Battery Technology & Materials | Innovation and Development (Lithium Manganese Iron Phosphate, Silicon Carbon Anode, LiFSI, Dry Electrode)

PART-0 Preface – The Evolution of the Energy Revolution

In fact, the innovation and development of human civilization has hardly made any significant progress in the past tens of thousands of years. The main changes came after the Industrial Revolution, and one of the most critical factors was the “change in the way energy is used.”

It can be said that the revolution of the energy system is the most fundamental source of development for human development to date. So far, there have been three major systemic revolutions:

(1) Agricultural civilization – biomass energy: some of the energy given by the sun at that time was attached to plants, trees, animals and other entities. In fact, the energy materials on the earth are all solar energy when they return to the source. In the agricultural civilization period, humans only used energy by using grass and trees to make fire and used animals to cultivate crops to achieve agricultural production. These biomass energies were essentially just energy storage converted by the sun in a short period of time. This was the starting point for humans to truly begin to use energy.

(2) Industrial Revolution – Fossil Energy: During the industrial civilization period, humans began to rely on extracting huge energy reserves (fossil energy such as coal, oil, and natural gas) and converting them into other forms of energy for use. From coal and steam engines in the 18th century, to oil and internal combustion engines in the 19th century, to electricity (secondary energy) in the 20th century, it gradually became a new way of energy utilization.

(3) Information Revolution – New Energy: Electricity, as a secondary energy source, is easy to produce, transport and use, and can also be easily converted into other forms of energy such as heat, mechanical energy, light energy, and sound energy. It is precisely because of the emergence and popularization of electricity that the infrastructure of the information revolution has been consolidated. The goal of the new wave of energy revolution is to achieve the transformation of electricity sources from high-carbon to low-carbon, while meeting the growing energy demand and ensuring energy supply, and solving the problems facing mankind such as environmental pollution and climate change through time mismatch.

Therefore, the new energy revolution essentially revolves around how to convert solar energy into “electricity” in a low-carbon manner and store and utilize it efficiently. This has led to the rapid development of wind, light, electricity and lithium-ion batteries as energy storage carriers.

This article mainly revolves around the iteration and development of the new generation of battery technology and materials. There are actually two main lines of innovation in the battery field:

(1) On the industrial side: one is to improve energy density (fast charging is actually also a solution); the other is to continuously reduce costs by achieving large-scale industrialization;

(2) On the consumer side: it is necessary to consider improving safety and higher cycle life at the same time to meet people’s better consumer experience.

The following types of innovations in materials and technologies are essentially centered around the above key points.

PART-1 Cathode | Lithium manganese iron phosphate (LFP)

❶ First principles: properties and defects of the material itself

The cathode materials of lithium-ion batteries mainly include

• Spinel structure materials(LiMn₂O₄);
• Layered structural materials (LiCoO₂、LiNiO₂、LiNi_1-x-yMn_xCo_yO₂);
• Olivine structural material (LiFePO₄、LiMnPO₄);
• Other transition metal oxides and phosphate series materials.

Since 2022, lithium iron phosphate batteries have rapidly increased their market share due to their low cost, excellent safety performance and long cycle life. In 2023, China’s cumulative installed capacity of lithium iron phosphate batteries reached 261.0GWh, a year-on-year increase of 42.1%, accounting for 67.3% of the total installed capacity, and once again became the mainstream of the market.

However, the actual discharge capacity of lithium iron phosphate is only 160-170mAh/g, which is lower than the capacity of the negative electrode material, and the energy density is close to the theoretical limit.

How to effectively improve energy density?

Introducing Mn elements on the basis of lithium iron phosphate can increase the voltage platform (from 3.4V to 4.1V), thereby increasing the energy density of the battery cell (up to 21% is expected). At the same time, since the radius of iron and manganese ions is similar, the two can be mixed at the atomic level to obtain lithium manganese iron phosphate that combines the advantages of both.

In other words, lithium manganese iron phosphate (LMFP) inherits the advantages of lithium iron phosphate such as high temperature, high safety, and high cycle times, and has higher energy density and better low temperature performance.

Therefore, lithium manganese iron phosphate has become an important direction for the development of cathode materials technology.

