Boosting the electrochemical performances of LiNi1/3Co1/3Mn1/3O2 cathodes via optimizing calcination temperature for lithium-ion batteries

The research of high-performance cathode materials for rechargeable lithium-ion batteries (LIBs) is highly desirable. The ternary layered oxide LiNi1/3Co1/3Mn1/3O2 (LNCM) is a promising cathode material for LIBs due to its high discharge voltage, large specific capacity, good thermostability, and low cost. However, the LNCM cathode still has certain limitations, including cationic mixing and low electronic conductivity. These drawbacks ultimately result in poor cycling stability, rapid voltage degradation, and capacity loss during high-rate cycling. To address these issues, we have established a feasible sol-gel method combined with calcination to prepare LNCM, which can significantly improve the electrochemical activity of the LNCM cathode. The developed LNCM−850/10 cathode displays an initial specific discharge capacity of 215.3 mAh g−1 at a current rate of 0.2 C and retains a high reversible capacity of 93.9 mAh g−1 after 200 cycles. In addition, the LNCM−850/10 cathode also exhibits excellent high-rate charge-discharge capability and high-rate cycling performance. These remarkable results are probably due to the low Li+/Ni2+ cation mixing degree, good particle morphology, and uniform particle size distribution of LNCM−850/10, which effectively improves the electronic conductivity and lowers the charge transfer resistance, while reducing the Li+ diffusion distance and accelerating the insertion/extraction of Li+. Our study demonstrates that careful control of the calcination temperature of sol-gel-synthesized LNCM precursors can promote the development of LNCM cathodes suitable for advanced LIBs.


Introduction
The widespread use of fossil fuels has caused severe environmental and ecological problems as well as energy crisis [1].In order to achieve sustainable development, the key is to develop renewable energy such as solar energy, wind energy, and tidal energy to reduce the dependence on fossil energy [2,3].However, their intermittent nature is not convenient for direct use in electronic devices, so it is necessary to develop energy storage systems.Lithium-ion batteries (LIBs) have attracted widespread attention among electrochemical energy storage systems due to their advantages of high energy/power density, long lifetime, and environmental friendliness [4][5][6][7][8].Cathode materials, as the core component, generally determine the performance of LIBs, but the design and synthesis of advanced cathode materials still remain a key challenge for the sustainable development of LIB technology [9][10][11][12].
Qing Han and Chenguang Bao contributed equally to this work.
Aiming to improve the reversible capacity and structural stability and reduce the cost of LiCoO 2 cathode, the nickelcobalt-manganese ternary layered oxide cathode material LiNi 1-x-y Co x Mn y O 2 has been extensively studied and widely used in power/energy storage battery systems [27][28][29][30][31][32].LiNi 1-x-y Co x Mn y O 2 with the layered structure of α-NaFeO 2 is a solid solution of LiCoO 2 , LiNiO 2 , and LiMnO 2 , and the predominant oxidation states of Ni, Co, and Mn are +2, +3, and +4, respectively.Among them, Ni 2+ mainly participates in electrochemical reactions to improve the reversible capacity of LiNi 1-x-y Co x Mn y O 2 , while Co 3+ not only stabilizes the layered structure of LiNi 1-x-y Co x Mn y O 2 and enhances its electronic conductivity but also suppresses the Ni 2+ /Li + mixing degree [30].Meanwhile, Mn 4+ does not participate in the charging and discharging process, but mainly stabilizes the LiNi 1-x-y Co x Mn y O 2 structure and reduces the cost of LiNi 1-x-y Co x Mn y O 2 .By controlling the Ni 2+ , Co 3+ , and Mn 4+ ratios, the electrochemical performance of LiNi 1-x-y Co x Mn y O 2 changes significantly.Therefore, by combining the advantages of these three components and overcoming their respective shortcomings, LiNi 1/3 Co 1/3 Mn 1/3 O 2 (LNCM) cathode exhibits high theoretical capacity, low cost, and excellent safety performance, which can effectively solve the existing problems in LiCoO 2 cathode [33][34][35][36][37][38].
However, LNCM also faces two critical issues: (1) the Ni 2+ /Li + cation mixing is easily generated during the fabrication process of LNCM or electrochemical chargedischarge processes due to the similar ionic radius of Ni 2+ (0.069 nm) and Li + (0.076 nm), which greatly hinders the Li + intercalation/deintercalation and further affects the cycling performance; (2) the low electronic conductivity of LNCM results in low initial coulombic efficiency and poor rate performance.Over the past few decades, extensive research has been conducted to address the challenges associated with LNCM.The primary approach has been to introduce other elements or metals into the structure of LNCM through ion doping [39][40][41][42].This strategy aims to improve the structural stability and ionic conductivity of LNCM, as well as its cycling and rate performance.In addition, carbon or metal oxides can be used to form a protective coating on the LNCM surface, prevent reactions between LNCM and electrolyte, and promote charge transfer, thereby improving the electrochemical performance of LNCM [43][44][45][46][47].Although these modification methods have shown some success in improving the performance of LNCM positive electrodes, the introduction of non-active elements through doping can lead to a loss of battery capacity.In addition, achieving a uniform, stable, and controllable surface coating remains a challenge.Therefore, it is imperative to design and optimize the morphology and microstructure of LNCM from its own structure to achieve the desired electrochemical performance [48][49][50][51][52][53][54][55].
Considering the above points, we used a simple sol-gel method to prepare an LNCM precursor and then obtained LNCM materials by precisely controlling the calcination temperature and time.The LNCM−850/10 material prepared at a sintering temperature of 850 °C and a sintering time of 10 h showed a complete and pure α-NaFeO 2 layered structure, lower Ni 2+ /Li + cation mixing, and uniform particle distribution.At a discharge rate of 0.2 C and within the voltage range of 2.5-4.6 V, the LNCM−850/10 sample exhibited an initial discharge capacity of 215.3 mAh g −1 and maintained a discharge capacity of 93.9 mAh g −1 after 200 cycles, demonstrating superior cycling performance compared to other samples.In addition, this material exhibited higher discharge capacity and cycling stability at high discharge rates.Our results demonstrate a facile synthesis approach for LNCM, a promising candidate cathode for high-performance LIBs.

