In order to prepare the 500 g of total solid material contained cathode slurry, a solid composition of Ni₈₀Co₁₅Al_0.5 (NCA 90 wt.% (Toda, Japan), 1% Graphite (G) (Graphite KROPFMUHLAG, Germany), 3% acetylene black (AB) (SN2A, France) and 6% PVDF (Kureha Corporation, Japan) was chosen. Accordingly, NMP (Sigma Aldrich) was added to the slurry preparation. The solid to liquid composition ratio was maintained as 60:40 (wt.%). The slurry preparation was initiated with a binder preparation by dissolving required amount of the binder (6% PVDF) in ~195 mL (60 vol.% of total 324 mL NMP). In addition, the solution was mixed at 1200 rpm for 10 min to prepare homogeneous binder using the planetary orbital mixture (Thinky Corporation, ARV-930 twin model, Japan).

The mixture of NCA, G and AB were dry mixed separately during the binder preparation using a dry mixture. The resultant dry mixed solid composition mixture was added to the binder solution and allowed for stirring with the sequential addition of NMP. Left over 129 mL NMP (40 vol.% in total NMP volume) was added to the slurry at different time intervals. Four different addition processes were tried to identify the effect of the sequential addition of NMP. The total planetary stirring/mixing time and the total added volume of NMP are kept constant and the interval time and NMP volume at each interval were varied as shown in Fig. 1. In the first method and the single step method, the required (129 mL) NMP was added to the slurry after stirring of binder and the dry mixed cathode solid composition (slurry mixture-1) and then allowed for 20 min planetary stirring at 800 rpm. In the two-step process, the total NMP was separated into two equal parts and the first addition was done into the slurry mixture-1 initially and allowed for 10 min stirring at 800 rpm and then the remaining amount was added and allowed for second 10 min stirring. Three and four step NMP addition process was done. The finally prepared four resultant cathode slurries were denoted as S-1, S-2, S-3, and S-4 (where S indicates the sequence and the number (1 to 4)) represents the number of NMP addition sequence steps in Fig. 1.

The electrode slurry properties are primarily evaluated by measuring the slurry viscosity and viscoelasticity. These properties are critical for understanding the fluid flow properties of the electrode slurry while pasting on the Al current collector. Viscosity measurements were carried out on Anton Paar Rheometer MCR 72–92 with a cone and plate configuration. The system is set at an angle of 4o, radius of 20 mm, truncation of 40 mm, and measuring position gap of 0.4 mm. Viscosity measurements were conducted at a constant temperature of 25°C controlled by several air-cooled Peltier temperature units. For each sample, ~3 mL of slurry sample was used and measured the viscosity was in a shear rate of 0.1 to 100/S. Viscoelasticity study carried out with angular frequency sweep test was in the range of 0.1 to 100 rad/s.

The Li-ion cell assembly was carried out in an environmentally controlled atmosphere, subjected to calendaring the electrodes prepared by single and different sequential addition of solvent. In-house made anode and a porous tri-layer poly propylene/polyethylene/polypropylene (2320, Celgard, USA) separator was used between the cathode and anode to prevent internal shorting of the cell. For preparation of the 18650 cylindrical cells, the calendared cathodes were used to prepare the cylindrical cell electrodes. The required length and width of the electrodes were achieved by vertical and horizontal slitting, respectively. Counter anode electrodes were prepared with graphite active material for different kind cathode electrodes. The Al and Ni tabs are welded to the bare area of the current collectors of cathode and anode electrodes respectively using the ultrasonic welding. The tab width 4 mm, thickness 0.1 mm and length 80 mm were used and welded in the energy mode for cathode 12 J and anode 18 J. The separator was wrapped between the two electrodes to the ionic flow between the electrodes during the charge and discharge processes. The anode and cathode electrodes are separated by two layers of separators used to prepare the jelly roll and the resultant jelly roll was inserted into the cylindrical can. The anode tab was welded at the bottom position of the cell and the cell was grooved to maintain the jelly roll structure. The cathode tab was welded to the top lid by resistive welding. 4 mL of electrolyte used is a mixture of the lithium slat and organic solvents (1.0 M LiPF₆ in EC: DEC: EMC, Kishida Chem. Corp., Japan) in the 1:1:1, molar volume (v/v%) followed by crimping the cell to form a cylindrical cell. During electrolyte filling 60 kPa pressure was maintained. The battery was kept aging for 48 h. A similar procedure was adopted to fabricate the pouch cell with anode and cathode electrodes being prepared by both single and multi-step processes (referred to as S-1, S-2, S-3, and S-4). We fabricated single stack pouch cell with the dimension of cathode (4 cm to 3 cm). The cathode loading of S-1 to S-4 are ~9.5 mg/cm² and anode coating is adjusted with cathode capacity. Then selected dimension are stacked with tri-layer separator and the suitable tabs are welded with corresponding electrode. After that, entirely covered with aluminium laminated pouch (grade PET C8) with 150 μM and it was degassed. Then electrolyte added under vacuum conditions sealed the pouch cell for further analysis. These cells are used to measure the EIS and charge discharge performance of the cells at ambient temperature.