Project	Lithium iron phosphate (LFP)	Nickel cobalt manganese oxide (NCM)	Lithium manganese iron phosphate (LMFP)
Crystal structure	Olivine structure	Layered	Olivine structure
Theoretical specific capacity (mAh/g)	170	273-285	170
Actual specific capacity (mAh/g)	130-140	155-220	130-140
Tapped density (g/cm³)	0.8-1.1	2.6-2.8	1.19
Pressed density (g/cm³)	2.20-2.60	3.40-3.80	2.3-2.5
Cycle life (times)	2,000-6,000	800-2,000	2000
Voltage range (v)	3.2-3.7	2.8-4.5	4.1
Thermal stability	Stable	Generally worsens with higher Ni content	Stable
Material cost	Low	Medium	Low
Advantages	Low cost, good safety, long cycle life	High energy density, relatively low cost	Low cost, high energy density, good low temperature performance
Disadvantages	Low energy density, poor low temperature performance	High temperature prone to bloating, poor circulation and safety	Low conductivity, low lithium ion diffusion coefficient

However, there are also obvious drawbacks that limit the further industrialization of lithium manganese iron phosphate. For example, extremely low conductivity(<10^-10 s/cm), Extremely small lithium ion diffusion coefficient(<10^-16 cm²/s), poor rate performance, poor stability, and The Jahn-Teller effect generated by Mn3+ during the charge and discharge process has a great influence on the structural stability and cycle performance of LMFP.

(Note: Jahn-Teller effect: During the charge and discharge process, manganese ions dissolve and deposit on the negative electrode surface, destroying the SEI membrane, causing the SEI membrane to continuously regenerate and repair, thereby consuming a large amount of active lithium and causing capacity loss; manganese dissolution reacts with the electrolyte, affecting material stability and capacity retention.)

❷ Key point: Material modification improves defects and enhances performance

All actions are aimed at improving electrical conductivity and lithium ion diffusion coefficient.

A key point to improve the performance of LMFP materials lies in the ratio of Mn to Fe.

Structurally, since the radius of Mn²⁺ and Fe²⁺ is similar, they can form a solid solution with any manganese/iron ratio. The principle of manganese-iron ratio adjustment: Since Mn has a higher voltage platform and Fe has better conductivity, different manganese-iron ratios will directly lead to differences in the performance of LMFP. When the manganese-iron ratio is high, Mn drives a significant increase in battery voltage and energy density, but excessive manganese content will destroy the solid solution structure due to the Jahn-Teller effect, resulting in the dissolution of active materials and rapid decay of cycle performance; when the manganese-iron ratio is too low, the voltage increase effect is limited and the energy density advantage over LFP is not obvious, which is meaningless.

Therefore, in order to give full play to the performance advantages of LMFP, the Mn content in the material is generally not less than 50%, but how to increase the Mn content without affecting its final performance is a key issue that the industry needs to solve! At present, in order to balance the contradiction between material cost and battery energy density, after theoretical calculation and practical demonstration, the Mn/Fe ratio is mainly concentrated between 6/4 and 8/2.

In addition to the synthesis process, modification technology is the key to improving LMFP defects. The main purpose is to improve the performance of conducting electrons/ions. At present, the mainstream modification technologies include: nano-sizing, carbon coating, ion doping, etc.

Nano-sizing: Reducing the particle size of the material to the nanoscale can, on the one hand, shorten the migration path of lithium ions, thereby improving the migration efficiency; on the other hand, it increases the specific surface area, allowing the material to contact the electrolyte more fully, reducing the interface impedance, and thus improving the material’s charge and discharge capacity and rate performance. Nano-sizing is the most basic and effective way to improve lithium manganese iron phosphate, but it also has certain negative effects. As the particle size of the material decreases, serious particle agglomeration will occur during the production and pulping process, affecting the uniformity of the material, reducing the compaction density of the pole piece, and resulting in a decrease in the volume energy density of the material. In addition, particles with large specific surface area will increase the contact area between the material and the electrolyte, increase the impact of the Jahn-Teller distortion of Mn³⁺, and ultimately have an adverse effect on battery performance, so it is often used in combination with other modification methods.

Carbon coating: Carbon coating is achieved by uniformly coating the carbon coating on the surface of the material (layered coating). The graphitized structure of the carbon material is conducive to the establishment of a three-dimensional fast conductive network, providing an effective channel for the diffusion of Li+ ions, and inhibiting the growth and agglomeration of crystal particles, thereby improving the external conductivity of the material. In addition, carbon coating can also inhibit the dissolution of manganese ions and reduce the impact of the Jan-Taylor effect on battery capacity and cycle stability. Commonly used carbon sources are sucrose, glucose, citric acid, graphene oxide (GO), reduced graphene oxide (rGO), etc. At the same time, doping atoms such as S, N, and P in the carbon coating is also a means to further improve the performance of LMFP. However, excessive carbon coating will reduce the tap density and compaction density of the material, resulting in a decrease in the volume energy density of the material.

