Tailoring Redox Couples of Li-Rich Mn-Based Cathode Materials by In-Situ Surface Reconstruction

Q: 11.1 What is in-situ surface reconstruction and how does it stabilize lattice oxygen in Li-rich Mn-based cathodes?

In-situ surface reconstruction is a modification strategy that introduces near-surface Ni²⁺ and PO₄³⁻ dopants into a Li-rich Mn-based layered oxide while simultaneously forming an amorphous Li₃PO₄ coating on the particle surface. DFT calculations show this co-doping lowers the TM-3d and O-2p energy levels, raising the oxidation potential required for lattice oxygen participation and stabilizing the nonbonding O-2p states that would otherwise form reactive O–O dimers and release O₂. In this study, the strategy reduced first-cycle O₂ release from 10.02 to 6.79 µmol/g and raised 700-cycle capacity retention from 35.9% to 77.0%, directly linking the electronic-structure change to measurable lattice oxygen stabilization.

Q: 11.2 How does tuning redox couples improve cycling stability in Li-rich Mn-based cathodes?

Li-rich Mn-based cathodes rely on two coupled redox couples — conventional transition-metal (cationic) redox and anionic (lattice oxygen) redox — to deliver their high specific capacity. When the oxygen redox couple operates at too low a potential relative to electrolyte stability limits, lattice oxygen becomes overly reactive and prone to irreversible O₂ release. By using Ni²⁺/PO₄³⁻ co-doping to raise the oxygen redox potential (shown by upward-shifted cyclic voltammetry peaks: 4.028 V and 4.637 V vs. 4.006 V and 4.626 V in the pristine material), the modified cathode's oxygen redox couple becomes more thermodynamically stable and less prone to irreversible side reactions — directly translating to higher capacity retention, reduced voltage fade, and improved high-temperature stability.

Q: 11.3 How was mechanical strength of the Li-rich cathode particles measured, and why does it matter?

Mechanical strength was measured using single-particle crush force testing on the IEST SPFT2000 Single Particle Mechanical Properties Test System , which applies a controlled compressive load directly to an individual cathode particle while recording force and displacement until fracture. This measurement matters because Li-rich cathode particles accumulate internal stress from lattice oxygen redox-driven volume changes during cycling, and particles that crack expose fresh surface to electrolyte, accelerating side reactions and capacity fade. In this study, the surface-reconstructed LLO-NP@LPO particles showed an average crush force of 8.143 mN — a 58% increase over the pristine PLLO particles' 5.154 mN — directly correlating with the intact particle morphology observed by SEM/TEM after 400 cycles and the substantially improved long-term capacity retention.

Q: 11.4 Why does a Li₃PO₄ coating suppress gas evolution in Li-rich cathodes?

The amorphous Li₃PO₄ coating formed during in-situ surface reconstruction acts as a physical barrier that reduces direct contact between the reactive cathode surface and the electrolyte, suppressing electrolyte oxidative decomposition and LiPF₆ decomposition byproduct formation at the interface. Because much of the CO₂ evolution in Li-rich cathodes originates from electrolyte and carbonate species reacting at the exposed cathode surface — rather than from the cathode's own lattice oxygen alone — physically isolating this interface with a stable, ion-conductive coating directly reduces CO₂ generation. In this study, CO₂ evolution dropped from 44.74 to 21.14 µmol/g with the Li₃PO₄ coating in place, while the coating's inherent Li⁺ conductivity avoided the rate-capability penalty that a purely insulating coating would cause.

Q: 11.5 Why does capacity retention in Li-rich Mn-based cathodes degrade faster at high temperature?

