Introduction

Li-ion batteries (LIBs), which possess the highest energy density among practical rechargeable batteries, are used in a wide range of applications such as portable electronic devices, electric vehicles, and stationary energy storage systems. In recent years, LIBs have been attracting intensive attention as a key technology that could contribute to the realization of a sustainable society, and further improvements in their energy and power densities are expected. Therefore, many researchers have made efforts to develop battery materials for advanced LIBs with higher performance for versatile applications. The battery performances of LIBs are greatly influenced by positive and negative electrode materials, which are key materials affecting energy density of LIBs. In commercialized LIBs, Li insertion materials that can reversibly insert and extract Li-ions coupled with electron exchange while maintaining the framework structure of the materials are used as both positive and negative electrodes. Because of the high reversibility of electrochemical Li insertion/extraction reactions of these materials, which is also classified as a topotactic reaction [1], LIBs succeeded in achieving longer cycle life when compared with the other commercialized rechargeable batteries. To meet the recent growing demand for advanced LIBs with higher energy and power densities, numerous efforts have been devoted to the exploration of novel Li insertion materials.

The electrode performance of Li insertion materials such as reversibility, cyclability, rate capability, and reaction kinetics is generally evaluated by several electrochemical measurements. For example, galvanostatic and potentiostatic charge/discharge tests, electrochemical impedance spectroscopy, and galvanostatic and potentiostatic intermittent titration techniques are often used as electrochemical techniques [2,3,4]. However, the detailed methods of theses electrochemical evaluations of Li insertion materials are often not described in literatures. In this article, we present a series of electrochemical evaluation protocols and methods of Li insertion materials including electrode preparation, cell assembly, and electrochemical measurements in the laboratory-scale research. In addition, the effects of battery components on the electrochemical properties of Li insertion materials are also discussed. The purpose of this paper is to provide basic knowledge and techniques for electrochemical evaluation of Li insertion materials to students and beginners in the field of battery research.

Lithium insertion materials

In the past four decades, various lithium-containing transition metal oxides have been discovered as positive electrode materials for LIBs. LiCoO2 is a layered oxide that can electrochemically extract and insert Li-ions for charge compensation of Co3+/Co4+ redox reaction and has been widely used from firstly commercialized LIBs to state-of-the-art ones [5]. Both Li and Co ions in LiCoO2 are located at octahedral sites in a cubic close-packed array of oxide ions, and LiO2 and CoO2 slabs are alternately stacked to form the α-NaFeO2-type layered structure with a two-dimensional conduction path for both Li-ions and electrons (Fig. 1a). LiCoO2 delivers a relatively high reversible capacity of ~ 180 mA h g−1 with a cut-off voltage of ~ 4.5 V (Fig. 1b). On charge (oxidation), Co ions in LiCoO2 are oxidized from Co3+ to Co4+, and Li-ions are released to the electrolyte solution to maintain charge neutrality. This reaction is described by the following equation:

Fig. 1
figure 1

a Comparison of schematic illustrations of the crystal structures for LiCoO2 (or LiNiO2), LiMn2O4, LiFePO4, graphite, Li(Li1/3Ti5/3)O4, and TiNb2O7. b Charge/discharge curves of various Li insertion positive/negative electrodes materials

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$${\mathrm{LiCo}{\mathrm{O}}}_{2}\to \square_{x}{\mathrm{Li}}_{1-x}{\mathrm{Co}}{\mathrm{O}}_{2}+x {{\mathrm{Li}}}^{+}+x\;{\mathrm{e}}^{-}$$
(1)

