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How Binder Affects Decline of Lithium Battery?
- Jun 20, 2018 -

The research on the decay mechanism of lithium-ion batteries is mostly focused on positive and negative materials. For example, many studies have shown that the loss of active materials, increase in internal resistance and other factors are the main factors causing the decline of lithium-ion batteries, and for the binder in lithium. The role played by the decay of ion batteries is still relatively small. In fact, although the proportion of the binder in the lithium-ion battery is very small (usually less than 5% of the active material), the binder plays a crucial role. In lithium ion batteries, the role of the adhesive is to bind the active material particles and the conductive agent particles together to form a stable system. However, in the process of charge and discharge, due to the presence of a certain volume change in the positive and negative electrodes, this stable structure will be destroyed, for example, the most common is the situation shown in the figure below, adhesives / conductive agents and active material particles Stratification occurred between the two, resulting in the loss of active materials, causing a decrease in the reversible capacity of lithium-ion batteries.

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In order to analyze the role of adhesives in the decay process of lithium-ion batteries, JM Foster of University of Portsmouth, UK, established a model to study the effects of particle shape and cyclic ratio on the adhesive properties of adhesives. Studies have shown that oval-shaped particles will significantly increase the strain on the upper and lower parts of the particles that the adhesive absorbs when the electrolyte expands. The large charge-discharge rate (more than 1C) will also significantly cause the adhesive on the left and right sides of the active material particles. Increased strain affects the cycling performance of the battery.

JM Foster's model mainly consists of three hypotheses: 1) The electrode is filled with electrolyte by spherical active substance particles and elastic porous binders, and the binder pores; 2) The active substance particles will participate in lithium insertion and delithiation. The volume expansion occurs; 3) The adhesive undergoes liquid-swelling upon contact with the electrolyte.

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 Based on the above assumptions, J.M. Foster used mathematical methods to model the motor. In the actual electrode, there are tens of millions of active material particles and a large amount of binder. It is obviously unrealistic to directly solve the entire electrode. Therefore, JM Foster adopts a simplified method. JM Foster thinks that in addition to the electrode edge Position, the internal force of the electrode is very uniform, so we can simplify the solution of the entire electrode to solve the single active material particles and the adhesive around it, so that the model's solution process is greatly simplified.

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The following figure a shows the stress distribution around the active material particles after the electrolyte is expanded by the binder. The following figure c shows the bonding between the P and E points of the active material particles after absorbing the electrolyte. From the figure, we can see that the strain at the point P near the surface of the electrode and the current collector increases as the absorption solution of the adhesive expands, and the strain at point E on the left and right sides of the particle increases. Falling, due to the fluidity of the adhesive, the adhesive pushes the adhesive from both the top and bottom of the active material particles to both sides of the active material under the effect of strain.

The figure b below shows the strain distribution of the adhesive around the active material particles during volume change. It can be noted from the figure that the adhesive stress distribution caused by the volume change of the active material is almost uniform, but careful study still found The binder strain on the left and right sides of the active material is still higher than the strain on the adhesive on the upper and lower ends of the active material, which indicates that the binder on the left and right sides of the active material particles is more likely to delaminate during the cycling process. However, in practice, we need to note that since the volume change of positive active material during cycling is very small (2-4% for NMC), the change in binder strain caused by the volume expansion of active material particles is actually much smaller than that due to PVDF. The volume expansion caused by the adhesive aspiration.

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The previous analysis was for spherical particles, but in practice we used particles with many other shapes, so J.M. Foster analyzed the influence of different particle shapes on the strain of the adhesive. The figure below shows the effect of different particle shapes on the strain distribution of the adhesive after pipetting. From the calculation results, the adhesive strain at the P-point of the elliptical particles is positive, while that at the E point. The knot strain is negative, which is consistent with the previous analysis. At the same time, it can be seen from the figure below that the arrangement direction of the elliptical particles also affects the strain of the adhesive. When the long side of the ellipse is parallel to the surface of the electrode, the strain of the adhesive is significantly increased.

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The figure below shows the strain of the adhesive at different charge rates (Fig. a is the strain of the adhesive in the positive electrode and Fig. b is the strain of the adhesive in the negative electrode). The slowest charging rate used in the calculation requires 3100h charging is completed, and the fastest charging rate requires only 0.031h to complete the charging. It can be seen from the figure that the high charging rate significantly increases the strain of the adhesive at the position of the E point of the active material particle, resulting in the adhesive and activity. Particle delamination problem. In general, the rapid charging of more than 1C rate will cause damage to the positive and negative adhesives, thereby affecting the life of lithium-ion batteries.

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JM Foster's work allows us to have a clear understanding of the distribution of the adhesive around the active material particles on the microscopic level, and the factors that affect the strain distribution of the adhesive—active material particle shape and charge/discharge rate. Conducted in-depth discussion, for electrode material design and lithium-ion battery formulation design has a certain guiding significance.