Lithium battery - process - chemical capacity division

1、Lithium ion battery

According to the latest White Paper on the Development of China's Lithium ion Battery Industry (2023 Edition),

 the global lithium-ion battery market achieved significant expansion in 2022, 

with shipments jumping to 957.7 gigawatt hours (GWh), a 70.3% increase compared to the previous year, highlighting the thriving development trend of the industry.

 Lithium ion batteries have a wide range of applications and have deeply penetrated into the field of new energy vehicles, serving as the core driving force for green travel; 

At the same time, it also plays an indispensable role in diversified fields such as power storage systems, consumer electronics markets, uninterruptible power supply systems, and military equipment.

The continuous progress of lithium-ion battery technology not only profoundly affects the transformation and upgrading of the new energy industry, 

but also closely relates to the future development of multiple key fields such as information technology, 

forming an important cornerstone for the improvement of national technological strength and the progress of human civilization. 

In 1990, Sony Corporation pioneered history by launching the world's first lithium-ion battery, 

which features a unique dual compound design where both the positive and negative electrodes can reversibly embed and deintercalate lithium ions,

 forming a high-performance secondary battery system. 

The structure of lithium-ion batteries is exquisite, mainly consisting of four core components: positive electrode, negative electrode, electrolyte, and separator.

During the charging process, the positive electrode material LiCoO2 undergoes an oxidation reaction, releasing lithium ions that cross the electrolyte solution, 

migrate to the negative electrode region, and combine with the graphite material of the negative electrode to form LiCX compounds, achieving the storage of electrical energy. 

When the battery is discharged, it becomes the energy source in the load circuit, 

and the chemical reaction occurring on the positive and negative electrodes happens to be the reverse of the charging process, ensuring the smooth release of electrical energy.

It is worth noting that the separator plays a crucial role in lithium-ion batteries.

 It not only has excellent ion conductivity, allowing lithium ions to freely shuttle, 

but also exhibits excellent electronic insulation, 

effectively preventing unnecessary electron flow between the positive and negative electrodes inside the battery, i.e. micro short circuits, 

thus ensuring the safe and stable operation of the battery.

According to diverse classification criteria, lithium-ion batteries exhibit rich types of classification.

 Based on the differences in electrolyte materials, they are clearly divided into two camps: liquid lithium-ion batteries and polymer lithium-ion batteries; 

From a morphological perspective, lithium-ion batteries cover various shapes such as cylindrical, square, and compact button shaped (also known as coin shaped);

 Furthermore, according to the different positive electrode materials, 

they can be further divided into series such as cobalt lithium oxide, nickel lithium oxide, manganese lithium oxide, etc.

Each material endows the battery with unique performance characteristics.

The application fields of lithium-ion batteries are becoming increasingly widespread, 

extending from precision microelectronics technology to various transportation vehicles, and their presence is everywhere. 

With the increasing demand for performance of technological equipment,

 the market's performance requirements for supporting battery components have become increasingly stringent,

 which has prompted battery manufacturers to constantly face severe challenges in optimizing production processes.

 It can be seen that lithium ion batteries stand out from the comparison of comprehensive performance and characteristics by virtue of its multiple significant advantages such as high rated voltage, 

excellent energy density, long service life, extremely low self discharge rate,

 and green environmental protection, becoming the leader in market applications and leading the trend of energy storage technology.

2、Production process of lithium-ion batteries

The entire manufacturing process of lithium-ion batteries is extremely complex and refined, 

and every step of the production process requires strict quality control and refined management. 

At the critical stage of achieving excellent performance in batteries, 

the formation and capacity separation process is of paramount importance in the post production stage, playing a decisive role in the performance of the final product.

 Given the complexity of production processes and the limitations of current technological levels, it is difficult to completely avoid small differences in the production process. 

Therefore, this article will not delve into the specific methods for improving production processes and optimizing consistency before chemical separation.

The preparation process of lithium-ion batteries is a complex system consisting of a series of highly specialized steps closely connected, 

including electrode preparation, liquid immersion, solid-state molding, composite coating, and electrolyte filling. 

Specifically, during the electrode preparation stage, the selected electrode materials need to undergo fine grinding,

 rigorous screening, and scientific mixing to ensure the desired particle size distribution and morphology. 