Herein, “□” indicates vacant octahedral sites formed by Li extraction. This process is highly reversible, and on discharge, Co ions are reduced to the trivalent state coupled with Li-ion insertion into crystal lattice. This process is achieved with relatively small volume change without the breaking of chemical bonds between Co and O ions, i.e., topotactic reaction. Theoretical capacity is calculated to be 274 mA h g−1 if all Li-ions are reversibly extracted and re-inserted. In recent years, Co ions, which have the material supply chain risk, are replaced by relatively abundant Ni ions, especially for electric vehicle applications. LiNiO2, which is isostructural with LiCoO2, has attracted intensive attention owing to its high reversible capacity of > 200 mA h g−1 [6]. In addition to the two-dimensional layered oxides, spinel-type LiMn2O4 with a three-dimensional Li conduction path has also been widely studied [7]. In LiMn2O4, Li and Mn ions are accommodated at tetrahedral and octahedral sites, respectively, and MnO6 octahedra share edges to form a three-dimensional framework structure (Fig. 1a). As shown in Fig. 1b, LiMn2O4 can be used as 4 V-class positive electrode materials, comparable to LiCoO2. Furthermore, LiNi0.5Mn1.5O4, where Mn in LiMn2O4 is partially substituted with Ni, is found to show a high operating voltage of 4.7 V with two-electron Ni2+/Ni4+ cationic redox [8]. Similar to LiNi0.5Mn1.5O4, high-voltage materials are proposed, but their commercial use is still hindered because of the unavoidable decomposition of electrolyte solutions. Although LiMn2O4 with abundant Mn ions has been used for electric vehicle applications in the past, it has been replaced by Ni-based layered materials and LiFePO4 made with abundant Fe ions. Olivine-type LiFePO4, which has one-dimensional Li conduction path, is widely used as cost-effective electrode materials based on abundant Fe ions, especially for electric vehicles, despite the low operating voltage of 3.4 V [9]. Because the electronic conductivity of LiFePO4 is as low as ~ 10−9 S cm−1 due to the corner-sharing of FeO6 octahedra (Fig. 1b) and existence of PO43− ions not contributing electronic conduction, carbon coating and particle size minimization during material synthesis are necessary to achieve a good electrochemical performance at high rate [10].

Li insertion materials have also been extensively studied as negative electrode materials for LIBs. Graphite is a typical layered material that can be used as negative electrode materials for LIBs. Carbon atoms are arranged in a hexagonal pattern within each graphene layer, and the layers are stacked in the ABAB packing (Fig. 1a). As shown in Fig. 1b, graphite is charged and discharged in the low voltage of 0–0.3 V and delivers a high theoretical capacity (372 mA h g−1). Upon charging, graphite is electrochemically reduced, and Li-ions are inserted into graphite to maintain electrical neutrality, as shown in the following equation [11].

$$\square{\mathrm{C}}_{6}+{\mathrm{Li}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{LiC}}_{6}$$
(2)

Although the graphene layer stacking is changed to the AA packing after Li insertion, this electrochemical Li insertion reaction is highly reversible without the destruction of a planner network of carbon atoms. In addition to carbonaceous materials, transition metal oxide, for instance, spinel-type Li(Li1/3Ti5/3)O4, is also applied for commercial LIBs (Fig. 1a). Li(Li1/3Ti5/3)O4 shows a flat voltage plateau via a two-phase reaction at 1.55 V vs Li/Li+ (Fig. 1b), and such a high operating voltage effectively suppresses the side reactions such as electrolyte decomposition and metallic Li deposition. In addition, the volume change of Li(Li1/3Ti5/3)O4 during cycling is almost negligible [12], which contributes to excellent cyclability of Li(Li1/3Ti5/3)O4. However, the low theoretical capacity of Li(Li1/3Ti5/3)O4 (175 mA h g−1) sacrifices energy density of batteries. To further increase the energy density, TiNb2O7 has attracted attention as an alternative negative electrode material [13]. TiNb2O7 has a monoclinic lattice, where MeO6 (Ti or Nb) octahedra share the corner and edges (Fig. 1a) and a higher theoretical capacity because of the enriched cationic vacant sites in the host structure with a high-valent transition metal ion, i.e., Nb5+ ions, while retaining a comparable operating voltage to Li(Li1/3Ti5/3)O4 (Fig. 1b). Operating voltages of Li-ion batteries are decided by differences in electrochemical potential between positive and negative electrode materials. By combining different positive and negative electrode materials, many Li-ion batteries with different operating voltages are fabricated.

