Li metal counter electrodes, in a half-cell configuration.The CPC-LCO electrodes and CPC-LFP electrodes have the mass loading of 5.73 and 10.76 mg cm À2 , respectively.At a slow C-rate (14 mA g À1 ), the accessed discharge-specific capacity of CPC-LCO and CPC-LFP is ~150 and 71 mAh g À1 , which is ~97% and ~45% of theoretical capacity.The thicker LCO electrodes with a thickness of 220 lm were also fabricated with the CPC technique, as shown in Figure 5b.Note that the mass loading of LCO particles reaches 42.27 mg cm À2 at 19.68 mA g À1 rate.It offered an areal capacity of 4.2 mAh cm À2 .The full cell was also assembled with the CPC-LCO electrode and a graphite anode.Figure 5c plots the capacity retention of the full cell when cycling between 3 and 4.3 V at a cycling rate of 14.2 mA g À1 (1 st cycle) and 28.4 mA g À1 for the sequential cycles.It shows that the capacity retention is 80.7% after 200 cycles.Comparing with the CPC-LFP electrodes, the CPC-LCO electrodes deliver their theoretical capacity, suggesting that all the LCO particles participate in the electrochemical reactions.We believe that the intrinsic electrical conductivity of the electrode materials plays an important role in their resulting electrochemical performance.A systematic investigation has been initialized at our lab to understand the impact of CPC process on the electrochemical properties of the CPC fabricated electrodes.

Conclusion
This work demonstrates a new electrode manufacturing method via the cold plasma process (CPC), which is free of solvents, polymers, carbon additives, drying, and calendering processes.The CPC electrodes provide an excellent mechanical strength which is up to an order of magnitude higher than the slurry-casted electrodes.The CPC enables the Energy Environ.Mater.2024, 7, e12503 fabrication of ultrathick electrodes (>220 lm) due to the unique binding mechanism.The thickness of the CPC-LFP cathode sheet can reach up to 220 lm with a mass loading above 42 mg cm À2 .With the CPC-LFP cathodes, the full cell delivered highly reversible cycling performance with an average coulombic efficiency of ~99.95% and a capacity retention of 81.6% after 500 cycles.However, only partial delithiation of LFP particles in the CPC-LFP electrode occurred during the electrochemical charge/discharge reactions, leading to half of their theoretical capacity.We attribute the partial participation of LFP to the low intrinsic electronic conductivity of the LFP particles, exacerbated after conductive surface coatings are removed by CPC.When using the LCO particles with a much higher intrinsic electrical conductivity than that of LFP particles, the CPC-LCO electrode delivers its theoretical reversible capacity at a cycling rate of 14.68 mA g À1 .To improve the electrochemical reactivities for LFP particles, we expect that surface engineering with non-carbon-based conductive coating materials would help increase electronic conductivity and stabilize the surface, enabling scalable, solvent-free manufacturing of high-energy density CPC-LFP electrodes.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.

Figure 1 .
Figure 1.Schematic process of solvent and calendering free LIB cathode fabrication via CPC.a) Schematic diagram of the cold plasma deposition system.b) An image taken during the cold plasma deposition process.c) A 5 cm 9 5 cm LFP cathode sheet via CPC.

Figure 4 .
Figure 4. X-ray characterization of charged and discharged CPC-LFP cathode.a) Voltage and time profile of CPC-LFP cathode charged to 4 V and discharged to 2.5 V at 14 mA g À1 versus lithium anode.b, c) In situ X-ray diffraction of LFP cathode at different charge/discharge state (points A, B, C, D, E, and F in a).d) Fe 2+ and Fe 3+ characterization at different thickness of the electrode after finishing discharge at 2.5 V via ex situ diffraction assessments with radially accessible tubular in situ X-ray (RATIX) cell (inserted plot and Figure S7, Supporting Information).

Table 1 .
Peel strength test of slurry casting and CPC-LFP cathode sheets.