Correlative Survey of Blended LiCoO 2 and LiMn 2 O 4 Cathode Materials for Lithium-ion Batteries

. We employed the lithium-ion battery model in Multiphysics to simulate the electrochemical characteristics of lithium-ion batteries. These batteries consist of a cathode mixture comprising LiCoO 2 and LiMn 2 O 4 , as well as quasi-graphite anode. Our simulations successfully replicated the discharge profiles of both unblended and blended cathodes across different current rates, aligning with results obtained from experiments. The energy and power densities of the blended cathode framework were regulated by adjusting the mix ratio in the simulation model. Additionally, the blended electrodes of LiCoO 2 and LiMn 2 O 4 demonstrated an above-average electrochemical performance, combining the characteristics of the two active materials.


Introduction
This study involves a systematic investigation of two cathode materials; LiCoO2 and LiMn2O4, to establish connections or relationships between the blended systems of the electrode materials.Because of their better energy density and similar calendar and cycle lives, lithium-ion batteries have supplanted nickel-metal hydride batteries in a wide range of electronic devices and electric cars [1].The interdisciplinary aspect of finding cathode materials for lithium-ion batteries was highlighted by Huang.Based on his deep understanding of chemical structure and bonding, Professor John B. Goodenough's early solid-state physics research provided the fundamental ideas underpinning lithium cobalt oxide, lithium manganese oxide, and LiFePO4.The author highlighted that, with the rapid advancement of rechargeable batteries, this interdisciplinary approach has expanded to include engineering, information sciences, and even artificial intelligence [2].There is hope that further study and technical advancement will soon result in better solutions, despite current problems including energy density and safety issues with batteries.However, to match the range needs of conventional internal combustion engine vehicles, Li-ion battery energy density improvements must continue.High energy density new electrode active materials can be developed to do this [3][4].Researchers have been investigating the creation of blended electrode active materials in an effort to meet the stringent standards of lithium-ion batteries for electric automobiles and other household electronics.Blended materials often have higher energy densities compared to traditional intercalation materials like graphite.This means they can store more energy per unit of mass or volume, which is crucial for increasing the energy capacity of batteries.Among them, LiMn2O4 and LiNiMnCoO2 have garnered significant attention due to their combined ability to provide high energy densities and excellent rate capability.However, LiMn2O4 has limitations, such as low energy density and poor cycling stability at elevated temperatures [5].To overcome these drawbacks, an alloyed electrode system integrates LiMn2O4 with LiNiMnCoO2 cathode active material possessing a high energy density.This allows the battery to retain additional energy, resulting in extended operational durations and heightened power density [6][7][8].
The elements influencing blended systems' advantageous electrochemical performances have been the subject of numerous prior studies [9][10].However, figuring out the ideal mix ratio and blend chemistry alone by trial and error can be difficult and expensive.Within this framework, simulation techniques offer a productive means of forecasting the electrochemical behavior of various mixes.Paul et al. conducted a study that compared coin cells' experimental and simulated galvanostatic discharge performances.These cells contained mixed cathodes with varying proportions of LiyMn2O4 and LiyNi0.8Co0.15Al0.05O2,covering a wide range of discharge rates.The researchers assumed equal thicknesses of the two active systems and their blends, as well as equal electrolyte volume ratios.Nevertheless, they were unable to carry out in-depth life investigations on the cells since they were using half-cells with Li metal as the negative electrode [11][12].In this work, we utilized the Multi-physics lithium-ion battery model to simulate the electrochemical characteristics of refined and blended LiMn2O4 and LiCoO2 cathodes.We next compared the results of these models to experimental data collected from pouch-full cells running at 25°C.Gaining knowledge about the cathode blend's lifetime and performance was the main goal of our research.Solid-phase diffusion and reaction kinetics are two examples of the physical phenomena that must be integrated into a multiphysics model in order to accurately represent the electrochemical properties of lithium-ion batteries.

