Understanding the Differences in Electrical and Mechanical Performance of Positive Electrodes with Various Ratios of LMFP Mixing
With the growing new energy vehicle market, the supply demand for lithium-ion batteries is also increasing rapidly. Automotive lithium-ion batteries have high requirements on energy density, cycle life, safety and cost, currently, the mainstream cathode materials for lithium batteries include lithium cobalt oxide (LCO), ternary material (NCM), and lithium iron phosphate (LFP). Although LCO and NCM cathodes have higher energy density, their cost and safety are inferior to those of LFP cathodes. Although the LFP cathode has good stability and safety, its energy density development is approaching the limit. Like the LFP positive electrode, the lithium iron manganese phosphate (LMFP) positive electrode also has good chemical stability and safety, at the same time, the doping of manganese element can increase the charging voltage of the material to 4.1V, making the theoretical energy density of LMFP batteries 15~20% higher than the LFP cathode. However, the current LMFP positive electrode is not perfect, and there are still problems such as voltage jump, low conductivity, and poor rate performance.
To take advantage of the above materials and meet different market demands, the strategy of hybrid cathodes emerged [1,2]. Two (or more) cathode materials with complementary properties are physically and mechanically mixed and used. When the advantages of one component material are fully utilized, the advantages of other component materials are used to make up for its disadvantages, and then a cathode material with good performance can be prepared, affordable lithium batteries meet people's balancing requirements for cycle performance, cruising range and safety. For example, H.S. Kim et al. [2] mixed NCM cathodes and LCO cathodes in different proportions, and found that as the proportion of NCM in the components increased, the reversible specific capacity and cycle stability of the battery have been significantly improved, but its rate performance has declined sequentially. When the mixing ratio is 1:1, the rate and cycle performance reach the dynamic optimum.
Different cathode materials have different operating voltages. Therefore, within a certain operating voltage range, the synergistic effect between materials needs to be considered when mixing multiple active particles. LMFP and NCM materials have similar discharge voltage windows, that is, under the same voltage window, the electrochemical properties of both materials can be exerted well. Therefore, blending the two materials may have better synergistic effects. This article uses IEST's PRCD3100 to study the resistivity, compaction density and stress-strain curves of mixed cathode materials with different proportions of LMFP and NCM under different pressures, and further analyzes the differences in electrical and mechanical properties of cathode materials under different mixing proportions.
1.Test Conditions
1.1 Testing Equipment
Figure 1 shows the powder resistivity & compaction density meter (PRCD3100) independently developed by IEST. The device can apply different pressures (up to 5T) to powder samples while, simultaneously collect the resistivity, conductivity, compaction density and other parameters of powder samples to assist researchers in studying the effects of different pressures on the electrical and mechanical properties of powder samples.
Figure 1. Schematic diagram of the Powder Resistivity & Compaction Density Meter (PRCD3100, IEST) and the two testing principles of powder resistivity.
1.2 Experimental Procedure
1.Prepare 6 different proportions of LMFP and NCM mixed cathode materials, respectively: 1.100%LMFP; 2. 80%LMFP+20%NCM; 3. 60%LMFP+40%NCM; 4. 40%LMFP+60%NC; 5. 20%LMFP+80%NCM; 6. 100%NCM.
2.In the range of 10~350MPa, with 20MPa as step interval, apply pressure step by step on the above 6 mixed positive electrodes with different proportions, and use the four-probe resistance test module and thickness test module that come with PRCD3100 to record the changes in resistivity and thickness under different pressures in real time, so as to obtain the changes in resistivity & compaction density of these six hybrid cathodes with pressure.
3.In the range of 10~350MPa, with a step interval of 20MPa, the above six mixed positive electrodes with different proportions are first pressurized to 350MPa in a stepwise manner, and then depressurized in a stepwise manner to 10MPa, the thickness changes of the entire process were recorded simultaneously to obtain the stress-strain curves of these six hybrid cathodes.
