What Is Battery reaction mechanism?

What Is Battery reaction mechanism?

Currently, there is no accurate and consistent understanding of the electrochemical reaction mechanism of LiFePO₄ in the industry. The application of the composite anion (PO₄)³⁻ makes iron-based compounds an ideal candidate material for lithium-ion battery cathodes. However, the crystal structure of LiFePO₄ limits its conductivity and lithium-ion diffusion performance, resulting in a decrease in the material's electrochemical performance. Unlike layered materials, the charge-discharge curve of LiMPO₄ typically has a very flat plateau, which is a typical characteristic of two-phase reactions, meaning that a phase transition process between LiMPO₄ and MPO₄ occurs during lithium-ion intercalation/deintercalation.

LiFePO4 undergoes a two-phase reaction mechanism during charging and discharging in a battery, i.e.

Reaction mechanism model

During charging, Li⁺ migrates from the FeO₆ layer, passes through the electrolyte, and enters the negative electrode. Fe²⁺ is oxidized to Fe³⁺, while electrons travel from the external circuit to the negative electrode via the contacting conductive agent and current collector. The discharge process is the reverse.

To describe this two-phase behavior, Padhi and Goodenough et al. first proposed the "core-shell model," which posits that the lithium-ion intercalation/deintercalation process occurs at the LiFePO₄/FePO₄ two-phase interface, as shown in Figure 4-3a.

During charging, the LiFePO₄/FePO₄ interface continuously moves from the surface towards the center, pushing towards the core. Li⁺ continuously migrates outward, and the outer LiFePO₄ continuously transforms into FePO₄. Lithium ions and electrons continuously pass through the newly formed two-phase interface to maintain an effective current, but the lithium-ion diffusion rate is constant under certain conditions. As the interface between the two phases shrinks, the diffusion of lithium ions will eventually be insufficient to maintain an effective current. The LiFePO₄ in the particle core will not be fully utilized, resulting in capacity loss. After charging is complete, unused LiFePO₄ will remain in the center of the particle.

Considering that lithium ions can simultaneously intercalate and deintercalate at multiple sites, Andersson et al. proposed the mosaic model to explain the initial capacity loss, as shown in Figure 4-3b. The mosaic model posits that although the lithium ion intercalation and deintercalation process is at the LiFePO₄/FePO₄ two-phase interface, the process can occur at any location within the particle. During charging, the FePO₄ region enlarges at different points on the particle, and the edges of these regions cross-contact, creating many unreactable dead zones, thus causing capacity loss. During discharging, the reverse reaction occurs, with lithium ions intercalating into the FePO₄ phase. The portion at the core where lithium ions are not intercalated results in capacity loss.

Figure 4-3 Lithium-ion intercalation/deintercalation model of lithium iron phosphate battery

Two theoretical models were developed simultaneously, but the core-shell model is more widely accepted by researchers, although the specific materials of the shell and core remain controversial. Based on these two models, it can be concluded that the diffusion kinetics of lithium ions and charge are the decisive factors for the practical application of the entire electrode material. In the preparation of lithium iron phosphate cathode materials, efforts are made to obtain particles with small and uniform particle size (nanoscale or microporous), using carbon coating (nanocarbon film) and ion doping to improve conductivity and lithium ion diffusion.

With a deeper understanding of LiMPO materials, it was found that these two models neglected the highly anisotropic characteristics of lithium ion transport in LiMPO materials. Laffont proposed a "New Core-shell Model" to correct the shortcomings of the "core-shell model." Building on this, Delmas studied LiFePO particles in different depletion states and proposed a "Domino-cascade Model," which effectively explains the fast charge and discharge performance of nanoscale particles, as shown in Figure 4-4.

Despite the significant differences between the aforementioned models, the core issue lies in the prediction and characterization of the two-phase interface. Since the kinetics of lithium insertion/extraction and the phase transition are highly dependent on the particle size, morphology, and physicochemical properties of the material, the above discussions (including conflicts between models) may be due to insufficient experimental conditions.

With the development of microscopy and spectroscopy, solid solution reactions and intermediate phases have been observed and detected during the phase transition of LiMPO4 materials, indicating that another phase transition mechanism may exist in LiMPO4 materials. In typical solid solution reactions, the cell parameters and cell volume exhibit continuous changes during phase transitions. Through some extreme testing conditions and characterization methods, such as ultra-small particles (nanoscale) and high-rate charge-discharge (above 10C), solid solution reactions and the existence of intermediate phases have been observed in LiMPO4.

Phase transitions during charge-discharge processes at room temperature. Lithium-ion batteries exhibit good reversibility during charge-discharge cycles, which is related to the structural similarity between the phase states after lithium-ion deintercalation/intercalation. During charge-discharge processes, the capacity decay of the battery is closely related to the phase transition kinetics. According to the structure of LiFePO4, the [100]pmnb direction is most favorable for lithium-ion migration, and the interface between the two phases moves along the c-axis during charge-discharge processes.

