In response to recent public demand for reducing carbon emissions using fossil fuels [1–4] portable technology is being developed for large-scale applications in electronic devices, and hybrid/full-electric vehicles (EV) [5,6].

Currently, Li-ion batteries are innovative energy storage technology [7–10] and are applied in electronic gadgets, such as mobile phones, and laptop computers; and EVs. Studies have attempted to improve the energy density of lithium-ion batteries for large-scale applications [11]. Consequently, the amount of nickel increased to more than 90%, and the cost was reduced by decreasing the amount of expensive Co [12].

However, the Ni-rich cathode material causes internal microcracks when Li-ions are extracted/inserted, resulting in volume expansion and disruption [13] and the rapid decrease in capacity owing to side reactions with electrolytes during long cycling degraded the electrochemical performance of LIBs and their limited applications [14,15].

First, to improve these limitations caused by volume expansion, various nanomaterial synthesis methods, such as the solution spray method [16], sol-gel method [17], and hydrothermal synthesis [18] are studied. Nanoparticles provide a large contact area between the electrode and electrolyte, reducing the Li-ion diffusion path distance and this facilitates the movement of Li-ions in the charge/discharge process by suppressing the volume expansion of nanoparticles [19,20]. However, side reactions occur between nanoparticles and electrolytes and the cycle characteristics deteriorate [21]. In this study, the nanocarbon layer was formed using a new approach, wherein, polyvinylpyrrolidone (PVP) was directly added during the synthesis process to suppress the side reaction with the electrolyte, which is a challenge in the high-nickel-based cathode materials. Comparing the surface coating state, interfacial resistance, and related electrochemical properties of Li[Ni_0.93Co_0.07]O₂ (NC) synthesized by electrospinning (E-NC) and those synthesized by co-precipitation (C-NC), we investigated the applicability of nanofiber-type E-NC for high-speed charge-discharge secondary batteries.

The surface microstructure of the E-NC and C-NC samples is observed using a SEM. Fig. 1(a) shows the nanofiber structure of the E-NC sample. Fig. 1(b) (an enlarged image of (a)) shows the porous structure of E-NC with a primary particle size of 50–100 nm. Fig. 1(c) shows the spherical particles of a C-NC sample. Fig. 1(d) (an enlarged image of (c)) shows a C-NC sample with a primary particle size of 250–300 nm. The morphology of the porous nanofibers of E-NC offers a larger surface area than C-NC, and promotes the efficient charge transfer and the Li-ions diffusion. Also, it is expected that the charge transfer resistance (R_ct) will be decreased, and the high-rate charge/discharge performance will be improved [22].

Fig. 2 shows the X-ray diffraction (XRD) pattern of the E-NC and C-NC samples. The diffraction peaks of both E-NC and C-NC indicate a hexagonal structure with an R3m space group without an impurity phase. E-NC sample shows double peaks at (006)/(012) and (018)/(110); This indicates that E-NC has a well-arranged hexagonal structure. Table 1. lists the lattice parameters of E-NC and C-NC samples. Excessed nickel limits Li⁺ diffusivity due to cation mixing [23]. E-NC shows a higher I(003)/I(104) ratio than C-NC, which means lower cation mixing. E-NC is mixed by a liquid phase method, it is expected that Li, Ni, and Co will be uniformly mixed atomically. C-NC is produced by the solid-state method of mixing a precursor and LiOH·H₂O is unlikely uniformly mixed because of the possibility of the atoms of lithium and nickel changing each other. Accordingly, with the high uniformity mixing of E-NC, it is expected to increase Li-ions diffusion, resulting in improve charge/discharge capacity at high rate.

Table 2 shows the carbon amount of the E-NC and C-NC samples. The carbon amount of the E-NC sample is three times higher than that of the C-NC sample, expecting the improvement in electrical conductivity, high-rate charge/discharge performance and decrease charge transfer resistance (R_ct).

Fig. 3 shows Ni 2p XPS of E-NC and C-NC. The ratio of Ni²⁺/Ni³⁺ of E-NC is lower than that of C-NC, suggesting that lower proportion of Ni²⁺. The lower Ni²⁺ leads with the lower Li⁺ and Ni²⁺ mixing, which is aligned well with the XRD results.