Subsequently, these electrode materials are carefully shaped into the desired form and density through advanced liquid immersion technology or solid-state molding processes to optimize their electrochemical performance.

Subsequently, the composite coating step is crucial, which involves uniformly coating a layer of electroactive composite on the electrode surface. 

This aims to significantly enhance the electrochemical activity of the electrode surface, thereby improving the overall performance of the battery.

 Finally, the battery enters the electrolyte filling stage, and through precise control of electrolyte injection, 

the formation process of the battery is completed, enabling the battery to reach its optimal working state. 

The entire production process of lithium-ion batteries is shown in Figure 3, and each step embodies the power of technology and the careful carving of craftsmen.

1） Formation process of lithium batteries

Battery formation, also known as battery aging, is an important process in the production of lithium-ion batteries. 

The principle is to activate the internal positive and negative electrode substances through specific charging 

and discharging methods to improve the charging and discharging performance of the battery, 

and to enhance the comprehensive performance of the battery in its initial manufacturing stage, such as self discharge, storage, etc. 

The core of lithium-ion formation is essentially a crucial initialization ceremony for the battery,

 which promotes the elegant formation and coverage of a crucial passivation film layer

 - the Solid Electrolyte Interface (SEI) - at the interface between the carbon negative electrode and the electrolyte on the carbon electrode. 

This SEI film is constructed from a complex lithium salt network, exhibiting excellent ion conductivity while ensuring electronic insulation and isolation. 

Its unique passivation properties effectively suppress excessive reactions of the negative electrode material, thereby significantly extending the cycle life and stability of lithium-ion batteries.

However, the birth of SEI film is not entirely guaranteed.

 During the construction process, it is inevitable to consume valuable lithium ions that migrate from the positive electrode. 

Once these lithium ions participate in the reaction and are embedded in the SEI film, they cannot return to the positive electrode embrace. 

 This phenomenon directly leads to irreversible capacity loss of lithium ions, limiting their free insertion and extraction, 

thereby exacerbating the accumulation of resistance at the electrode/electrolyte interface and causing hysteresis in voltage response.

More profoundly, the quality of SEI film is directly related to the electrochemical performance map of lithium-ion batteries, 

including but not limited to the length of cycle life, stability, self discharge rate, and safety performance.

 Therefore, SEI film is not only a silent guardian in the electrochemical journey of lithium-ion batteries,

 but also a key role in determining their comprehensive performance, and its importance is self-evident.

Given that the SEI film is formed during the initial charging stage and its main body is mostly constructed during the first charge discharge cycle, 

the formation process, as the direct shaper of this critical process, has a particularly profound impact on the overall performance of the battery. 

The key to optimizing the formation process lies in accurately controlling core parameters such as formation voltage, temperature, external pressure, and current. 

The subtle changes in these factors can significantly affect the quality of the SEI film, which in turn affects the overall performance of the battery.

① Formation voltage: Research has shown that 3.5V is considered the ideal voltage threshold for lithium-ion battery formation. 

Although increasing the voltage may temporarily increase the initial charging capacity,

 its long-term charging and discharging efficiency is inferior to the 3.5V setting, 

and it is easy to induce the phenomenon of "white spots" on the negative electrode surface - this is actually an intuitive manifestation of irreversible deposition of lithium or its compounds,

 which not only reduces the first effect, but also increases the internal resistance of the electrode, posing a threat to cycling performance. 

Under high voltage conditions, batteries will face more severe problems such as decreased charging and discharging efficiency,

 increased internal resistance, and decreased cycling performance.

② Formation temperature: Experimental data generally indicates that the optimal temperature range for SEI film formation is 20-35 ℃.

 However, in order to improve battery cycling and storage performance,

 most manufacturers tend to choose temperatures slightly higher than this range (30-60 ℃) for chemical conversion.

 Under high temperature conditions, the SEI film reaction is more complete, promoting the membrane's liquid absorption and reducing inflation.

 However, excessive high temperature can also accelerate the dissolution of SEI film and the co embedding of solvent molecules, weaken its stability,

 and instead damage the electrode protection effect, reducing the cycling performance of the battery. 

