A lithium-ion battery does not usually fade because lithium simply disappears from the sealed cell. One major problem is that some of its lithium stops participating in the reactions that store and release energy.
Much of that story unfolds inside a nanometre-scale layer called the solid electrolyte interphase, or SEI. It forms on the anode during a battery’s earliest charging cycles and is essential to the cell’s operation. But if it keeps evolving, it can consume electrolyte and active lithium while increasing resistance. A 2023 review in Advanced Energy Materials describes those effects as contributors to capacity fade and poorer power delivery.
The SEI is therefore both protector and participant in ageing. It is not the only reason a battery loses capacity, but it is one of the most important.
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The film that builds itself the first time you charge
Inside a typical lithium-ion cell, two electrodes are separated by an electrolyte, often made from a lithium salt such as LiPF₆ dissolved in organic carbonate solvents. During the cell’s initial charging cycles, the electrolyte is not completely stable at the low voltage of the negative electrode, which is commonly graphite.
Some electrolyte molecules decompose at that surface. Their reaction products, including organic compounds and inorganic salts such as lithium fluoride and lithium carbonate, accumulate into a thin, chemically complex film. This is the SEI.
A functional SEI conducts lithium ions but largely blocks electrons. That combination matters because it lets lithium continue moving into and out of the graphite while limiting further electrolyte decomposition.
Without adequate passivation, the electrolyte would continue reacting with the anode. The battery could lose electrolyte and active lithium much faster. The SEI is therefore not a simple defect. It is a protective layer that can become a source of degradation when it remains unstable or continues growing.
Why a good film can become a problem
The SEI is not a uniform sheet with identical properties throughout. Its composition, thickness and permeability depend on the electrode material, electrolyte formulation, temperature, voltage and charging conditions.
Graphite changes volume as lithium enters and leaves it. The change is modest compared with silicon, but it can still place stress on the interphase. Silicon-containing anodes undergo much larger volume swings, making repeated cracking, fresh-surface exposure and interphase reformation especially difficult to control.
A 2021 Nature Nanotechnology study used three-dimensional imaging to examine silicon and its SEI. The researchers found that electrolyte could penetrate pathways within the silicon structure and that SEI growth could extend inward, disrupting electron-conduction pathways and creating electrically isolated, inactive silicon.
That is more complicated than a film simply becoming uniformly thicker. Depending on the cell chemistry, degradation can involve interphase growth, cracking, pore formation, lost electrical contact and changes within the active electrode material itself.
The lithium that stops cycling
Whenever additional SEI forms, some lithium can become chemically incorporated into its reaction products. Those lithium atoms remain inside the battery, but they are no longer part of the inventory that shuttles between the electrodes during normal charging and discharging.
This loss of cyclable lithium helps explain why capacity often declines gradually rather than vanishing at once. Continued interphase growth can also increase resistance, while damage elsewhere in the cell can isolate active material or slow charge transfer.
A TechTimes overview of smartphone-battery degradation gives roughly 300 to 500 full cycles as a common range before a noticeable capacity decline. That is a broad rule of thumb, not a universal specification. Actual retention varies with the battery design, device, temperature and charging pattern.
The lithium reservoir has not simply emptied. Part of it may be trapped in interphase products, while other portions of the cell may have become less able to store or transport it effectively.
Heat accelerates the unwanted reactions
Higher temperatures generally accelerate the parasitic reactions involved in battery ageing. Heat can encourage further electrolyte decomposition and interphase change even while a battery is resting, particularly when the cell is held at a high state of charge.
This is why charging a hot phone on a car dashboard or running a laptop under heavy load on a surface that blocks ventilation can be harder on the battery than charging the same device in a cooler environment. The exact ageing rate cannot be reduced to one universal temperature formula because it depends on the cell chemistry and operating conditions.
Cold presents a different problem. Lithium ions move more slowly at low temperatures. If a cold cell is charged too aggressively, metallic lithium can plate onto the anode rather than entering the graphite normally. That can reduce usable capacity and, under severe conditions, create a safety risk.
The safest practical response is simple: avoid charging a device when it is already very hot, and allow an extremely cold device to warm toward its normal operating range before fast charging it.