Sol-gel synthesis of LNCM materials
All chemicals were analytically pure and used without further purification.The starting reactants CH 3 COOLi•2H 2 O and M(CH 3 COO) 2 •4H 2 O (M = Ni, Co, Mn) were accurately weighed according to the proportions of each element in the chemical formula of LNCM.CH 3 COOLi•2H 2 O was maintained in a 5% molar excess to account for volatilization of Li during subsequent calcination.Citric acid (C 6 H 8 O 7 •H 2 O) was used as a chelating agent in a 3:1 molar ratio to Ni(CH 3 COO) 2 •4H 2 O.The complexing agent and the original reactants were transferred to separate beakers A and B, respectively, and then dissolved in distilled water under ultrasonic oscillation for 20 minutes.Beaker A was maintained at 80 °C with constant stirring, and the chelating agent in beaker B was slowly added to beaker A. After the addition was complete, the mixture was heated and stirred at 80 °C until the water in the beaker slowly evaporated and the aqueous solution became a pink sol state.The sol-state reactants were dried in a vacuum drying oven at 80 °C for 12 h to obtain a pink LNCM precursor.The precursor was thoroughly ground under an infrared lamp and transferred to a crucible for calcination.Calcination temperatures were set at 450, 700, 750, 800, 850, and 900 °C with a heating rate of 2 °C min −1 and a calcination time of 10 h.After calcination, the LNCM materials were thoroughly ground and the target products were obtained.For the sake of clarity, the synthesized LNCM samples were referred to as LNCM−450/10, LNCM−700/10, LNCM−750/10, LNCM−800/10, LNCM−850/10, and LNCM−900/10, respectively.

Preparation of working electrodes
First, the electrode active material LNCM, the conductive additive Ketjen Black (KB), and the binder polyvinylidene fluoride (PVDF) (5% by weight, dissolved in N-methyl-2-pyrrolidone) were weighed in an 8:1:1 mass ratio.LNCM and KB were then transferred to an agate mortar and ground uniformly and then added to the volumetric flask (5 mL) containing PVDF.The mixture was magnetically stirred for 24 h until a smooth slurry was formed.The prepared slurry was then applied to aluminum foil to form a uniformly thick film.The film was vacuum dried at 60 °C for 8 h and then cut into circular electrode sheets to form the working electrodes for use.

