Preparation and assembly testing methods for sulfide solid-state batteries

In recent years, the rapid development of sulfide solid electrolytes, including Li2S-SiS2, Li2S-B2S3, Li2S-P2S5, Li (10 ± 1) MP2S12 (M=Ge, Si, Sn, Al or P), and Li6PS5X (X=Cl, Br, I), particularly Li10GeP2S12 (LGPS), which exhibits extremely high room temperature lithium ion conductivity exceeding that of liquid electrolytes, has partially solved the problem of insufficient intrinsic conductivity in solid electrolytes.

Figure 1 (a) shows an all solid state lithium battery using Li10Ge2PS12 ceramic solid electrolyte powder with a room temperature conductivity exceeding 5mS/cm, LiCoO2 cathode material, 99% · (30Li2S · 70P2S5) · 1% P2O5 electrolyte as the negative side modification electrolyte, and metallic lithium as the negative electrode. It can discharge normally at room temperature and light up LED lights. The schematic diagram of its core component structure is shown in Figure 1 (b), from which it can be seen that the positive electrode layer, inorganic solid electrolyte layer, and lithium foil are tightly pressed in the mold. Below is a detailed introduction to the preparation methods and processes of its various components.

Figure 1 Sulfide Solid Electrolyte Based All Solid State Lithium Battery

1. Preparation method of positive electrode

The Young's modulus of sulfide electrolyte powder is around 20GPa, with high adhesion and compressibility, making it prone to plastic deformation. After cold pressing, the grain boundary impedance is small. Therefore, when preparing the positive electrode layer, it is suitable for direct dry mixing with the positive electrode powder [Figure 2 (a)]. When dry mixing, conductive agents, sulfide electrolytes, and positive electrode materials are added to the mortar simultaneously, and then manually ground or mechanically mixed in a mixer. It should be noted that the compatibility between different positive electrode materials and electrolytes, different conductive agents, and the applicability of different positive electrode coatings need to be considered under actual conditions.

Figure 2 Preparation method of positive electrode for sulfide solid electrolyte based all solid state lithium batteries

When preparing sulfide batteries in large quantities, the wet coating process [Figure 2 (b)] may be more suitable for scaling up. This is because in order to provide the mechanical properties required for high-throughput roll to roll processes, polymer adhesives and solvents are needed to make thin film electrolyte layers and electrode layers. In addition, the presence of flexible polymers in electrolytes/electrodes can effectively buffer the stress and strain generated by repeated charging and discharging cycles, and alleviate problems such as crack formation and particle detachment.

However, the following issues need to be noted during the preparation process: ① The polymer binder should be dissolved in non-polar or less polar solvents (such as xylene) with negligible reactivity with sulfides; ② Polymer adhesives with strong bonding ability should be used, otherwise excessive polymer will have adverse effects on conductivity, electrolyte/electrode thermal stability; ③ Polymer adhesives need to have high flexibility. Although polymers such as polystyrene (PS) and polymethyl methacrylate (PMMA) can dissolve in xylene, they are extremely hard after solvent drying, causing electrolyte/electrode crushing. Therefore, most works choose nitrile rubber (NBR) and styrene butadiene rubber. However, the problem with rubber is that it cannot generate ion conductivity internally, which leads to a significant decrease in the electrochemical performance of batteries even with only a small amount of nitrile rubber. Therefore, the use of polymers with high ionic conductivity, high thermal stability, solubility in non-polar or less polar solvents, and insoluble polysulfides is the direction for the future development of wet coating of sulfide electrolytes.

However, the wet pulping process mentioned above will use a large amount of solvents, which will inevitably result in some small molecules of solvents remaining in the mixture, leading to side reactions and a decrease in electrolyte conductivity and severe degradation of battery life; The degree of encapsulation of active materials by polymer binders in solution is difficult to control, which can easily lead to charge transfer failure; The volatilization of solvents leads to lower density of electrode plates, which is not conducive to the dynamic process of the battery; In addition, the emission and recovery of solvents after scaling up are also unavoidable issues.

Therefore, the use of PTFE dry coating technology [Figure 2 (c)] has become another option. It mainly includes three steps: ① dry mix electrolyte, electrode, and PTFE ball milling; ② Roll the powder into a thin film; ③ Roll and shape the thin film with the current collector. Due to the extremely low intermolecular interaction force between fluorine and carbon chains in PTFE, the flexibility of the molecular chains is good. High molecular weight PTFE fine powder particles will undergo fibrosis under directional force, that is, the particles inside the particles will be arranged in a certain direction under shear force to form fibrous and mesh structures. Therefore, a large number of active materials, electrolytes, and conductive carbon can be tightly but not completely covered together.

