Surfactant-coassisted sol–gel synthesis to prepare LiNi _y Mn _y Co_{1 − 2y} O₂ with improved electrochemical behavior

Abstract

Layered LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds with interconnected spherical particles are synthesized using a novel surfactant-coassisted sol–gel process. A combination of cetyltrimethyl ammonium bromide as surfactant and citric acid as chelating agent results in the formation of hydrophobically nucleated transparent gel, which upon furnace calcination yields phase pure and growth-controlled spherical particles of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds. The compounds containing Co³⁺ ions, viz., LiNi_0.45Mn_0.45 C_0.1O₂ and LiNi_0.4Mn_0.4Co_0.2O₂ cathodes, exhibit good cation ordering and better electrochemical performance than LiNi_0.5Mn_0.5O₂ cathode. The study demonstrates the superiority and suitability of LiNi_0.4Mn_0.4Co_0.2O₂ cathode over LiNi_0.5Mn_0.5O₂ and LiNi_0.45Mn_0.45Co_0.1O₂ cathodes at 1 C rate and up to 4.6 V limit.

Introduction

Majority of the commercialized cathode materials for lithium ion batteries are based on LiCoO₂, but cobalt has environment-related problems that leave the door open to exploit alternative cathode materials [1]. Over the past several years, there have been concerted efforts to design Mn- or Ni-based layered LiMO₂ compounds with a view to find a less expensive and safer cathode material than LiCoO₂. However, LiMnO₂ and LiNiO₂ of LiMO₂ family have hampering issues like difficulty in the synthesis of stoichiometric and stable compounds and thus remain unsuitable for practical applications.

In an electronic stabilization approach, when Mn is partially substituted by Ni in equal concentration such as 1:1 in LiMn_0.5Ni_0.5O₂ compound, Ni is found to be in +2 state and Mn in +4 state, thus eliminating the adverse effects of Jahn–Teller distortion prone Mn³⁺ ions and becomes a successful cathode [2–4]. Basically, LiNi_0.5Mn_0.5O₂ cathode offers all the advantages of LiCoO₂ and a higher discharge capacity of 250 mA h g⁻¹ (when charged up to 4.8 V) along with an apparently better chemical stability resulting from the electrochemical redox process of Ni²⁺/Ni⁴⁺ via Ni³⁺ [2–6]. It also reduces the nickel content of LiNiO₂ electrode by 50 % without compromising its theoretical capacity (274 mA h g⁻¹).

However, the major drawback of LiMn_0.5Ni_0.5O₂ compound is that ~0.1Li⁺ per formula unit is found in the transition metal layer of the O3-layered structure, which displaces an equivalent amount of Ni²⁺ to Li⁺ layer [7, 8]. Such a disorder or mixing of Li⁺ and Ni²⁺ in alternative layers may also result from the fact that they have similar ionic radii (Li⁺ = 0.76 Å and Ni²⁺ = 0.69 Å). On the other hand, presence of cobalt plays a critical role in minimizing the transition metal content in the lithium layer and enhances the electronic conductivity [9, 10]. Hence, lithium–nickel–manganese compounds with smaller amount of cobalt would be a better choice to prepare tailored cathode materials with better physical and electrochemical properties. Since the charge compensation in LiNi_0.5Mn_0.5O₂ proceeds via Ni²⁺ and Mn⁴⁺ in the transition metal layers, replacement of Ni²⁺ with an equal amount of M³⁺ (Co³⁺) and Mn⁴⁺ will maintain the charge neutrality. Based on this simple concept, two solid solutions, viz., LiNi_0.45Mn_0.45Co_0.1O₂ and LiNi_0.4Mn_0.4Co_0.2O₂, were chosen for the present study along with the parent LiNi_0.5Mn_0.5O₂ cathode.

It is well known that the electrochemical performance of any electrode material is highly sensitive to the synthesis procedure, which strongly affects the cationic distribution in lithium-transition metal oxides [11–13]. Towards this direction, literature is replete with results of this research group [14, 15] and other groups [16–19] on the citric acid-assisted sol–gel method, where the citric acid acts as a chelating agent to produce a transparent gel and to form the desired cathode materials. Quite different from such an approach, cetyltrimethyl ammonium bromide (CTAB), a known quaternary ammonium salt, has been exploited in the present study along with the chelating citric acid in order to obtain interconnected spherical morphology.

