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The use of battery material blending and the synergistic effect between different materials is an effective method for designing electrode materials for lithium-ion batteries. Research has shown that blending between two or more materials has the potential to improve the overall performance of electrodes.
This strategy not only utilizes the synergistic effect between materials to compensate for the shortcomings of the materials themselves, but also often generates spillover effects. Compared with individual materials, blended materials exhibit more balanced and superior comprehensive properties.The so-called blending refers to the physical or mechanical combination of two or more objects.
Unlike common modification methods such as hybridization and coating, blending retains the pure components of the blended material, and the operation process is simple, with low equipment requirements, high consistency in composition and performance, and easy industrial production.Blending modification of lithium battery cathode materialsAt present, various positive electrode materials have been successfully developed and applied, including LiCoO2, lithium manganese oxide, lithium iron phosphate, and ternary materials.
① Blending of high and low safety positive electrode materialsLayered materials are commonly used as positive electrode materials for power batteries due to their high energy density, but their poor thermal stability limits their further development and application.
Researchers have proposed a unique blending method to enhance the safety of layered materials by blending layered lithium cobalt oxide and lithium iron phosphate. As shown in Figure 1, they demonstrated a double-layer electrode of lithium cobalt oxide and lithium iron phosphate. The lithium iron phosphate layer is used as both an active material and a resistive barrier layer for overcharging and thermal runaway protection. The blended electrode not only enhances its resistance to overcharging currents, but also reduces the surface temperature during overcharging to 80 ℃, significantly improving the safety of lithium cobalt oxide.Research has found that when LiCoO2 and LiMn2O4 are blended in a mass ratio of 3:2, the cost significantly decreases, and the stable capacity reaches up to 137mAh/g, while the capacity retention rate reaches 92.3% after 5 cycles of overcharging. When lithium cobalt oxide and lithium iron phosphate are blended in a mass ratio of 1:1, their overcharging resistance and thermal stability are significantly improved.
Under 3C/10V overcharging conditions, only swelling occurs, and there is no smoke or fire phenomenon; Moreover, under short-circuit and overcharging conditions, the surface temperature of the blend material is significantly lower than that of lithium cobalt oxide material.
Figure 1 Schematic diagram of lithium iron phosphate/lithium cobalt oxide double-layer electrode blending② Blending of high and low voltage positive electrode materialsResearchers blended lithium iron phosphate with lithium manganese oxide, and the experimental results showed that when the mass ratio of the two was 5:5, the average working voltage of the blended positive electrode graphite full battery could be increased to 3.64V, which was significantly improved compared to the pure lithium iron phosphate graphite voltage platform.
Moreover, due to the fact that lithium iron phosphate is a sub micron sized particle, while lithium manganese oxide is a micron sized particle, it can fill the gaps in lithium manganese oxide, effectively blocking direct contact between the electrolyte and lithium manganese oxide, inhibiting manganese dissolution, and improving electron transfer between particles, reducing electrode internal resistance, and enhancing cycling performance and stability.③ Blending of reversible/irreversible cathode materialsWith the increasing demand for high-energy density batteries in lithium-ion batteries, the application of alloy negative electrodes has attracted attention. However, alloying negative electrodes generally have lower Coulombic efficiency (≤ 85%), which does not match the higher Coulombic efficiency of the positive electrode material. During the first charge and discharge process, some active lithium ions in the positive electrode material are lost, reducing the utilization rate of the positive electrode and the energy density of the battery.
Researchers have coated a layer of Li2S with irreversible lithium removal capacity on the surface of lithium iron phosphate cathode materials.Due to the initial charging of Li2S in the positive electrode operating voltage range (2.5-4V), the lithium removal capacity reaches up to 1093mAh/g, while during discharge, the lithium insertion capacity is only 9mAh/g. So, after blending a small amount of Li2S with the lithium iron phosphate cathode, the first charge lithium removal capacity can reach 200mAh/g. During discharge, only lithium iron phosphate plays a role, and its lithium insertion capacity is only 156mAh/g; When the blended electrode is matched with the Si/graphite negative electrode, the excess 44mAh/g can be used to compensate for the lithium ion consumption during the first charge and discharge of Si/graphite, resulting in the positive electrode receiving up to 150mAh/g of lithium ions, making the utilization rate of the positive electrode material close to 100%.
