Analysis of Key Factors Affecting the Lifespan of Lithium Iron Phosphate Batteries

This article focuses on analyzing the aging mechanisms of lithium iron phosphate (LFP) batteries under different operating conditions and examines the key factors and optimal operating parameters for extending the life of LFP batteries from multiple dimensions, including cell design, cell manufacturing, and application. This research provides technical support for the development and application of long – life energy – storage systems.

I. Aging Mechanisms of Lithium Iron Phosphate Batteries

Lithium-ion batteries consist of multiple components, including the cathode material, anode material, electrolyte, and separator. Capacity fading of these batteries involves a series of physical and chemical changes. This paper summarizes and analyzes the performance – fading mechanisms and their impact modes for lithium iron phosphate batteries, as shown in Figure 1.

Factors affecting the performance – fading of lithium iron phosphate batteries mainly include operating time, temperature, state of charge (SOC), current, and mechanical pressure. Specifically:

• • Time primarily affects the growth of the solid-electrolyte interphase (SEI) film.
  • High temperatures influence the growth and decomposition of SEI film and the decomposition of the electrolyte and binder.
  • Low temperatures can lead to lithium metal plating.
  • High SOC not only has the same impact as high temperatures but also causes graphite exfoliation and lithium plating.
  • Low SOC affects the decomposition of the SEI film and the corrosion of the current collector.
  • Current density mainly affects graphite exfoliation, the structure of the cathode material, and the structure of the SEI film.
  • Mechanical pressure affects the structure of material particles and the electrical contact between materials. Excessive mechanical pressure can also trigger lithium plating.

In summary, the main reasons for lithium-ion battery capacity loss are the loss of active lithium, the loss of cathode and anode materials, and the increase in internal resistance. Among them, lithium plating at the anode and the decomposition of the SEI film affect the loss of active lithium; graphite exfoliation, changes in electrode material structure, disorder of the cathode material structure, electrode particle rupture, dissolution of transition metals, and corrosion of the current collector affect the performance of the cathode and anode.

II. Key Factors for Extending the Life of Energy – Storage Systems

The key factors include the development of long-life batteries, consistency control, and application conditions of energy-storage systems.

1. Design of Lithium Iron Phosphate Batteries

The development of batteries with different performance levels requires matching material systems and design formulations. Based on experimental data and experience, the requirements for main materials and design parameters in the development of long-life batteries are optimized.

1. 1. Main Materials for Lithium Iron Phosphate Batteries
      • The main materials include cathode material, anode material, electrolyte, and separator. The selection strategy for long-life main materials is as follows:
        • The cathode material should be highly structurally stable (achieved through element doping and carbon coating).
        • The anode material should be graphite with a low expansion coefficient and an appropriate particle size to reduce particle breakage and damage to the SEI film.
        • The electrolyte should contain film-forming additives and an appropriate salt concentration to enhance the stability of the SEI film.
        • The separator should be a coated separator to reduce electrode sheet wrinkling and improve cycle performance.
   2. Design Parameters for Lithium Iron Phosphate Batteries
      • • The design parameters that affect life mainly include slurry formulation, electrode sheet surface density, electrode sheet compaction, N/P ratio, group margin, and electrolyte injection volume. The optimization requirements for long-life design parameters are as follows:
          • Increase the number of conductive agents and binders to reduce electrode shedding during cycling.
          • Reduce the coating surface density of the electrode sheet to shorten the lithium-ion transport channel, with a cathode surface density of ≤ 380 g/m².
          • Lower the compaction density of the electrode sheet to increase its porosity, with a cathode compaction density of ≤ 2.55 g/cc and an anode compaction density of ≤ 1.58 g/cc.
          • Increase the N/P ratio to reduce the risk of lithium plating at the anode, with a design N/P ratio of ≥ 1.12.
          • Select an appropriate group margin, ranging from 85 – 92% (for prismatic lithium iron phosphate batteries).
          • Increase the electrolyte injection volume, with an injection coefficient of ≥ 4.0 g/Ah.