Initially, CTAB gets absorbed on the surface of precursor particles with its hydrophobic head inside and hydrophilic tail oriented away from the surface and thereby minimizes the friction between individual metal particles. Further, formation of metastable chelating complex in the form of a transparent gel is aided by the addition of citric acid and by adjusting the pH with NH₄OH. Herein, the selective capping effect of CTAB by its hydrophobic regions nucleating the surface plays a vital role in forming spherical particles without agglomeration via strong surface interaction and controls the growth of the same even upon high-temperature calcinations. The chelating citric acid forms a transparent gel at an alkaline pH (consisting of CTAB-facilitated spherical particles) and renders interconnectivity between the nano spherical particles to result in the formation of spherically interconnected surface morphology with a porous structure (Scheme 1). Such a formation of desired morphology with porosity will facilitate easy penetration of electrolyte and facile lithium ion transport, which is the significance of the study.

Scheme 1

Formation of interconnected spherical particles facilitated by CTAB-coassisted sol–gel method

To sum up, the present study deals with the exploration of CTAB-coassisted sol–gel method in synthesizing phase pure and tailored cathodes of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] family, wherein synergistic effect of synthesis method and the select concentration of Co³⁺ ions in improving the electrochemical performance has been investigated. Particularly, cycleability at extended voltage limits (4.5–4.8 V) and at different rates (0.1–1 C) has been studied.

Materials and methods

Synthesis procedure

The LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] active materials planned for the study were synthesized by adopting surfactant (CTAB) coassisted sol–gel method. Stoichiometric ratios of high purity starting materials, viz., LiCOOCH₃, (CH₃COO)₂Co, (CH₃COO)₂Mn, and (CH₃COO)₂Ni (Alfa Aesar), were dissolved in water with stirring to get a homogeneous solution. To the solution, 0.01 M of CTAB surfactant was added dropwise and the stirring was continued for 1 h. Followed by this, an optimized amount of 1 M of citric acid as chelating agent was added. Further, ammonia solution was added drop wise to adjust the pH value of the solution between 8.5 and 9 and the process of stirring and heating (80 °C) was continued to get a transparent gel. The gel thus obtained was aged for 24 h in an oven and furnace-heated to 350 °C for about 12 h followed by calcination at 900 °C for about 5 h in air using alumina crucibles. Ultrafine LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] powders obtained after 900 °C were ground and subjected to characterization studies.

Details pertaining to electrode fabrication and the subsequent 2032 coin cell assembly are reported elsewhere [20]. Electrochemical characterizations were carried out on freshly fabricated 2032 coin cells consisting of lithium anode, synthesized LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] cathodes individually, and a nonaqueous electrolyte consisting of 1 M LiPF₆ dissolved in 1:1 v/v EC/PC with a Celgard separator.

Physical and electrochemical characterizations

Phase characterization was done by powder X-ray diffraction technique on a PANalytical X'pert PRO X-ray diffractometer using Ni-filtered Cu Kα radiation (=1.5406 Å) in the 2θ range 15–70° at a scan rate of 0.04° s⁻¹. The lattice parameters were determined using the HighScore Plus software package. Surface chemical compositions of individual cathode materials were measured using X-ray photoelectron spectroscopy on a Physical Electronics model 5400 X-ray photoelectron spectrometer using an Mg Kα excitation source. The surface morphology and particle size of synthesized samples were examined by scanning electron microscopy (SEM, HITACHI S–3000 H, and Japan) and transmission electron microscopy (TEM, FEI-Tecnai-20 G2). Electrochemical cycling behavior (CV) was performed using an AutoLab electrochemical workstation and the charge–discharge studies were carried out on ARBIN charge–discharge cycle life tester.

