For power lithium-ion batteries, we are most concerned about the indicators of energy density and power density, energy density is about the range of the vehicle, power density is about the power performance of electric vehicles. How to improve the performance of lithium-ion battery multiplier designers have their own unique insights, I dare to talk here about some of my ideas to improve the performance of lithium-ion battery multiplier, I hope to throw a brick to draw jade.
1. Material selection
Generally speaking, to improve the performance of power battery multiplier is mainly from the choice of materials. At present, the common high nickel ternary materials and the traditional lithium cobaltate material ionic and electronic conductivity comparison [1], room temperature 20 ℃, LCO material electronic conductivity of the lowest only 5x10-8S/cm, while NCM111 material electronic conductivity up to 2.2x10-6S/cm, with the further increase in nickel content, the electronic conductivity of ternary materials also increased significantly, NCM8111 material electronic conductivity. The electronic conductivity of NCM8111 material is even up to 4.1x10-3S/cm. The ionic conductivity also shows the same trend. LCO material has an ionic conductivity of only 2.3x10-7S/cm at 20°C, while NCM111 material has an ionic conductivity of 3.2x10-6S/cm, NCM532 bit 1.7x10-3S/cm, and NCM622 3.4x10-3S/cm, NCM811 6.3x10-3S/cm, so whether from the electronic conductivity or ionic conductivity ternary materials, especially high nickel ternary materials or NCA materials are more suitable for multiplier lithium-ion batteries, of course, in addition to these intrinsic characteristics of the material, its multiplier performance is also affected by multiple factors such as morphology For example, the surface area of small particles is larger, and the diffusion distance of Li+ inside the particles is shorter, so theoretically it will have better multiplier performance.
There are a wide variety of options for anode materials, for example, graphite materials of the intermediate phase class with small particles have better performance in terms of multiplicity performance, and the evaluation of graphite materials of different types and particle sizes by S.R. Sivakkumar, J.Y. Nerkar,A.G. Pandolfo [5] of the Energy Technology Division of the Commonwealth Scientific and Industrial Organization (CSIRO), Australia, showed that The smaller the particle size of graphite materials, the higher the multiplicity performance, and the reduction of graphite surface coating thickness can also improve the multiplicity performance of graphite negative electrodes. However, the reduction of particle size also brings a series of problems, such as the reduction of reversible capacity and compaction density, and the study also shows that although the above measures can improve the discharge multiplier performance of graphite anode, it is difficult to effectively improve the charging multiplier performance of graphite anode.
Li4Ti5O12 material itself has a high Li+ diffusion coefficient (10-16-10-15m2/S) [2], while lithium titanate materials are often made into nanoscale particles in production because of their low electrical conductivity, thus further increasing the active area and reducing the diffusion distance of Li+, and lithium titanate batteries therefore have very excellent multiplier performance and can achieve fast charging However, the voltage platform of lithium titanate material is 1.55V, and the theoretical reversible capacity is 170mAh/g, resulting in low specific energy of the battery, which seriously affects the range of electric vehicles, which is also the root cause of the recent crisis of Yinlong, as the saying goes, the success is also Xiao He, the failure is also Xiao He. In order to solve these problems of lithium titanate, while retaining the advantages of its high rate performance, scientific researchers have made a lot of efforts, Japan Toshiba Corporation [3] developed a new anode material of niobium titanium oxide NTO, the reversible capacity of the material up to 341mAh/g is much higher than LTO material, close to graphite material, but with the advantage of high pressure solid density, the bulk energy density reached The material also retains the characteristics of fast charging, from 0% SoC charging to 90% SoC in as little as 6 min, almost perfectly meeting the needs of electric vehicles.
In a recent article published in Nature Kent J. Griffith [4] introduced the latest research results from the University of Cambridge: Nb16W5O55 and Nb18W16O93 materials, which have a reversible capacity of over 200 mAh/g at C/5 multiplicity, and Li+ diffusion coefficients of 10-13-10-12 m2/ The diffusion coefficient of Li+ in both materials reaches 10-13-10-12m2/S, which is much higher than that of LTO (10-16-10-15m2/S) materials, so that the excellent multiplication performance can be achieved at the micron level particle size, and the larger particles not only reduce the active material/electrolyte interface area and the occurrence of side reactions, but also greatly increase the compaction density of the materials. Therefore, the two materials perform exceptionally well in terms of capacity per unit volume, crushing all anode materials.
2. Formulation optimization
Another key to determine the multiplier performance of lithium-ion battery is the battery formula design. The ionic conductivity mainly includes the diffusion of Li+ in the electrolyte, pores inside the electrode and inside the active material, the electronic conductivity is mainly the conductivity between the active material particles, and the electronic conductivity can be divided into "short-range conductivity" and "long-range conductivity", for example, carbon black as the representative of the conductive agent is mainly responsible for the short-range conductivity, to Carbon fiber, carbon nanotubes as the representative of the conductive agent is mainly responsible for the long-range conductivity. Samantha L. Morelly of Drexel University [6] showed that the key to the multiplier performance of lithium-ion batteries is not the ion diffusion process we usually think of, but more on the electron conductivity. For example, the multiplicity performance of electrodes with 3% carbon black is significantly better than that of electrodes with 2.5%, but according to the "ion transport" limitation theory, more carbon black means more tortuous Li+ diffusion channels, which will reduce the multiplicity performance of Li-ion batteries. The study also shows that the short-range conductivity provided by the carbon black adsorbed on the surface of NCM particles is more effective in improving the multiplicity performance of Li-ion batteries than the long-range conductivity.
It is not difficult to achieve high multiplicity performance alone, but the difficulty lies in the balance between multiplicity performance and energy density, which is generally contradictory to each other, and it is very difficult to find a balance between them. The combination of coating thickness and compaction density (70um and 2.9g/cm3) will lead to a sharp decrease in electrode porosity when the compaction density is too high, resulting in an increase in ion diffusion impedance, and an increase in contact impedance when the compaction density is low, so only a suitable compaction density can ensure the excellent multiplicity performance of lithium-ion batteries while taking into account the characteristics of high energy density.
3. Choice of battery structure
How to control the temperature of the discharge process for multiplier batteries is also a very important issue, in the process of high current discharge lithium-ion batteries will generate a lot of heat, the accumulation of heat in the internal lithium-ion battery will lead to an increase in temperature, resulting in a large temperature gradient, so the internal decay of the lithium-ion battery is inconsistent, affecting the life of the lithium-ion battery. How to choose a suitable structure becomes especially important. Stephan Kosch et al. of Technical University of Munich, Germany [8] studied the influence of the shape and position of the lithium-ion battery lugs on the thermal characteristics of large-size lithium-ion batteries through a two-dimensional electric-thermal polarization model and found that the width of the lugs and the thickness of the collector fluid affect the temperature distribution of the lithium-ion battery during discharge. The narrower the lug and the thinner the collector, the greater the temperature distribution inhomogeneity in the battery.
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