Fig. 2 shows the non-Newtonian fluid behaviour of the electrode slurries (S-1, S-2, S-3, and S-4) prepared by sequential addition of NMP solvent for homogeneous solution [15]. It is worth to note that the viscosity of the slurry decreases with increase of the shear rate which is clear evidence of thinning behaviour of the electrode [21,22]. A more uniform coating can be expected with the less viscous slurries at high shear rates and the S-3 and S-4 are showing less viscosity at a higher shear rate than the S-1 and S-2. Fig. S1 shows the viscoelastic representation of the relationship between the viscous (liquid like) and elastic (solid like) behaviour of the electrode material. The desired viscoelasticity contained slurry samples have the better dispersion and stability. It is observed that the S-1 sample shows the storage modulus (G′) is larger than the loss modulus (G″) which indicates the solid like gel behaviour [23]. In the multi-mode mixing sequence, (S-2 to S-4) the slurries are showing liquid like behaviour which gains higher G″ than that of the G′, indicates that the nature of the slurry viscous is low. The low dependence of G′ to the frequency was observed in the S-3 and S-4 samples as compared to S-1 and S-2 (Fig. 3). The lower frequency dependence on the G′ was due to the formation of the bridging flocculation between the binder and cathode active particles and maintains an optimal interactive distance between the cathode particles and the binder which appears to be an optimal viscoelastic nature of the slurry. It indicates that the particles are well dispersed and stabilized, usually have relatively smaller G′ as compared to the G″ and shows a wider linear viscoelastic regime [10,21]. The viscoelastic representation of relationship between viscous (liquid) and elastic (solid) behaviour of electrode material shown in supplementary section.

Well dispersed and uniform distribution of the conducting carbon particle on the active materials in presence of the binder in the Li-ion cells can be mapped by the SEM. The Fig. 4 shows the SEM images of the cathode electrode with different addition of the NMP solvent to the mixture of NCA, conductive carbon and graphite. Fig. 4(a) shows that the sample S-1, the mixing of the conductive carbon materials (AB and graphite) in the cathode mixture is not well dispersed. The binder was observed on the surface of the cathode particles indicating that there is no uniform network formation [20]. The dispersion of the conductive additive increases with an increase of the mixing process steps as observed in the S-3 (Fig. 4(c)) and S-4 (Fig. 4(d)) samples that is the dispersion of the conductive particles on the surface of the active cathode particles. S-3 sample in Fig. 4(c) shows the uniform wrapping up on the cathode active particle with the conductive additive and binder. Further, by increasing the mixing process steps, the interaction between the binder and the conductive carbon can disturb and solid particles come together via the van der Waals forces and aggregate together. Hence, the amount of solvent utility will reduce and the slurry starts attaining the sol in nature. The G″>G′ modulus conforms the sol in nature and it further extends with increasing to the S-4 sample.

After coated samples, calendaring is most important part and in a biaxial way, because the electrode densification used for increase volumetric energy density of the cells and also important in improves the electrical contact, adhesiveness and capacity retention. Here, the coated electrode applied 0.5 Tons of pressure in the first pass and it reduces the thickness into 5% from its initial electrode thickness. After that, we applied 4 Tons of pressure for more densification of electrode to reduce upto 27–28% of its thickness. Fig. 5 shows that the compaction of electrode with reduce in porosity of the electrode. The particle agglomeration is less due void obtained more in S-1 shown in Fig. 5(a). The better agglomeration and decreased in porosity of S-2 shown in Fig. 5(b). Fig. 5(c) shows the uniform wrapping up on the cathode active particle with the conductive additive, binder, reduces the porosity and also obtained uniformity of the electrode for S-3. The S-4, further by increasing step of solvent it starts to affect the agglomeration and the materials turns into sol nature. The stability of slurry was affected more steps to add solvent and bridging flocculation of cathode and binder materials shown in Fig. 5(d).