Result and discussion

Powder diffraction studies—structural characterization

Powder diffraction pattern recorded for LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds are depicted in Fig. 1a–c. All the diffraction peaks could be indexed to α-NaFeO₂-type of structure (space group R-3 m) that forms an fcc lattice with a distortion in the c direction, as indicated by the clear splitting of the (006)/(102) and (108)/(110) peaks. It is well known that a good resolution of (006)/(102) and (108)/(110) diffraction pairs is typical of an ideal layered structure with superior crystallinity, good hexagonal ordering, and better intercalation characteristics [21, 22]. Figure 1 evidences the presence of distinct splitting of (006) and (102) doublets at 2θ = 38.5° and those of (108) and (110) at 2θ = 65° for LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.45 and 0.4] compounds, which is in favor of the formation of good layered structure. It is further understood from the cell parameters and the degree of cation ordering derived from XRD (Table 1), i.e., the increasing c/3a ratio from 1.637 to 1.652 with the increase in cobalt content substantiated the presence of perfectly formed layered lattice [23, 24]. Similarly, the I ₍₀₀₃₎/I ₍₁₀₄₎ ratio increases from 1.15 to 1.38 which is greater than unity with an increase in the concentration of cobalt and the same is an indication of lowering the degree of cation mixing [25, 26]. Hence the role of Co³⁺ in minimizing the cation mixing of pristine LiNi_0.5Mn_0.5O₂ lattice could be understood. The R value refers to the ratio of the sum of diffraction intensity of (006) and (102) to the diffraction intensity of (101) which measures the cation disorder. This is found to be the lowest for LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.45 and 0.4] compounds and suggests that the hexagonal ordering is better.

Fig. 1

X-ray diffraction pattern of a LiNi_0.5Mn_0.5O₂; b LiNi_0.45Mn_0.45Co_0.1O₂; and c LiNi_0.4Mn_0.4Co_0.2O₂ synthesized by CTAB-coassisted sol–gel method

Table 1 Physical parameters calculated for LiNi _y Mn _y Co_1−2y O₂ [y = 0.5, 0.45, and 0.4] compounds

Electron microscopy studies—particle size

Presence of 200 nm secondary particles of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds prepared by surfactant-coassisted sol–gel method are evidenced by transmission electron microscopy (Fig. 2). Particularly, homogeneous distribution of spherical particles with interconnected crystals is seen from the TEM images (Fig. 2a–c). Figure 2d shows the representative SEM image of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds, wherein spherical particles with interconnected morphology is found to get maintained even after furnace calcination at 900 °C. Unlike citric acid-assisted sol–gel method (inset of Fig. 2d), wherein presence of crystalline particles is seen without any desired spherical or interconnected morphology, the currently adopted CTAB-coassisted sol–gel method has produced interconnected spherical particles and the same could be attributed to the selective capping (or) nucleation effect of CTAB as a surfactant. Further, LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds synthesized with CTAB surfactant shows uniformly distributed and size reduced (200 nm) primary particles that are properly interconnected to form a porous network, which in turn would facilate better penetration of electrolyte and improve the electrochemical performance. Hence, it is learnt from the current study that the newly adopted surfactant-coassisted sol–gel method is advantageous over the conventional solid state [27], mixed hydroxide [28], and solution-assisted combustion [29] methods in synthesizing layered LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds with desired morphology.

Fig. 2

TEM images of CTAB-coassisted sol–gel synthesized: a LiNi_0.5Mn_0.5O₂; b LiNi_0.45Mn_0.45Co_0.1O₂; c LiNi_0.4Mn_0.4Co_0.2O₂; and d typical SEM image of LiNi _y Mn _y Co_{1 − 2y} O₂. Inset typical SEM image of citric acid-assisted sol–gel synthesized LiNi_0.4Mn_0.4Co_0.2O₂