Ion doping: Carbon coating can only improve the external conductivity of the material, and the internal conductivity of the material can be improved by doping ions in the material. A small amount of ion doping will not change the olive-type crystal structure of LiMnFePO₄, but can also cause defects or electron holes in the original lattice, promote the expansion of Li+ diffusion channels, increase the carrier density of the material, and thus improve the conductivity of the material itself. It is considered to be one of the most effective and direct methods to improve conductivity; at the same time, some ions can also inhibit the dissolution of manganese ions, inhibit the Jahn-Teller effect, and improve the capacity and cycle stability of the material. At present, Mg²⁺、Ni⁴⁺、Co²⁺、Nb⁵⁺, etc. are introduced into LMFP materials, and combined with carbon coating, LMFP is double-modified to improve the performance of the material. Among them, the method of doping Mg²⁺ is the most widely used and studied. However, if the doping is excessive, severe lattice distortion will occur, blocking the transmission channel of Li+, resulting in a decrease in the conductivity of the material. How to select doping ions and determine the doping amount is the key issue that needs to be determined in this modification method.

All modification strategies have their own advantages and disadvantages. The ultimate realization must be to integrate these modification methods, explore a set of effective combination strategies and implement them.

❸ Industrialization: Combined use with ternary materials

There are two major application directions of lithium manganese iron phosphate: one is to directly replace lithium iron phosphate; the other is to use it in combination with ternary materials.

(1) Reasons for replacing lithium iron phosphate

The production process of lithium iron phosphate is similar to that of lithium iron phosphate, and the production equipment is compatible, so the overall switching cost is relatively low.

If lithium iron phosphate is to be prepared by tangent, it is only necessary to treat the equipment to eliminate the residual manganese element; in terms of production process, a new grinding process of manganese source is required, and the sintering temperature and process are slightly different, and the crushing strength is increased.

In terms of performance and cost, the theoretical energy density of lithium iron phosphate can be increased by 15%-20% compared with LFP, but the price is only 5%-6% higher, which is more cost-effective. This is the opportunity point for replacing lithium iron phosphate.

At present, the only constraint on large-scale commercialization is the technology and process itself. There is no stable and consistent evaluation of mass-produced products, such as low first efficiency, insufficient circulation, weak stability, etc.

(2) Combined use with ternary materials

Compared with replacing LFP, the combined use with ternary materials is more promising.

By combining the poorly conductive lithium manganese iron phosphate material with the excellently conductive ternary material, the battery equipped with this composite positive electrode material can have both the high energy density and high power characteristics of the ternary material and the high safety and low cost advantages of lithium manganese iron phosphate.

In December 2021, CATL spent 413 million yuan to acquire 60% of Litai Lithium Energy’s shares and entered the lithium manganese iron phosphate track. Litai currently has an annual production capacity of 2,000 tons of lithium manganese iron phosphate, and plans to build a new annual production capacity of 3,000 tons of lithium manganese iron phosphate.

In 2022, CATL announced the launch of its new battery product – M3P battery. M3P battery is a battery developed by CATL based on a new material system. Its energy density is higher than that of lithium iron phosphate (energy density is 210Wh/kg) and its cost is better than that of ternary battery – directly hitting the two long-term pain points of LFP and ternary battery.

However, as to whether it is LMFP, CATL did not admit it, but called it “ternary phosphate system”, which is mainly generated by doping metal elements such as magnesium, zinc, aluminum, and manganese, and replacing certain iron elements at certain sites. And from the lithium manganese iron phosphate technology patent published by CATL, it can be seen that metal elements such as manganese, magnesium, and aluminum are also doped. It can be seen that both statements are more or less related to lithium manganese iron phosphate.

So far, CATL’s M3P battery has been installed in 6 models, including the modified Model 3, Zhijie S7, Chery Xingjiyuan ES, etc.

PART-2 Anode – Silicon Carbon Anode Materials

❶ First principles: properties and defects of the material itself

Following the evolution diagram of the “high energy route” in the above figure, it can be seen that the capacity of the negative electrode graphite is close to the theoretical limit (372 mAh/g), while the silicon negative electrode has a much higher theoretical specific capacity than graphite (4200 mAh/g).

• Li+C→LiC₆​: One carbon atom absorbs 0.17 Li+ ions;
• 5Si+22Li⁺+22e⁻=Li₂₂Si₅: One silicon atom absorbs 4.4 Li+ ions.

The graphite negative electrode is a layered material, and the storage of lithium ions between graphite layers during the charging process is a typical intercalation reaction. The crystal of the silicon negative electrode is a regular tetrahedral structure, and the lithium ions combine with silicon atoms during the charging process to form an alloying reaction. Since the silicon negative electrode has a stronger lithium insertion ability than the graphite negative electrode, the theoretical specific capacity of the silicon negative electrode is significantly higher than that of graphite.

However, silicon particles will cause huge volume expansion and contraction during the alloying/de-alloying process. When silicon and lithium form the Li₁₅Si₄ phase, the corresponding maximum volume expansion can reach 300%. According to relevant research data, the maximum expansion of pure graphite pole pieces is ~19%, while the maximum expansion of pure silicon pole pieces is ~300%.