Preparation of porous electrodes and electrochemical cells

Because Li insertion materials described in the former section are inorganic ceramic materials and obtained as powder form with particle sizes ranging from 100 nm to 20 μm, porous electrodes with polymer binders are prepared and used for electrochemical characterization. Slurry coating method is generally adopted to practical fabrication process of both positive and negative porous electrodes in LIBs as described in Fig. 2a. Porous electrodes consist of an active material (e.g., LiCoO2 powder), conductive agent (e.g., acetylene black, which is nanosized carbonaceous materials), and polymer binder (e.g., poly(vinylidene fluoride)). These materials are mixed in the appropriate weight ratios using a mortar and pestle, and then thoroughly mixed with N-methylpyrrolidone using a planetary centrifugal mixer, forming a slurry. The obtained slurry is pasted onto Al foil as a current collector with a doctor blade. Note that Cu current collector is used for the negative electrode materials with low redox potential (< 1 V vs. Li/Li+) to prevent the electrochemical alloy reaction of Al foil with Li. The porous electrode sheet is dried at 80 °C and then cut into a disk with a punching cutter followed by drying in a vacuum at 120 °C to prepare a porous electrode. Figure 2b shows the cross-sectional scanning electron microscopy (SEM) image and the corresponding energy-dispersive X-ray spectroscopy (EDX) maps of LiCoO2 porous electrode. These images prove that LiCoO2 particles and conductive carbons are uniformly dispersed in the porous electrode, forming an electron-conducting path from current collector to the top of the porous electrode layer. In addition, a uniform distribution of F elements indicates that PVdF binds active material and conductive agent to current collector.

Fig. 2
figure 2

a Images of fabrication process for a LiCoO2 porous electrode. Cross-sectional SEM and EDX mapping images of a LiCoO2 porous electrode are also shown in b

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To examine the electrochemical properties of Li insertion materials in the porous electrode, electrochemical testing in the laboratory-scale research is generally conducted using 2032-type coin cells with a porous electrode and Li metal electrode, so-called a half-cell. Coin cells are assembled in the configuration shown in Fig. 3a. Note that all the cell components must be dried carefully, and cell assembly needs to be done in an Ar-filled glovebox to avoid exposure to air and moisture. Metallic Li is highly reactive with moisture. The order of cell assembly is as follows: a bottom case, gasket, porous electrode, separator, electrolyte solution, metallic Li, spacer, wavy washer, and top case. To prevent the direct contact between positive and negative electrodes (i.e., internal short circuit), the separator size should be a larger diameter than that of both the porous electrode and metallic Li. In addition, the positive electrode area is preferred to be smaller than the Li negative electrode area to ensure the uniformity of electrochemical reactions of porous electrode. After the sealing of coin cells using a crimper, the cells can be electrochemically evaluated outside the glovebox. Among many different types of commercial LIBs, cylindrical cells are the most widely used because of their low cost and high energy density. In cylindrical cells, positive and negative porous electrode sheets are overlaid via a separator, and they are wounded into a cylinder shape, which effectively reduces the dead space in the cell (Fig. 3b). Moreover, cylindrical cells are produced by a roll-to-roll process [14], i.e., continuous manufacturing method for preparation of porous electrodes and integration of porous electrode sheet and separator, which enables and effectively reduces a manufacturing cost through stable and efficient mass production of batteries.

Fig. 3
figure 3

Configurations of the two-electrode coin cell used in laboratory and cylindrical cell used in commercial applications

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Electrochemical characterizations of Li insertion materials

