How to better improve the performance of lithium-ion battery rate

For power lithium-ion batteries, the most important indicators we focus on are energy density and power density. The energy density is related to the cruising range of the vehicle, and the power density is related to the dynamic performance of the electric vehicle. How to improve the performance of lithium-ion battery rate designers have their own unique insights, I’ll talk about some of my ideas to improve the performance of lithium-ion battery rate. I hope to be meaningful.

1. Material selection

Generally speaking, the improvement of power battery rate performance is mainly based on the choice of materials. For example, we used to have the article "Ion Conduction, Electronic Conductive Stupidity Is Unclear? You Want to Know It Is Here!" The ion and electron conductivity of nickel ternary materials and traditional lithium cobalt oxide materials, at 20 °C, the electronic conductivity of LCO materials is only 5x10-8S/cm, while the electronic conductivity of NCM111 materials is up to 2.2x10-6S/cm, with the further increase of nickel content, the electronic conductivity of ternary materials is also significantly improved. The electronic conductivity of NCM8111 material is 4.1x10-3S/cm, and the ionic conductivity is also shown. In the same trend, the ionic conductivity of the LCO material is only 2.3x10-7S/cm at 20 °C, while the ionic conductivity of the NCM111 material is 3.2x10-6S/cm, the NCM532 is 1.7x10-3S/cm, and the NCM622 is 3.4x10. -3S/cm, NCM811 material is up to 6.3x10-3S/cm, so whether it is from the electronic conductivity or ionic conductivity, the ternary material, especially the high nickel ternary material or NCA material is more suitable for the magnification type. Lithium-ion battery, of course, in addition to materials These intrinsic properties of the outer, rate capability which is also affected by the morphology of multiple factors, such as small particles of material greater surface area, Li + diffusion distance is shorter in the interior of the particle, thus theoretically better rate capability.

There are many types of anode materials, such as graphite particles with small particles of mesophases, which have a good performance in rate performance. SRSivakkumar, JYNerkar, AGPandolfo Energy Technology Department of the Commonwealth Scientific and Industrial Organization (CSIRO) Evaluation of different types and sizes of graphite materials shows that the smaller the particle size of graphite materials, the higher the rate performance, and the reduction of graphite surface coating thickness can also improve the rate performance of graphite anodes. However, the particle size reduction also brings a series of problems, such as the reduction of the reversible capacity and the decrease of the compaction density. At the same time, the research also shows that although the above measures can improve the discharge rate performance of the graphite anode, it is difficult to effectively improve the graphite anode charging rate performance.

The Li4Ti5O12 material itself has a high Li+ diffusion coefficient (10-16-10-15m2/S) [2], and the lithium titanate battery material is often made into nano-sized particles due to its low conductivity, so further The active area is increased and the diffusion distance of Li+ is reduced. The lithium titanate battery has excellent rate performance and can achieve fast charging, which is why Dong Mingzhu is interested in Yinlong, but the voltage platform of lithium titanate material. For 1.55V, the theoretical reversible capacity is 170mAh/g, resulting in lower battery specific energy, which seriously affects the cruising range of electric vehicles. This is also the root cause of Yinlong's recent crisis. It is said that Cheng is also Xiao He, and Xiao He is also defeated. In order to solve these problems of lithium titanate while retaining the advantages of its high rate performance, researchers have made a lot of efforts, the reversible capacity of the material, the neodymium titanium oxide compound NTO new anode material developed by Toshiba Corporation of Japan. Up to 341mAh/g is much higher than LTO material, close to graphite material, but with the advantage of high pressure solid density, the volumetric energy density reaches twice that of graphite anode, and the material retains the characteristics of fast charging. 0% SoC charging to 90% SoC takes only 6 minutes at the earliest, almost exactly meets the needs of electric vehicles. At present, Toshiba has announced a cooperation agreement with Sojitz and Brazilian mining company CBMM to jointly develop and produce the material.