The silicon anode material repeatedly expands and contracts during the process of lithium insertion and extraction. The huge volume expansion will cause the silicon material particles to pulverize and fail, making the electrical contact between the silicon particles and the conductive agent or current collector worse, or even detach from the electrode. Secondly, it will cause the SEI film to constantly rupture and regenerate. This process will consume a large amount of active lithium and electrolyte, and form a thick and uneven SEI film, thereby accelerating the capacity decay and aging of the battery.

The above defects are actually caused by the volume expansion of the silicon negative electrode material itself during the process of lithium insertion and extraction, which leads to problems such as low initial efficiency, electrode expansion, effective material shedding, rapid cycle attenuation, low conductivity, and ultimately causes complete failure of the battery.

❷ Key point: Control volume expansion to improve defects and enhance performance

Therefore, the industry focuses on modifying negative electrode materials mainly to control expansion, which is generally done through nano-sizing, surface coating, composite, cavity construction, etc.

After years of continuous iterative evolution, silicon anode materials have developed four generations of products.

1. Sanded Silicon Carbon;
2. Silicone coated;
3. Pre-lithium pre-magnesium silicon oxide;
4. Vapor Deposition Silicon Carbon.

The previous generations of product routes all had inevitable technical problems, or the cost could not be controlled (such as the high cost of pre-lithiation, etc.), and the logic of large-scale industrialization was difficult to implement.

CVD silicon-carbon anode has the possibility of actual implementation and large-scale mass production.

The new vapor-deposited silicon-carbon anode material is a lithium battery anode material obtained by uniformly depositing silicon nanoparticles inside a porous carbon material skeleton. Compared with traditional silicon-oxygen, ground silicon-carbon and other anode materials, the use of vapor-deposited silicon-carbon significantly improves the battery’s first-cycle efficiency, energy density, cycle performance, cell expansion and other performance, and the future cost reduction path through silane gas is also clearly visible.

Guaranteed performance and reduced costs are the reasons why this technology route has attracted much attention in the industry, and has also become the mainstream solution for large-scale industrialization.

This technical route is basically based on the technical process characteristics of Group 14 in the United States. The core technical process is mainly divided into three steps:

(1) Preparation of porous carbon skeleton. Generally, a resin-based or bio-based carbon source is used. First, carbonization is carried out under inert gas protection, and then physical or chemical methods are used for activation etching to increase the surface area and widen the pores.

(2) Deposition of nano-silicon particles inside porous carbon by silane. The deposition of silane inside porous carbon is generally divided into two steps: adsorption and deposition (cracking). The current mainstream in the industry uses a fluidized bed as a reactor to achieve the uniformity of silane adsorption and cracking inside porous carbon as much as possible, and finally obtain nano-silicon particles.

(3) Carbon layer coating. After the silicon particles are deposited, a carbon source gas (acetylene/ethylene, etc.) will be introduced into the fluidized bed to uniformly coat the surface of the material with a carbon layer to improve the conductivity of the overall material and reduce side reactions, and finally obtain vapor-deposited silicon carbon.

❸ Key to industrialization: silane gas, porous carbon, fluidized bed

In the preparation process of CVD silicon-carbon anode, “silane gas and porous carbon” are indispensable key raw materials in the upstream raw materials.

Generally speaking, it takes about 0.6 tons of silane + 0.5 tons of porous carbon to produce 1 ton of silicon-carbon anode. The prices of these two raw materials are basically 180,000-220,000 yuan/ton. It can be said that the prices of these two raw materials directly determine the overall cost of silicon-carbon anode.

At present, the price is still high. If the overall cost of silicon-carbon anode is reduced to less than 200,000 yuan according to the future industry cost reduction logic, it will have a very strong cost-effective advantage, and this is very likely to be achieved.

(1) Porous carbon: Industrialization has just begun, the technical route has not yet been determined, and there is room for cost reduction

Porous carbon nanomaterials have the properties of carbon materials, such as high chemical stability, good conductivity, and low price. At the same time, the introduction of pore structure makes it have large specific surface area, controllable pore structure, and adjustable pore size (microporous carbon <2nm, mesoporous carbon 2-50nm, macroporous carbon >50nm).

As a carbon source, porous carbon controls the volume expansion of silicon-carbon negative electrode through a nanoporous carbon skeleton. Therefore, the quality of the carbon skeleton directly determines the mass production capacity of the product (specific surface area, pore size distribution, open porosity, pore volume, porosity uniformity, etc.).

The volume expansion of silicon embedded in lithium is buffered by the gaps inside the porous carbon, and electrons and ions can be effectively transferred. At the same time, the carbon layer coating reduces the direct contact between bare silicon and electrolyte, inhibits the repeated growth of SEI film, and thus improves the cycle life and first effect of lithium batteries.