Theoretical Background
A battery, which has one or more electrochemical cells inside of it, serves as an energy storage device.Through electrochemical processes, it converts the chemical energy contained in its active components to electrical energy [13].Anode, cathode, and electrolyte are the main components of a battery.A separator and current collector are the auxiliary parts that complete the battery.The next generation of lithium-ion batteries is being developed by researchers from government laboratories, agencies, and university institutions with the goal of improving characteristics such as charge/discharge capacity.However, battery creation can occasionally become more of an art than a simple science due to the complex interactions between materials science, physics, and engineering.The problem is that scientists and engineers are limited in the chemical elements and configurations they can use to create batteries.Finding the elements that work best with lithium-ion electrodes has been the subject of extensive research and continuous efforts [14].While commercially available cathode active materials such as lithium nickel cobalt aluminum oxide (LiNiCoAlO2 or NCA), lithium iron phosphate (LiFePO4 or LFP), lithium nickel manganese cobalt oxide (LiNiMnCoO2 or NMC), lithium manganese oxide (LiMn2O4 or LMO), and lithium cobalt oxide (LiCoO2 or LCO) are used, anode active materials like lithium titanate (Li4Ti5O12), graphite, and amorphous carbon are also employed [6][7][8].For example, Zheng et al. conducted a study on a comprehensive understanding of electrode thickness effects on the electrochemical performances of Li-ion battery cathodes composed of LiMn2O4 and Li(Ni0.6Co0.2Mn0.2)O2.These materials were chosen to address their respective limitations.Through experiments and simulations, it was shown that the addition of Li(Ni0.6Co0.2Mn0.2)O2 to LiMn2O4 reduced the dissolution of Mn 2 ions, resulting in a 21% suppression of capacity fade in LiMn2O4 after 100 cycles.Consequently, the cycle performance of LiMn2O4 as an active material improved when combined with Li(Ni0.6Co0.2Mn0.2)O2as the cathode active material [15].In another study carried out by K. Hyun-Soo, a blend of LiNi0.8Co0.15Al0.05O2and Li1.1 Mn1.9O4 in a 1:1 weight ratio was investigated.The blended cathode exhibited favorable capacity retention compared to pure Li1.1Mn1.9O4.The researchers concluded that the addition of LiNi0.8Co0.15Al0.05O2enhanced the capacity retention of Li1.1Mn1.9O4.In addition to the aforementioned research, a large body of literature has been written about hybrid cathode systems that combine several cathode active materials in an effort to produce harmonious characteristics in lithium-ion batteries.Due to its potential to be a very thermally stable cathode material compared to other commercially available layered oxides, LiMn2O4 has drawn a lot of study attention [16].But it's important to recognize that LiMn2O4 has low energy density as well [17][18].There hasn't been much research done on the complementary impacts of two refined cathode materials, LiMn2O4 and LiCoO2, on one another in a cathode system.LiCoO2 has shown to be a valuable cathode material because of its noteworthy characteristics, which include high energy density, voltage stability, cycling efficiency, and rate flexibility.These qualities have established it as a favored selection in a range of applications where these characteristics are paramount.Hence, we investigated two blended positive electrode, comprising a mixture of LiMn2O4 and LiCoO2.

