The industry is enthusiastic about the participation of sulfide solid-state batteries, stating that they have entered the "last mile" of commercialization.

However, reality seems to be more complex. A battery technology that is about to mature should have clearer breakthroughs and engineering signals in its upstream and downstream key materials, but why do we see a scene of path differentiation and uncertainty in the new generation of current collectors?

The "temperature difference" between theory and reality may reveal a profound technological revolution taking place in the seemingly traditional field of current collectors.

It is precisely in this context that the innovative keywords of current collectors have shifted from traditional "lightweight, high-strength, and stretchable" to more complex "3D/porous, high-temperature resistant, corrosion-resistant, and non-metallic".

Traditional metal foil is upgrading to cope with corrosion and expansion

The chemical erosion of copper by sulfide solid electrolytes, as well as the severe volume expansion of silicon-based and lithium metal negative electrodes during charge and discharge processes, constitute one of the many bottlenecks in the breakthrough of all solid state battery technology.

Surface modification based on existing mature processes has become the fastest response solution in the industry.

Recently, Nord Group announced the launch of its world's first high-temperature resistant double-sided nickel plated copper foil by 2025, which has attracted high market attention. According to its product information, this nickel plating layer with a thickness in the range of 0.5-0.9 μ m has a dense, flat, and pore free microstructure.

The nickel plating layer can effectively resist sulfide corrosion, and its dense physical properties are more helpful in solving the interface separation problem caused by the different thermal expansion coefficients of solid electrolytes and current collectors.

At the same time, it enables the copper foil to have an antioxidant capacity of over 30 hours at a high temperature of 150 ℃, and can still persist for more than 24 hours at the limit of 200 ℃.

This marks the deepening of the layout of leading companies in the field of solid-state batteries. Nord Corporation has been developing porous copper foils since 2018 and has continuously launched specialized copper foils for high-temperature and corrosion-resistant products in recent years, demonstrating its systematic layout of providing material support for various technological routes of solid-state batteries.

There is actually a practical consideration behind this. It is understood that using stainless steel or nickel as metal foil directly to cope with sulfide corrosion is too costly. Therefore, double-sided nickel plating on existing copper foils is considered a solution that combines performance and cost competitiveness.

However, the newly added electroplating process not only lengthens the manufacturing process and increases costs, but may also introduce new potential risks such as coating substrate adhesion.

While upgrading traditional metal current collectors, another technological route - composite current collectors, has also begun to intersect with solid-state battery technology due to its unique advantages in high safety and lightweight.

Jiemei Electronics' subsidiary, Rouzhen Technology, recently announced that it has signed a "Strategic Cooperation Framework Agreement" with a solid-state battery manufacturing enterprise. Both parties will jointly design and develop a "high safety and lightweight composite current collector", which will be produced and supplied by Soft Shock Technology according to each other's needs.

At the same time, Yinglian Corporation also stated that it will rely on its technological advantages in vapor deposition technology to develop "lithium metal/composite current collector negative electrode integrated materials" for solid-state batteries. This move aims to integrate the negative electrode and current collector into one, further improving the energy density and integration of the battery.

These two developments indicate that composite current collectors are serving as a relatively mature innovation platform, actively seeking integration points with next-generation battery technology.

Moving towards 3D structures to seek better solutions

A more profound transformation occurred in the three-dimensional structure of the current collector. The team led by Professor Cui Yi from Stanford University proposed in a paper published in Nature Energy in 2024 that porous current collectors are the next generation of development direction for composite current collectors.

At present, this exploration is diverging into two clear paths.

The first path is 3D metal mesh fluid with foam copper, foam nickel, foam aluminum as raw materials.

This path aims to provide buffer space for high expansion negative electrodes such as lithium metal and silicon-based by constructing a three-dimensional metal skeleton, and to reduce local current density due to their large specific surface area, thereby suppressing lithium dendrite growth.
The domestic industry has made rapid progress, and Defu Technology has developed multiple solutions such as porous copper foil, atomized copper foil, and core foil (i.e. composite current collector), and some products have achieved mass shipment.

According to its introduction, there are various preparation processes for porous copper foil, including laser drilling, chemical etching, foam forming, etc. Its research and development project aims to obtain copper foil with controllable pore morphology through innovative "printing template electrolysis".

The sense of smell of industrial capital is equally keen. Sanfu Xinke has teamed up with Dongfeng Group to jointly invest in Fio New Materials, which is mainly engaged in foam copper (that is, "3D copper foil").

Sanfu Xinke once pointed out that the core of foam copper manufacturing is the electroplating process, which can provide Fiore with an overall solution from electroplating equipment to special chemicals, and deeply empower in the follow-up foam copper process improvement and equipment upgrading. This indicates that the upstream and downstream of the industrial chain are accelerating the industrialization process of 3D collectors through the binding of capital and technology.

However, simple porous structures still face challenges. Research has found that homogeneous porous current collectors cannot effectively guide lithium metal to deposit towards the bottom, which can easily lead to the phenomenon of "top growth".