From the technical route, porous carbon can be divided into two types: resin-based and wood-based.

From the perspective of performance, resin carbon is superior. Resin-based spherical porous carbon reduces surface stress, reduces expansion rate, and improves cycle performance; micropores have good consistency and uniform pore size distribution, which is beneficial to uniform deposition of silane and is not easy to break under high compaction. However, the degree of graphitization of the resin-based carbon is lower and the conductivity is average.

From the perspective of cost/performance ratio, wood carbon is superior. The raw material of wood carbon is lignin, which is industrial waste from industries such as papermaking. It has low cost, wide sources, and environmental benefits. The cost of resin-based carbon is expected to be 30,000-50,000 yuan/ton higher than that of wood carbon (mainly because the raw material cost is higher than that of lignin, depending on the price of raw materials such as phenolic formaldehyde). The current price of resin porous carbon is 300,000 yuan/ton, which is much higher than the 150,000-200,000 yuan of wood carbon.

At present, some manufacturers use petroleum coke to make porous carbon, which can effectively balance high performance and low cost, and may have better conductivity. The source is also very stable, which deserves further attention.

So far, porous carbon is still an emerging industry, the pattern has not been clear, and the technical route has not been finalized. The intrinsic carbon provided by the listed company Yuanli Co., Ltd. can basically meet the current demand, but there is still a lot of room for improvement, especially silicon-carbon negative electrode manufacturers are actively and independently developing porous carbon.

(2) Silane gas: a mature industry, expansion cycle, and huge room for cost reduction

Silane is a large class of compounds composed of two elements, silicon and hydrogen. Its general chemical formula is Si_nH2_n+2, which actually covers many substances. The so-called “silane” specifically refers to silane (chemical formula is SiH₄). This special compound is known as “flowing, pure silicon”. Because it can efficiently generate high-purity silicon through pyrolysis reaction, it is another important raw material in the manufacture of CVD silicon-carbon negative electrode. It is mainly used as a high-purity silicon source, which undergoes thermal cracking inside porous carbon and then deposits into nano-silicon.

Electronic grade silane gas is a high-purity electronic special gas, which is mainly obtained by various reaction distillation and purification of silicon powder, hydrogen, silicon tetrachloride, catalyst, etc. Purity 3N~4N is called industrial grade silane, and purity above 6N is called electronic grade silane gas.

As a gas source for carrying silicon components, silane gas has high purity and can achieve fine control. It is an important special gas that cannot be replaced by other silicon sources. High-purity silane gas can ensure the uniformity and density of CVD deposition, thereby improving the electrochemical performance of silicon anodes.

The current market competition for silane gas is mainly the competition between existing companies to quickly expand production to fill market gaps, as well as to capture market opportunities, take the lead in emerging application fields such as silicon-carbon anodes and electronic-grade polysilicon, and obtain high-quality customers.

In the past two years, due to the rapid growth of demand, silane production capacity has become tight, but there are no particularly obvious barriers. Existing players and new entrants have announced very aggressive expansion plans.

According to incomplete statistics, it is expected that by the end of 2025, silane production capacity will reach 43,000 tons, which is four times that of the end of 2023. This does not include granular silicon companies such as GCL, which have huge self-use silane production capacity and costs far lower than silane manufacturers. If they convert their surplus production capacity into external sales in the future, it will have a severe impact on silane prices, and the cost is expected to be only 30,000-40,000 yuan/ton.

Therefore, the production cost of silane gas will surely drop significantly in the future, providing a clear and feasible path for the price of silicon-carbon negative electrode (silane gas accounts for more than 60% of the total cost) to drop from 400,000 yuan to 200,000 yuan (or even around 100,000 yuan).

(3) Deposition equipment: Fluidized bed method has become the mainstream, but there is still room for breakthroughs

Deposition equipment is the key issue that needs to be solved for the large-scale industrialization of CVD silicon-carbon negative electrodes. Currently, the mainstream equipment includes rotary kilns and fluidized beds.

A. Rotary kiln: Silane is very likely to spontaneously combust and explode during deposition in a rotary kiln, posing a safety hazard. In addition, the solid precipitation produced by combustion leads to a low utilization rate of silane in the rotary kiln. Too much silane is wasted, resulting in a high cost for mass-produced silicon-carbon. Rotary kiln equipment is relatively mature and simple, but it is prone to poor material performance due to uneven silane deposition and imperfect coating.

B. Fluidized bed: The specific advantages of fluidized bed are: 1) Cost control: high silane utilization rate, reducing raw material consumption; 2) Feasibility: fluidized bed equipment meets the requirements of high airtightness and high gas pressure, and has better safety; 3) Product performance: fluidized bed has the advantages of high specific capacity, high initial efficiency, high cycle stability, and high rate performance.