Experimental
In this study, we utilized two cathode active materials: LiMn2O4 obtained from Soundon New Energy Co., Ltd., China, and LiCoO2 obtained from Shandong Gelon Lib Co., Ltd., China.To conduct experimental investigations on the electrochemical properties, we fabricated 906090 pouch-type cells.These cells had cathodes with three different mix ratios of LiMn2O4 and LiCoO2, while the anode was made of artificial graphite from Shenzhen Sinuo Industrial Development Co. Ltd.The cathode composition of the electrodes consisted of 92 wt% blended active materials, 4 wt% conductive additive (Soundon New Energy Co., Ltd., China), and 4 wt% polyvinylidene fluoride (PVDF) from Kureha Corporation, Japan, serving as a binder.For the preparation of the electrode slurry, N-methyl pyrrolidone (NMP) from Sigma Aldrich, USA, was used as the solvent.The cathode slurry was applied onto 15µm-thick aluminum foil (Shandong Linxu Materials Co., Ltd., China) using the doctor blade procedure, followed by drying at 120°C for 60 minutes to remove the solvent.The electrodes were punched, and then dried again under vacuum at 60°C to get rid of any last traces of water and NMP solvent.SEM, specifically the Phenom Prox from the Netherlands, was used to analyze the surface morphology of the electrodes.
Three distinct blended cathodes were made, each with a density that was roughly equal.The electrode thickness and porosity were both meticulously regulated to be equivalent in order to guarantee comparable liquid phase outcomes within the cells.
We used aluminum and polypropylene separators from Dongguan Mochuang New Energy Materials, China, for the assembly of the pouch cells.The liquid electrolytes were composed of 1.15 M LiPF6 in a mixture of ethylene carbonate, ethylmethyl carbonate, and diethyl carbonate (3/7 by volume) obtained from Dongguan Tianfeng Power Material Co., Ltd., China.After construction, the cells were aged for 16 hours and then subjected to cycling at a constant current (CC) of 0.1C for both the charging and discharging processes.This initial cycling step served as a formation process and was conducted using a charge/discharge cycler from Dongguan ZhengRui Automatic Equip.Techn.Co., Ltd., China, at a temperature of 25°C.Following the formation step, a stabilization process was performed for three cycles at a 0.3C rate.To assess the rate capability, we tested three cells for each cathode composition at various current rates: 0.3C, 0.6C, 1C, 2C, 4C, 6C, 8C, and 10C, with each rate maintained for three cycles.The discharge processes at different C rates were followed by a constant current/constant voltage (CC/CV) charging at a 0.3C rate.In all cases, we applied an upper limited charge voltage of 4.2 V and a lower cut-off voltage of 3.0 V. To establish a reference rate (0.3C), the capacities of all the cells were normalized to their respective 0.3C capacities.The electrochemical reactions that occur during charge/discharge process in the Lithium-ion cell is as shown in Figure 1.

Determination of Morphology
The morphology of the blended and unblended positive electrodes was examined in the study using scanning electron microscopy (SEM), which gave important insights into the structural properties and particle configurations within the cathode.The micrographs shown in Figure 2 showed the unblended electrode next to the blended electrode that had a weight ratio of 5:5 between LiMn2O4 and LiCoO2.The two active materials were evenly mixed within the electrode structure, as seen by the blended electrode's SEM image.This consistency is the outcome of the electrodes' wet thicknesses and loading densities being carefully controlled, which guaranteed that the cathode's composition would not change over the course of the investigation.The gaps between the LiCoO2 particles were effectively filled with LiMn2O4 particles and carbon black conductive material, leading to a well-dispersed and homogenous mixture of active materials.
The blended electrode demonstrated effective interaction between the conductive additives and the active components by eliminating inter-particle gaps and attaining enhanced dispersion.Better charge transfer and ion diffusion kinetics were encouraged by this improved contact, which improved the cathode's overall electrochemical performance.A cathode with harmonic qualities was produced by combining LiMn2O4 and LiCoO2 in a weight ratio of 5:5, which addressed the drawbacks of each material separately and enhanced battery performance, capacity retention, and cyclability.The study's methodology is further supported by the quantitative measurements of the electrode properties, such as loading densities and thicknesses, shown in Table 1.We were able to fairly and accurately compare the various positive electrode compositions under study by maintaining uniformity in the electrode composition and managing the electrode's wet thickness.
All things considered, the SEM analysis of the electrode morphology was really helpful in comprehending the structural characteristics and particle distribution of the cathode.This information is essential for creating cathode materials with enhanced electrochemical behavior and lifespan, as well as maximizing the performance of Li-ion batteries.

