The bumpy road for lithium-ion batteries

by:CTECHi     2021-09-21
The bumpy lithium-ion battery, the technology is still on the way to maturity. The invention of lithium-ion battery has a history of 49 years, and its safety, energy density, and price are still not close to the people. Looking at all kinds of information, all of them say that the process of lithium-ion batteries is complicated, there are many preparation processes, thermal runaway is prominent, and the verification of failure decomposition is not easy to verify. Let me popularize the knowledge of lithium-ion batteries first, and I don’t like to skip it first. The capacity of a lithium ion battery is determined by the lithium ion content in the positive electrode active material. Excessive positive electrode and uneven distribution of lithium ion gradient in the electrolyte will cause lithium precipitation and dendrite formation, which is likely to cause safety threats. The electrolyte is used to transport lithium ions, and the quantity must be guaranteed. The water in the battery will consume lithium ions, affect the capacity, and at the same time easily react with the electrolyte, causing swelling. The diaphragm is used for filtration and cannot be damaged, otherwise the positive and negative electrodes will contact and short-circuit. If you work at high temperatures, the battery capacity will drop faster, and the life will be shortened. In particular, the abuse of lithium-ion batteries is clear, and the most important ones are heat abuse, electricity abuse and mechanical abuse. Overheating and fire exposure are heat abuse. Overcharge, overdischarge and external/internal short circuit are electrical abuses. Mechanical abuse conditions include collisions, penetrations, and bending. All in all, for lithium-ion battery processing, the most important thing is to learn failure decomposition. The purpose is to find the cause of the quality problem, find the correct conclusion in a short time, and choose practical measures. Failure decomposition is very simple, including: clear decomposition object, determine failure mode, study failure mechanism, determine the cause of failure, and propose preventive measures (including design improvements). Regardless of whether a failure event is a major accident or a minor failure, the cause always includes six aspects: operators, mechanical equipment systems, materials, manufacturing processes, environment, and management. The handling methods are as follows. First, compare methods. Choose a system that has no failure and can be compared with the failed system in order to find the difference. This will help to find out the cause of the failure as soon as possible. Second, the historical approach. The objective basis of the historical approach is the movement and causality of the material world. It is based on the past performance and changing laws of the equipment under the same service conditions to infer the possible reasons for the current failure. Third, the logical approach. It is to decompose, compare, synthesize and summarize the information obtained from background data (design, materials, manufacturing, etc.) and failure site investigation materials, as well as decomposition and detection, to make judgments and inferences, and then to derive the possible reasons for the failure. In terms of lithium-ion battery failure, there are two important categories: performance failure and safety failure. Performance failure means that the performance of lithium-ion batteries cannot meet the requirements and related indicators. The important ones are capacity degradation or diving, short cycle life, poor rate performance, poor consistency, easy self-discharge, high and low temperature performance degradation, etc. Safety failure is the failure of lithium-ion batteries with certain safety risks due to improper use or abuse. The important ones include thermal runaway, flatulence, liquid leakage, lithium evolution, short circuit, swelling deformation, etc. However, fundamentally speaking, the failure is ultimately due to the destruction of material properties and structure. The author is a master of materials and knows that great and noble creations are behind the innovation of materials. Material failure mainly refers to the abnormalities in the structure, properties, and morphology of materials and the mismatch between materials. For example, in the cathode material, due to the inconsistency of local Li+ deintercalation rate, the initiation of particles ruptures due to uneven stress on the material. The silicon anode material breaks and pulverizes due to the volume expansion and contraction during the charge and discharge process, the electrolyte is analyzed or deteriorated under the influence of humidity and temperature, and the graphite anode is co-embedded with the solvent of the additive propylene carbonate (PC) in the electrolyte. The problem is that N/P (the ratio of the capacity of the negative electrode sheet to the capacity of the positive electrode sheet) is too small to cause lithium evolution. For electric vehicles and hybrid vehicles that sell well today, the core technology is the battery. Compared with 3C consumer batteries, although the power lithium battery has a high price and poor safety performance, it has a large specific energy, a long cycle life, and a wider use scene. Wide sea diving, sky high the birds to fly. With the advancement of the times, the technology of power lithium batteries changes with each passing day, and future improvements should also be concentrated on capacity and structure. One thing is certain is that no matter which technical route the battery manufacturer adopts, it must meet the conditions of high safety, wide environmental temperature range, strong charging and discharging functionality, good rate discharge usability, and low cost.
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