The blended electrode not only increases the energy density of the battery, but also reduces the cost of the battery.Blending modification of lithium battery negative electrode materials
① Blending of high and low capacity negative electrode materialsGraphite is the mainstream negative electrode material for lithium-ion batteries. Its structure is stable during charging and discharging, with small volume changes, and it itself has good conductivity. However, due to its low theoretical capacity, it is difficult to meet the high capacity requirements of the next generation of lithium-ion batteries. Silicon not only has high capacity but also abundant resources, but its volume expansion during charging and discharging processes results in poor cycling stability. Improving the cycling stability of silicon negative electrodes is an urgent challenge that needs to be overcome, among which silicon carbon blending is an effective strategy for balancing the performance of silicon negative electrodes.Researchers blended graphite with 5%, 10%, 15%, and 20% silicon, and found that the capacity increased with the increase of Si content, while the irreversible capacity also increased. When the blending ratio of Si is 20%, the reversible capacity can reach up to 830mAh/g, which is twice the capacity of pure graphite, and the Coulombic efficiency in the first week is 83%.
Its comprehensive performance is balanced.Researchers have blended graphite with phosphorus doped silicon materials by ball milling, resulting in a uniform composition of the blended material and utilizing the excellent conductivity of graphite to enhance the power density of the material;
At the same time, due to the effective coating of graphite on the silicon surface, the silicon material is isolated from the electrolyte, avoiding direct production of SEI on the silicon surface and improving the cycling stability of the composite electrode. As shown in Figure 2, when the ratio of graphite to doped silicon is 1:1, the mixed electrode exhibits an initial capacity of 1427mAh/g, and the capacity can still reach 883.4mAh/g after 200 cycles.
Figure 2: Schematic diagram of (a) synthesis of graphite silicon (doped with phosphorus) room temperature composite material, (b) EDX spectrum, and (c) charge discharge curve
② Blending of High and Low Coulombic Efficiency Negative Electrode MaterialsThe low Coulombic efficiency of hard carbon materials with amorphous structures during initial charge and discharge limits the improvement of battery energy density. Researchers used mechanical ball milling to blend graphite and hard carbon, and the graphite/pyrolysis carbon blend material has the characteristics of both graphite and pyrolysis carbon materials.As the proportion of graphite increases, the first cycle Coulombic efficiency of the blended negative electrode increases. When the mass ratio of graphite to hard carbon is 2:1, its first cycle Coulombic efficiency increases to 76%, significantly higher than the Coulombic efficiency of pure hard carbon materials at 69%, and its rate performance and cycle life are also better.
In addition, as an ideal negative electrode material for sodium ion batteries, hard carbon materials have excellent overall performance. However, due to the lower carbonization temperature (below 1000 ℃), the specific surface area is larger, and there are more surface defects and impurity atoms, resulting in more available sodium ions being consumed for the first time. Soft carbon structures have higher orderliness and fewer surface defects, resulting in higher Coulombic efficiency in the first week, but their specific capacity is lower than that of hard carbon. Researchers have designed blended negative electrode materials with different ratios of hard and soft carbon to be used as low-cost sodium ion battery negative electrodes. When the mass ratio of hard carbon to soft carbon is 5:2, the blended electrode exhibits a maximum capacity of 282mAh/g and an initial Coulombic efficiency of up to 80%, which is more than twice the first week Coulombic efficiency of pure hard carbon at 37%.
SummaryBy blending and modifying multiple materials, compared with a single material, it can achieve synergistic effects such as reducing capacity loss, improving battery life, and enhancing safety performance. This can provide ideas for process optimization and cost reduction in actual production. In summary, the selection of lithium-ion battery material systems is a comprehensive consideration of energy density, safety, recyclability, and manufacturing costs.
Blending modification can be an important technical means to reduce manufacturing costs while meeting certain performance requirements of lithium-ion batteries.(Reference source: Zhong Xingguo. Material blending to improve the performance of hard carbon negative electrodes in lithium-ion batteries; Zhang Lin et al.. Research progress on blending modification of lithium-ion battery positive electrode materials)
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