As the world's top university, the University of Cambridge is also working on the development of high-capacity, high-magnification high-performance lithium-ion battery anode materials. In a recent article published in Nature, KentJ. Griffith introduced the latest research from Cambridge University. Results: Nb16W5O55 and Nb18W16O93 materials, the reversible capacity of these two materials exceeds 200mAh/g at C/5 rate, and the diffusion coefficient of Li+ in both materials reaches 10-13-10-12m2/S, which is much higher than LTO ( 10-16-10-15m2/S) material, so it can achieve excellent rate performance on micron-sized particle size. Larger particles not only reduce the active material/electrolyte interface area, but also reduce the occurrence of side reactions. The compaction density of the material is greatly increased, so the two materials perform exceptionally well in terms of unit volume capacity, and all negative electrode materials are rolled.

2. Formula optimization

Another key to determining the performance of lithium-ion battery rate is the design of the battery. There are two kinds of conductive forms of "ion conduction" and "electron conduction" inside the lithium ion battery. The ion conduction mainly includes Li+ in the electrolyte, the internal pores of the electrode and The internal diffusion of the active material, the electronic conduction is mainly the conduction between the active material particles, and the electronic conduction can be further divided into "short-range conduction" and "long-range conduction", for example, the conductive agent represented by carbon black is mainly responsible for Short-range conductive, conductive agent represented by carbon fiber and carbon nanotube is mainly responsible for long-distance conduction. The rate performance of lithium-ion batteries is a comprehensive manifestation of several conductive forms. Research by Samantha L. Morelly et al. of Drexel University in the United States has shown that the key to affecting the rate performance of lithium-ion batteries is not what we usually think of as "ion diffusion." The process is more dependent on electronic conductivity. For example, the rate of performance of an electrode with 3% carbon black is significantly better than that of 2.5%, but according to the theory of "ion transport" limitation, more carbon black means the more tortuous Li+ diffusion channel will reduce the rate performance of lithium-ion batteries. At the same time, this study shows that the short-range conductivity provided by carbon black adsorbed on the surface of NCM particles can improve the rate performance of lithium-ion batteries compared to long-range conductivity bigger.

It is not difficult to achieve high rate performance simply. It is difficult to balance the rate performance with the energy density. Generally speaking, the ratio performance and energy density are contradictory. It is very difficult to find a balance between the two. Kazuaki Kisu et al. of Tokyo University of Agriculture and Technology in Japan obtained the best combination of coating thickness and compaction density (70um and 2.9g/cm³) by analyzing the impedance of NCM electrodes of different coating thicknesses and compacting densities. When the compaction density is too high, the electrode porosity will drop sharply, resulting in an increase in ion diffusion resistance and a lower compaction density will lead to an increase in contact resistance. Therefore, only the appropriate compaction density can guarantee the excellent multiplier performance of lithium ion battery and also take into account the characteristics of high energy density.

3. Choice of battery structure

How to control the temperature during discharge of the rate battery is also a very important problem. During the high current discharge process, the lithium ion battery will generate a large amount of heat. The accumulation of heat inside the lithium ion battery will cause the temperature to rise. Large temperature gradients, so the internal decay of lithium-ion batteries is inconsistent, affecting the life of lithium-ion batteries. How to choose a suitable structure becomes more important. By electrical and thermal polarization model of two-dimensional shape and the location of the lithium ion battery pole ear for large size thermal characteristics of the lithium ion battery research found that the influence of the width of the ear and set the thickness of the fluid is for lithium ion battery in the temperature distribution in the process of discharge, the narrower the pole ear, the thinner the battery set fluid temperature distribution inside the nonuniformity, the greater the also found that when the battery pole ear on the battery terminals can effectively reduce discharge when the internal temperature of the battery in the process of inhomogeneity.

By selecting suitable materials, formulations and structures, the internal impedance and polarization of the lithium ion battery during large-rate discharge can be reduced, the temperature non-uniformity can be reduced, and the rate performance of the battery can be effectively improved. Improving the rate performance is a comprehensive project, which needs to be considered from multiple factors. What I have introduced is just a drop in the ocean. It is hard to avoid some omissions and omissions in knowledge. I hope you will correct me and put forward your own views.

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