In general, fluidized bed is the mainstream choice for producing vapor-deposited silicon-carbon anode materials, which can make silane deposition more uniform, silane utilization rate higher, and the products produced have good coating uniformity and high purity.

However, silane adsorption and cracking require high pressure and high temperature environment, as well as gas-solid separation at the back end, so the reliability, stability and safety requirements of fluidized bed equipment are very high. The industry is still dominated by small equipment (100L/20kg).

Therefore, large-scale and continuous production equipment is still a difficulty that restricts the industrialization of vapor-deposited silicon-carbon anodes.

PART-3 Electrolyte – Lithium Salt LiFSI

❶ First principles: LiFSI has better overall performance

As one of the four main materials of lithium batteries, electrolyte is the carrier of ion transmission in the battery, and plays the role of conducting lithium ions between the positive and negative electrodes.

Mainstream lithium battery electrolytes are usually prepared from electrolyte lithium salts (solutes), high-purity organic solvents, various additives and other raw materials in a certain proportion. Lithium salts, that is, the solutes in the electrolyte, are the most core and cost-intensive components of the electrolyte.

Since the commercialization of lithium-ion batteries, the most commonly used main salt is lithium hexafluorophosphate (LiPF6), which is used in various batteries for its stable electrochemical window and good system compatibility. However, it has defects such as unstable chemical properties, limited efficiency in low temperature environments, and poor thermal stability. In particular, it is extremely sensitive to moisture and will decompose to produce hydrogen fluoride during use, causing battery failure, making it increasingly difficult to support the current growing requirements for improving the comprehensive performance of batteries.

In recent years, new lithium salts represented by LiFSI have shown significant advantages in conductivity, thermal stability, chemical stability, and battery performance, which are more in line with the future development direction of high energy density, high power density, and high safety lithium batteries. In particular, the permeability and mass proportion of LiFSI in the electrolyte formula have been greatly improved in recent years, and there is a trend of partially replacing LiPF6 as the main salt.

Item		LiFSI	LiPF6
Basic physical properties Battery performance	Decomposition temperature in solution	>200℃	>80℃
	Oxidation voltage	≤4.5V	>5V
	Hydrolysis	Resistant to hydrolysis, no HF generation	Easy to hydrolyze, produce HF
	Electrical conductivity	High	Slightly low
	Chemical stability	Stable	Unstable
	Cycle life	LiFSI has more advantages
	Low temperature performance	LiFSI has more advantages
	High temperature resistance	LiFSI has more advantages
	Gas expansion	Prevent battery bloating	Bloating will occur

❷ Key point: Control volume expansion to improve defects and enhance performance

The global commercial production processes of LiFSI are basically similar, and are mainly divided into four steps: chlorination, fluorination, salt formation, and purification.

(1) Chlorination: In this step, aminosulfonic acid, thionyl chloride and chlorosulfonic acid react to prepare HClSI (chlorosulfonyl imide). This is a relatively mature process, and there are usually two routes: one route is to use thionyl chloride as a raw material. This method has a relatively low cost, but the reaction yield is low, and the reaction will produce more by-products, resulting in a large amount of sulfate and chloride impurities in the product, which is difficult and costly to purify later. The other route is to use chlorosulfonyl isocyanate. This method has a higher product purity, but the cost is relatively high.

(2) Fluorination: The core of this step is to introduce F ions. The optional F-containing raw materials include HF (hydrofluoric acid), metal fluorides, NH4F (ammonium fluoride), etc. At present, NH4F is widely used by overseas companies, and HF is mostly used in China. Different raw material selections will affect the purity and cost of the final product.

(3) Salt formation: The core of this step is to introduce Li ions. Commonly used lithium compounds include LiOH (lithium hydroxide), Li2CO3 (lithium carbonate), LiF (lithium fluoride), and LiX (such as LiCl, LiBr, etc.). Different lithium compounds require different reaction solvents and temperatures, and different products require different reaction environments and products, which affects the properties of the final product.

(4) Purification: Since LiFSI is a battery-grade product, the purity requirement is very high (above 99.9%), and the purification process is the key to controlling product quality. The various raw materials selected in the previous processes and the resulting by-products will affect the performance of the final product. Therefore, the reagents and methods used by different companies in the purification process are different, which is also one of the process barriers.

❸ Industrialization: Continuous cost reduction and continuous increase in added amount

At present, the preparation process of LiFSI is relatively mature, and the yield of leading enterprises is basically above 95%. Cost and price are key factors hindering further large-scale substitution, especially compared with the price of lithium hexafluorophosphate.