Elevated temperature accelerates nearly all of the degradation pathways that already threaten Li-rich Mn-based cathodes at room temperature — lattice oxygen release, electrolyte oxidative decomposition, transition-metal migration, and CEI growth all proceed faster with increased thermal energy. In this study, pristine PLLO retained only 32.1% of its capacity after 120 cycles at 55°C and 1C — far worse than its already-limited room-temperature performance — because the reactive, unstabilized lattice oxygen and unprotected surface accelerated these parasitic reactions. The surface-reconstructed LLO-NP@LPO retained 85.9% capacity under identical high-temperature conditions, demonstrating that stabilizing lattice oxygen and protecting the cathode-electrolyte interface addresses the same underlying degradation mechanisms that are simply accelerated, not fundamentally different, at elevated temperature.

Academic Application, Single Particle Mechanics Case

Updated on 2026/07/09

Abstract

Li-rich Mn-based cathodes deliver exceptional specific capacity because they combine cationic and anionic (lattice oxygen) redox activity. However, lattice-oxygen participation at high voltages generates unstable electronic holes and reactive O²⁻ species that ultimately form O–O dimers and release O₂ — causing irreversible oxygen loss, cation vacancy formation, structural collapse, and rapid capacity/voltage fade. This study summarizes a recently reported in-situ surface reconstruction strategy that introduces near-surface Ni²⁺ and PO₄³⁻ into Li-rich layered oxides while simultaneously forming an amorphous Li₃PO₄ coating. DFT calculations and comprehensive electrochemical/structural characterization show this modification lowers TM-3d/O-2p energy levels, raises the oxygen redox potential, stabilizes lattice oxygen, suppresses gas evolution (O₂, CO₂), and markedly improves cycling, rate performance, and high-temperature stability. Key results: capacity retention increases from 35.9% to 77.0% after 700 cycles at 1C, and capacity retention at 55°C rises from 32.1% to 85.9% after 120 cycles at 1C. Mechanical robustness was directly validated using the IEST SPFT-2000 Single Particle Mechanical Properties Test System, which measured single-particle crush force increasing from 5.154 mN to 8.143 mN after surface reconstruction — direct evidence that the modified cathode particles resist stress-induced fracture more effectively during cycling.

📄 Source Paper

Xutao Zhu, Xujia Xie, Jie Lin*, Yuanyuan Liu, Guiyang Gao, Yong Yang, Yinggan Zhang*, Weicheng Xiong, Yidi Jiang, Qiyuan Li, Dongliang Peng*

Tailoring redox couples of Li-rich Mn-based cathode materials by in-situ surface reconstruction for high-performance lithium-ion batteries

DOI: 10.1016/j.nanoen.2024.110588
| Journal: Nano Energy
| Institutions: Xiamen University

✓ IEST SPFT-2000 Single Particle Mechanical Properties Test System used in this research

1. Challenges for Li-Rich Mn-Based Cathode Redox Couples and Lattice Oxygen

Li-rich Mn-based layered oxides (LLOs) exploit both transition-metal redox and lattice-oxygen redox to reach high specific capacities, making them promising candidates for next-generation high-energy batteries. However, when cycled at high cutoff voltages or elevated temperature, lattice oxygen oxidation creates electronic holes and reactive oxygen species (O²⁻ → Oⁿ⁻ / O–O dimers), which can dimerize and evolve as O₂ (bond length <1.5 Å). This irreversible oxygen loss triggers Li de-intercalation, transition-metal migration, cation vacancy clustering, and layered-to-spinel/rocksalt phase transformation — processes that accelerate capacity and voltage decay. Moreover, an unstable cathode–electrolyte interface (CEI), electrolyte oxidation, and stress accumulation further degrade particle integrity and kinetics. A unified design guideline for tuning LLO redox couples and stabilizing lattice oxygen has remained elusive.

A recent study from Xiamen University, led by Professors Peng Dongliang, Lin Jie, and Zhang Yinggan, presents an innovative solution. Inspired by high-voltage polyanionic cathodes, the team developed an “in-situ surface reconstruction” strategy involving near-surface co-doping and coating. This approach successfully tailors the redox couples and enhances the structural stability of LLOs. The research results were published in Nano Energy under the title “Tailoring Redox Couples of Li-Rich Mn-Based Cathode Materials by In-Situ Surface Reconstruction for High-Performance Lithium-Ion Batteries.”