Therefore, choosing a temperature slightly higher than the optimal temperature for formation aims to balance SEI film quality, production efficiency, and speed.

③ External pressure: Applying an appropriate amount of rolling pressure during the charging stage is an effective means of eliminating gas generation during the formation process. 

These gases are mainly derived from the decomposition of organic matter under high pressure, and their presence can reduce the charging capacity of batteries. 

Research has shown that eliminating gas through external pressure can not only increase the formation capacity, but also significantly improve the rate performance and cycle life of batteries. 

In addition, appropriate pressure can also help suppress lithium deposition in the negative electrode, further improving battery performance.

④Formation current: The structure and composition of SEI film are deeply influenced by current density, 

and its interior can be subdivided into a dense and uniform inner layer and a loose and porous outer layer,

 which are rich in inorganic substances and organic products, respectively. 

The difference in ion diffusion rate and migration number under different current densities leads to changes in the chemical reaction pathway and products on the negative electrode surface,

 which in turn affects the properties of the SEI film. Therefore, precise control of the formation current density is crucial for obtaining SEI films with uniform structure and excellent performance.

The current research has revealed the unique advantages of the low current density formation strategy in optimizing the SEI film structure. 

This strategy significantly promotes the enrichment of inorganic components in the inner layer of the SEI film, 

thereby bringing a qualitative leap to the electrochemical performance of lithium-ion batteries. 

Using advanced characterization techniques such as electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM),

we intuitively observed that the SEI film generated on the negative electrode surface exhibited a more ideal morphology after chemical treatment with lower current density: 

the inorganic composition of the inner layer dominated, and compared to the outer layer, the inner layer was thicker, more uniform, and denser. 

This structure not only achieves comprehensive coverage and tight wrapping of the negative electrode material, 

effectively blocking direct contact between the electrolyte and the negative electrode, 

but also greatly enhances the stability of the SEI film,

 providing a more solid protective barrier for lithium-ion batteries, thereby improving performance in multiple dimensions such as cycle life, 

charging and discharging efficiency, and safety.

Currently, the formation process of lithium-ion batteries generally follows a two-stage strategy, which involves adding a pre formation step before formal formation. 

The core of the pre formation stage is to use a small current (usually in the range of 0.02C to 0.05C) and a voltage of about 2.5V for initial charging,

 which aims to promote the formation of a stable and dense SEI film on the surface of the battery. 

However, it is worth noting that maintaining this low current charging mode for a long time is beneficial for the stable construction of SEI film,

 but it also comes with the risk of gradually increasing film impedance, 

which may have adverse effects on the rate performance and cycle life of the battery.

 In addition, relying solely on low current for chemical conversion will significantly prolong the entire production process, posing a challenge to the production efficiency of the workshop.

To overcome the above difficulties, researchers have proposed an innovative solution - introducing stepped current conversion technology during the constant current charging stage. 

This technology, through carefully designed current step changes, can not only effectively reduce the polarization phenomenon of batteries during charging, improve charging capacity,

 but also significantly shorten charging time and achieve a significant improvement in formation efficiency.

 This discovery provides a new perspective and path for optimizing the formation process of lithium-ion batteries,

 and is expected to drive a dual leap in production efficiency and product quality in the battery manufacturing industry in the future.

2) Capacity division process of lithium-ion batteries

The capacity division process, as a key link in the battery production process, 

closely follows the formation process and aims to comprehensively evaluate the various performance indicators of the battery through charge discharge cycle testing, 

and accurately assemble based on this.

 This process covers various capacitance methods such as single parameter, multi parameter, 

dynamic characteristics, and electrochemical impedance spectroscopy (EIS), each showing their own unique abilities.

The single parameter method focuses on a single key indicator, such as capacity or voltage, which may be simple but somewhat singular.

The multi parameter rule is more comprehensive, taking into account multiple dimensions such as capacity, voltage, self discharge rate, and average internal resistance. 

It uses the external performance of the battery as the capacity benchmark, providing rich basis for accurate screening.

The dynamic characteristic capacity division method is deeply integrated into the battery,

capturing its dynamic response characteristics by simulating the charging and discharging curves under actual operating conditions, 

and achieving precise capacity division based on the inherent characteristics of the battery.