According to Xinrong Information data statistics, the global output of lithium hexafluorophosphate in 2023 will be 159,000 tons, the effective production capacity will be 360,000 tons, the overall capacity utilization rate will be 44%, the market supply will be in surplus, and the capacity will be seriously oversupplied.

After a short-term rapid rise in the price of LiPF6 from 2021 to the beginning of 2022, with the serious overcapacity, the price has also fallen sharply, and is currently basically stable at around 60,000 yuan.

Previously, the LiFSI addition ratio of the world’s leading battery companies was between 0.5% and 2%. At present, the mainstream formula of adding LiFSI has increased to 2%-15%. In 2025, the global demand for lithium-ion battery electrolyte will reach 2.16 million tons. Based on the addition ratio of 5%, the global market demand for LiFSI is about 108,000 tons; even if the addition ratio is increased to 10%, the global market demand is only 210,000 tons. Due to cost and technical considerations, this addition ratio is unlikely to be achieved in the short term.

According to the Xinlang Information Database, the global total production capacity of LiFSI will exceed 250,000 tons in 2025. This means that by 2025, the production capacity of LiFSI will be twice the demand, and the era of full surplus of LiFSI will also come.

As of the end of December 2023, the price of liquid LiFSI is 100,000 yuan/ton, a year-on-year decrease of 72.22%; the price of solid LiFSI is 130,000 yuan/ton, a year-on-year decrease of 68.29%.

Therefore, in comparison, the price of LiFSI is nearly double that of LiPF6, and it is very likely that the situation of quantity-for-price exchange will continue in the future.

On the battery side, Tesla’s 4680 battery requires at least 20% LiFSI addition (the more the better); CATL’s Kirin battery supercharge and battery performance also require at least 20% LiFSI addition (the more the better), and use FEC additives to form lithium fluoride at the negative electrode, with a small ion radius, which can repair cracks in time; in semi-solid batteries and solid-state batteries, LiFSI must be used as the main salt due to battery efficiency and safety requirements.

Type	Liquid battery	First generation condensed matter battery	Second generation condensed matter battery
Electrolyte	Solvent+LiPF6+Additive	Polymer+oxide solid electrolyte+solvent+LiPF6/LiFSI+additive	Polymer + oxide solid electrolyte + solvent + LiFSI + additive
Separator	Yes	Retention+LATP coating	Retention + LLZO coating
Anode	Graphite	Carbon silicon negative electrode	Lithium composite negative electrode / lithium metal
Cathode	Ternary/LiFePO4	High nickel	High nickel high voltage / ultra-high nickel 9 series / lithium-rich manganese base
Packaging	Wound/Laminated+Square/Cylindrical/Soft pack	Wound/laminated+square/soft pack	Lamination + soft pack
Energy density	<300Wh/kg	>300Wh/kg	>450Wh/kg

Therefore, the solution of LiFSI as the main salt in the future is certain, and the downstream demand is urgent. The key lies in production capacity and price.

PART-4 Electrode – Dry Electrode

Dry electrode is a new way to produce electrodes.

The traditional wet process is to mix active materials, conductive agents, and adhesives in proportion in solvents such as NMP, first complete the slurry process, then coat it on the surface of the current collector as required through a slit coating die, then bake and dry to remove the solvent (NMP recovery), and roll it.

The dry process is to dry-mix the active particles, conductive agents, and then add adhesives, form a self-supporting film under the fibrillation of the adhesive, and finally roll it to cover the surface of the current collector.

The advantages over the traditional wet process are:

(1) Reduced costs: Dry electrode technology saves the cost of solvents, solvent evaporation and recovery, and coating and drying, and also reduces the impact on the environment. According to incomplete estimates, a production line that produces 1 million lithium-ion batteries (20.5Ah, 3.7V) using a dry process can save about 56% (about RMB 850 million) of production costs each year.

	Dry process	Wet process	Notes
Raw material mixing stage
Active particles	Required	Required
Conductive agent	Required	Required
Adhesive	Required (both positive and negative electrodes are PTFE)	Required (PVDF for positive electrode, SBR and CMC composite for negative electrode)
Solvent	Not required	Required (NMP for positive electrode, deionized water for negative electrode)	NMP accounts for 5% of the battery cell manufacturing cost
First stage Finished product form	Powder after fiberization	Mixed slurry

Electrode coating process			The energy costs for drying and recycling account for 54% of the energy costs consumed by the battery
Self-supporting film/slurry coating	Needed	Need
Baking	Not needed	Need
Solvent recovery	Not needed	Need
The second stage finished product form	Self-supporting film + metal current collector	Mixed material + metal current collector
Pole rolling process
Roller press	Required	Need
The third stage finished product form	Wide electrode	Wide electrode	Dry coating process has fewer steps and the overall manufacturing cost is reduced by 18%

(2) Improve electrode uniformity: Dry electrode technology does not use solvents during the mixing process, so it can achieve uniform distribution of the various components of the electrode material and avoid electrode stratification caused by solvent evaporation.

