Prolonged overcharging does more than add charge—it changes the nickel electrode’s phase chemistry. Once the electrode’s normal redox reaction and self-discharge processes can no longer accommodate the applied current, oxygen evolution begins. Continued overcharge promotes an amorphous H–Ni–O secondary phase, commonly represented as HNi₂O₃, which redirects subsequent discharge into a lower-potential reaction pathway and makes part of the capacity unusable at the normal operating voltage.
The apparent capacity loss is often a capacity displacement rather than immediate disappearance of active material: capacity moves from the normal high-voltage plateau, approximately 1.15–1.3 V in practical cell measurements or about 1.34 V versus hydrogen in the referenced thermodynamic description, to a secondary plateau near 0.8–1.0 V, approximately 0.78 V versus hydrogen.
How Overcharging Changes the Nickel Electrode
The normal reaction reaches its practical limit
A nickel-based positive electrode normally stores charge through reversible nickel oxyhydroxide/hydroxide chemistry. During overcharge, however, the applied current eventually exceeds what the compositional phase transformation and electrode self-discharge processes can absorb.
At that point, additional current is diverted into parasitic reactions rather than useful nickel redox storage.
Oxygen evolution becomes the competing reaction
In alkaline aqueous systems, excess anodic current can drive oxygen evolution at the electrolyte/NiOOH interface:
[ 2\mathrm{OH^-} \rightarrow \mathrm{H_2O} + \frac{1}{2}\mathrm{O_2} + 2e^- ]
The generated electrons flow into the current collector, so the test system continues to record charge input even though the nickel active material is no longer storing an equivalent amount of useful charge.
Why prolonged overcharge is more damaging than brief overcharge
A short overcharge may mainly produce temporary gas evolution and limited chemical disturbance. Prolonged overcharging sustains the high-potential, oxygen-evolution environment long enough to alter the electrode’s phase composition and microstructure.
This is the point at which overcharge changes from an electrical operating condition into a materials degradation mechanism.
Why a New Low-Voltage Plateau Appears
Formation of an amorphous secondary phase
Extended oxygen evolution is associated with gradual formation of an amorphous H–Ni–O phase, described in the reference as HNi₂O₃. The phase is not merely an inert deposit; it changes which combinations of solid phases are thermodynamically accessible during the next discharge.
In simplified compositional terms, the overall solid composition shifts along the HNiO₂–HNi₂O₃ tie line.
The discharge path enters a different phase region
Without substantial secondary-phase formation, discharge follows the phase relationships associated with the normal high-potential reaction. Once HNi₂O₃ is present, part of the discharge composition moves into a different ternary phase subtriangle.
That region is bounded by H₂NiO₂, HNi₂O₃, and NiO and has a lower thermodynamic potential, approximately 0.78 V versus hydrogen.
The voltage curve reveals the phase change
The result is a two-plateau discharge signature:
- A primary plateau at the normal nickel-electrode operating potential.
- A secondary low-voltage plateau near 0.8–1.0 V in practical battery measurements.
The lower plateau is evidence that some material is still electrochemically reactive, but it is reacting through a less favorable phase pathway.
Why This Looks Like Capacity Loss
Capacity becomes inaccessible at the normal operating voltage
In many applications, the cell is considered depleted when its voltage falls below the useful operating range. Capacity delivered only on the low-voltage plateau therefore cannot be used, even though a deep, slow discharge may recover it electrically.
This is why the phenomenon is commonly associated with the memory effect: the electrode appears to have lost capacity under normal cycling, while some of that capacity has actually shifted to a lower potential.
Routine cycling can hide the early degradation
Partial-depth discharge cycles may terminate before reaching the secondary plateau. Standard cycling can therefore report apparently acceptable performance while the fraction of capacity available at the primary plateau is steadily declining.
A periodic, controlled full discharge is needed to separate primary-plateau capacity from secondary-plateau capacity.
The charge record can become misleading
Because oxygen evolution consumes part of the applied charge, the charge passed by the tester no longer corresponds directly to reversible nickel capacity. Gas evolution, corrosion, and side reactions can reduce charge efficiency while the instrument continues to report the programmed charge input.
Additional Damage That Can Make the Loss Permanent
Gas evolution and electrode swelling
Oxygen generation can cause gassing and contribute to chemical corrosion or physical swelling. These effects may damage electrical contacts, alter porosity, and reduce the electrode’s ability to use its active material efficiently.
Thus, the initial plateau shift may be partly reversible, while prolonged exposure can create irreversible structural degradation.