X-ray photoelectron spectroscopy-elemental composition of cathode material

X-ray photoelectron spectroscopy (XPS) was employed to confirm the oxidation state of metal ions present in LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds. From Fig. 3, it is evident that the binding energy (BE) values of Ni 2p_3/2, Co 2p_3/2, and Mn 2p_3/2 of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds are in good agreement with the reported values [30, 31]. The curve fitting of LiNi_0.5Mn_0.5O₂ Ni 2p_3/2 signal at 855 eV indicates [30, 31] a slight shift in energy towards higher side compared to those of LiNi_0.45Mn_0.45Co_0.1O₂ and LiNi_0.4Mn_0.4Co_0.2O₂ compounds, which may be correlated to the effect of cationic mixing. However, the presence of an additional and a characteristic satellite peak noted with the Ni 2p_3/2 signal, corresponding to the multi-electron excitation behavior, confirms the oxidation state of Ni as +2 in all the LiNi _y Mn _y Co_{1 − 2y} O₂ compounds. The best fit for the Co 2p_3/2 spectrum gives a BE value, which is matching with the value reported by Shaju et al. [32] for Co³⁺ ions. Similarly, the Mn XPS spectra that has a major peak located at a binding energy value of 642.2 eV for LiNi_0.45Mn_0.45Co_0.1O₂ and LiNi_0.4Mn_0.4Co_0.2O₂ compounds matches with the BE reported for Mn⁴⁺ ions [33]. Here again, a small shift towards higher energy side is observed for LiNi_0.5Mn_0.5O₂ compounds, which may also be correlated to the possible presence of cation mixing. Based on these results, the individual oxidation state of Ni, Mn, and Co atoms in the currently synthesized LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds has been confirmed as +2, +4, and +3, respectively, and the charge neutrality of synthesized compounds is thus understood.

Fig. 3

X-ray photoelectron spectroscopy recorded for LiNi _y Mn _y Co_{1 − 2y} O₂ compound

Electrochemical characterizations

The electrochemical behavior of synthesized cathodes was evaluated using 2032 coin cells containing lithium foil as anode and the synthesized compounds as cathode individually, wherein the investigation was carried out initially at C/10 rate in the potential range between 2.5 and 4.6 V. Figure 4 shows the charge and discharge profile of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] cathodes, wherein the first discharge capacity of LiNi_0.5Mn_0.5O₂, LiNi_0.45Mn_0.45Co_0.1O₂, and LiNi_0.4Mn_0.4Co_0.2O₂ is found to be 210, 222, and 206 mA h g⁻¹, respectively. Subsequently, an initial irreversible capacity loss of 30, 22, and 16 mA h g⁻¹ has been exhibited by LiNi_0.5Mn_0.5O₂, LiNi_0.45Mn_0.45Co_0.1O₂, and LiNi_0.4Mn_0.4Co_0.2O₂ cathodes, which is not unusual. Further, such a reduced irreversible capacity loss of 16 mA h g⁻¹ exhibited by the currently synthesized LiNi_0.4Mn_0.4Co_0.2O₂ cathode indicates the superiority of the same, based on the reported behavior [10, 26] in the literature. On the other hand, progressive discharge capacity values of 140, 172, 179 mA h g⁻¹ have been observed for LiNi_0.5Mn_0.5O₂, LiNi_0.45Mn_0.45Co_0.1O₂, and LiNi_0.4Mn_0.4Co_0.2O₂ cathodes at the end of the 50th cycle (Fig. 4), thus suggesting that 20 % cobalt addition is beneficial in minimizing the capacity fade of native LiNi_0.5Mn_0.5O₂ cathode, via hexagonal ordering of lithium and nickel ions. Further, maintenance of discharge plateau at 3.8 V and the absence of plateau around 4.0 V exhibited up to 50 cycles confirm the absence of possible structural changes of layered LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] cathodes into spinel-related phases [34]. Such an improved structural stabilization is attributed to the presence of Mn⁴⁺ that maintains the oxide network favorable for facile lithium intercalation/de-intercalation upon extended cycling.

Fig. 4

Voltage vs. capacity behavior of LiNi _y Mn _y Co_{1 − 2y} O₂ (y = 0.5, 0.45, and 0.4) cathode materials in the potential range of 2.5 ~ 4.6 V corresponding to a first cycle and b 50th cycle