Therefore, designing 3D structures with a "lithium affinity gradient" is becoming a new research and development hotspot, aiming to fundamentally improve battery safety and cycle life.

The second path is the more cutting-edge porous carbon based current collector, pursuing "demetalization".

This path uses graphene, carbon nanotubes, and other materials as raw materials, aiming to completely eliminate metals while achieving high corrosion resistance and ultimate low mass density.

The Oak Ridge National Laboratory (ORNL) in the United States has launched a carbon fiber carbon nanotube polymer integrated current collector.

In this scheme, carbon fiber provides the mechanical framework, carbon nanotubes construct a three-dimensional conductive network, and polymers are responsible for bonding. Its unit area mass is only about 1.5 mg/cm ², far lower than the traditional copper foil's 8.7 mg/cm ², which can significantly improve the battery's specific energy and fundamentally avoid metal corrosion.

The research from Tsinghua University also points out that in the era of silicon-based negative electrodes, carbon nanotube current collectors should have multiple functions such as conductivity, reinforcement, and adhesion. The study also envisioned a future form of "integrated dry multi-stage electrode": combining carbon nanotube networks with silicon materials in situ, and then composite carbon based current collectors.

But new materials also come with new challenges, such as the welding process of carbon based current collectors, which is a difficult problem that must be overcome before its large-scale application.

Difficulties in process synergy: dry electrode calls for innovation in current collectors

As solid-state batteries move towards industrialization, the dry electrode process is highly anticipated by the industry due to its potential for solvent-free, high efficiency, and high energy density, but it also poses a challenge to traditional current collectors. Currently, the response from the material side seems to be lagging behind.

In the dry process, the electrode is composed of a pre prepared active material "dry membrane" and a current collector through hot pressing composite. The core bottleneck here lies in the interface integration between the two.

Due to relying solely on a small amount of adhesive on the surface of the dry film to achieve bonding, it can lead to low bonding strength and high interface resistance between the film and the current collector.

This not only causes excessive internal resistance of the battery, affecting its performance, but more seriously, there is a risk of the membrane peeling off from the current collector as a whole during subsequent high and low temperature cycling tests.

This means that the surface of the current collector used in the dry process can no longer be a simple metal foil, but must have the characteristic of firmly bonding with the dry powder film.

However, the current situation is that there has not been a large-scale emergence of innovative fluid collection products to meet this demand. On the contrary, equipment companies were the first to propose solutions from a process perspective.

For example, the recently highly anticipated equipment company Xinyu Ren's "dry powder direct coating thermal composite technology" is one of the observation windows.

This technology changes the two-step process of "film making first, then composite", and instead uses a specially designed die to evenly sprinkle the dry powder mixture directly onto the current collector pre coated with conductive adhesive, and then forms it in one go through a heated roller press.

In principle, this process improvement reduces the dependence on the bonding strength between active material particles and can better maintain the porosity of the electrode, especially suitable for manufacturing high-capacity thick electrodes that are conducive to fast charging.

But this precisely highlights a fact: one of the core components of the solution is a "pre coated conductive adhesive current collector", and ensuring uniformity and consistency is even more difficult.

Domestic research has clearly pointed out that there is currently more research on the "dry process membrane" itself, but relatively less research on the "dry process electrode" (i.e. the final product of membrane and current collector). This also reflects the reality of poor coordination between upstream and downstream links in the formation process of emerging industrial chains.

The progress of technology is calling for the common innovation of equipment and materials, but the innovation of exclusive current collector materials that can deeply solve the problem of dry bonding may still be a gap that urgently needs to be filled in the industry.

Facing multiple challenges, looking forward to a real breakthrough

Thus, we can see that the challenges faced by current collectors are multidimensional: there are material performance competitions from battery chemistry systems (dealing with corrosion and expansion), as well as process adaptation challenges from advanced manufacturing (compatible with dry electrodes).

By integrating these coexisting challenges with diverse solutions, we can provide a clearer answer to the question raised at the beginning of the article:

The so-called "last mile" temperature difference of solid-state batteries is due to the fact that their breakthrough is not a linear progress of a single technology, but the collaborative operation of the entire industry chain ecology on multiple battlefields, and the current collector is precisely a microcosm of this complex situation.

On the battlefield of materials, there are many paths from coating "armor" to composite "sandwich", and then to 3D "skeleton" and demetalization, but there is no absolute king that balances performance, cost, and reliability yet.

On the battlefield of technology, the introduction of dry manufacturing has exposed the lack of synchronization between material and equipment innovation.

This multidimensional uncertainty has led to a clear sense of fragmentation in industrial development. Just as composite current collectors have been introduced in previous years, but the progress of large-scale downstream applications is still slow, the "black box effect" between various links in the industry still exists.

The so-called 'impossible triangle' ultimately aims to reach a compromise on one of them at some point.

Before downstream battery companies can fully validate and ultimately make a decision on these new current collectors that cross materials and processes, this innovation competition around current collectors is likely to continue.