The dual plateaus arise from sequential oxidation of different transition-metal states. In substituted nickel–manganese spinels such as LiNiᵧMn₂₋ᵧO₄, the approximately 4.1 V plateau is associated primarily with Mn³⁺ → Mn⁴⁺ oxidation, while the approximately 4.7 V plateau appears after manganese has been fully oxidized and lithium extraction proceeds through the Ni²⁺/Ni⁴⁺ redox system. The difference in voltage reflects the different binding energies of the electrons being removed from manganese and nickel.
Core takeaway: Crystal-field splitting separates the transition-metal 3d states, while the distinct energies of the Mn and Ni electronic levels force lithium extraction to occur in two stages. Lower-binding-energy Mn (e_g) electrons are removed first near 4.1 V; higher-binding-energy Ni (e_g) electrons are removed later near 4.7 V.
Why Two Voltage Plateaus Appear
Lithium extraction is also transition-metal oxidation
During delithiation, lithium ions leave the spinel structure and electrons are removed from transition-metal redox centers to maintain charge neutrality.
Because manganese and nickel do not hold their redox electrons at the same energy, their oxidation reactions occur at different electrochemical potentials. Each dominant oxidation process therefore produces a separate voltage plateau.
The 4.1 V plateau is manganese-controlled
The lower plateau, near 4.1 V, corresponds primarily to the oxidation of Mn³⁺ to Mn⁴⁺.
The electron removed during this process is associated mainly with a manganese (e_g) orbital. Its relatively lower binding energy, approximately 1.5–1.6 eV in the reference description, makes manganese oxidation energetically accessible before nickel oxidation.
The 4.7 V plateau is nickel-controlled
Once the available manganese is fully converted to Mn⁴⁺, further delithiation must oxidize nickel.
The higher plateau, near 4.7 V, is therefore associated with the low-spin Ni²⁺/Ni⁴⁺ redox couple. Removing the relevant electron from the nickel (e_g) state requires a higher binding energy, approximately 2.1 eV, which corresponds to a higher cell voltage.
How Crystal-Field Splitting Sets the Redox Sequence
Octahedral coordination divides the 3d states
The manganese and nickel ions occupy octahedral sites in the spinel framework. In an octahedral crystal field, their 3d orbitals split into lower-energy (t_{2g}) states and higher-energy (e_g) states.
This splitting does not merely describe the structure’s electronic configuration. It determines which orbitals participate in oxidation and how strongly their electrons are bound.
The (e_g) states are central to the observed plateaus
The redox electrons involved in the two plateaus are described primarily as occupying transition-metal (e_g) states.
The Mn and Ni (e_g) levels are not equivalent: their different local electronic environments and binding energies produce separate oxidation potentials rather than one continuous plateau.
Voltage reflects electron-removal energy
A higher electrode voltage indicates that lithium extraction is coupled to removal of electrons from a deeper, more strongly bound electronic state.
Accordingly, the nickel-related process appears at a higher voltage than the manganese-related process because the relevant Ni (e_g) electrons require more energy to remove.
The Electronic Mechanism in Sequence
Stage one: oxidation of Mn³⁺
Initially, manganese contains Mn³⁺ species that can be oxidized to Mn⁴⁺.
As lithium is extracted, electrons are removed from the lower-binding-energy Mn (e_g) states. This produces the first plateau near 4.1 V.
Stage two: exhaustion of the manganese redox capacity
The first plateau ends when the manganese participating in this reaction has been converted to Mn⁴⁺.
Mn⁴⁺ is then no longer the active source of the same oxidation charge, so the next available redox process involves nickel.
Stage three: oxidation through nickel states
Further lithium removal oxidizes nickel through the effective Ni²⁺/Ni⁴⁺ redox process.
Because the nickel (e_g) electrons have higher binding energy, this second process occurs near 4.7 V, producing the upper voltage plateau.
Why the Plateaus Matter in Practice
They identify which element stores charge
The two plateaus provide an electrochemical signature of the active redox centers.
The lower-voltage capacity is mainly manganese-associated, whereas the high-voltage capacity is mainly nickel-associated. This distinction helps separate the material’s nominal composition from the specific transition-metal reactions actually occurring during cycling.
They define usable energy and capacity
The high-voltage nickel process can substantially increase the cell’s average discharge voltage and energy density.
However, the measured practical capacity depends on how completely each redox process proceeds and whether the material remains structurally and electrochemically stable at the upper cutoff voltage.
They require controlled electrochemical testing
Evaluating the plateaus requires carefully assembled laboratory cells and controlled charge–discharge measurements.
The resulting voltage profile can reveal the position, separation, and relative capacity of the manganese- and nickel-associated reactions, while also showing polarization or degradation that may obscure the ideal electronic picture.
Understanding the Trade-offs
High voltage increases energy but raises stability demands
The approximately 4.7 V nickel plateau is valuable because it raises operating voltage and energy density.
At the same time, operation at such high potentials places greater demands on the electrode, electrolyte, interfaces, and cell construction. The electronic origin of the plateau does not by itself guarantee long-term cycling stability.
The ideal two-plateau picture can be broadened
Real materials may not display perfectly sharp plateaus.
Cation disorder, compositional variation, local structural distortions, polarization, and nonuniform reaction kinetics can broaden or shift the observed features. The two-step redox mechanism remains the underlying interpretation, but the measured profile may be less ideal than the schematic 4.1 V and 4.7 V values.
Nominal oxidation states are a simplified description
Descriptions such as Mn³⁺, Mn⁴⁺, Ni²⁺, and Ni⁴⁺ are useful for tracking charge compensation.
In a real solid, electronic charge can be distributed across metal–oxygen bonds rather than localized perfectly on one ion. Thus, the oxidation-state notation should be understood as a chemically meaningful model of the dominant redox processes, not necessarily as a complete description of electron density.
Making the Right Choice for Your Goal
The most useful interpretation depends on what you are trying to determine from the voltage profile.
- If your primary focus is identifying the redox mechanism: Assign the approximately 4.1 V plateau to Mn³⁺/Mn⁴⁺ oxidation and the approximately 4.7 V plateau to the subsequent Ni²⁺/Ni⁴⁺ process.
- If your primary focus is explaining the voltage difference: Compare the binding energies of the Mn and Ni (e_g) electrons; the higher Ni binding energy produces the higher oxidation potential.
- If your primary focus is evaluating practical cell performance: Measure the complete charge–discharge profile under controlled conditions, because accessible capacity, polarization, and high-voltage stability determine how much of the theoretical two-plateau behavior is usable.
- If your primary focus is interpreting material changes: Treat shifts or broadening of the plateaus as evidence that composition, local ordering, kinetics, or structural stability may be affecting the ideal redox sequence.
The dual plateaus are best understood as an electronic fingerprint of sequential manganese and nickel oxidation governed by crystal-field-split transition-metal states.
Summary Table:
| Plateau | Voltage (V) | Redox Couple | Electron State | Binding Energy |
|---|---|---|---|---|
| Lower | ~4.1 | Mn³⁺/Mn⁴⁺ | Mn e_g | ~1.5–1.6 eV |
| Upper | ~4.7 | Ni²⁺/Ni⁴⁺ | Ni e_g | ~2.1 eV |
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