2. Design Concept: In-Situ Surface Reconstruction to Tailor Redox Couples

The reported strategy uses near-surface co-doping (Ni²⁺ and PO₄³⁻) plus formation of an amorphous Li₃PO₄ surface layer (denoted LLO-NP@LPO). DFT calculations indicate that introducing Ni²⁺ and PO₄³⁻ shifts TM-3d/O-2p bands to lower energies: the TM-3d–O-2p center moves from 4.556 eV to 4.028 eV, while nonbonding O-2p states shift markedly (0.065 eV → −1.287 eV). This band lowering raises the oxidation potential for lattice oxygen and reduces the reactivity of nonbonding O-2p orbitals, increasing oxygen redox reversibility. Notably, the O-2p nonbonding shift (≈1.352 eV) exceeds the TM-3d–O-2p shift (≈0.528 eV), indicating greater stabilization of oxygen states and enhanced overlap between O-2p nonbonding orbitals and TM orbitals — factors that favor reversible oxygen redox.

Figure 1. (a) Schematic diagram of molecular orbital energy levels for transition metal 3d orbitals and oxygen 2p orbitals. (b) Energy level diagrams of Ni^3+/2+/Co^3+/2+ redox pairs in olivine LiMPO4 anode and ternary NCM anode. (c, e) State density maps of O 2p states and TM 3d orbitals in PLLO and LLO-NP@LPO obtained by DFT calculations. (d, f) Electronic structure maps based on the state density maps and schematic diagrams of the two crystal structures used for the calculations.

Figure 2. (a) SEM and EDS maps of LLO-NP@LPO. (b, c) XPS maps of Ni 2p and P 2p. (d) XRD Rietveld refinement of PLLO and LLO-NP@LPO. (e–g) HR-TEM images of LLO-NP@LPO and PLLO. (h–k) Enlarged HR-TEM and FFT images of surface regions. (l) Magnified near-surface region. (m) Schematic of the near-surface crystal structure after in-situ surface reconstruction.

3. Structural and Compositional Evidence of Near-Surface Reconstruction

Microscopy and spectroscopy confirm the designed near-surface changes and coating:

SEM/EDS and XPS show uniform particle morphology and surface distribution of Ni, Co, Mn, O, and P. Surface Ni²⁺ fraction rises to 42.79% in LLO-NP@LPO versus 30.11% in PLLO, decreasing toward bulk after etching (200 s) — consistent with a gradient, near-surface doping profile.
XRD (Rietveld refinement) reveals both R-3m LiTMO₂ and C2/m Li₂MnO₃ phases in both samples. The (003) peak in LLO-NP@LPO shifts to lower angle (slightly increased c-axis), and the (003)/(104) intensity ratio drops from 2.14 (PLLO) to 2.04 (LLO-NP@LPO), indicating modest Li/Ni mixing (<5%, recommended range) and structural accommodation of PO₄ groups. Li₂MnO₃ fraction decreases from 53.2% to 51.6%, reducing Li–O–Li motifs prone to irreversible oxygen loss.
HRTEM shows a ≈3.2 nm amorphous Li₃PO₄ coating and evidence of transition-metal occupation at former lithium sites in the near surface — consistent with the intended reconstruction.

Figure 3. (a-b) Cyclic voltammograms of LLO-NP@LPO and PLLO. (c-d) Cyclic dQ/dV curves and (e-h) corresponding 2D contour plots. initial charge/discharge curves at (i) 0.2C at 30 °C, (j) cyclic performance, and (k) average discharge voltage at 1C. initial charge/discharge curves at (l) 0.2C at 55 °C, (m) cyclic performance, and (n) average discharge voltage at 1C.