Preparation method of 2 negative electrodes

The ternary sulfide electrolyte with thio LISICON structure has a high conductivity, but according to experimental and computational reports, the spontaneous and gradually extending interface reactions between metallic lithium and LGPS, Li10Sn2PS12, etc. will produce some low ion conductivity such as Li2S, Li3P, and high electron conductivity such as Li15Ge4 interface phases, leading to an increase in Li/LGPS interface impedance and short circuits in all solid state lithium batteries, seriously restricting the development of its high energy density all solid state lithium batteries. There are currently three main solutions to improve the chemical/electrochemical stability of lithium metal in sulfide electrolytes, especially ternary sulfides containing germanium, tin, zinc, etc.

(1) Treat the surface of lithium metal to generate a surface ion conductivity modification layer in situ to protect the sulfide electrolyte. As shown in Figure 3 (a), Zhang et al. increased the contact area between the modification layer and metallic lithium by controlling the LiH2PO4 protective layer formed by the reaction between Li and pure H3PO4, avoiding direct contact between metallic lithium and LGPS, preventing the infiltration of mixed ion electron conductivity into the interior of LGPS, and improving the problem of slow lithium ion dynamics at the interface. The results showed that the modification of LiH2PO4 significantly improved the lithium stability of LGPS. LCO/LGPS/LiH2PO4 Li all solid state lithium batteries can provide ultra long cycle life and high capacity, that is, at 25 ℃ and 0.1C rate, their reversible discharge capacity for the 500th cycle is maintained at 113.7 mA · h/g, with a retention rate of 86.7%. Li/Li symmetric batteries can stably cycle for more than 950 hours at a current density of 0.1mA/cm2.

Figure 3: Negative electrode modification method for sulfide solid electrolyte based all solid state lithium batteries

(2) Using a transition layer sulfide electrolyte that is stable for metallic lithium to protect the other layer. As shown in Figure 3 (b), Yao et al. proposed an LGPS/LPOS double-layer electrolyte structure to improve ion conductivity and stability at the LGPS/Li interface, and achieved good results in various battery systems. However, thicker double-layer electrolytes may reduce the overall mass and energy density of the battery. The assembly method is to first cold press a layer of electrolyte, then cold press another layer of electrolyte on its surface, and then superimpose the positive and negative electrodes to apply pressure together.

(3) Generate a modified layer in situ on the surface of the electrolyte (electrolyte/electrode interface). As shown in Figure 3 (c), Gao et al. added 1 mol/LLiTFSI DOL-DME electrolyte droplets to the LGPS/Li interface to generate organic inorganic mixed lithium salts such as LiO - (CH2O) n-Li, LiF, - NSO2 Li, Li2O, etc. The Li/LGPS/Li symmetric battery was stably cycled at 0.1mA/cm2 for 3000 hours. Chien et al. used solid-state nuclear magnetic imaging to study and found that there was a significant loss of interface Li after cycling in Li/LGPS/Li symmetric batteries. However, coating PEO-LiTFSI can improve the shortcomings of interface Li and its uneven deposition. The above methods have improved the compatibility between sulfide electrolytes and metal lithium negative electrodes to a certain extent, but at the same time, there may also be issues such as unclear principles of drip adding electrolytes and reduced thermal stability of electrolytes due to the addition of polymers.

Figure 4 Assembly method for sulfide solid electrolyte based all solid state lithium batteries

2.3 Assembly method of sulfide solid electrolyte based all solid state lithium batteries

In terms of assembly of sulfide solid electrolyte based all solid lithium batteries, as shown in Figure 4, the main steps are as follows: ① Electrolyte pressure molding, generally with a pressure of 120-150MPa; ② The positive electrode is pressurized and formed, and a steel plate is attached as the current collector, with a general pressure of 120-150MPa; ③ Negative electrode pressure forming, for metallic lithium, the general pressure is 120-150 MPa, and for graphite, the general pressure is 250-350 MPa, with steel sheets attached as current collectors; ④ Tighten the battery bolts. It is important to note that the reading on the oil compressor gauge should be converted based on the actual shape of the battery mold, and to prevent battery short circuits during assembly.