The accelerating global transition towards electric mobility and grid-scale energy storage has precipitated a surge in the production and deployment of lithium-ion batteries (LIBs). Among various cathode chemistries, lithium iron phosphate (LiFePO4 or LFP) batteries have garnered dominant market share in specific sectors, notably electric vehicles and stationary storage, due to their compelling advantages: exceptional thermal and chemical stability, long cycle life, and relatively lower cost stemming from the abundance of iron and phosphate. Consequently, a vast wave of end-of-life LiFePO4 batteries is anticipated to enter the waste stream in the coming years. The efficient recycling of these spent batteries is no longer merely an economic consideration but a strategic imperative. It addresses critical concerns regarding the sustainable supply of critical raw materials, notably lithium—a metal pivotal to the clean energy transition—and mitigates potential environmental hazards posed by improper disposal of battery components such as fluorinated electrolytes.
Conventional recycling paradigms for spent LiFePO4 batteries often face significant technical and economic hurdles. Direct hydrometallurgical processes, which involve acid leaching of the whole black mass, typically consume large quantities of reagents, generate substantial acidic waste streams requiring neutralization, and necessitate complex subsequent separation steps to isolate lithium from the concurrent dissolution of iron and phosphorus. Pyrometallurgical smelting, while robust, is energy-intensive, often leads to the loss of lithium into slag phases, and produces iron alloys that require further processing. Therefore, developing a targeted, efficient, and environmentally benign method for the selective extraction of lithium from spent LiFePO4 cathode materials is of paramount importance. This study investigates and optimizes an ammonium sulfate ((NH4)2SO4)-assisted oxidation roasting process, followed by simple water leaching, as a novel route to preferentially recover lithium, leaving iron and phosphorus in a solid residue amenable for alternative recovery pathways.
1. Materials and Experimental Methodology
1.1. Feedstock Characterization
The feedstock for this study was a black mass powder obtained from the mechanical pre-treatment (crushing, sieving, and separation) of spent LiFePO4 battery cells, supplied by a recycling enterprise. The primary active material was LiFePO4, accompanied by residual conductive carbon (e.g., carbon black), binder (e.g., PVDF), and traces of current collector fragments (Al). The elemental composition of the received powder was quantitatively determined, and the results are summarized in Table 1. The presence of ~4.1 wt.% Li confirms the high value of this waste stream, while elements like F, Al, and others are typical impurities originating from the electrolyte (LiPF6 salts) and cell hardware.
| Element | Content (wt.%) |
|---|---|
| Fe | 33.30 |
| P | 18.63 |
| Li | 4.12 |
| C | 4.11 |
| F | 1.18 |
| Al | 0.17 |
| Others (Na, Pb, Ca, K) | ~0.20 |
1.2. Principle and Experimental Procedure
The core principle of the proposed method is a solid-state reaction between LiFePO4, (NH4)2SO4, and oxygen from air during roasting. The intended overall reaction can be represented as:
$$4\text{LiFePO}_4 + \text{O}_2 + 2(\text{NH}_4)_2\text{SO}_4 \rightarrow 4\text{FePO}_4 + 2\text{Li}_2\text{SO}_4 + 4\text{NH}_3 \uparrow + 2\text{H}_2\text{O} \uparrow$$
This reaction is designed to selectively convert lithium into water-soluble lithium sulfate (Li2SO4) while transforming iron into insoluble iron(III) phosphate (FePO4). The ammonia gas is released, leaving no nitrogenous residue in the calcine. The subsequent water leaching step then selectively dissolves Li2SO4, achieving lithium separation.
The experimental procedure was systematic. Precisely weighed amounts of the spent LiFePO4 powder and analytical grade (NH4)2SO4 were mixed according to predefined molar ratios (n((NH4)2SO4)/n(LiFePO4), termed the ‘material ratio’). The mixture was manually ground in an agate mortar to ensure homogeneity and then placed in an alumina crucible. The crucible was transferred to a preheated muffle furnace. The roasting was conducted under static air at varying temperatures (250–600 °C) and durations (0.5–2.5 h), with a controlled heating rate of 15 °C/min. After roasting, the calcine was allowed to cool inside the furnace to ambient temperature, then weighed to check for mass loss.
The roasted product (calcine) was subjected to water leaching to extract lithium. The leaching was performed at 65 °C for 1.0 hour with constant stirring, using deionized water at a fixed liquid-to-solid ratio. After leaching, the slurry was filtered. The filtrate was collected for lithium concentration analysis via Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), and the filter cake was dried for further analysis. The lithium leaching efficiency (ηLi) was calculated using the following formula:
$$\eta_{Li} (\%) = \frac{C_{Li} \times V}{m_0 \times w_{Li}} \times 100\%$$
where \(C_{Li}\) is the lithium concentration in the leachate (g/L), \(V\) is the volume of the leachate (L), \(m_0\) is the mass of the calcine charged for leaching (g), and \(w_{Li}\) is the mass fraction of lithium in the calcine, back-calculated from the initial feedstock composition and roasting mass change.