The cycling behavior of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] cathodes at 0.1 and 0.2 C rate is furnished in Fig. 5. As mentioned already, an initial discharge capacity of 210, 222, and 206 mA h g⁻¹ has been exhibited by LiNi_0.5Mn_0.5O₂, LiNi_0.45Mn_0.45Co_0.1O₂, and LiNi_0.4Mn_0.4Co_0.2O₂ cathodes at 0.1 C rate with reduced second cycle discharge capacity values such as 176, 213, and 192 mA h g⁻¹, respectively. Interestingly, LiNi_0.4Mn_0.4Co_0.2O₂ cathode with 192 mA h g⁻¹ of second cycle discharge capacity exhibited better capacity retention (93 %) and lesser capacity fade (7 %) up to 50 cycles. In other words, the overall capacity retention of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] cathodes calculated for 50 cycles (with respect to second cycle discharge capacity value) is found to be 78, 82, and 93 %, respectively. Similarly, when charge–discharge cycling was performed at 0.2 C rate, LiNi_0.4Mn_0.4Co_0.2O₂ cathode exhibited a capacity as high as 160 mA h g⁻¹ along with a capacity retention of 98 %, compared to those of LiNi_0.5Mn_0.5O₂ (142 mA h g⁻¹) and LiNi_0.45Mn_0.45Co_0.1O₂ (150 mA h g⁻¹) with a capacity retention of 89 and 95 %, respectively. Such an improved electrochemical behavior of LiNi_0.4Mn_0.4Co_0.2O₂ cathode has been endorsed to the synergistic effect of spherically formed interconnected particles obtained from CTAB-coassisted sol–gel method and the optimum cobalt substitution (20 %) that minimizes the cation disorder.

Fig. 5

Cycling performance of LiNi _y Mn _y Co_{1 − 2y} O₂ (y = 0.5, 0.45, and 0.4) cathode under the influence of 0.1 and 0.2 C rate

Further, with a view to investigate the rate capability behavior of LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] cathodes, electrodes of synthesized compounds were subjected to charge–discharge cycling with different C rates and the observed capacity values are depicted in Fig. 6. The discharge capacity of LiNi_0.5Mn_0.5O₂ is 210 mA h g⁻¹ at a 0.1-C rate and the same gets decreased to 70 mA h g⁻¹ at 1 C rate. Interestingly, 10 % of cobalt-doped LiNi_0.45Mn_0.45Co_0.1O₂ cathode exhibits the highest discharge capacity of 222 mA h g⁻¹ (compared to the rest of the cathodes at 0.1 C rate) but suffers from significant capacity fade, as evident from the reduced capacity value of 94 mA h g⁻¹ exhibited observed at 1 C rate. On the other hand, LiNi_0.4Mn_0.4Co_0.2O₂ cathode exhibits a nominal discharge capacity of 206 mA h g⁻¹ at 0.1 C rate and an appreciable capacity value of 163, 131, and 109 mA h g⁻¹ at higher discharge rates such as 0.2, 0.5, and 1 C rate, respectively. As mentioned earlier, such an improved rate capability behavior of LiNi_0.4Mn_0.4Co_0.2O₂ cathode is also attributed to the optimum addition of cobalt (20 %) that minimizes the cation disorder in the pristine LiNi_0.5Mn_0.5O₂ lattice and the formation of interconnected spherical grains obtained from CTAB-coassisted sol–gel method.

Fig. 6

Rate capability behavior of LiNi _y Mn _y Co_{1 − 2y} O₂ (y = 0.5, 0.45, and 0.4) cathodes at different C rates

Based on the encouraging results obtained for LiNi_0.4Mn_0.4Co_0.2O₂ cathode, an explorative attempt to investigate the CV behavior of the same at different voltages such as 4.4, 4.5, 4.6, 4.7, 4.8, and 4.85 V has been made with the scan rate of 0.1 mV s⁻¹ (Fig. 7), mainly to understand the effect and extent of individual transition metals in imparting the specific capacity. The CV recorded with 4.4 V cutoff voltage limit consists of a redox couple at 3.81 and 3.68 V corresponding to Ni²⁺/Ni⁴⁺ pair, and Mn⁴⁺, being an electrochemically inactive species, gives rise to no CV signal throughout the study, which is justifiable. Alternatively, CV recorded up to 4.6 V and at higher cutoff voltage limits (4.7 to 4.85 V) consists of another redox pair at 4.58 and 4.51 V, corresponding to the presence of Co³⁺/Co⁴⁺ redox pair. Hence, apart from an understanding on the facile intercalation and de-intercalation behavior of Li⁺ ions in LiNi_0.4Mn_0.4Co_0.2O₂ cathode and the respective oxidation states of Ni⁺², Co⁺³, and Mn⁺⁴ ions, significant role of Co⁺³ ions in improving the electrochemical behavior of the same has also been substantiated from CV studies.

