EverExceed have recently introduced a new technology of low temperature
Lithium iron phosphate battery, which can be charged even under 0°C and in negative temperature. In a series of 5 technical articles we are going to elaborately describe the details of this revolutionary technology. In this article we are going to talk about the “Material improvement” of the low temperature technology lithium battery.
Material improvement:
Introduction:
In the first two articles we discussed about "performance" and "principle". In this article, let's talk about how to improve the low-temperature performance.
Before mentioning the improvement of materials, let's review the charging process of lithium-ion battery, which is divided into four steps:
1) Lithium ions are de intercalated from the positive particles and enter the electrolyte
2) Transfer of lithium ion in electrolyte
3) Lithium ion contacts the negative electrode through SEI film
4) Intercalation and diffusion of lithium ion in negative electrode
The content of material improvement to be discussed next in this paper is also expanded one by one from the above four points.
——Lithium ions are de intercalated from the positive particles and enter the electrolyte
This is the initiation journey of lithium ion movement in the charging process, and it is also the easiest step with the least resistance among the four steps. The resistance of lithium ion cathode de intercalation mainly depends on the structure of cathode material. Lithium cobaltate has a layered structure, and lithium ions can be freely de embedded and embedded from the front, back, left and right directions. Therefore, it has a good performance even at low temperature. The molecular structure of lithium cobaltate is shown as follows:
Compared with layered lithium cobalt oxide, lithium iron phosphate has olivine structure. In this structure, PO4 limits the volume change of crystal structure, so the impedance of lithium ion intercalation and de intercalation is greater, and the relative low-temperature performance is not as good as lithium cobalt oxide.
In addition, for active material particles, the smaller the particles, the shorter the lithium ion migration path. At room temperature, due to the rapid diffusion of lithium ions, the influence of large and small particles on the capacity is not obvious, but at low temperature, the advantages of small particle materials will begin to appear. The comparison results of the capacity of particles of the same material at different temperatures are as follows:
Lithium ion is removed from the cathode with the least hindrance and happily comes to the electrolyte. In the electrolyte, the degree of hindrance depends on the ionic conductivity of the electrolyte at low temperature. In order to ensure the low-temperature performance of electrolyte, the content of high melting point solvent EC (melting point 39 ~ 40 ℃) needs to be reduced, generally 15% ~ 25%. Some low melting point PCs (melting point - 48.8 ℃) can be added, but film-forming additives should be added at the same time to avoid peeling of graphite layer caused by PC. The schematic diagram is as follows:
High ionic conductivity is the standard configuration of low-temperature electrolyte, but high ionic conductivity at room temperature does not necessarily mean better low-temperature performance. The key to the problem is to ensure the ionic conductivity at low temperature. Ionic conductivity is determined by dielectric constant and viscosity. Dielectric constant refers to the amount of Li + in free state at the same lithium salt concentration. Naturally, the more, the better; Viscosity refers to the resistance to Li + transfer. Naturally, the smaller the resistance, the better.
Conclusion:
To match the requirement of cold countries where you need reliable
energy storage solution in outdoor application, EverExceed research and development engineers were working for long time for a suitable solution and thus came the new technology. So for your cold temperature energy storage solution choose EverExceed as your brand with complete reliability.