What engineers are doing about it
Battery engineers cannot simply eliminate the SEI from conventional lithium-ion cells because a stable passivation layer is necessary. Instead, they try to control its chemistry, structure and mechanical behaviour.
One approach uses electrolyte additives that react before the main solvent and help form a more stable interphase. Fluoroethylene carbonate and vinylene carbonate are widely studied examples, particularly in cells containing silicon. Their effects depend on the full electrolyte and electrode formulation, so no single additive is ideal for every battery.
Researchers are also developing more specialised molecules. A 2021 Nature Communications study of dioxolone-derived additives reported 81.5 percent capacity retention after 400 cycles at 1C in the tested NCM811/silicon-carbon cells. In a separate fast-charging test at 3C, the formulation showed 1.9 percent capacity fading after 100 cycles.
Those are laboratory results for a particular cell design, not a promise for every phone or electric vehicle. They nevertheless illustrate the goal: create an interphase that remains ion-conductive, resists repeated mechanical stress and limits further reactions.
The solid-state gamble
Solid-state batteries replace the conventional liquid electrolyte with a solid ion-conducting material. Some designs use ceramics, polymers or sulphide-based compounds. Removing flammable liquid components could improve safety, but it does not make interface chemistry disappear.
In January 2026, researchers at Switzerland’s Paul Scherrer Institute reported a mild-sintering process for the argyrodite electrolyte Li₆PS₅Cl, combined with a 65-nanometre lithium fluoride coating on a lithium-metal anode. According to the ESS News account of the Advanced Science study, the tested full cell retained about 75 percent of its initial capacity after 1,500 cycles, while stable cycling continued beyond 2,700 cycles under the reported test conditions.
Lead author Jinsong Zhang described the high-voltage cycle stability as among the best reported at the time. The coating helped suppress electrolyte decomposition at the lithium interface and acted as a barrier against dendrite penetration. It was an engineered passivation layer rather than an uncontrolled film left to develop on its own.
Interfacial degradation also matters on the cathode side. A 2025 Nature Communications study found that chemical degradation at the cathode and solid-electrolyte interface affected charge-transfer behaviour and mechanical degradation in sulphide-based solid-state cells.
Solid-state technology therefore changes the interphase problem rather than abolishing it. The interfaces can potentially be designed more deliberately, but they remain central to performance and lifespan.
What this means for the phone in your pocket
A user cannot remove an SEI that has already formed, but charging conditions can influence how quickly a battery ages. Heat and prolonged time at a very high state of charge are two conditions worth limiting when convenient.
Modern devices increasingly handle some of this automatically. Apple says its Optimized Battery Charging feature can delay charging past 80 percent when an iPhone is expected to remain connected to power for an extended period. Other manufacturers offer similar adaptive-charging or user-selectable charge-limit features.
There is no need to treat a phone like laboratory equipment. Charging to 100 percent when the full range is needed is normal use. A practical approach is to avoid unnecessary heat, use the manufacturer’s battery-protection settings and avoid leaving a device fully charged in a hot environment for long periods.
Fast charging is also a managed trade-off rather than an automatic battery killer. Higher charging power can produce more heat, but modern devices regulate current and temperature. Letting a hot device cool before charging is more useful than worrying about an occasional fast charge.
A film you cannot see, doing essential work
The SEI is one of battery chemistry’s central compromises. It forms because the electrolyte is unstable against the charged anode, then protects that anode from more extensive decomposition. When the interphase remains stable, the battery can cycle efficiently. When it keeps changing, it can consume active lithium, increase resistance and contribute to capacity loss.
Researchers now use imaging, spectroscopy, simulations and operando experiments to follow processes that were once hidden inside sealed cells. In July 2026, the Institut Laue-Langevin reported tracking lithium movement in real time inside a working solid-state battery using neutron diffraction. The experiment revealed uneven lithium extraction within a thick positive electrode, showing how much complexity can remain even when the liquid electrolyte is gone.
If an ageing phone no longer lasts through the day, the SEI may be part of the explanation, but it is not necessarily the only culprit. The real story is a network of slowly accumulating chemical and mechanical changes. The invisible film on the anode is one of the most consequential of them because the battery needs it to work, even as its continued evolution helps determine how long that work can continue.