(1) LiFePO₄/FePO₄ The ratio of LiFePO₄/FePO₄ changes continuously with the battery charge-discharge reaction (the value of x in LiₓFePO₄ changes continuously). As lithium ions are extracted, the intensity of the diffraction peak produced by LiFePO₄ gradually decreases. When δ>0.2, the diffraction peak of Li₁₋δFePO₄ begins to disappear, and the intensity of the diffraction peak produced by FePO₄ gradually increases. Conversely, as lithium ions are inserted, the intensity of the diffraction peak produced by FePO₄ gradually decreases, and the intensity of the diffraction peak produced by Li₁₋δFePO₄ gradually increases.

(2) LiₓFePO₄/Li₁₋yFePO₄ LiₓFePO₄ at room temperature is a mixture of Fe³⁺/Fe²⁺ mixed-valence mesophase LiₐFePO₄/Li₁₋βFePO₄. α and β represent the carrier density and hopping probability during charging and discharging, respectively. Powder neutron diffraction revealed that the optimal values for α and β are 0.05 and 0.11, respectively. Factors such as ion doping, temperature, transition metal, particle size, and non-equilibrium states at overpotential all affect the values of α and β. Increasing the values of α and β will improve the kinetic performance of the electrode reaction during charging and discharging at room temperature.

At 450℃, a solid solution of LiₓFePO₄ exists, while at room temperature, two metastable phases exist: Li₀.₇₅FePO₄ and Li₀.₅FePO₄. Above 500℃, LiₓFePO₄ begins to decompose into non-olivine compounds; the composition and content of these phosphates or phosphides depend on the value of x. Between 400 and 500℃, only a solid solution of LiₓFePO₄ exists.

The changes during cooling are much more complex than those during heating. The composition of the mixture during cooling depends on the value of x and the thermal process. Upon cooling, LiₓFePO₄ first decomposes into a mixture of two non-olivine phases, the proportions of which depend on the initial value of temperature and x. When the temperature is below (140±20℃), the two-phase system becomes a more complex system, in which LiFePO₄ and FePO₄ coexist with two other olivine-type compounds, Liₓ₁FePO₄ and Liₓ₂FePO₄. Aging this mixture at room temperature causes the four-phase system to gradually transform into a two-phase system of LiFePO₄ and FePO₄.

Battery reaction mechanism

FePO₄ exists in several structures: ① After complete delithiation of LiFePO₄, orthorhombic FePO₄ is formed; ② Triclinic FePO₄ has a quartz-like structure, with all cations tetrahedral coordinated; ③ Monoclinic and orthorhombic FePO₄ can be prepared from their respective hydrates. All these crystalline forms of FePO₄, as well as amorphous FePO₄, can be transformed into triclinic FePO₄ upon heating.

The transformation from LiFePO₄ to FePO₄ is slow and incomplete, but complete when the temperature exceeds 500℃. Under battery operating conditions, the cathode material is kinetically stable. During the synthesis of LiFePO₄, it is essential to ensure the absence of FePO₄. If present, triclinic FePO₄ will be generated upon heating, resulting in a non-electrochemically active glassy phase on the material surface at high temperatures.

Ion doping can improve the conductivity of materials. P-type semiconductor conductive materials with conductivity reaching 10⁻² S/cm are obtained through ion doping. Doping is a very complex process: on the one hand, density functional theory (DFT) calculations of the electronic structure of LiFePO₄ under the local density approximation (LDA) and generalized gradient approximation (GGA) show that the material should exhibit characteristics of a metallic or semiconductor material, with a conduction band and valence band width of approximately 0.3 eV, which is inconsistent with the low conductivity actually detected. On the other hand, considering the interactions of electron orbitals and Coulomb interactions after ion doping, an improved valence band structure is theoretically feasible.

DFT calculations of Mg- or Cr-doped LiFePO₄ show that the maximum density of electronic states is located near the Fermi level, which explains the metallic conductivity of the doped material. The change in conductivity caused by ion doping may be related to the following factors:

1) The edges of the charge carrier regions are metallized.

2) Ion doping narrows the width of the valence band and conduction band.

3) Exceeding a certain critical concentration, the electron wavefunction of the dopant ions leads to the formation of a conduction band.

4) The type, concentration, and distribution of dopant ions.

5) In many M-O metal oxides, a metal conduction band appears when the M-M bond distance is less than 3 × 10⁻¹⁰ m.

6) During synthesis, the addition of organic carbon causes carbon coating of the material, creating an effective conduction path.