To elucidate the reaction mechanisms and phase transformations, the raw material and selected calcine products were characterized by X-ray Diffraction (XRD, Cu Kα radiation) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS).
2. Results and Discussion: Optimization of Process Parameters
2.1. Influence of Roasting Temperature
Roasting temperature is a critical parameter governing the kinetics and thermodynamics of the solid-state reaction. Its effect on lithium recovery was investigated by fixing the material ratio at 0.5 and the roasting time at 1.0 hour. The results, detailed in Table 2, show a clear trend.
| Roasting Temperature (°C) | Lithium Leaching Efficiency (%) | Observations |
|---|---|---|
| 250 | 35.42 | Incomplete reaction, low conversion. |
| 300 | 52.18 | Moderate improvement. |
| 400 | 70.13 | Significant enhancement, optimal point under these conditions. |
| 500 | 45.67 | Sharp decline in efficiency. |
| 600 | 19.63 | Very low recovery. |
The leaching efficiency increased from 35.42% at 250°C to a maximum of 70.13% at 400°C. This increase is attributed to enhanced diffusion rates and reaction activity at elevated temperatures, promoting the sulfation of lithium. However, further temperature increases above 400°C proved detrimental. This is primarily due to the thermal instability of (NH4)2SO4, which begins to decompose before it can effectively react with the LiFePO4 cathode material. At temperatures exceeding 500°C, (NH4)2SO4 undergoes significant auto-decomposition:
$$3(\text{NH}_4)_2\text{SO}_4 \xrightarrow{\text{High T}} 4\text{NH}_3 \uparrow + 3\text{SO}_2 \uparrow + 6\text{H}_2\text{O} \uparrow + \text{N}_2 \uparrow$$
This premature decomposition drastically reduces the availability of the sulfating agent, leading to incomplete lithium conversion and consequently low water leaching efficiency. Therefore, 400°C was identified as the optimal roasting temperature for balancing reaction completeness and avoiding reagent loss, forming a crucial foundation for the recycling of LiFePO4 battery materials.
2.2. Influence of (NH4)2SO4 to LiFePO4 Material Ratio
The stoichiometry of the reagent is paramount for maximizing lithium extraction. Using the optimal temperature of 400°C and a fixed time of 1.0 h, the material ratio was varied. The results are consolidated in Table 3.
| Material Ratio (mol/mol) | Lithium Leaching Efficiency (%) | Mechanistic Implication |
|---|---|---|
| 0.25 | 45.15 | Severe deficiency of sulfating agent. |
| 0.50 | 70.13 | Sub-stoichiometric, reaction limited. |
| 0.75 | 82.74 | Near-optimal, sufficient reagent for main reaction. |
| 1.00 | 78.21 | Slight decrease, possible particle coating/blocking. |
| 1.25 | 72.89 | Excess reagent leading to negative effects. |
The efficiency increased steadily from 45.15% at a ratio of 0.25 to a peak of 82.74% at a ratio of 0.75. The increase is straightforward: a higher concentration of (NH4)2SO4 provides more sulfating species, driving the desired reaction forward. The theoretical stoichiometric ratio from the main reaction equation is 0.5. The fact that the best result was achieved at 0.75 suggests that a slight excess is beneficial, likely compensating for minor side reactions or incomplete gas-solid contact. However, when the ratio exceeded 0.75, a decline in efficiency was observed. This can be explained by the physical behavior of (NH4)2SO4, which melts at ~280°C. An excessive amount can form a molten salt layer that coats the LiFePO4 particles, potentially hindering the diffusion of oxygen (O2)—a crucial reactant in the oxidation step of the process—to the particle surface. This mass transfer limitation inhibits the overall oxidation-sulfation reaction sequence. Thus, a material ratio of 0.75 was deemed optimal for the selective recovery of lithium from waste LiFePO4 battery cathodes.