Characterization techniques
The thermostability of the LNCM precursor was investigated by thermogravimetric (TG) analysis using a Linseis STA PT1600 instrument under air atmosphere with a heating rate of 10 °C min −1 and a temperature range of 25 to 1000 °C.The phase compositions of the synthesized LNCM materials were confirmed by X-ray diffraction (XRD) using a Rigaku MiniFlex 600 instrument with Cu Kα radiation (λ = 1.54056Å) at a voltage of 40 kV and a current of 15 mA.The diffraction range was set to 10-80°, fixed at a scan rate of 10° min −1 .The microscopic morphologies and internal structures of the synthesized LNCM samples were analyzed by field emission scanning electron microscopy (SEM) using an FEI-Quanta 250 FEG instrument.

Electrochemical measurements
In the assembly process of CR2016 button cell batteries, preprepared aluminum foil electrodes containing LNCM active materials and lithium foil are used as the positive and negative electrodes, respectively.The 1 mol dm −3 LiPF 6 solution in a solvent of dimethyl carbonate, ethylene carbonate, and ethyl methyl carbonate with a volume ratio of 1:1:1 was used as the electrolyte.The separator used is a microporous polypropylene membrane (Celgard PP2075).The assembly process takes place in a glove box filled with argon gas to reduce the oxygen and water content below 0.1 ppm.After assembly, the batteries are sealed and allowed to rest for 12 h before undergoing electrochemical performance testing.
Constant current charge-discharge testing is performed using the Land 2001A multi-channel battery tester (Wuhan Land electrochemical equipment company, China).Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are conducted on the CHI 660D electrochemical workstation (Shanghai, China).EIS test conditions include an amplitude of ±5 mV, open-circuit voltage testing, and a frequency range of 100 kHz to 10 mHz.All electrochemical experiments are performed at room temperature in the voltage/potential range of 2.5-4.6 V. Specific capacities (mAh g −1 ) and current densities (C or mA g −1 ) are based on the mass of the LNCM samples.

