Sulfur-based all-solid-state batteries are expected to replace current lithium-ion batteries because of their superior safety performance. However, in the preparation process of all-solid-state battery slurry, there are incompatible polarities among solvent, binder and sulphide electrolyte, so there is no way to achieve large-scale production at present. At present, the research on all-solid-state battery is mainly carried out on the laboratory scale, and the volume of the battery is relatively small. The large-scale production of all-solid-state battery is still towards the existing production process, that is, the active substance is prepared into slurry and then coated and dried, which can have lower cost and higher efficiency.

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Difficulties faced

Therefore, it is difficult to find suitable polymer binder and solvent to support the liquid solution. Most sulfur-based solid electrolytes can be dissolved in polar solvents, such as the NMP we currently use. So the choice of solvent can only be biased to non-polar or relatively weak polarity of the solvent, which means that the choice of binder is also correspondingly narrow – most of the polar functional groups of the polymer can not be used!

This is not the worst problem. In terms of polarity, binders that are relatively compatible with solvents and sulfide electrolytes will lead to reduced bond between aggregates and active substances and electrolytes, which will undoubtedly lead to extreme electrode impedance and fast capacity decay, which is extremely detrimental to battery performance.

In order to meet the above requirements, the three main substances (binder, solvent, electrolyte) can be selected, only non-polar or weak polar solvents, such as para-(P) xylene, toluene, n-hexane, anisole, etc., using weak polar polymer binder, Such as butadiene rubber (BR), styrene butadiene rubber (SBR), SEBS, polyvinyl chloride (PVC), nitrile rubber (NBR), silicone rubber and ethyl cellulose, in order to meet the required performance.

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In situ polar – non-polar conversion scheme

In this paper, a new type of binder is introduced, which can change the polarity of electrode during machining by means of protection-de-protection chemistry. The polar functional groups of this binder are protected by non-polar tert-butyl functional groups, ensuring that the binder can be matched with the sulfide electrolyte (in this case LPSCl) during the preparation of the electrode paste. Then through the heat treatment, namely the drying process of the electrode, the tert-butyl functional group of the polymer binder can be thermal split, to achieve the purpose of protection, and finally get the polar binder. See Figure A.

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BR (butadiene rubber) was selected as polymer binder for sulfide all-solid-state battery by comparing the mechanical and electrochemical properties of the electrode. In addition to enhancing the mechanical and electrochemical properties of all-solid-state batteries, this research opens up a new approach to polymer binder design, which is a protection-de-protection-chemical approach to keep electrodes in the appropriate and desired state at different stages of electrode manufacturing.

Then, polytert-butylacrylate (TBA) and its block copolymer, polytert-butylacrylate – b-poly 1, 4-butadiene (TBA-B-BR), whose carboxylic acid functional groups are protected by thermolyzed T-butyl group, were selected in the experiment. In fact, TBA is the precursor of PAA, which is commonly used in current lithium ion batteries, but cannot be used in sulfide-based all-solid lithium batteries because of its polarity mismatch. The strong polarity of PAA can react violently with sulfide electrolytes, but with the protective carboxylic acid functional group of T-butyl, the polarity of PAA can be reduced, allowing it to dissolve in non-polar or weakly polar solvents. After heat treatment, the t-butyl ester group is decomposed to release isobutene, resulting in the formation of carboxylic acid, as shown in Figure B. The products of the two polymer deprotected are represented by (deprotected) TBA and (deprotected) TBA-B-BR.

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Finally, the paA-like binder can bond well with NCM, while the whole process takes place in situ. It is understood that this is the first time an in situ polarity conversion scheme has been used in an all-solid-state lithium battery.

