As GWh level orders begin to emerge, a commercial closed loop for solid-state batteries seems to be taking shape.

However, the current order delivery is not supported by a disruptive new production model, but mainly by the transformation and innovation of traditional wet coating equipment for lithium-ion batteries.

This batch of early but significant commercial orders is like a stress test, forcing the industry to confront a core contradiction: how to use the existing "wet" toolbox to manufacture a truly next-generation battery when the ultimate "dry" process is far from mature?

Replacing the liquid electrolyte and separator in traditional batteries with a solid electrolyte membrane is the core step towards solidification.

The key to this process is to minimize the electrolyte layer as much as possible to maximize energy density, and wet coating technology is currently the mainstream choice to achieve this goal. But this is not a simple material replacement, it fundamentally reshapes the fundamental physics of the process.

High solid content slurry triggers fundamental manufacturing transformation

To understand the difficulty of this transformation, it is necessary to first clarify that wet coating of solid-state batteries is far from simply replacing materials, it fundamentally changes the physical nature of the slurry system.

The ideal battery slurry needs to have rheological properties of "shear thinning": the viscosity is low enough during coating and pumping to ensure fluidity, and the structural strength can be maintained during static drying to prevent particle settling.

The solid content of traditional lithium battery paste is usually between 60% and 70%, and its control core lies in the uniform mixing of μ m level active materials and nanoscale conductive agents.

When solid electrolytes are introduced as the main solid component, the entire system transforms into a "rich solid phase". The shortening of particle spacing leads to a sharp increase in van der Waals forces and other interactions, which not only significantly increases the overall viscosity of the slurry, but also makes it easier to form hard agglomerates that are difficult to disperse.

These gatherings cause defects such as uneven coating thickness, streaks, pinholes, etc. during coating, ultimately damaging battery performance.

The deeper challenge lies in the evolution of the entire system from a relatively simple "bimodal particle" (μ m level active substance+nanoscale conductive agent) to a complex "multimodal particle" blend system.

Solid electrolytes, as the third main solid component, have unique particle size, density, and surface properties that elevate engineering challenges to achieving uniform blending of three or more solid particles with distinct physical and chemical properties in a crowded suspension system.

This leads to a new failure mode. For example, NMC positive electrode materials with higher density and solid electrolytes with relatively lower density will produce different settling rates under a gravitational field.

This means that even if the coating is uniform at the moment of application, vertical "layering" may occur inside the film during subsequent settling and drying processes. This' vertical non-uniformity 'can cause local ion conduction path blockage, increase interface impedance, and pose a direct threat to the long-term reliability of the battery.

Oxide Route: Path Differentiation between Performance Priority and Compatibility Priority

Furthermore, different solid-state electrolyte chemical systems pose vastly different challenges for wet coating.

For the oxide route, its electrolyte is essentially a hard and brittle ceramic material. When these μ m grade ceramic particles are made into high solid slurry and transported under high pressure, they act like "flowing sandpaper" and cause severe abrasion to the pipeline, especially the precision processed narrow slot coating die. This not only shortens the service life of expensive equipment, but the metal debris generated by its wear and tear may also mix into the slurry, posing a safety hazard.

In this context, there is a significant differentiation in the industrial path.

One is represented by QuantumScape from the United States, which prioritizes seeking fundamental breakthroughs in material performance.  

They faced the natural defect of high contact impedance between oxide particles and chose to add a high-temperature sintering process of up to 700 ° C or even 1000 ° C after wet coating. The goal is to melt ceramic particles into a dense continuous network through high temperature, fundamentally solving the problem of ion transport.

It is reported that QuantumScape's latest progress is a 25 fold increase in the efficiency of its heat treatment equipment. However, the contradiction of this choice lies in its pursuit of the ultimate material performance, which runs counter to the efficiency oriented roll to roll continuous production concept of modern battery manufacturing. High temperature will directly destroy the current collector, binder, and even the active material itself.

Secondly, represented by companies such as Penghui Energy, there is a greater emphasis on process compatibility and commercial efficiency.

Penghui Energy claims that its electrolyte wet coating process can bypass sintering, achieve high energy density of 280-300Wh/kg, and the cost is expected to be on par with traditional lithium batteries.

Behind this statement, another more compatible strategy is revealed. It is widely believed in the industry that this does not mean that pure thick ceramic membranes are made by wet process, but that the role of oxides is changed from the leading role to supporting role through the ingenious design of the material system.

For example, only a small amount of oxide powder (several wt%) is used as the "filler" or "interface modifier" to mix with the positive paste, or form a composite layer with the polymer, or use the sol gel gel and other methods to prepare the ultra-thin interface.

The essence of this method is to sacrifice the "purity" of a portion of the electrolyte in exchange for maximum compatibility with the existing large-scale production system, in order to seize the opportunity in terms of commercialization speed.

Sulfide route: a difficult balance between performance and process constraints

If the oxide route exhibits strategic divergence, then the sulfide route faces a series of interlocking and difficult to break free technical constraints.

Compared to oxides, sulfide systems were once considered to require more dry processes to avoid their chemically unstable characteristics.

However, the actual choices made by the industry present a completely different picture. According to the observation of Gaogong Lithium Battery at CIBF in 2025, wet coating has become the mainstream method for most enterprises to prepare sulfide solid electrolyte membranes.
The motivation behind this is clear and pragmatic.