4. Electrochemical Performance: Improved Redox Reversibility, Cycling, and High-Temperature Stability

Electrochemical probes quantify the benefits to redox couples and lattice oxygen behavior:

Cyclic voltammetry: First-cycle oxidation peaks shift upward in LLO-NP@LPO (e.g., 4.028 V and 4.637 V vs. PLLO at 4.006 V and 4.626 V), indicating higher redox potentials for both TM and oxygen redox couples, consistent with DFT.
Long-term cycling at 1 C: After 700 cycles, capacity retention improves from 35.9% (PLLO) to 77.0% (LLO-NP@LPO). Average discharge-voltage retention rises from 68.6% to 75.1%; per-cycle voltage fade reduces from 1.58 mV to 1.27 mV.
Elevated temperature (55 °C): At 1 C and after 120 cycles, LLO-NP@LPO retains 85.9% capacity (238.7 mAh g⁻¹), whereas PLLO retains only 32.1% (90.7 mAh g⁻¹). Voltage retention at 55 °C improves from 67.9% (PLLO) to 82.3% (LLO-NP@LPO).
Rate capability: LLO-NP@LPO shows superior kinetics across multiple C-rates, attributed to enlarged interlayer spacing and the fast-ion Li₃PO₄ surface conductor.

Figure 4. (a) In situ XRD curves of the first two cycles of (a) PLLO and (b) LLO-NP@LPO and the corresponding numerical c-axis curves of charging and discharging. (c) 3D contour plots of the evolution of the (003) peak for the first two cycles of PLLO and (d) LLO-NP@LPO.

LLO-NP@LPO vs PLLO: Key Performance Data

Table 1. Summary of electrochemical, gas evolution, impedance, and mechanical data comparing pristine PLLO and surface-reconstructed LLO-NP@LPO Li-rich Mn-based cathode.
Metric	PLLO (Pristine)	LLO-NP@LPO (Reconstructed)
Capacity retention (700 cyc, 1C)	35.9%	77.0%
Capacity retention (55°C, 120 cyc, 1C)	32.1% (90.7 mAh/g)	85.9% (238.7 mAh/g)
Voltage retention (1C)	68.6%	75.1%
Voltage fade per cycle	1.58 mV	1.27 mV
Voltage retention at 55°C	67.9%	82.3%
First-cycle O₂ release (DEMS)	10.02 µmol/g	6.79 µmol/g
First-cycle CO₂ evolution (DEMS)	44.74 µmol/g	21.14 µmol/g
Initial charge-transfer resistance ($R_{ct}$)	90.27 Ω	77.87 Ω
$R_{ct}$ after 100 cycles	64.05 Ω	26.09 Ω
Single-particle crush force (SPFT2000)	5.154 mN	8.143 mN

5. Probing Gas Evolution, Impedance, and Ion Diffusion Related to Lattice Oxygen

Direct and kinetic measurements support reduced oxygen loss and better interfacial stability:

DEMS: O₂ release in the first cycle drops from 10.02 µmol g⁻¹ (PLLO) to 6.79 µmol g⁻¹ (LLO-NP@LPO); CO₂ evolution drops from 44.74 to 21.14 µmol g⁻¹, indicating suppressed electrolyte decomposition and carbonate oxidation.
EIS: Initial charge-transfer resistances (Rct) are 90.27 Ω (PLLO) and 77.87 Ω (LLO-NP@LPO). After 100 cycles, Rct decreases to 64.05 Ω (PLLO) and 26.09 Ω (LLO-NP@LPO), showing a more stable and lower-resistance electrode/electrolyte interface in the modified sample.
GITT/Diffusion: Post-100 cycles, the Li⁺ diffusion coefficient (D_Li⁺) for LLO-NP@LPO remains higher than PLLO at high states of charge where oxygen redox dominates, demonstrating improved ion transport under conditions that typically slow kinetics.

Figure 5. DEMS test during charging and discharging of (a) PLLO and (b) LLO-NP@LPO. Electrochemical impedance spectra of PLLO and LLO-NP@LPO before cycling (c) and after 100 cycling cycles (d). (e, f) GITT curves and corresponding calculated Li-ion diffusion coefficients at 0.2C after 100 cycling cycles.