7) The appearance of Fe₂P. During synthesis, the addition of excess carbon reduces the phosphate.

Ion doping and conductivity

8) The Fe³⁺/Fe²⁺ redox pair acts as a catalyst in the reduction of LiFePO₄.

LiFePO₄ exhibits reactivity with commonly used electrolytes. The electrochemical behavior of the material is highly correlated with its surface chemistry within the electrolyte. Generally, a passivation film forms on the material's surface. This film facilitates lithium-ion diffusion, prevents loss of active material, and must withstand the volume and surface changes during lithium-ion insertion/extraction. Carbon-coated LiFePO₄ surface films contain compounds such as LiF, LiPF₆, LiₓFᵧ⁻, and LiₓPOᵧFᶻ⁻.

Common electrolytes typically contain alkyl carbonates and lithium salts. The cathode material undergoes many possible reactions in the electrolyte. For example, in LiPF₆ solutions, the acid-base reaction between LiFePO₄ and trace amounts of HF is unavoidable. The presence of HF in the electrolyte has two detrimental effects: firstly, the substitution reaction between iron ions and protons; and secondly, the reaction of Li ions and F ions on the particle surface to form LiF, which hinders Li⁺ diffusion.

Iron ions dissolve in electrolytes. Tests on the iron ion dissolution of LiFePO₄ in different electrolytes revealed the following:

1) In electrolytes free of acidic contaminants, even at elevated temperatures, the dissolution of iron ions and the resulting mass loss of the active material are negligible.

2) Higher solution acidity leads to easier iron ion dissolution.

3) Higher temperature leads to easier iron ion dissolution.

4) Higher carbon content within the material results in greater material stability.

The contact area between the active material and the binder is most susceptible to corrosion. This corrosion can be avoided by using an alkaline mesophase or applying acidic scavenging additives. In lithium-ion batteries using LiFePO₄ as the cathode material, non-acidic electrolytes or carbon addition or coating of LiFePO₄ can be used to prevent mass loss.

The kinetic characteristics of LiFePO₄ cathode materials are not yet fully understood. It is generally believed that particle size and distribution, conductivity, ion diffusion, kinetics during phase transitions (charge-discharge process), and carbon coating/doping all affect the battery's performance at different charge-discharge rates. Uniform carbon doping means that lithium ions and electrons can be inserted and extracted at the same location in the active material, reducing electrode polarization.

(1) Influence of Conductivity on Capacity The low conductivity of pure LiFePO₄ directly leads to a decrease in the high-rate discharge capacity of the battery. The conductivity of pure LiFePO₄ is approximately 10⁻⁹ S/cm, and the discharge capacity drops sharply from 148 mA·h/g at a 0.2C discharge rate to 85 mA·h/g at a 5C discharge rate. The high-rate discharge capacity of the cathode material does not always increase with increasing conductivity. At low conductivity, an increase in conductivity improves the electrochemical kinetics of the material. When the material conductivity exceeds a certain critical value, conductivity is no longer the determining factor for the material's rate capacity. LiFe₀.₉Ni₀.₁PO₄ (1.0 × 10⁻⁷ S/cm), with its low conductivity, exhibits better high-rate discharge capacity than LiFePO₄ (4.0 × 10⁻⁶ S/cm), with discharge capacities of 90 mA·h/g and 55 mA·h/g, respectively, at a 10C discharge rate. This suggests that lithium-ion diffusion may have replaced conductivity as the decisive factor in the electrochemical properties of lithium-ion batteries.

(2) Lithium-ion Diffusion Lithium-ion diffusion is determined by both internal and external factors. External factors include particle size, distribution, and morphology. Internal factors mainly refer to the lithium-ion diffusion coefficient. The lithium-ion diffusion coefficient is a constant value; the diffusion ability of lithium ions decreases with increasing particle size because the diffusion path of lithium ions within the particle increases. The diffusion ability of lithium ions is inversely proportional to the square of the particle size and directly proportional to the lithium-ion diffusion coefficient. Particle size has a greater impact on lithium-ion diffusion than diffusion coefficient. Numerical calculation of the lithium-ion diffusion coefficient must be combined with specific measurement methods and theoretical models. The main measurement methods are galvanostatic titration (GITT) and electrochemical impedance spectroscopy (EIS or AC Impedance).

(3) Two-dimensional scale electrodes: Thin-film electrodes enhance electrode activity by increasing surface area. In thin-film electrodes, electrons enter the current collector while lithium ions enter the electrolyte from the opposite direction. With the formation of the FePO₄ layer, the resistance to electron movement decreases, while the resistance to lithium-ion movement increases. FePO₄ first nucleates at crystal defects and then grows in all directions, inhibiting lithium-ion diffusion until lithium ions cannot escape in the [100] direction.