Firstly, this is currently the most reliable path to thin the membrane to the 20 μ m level while ensuring large-scale production. The GAC team has confirmed that a high ion conductivity of 1.8mS/cm can be achieved under this process.

Secondly, the wet process introduces flexible polymers for sulfides, which actually provide the necessary mechanical strength for winding production of fragile electrolyte membranes, while effectively buffering stress during battery cycling, preventing particle cracking and detachment.

However, behind this advantage lies a profound internal contradiction. The chemical fragility of sulfide materials to polar solvents such as NMP is the core challenge of the entire technology chain. It will cause material decomposition and release highly toxic hydrogen sulfide gas. This brings two major challenges:

Firstly, the selection range of solvents is strictly limited: only non-polar or weakly polar solvents can be used, and the selection range is extremely narrow. Moreover, these solvents themselves often have problems such as toxicity, strong volatility, and difficult recovery.

The second is that the adhesive faces an inherent contradiction between solubility and adhesion: the limitation of the solvent further transmits to the adhesive. A research and development team from a university pointed out that rubber based adhesives that are soluble in non-polar solvents often have insufficient bonding strength; However, commercial adhesives with strong adhesion cannot effectively dissolve in non-polar solvents due to the presence of polar groups.

The difficulty in balancing solubility and adhesion of binders has become one of the core bottlenecks in the current sulfide wet process.

To break through this series of constraints, the industrial chain is struggling to explore. In terms of coating methods, low-cost scraper coating is the preferred choice for various pilot lines, but its accuracy and uniformity defects make it difficult to achieve mass production.

Therefore, slit coating with high precision and system closure is considered an inevitable choice for achieving large-scale safety production due to its ability to effectively isolate sulfides from the external environment.

Response and systemic challenges of equipment manufacturers

The manufacturing challenges of battery technology are rapidly transforming into business opportunities for upstream equipment manufacturers. Faced with the demands of downstream customers, equipment companies are actively deploying and synchronously launching the commercialization process.

The high-temperature coating system recently released by Ernst provides a highly targeted solution.

It directly targets the pain points of high solid content slurry such as high viscosity, poor flowability, and difficulty in controlling thermal stability and process accuracy. By heating the coating process to a temperature range of 40 ℃ -65 ℃ (temperature accuracy controlled within ± 1 ℃, CPK ≥ 1.67), the flowability of the slurry and coating uniformity are significantly improved.

This innovation is not only a "remedial lesson" for solid-state batteries, but also a key process support for liquid batteries based on high nickel materials to break through the bottleneck of specific energy and improve energy density for the entire industry.

In addition, Manest also demonstrated a "dry wet mixed solid-state battery electrode manufacturing solution" covering the front and rear sections, and used wet slit coating technology combined with sand mill treatment to ensure the ultimate uniformity of the coating when finally applying a thin layer of solid electrolyte.

The layout of other leading device manufacturers also confirms this trend. Pioneer Intelligence has adopted a more robust approach to parallel research and development of dry and wet solid-state coating systems.

Its wet process, through the use of a special coating structure, has been able to achieve high-speed and wide production of solid-state electrodes, and can meet the mass production needs of thicknesses ranging from 10 μ m to 60 μ m, demonstrating its deep technical reserves in precision control. At present, multiple sets of solid-state battery core devices have also been shipped.

The landing of commercial orders is the most direct signal of market maturity. Yinghe Technology has successfully delivered a batch of solid-state wet coating equipment to a leading domestic battery enterprise.

At the same time, the company is also laying out composite transfer printing and continuous composite equipment, aiming to solve the production line level problem of efficient composite between electrodes and electrolyte membranes.

Putailai has also revealed that its new equipment related to solid-state batteries, including wet coating machines, has obtained orders and partially delivered them.

However, having advanced equipment only partially solves the problem. The deeper challenge may lie in the deep coupling between materials, processes, and the final product form, which has given rise to new process difficulties.

A research and development team from a university has pointed out that the electrolyte membrane prepared by wet coating fabric is still difficult to balance high ionic conductivity and high flexibility.

Behind this is still the problem of narrow screening range for adaptive adhesives, as well as difficulty in regulating their molecular weight and structure. The choice of binder and its micro nano scale distribution in the membrane directly determine the performance of the final electrolyte membrane. This indicates that even wet routes that appear compatible with existing equipment require a high degree of resonance between materials and processes.

Conclusion

Looking at the current situation, the industrialization of solid-state batteries is not an ideal path of linear evolution, but a dynamic process of constantly seeking local optimal solutions under multiple contradictions and constraints.

From the microstructure management of slurries, to the wear of hard particles, to the deep binding of chemical systems and process equipment, the industrialization challenges of solid-state batteries are expanding from the traditional electrochemical field to the fields of mechanical engineering, tribology, powder metallurgy, and precision process control on a large scale.

This means that companies that can succeed in the field of solid-state battery manufacturing in the future must possess strong interdisciplinary integration capabilities.

Wet coating, as a "compromise" technology inherited from traditional liquid battery production lines, is currently the most realistic and economical path to promote the large-scale production of solid-state batteries.

In the long run, the environmental and cost pressures brought about by the extensive use of solvents will always exist. Therefore, while the wet process continues to iterate and optimize, the continuous evolution of the dry process remains an unshakable ultimate goal in the industry.