Results and discussion
The LNCM precursor was prepared by a simple sol-gel method.In order to optimize the sintering temperature of the LNCM precursor, TG analysis of the LNCM precursor was performed.As depicted in Fig. 1a, the TG curve undergoes a four-step weight loss process.The first weight loss of 6.87% from 25 to 200 °C is attributed to the release of adsorbed and lattice water molecules in the LNCM precursor [45].The second weight loss of 46.81% occurs between 200 and 350 °C and is attributed to the volatilization of carbonization products such as CO, CO 2 , and H 2 O.The third weight loss rate slows down between 350 and 600 °C, with a weight loss of 19.53%, which can be attributed to the continued decomposition of residual citric acid and the gradual formation of LNCM.Upon further heating to 1000 °C, there is no significant weight loss (0.66%), which is mainly due to nucleus growth and phase transition between different phases.Interestingly, there is a very slight increase in mass around 700 °C in the TG curve [56,57], suggesting that the LNCM precursor is undergoing a substantial phase transition process that may eventually produce a structurally intact and pure LNCM crystal material.According to the TG results, we prepared different LNCM samples by calcining the LNCM precursor at 450, 700, 750, 800, 850, and 900 °C for 10 h, respectively.For convenience, the as-synthesized LNCM samples were designated as LNCM−450/10, LNCM−700/10, LNCM−750/10, LNCM−800/10, LNCM−850/10, and LNCM−900/10, respectively.
The crystal structures of the LNCM samples obtained by calcination at different temperatures were first verified by XRD analysis.Figure 1b shows that the diffraction pattern of the LNCM−450/10 sample obtained at the lower calcination temperature is incomplete, indicating that the LNCM precursor has not been completely transformed into the crystalline LNCM material.On the other hand, other samples exhibit clear characteristic peaks with similar peak shapes and peak positions that are well indexed to the classical layered framework of α-NaFeO 2 (JCPDS No. 09-0063).It is worth noting that impurity peaks appeared around 2θ = 31° in the LNCM−900/10 sample (Fig. 1b).Compared with JCPDS standard cards, the impurities were identified as Co 2 O 3 , Mn 3 O 4 , and other impurities [53], indicating that too high temperature can cause Li-O bonds to escape from LNCM, leading to the formation of new impurity phases.Furthermore, according to the relevant literature, the quality of the layered LNCM material can be determined by the degree of splitting between the (006)/ (102) and (108)/(110) doublets [58].A higher degree of splitting indicates a better layered framework and crystallinity of the LNCM material.With the exception of LNCM−450/10, the (006)/(102) and (108)/(110) doublets of the other samples are significantly split, indicating that they have a well-crystallized layered structure [59,60].
To further investigate the effect of sintering temperature on the crystal structures of the as-prepared LNCM samples, the variations of the lattice parameters are obtained by Rietveld refinement (Fig. 1d).As the calcination temperature is increased, the values of a, c and crystal cell volume of the as-prepared samples first increase and then decrease, with a maximum value observed at 850 °C for the LNCM−850/10 sample.However, the unit cell volume of LNCM−900/10 is significantly smaller than that of LNCM−850/10, which corresponds to the appearance of impurity peaks for LNCM−900/10 in the above XRD patterns.The presence of impurity phases indicates that the LNCM−900/10 sample has undergone partial transformation, resulting in a smaller cell volume.Furthermore, the c/a ratio of the LNCM is a useful indicator of the longitudinal interlayer spacing, which determines the ability of Li + to diffuse during charging and discharging.Typically, a higher c/a value indicates greater diffusion capability and therefore better performance.In comparison, the c/a ratios of LNCM−800/10 (4.98268) and LNCM−850/10 (4.98212) are the largest and almost identical, implying that the longitudinal interlayer spacing of these two samples is relatively large, which is favorable for Li + insertion/extraction.The characteristic peak I (003) /I (104) of the XRD pattern is commonly used to estimate the degree of Ni 2+ /Li + mixing in LNCM.LNCM−850/10 has a refined I (003) /I (104) value of 1.3330, which is the highest among all samples except LNCM−900/10.This indicates that LNCM−850/10 has the lowest degree of Ni 2+ /Li + mixing.However, the I (003) /I (104) value of LNCM−900/10 is not significant due to the presence of impurity phases and non-standard structures.Therefore, LNCM−850/10 is the optimal material due to its largest unit cell volume, favorable c/a ratio for Li + diffusion, and lowest degree of Ni 2+ /Li + mixing.
The influence of calcination temperature on the morphology and structure of the as-prepared products was also investigated.Figure 2 displays SEM images of LNCM materials obtained by calcining for 10 h at different temperatures.The particle sizes of the LNCM materials increase with increasing calcination temperature, indicating that the microstructure of the LNCM material is significantly affected by the calcination temperature.During calcination at 900 °C, small particles are detected on the LNCM bulk material, which are identified by XRD analysis as impurities produced under high-temperature calcination conditions.The presence of these impurities adversely affects both the particle homogeneity of LNCM and its electrochemical performance.In contrast, SEM images also show that the LNCM materials synthesized at 750, 800, and 850 °C have relatively uniform particle diameters, suggesting that these temperatures are favorable for the growth of LNCM.However, the LNCM−750/10 sample obtained at 750 °C contains some larger particles, while the particles in LNCM−800/10 and LNCM−850/10 are smaller and have similar sizes.Based on the XRD results, it is believed that the internal structure of LNCM−850/10 is superior, which is also confirmed by the subsequent electrochemical performance test.
Figure 3c displays the initial charge-discharge curves of LNCM prepared for 10 h at different calcination temperatures within the voltage range of 2.5-4.6 V and at a discharge rate of 0.2 C. Obviously, the initial discharge-charge curves of LNCM−850/10 electrode exhibit the highest discharge platform and the most symmetrical shape, indicating superior electrochemical reversibility and less polarization compared to other samples.This explains why the LNCM−850/10 cathode has better cycling stability and discharge specific capacity than other cathodes [58,59].With increasing number of charge-discharge cycles, all cathodes show a trend of decreasing charge and discharge specific capacitance and In addition, the discharge specific capacity and discharge voltage of LNCM−850/10 after 200 cycles are still higher than those of other samples, which are 93.9 mAh g −1 and 3.3853 V, respectively, demonstrating high discharge specific capacity and better cycling performance in the first cycle.
CV curves were performed for the prepared LNCM samples in the voltage window of 2.5-4.6 V and at a scan rate of 0.1 mV s −1 during the first five cycles and the fifth cycle to compare the diffusion kinetics of Li + .The results, shown in Fig. 4d and Fig. 5, show that all five materials have a pair of oxidation-reduction peaks with similar peak shapes, indicating similar oxidation-reduction reactions.The CV curve of LNCM−850/10 is the most stable, with the five cycles almost overlapping.The other four materials exhibit varying degrees of right-shifted oxidation peaks and left-shifted reduction peaks.The degree of overlap of the CV curves represents the reversibility of the oxidation-reduction reactions, while the peak position is related to the magnitude of the polarization [65,66].This indicates that LNCM−850/10 has good oxidationreduction reversibility.Furthermore, LNCM−850/10 has the smallest peak potential difference of 0.14 V, indicating the smallest degree of polarization and the largest peak current density, as well as excellent high-rate chargedischarge performance.These results are consistent with the electrochemical property of LNCM−850/10 in practice.[47,54], the charge transport resistance (R ct ), which is the size of the arc diameter in the mid-frequency region of the EIS spectrum, mainly limits the behavior of electrode materials in the electrochemical process.From Fig. 6a, it can be seen that the R ct of LNCM−850/10 is the smallest, followed by LNCM−800/10 and LNCM−700/10; the smaller charge transport resistance of LNCM−850/10 enables it to react faster during the electrode reaction process.
In addition, the EIS spectra of LNCM−700/10, LNCM−750/10, LNCM−800/10, LNCM−850/10, and LNCM−900/10 were analyzed using ZsimpWin software.The schematic used for the analysis is illustrated in Fig. 6b.The R s parameter delegates the solution resistance of the battery, which is indicated by the small intercept in the highfrequency region of Fig. 6a