Galvanostatic and potentiostatic charge and discharge test

Galvanostatic (constant current) charge/discharge test is the most popular experimental technique to study the electrochemical behavior of Li insertion materials. Figure 4a shows the current protocol and voltage profile in a galvanostatic charge/discharge measurement for a typical Li cell. The cell is charged at a constant current (I) until the cell voltage reaches certain voltage followed by a resting time. After the rest, the reverse current (− I) is applied to the cell in the same way, and the cycle is repeated. Current rate, which is often expressed by C rate, is an important parameter in a galvanostatic charge/discharge test. A 1 C rate represents the current rate at which a cell is fully charged or discharged for 1 h. At first, to assess the electrochemical properties of the Li insertion materials under nearly equilibrium conditions, the cell is typically charged and discharged at a low current rate less than 0.1 C. Then, the current rate is increased to evaluate the rate capability and cyclability. Cut-off voltage is also another critical parameter to be considered. Testing with different cut-off voltages provides the maximum available reversible capacity of the electrode materials. The voltage protocol and current profile in a potentiostatic (constant voltage) charge/discharge test are shown in Fig. 4b. When a constant voltage is applied to the cell, the faradaic current is observed, and then, it decays with time and eventually reaches zero. Subsequently, the cell is rested at open circuit condition for a duration and then polarized reversely for discharge. In both the galvanostatic and potentiostatic charge/discharge tests, the reversibility of electrochemical Li insertion/extraction reactions is evaluated by Coulombic efficiency, which is defined as (discharge capacity)/(charge capacity). If Coulombic efficiency is close to 100%, Li insertion/extraction reaction is highly reversible without side reactions, e.g., electrolyte decomposition.

Fig. 4
figure 4

Protocols of a galvanostatic and b potentiostatic charge/discharge tests, c GITT, and d PITT

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Galvanostatic intermittent titration technique (GITT) and potentiostatic intermittent titration technique (PITT) have been commonly used to understand the kinetic properties of porous electrodes. Figure 4c and d show the schematic diagrams of GITT and PITT. In a GITT measurement, the cell is charged (or discharged) at a constant current (I) for certain interval without full charge and then rested to allow the cell voltage to relax to the equilibrium state (Fig. 4c). This procedure is repeated until the cell voltage reaches to the highest/lowest cut-off voltages, thereby obtaining quasi-open-circuit voltage curves and polarization profiles during charging/discharging. In addition, GITT allows us to determine the electrode kinetics and apparent chemical diffusion coefficient of Li-ion in the Li insertion material depending on the current decay during rest time (relaxation processes) [3, 15]. PITT is performed by measuring current response to applied small potential steps of ΔE (Fig. 4d). The potential pulses are applied repeatedly until the upper or lower voltage cut-off is reached. From the PITT measurement, apparent chemical diffusion coefficients of Li-ion in the electrode can also be calculated as well as GITT [2, 16]. Single-phase or two-phase reaction is also distinguished from each other based on the current response [17].

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is a powerful tool to study the electron and ion transport in the porous electrode and charge transfer reaction from/to transition metal ions, coupled with lithium extraction/insertion from/into insertion materials and solvation/desolvation into/from electrolyte solutions, depending on different time scales [18]. In a potentiostatic EIS measurement, sinusoidal alternating voltage is applied to an electrochemical cell and its current response is monitored to obtain the resulting complex impedance (Z) according to Ohm’s law (Fig. 5a). EIS is performed in a wide range of frequency (f), which allows to distinguish the internal processes with different time constants in the cell. When a sinusoidal potential signal is applied, delays of current response (phase shift θ) are generally observed as shown in Fig. 5a. Current delays originate from the presence of non-faradaic (capacitive) components, e.g., the formation of electrical double layers. Bode plots are useful to display the frequency dependence of impedance from the obtained EIS data. In Bode plots, log |Z| and phase shift θ are plotted versus log f, and the slopes of these plots reflect dominant impedance response in a certain frequency range (Fig. 5b). Nyquist plots (or Cole-Cole plots), which are complex plane plots of the negative of the imaginary part (–Zʺ) versus the real part (Zʹ) of the complex impedance, are an alternative representation to display variations of impedance with frequency. Figure 5c shows a typical Nyquist plot for an electrochemical cell with lithium insertion materials in the porous electrode and metallic Li. By fitting the Nyquist plot to an equivalent circuit of resistors R and capacitors C shown in Fig. 5d, each resistive component of the impedance can be evaluated individually, where Z is the Warburg impedance used for modeling diffusion behavior in the porous electrode. The impedance analysis with the Nyquist plot is helpful to understand the electrochemical reaction kinetics and degradation mechanism of the electrode materials [19,20,21,22]. As shown in Fig. 5c, two semicircles appear in the Nyquist plot for porous electrode, where high-frequency component is related to coupled Li-ion/electron conduction in the porous electrode and another low-frequency component associated with charge transfer reaction from/to transition metal ions coupled with lithium-ion extraction/insertion from/to insertion materials (Fig. 5e). This process is also influenced by solvation/desolvation of Li-ions in electrolyte solutions [23]. A constant phase element (CPE) is often used for the analysis of EIS instead of a pure capacitor. In general, better fitting results are obtained with CPE, and electrode roughness and current non-uniformity for porous electrodes are often used for the physical explanation of CPE even if its true nature is unknown [24].