Performance Rate
Figure 3a in the study presents a comparison of the low-rate discharge profiles (0.3C rate) of three positive electrodes: unblended LiMn2O4, unblended LiCoO2, and the blended cathode with a 5:5 weight ratio of LiMn2O4 and LiCoO2.Notably, the blended cathode in this study exhibits identical initial and final discharge characteristics to that of unblended LiCoO2 at the low-rate discharge stage.This suggests that, at this point, the LiCoO2 component is mostly responsible for the current delivery.In the unblended LiMn2O4 electrode system, however, an increased elevation is seen at cell output potential difference values lower than 3.8 V.This implies that LiMn2O4 electrochemical behavior manifests itself in this potential range.The distinct discharge characteristics observed in this area suggest that LiMn2O4 plays a major role in the blended cathode's electrochemical performance.The discharge characteristics for the three cathodes; unblended LiMn2O4, unblended LiCoO2, and the blended cathode with a 5:5 weight ratio are directly compared at a higher rate (4C rate) in Figure 3b.The blended cathode's initial discharge characteristics seem to be an average of the two unblended cathodes at the 4C rate.At this higher rate, unblended LiMn2O4 performs the best out of the three cathodes.The direct comparison of the discharge characteristics at the 4C rate indicates that the presence of LiMn2O4 in the blended cathode contributes to improved rate performance compared to unblended LiCoO2.The combined cathode with a 5:5 weight ratio of LiMn2O4 and LiCoO2 may display a mix of the electrochemical characteristics seen in the unblended cathodes, according to this theory.At low rates, the LiCoO2 component predominates, but the LiMn2O4 component improves the cathode's rate performance.
. Overall, the results indicate that the blended cathode with the 5:5 weight ratio provides a synergistic effect, where the combination of LiMn2O4 and LiCoO2 leads to improved electrochemical performance compared to the individual unblended cathodes.This discovery is crucial to comprehending how the cathode material composition affects the Li-ion battery's rate capability and overall performance.High-performance lithium-ion battery design and optimization may be significantly impacted by the blended cathode's enhanced rate performance.The unblended LiMn2O4 cathode's experimental and simulated rate performances are shown in Figure 4a of the study at different discharge rates ranging from 0.3C to 4C.The lithium-ion intercalation and de-intercalation reactions take place in two stages, as shown by the two elevations seen in the discharge curve of the LiMn2O4 spinel cathode.For all discharge rates, the modeling findings are in close agreement with the actual values, especially for the early phases and the lower cut-off voltage of 3.0 V.This agreement between simulation and experiment validates the accuracy of the model in representing the electrochemical behavior of the unblended LiMn2O4 cathode.In Figure 4b, the experimental and simulated rate performances of the unblended LiCoO2 electrode system are shown at discharge rates from 0.3C to 4C.Comparing Figure 4a and Figure 4b, it can be concluded that the rate capabilities of the unblended LiMn2O4 cathode are superior to those of unblended LiCoO2.Once again, there is good agreement between the simulated discharge profiles and the experimental results, particularly at the initial stages and cut-off voltage of 3.0 V.At high discharge rates, there is a small variation, but overall, the simulation accurately depicts the main features of the experimental behavior.These results verify that the simulation's model accurately captures the unblended LiMn2O4 and LiCoO2 cathodes electrochemical behavior, offering important information on their rate capacities and discharge profiles.Indeed, Figure 4 demonstrates that the regularized discharge capacity is dependent on the discharge rate (C-rate).The discharge curve tends to slope downward with increasing C-rate, suggesting a decline in the cell's total capacity.The main reason for this decrease in capacity is the polarization brought on by the cell's internal resistance.To maximize the design of Li-ion batteries for particular applications and comprehend performance constraints, the model must be able to forecast the rate capabilities and discharge behavior of the unblended cathodes with sufficient accuracy..