Gamma-phase formation and mechanical stress
Overcharge, particularly under conditions involving high KOH concentration, can promote conversion of desirable beta-nickel oxyhydroxide toward gamma-nickel oxyhydroxide.
The gamma phase incorporates potassium ions and water, producing interlayer expansion and substantial volume changes. Although this phase can theoretically support higher capacity, repeated expansion and contraction mechanically damage the electrode matrix and shorten cycle life.
Interfacial barrier formation
A separate contributor to second-plateau behavior is formation of a semiconductive barrier between the sintered substrate and active material, especially near the end of discharge. Continued discharge then requires an additional voltage drop—reported as approximately 0.3–0.4 V—to drive sufficient electronic conduction through the barrier.
This mechanism should not be confused with the thermodynamic plateau shift caused by HNi₂O₃. Both can produce low-voltage discharge, but one is primarily a phase-pathway change, while the other is an electronic and interfacial transport problem.
Understanding the Trade-offs
More overcharge can temporarily mask degradation
In some test protocols, additional overcharge may appear to restore charge acceptance or normal efficiency temporarily. It can drive the electrode through reactions that make capacity appear more accessible during subsequent cycles.
The cost is accelerated oxygen evolution, electrolyte dry-out, corrosion, swelling, and possible shorting. Using overcharge as a recovery method can therefore conceal rather than solve the underlying degradation.
High capacity does not guarantee useful capacity
A deep discharge may show that substantial charge remains in the electrode. That result does not mean the cell retains equivalent practical performance if the charge is delivered only at the low-voltage plateau.
Performance should be reported by voltage region, not only by total measured discharge capacity.
Plateau shift and irreversible damage must be separated
A reversible or partially reversible phase redistribution can shift capacity between plateaus. Physical destruction of the active-material network, severe corrosion, or persistent interfacial resistance can permanently remove that capacity from practical use.
Diagnostic testing should therefore track both the plateau locations and the total recoverable capacity over time.
How to Diagnose and Control the Problem
Precision charge-discharge systems and potentiostats can identify when the primary plateau begins to shrink and the secondary plateau grows. The most informative measurements combine voltage-resolved capacity, charge efficiency, controlled overcharge exposure, and periodic low-rate reconditioning.
Use voltage-resolved capacity measurements
Record the capacity delivered above and below the normal operating-voltage threshold. This distinguishes a true loss of electrochemical activity from a redistribution of capacity toward the low-voltage plateau.
Control the overcharge regime
Define charge cutoffs, current density, temperature, and electrolyte conditions carefully. The goal is to avoid sustained operation in the oxygen-evolution regime unless that regime is deliberately being studied.
Include periodic diagnostic discharges
A low-rate, full discharge can reveal capacity that routine partial cycling misses. Repeating this measurement at defined intervals allows researchers to trend primary-plateau loss, secondary-plateau growth, and total residual capacity separately.
Correlate electrical data with physical characterization
Voltage curves should be interpreted alongside evidence of gassing, swelling, corrosion, phase transformation, and interfacial resistance. Electrical signatures alone may identify degradation but cannot always distinguish the responsible mechanism.
Making the Right Choice for Your Goal
- If your primary focus is understanding the memory effect: Track the transfer of capacity from the normal high-voltage plateau to the approximately 0.8–1.0 V secondary plateau using periodic low-rate full discharges.
- If your primary focus is preventing overcharge damage: Limit charge duration and current once useful nickel redox capacity is exhausted, thereby reducing sustained oxygen evolution and secondary-phase formation.
- If your primary focus is distinguishing reversible from permanent capacity loss: Compare recovery after controlled reconditioning with long-term trends in charge efficiency, plateau position, swelling, and interfacial resistance.
- If your primary focus is improving cycle life: Control electrolyte concentration and charge cutoff conditions to limit gamma-phase formation, volume-change damage, corrosion, and gas-related degradation.
The most reliable interpretation is to treat a growing low-voltage plateau as a diagnostic signature of altered phase chemistry and transport—not simply as evidence that all active material has vanished.
Summary Table:
| Cause | Effect | Outcome |
|---|---|---|
| Oxygen evolution | Sustained high-potential environment | Amorphous HNi2O3 formation |
| HNi2O3 formation | New low-voltage plateau near 0.8-1.0 V | Capacity shift from high to low voltage |
| Gamma-phase formation | Interlayer expansion and stress | Mechanical damage and shorter cycle life |
| Interfacial barrier | Additional voltage drop (0.3-0.4 V) | Transport resistance, low-voltage discharge |
| Routine partial cycling | Hides low-voltage capacity | Apparent capacity loss |
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