The EIS capacitance partitioning method utilizes electrochemical impedance spectroscopy technology, 

combined with preset conditions and equivalent circuit parameters, to perform impedance vector clustering analysis on batteries, 

further enhancing the scientific and accurate nature of capacitance partitioning.

In practice, the multi parameter method and the dynamic characteristic method often complement each other. 

The former grasps the external characteristics, while the latter understands the internal mechanism and works together to achieve the optimal capacity distribution effect.

The charging process is ingeniously divided into two stages: constant current and constant voltage. 

Although the former is accompanied by polarization phenomena, it lays the foundation for the elimination of polarization in the subsequent constant voltage stage. 

The shorter the duration of the constant voltage stage, the better the polarization control, and the better the battery performance naturally.

The reason why the capacity separation process is important lies in the unavoidable performance differences between individual cells. 

When building high-capacity battery systems, this difference will directly affect overall performance and lifespan. 

Especially capacity deviation, it is not only the focus of current consistency screening, but also the key to ensuring stable operation of battery packs under complex working conditions. 

By using the capacity separation process to classify and screen batteries, the efficiency of individual cells and modules can be significantly improved, and the overall lifespan of the battery system can be extended.

Looking ahead to the future, the lithium-ion battery capacity separation process will not only continue to emit light and heat in traditional fields such as consumer electronics,

 electric vehicles, and energy storage, 

but is also expected to deeply integrate with cutting-edge technologies such as fast charging and supercapacitors, jointly promoting the innovation and development of battery technology. 

At the same time, continuous attention and research on battery cycle life and safety are also key to ensuring the continuous progress of this technology and benefiting society.

3) The practical significance of SEWF series current sensing resistors for chemical and capacitive processes

The formation and capacity separation process of lithium-ion batteries, as the core link in the battery manufacturing process,

 is crucial for improving the overall performance and capacity of the battery through precise design and in-depth research. 

With the continuous evolution of lithium-ion battery technology, we are expected to witness a dual leap in battery performance and capacity,

 thereby promoting its popularization and deepening in a wider range of application fields.

It is not difficult to see from the above analysis that lithium-ion batteries have extremely strict control requirements for charge and discharge voltage and current during the formation and capacity division stage. 

The fine regulation of current density in the formation process is directly related to the composition and structure optimization of SEI film (solid electrolyte interface facial mask), 

and then affects the long-term stability and safety of the battery. 

As a key step in determining battery capacity and average voltage reference, the precision control level of capacity division directly determines the scientific and reasonable subsequent battery gear division.

To achieve this goal, precise detection equipment and efficient consistency screening methods are indispensable. 

The accuracy of these devices needs to be guided by the optimization of battery pack performance, strictly following established standard ranges. 

For example, the current accuracy of charging and discharging equipment needs to be maintained between 0.05% and 2%, 

and the voltage accuracy needs to be controlled within a narrow range of 0.05% to 0.1%. 

This means that the uncertainty of capacity measurement must be strictly limited within the allowable deviation range of consistency to ensure the accuracy of screening results.

However, traditional testing equipment on the current market often fails to fully meet the high-precision requirements mentioned above, 

and charging and discharging equipment that pursues higher current accuracy is inevitably accompanied by a significant increase in cost. 

Therefore, how to effectively control costs while ensuring accuracy has become a major challenge for battery manufacturing enterprises.

In the implementation process of battery formation system, the system accuracy is usually set within the high-precision range of 0.01% to 0.05%, 

which undoubtedly puts more stringent requirements on the performance of various components of the system. 

Among them, the stability of the sampling resistor is one of the key factors affecting the accuracy of the system. 

Its temperature coefficient, power coefficient, and resistor structure design all need to be carefully designed and optimized to ensure that the system can operate stably within the predetermined accuracy range for a long time.

In summary, the formation and capacity division process of lithium-ion batteries is not only a technology intensive task, but also a process that requires continuous innovation and optimization. 

Through continuous technological research and equipment upgrades, 

we are expected to further enhance the performance and reliability of lithium-ion batteries, laying a solid foundation for their application in a wider range of fields.