2. Results Analysis
Figure 2 shows the change curves of resistivity (a) and compaction density (b) with pressure of 6 different ratios of LMFP and NCM mixed cathode materials. As can be seen from Figure 2(a), the electrical conductivity of the six cathode materials gradually decreases with increasing pressure, it shows that high pressure can help improve the contact resistance between powder materials, thereby improving the material's electron transport capability. In addition, the resistivity of 100% LMFP powder is the largest among the six, regardless of whether it is under small pressure or high pressure. That is, the conductivity of pure LMFP powder is the worst, which also limits the rate performance of the material. With the gradual addition of NCM powder, the resistivity of the mixed positive electrode also gradually decreases throughout the test pressure range, that is, the electrical conductivity of the cathode material gradually becomes better with the addition of NCM powder, until it approaches the electron transport capacity of pure NCM powder. In mixed granular materials, electrons are conducted through solid particles.
The material's own electrical conductivity, particle size distribution, and contact state between particles will all affect the electronic conductivity. Typically, an electrode sheet contains active material, conductive carbon, and a binder. In the current research, the impact of the type and proportion of the conductive agent in the electrode piece on the electronic conductivity of the electrode piece is mainly considered. Especially for the positive electrode, since the electronic conductivity of the active material is very low, conductive additives are used to ensure good electronic conductivity. However, in addition to conductive carbon, the type and volume fraction of active materials also have an impact on electrical conductivity. Therefore, the impact of the electronic conductivity of the active material itself on battery performance should also be taken into consideration. This hybrid material electrode can exert the synergistic effect of the two, that is, using NCM to avoid the shortcomings of poor electrical conductivity of LMFP powder.
It can be seen from the changing trend of compacted density with pressure in Figure 2(b) that the compacted density of 100% LMFP powder is the smallest, and with the continuous addition of NCM powder, the compacted density of the mixed cathode material is gradually increasing. The compacted density is also related to the mechanical properties and particle size distribution of the active particles. In mixed materials, during the compression process of two kinds of particles, the particles are in closer contact with each other, small particles fill the gaps between large particles, and the gap pressure is reduced; when the pressure continues to increase, the active particles break and cracks form between secondary particles. Without affecting the electrolyte infiltration and specific capacity development, the greater the compaction density of the positive electrode material and the smaller the thickness of the electrode, the higher the battery capacity will be, and the higher the battery volume energy density will be. The better it can meet the current market demand for the energy density of lithium batteries.
To sum up, adding NCM materials to LMFP cathode materials can effectively improve the electron transport capability and compaction density of LMFP materials. However, it is worth noting that although in this study, the improvement effects of these two parameters showed a monotonically getting better trend with the continuous addition of NCM, it does not mean that the more NCM materials are mixed, the performance of the hybrid cathode will also be better. This also requires a comprehensive evaluation of the cycle performance, rate performance, safety performance and cost advantages of the hybrid cathode after being prepared into a battery, and finally determine the optimal mixing ratio to balance market demand and production costs. X.X. Zhao et al[3] prepared a mixture of NCM and LMFP, and used the NCM-LMFP mixture as the cathode to assemble a 18650 full battery. The overall performance of the battery was better than that of a single material NCM or LMFP battery, including superior rate capability, good cycle stability and high and low temperature performance.
Figure 2. (a) Resistivity change curve and (b) compaction density change curve of 6 different ratios of LMFP and NCM mixed cathode materials with pressure.
Figure 3(a) shows the stress-strain curves of 6 hybrid cathodes with different proportions during pressurization and pressure relief (pressure range is 10~350MPa, step interval is 20MPa), help analyze the differences in mechanical properties of different hybrid cathodes. First, the strains of the six types of mixed cathode powders after pressure relief cannot return to the original point. That is, all the mixed cathode powders studied in this article have a certain proportion of irreversible deformation. Subsequently, the changes in the maximum deformation, irreversible deformation and reversible deformation of these six samples with the NCM addition ratio were statistically calculated.