The mechanical strength of the electrodes with different step addition of NMP was studied by peel strength. A 25-mm-wide and 150-mm-long cathode with changing binder percentage was connected to 3 M adhesive tape, and a highly accurate micromechanical test instrument was used to quantify the cathode sample peel strength. The electrode pasted on adhesive tape was peeled off at an angle of 180 degrees while moving at a constant speed of 20 mm/min. Force/displacement graphs were created by continually measuring the applied load [24]. Fig. 6(a–d) shows the peel strength of the electrode (S-1 to S-4). Here, peel extension is the displacement of the upper grasp, and peel strength is calculated by dividing the force by the sample width. Using data with peel extension ranging from 25 mm to 150 mm, the median and standard deviation of the peel strength were determined. The results are given in Table 1. Fig. 6(a) and (b) shows the high voids present on the surface of the electrode. For Fig. 6(c), uniform peel range was obtained due to less voids on the electrode. As it is same like S-4 shows higher in voids because of converting into sol nature of the electrode composite. Fig. 6(e) and (f) are before and after peel strength study of the electrode, respectively.

EIS studies were carried out on the pouch cell to understand the electrode behaviour with sequential addition of solvents process in the cathode mixing method. The cells were prepared with different cathode electrode slurries (S-1 to S-4) and the EIS characteristics are presented in Fig. 7. The EIS spectra profile reveals that the S-1 to S-4 samples follow almost similar behaviour with the formation of semicircle at high frequency region followed by a slope in the low frequency (Fig. 7). The EIS characteristics represents the charge transfer resistance (R_ct) and the Warburg impedance corresponding to Li-ion diffusion in the electrode. The R_ct values of the S-3 and S-4 are lesser than the S-1 and S-2 R_ct values. The low R_ct value of the electrodes indicate that the S-3 electrode had better electronic conductivity as compared to other electrodes (S-1, S-2, and S-4). The EIS results further support the flocculation formation that is wrapping up of the NCA cathode particle with conductive additives and binder which improves the stability and conductivity of the electrode. The low R_ct of the electrodes indicate that the S-3 electrode had better electronic conductivity as compared to other electrodes (S-1, S-2, and S-4). Our electrode is fitted with R₁+C₁/(R₂+C₂/R₃+W₃) circuit.

Fig. 8 and 9 shows the electrical performance of the cells was carried on pouch and cylindrical type of configuration with capacity of cell 20 and 1500 mAh, respectively, using layered NCA cathode with respective compositions as given in experimental section (S-1 to S-4). The graphite was used as negative electrode and the N/P capacity ratio maintained 1.15. The galvanostatic tests were carried out in the potential window of 2.8 V to 4.1 V at 0.2 C rate at room temperature for pouch cell and all subjected to three formation charge-discharge cycles at C/10 current rate and processed for degassing after completion of formation cycling. Fig. 8 shows the charge-discharge profile for the first cycle of pouch cells after formation. The NCA (S-1 to S-4) composition showed the higher cell charge/discharge capacity of 16.52, 18.31, 20.24, 18.75 mAh. The observed high columbic efficiency with high discharge capacity for the NCA S-3. We studied using commercial 18650 cylindrical cells are fabricated with the composition on high and low cycling efficient NCA cathodes S-1 and S-3, respectively. Fig. 9(a) showed the first charge-discharge profiles of 18650 cylindrical cells at 0.2 C rate charge and 0.5 C discharge rate between potential window of 4.1 and 2.8 V. The observed discharge capacities are 1.46 Ah and 1.53 Ah for the S-1 and S-3, respectively. The cyclic life behaviour of the S-1 and S-3 containing 18650 cylindrical cells to compare the capacity retention between the single step mixing electrode cylindrical cell and the high-performance mixing electrode cylindrical cell. The capacity of S-1 and S-3 cathode made cylindrical cells at 1.25 and 1.42 mAh for the capacity retention of 85.7 and 92.9%, respectively and Fig. 9(b) shows 150 cycles. This indicates that the three-step mixing (S-3) electrode show excellent high cycling performance.