In today’s energy field, lithium-ion batteries have become the core power source for various electronic devices and electric vehicles due to their many advantages such as high energy density and long cycle life. However, with the widespread application of lithium-ion batteries, their failure problems have gradually become prominent, which not only affects the normal operation of the equipment, but also may cause safety hazards. The common failure types in the design of lithium-ion battery cells are worth exploring in depth. The main types are as follows.
Low capacity
Without considering the influence of raw materials, design system and process, only considering the design itself, common design errors lead to low capacity as follows:
(1) CB value design is too large, resulting in reversible lithium loss: Due to the large CB value, the Li of the positive electrode cannot be completely returned to the positive electrode after being embedded in the negative electrode during discharge. The larger the excess of the negative electrode, the more lithium loss, the lower the first efficiency, and the lower the capacity.
[CB value refers to the battery capacity loss of lithium-ion batteries during discharge. The larger the CB value, the faster the battery capacity decays and the shorter the battery life; the smaller the CB value, the slower the battery capacity decays and the longer the battery life]
(2) Insufficient CB value design leads to irreversible lithium loss: Due to insufficient CB value, the Li of the positive electrode cannot be embedded in the negative electrode and is directly precipitated on the surface of the negative electrode, resulting in irreversible lithium loss, which is manifested as low initial efficiency and low capacity. This type of defect can be confirmed by disassembly to confirm whether there is lithium precipitation. If it is not obvious, it can be confirmed by appropriate overcharging.
(3) Insufficient liquid retention leads to low capacity: This type of situation generally manifests as large internal resistance. Due to insufficient electrolyte, there is not enough electrolyte in the pores of the electrode or diaphragm. During the charge and discharge process, Li+ cannot be completely removed or embedded in the negative electrode. The lithium that cannot be embedded in the negative electrode will precipitate on the surface of the negative electrode, resulting in low capacity. This type of low capacity phenomenon can be determined by disassembling the battery, checking the surface of the electrode and the infiltration of the battery cell, and whether there are black spots.
(4) The designed compaction density is too high, so the electrolyte cannot enter the particle pores: In this case, the battery can be directly disassembled to see if there are black spots on the surface of the negative electrode, whether the electrode is relatively brittle, whether the pressure is abnormal during cold pressing, and whether the slightly lower compaction can be improved to make a comprehensive judgment.
(5) The designed formation charging system is unreasonable: The purpose of formation is to remove side reactions and form a dense SEI film at the same time. The first efficiency loss mainly occurs in the formation stage. If the charging temperature is too high, the film formation reaction will be aggravated, the SEI aging speed will be faster, the inorganic lithium salt content will be higher, and the first efficiency will be lower.
The internal resistance is too large
The internal resistance directly affects the rate performance and power performance, and will also have a certain impact on the cycle. In terms of design, the internal resistance is also reduced as much as possible. If the internal resistance is large, without considering the process system material problem and only considering the design, the general reasons are as follows:
(1) The internal resistance of the selected current collector is large
(2) The compaction is too large, resulting in increased impedance
(3) Insufficient electrolyte
(4) The welding area is not enough, and the tab connection impedance is large
(5) The tab design position is unreasonable, resulting in uneven current distribution and high impedance
(6) The surface density design is unreasonable, resulting in excessive thickness of the pole piece and increased impedance
(7) The formation design is unreasonable
(8) The moisture control design is unreasonable
The battery cell is too thick
(1) The Group Yield (the ratio of the diameter of the battery cell to the diameter of the shell) design is unreasonable: The Group Yield design is too large, resulting in battery expansion exceeding the design value
(2) The compaction is unreasonable, resulting in excessive expansion
(3) The winding battery cell is deformed, resulting in excessive thickness
(4) The injection volume is too large, resulting in the battery cell being soft and deformed
(5) The expansion coefficient design is unreasonable, resulting in excessive actual expansion
Lithium deposition
Lithium deposition is a common aging failure phenomenon of lithium-ion batteries. The manifestation is mainly a layer of gray, grayish white or grayish blue material on the surface of the negative electrode. These materials are metallic lithium precipitated on the surface of the negative electrode. There are many reasons for lithium precipitation. From the design point of view, they are as follows:
(1) The CB value is too low, resulting in lithium precipitation
(2) Insufficient electrolyte leads to black spot lithium precipitation
(3) The compaction density is too large, resulting in insufficient infiltration and black spot lithium precipitation
Fast cycle decay
The capacity decay of lithium-ion batteries is mainly divided into
Reversible capacity decay: Reversible capacity decay can be restored by adjusting the battery charging and discharging system and improving the battery use environment.
Irreversible capacity decay: Irreversible capacity decay is an irreversible change in the battery that produces irreversible capacity loss.
The root cause of battery capacity decay failure lies in the failure of materials, and is closely related to objective factors such as battery manufacturing process and battery use environment. From the material point of view, the main causes of failure are structural failure of positive electrode materials, transitional growth of SEI on the negative electrode surface, decomposition and deterioration of electrolyte, corrosion of current collector, and trace impurities in the system. From a design perspective, common reasons for rapid capacity decay are as follows:
(1) Unreasonable CB design
(2) Excessive compaction density
(3) Excessive coating surface density
(4) Insufficient electrolyte
(5) Unreasonable overhang
(6) Insufficient current capacity of the tab leads to excessive temperature rise