Fig. 5
figure 5

a Current and voltage profiles, b Bode plots, and c Nyquist plot of EIS measurement. An equivalent circuit used in the fitting of impedance and schematic illustrations for the assignment of impedance for porous electrodes with different time scales are also shown in d and e, respectively

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Impacts of battery components

The effects of battery components on the electrode performance of Li insertion materials are discussed in this section. Cyclability of Li insertion electrodes is strongly influenced by electrolyte solutions and polymer binders. Figure 6 compares the galvanostatic charge/discharge curves of the Li/LiCoO2 cells with different electrolyte solutions, i.e., 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) and 1 M LiPF6 in methyl 3,3,3-trifluoropropionate (MTFP), and different polymer binders, poly(vinylidene fluoride) (PVdF), and carboxymethyl cellulose (CMC)/styrene-butadiene rubber (SBR). Although the reversible capacity of each cell is almost the same at the first cycle, the clear difference in cycle performance is noted in the subsequent cycles. For the cell with EC/DMC-based electrolyte (Fig. 6a), the polarization is increased, and reversible capacity is reduced on electrochemical cycles, probably due to the continuous interfacial impedance growth associated with irreversible electrolyte decomposition and surface degradation of LiyCoO2. On the other hand, the increased polarization and reduced reversible capacity on continuous cycles are mitigated to some extent for the cell with MTFP-based electrolyte because of the higher oxidative stability of the electrolyte and formation of an effective passivation layer on the LiCoO2 surface (Fig. 6b). The cyclability of LiCoO2 electrode is also improved by using functional polymer binders. As shown in Fig. 6c and d, the use of SBR/CMC binder can effectively suppress the capacity degradation of high-voltage LiCoO2 electrodes in both the EC/DMC- and MTFP-based electrolytes, which can be attributed to the high adhesive strength and effective passivation ability of SBR/CMC [25]. Consequently, the combined use of functional electrolyte and binder achieves no capacity fading for 50 cycles under high-voltage operation [26].

Fig. 6
figure 6

Comparison of charge/discharge curves of LiCoO2 porous electrodes with a, b PVdF and c, d SBR/CMC binders in a, c 1 M LiPF6 in EC/DMC and b, d 1 M LiPF6 in MTFP electrolyte solutions at 25 mA g−1 at room temperature. The sample loading of LiCoO2 for both electrodes ranges from 4.5 to 5.0 mg cm−2. Reprinted with permission from Ref. [26]. Copyright 2023 American Chemical Society. Chemical structural formulas of electrolyte components and binder polymers are also shown in e