We measured the electrochemical properties using galvanostatic charge-discharge curves. Fig. 4(a) shows that the initial discharge capacity of E-NC is 206 mAh g⁻¹ at 0.1C, which is higher than that of C-NC (189.2 mAh g⁻¹). Fig. 4(b) shows the differential capacity (dQ/dV) measured at 0.1C. In the discharge state, the peak for E-NC shifted to a higher potential at 4.1–4.3V, this is a result of the improved electron transport by the carbon coating and shorten diffusion length of Li-ions by reducing the particle size [23].

Fig. 5(a) shows the cycling performance of E-NC and C-NC. After 60 cycles at 0.5C, E-NC maintains a capacity of 152.12 mAh g⁻¹ (76.2%), which is higher than that of C-NC (115.72 mAh g⁻¹ = 68%). Fig. 5(b) shows the discharge capacity rate at various rates from 0.1C to 5C. As the rate increased, E-NC maintains 118.32 mAh g⁻¹ (61.7%), whereas C-NC shows a dramatic decrease at 5C and maintains 78.22 mAh g⁻¹ (45.7%). Because of high level cation mixing of C-NC, high-speed charge/discharge performance is deceased due to limit of the movement of Li-ions. E-NC with carbon-coated nanomaterials increases ionic conductivity, electronic conductivity, improved high-rate charge and discharge performance by increasing the surface area due to the reduced size of primary particles [24].

Fig. 6 shows the resistance measurements of the E-NC and C-NC samples before cycle and afer 20 cycles, measured at 4.3 V to study the surface resistance of the cell. The first semicircle represents the solid electrolyte interface resistance (R_f) and the second semicircle represents the charge-transfer resistance (R_ct). A transparent semicircle is formed between R_f and R_ct after 20 cycles. From Table 3, the R_f value of E-NC increases from 3.5 Ω to 7.3 Ω, and that of C-NC is from 2.6 Ω to 11 Ω, R_ct value of E-NC increases from 5.12 Ω to 7.7 Ω. The change in R_f and R_ct are lower than that of C-NC increased from 5.27 Ω to 11.7 Ω after 20 cycles. The porous nanofiber structure E-NC increases the surface area by reducing the particle size, which shortenes the Li-ions diffusion path. [25]. According to the relationship between Z′ and ω^−1/2 as shown in Fig. 5(c), the Li-ion diffusion coefficient of E-NC and C-NC electrodes are calculated as 1.75×10⁻¹⁰ and 9.82×10⁻¹¹ cm² s⁻¹, respectively. The higher ions diffusion is aligned well with the better electrochemical performance. In addition, the electric conductivity is improved by the carbon coating, therefore the charge transfer resistance (R_ct) is decreased, and the high-rate charge and discharge performance is improved [26].

In this study, PVP-based direct carbon-coated porous nanostructure E-NC and its properties are compared with C-NC (synthesized via co-precipitation). SEM image shows primary particle size of E-NC has a nanorod structure with a diameter range of 50–100 nm, C-NC has a spherical structure with a diameter range of 250–300 nm. XRD indicates that E-NC shows lower cation mixing than C-NC. E-NC were synthesized using the liquid mixing method to uniformly mixed atomically, C-NC is synthesized by a solid-state method by mixing LiOH · H₂O based on lithium diffusion, and the atomic states of lithium and nickel presumably change each other. The electrochemical results shows that E-NC had an initial discharge capacity 206 mAh g⁻¹ at 0.1C (17 mA g⁻¹), which is an improvement of 10% compared to that of C-NC. At a high-rate of 850 mA g⁻¹, E-NC maintained 118.32 mAh g⁻¹ (61.7%) compared to that of C-NC (78.22 mAh g⁻¹ (45.7%)), the high-rate capacity was improved by more than 50%. The impedance of the samples was analyzed using EIS. After 20 cycles, at 4.3V, the charge-transfer resistance of E-NC increases from 5.12 Ω to 7.7 Ω, and that of C-NC increases from 5.27 Ω to 11.7 Ω, indicating a decrease in resistance of more than 35% compared with C-NC.

The results of this study can be summarized as follows: 1. Shortening the Li-ion diffusion path promoted the movement of Li-ions, 2. synthesizing by liquid mixing method reduced cation mixing, and 3. carbon coating improves electrical conductivity, resulting in excellent rate performance.

This study proposes the possibility of using nanomaterials as cathodes for applications in EVs with high-rate charge and discharge performance.