2.Production of Lithium Iron Phosphate Batteries

1. Environment
   • Dust and moisture in the environment can affect the self-discharge, life, and safety performance of lithium iron phosphate batteries. Excessive moisture can affect the stability of the electrode sheet and electrolyte and the stability of the SEI film. Currently, the dew point control for the electrolyte injection process is ≤ – 36℃, and the dust cleanliness is ≤100,000-class.
2. Production Processes
   • • • The battery production process is complex, taking about 14 days from material input to the final output of the lithium iron phosphate battery. The key manufacturing process requirements for battery life are as follows:
         • Pressing the cathode electrode sheet can reduce electrode sheet wrinkling, reduce lithium plating at the anode and black spots during cycling, and improve the cycle performance of lithium iron phosphate batteries. Most battery manufacturers have now adopted the pressing process.
         • The hot-pressing parameters of the wound core, including pressure and time, can affect the interface of the lithium iron phosphate battery and its cycle performance.
         • Drying time and temperature affect the internal moisture content of the lithium iron phosphate battery. Currently, the moisture standards for the positive and negative electrode sheets are as follows: positive electrode sheet moisture ≤ 350ppm and negative electrode sheet moisture ≤ 250ppm.
         • The formation process affects the stability of the SEI film in the battery and is a key core process for battery manufacturers. Each battery manufacturer has its unique formation process. Based on the film-forming mechanism, the current formation process generally uses a low current (around 0.1C) and a high temperature (45℃) for formation.
         • Compared with the winding process, the stacking process can reduce capacity loss caused by electrode deformation and lithium plating at the R-corner, thereby improving cycle life.
         • Pre – lithiation can compensate for the loss of active lithium ions during battery cycling and improve cycle life.

3. Inconsistencies in Lithium Iron Phosphate Batteries

Inconsistencies are mainly manifested in differences in capacity, internal resistance, self-discharge rate, life, state of charge (SOC), and working voltage. Research shows that the causes of battery inconsistency mainly include differences in the production process and inconsistent usage conditions. In practical applications, these two aspects interact and gradually increase battery inconsistency.

4. Storage of Lithium Iron Phosphate Batteries and Energy – Storage Systems

The calendar life of batteries and energy-storage systems is strongly related to storage time, SOC, and temperature. The higher the storage temperature of the battery, the faster the capacity fades. For example, capacity fading at 60℃ is about 2.4 times faster than at 25℃. The higher the storage SOC of the battery, the faster the capacity fades. Capacity fading at a SOC of 25 – 40% is 15 – 30% slower than at a SOC of 100%. Compared with SOC, temperature has a more pronounced impact on calendar life.

5.Operation of Lithium Iron Phosphate Batteries and Energy – Storage Systems

The life of batteries and energy-storage systems is strongly related to operating rates, temperature, SOC range, and pre-tension. Based on experimental data and research, the impacts of these four factors on the life of energy-storage systems are summarized as follows:

1. Rate
   • Operating energy – storage systems at low rates is beneficial for extending cycle life. As the charging and discharging rate increases, the capacity-fading rate of the battery accelerates significantly. Research shows that after rate aging tests, the structure of the lithium iron phosphate material does not show obvious aging with battery capacity fading. The decomposition of the SEI film on the surface of the graphite and the formation of lithium deposits are the main reasons for capacity fading.
2. Temperature
   • As the operating temperature of the energy-storage system increases, battery capacity fading accelerates. Based on a temperature of 25℃, for every 1℃increase in battery temperature, the life is reduced by about 100 cycles. Research shows that with the increase in ambient temperature, the performance degradation of the cathode material is an important cause of full battery capacity fading.
3. SOC Range
   • As SOC increases, battery capacity gradually increases. As shown in Figure 2, the cycle capacity fading in the 75 – 100% SOC range is about twice as fast as in the 0 – 25% SOC range. Therefore, it is recommended to use energy-storage systems at low SOC levels. The main reasons for the rapid capacity fading during high-SOC charging and discharging are as follows: first, high SOC increases the risk of lithium plating; second, the structural stability of lithium iron phosphate materials at high SOC is poor, leading to the dissolution of Fe. Fe ions can exacerbate the decomposition of the SEI film, triggering capacity fading.
4. Pre-tension

Both excessive and insufficient pre-tension in energy-storage systems can reduce the life cycle. Insufficient pre-tension can lead to poor contact at the electrode interfaces due to battery expansion during cycling, causing lithium plating at the anode. Excessive pre-tension can affect the diffusion rate of lithium ions through the separator and electrodes, leading to extensive lithium plating at the anode and rapid capacity loss. As the battery cycles, pre-tension gradually increases due to internal side reactions and expansion of the anode material. The initial pre-tension for battery assembly is currently recommended to be around 300 kgf, and it can increase by about 10 times at the end of the cycle life (SOH = 60%).

III. Conclusion and Outlook

This paper focuses on analyzing the key factors and optimal operating conditions for extending the life of energy – storage systems and summarizes the aging mechanisms of batteries under different operating conditions, providing technical support for the development and application of long – life energy – storage systems. To improve the profitability of energy-storage systems, the next key challenge is to study the coupling mechanism between calendar life and cycle life and to establish an accurate life-prediction model for complex operating conditions.