Conclusions
In the present study, the sol-gel technique combined with calcination is used to prepare LNCM samples for use as cathode materials in LIBs.Our results demonstrate the critical role of the calcination temperature of the LNCM precursor on the electrochemical properties of the resulting LNCM materials.The relationship between crystal structures, particle morphologies, and lithium storage performance of LNCM materials has been extensively investigated.Among these prepared LNCM samples, LNCM−850/10 exhibits a uniform particle distribution and a highly ordered layered structure without any impurities.As a result, the LNCM−850/10 cathode exhibited the highest first discharge capacity and excellent cycling stability.In addition, the LNCM−850/10 sample prepared by this simple approach showed excellent CV behavior, demonstrating its stable crystal structure and great application potential as a promising LIB cathode material.Further research to improve the electrochemical properties of the LNCM cathode will be addressed elsewhere.
Funding This work was supported by the Natural Science Foundation of Henan, China (No. 222300420138), for the project entitled "Design, synthesis, and oxygen production performance of cobalt-titanium encapsulated polyoxotungstates with photocatalytic function orientation" and by the Ph.D. Programs Foundation of Henan University of Technology (No. 2021BS0027) for the project entitled "Polyoxometalate-based functional materials and their applications in the field of energy storage."

Fig. 1 a
Fig. 1 a TG curve of LNCM precursor.b XRD pattern of LNCM at different calcination temperatures and the locally magnified XRD pattern of LNCM−900/10 in the insert.c Schematic of the layered

Fig. 5 10 Fig. 6 a
Fig. 5 CV curves of LNCM−700/10 (a), LNCM−750/10 (b), LNCM−800/10 (c), LNCM−850/10 (d), and LNCM−900/10 (e) at a scanning rate of 0.1 mV s −1 for the first five cycles.f CV curves . The R SEI parameter delegates the SEI film resistance of the electrode material.The R ct parameter represents the charge transfer resistance of the electrode process, as indicated by the approximate semicircle in the mid-frequency region of Fig. 6a.The CPE parameter represents the electrical double layer capacitance.The W S parameter represents the Warburg impedance of the electrode process, indicated by the diagonal line in the low-frequency region of Fig. 6a.The specific parameters obtained are listed in Fig. 6b.Among them, we mainly focus on the R ct value, which is 49.46 Ω (LNCM−850/10) < 57.62 Ω (LNCM−800/10) < 118.2 Ω (LNCM−700/10) < 207.5 Ω (LNCM−750/10) < 597.4 Ω (LNCM−900/10).From these values, we can see that the LNCM−850/10 has the lowest R ct value, indicating the lowest charge transfer resistance.This corresponds to the better electrochemical performance of the battery assembled with the LNCM−850/10.