As for the temperature of heat treatment, no obvious mass loss was observed at 120℃, while the corresponding mass of butyl group was lost after 15h at 160℃. This indicates that there is a certain temperature at which butyl can be removed (in actual production, this temperature time is too long, whether there is a more appropriate temperature or condition to improve the production efficiency needs further research and discussion). Ft-ir results of materials before and after deprotection also showed that solid electrolyte did not interfere with the deprotection process. The adhesive film was made with the adhesive before and after deprotection, and the result showed that the adhesive after deprotection had stronger adhesion with the fluid collector. In order to test the compatibility of the binder and electrolyte before and after deprotection, XRD and Raman analysis were carried out, and the results showed that the LPSCl solid electrolyte had good compatibility with the tested binder.

Next, make an all-solid-state battery and see how it performs. Using NCM711 74.5%/ LPSCL21.5% /SP2%/ binder 2%, the stripping strength of pole sheet shows that the stripping strength is the largest when binder tBA-B-BR is used (as shown in Figure 1). Meanwhile, the stripping time also has an impact on the stripping strength. The deprotected TBA electrode sheet is brittle and easy to fracture, so TBA-B-BR with good flexibility and high peel strength is selected as the main binder to test the battery performance.

Figure 1. Peel strength with different binders

The binder itself is ionic insulating. In order to study the effect of the addition of binder on ionic conductivity, two groups of experiments were conducted, one group containing 97.5% electrolyte +2.5% binder and the other group containing no binder. It was found that the ionic conductivity without binder was 4.8×10-3 SCM-1, and the conductivity with binder was also 10-3 order of magnitude. The electrochemical stability of TBA-B-BR was proved by CV test.

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Half battery and full battery performance

Many comparative tests show that the deprotected binder has better adhesion and has no effect on the migration of lithium ions. Using different binder made half cell to test the electrochemical properties, various experimental half cell respectively by mixed with binder the positive, no binder of the solid electrolyte and Li – In the electrode of single factor experiments, not mixed with binder In the solid electrolyte, to prove that the different influence on the anode binder. Its electrochemical performance results are shown in the figure below:

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In the figure above: a. is the half-cell cycle performance of different binders when the density of the positive surface is 8mg/cm2, and B is the half-cell cycle performance of different binders when the density of the positive surface is 16mg/cm2. It can be seen from the above results that (deprotected) TBA-B-BR has significantly better battery cycle performance than other binders, and the cycle diagram is compared with the peel strength diagram, which shows that the mechanical properties of the poles play an important role in the performance of cycle performance.

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The left figure shows the EIS of NCM711/ Li-IN half cell before the cycle, and the right figure shows the EIS of half cell without the cycle of 0.1c for 50 weeks. The EIS of half cell using (deprotected) TBA-B-BR and BR binder respectively. It can be concluded from the EIS diagram as follows:

1. No matter how many cycles, the electrolyte layer RSE of each battery is around 10 ω cm2, which represents the inherent volume resistance of electrolyte LPSCl 2. The charge transfer impedance (RCT) increased during the cycle, but the INCREASE of RCT using BR binder was significantly higher than that using tBA-B-BR binder. It can be seen that the bonding between active substances using BR binder was not very strong, and there was loosening in the cycle.

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SEM was used to observe the cross-section of pole slices in different states, and the results are shown in the figure above: a. Tba-b-br before circulation (deprotection); B. before circulation BR; C. TBA-B-BR after 25 weeks (deprotection); D. after 25 weeks BR;

Cycle before all electrodes can be observed closely contact between active particles, can only see small holes, but after 25 weeks cycle, can see the obvious change, used in c (take off) associates – b – the positive activity of the BR most particles or no cracks, and using the electrode activity of BR binder particles there are a lot of cracks in the middle, As shown in the yellow area of D, in addition, electrolyte and NCM particles are more seriously separated, which are important reasons for battery performance attenuation.

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Finally, the performance of the whole battery is verified. The positive electrode NCM711/ negative electrode graphite can reach 153mAh/g in the first cycle and maintain 85.5% after 45 cycles.

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A brief summary

In conclusion, in all-solid-state lithium batteries, solid contact between active substances, high mechanical properties and interface stability are the most important to obtain high electrochemical performance.