Figure 6. XPS maps of (a, b) O 1s (c, d) P 2p in PLLO and LLO-NP@LPO samples.

6. Mechanical Robustness and Microstructural Preservation

Mechanical testing and microscopy reveal preserved particle integrity:

Single-particle compression tests — performed using the IEST SPFT2000 — show the average crush force increases from 5.154 mN (PLLO) to 8.143 mN (LLO-NP@LPO), indicating higher mechanical resilience and greater resistance to stress-induced fracture during repeated cycling.
After 400 cycles, SEM/TEM show PLLO exhibits particle cracking, internal lattice distortion, and partial conversion to spinel/rocksalt phases; by contrast, LLO-NP@LPO retains intact secondary particles, clearer layered lattice fringes, and a more uniform CEI.

Figure 7. (a, b) Single particle crushing test curves of PLLO and LLO-NP@LPO, measured with the IEST SPFT2000. (c, d) Optical images before/after crushing. (e, f) SEM and (g, h) TEM images after 400 cycles.

7. Mechanistic Summary: Tuning Redox Couples and Stabilizing Lattice Oxygen

In-situ surface reconstruction achieves three synergistic effects:

Electronic-structure tuning: $Ni^{2+}$ and $PO_4^{3-}$ lower TM-3d and O-2p energy levels, increasing oxygen redox potential and stabilizing nonbonding O-2p states — this makes oxygen oxidation more reversible and reduces irreversible $O_2$ release.
Interfacial protection: An amorphous Li₃PO₄ coating prevents direct electrolyte attack, suppresses LiPF₆ decomposition products at the surface, and reduces CO₂ evolution.
Kinetics and mechanics: Enlarged lattice spacing and a fast-ion surface layer enhance Li⁺ diffusion; improved near-surface composition raises mechanical strength and mitigates stress accumulation and particle collapse.

Together these mechanisms explain the observed gains in capacity retention, voltage stability and high-temperature performance.

8. Summary and Outlook

The in-situ near-surface reconstruction strategy — combining $Ni^{2+}$ and $PO_4^{3-}$ near-surface doping with an amorphous $Li_3PO_4$ coating — provides an effective route to tune redox couples and stabilize lattice oxygen in Li-rich Mn-based cathodes. The approach reduces gas evolution, lowers interfacial impedance, sustains $Li^+$ diffusion under high-voltage conditions, and strengthens particle mechanics, which together yield substantially improved cycling and thermal stability. Future work should explore optimization of doping depth, $PO_4$ coverage, and compatibility with practical electrode loadings and electrolytes, and quantify tradeoffs between increased redox potential and long-term electrode conductivity.

9. Original Paper

Zhu Xutao, Xie Xujia, Lin Jie, Liu Yuanyuan, Gao Guiyang, Yang Yong, Zhang Yinggan, Xiong Weicheng, Jiang Yidi, Li Qiyuan, Peng Dongliang*. Tailoring Redox Couples of Li-Rich Mn-Based Cathode Materials by In-Situ Surface Reconstruction for High-Performance Lithium-Ion Batteries. Nano Energy, 2024. DOI: 10.1016/j.nanoen.2024.110588

10. Related Test Equipment Recommendation

You may want to learn about our equipment: IEST Single Particle Mechanical Properties Test System (SPFT2000)

IEST Single Particle Force Properties Test System and Test schematic

Application:

Testing the crushing strength of battery material particles
Can be used to evaluate the pressure resistance of the material
Guide the rolling process
Materials with high mechanical strength will have better subsequent cycle stability — directly demonstrated in this study by the crush-force increase from 5.154 mN to 8.143 mN correlating with improved 700-cycle capacity retention

11. FAQs

11.1 What is in-situ surface reconstruction and how does it stabilize lattice oxygen in Li-rich Mn-based cathodes?

In-situ surface reconstruction is a modification strategy that introduces near-surface Ni²⁺ and PO₄³⁻ dopants into a Li-rich Mn-based layered oxide while simultaneously forming an amorphous Li₃PO₄ coating on the particle surface. DFT calculations show this co-doping lowers the TM-3d and O-2p energy levels, raising the oxidation potential required for lattice oxygen participation and stabilizing the nonbonding O-2p states that would otherwise form reactive O–O dimers and release O₂. In this study, the strategy reduced first-cycle O₂ release from 10.02 to 6.79 µmol/g and raised 700-cycle capacity retention from 35.9% to 77.0%, directly linking the electronic-structure change to measurable lattice oxygen stabilization.