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Recently, several research groups reported that highly concentrated electrolytes (HCEs) are effective in improving the reversibility and cyclability of Li batteries [27,28,29,30,31,32,33]. Here, we emphasize that the battery performances of Li cells with HCEs strongly depend on the battery components. The use of HCEs containing LiN(SO2F)2 (LiFSA) salt has obvious benefits for both positive and negative electrode materials [34, 35]. However, stainless steel (SS), which is widely used as a case material for laboratory electrochemical cells, severely corroded during charging at high voltage in the LiFSA-based electrolytes even at high salt concentrations, and this fact has often been overlooked. In contrast, the oxidative corrosion of Al current collector is suppressed using LiFSA-based HCEs [36, 37]; therefore, the case materials of electrochemical cells for the positive electrode side need to be made with Al when LiFSA-based HCEs are used as electrolyte. Charge/discharge curves and capacity retention of a nickel-rich layered material in Li cells made with SS and Al using 5.5 M LiFSA/DMC electrolyte solution are shown in Fig. 7a and b, respectively. The SS cell exhibits a large irreversible capacity and continuous capacity fading upon cycles due to the oxidative corrosion of SS during charge. In contrast, when an Al cell is used as electrochemical cell, highly reversible charge/discharge behavior and almost no capacity fading for 200 cycles are observed. Thus, the use of Al cell or Al-coated SS cell is required to obtain the reversible cycling with LiFSA-based HCEs. Poor wettability of conventional polyolefin separators toward HCEs with high viscosity is another critical issue to be addressed for practical applications. Very recently, we reported that the electrolyte wettability issue is solved by using the functional polyolefin separator coated on both sides with the meta-aramid layer (Fig. 7c, d) [34]. The polar functional groups in the aramid-coating layer provide a strong affinity of polyolefin separator to HCEs, resulting in the improved electrolyte wettability. Although porous glass fiber filters, which have large pores of 5–10 μm (Fig. 7e), are often used as separators for HCEs, Li dendrites easily penetrate the separator during repeated charge/discharge cycling for the Ni-rich layered material with the glass fiber filter, resulting in an internal short circuit (Fig. 7f). In contrast, the use of aramid-coated polyolefin separator possessing nanosized pores (Fig. 7e) and high electrolyte wettability effectively suppresses the penetration of dendritic Li metal through the separator during cycling and enables stable operation of the Ni-rich layered material for 200 cycles (Fig. 7f).

Fig. 7
figure 7

a Charge/discharge curves and b capacity retention of Ni-rich layered material in Li cells made with SS (top) and Al (bottom) case using the LiFSA:DMC = 1:1.1 (molar ratio) electrolyte solution at 50 mA g−1 at room temperature. c Photographs of wettability of a polyolefin separator (left) and aramid-coated polyolefin separator (right) to the highly concentrated electrolyte. d Chemical structural formula of meta-aramid used for coating. e SEM images of the glass fiber filter and aramid-coated polyolefin separator. f Charge/discharge curves of Ni-rich layered material in Li cells with a glass fiber filter (top) and aramid-coated polyolefin separator (bottom). c–e were reprinted from Ref. [34]. Copyright 2023 Authors, licensed under a Creative Commons Attribution (CC BY) License

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Factors affecting electrode kinetics

Rate capability, which is another critical battery performance in practical LIBs, strongly depends on the composition of electrolyte solution. Figure 8 shows the comparison of discharge curves of Li/LiCoO2 cells with sulfolane (SL)–based highly concentrated electrolytes containing different anions, FSA−, N(SO2CF3)2− (TFSA−), and BF4−, measured at various current densities. With increasing current density, the discharge capacity of each cell decreases. This is because the polarization for discharging increases with current density, and the cell attains the cut-off voltage (3.0 V) prior to achieving the full discharge capacity of LiCoO2. The cell with LiFSA:SL = 1:3 (in a molar ratio) electrolyte, which possesses the highest ionic conductivity [38,39,40], exhibited the highest rate performance among the tested cells (Fig. 8a). The resistance of bulk electrolyte is inversely proportional to ionic conductivity; therefore, the highest rate capability observed for the LiFSA:SL = 1:3 electrolyte is justifiable. The cell containing LiTFSA:SL = 1:3 electrolyte exhibited a smaller polarization at the beginning of discharge and resulted in higher rate capability when compared with that containing LiBF4:SL = 1:3 even though the ionic conductivities of both the electrolytes are comparable (Fig. 8b, c) [38,39,40]. The initial polarization includes the voltage drops originating from not only bulk resistance of electrolyte but also resistance related to Li-ion/electron conduction in the porous electrode [41,42,43]. Because the ohmic drop associated with electron conduction in LiyCoO2 is expected to be the same for both cells, the difference in ohmic drop is influenced by Li-ion migration at the surface of LiyCoO2 coupled with adsorption/desorption of Li-ions [23]. In addition, the charge transfer coupled with Li extraction/insertion and solvation/desolvation of Li-ions may also influence the ohmic voltage drop. These interfacial phenomena are affected by a nature of salts. For instance, LiTFSA is known as highly dissociative Li salts with a weak Lewis basic anion, and therefore, it is expected that the resistance at the LiCoO2/electrolyte interface is decreased as increasing Li-ion activity in the LiTFSA electrolyte [44]. Moreover, the contribution of Li metal electrode on voltage drop cannot be eliminated, especially for LiBF4 because of the instability of BF4− against metallic Li [45].