The findings show that, especially for the unblended LiCoO2 active material, the capacity difference between various discharge rates is more noticeable in the low output potential difference range.The loss of discharge capacity increases with the discharge rate (c-rate), primarily because of the electrode's contribution from Li-ion diffusion.During discharge, a significant gradient in the concentration of Li-ions is formed within the active material particles when the c-rate is elevated.As a result, the electrode potential drops sharply to the cut-off voltage, which lowers the cell's capacity to discharge energy [12].The smaller solid-phase Li-ion diffusion coefficient of the unblended LiCoO2 active material, compared to that of the unblended LiMn2O4 active material (as shown in Table 1), supports this observation.The lower Li-ion diffusion coefficient of LiCoO2 implies a slower rate of Li-ion diffusion within the electrode, leading to a larger concentration gradient and more pronounced capacity loss at higher c-rates.This finding emphasizes the significant influence of the discharge rate on the discharge capacity and polarization effects in the studied lithium-ion battery system.The study also presented how crucial it is to take the cathode's contact resistance into account while running simulations.Simulated discharge profiles that initially assumed zero contact resistance in both the positive and negative electrodes differed from the experimental results, especially with regard to the distance between the discharge curves.As seen in Table 1, the cathode's contact resistance was raised to a noteworthy value in order to resolve this problem.The computed and experimental findings agreed better with this adjustment, particularly for low c-rates and a cut-off voltage of 3.0 V. To ensure that the simulations match the experimental observations and appropriately represent the electrochemical behavior of the battery system, contact resistance must be taken into account.This allows for a more comprehensive understanding of the battery's performance and behavior, leading to better optimization and design for practical applications.The experimental and simulated rate performances of the blended cathode with a 5:5 weight ratio of LiMn2O4 spinel and LiCoO2 are shown in Figure 5.The graph unequivocally shows that the blended electrode system's rate performance is much better than the unblended LiCoO2 cathode's.The simulated and experimental data clearly show the blended cathode's better rate performance.The agreement between simulation and experiment is good at low discharge rates, similar to the previous situations, while there may be some differences between the simulated and experimental results at high discharge rates.This indicates that the model, with unblended LiMn2O4 cathode under high-rate conditions, especially at the cut-off voltage of 3.0 V, is responsible for its superior performance at high discharge rates.At these high rates, however, the energy density of the mixed cathode system and the unblended LiCoO2 system is lower.Furthermore, it is observed that at rates below 6C, the energy density of the active material with a 0.5 weight fraction of LiMn2O4 is lower than the mean value of the energy density of the two unblended active materials (LiMn2O4 and LiCoO2).However, when the rate exceeds 6C, the energy density of the active material with a 0.5 weight fraction of LiMn2O4 surpasses the mean value of the energy density of the two unblended active materials.These findings demonstrate how the electrode compositions behave in a rate-dependent manner and how that affects the battery system's specific energy.
The behavior seen in Figure 6 can be explained by observing varying proportions of current contribution from the two unblended active materials in the blended cathode at varied discharge rates.The active material LiMn2O4 contributes a larger portion of the current at discharge rates below 6C, while LiCoO2 contributes a larger portion of the current at rates above 6C.The variations seen in Figure 6, where the specific energy of the blended cathode system varies with the discharge rate, are a result of this dynamic behavior, which also affects the cell's specific energy.These composite electrode materials have the potential to be used in the fabrication of a powerenergy hybrid system, as demonstrated by the rate-dependent contributions of each active material in the blended cathode.One active material with comparatively higher power characteristics can control the discharge pulses in a power-energy hybrid system, delivering high-power bursts when required, while another active material with comparatively higher energy characteristics can preserve the system's energy content when the power demand is lower [8].This characteristic allows for a more balanced usage of the strengths of the active .19184/cerimre.v7i1.44367 .19184/cerimre.v7i1.44367 Computational and Experimental Research in Materials and Renewable Energy (CERiMRE) Volume 7, Issue 1, page 49-63 eISSN : 2747-173X Submitted : November 22, 2023 Accepted : May 24, 2024 Online : May 31, 2024 DOI : 10.19184/cerimre.v7i1.44367

Figure 3a .
Figure 3a.Cathode electrode discharge profiles from experimental results consisting unblendedLiMn2O4, LiCoO2 and equal ratio of the mix at 5C rate.

Figure 3b .
Figure 3b.Cathode electrode discharge profiles from experimental results consisting unblendedLiMn2O4, LiCoO2 and equal ratio of the mix at 0.3C rate.

Figure 6 .
Figure 6.Specific energy versus weight ratio at different discharge rates

Table 1 .
Design variables used to model the battery