NMC cathode was prepared by casting a slurry consisting of 93% (NMC, Ecopro, Korea) as active material, 4 wt.% of SP (TIMCAL SUPER P^TM), VGCF (Showa Denko), KB (EC 300 J, AkzoNobel), or CNT (LG Chem, South Korea) as conductive additive, and 3 wt.% polyvinylidene fluoride (PVdF, Kreha, Japan, M_w ≈ 350,000) as polymeric binder in N-methyl-2-pyrrolidone (NMP, Sigma Aldrich). The physical properties of the different CC are depicted in Table 1. The viscous slurry was cast on an aluminum foil (15 µm, San-A Aluminum, Korea) via doctor blade apparatus. The coated slurry was dried at 120°C for 2 h and then pressed with a roll pressing machine (WV-60, Samyang 60, South Korea). A composite electrode without conductive carbon was also prepared for comparison. The thickness, densities, and loading levels were controlled to about 49 μm, 2.1 g cm⁻³ and 15 mg cm⁻² respectively, for all the composite electrodes.

The surface morphology and electrical resistance of the electrodes were measured using field-emission scanning electron microscope (FE-SEM, HITACHI SU5000, Japan) and linear four-probe system (CMT-100, AIT-TU, Korea). For the surface resistance measurement, the dried and pressed electrodes, consisting of the cathode material, the CC and the PVDF binder was used. The cathodes were cut using an argon-ion milling system (Hitachi IM4000Plus, Japan) for cross-sectional analysis. The adhesion strength between the particles of the composite film (A_mid), and the composite film and the current collector (A_C/Al) were measured using SAICAS (Diapla Winters Company limited, Japan). A boron nitride blade with a width of 1 mm, fixed at a shear angle of 45° was used. To measure the adhesion strength between the composite and the current collector (A_C/Al), the blade of the instrument moves vertically with a fixed load of 0.5 N until it contacts the current collector where the vertical force is changed to 0.2 N (region A, Fig. 3a). The blade then moves in the horizontal direction employing this new vertical force (region B, Fig. 3a). The results were measured at a constant vertical fixed load mode. On the other hand, the adhesion strength between the particles of the composite (A_mid) was measured at a constant rate mode. In a typical A_mid measurement, the blade moves with a speed of 0.2 μ·ms⁻¹ in the vertical direction until it gets to the center of the coated film (region A, Fig. 3b) and then move in the horizontal direction (region B, Fig. 3b). A_C/Al and A_mid were obtained by dividing the horizontal forces in each case by the width of the blade.

The electrodes were punched into a spherical disc with a diameter of 14 mm. The half cells assembled in a coin-type cell (2032) and in an argon-filled glove box consisting of lithium metal (16 mm, 99.99% Aldrich) as counter and reference electrode, and NMC with the different CC as the working electrode. 1.2 M LiPF₆ (EC/DMC =1:3 volume ratio) and polyethylene separator (PE, 18 mm, 20 μm, celgard) were used as the electrolyte and separator, respectively.

Cyclic voltammograms (CVs) of the cathode with the different CC were measured at a scan rate of 0.1 mV s⁻¹ and a potential range of 2.5-4.4 V using Autolab (ECO CHEMIE PGSTAT 100). Charge and discharge tests were measured at room temperature in the potential range of 2.8-4.4 V, charging in constant current/constant voltage (CC/CV) mode at 0.1 C-rate and discharging at 0.1, 0.2, 0.5, 1, 2, and 5 C in CC mode using a battery cycler (PNE Solution, Korea). The cycle performance was measured for 100 cycles, charging in CC/CV mode at 1 C and discharging at 1 C in CC mode at the same voltage range. The rate performance was conducted at increasing rates of 0.2, 0.5, 1, 2, and 5 C for charging (CC/CV mode) and discharging (CC mode) in the voltage range of 2.8-4.4 V. All the cells were kept for 6 h before electrochemical measurement.

The CV curves of the NMC with the various CC are shown in Fig. 4. All the samples showed a cathodic peak around 3.7 V and an anodic peak around 3.9 V, which resonates with other publications [29,30]. These peaks can be attributed to the redox reaction of Ni²⁺/Ni⁴⁺, during the insertion and deinsertion of lithium ions into the crystal structure of NMC. There is no peak corresponding to the reduction of Mn at 3.0 V; thus, it has an oxidation state of +4. According to Table 3, the oxidation-reduction peak separation ΔE (E_oxidation-E_reduction) increases in the order of NMC-CNT < NMC-KB < NMC-VGCF < NMC-SP, which means the composite with CNT and KB poses the least polarization; hence, the excellent rate capability observed in Fig. 5.

In addition, the charge-discharge capacities of the NMC with the different CC are depicted in Fig. 5. The curves of all the composites showed a similar charge-discharge plateau with a significant amount of capacity around 3.5-4.2 V, which corresponds to the redox transition of Ni²⁺/Ni⁴⁺ in the structure of NMC. The fiber-like CC form a continuous conductive network connecting the different particles in the composite. This ensures the efficient passage of electrons through the composite. On the other hand, the particle-like CC lacks this continuous conductive network, and connects adjacent active material through direct point contact. This credible theory has been propagated by many researchers [8,10,12]. Nonetheless, results from A_C/Al and A_mid indicate that the adhesion strength of the composite film on the current collector and the adhesion property between the particles of the composite are highly influenced by CC. The composite with VGCF and SP (Fig. 5a and 5b) demonstrated the highest polarization at a high C-rate with their discharge profiles shifting quickly toward lower potentials compared to the composites with CNT and KB (Fig. 5c and 5d). This phenomenon may be attributed to the reduced adhesion strength between the composite film and the current collector. Looking at Fig. 5 again, it can be seen that the composite with KB and CNT showed the best rate capability. There is strong evidence indicating that the high conductivity and the better adhesion played a synergistic role in providing these composite electrodes the best electrochemical performance. The cycle performance of the NMC electrodes with the different CC is depicted in Fig. 6a. The electrodes with CNT and KB, in particular, showed stable and higher capacity after 20 cycles. On the other hand, the electrodes with VGCF and SP showed decreasing capacity with an increasing number of cycles. This behavior can be closely related to the excellent adhesion exhibited by the composite consisting of CNT and KB due to their structure and particle size. Fig. 2b shows the rate capability of the NMC electrodes with the different CC. It can be observed that the electrodes with CNT and KB showed a noticeable increase in capacitance at lower C-rates compared to the electrodes with VGCF and SP. This can be attributed to the low resistance and better adhesion demonstrated by the composite with CNT and KB compared to the electrodes with VGCF and SP. This suggests that employing conductive particles with relatively smaller particle size is significantly improves the adhesion of composite electrodes. Multiple CC (two or more conductive particles) with reduced weight composition can be utilized to achieve better adhesion and thus enhance the electrochemical performance of composite electrodes. Finally, the surface treatment of current collectors may also be encouraged to reduce the interfacial resistance between the current collectors and the composite film.