Fig. 7

Cyclic voltammograms of LiNi_0.4Mn_0.4Co_0.2O₂ cathode at different cutoff voltages

The charge–discharge behavior of LiNi_0.4Mn_0.4Co_0.2O₂ cathode synthesized using surfactant-coassisted sol–gel method at different cutoff voltages (0.1 C rate) has been furnished in Fig. 8 and the observed capacity fade values are given in Table 2. An increasing capacity value from 194 to 228 mA h g⁻¹ has been observed with an increase in the upper cutoff voltage from 4.5 to 4.8 V. Simultaneously, an increasing capacity fade of about 15 and 17 % has been observed up to 30 cycles, when the cell was charged to 4.7 and 4.8 V limits, respectively. This could be correlated to the slight shift in redox peaks observed in CV exhibited by LiNi_0.4Mn_0.4Co_0.2O₂ cathode, especially when cycled beyond 4.6 V. Interestingly, LiNi_0.4Mn_0.4Co_0.2O₂ cathode exhibited reduced capacity fade behavior (7 %) and an enhanced capacity value (192 mA h g⁻¹), especially when cycled up to 4.6 V compared to those of 4.7 and 4.8 V limits. On the other hand, cycling up to 4.5 V renders much reduced capacity fade behavior (5 %) but at the expense of highly reduced initial and progressive capacity values (180 mA h g⁻¹). Therefore, it is evident from the present study that LiNi_0.4Mn_0.4Co_0.2O₂ cathode synthesized by CTAB-coassisted sol–gel method exhibits lesser capacity fade (7 %) and higher capacity value (192 mA h g⁻¹) at 4.6 V limit, compared to those reported at reduced potential windows such as 4.3 and 4.5 V [11, 26]. Such an improved electrochemical behavior of LiNi_0.4Mn_0.4Co_0.2O₂ cathode has been endorsed to the synergistic effect of interconnected and spherically shaped particles (~200 nm) derived from surfactant-coassisted sol–gel method and the perfect layeredness obtained by using an optimum (20 %) amount of cobalt in the solid solution.

Fig. 8

Cycling performance of LiNi_0.4Mn_0.4Co_0.2O₂ cathode at different cutoff voltages (2.5 ~ 4.5, 4.6, 4.7, and 4.8 V)

Table 2 The initial and 30th cycle discharge capacity values and corresponding capacity fade of LiMn_0.4Ni_0.4Co_0.2O₂ cathode at different cutoff voltages

Conclusions

LiNi_0.5Mn_0.5O₂, LiNi_0.45Mn_0.45Co_0.1O₂, and LiNi_0.4Mn_0.4Co_0.2O₂ compounds were prepared at 900 °C using CTAB-coassisted sol–gel method. The Co⁺³ introduction in LiNi_0.5Mn_0.5O₂ compound helps in increasing the reversible capacity and rate capability. Among the LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds, LiNi_0.4Mn_0.4Co_0.2O₂ cathode exhibited superior discharge capacity (192 mA h g⁻¹) and with an excellent capacity retention (93 %) for 50 cycles, and its rate capability at 1 C rate and cycling behavior up to 4.6 V are found to be satisfactory compared to those of LiNi_0.5Mn_0.5O₂ and LiNi_0.45Mn_0.45Co_0.1O₂ cathodes. The study recommends that the currently adopted surfactant-coassisted sol–gel method is yet another potential approach to synthesize LiNi _y Mn _y Co_{1 − 2y} O₂ [y = 0.5, 0.45, and 0.4] compounds with interconnected spherical grains which are responsible for the improved electrochemical properties. Further, the suitability of LiNi_0.4Mn_0.4Co_0.2O₂ cathode for high-rate (1 C) lithium battery applications is believed to result from the synergistic effect of synthesis method and the selection of optimum amount of (20 %) Co³⁺ concentration that renders required cation ordering.