11.2 How does tuning redox couples improve cycling stability in Li-rich Mn-based cathodes?

Li-rich Mn-based cathodes rely on two coupled redox couples — conventional transition-metal (cationic) redox and anionic (lattice oxygen) redox — to deliver their high specific capacity. When the oxygen redox couple operates at too low a potential relative to electrolyte stability limits, lattice oxygen becomes overly reactive and prone to irreversible O₂ release. By using Ni²⁺/PO₄³⁻ co-doping to raise the oxygen redox potential (shown by upward-shifted cyclic voltammetry peaks: 4.028 V and 4.637 V vs. 4.006 V and 4.626 V in the pristine material), the modified cathode’s oxygen redox couple becomes more thermodynamically stable and less prone to irreversible side reactions — directly translating to higher capacity retention, reduced voltage fade, and improved high-temperature stability.

11.3 How was mechanical strength of the Li-rich cathode particles measured, and why does it matter?

Mechanical strength was measured using single-particle crush force testing on the IEST SPFT2000 Single Particle Mechanical Properties Test System, which applies a controlled compressive load directly to an individual cathode particle while recording force and displacement until fracture. This measurement matters because Li-rich cathode particles accumulate internal stress from lattice oxygen redox-driven volume changes during cycling, and particles that crack expose fresh surface to electrolyte, accelerating side reactions and capacity fade. In this study, the surface-reconstructed LLO-NP@LPO particles showed an average crush force of 8.143 mN — a 58% increase over the pristine PLLO particles’ 5.154 mN — directly correlating with the intact particle morphology observed by SEM/TEM after 400 cycles and the substantially improved long-term capacity retention.

11.4 Why does a Li₃PO₄ coating suppress gas evolution in Li-rich cathodes?

The amorphous Li₃PO₄ coating formed during in-situ surface reconstruction acts as a physical barrier that reduces direct contact between the reactive cathode surface and the electrolyte, suppressing electrolyte oxidative decomposition and LiPF₆ decomposition byproduct formation at the interface. Because much of the CO₂ evolution in Li-rich cathodes originates from electrolyte and carbonate species reacting at the exposed cathode surface — rather than from the cathode’s own lattice oxygen alone — physically isolating this interface with a stable, ion-conductive coating directly reduces CO₂ generation. In this study, CO₂ evolution dropped from 44.74 to 21.14 µmol/g with the Li₃PO₄ coating in place, while the coating’s inherent Li⁺ conductivity avoided the rate-capability penalty that a purely insulating coating would cause.

11.5 Why does capacity retention in Li-rich Mn-based cathodes degrade faster at high temperature?

Elevated temperature accelerates nearly all of the degradation pathways that already threaten Li-rich Mn-based cathodes at room temperature — lattice oxygen release, electrolyte oxidative decomposition, transition-metal migration, and CEI growth all proceed faster with increased thermal energy. In this study, pristine PLLO retained only 32.1% of its capacity after 120 cycles at 55°C and 1C — far worse than its already-limited room-temperature performance — because the reactive, unstabilized lattice oxygen and unprotected surface accelerated these parasitic reactions. The surface-reconstructed LLO-NP@LPO retained 85.9% capacity under identical high-temperature conditions, demonstrating that stabilizing lattice oxygen and protecting the cathode-electrolyte interface addresses the same underlying degradation mechanisms that are simply accelerated, not fundamentally different, at elevated temperature.

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