(3) Increase electrode active material loading: Dry electrode technology can easily control the electrode thickness and the uniformity of thick electrodes without generating cracks. It has unique advantages in the preparation of thick electrodes and is suitable for the preparation of ultra-high loading electrodes.

(4) Adaptation to sulfide solid electrolytes: Dry electrode technology avoids the use of organic/polar solvents and requires only a very small amount of adhesive during the preparation process. It is particularly suitable for the preparation of sulfide all-solid-state batteries. Since no solvents that react with sulfide solid electrolytes are used, the dry process helps to better prepare sulfide solid electrolyte membranes and maintain their high ionic conductivity.

(5) Better performance: The cycle performance, durability and impedance of dry process batteries are better under laboratory conditions. In the wet process, after the battery has undergone 500 cycles, the internal stress of the active particles continues to accumulate, resulting in cracks in the cross section, which ultimately reduces the battery performance. Under the dry process, the fiber mesh is coated on the surface of the active material. After 500 cycles of charge and discharge, the mesh structure remains intact and there are fewer cracks on the surface of the particles. At the same time, the fibrillated mesh structure can inhibit the volume expansion of the active material and prevent the particles from falling off the current collector, thereby enhancing stability and improving electrical performance.

Dry/wet battery comparison
	Dry process	Wet process	Notes (laboratory conditions: NCM811, all wet negative electrode)
Cycle performance	High (capacity retention rate 95%)	Low (capacity retention rate 90%)	500 cycles, 1C rate But it is currently not suitable for industrial conditions, and the cycle performance under industrial conditions is not as good as the wet method
Durability	High	Low	Under the wet method, the gaps between active particles increase
Voltage platform	No difference	No difference	Both show two voltage platforms at 3.0~4.3V
Specific capacity (mAh/g)	Low (171.8)	High (176.6)	1C rate
AC impedance	Low	High	Fresh battery, 50 cycles
Active particle morphology	1) Complete fibrillation network structure 2) Fewer cracks on particle surface	1) Particle volume expands and cracks appear on the surface 2) Cracks appear inside the particle section	500 cycles
Others	1) Less gas generation during circulation 2) High requirements for PTFE adhesion	Wet process adhesives are prone to aging, causing active substances to fall off and reducing durability	The electrolyte will react on the surface of the de-lithiated positive electrode and the lithiated negative electrode to produce gas

Since dry electrodes do not require the use of liquid solvents, they have the advantages of simple process, environmental protection, low cost, and good electrode performance, which perfectly adapts to the development trend of the next generation of batteries (especially solid-state batteries).

However, at this stage, dry electrodes have high requirements for equipment and adhesives, and the technology has not yet matured. There are still many process difficulties to be solved, and it will take some time before the industry explodes.

Difficulties in dry-process lithium battery technology
Technical difficulties	Reason	Solution
Higher bonding requirements	Lithium batteries have a small specific surface area, and the expansion coefficient caused by lithium ion insertion and extraction is high. The dry method is prone to powder loss after multiple cycles	Modify the adhesive PTFE to enhance the bonding effect, such as mixing PTFE with non-fibrillated adhesives and reducing the particle size of other adhesives; use new material PTFE-AGT powder instead of PTFE
Maintaining active materials from damage	Dry-mixing film-making process may destroy active substances	Improving the performance of fibrillating film machines
Increased internal impedance	1. The interface impedance between aluminum foil and active material is large 2. Polarization problems may occur under high current 3. Solid-solid interface impedance is large	1. Coating conductive carbon on the surface of aluminum foil and performing corona treatment. The AC internal resistance of aluminum foil treated with corona treatment can be reduced by about 20% 2. Changing the positive and negative electrode materials, increasing the conductivity of the electrolyte, and reducing the thickness of the electrode sheet 3. Introducing a higher pressure roller press to offset the force between solid-solid molecules
PTFE is prone to react with the negative electrode	PTFE’s LUMO orbital energy is lower and it is easier to accept electrons, resulting in a decrease in battery capacity	A layer of conductive carbon is coated on the surface of PTFE for passivation modification to weaken the reaction between PTFE and the negative electrode
Higher requirements for positive electrode rolling	1. The positive electrode active material has high electrochemical activity and is prone to chemical changes during rolling 2. The self-supporting film is still prone to powder loss after rolling	1. Add additives and improve electrode formula 2. Replace roller pressing equipment
Higher requirements for uniform adhesive distribution	PTFE adhesive has large particle size, which weakens the adhesion effect	Replace the calendering equipment and roll it multiple times