2.3. Influence of Roasting Time
Holding the temperature at 400°C and the material ratio at 0.75, the duration of isothermal roasting was investigated. The findings are summarized in Table 4.
| Roasting Time (h) | Lithium Leaching Efficiency (%) | Interpretation |
| 0.5 | 75.33 | Reaction proceeding but not complete. |
| 1.0 | 82.74 | Maximum efficiency, reaction essentially complete. |
| 1.5 | 82.10 | Plateau, no significant improvement. |
| 2.0 | 81.87 | Stable, prolonged heating unnecessary. |
| 2.5 | 81.95 | Confirmation of the plateau. |
The data indicates that the reaction proceeds rapidly, with 75% efficiency achieved within just 30 minutes. Extending the time to 1.0 hour allowed the reaction to reach near-completion, yielding the maximum leaching efficiency of 82.74%. Further extension of roasting time from 1.0 to 2.5 hours resulted in a stable plateau with no statistically significant improvement in lithium recovery. This demonstrates that under the optimal temperature and reagent ratio conditions, the sulfation reaction for lithium extraction from spent LiFePO4 battery material is kinetically favorable and reaches equilibrium within one hour. Prolonged roasting is therefore economically and energetically unjustified.
3. Mechanistic Insights from Phase and Microstructural Analysis
3.1. Phase Transformation Analysis (XRD)
XRD analysis of the calcine produced under optimal conditions (400°C, Ratio=0.75, Time=1.0 h) provides direct evidence of the chemical transformations. The diffractogram showed the disappearance of characteristic LiFePO4 peaks and the simultaneous emergence of new phases. The primary phases identified were lithium sulfate (Li2SO4, PDF#00-027-0736) and iron(III) phosphate (FePO4, PDF#01-070-1202), confirming the success of the intended main reaction pathway for lithium extraction from the LiFePO4 battery cathode.
However, the presence of other phases, most notably lithium-iron phosphate (Li3Fe2(PO4)3), was also detected. The formation of such ternary or complex phases is a competing reaction pathway during the oxidative roasting process, likely proceeding via a reaction such as:
$$3\text{LiFePO}_4 + \frac{1}{2}\text{O}_2 \rightarrow \text{Li}_3\text{Fe}_2(\text{PO}_4)_3 + \text{FeO} \text{ (or other Fe-O intermediates)}$$
This side reaction effectively “traps” lithium in an insoluble olivine or NASICON-type structure, rendering it inaccessible to the subsequent water leach. The formation of Li3Fe2(PO4)3 and similar refractory phases is the primary scientific reason the observed maximum lithium leaching efficiency was capped at approximately 83%, rather than approaching 100%. It represents a key limitation and an area for further process refinement in the recycling of LiFePO4 batteries.
3.2. Elemental Distribution and Residue Purity (SEM-EDS)
SEM images of the optimal calcine revealed a heterogeneous microstructure with agglomerated particles of varying sizes, which is typical for solid-state reaction products. Crucially, EDS point analysis and elemental mapping were performed on multiple regions of the calcine. The spectra consistently showed strong signals for Fe, P, O, and S (the latter from Li2SO4), along with trace carbon. Most significantly, no nitrogen (N) signal was detected above the background noise level. This observation provides definitive experimental proof that the ammonium ions from (NH4)2SO4 were completely decomposed and volatilized as ammonia (NH3) gas during the roasting stage, as per the reaction mechanism. Consequently, the process does not introduce nitrogenous impurities into the final solid residue (primarily FePO4), which is a major advantage for its potential subsequent use or disposal. The clean separation of the reagent by-product underscores the environmental benignity of this ammonium sulfate-assisted route for processing spent LiFePO4 battery materials.
4. Conclusions and Perspective
This comprehensive study demonstrates that ammonium sulfate-assisted oxidation roasting followed by water leaching is a highly effective and selective method for recovering lithium from spent LiFePO4 battery cathode material. Through systematic parameter optimization, the ideal process conditions were established as a roasting temperature of 400 °C, an (NH4)2SO4 to LiFePO4 molar ratio of 0.75, and a roasting duration of 1.0 hour. Under these conditions, a lithium leaching efficiency of 82.74% was achieved.
The mechanism involves the in-situ sulfation of lithium to form water-soluble Li2SO4 and concurrent oxidation/transformation of iron to insoluble FePO4. Analytical characterization confirmed the desired phase formation and, importantly, verified the complete removal of ammonium as gaseous NH3, leaving no nitrogen contamination in the residue. The primary factor limiting complete lithium extraction was identified as the competing formation of insoluble lithium-iron phosphate phases (e.g., Li3Fe2(PO4)3) during roasting.
This process offers distinct advantages for the recycling of LiFePO4 batteries: it is selective for lithium, uses a low-cost and widely available reagent, operates at a moderate temperature, avoids strong acids in the primary leaching step, and generates a nitrogen-free iron phosphate residue. Future work should focus on strategies to suppress the formation of refractory Li-Fe-P-O phases, perhaps through the use of additives or more controlled atmospheric conditions, to push lithium recovery efficiencies closer to quantitative levels. Furthermore, integrating this selective lithium extraction step with efficient recovery processes for the FePO4 residue will be essential for achieving a true closed-loop recycling solution for the entire LiFePO4 battery system.