Introduction
Lithium-ion batteries (LIBs) are pivotal in advancing energy storage systems, particularly for grid-scale applications. Among cathode materials, lithium iron phosphate (LiFePO44) has dominated due to its stability and safety. However, its low voltage plateau (~3.4 V) limits energy density. Lithium manganese iron phosphate (LiMnxxFe1−x1−xPO44) emerges as a promising alternative, offering a higher voltage platform (~4.1 V for Mn2+/3+2+/3+) and improved energy density. Despite these advantages, challenges such as Mn dissolution, polarization, and reduced cycle life hinder its adoption in energy storage battery. This study systematically evaluates LiMnxxFe1−x1−xPO44 (x = 0.4, 0.5, 0.6) against LiFePO44 under grid energy storage conditions, focusing on energy efficiency, capacity retention, and degradation mechanisms.
Material Synthesis and Characterization
LiMnxxFe1−x1−xPO44 was synthesized via a high-temperature solid-state method. Precursors included Li22CO33, MnC22O44·2H22O, FeC22O44·2H22O, and NH44H22PO44, with glucose (14 wt%) as a carbon source. The mixtures were ball-milled (350 rpm, 6 h) and sintered at 760°C under N22 for 8 h. Commercial LiFePO44 served as the baseline.
Structural and Morphological Analysis
XRD confirmed the olivine structure (Pnmb space group) for all samples. Increasing Mn content expanded lattice parameters due to Mn2+2+ (0.081 nm) replacing Fe2+2+ (0.075 nm):a=10.34 A˚, b=6.01 A˚, c=4.70 A˚ (x = 0.4)a=10.34A˚,b=6.01A˚,c=4.70A˚(x = 0.4)
SEM revealed nano-sized primary particles (<500 nm) with uniform Fe/Mn distribution (EDS mapping). Carbon coating thickness increased with Mn content (17 nm for x = 0.6 vs. 9 nm for LiFePO44), enhancing electronic conductivity.
Electrochemical Performance in Energy Storage Battery
Rate Capability and Energy Efficiency
Full-cell tests (2.7 Ah soft-pack batteries) under constant power (CP) modes highlighted critical trade-offs:
Table 1: Rate Performance at 25°C (0.50 P Reference)
| Mn/Fe Ratio (x) | Charge Capacity Retention (2.00 P) | Discharge Capacity Retention (2.00 P) | Energy Efficiency (%) |
|---|---|---|---|
| 0.4 | 86.0% | 95.2% | 95.3 |
| 0.5 | 83.7% | 94.8% | 95.1 |
| 0.6 | 80.5% | 94.5% | 94.8 |
| LiFePO44 | 92.4% | 96.5% | 96.2 |
Energy efficiency (ηη) was calculated as:η=EdischargeEcharge×100%η=EchargeEdischarge×100%
Higher Mn content degraded ηη due to sluggish Mn2+/3+2+/3+ kinetics and increased polarization.
Direct Current Resistance (DCR)
DCR measurements revealed Mn-dependent impedance rise:
Table 2: DCR at 50% SOC
| Mn/Fe Ratio (x) | Charge DCR (mΩ) | Discharge DCR (mΩ) |
|---|---|---|
| 0.4 | 45.2 | 38.7 |
| 0.6 | 58.9 | 49.3 |
| LiFePO44 | 32.1 | 28.5 |
The Mn2+/3+2+/3+ redox reaction exhibited slower kinetics than Fe2+/3+2+/3+, contributing to higher DCR.
Long-Term Cycle Stability
Room-Temperature Cycling (25°C, 0.50 P)
After 190 cycles, LiMnxxFe1−x1−xPO44 showed capacity fading proportional to Mn content:
Table 3: Capacity Retention at 25°C
| Mn/Fe Ratio (x) | Initial Capacity (Ah) | Capacity Retention (%) |
|---|---|---|
| 0.4 | 2.65 | 93.2 |
| 0.6 | 2.63 | 89.8 |
| LiFePO44 | 2.58 | 96.5 |
High-Temperature Cycling (45°C, 1.00 P)
After 950 cycles, Mn dissolution exacerbated degradation:
Table 4: High-Temperature Performance
| Material | Initial Capacity (Ah) | Capacity Retention (%) | Mn Deposition (wt%) |
|---|---|---|---|
| LiMn0.60.6Fe0.40.4PO44 | 2.67 | 86.6 | 0.05455 |
| LiFePO44 | 2.61 | 88.3 | 0.00121 |
Mn3+3+ Jahn-Teller distortion and disproportionation (Mn3+3+ → Mn2+2+ + Mn4+4+) accelerated electrolyte decomposition, depleting active Li++.
Degradation Mechanisms in Energy Storage Battery
Post-mortem analysis identified Mn/Fe deposition on graphite anodes (Figure 10-11 in original text). Key reactions include:
- Mn Dissolution:
Mn3+→Mn2++Mn4+ (disproportionation)Mn3+→Mn2++Mn4+(disproportionation)Mn2++2e−→Mn0 (deposition)Mn2++2e−→Mn0(deposition)
- Electrolyte Decomposition:
LiPF6+H2O→LiF+POF3+2HFLiPF6+H2O→LiF+POF3+2HF
HF further corrodes Mn/Fe, forming resistive SEI layers.
Strategies for Optimizing Energy Storage Battery
To mitigate Mn-related issues:
- Surface Coating: Apply Al22O33 or Li33PO44 to suppress Mn dissolution.
- Doping: Mg or Zr doping stabilizes the Mn3+3+ redox couple.
- Particle Size Control: Sub-100 nm particles shorten Li++ diffusion paths.
Conclusion
LiMnxxFe1−x1−xPO44 offers higher voltage and energy density than LiFePO44, making it a candidate for energy storage battery. However, increasing Mn content degrades energy efficiency (94.8–95.3% vs. 96.2% for LiFePO44) and cycle life (86.6% retention after 950 cycles). Mn dissolution and polarization remain critical challenges. Future work should focus on surface engineering and dopant integration to enhance stability. For grid-scale energy storage battery, LiMnxxFe1−x1−xPO44 requires further optimization to match LiFePO44’s reliability.