In the process of making positive and negative electrodes of lithium-ion batteries, some battery additives are often added to improve certain properties of the electrodes. The discharge capacity and some electrochemical properties of the electrodes are not only related to the properties of the active materials in the electrodes, but also closely related to the amount of these additives.
Among them, the battery binder is an important additive in the electrodes of lithium-ion batteries. Since lithium-ion batteries use organic electrolytes with low conductivity, the electrode area is required to be large. In addition, the battery assembly must go through processes such as rolling, slitting, and winding (processing the electrode sheet structure) before it can enter the battery case. This requires that the bonding between the active materials in the electrodes and between the active materials and the current collector must be quite strong, and there must be no active material powder or sheet falling off.
This requires the binder to have the following properties:
(1) Maintain thermal stability at 130-180°C when dry;
(2) Have good bonding effect on active particles;
(3) Have good chemical stability;
(4) Not easy to burn;
(5) Can be wetted by organic electrolytes.
In the past, nickel-cadmium and nickel-metal hydride batteries used aqueous solution systems, and the binder can use water-soluble polymer materials such as PVA, CMC or PTFE water-dispersed emulsions. The electrolyte of lithium-ion batteries is a highly polar carbonate organic solvent system, so the binder must be able to meet the above requirements, especially chemical stability, and not be reduced under the negative potential of lithium in the negative electrode, and not be oxidized when the positive electrode is overcharged or other oxygen is generated. During the charging and discharging process of lithium-ion batteries, as lithium ions are embedded and removed, the active materials will expand and shrink, and the binder is required to play a buffering role in this regard.
Among various fluorine-containing binders, PVDF is widely used in lithium-ion batteries due to the following characteristics:
- The larger the molecular weight of PVDF, the stronger the adhesion. If the molecular weight increases to 50,000, the adhesion doubles;
- The thermal stability value of PVDF is as high as 375-400℃, the heat treatment shrinkage is small (3%), the melting temperature is 170℃, and the softening temperature is 166℃;
- The swelling saturation in DMC is 7;
- It does not react with the positive and negative electrodes and has good chemical stability.
In practical applications, it must be emphasized that the viscosity of the binder varies with the density of the active material. When the density of the active material is high, the viscosity of the binder must be increased to prevent the active material from precipitating and causing uneven slurry, otherwise it will be diluted.
In addition, PVDF interacts with NMP to form an adhesive that has strong adhesion to the coating, but the preparation skills must also be paid attention to. During preparation, it must be heated and stirred, and the heating temperature should be controlled at 50-70℃. It should be used for coating immediately after preparation and should not be stored for a long time. Similarly, the coated electrode should not be stored for too long before assembly.
When the proportion of PVDF binder in the electrode is too small, the electrode active material, the conductive agent and the current collector are not in close contact, and the active material has a detachment phenomenon, resulting in low utilization of the active material and low discharge capacity;
When the proportion of PVDF binder in the electrode is large, on the one hand, the amount of active material is reduced while the total amount remains unchanged, and on the other hand, the binder wrapped around the active material also greatly reduces the effective area of the active material, the utilization of the active material in the electrode is low, and the discharge capacity is inevitably reduced.
Therefore, the amount of PVDF binder added should be neither too much nor too little, and the proportion should be appropriate.