Fig. 8
figure 8

Rate capabilities of LiCoO2 porous electrodes with the a LiFSA:SL = 1:3, b LiTFSA:SL = 1:3, and c LiBF4:SL = 1:3 electrolyte solutions at room temperature. Chemical structural formulas of SL and Li salts are shown in the inset of a–c

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The rate capability of Li batteries is also affected by mass loading of active material (and areal capacity) in the porous electrode. Heavier loading results in larger areal capacity, leading to the development of batteries with higher volumetric energy density. Figure 9a shows rate capability of LiCoO2 porous electrodes with different mass loadings of active material at various current rates (25–4000 mA g−1). Upon increasing the loading weight of LiCoO2, the discharge capacity is decreased more significantly at high current rates. This is because areal current density (mA cm−2) becomes larger with increasing the loading weight of LiCoO2 at the same current rate, leading to the larger ohmic drop at the beginning of discharge. In addition, the voltage drop caused by concentration polarization during discharging is steeper for the thicker electrode with a higher mass loading because the diffusion limiting current decreases as the porous electrodes become thicker [46]. Note that the discharge voltage for the cell with the heaviest loading (i.e., 15 mg cm−2) and thickest electrode is abruptly decreased during discharge at higher current rates > 800 mA g−1. This suggests that the supply of Li-ion from electrolyte bulk to porous electrode becomes zero (i.e., a diffusion-limited condition). Apparently, thinner electrodes seem to be advantageous for achieving high-rate capability, but both areal capacity and volumetric energy density are sacrificed for thinner electrodes. Figure 9b plots the normalized discharge capacity as a function of current density. The normalized capacity of each sample decreases with increasing the current density. Notably, from the comparison at the same current density over 1 mA cm−2, the thinner electrode with a lower sample loading delivers a smaller areal capacity. This is because a current density based on the mass of LiCoO2 becomes higher for the thinner electrode at the same current density, resulting in a larger polarization caused by the limitation of solid-state Li-ion migration inside LiCoO2 particle at higher current density. In contrast, even for the thickest electrode, the discharge capacity deceases dramatically at currents above 20 mA cm−2 due to the mass transfer limitation in the electrolyte. Therefore, further improvement of ionic conductivity is necessary to develop rechargeable batteries with larger volumetric energy density and superior power density in the future [47].

Fig. 9
figure 9

Rate capabilities of LiCoO2 porous electrodes with different mass loadings. a Charge/discharge curves and b normalized discharge capacity versus current density

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Conclusions

Fundamental methods of electrochemical characterizations of Li insertion materials including electrode preparation, cell assembly, and electrochemical measurements are systematically described. The basic electrochemical techniques, galvanostatic charge/discharge tests, EIS, GITT, and PITT, are introduced, and the impacts of battery components on the electrochemical properties of Li insertion materials are also discussed. The electrode reversibility is greatly influenced by electrolyte solutions and polymer binders. In particular, LiFSA-based highly concentrated electrolytes are effective in improving reversibility of high capacity of Li insertion materials coupled with the combined use of Al-bodied cell and functional separator. Furthermore, compositions of electrolyte solutions and mass loading of active material exert significant effects on the rate capability of Li insertion materials. This paper provides basic knowledge and technique for electrochemical evaluation of Li insertion materials to students and beginners in the field of batteries.