Microscopic Differences Among Ammonium, Sodium, and Iron Methods in the Preparation of Iron Phosphate

The mainstream synthesis processes for iron phosphate include the ammonium method, sodium method, and iron method. Although all three iron phosphate synthesis processes are based on co-precipitation, there are fundamental differences in the precipitation crystallization and growth mechanisms due to variations in raw material systems and microscopic synthesis environments. These differences lead to significant distinctions in the microscopic morphology, physical properties, and electrochemical performance of the resulting products. The primary particle morphology (spherical, flake-shaped, irregular) and secondary particle packing structure (dense, porous) of the precursor directly determine the tap density and electrochemical performance of lithium iron phosphate.

I. Precipitation Mechanism of Ammonium Method Iron Phosphate

The ammonia method for producing iron phosphate primarily uses monoammonium phosphate and ferrous sulfate as raw materials, with ammonia water serving as the precipitating agent and phosphoric acid as the pH regulator. When ammonia water is added to an acidic solution containing Fe³⁺ and PO₄³⁻, it reacts with H⁺ ions to form ammonium ions (NH₄⁺). The buffer system constituted by the ammonia-ammonium ion pair (NH₃/NH₄⁺) maintains the reaction solution's pH within a stable range. Additionally, NH₃ in the solution preferentially coordinates with Fe³⁺ to form soluble [Fe(NH₃)n]³⁺ complexes. This complexation reduces the instantaneous concentration of free Fe³⁺ ions, preventing explosive nucleation. When the slowly released Fe³⁺ ions encounter PO₄³⁻ ions and their concentration product exceeds the solubility product of iron phosphate, uniform and evenly distributed crystal nuclei gradually form.

Under stable supersaturation, the crystal growth rates on different facets of ferric phosphate dihydrate vary. In the ammonia method synthesis environment, crystals typically exhibit preferential growth along the (010) crystal plane. Consequently, the primary crystallites of ammonium method ferric phosphate dihydrate often appear as two-dimensional nanoplates. Driven by the Ostwald ripening effect, smaller crystallites with high surface energy in the solution gradually dissolve and redeposit onto larger plates. This promotes an increase in nanoplate thickness, boundary regularization, and continuous improvement in product crystallinity. Furthermore, to reduce the total system energy and minimize steric hindrance, the newly formed primary nanoplates, due to their high surface energy, spontaneously assemble into more stable, micrometer-sized, dense spherical secondary particles via non-covalent interactions such as electrostatic forces, van der Waals forces, or hydrogen bonding, through face-to-face stacking or curling. This structure provides a foundation for high tap density and excellent electrochemical performance of the material. The optimization of the ammonium method process primarily involves fine-tuning parameters such as reaction temperature, ammonia concentration, feeding rate, aging temperature, and time. This allows for precise control over the crystallization rate and morphology formation pathway of ferric phosphate dihydrate to achieve the targeted physical properties.

II. Precipitation Mechanism of Sodium Method Iron Phosphate

Compared to the ammonium method for producing iron phosphate, the crystallization process of the sodium method is considerably faster. When a strong alkali NaOH solution is added to an acidic reaction mixture containing Fe³⁺ and PO₄³⁻ ions, the local pH of the reaction system rapidly increases. This results in extremely high instantaneous supersaturation, causing Fe³⁺ and PO₄³⁻ ions to combine quickly. Within a short period, a massive number of amorphous iron phosphate nano-scale crystal nuclei are generated explosively in the solution.During the initial precipitation stage, the nucleation rate far exceeds the crystal growth rate. The large quantity of micro-nuclei possesses excessively high surface energy, placing them in a thermodynamically unstable state. To lower the total energy of the system, these primary micro-crystallites spontaneously and rapidly attract each other through van der Waals forces, hydrogen bonding, or electrostatic interactions, forming spherical or spherical-shell-like secondary aggregates. Simultaneously, newly formed, smaller particles in the solution continuously dissolve and redeposit onto these aggregates, causing them to grow larger.