The results are shown in Figure 3(b). Regardless of the maximum deformation amount (black), irreversible deformation amount (orange) or reversible deformation amount (gray), the three curves all show a "U" shaped change pattern the deformation amount of pure LMFP and pure NCM powder is the largest, while the deformation amount of mixed powder is relatively small. The lowest point of the three appears at 60%, that is, the mixed cathode of 40% LMFP + 60% NCM has the smallest deformation after pressure, and the irreversible variable is also the smallest. When preparing pole pieces, thickness is a key process indicator. To ensure that the thickness of the final pole piece is controllable, it is generally hoped that the thickness rebound of the active material after being compressed is minimal. It can be seen from the stress-strain experiment in Figure 3 that the mechanical behaviors of mixed positive electrodes with different proportions are different, and the electrode piece preparation process also needs to be adjusted according to different mechanical feedback to give different process parameters.
Figure 3. (a) shows the stress-strain curves of 6 hybrid cathodes with different proportions during the pressurization and pressure relief processes. (b) shows how the maximum deformation, irreversible deformation, and reversible deformation of the six hybrid cathodes change with the addition ratio of NCM.
In addition, T. Liebmann et al[4] also conducted a systematic study on how the electrochemical properties of the components affect the behavior of the mixed electrode for several mainstream cathode materials, namely olivine LFP, layered NCM and spinel LMO. The results show that the basic electrochemical performance of the hybrid electrode obeys the physical mixture model and can be predicted accordingly based on the component properties of different mass fractions, including thermodynamic properties, such as equilibrium potential and specific capacity curves, entropy distribution and derived properties.
But kinetic parameters, such as exchange current density and lithium diffusion coefficient in the active material, are often a function of the charge state, and these properties do not conform to the predictions of blending physical models. At the same time, it was also found that the microstructural properties of the blends will produce different electron and ion penetration networks in the electrodes, thus affecting battery performance. The electrical conductivity and mechanical properties of the particles are key parameters that determine the microstructural characteristics of the electrode. Therefore, work in this area deserves further in-depth research.
3. Summary
This article uses the powder resistivity & compaction density meter (PRCD3100) developed by IEST to test the resistivity and compaction density of six LMFP+NCM hybrid cathodes with different proportions under different pressures, and study the changes in their mechanical properties. From the perspective of electrical properties, with the gradual addition of NCM powder, the resistance transport capacity of the hybrid cathode is gradually improving; from the perspective of compaction density, the continuous addition of NCM powder is also conducive to improving the overall compaction density of the hybrid cathode material.
Judging from the stress-strain curve, the hybrid cathode with a composition of 40% LMFP + 60% NCM has the smallest deformation amount after being compressed, and the irreversible variable is also the smallest. However, it is worth noting that in addition to the parameters studied in this article, it is also necessary to comprehensively evaluate the cycle performance, rate performance, safety performance, cost advantages and other parameters of the hybrid cathode after being prepared into a battery, to ultimately determine the optimal mixing ratio. To balance market demand with production costs.
4. Reference Materials
[1] T. Or, S.W.D. Gourley, K. Kaliyappan, A.P. Yu and Z.W. Chen, Recycling of mixed cathode lithium‐ion batteries for electric vehicles: Current status and future outlook. Carbon Energy 2 (2020) 6-43.
[2] H.S. Kim, S.I. Kim and W.S. Kim, A study on electrochemical characteristics of LiCoO2/LiNi1/3Mn1/3Co1/3O2 mixed cathode for Li secondary battery. Electrochimica Acta 52 (2006) 1457-1461.
[3] X.X. Zhao, L.W. An, J.C. Sun and G.C. Liang, LiNi0.5Co0.2Mn0.3O2-LiMn0.6Fe0.4PO4 mixture with both excellent electrochemical performance and low cost as cathode material for power lithium-ion batteries, Journal of Electrochemical Society 165 (2018) A142-A148.
[4] T. Liebmann, C. Heubner, M. Schneider and A. Michaelis, Understanding kinetic and thermodynamic properties of blended cathode materials for lithium-ion batteries, Materials Today Energy, 22 (2021) 100845.