To prepare ideal secondary spherical or quasi-spherical particles composed of uniform primary nano-particles with appropriate porosity, it is essential to stably control the alkali concentration and pH value of the reaction system. These factors directly influence the nucleation rate and quantity, ultimately determining the original size of the primary particles. Low reactant concentration and slow feed rates favor the formation of small and uniform nuclei, making it easier to achieve a loose and porous spherical structure. In contrast, high concentration and fast feed rates tend to produce large and dense secondary particles. The mixing of the iron source and phosphorus source in the alkali solution must remain homogeneous. Any local variation in concentration or pH can lead to regional explosive nucleation, resulting in fine crystals that are difficult to grow or assemble into ideal spherical shapes. This often leads to excessively fine product slurries and low tap density.Such a precipitation mechanism inevitably leaves room for Na⁺ inclusion and residual contamination. The trapped Na⁺ ions can only be removed through extremely slow ion diffusion, resulting in very low washing efficiency and often incomplete purification. Therefore, one of the core challenges and major cost factors in optimizing the sodium method process is the complex, multi-stage washing process requiring large volumes of water. Moreover, it is fundamentally difficult to reduce impurity levels to the standards achieved by the ammonium method product. This explains, from a chemical principle perspective, the differences in cost-effectiveness and market share between the iron phosphate products obtained by these two methods.

III. Precipitation Mechanism of Iron Method Iron Phosphate

The precipitation and crystallization principle of the iron-method iron phosphate differs from both methods mentioned above. It is a seemingly simple but actually quite complex multi-step coupled oxidation-precipitation process. First, iron powder reacts with phosphoric acid to form a soluble ferrous dihydrogen phosphate (Fe(H₂PO₄)₂) solution. When a certain concentration of hydrogen peroxide is introduced into the ferrous dihydrogen phosphate solution, Fe²⁺ is oxidized to Fe³⁺. The Fe³⁺ then combines with phosphate ions in the solution to form microcrystalline particles.After hydrogen peroxide is introduced into the reaction system, Fe²⁺ in the system is rapidly oxidized to Fe³⁺. However, since the reactant being oxidized is Fe(H₂PO₄)₂, the forms of phosphate polyanions present in the system are not uniform. The actual combination process of Fe³⁺ and PO₄³⁻ is likely an ionization equilibrium process gradually driven by the precipitation reaction. The rapid formation, disordered stacking, and growth of crystal nuclei occur within a dynamic interplay of growth and dissolution.The primary microcrystallites of iron-method iron phosphate tend to grow along certain crystal planes, forming irregular plate-like or flake-like structures. To achieve a more stable form, these plate-like or flake-like primary particles typically stack face-to-face, forming loose secondary agglomerates with blocky or flower-cluster-like structures. Consequently, although traditional iron-method iron phosphate products have high purity, they suffer from drawbacks such as low tap density and poor batch-to-batch consistency. This inherent characteristic is "inherited" in lithium iron phosphate, manifesting as low product compaction density and difficulty in ensuring consistent batch quality.

Early-stage iron-based lithium iron phosphate production had limited capacity and was primarily used in small-scale applications for mid- to low-end lithium iron phosphate materials. After 2024, driven by market demand for cost-effective and environmentally friendly lithium iron phosphate production processes, the iron-based method began gaining favor due to its inherent near "zero-waste emission" advantage. This has led to comprehensive technological upgrades, gradually overcoming the morphological limitations of traditional processes. For instance, by adopting continuous crystallization optimization, dense quasi-spherical secondary agglomerates formed from stacked primary layered particles can now achieve a product tapped density exceeding 1.0 g/cm³. Lithium iron phosphate materials produced by leading iron-based method manufacturers using such optimized iron phosphate have reached third-generation plus or even higher performance levels.Thus, the differences in the morphological characteristics and physical properties of lithium iron phosphate fundamentally stem from variations in their micro-precipitation and crystallization environments. However, the evolution of any technological pathway is never merely an isolated iteration. The development and refinement of each new material or process result from adapting to the diverse and ever-changing market demands. There has never been a permanently optimal synthesis method. In this dynamic and rapidly evolving competitive landscape, only enterprises that deeply understand downstream needs and continuously innovate their processes accordingly can truly achieve lasting competitiveness.