As the core medium for lithium-ion transport within the battery, the electrolyte plays a crucial role as an "ion conductor," "electrode interface protector," and "charge transfer bridge." Its quantity must be strictly matched to the battery's electrode size, porosity, and encapsulation volume to ensure sufficient wetting of the electrode active materials and unobstructed lithium-ion transport pathways. Insufficient electrolyte (referred to in the industry as "under-electrolyte") is not simply a matter of insufficient medium; it disrupts the battery's internal electrochemical balance, triggering a series of chain reactions leading to performance degradation and safety failure. Most of this damage is irreversible, severely impacting the battery's service life and safety. The following analysis, based on the actual working principles of batteries, details its specific negative impacts and underlying mechanisms.
To facilitate clear understanding, let's first clarify the core premise: The core function of the electrolyte is to dissolve lithium salts (such as LiPF6, LiFSI, etc.), providing freely moving lithium ions, while simultaneously wetting the positive and negative electrode active materials (such as ternary materials, lithium iron phosphate, graphite, etc.) and the separator, constructing a stable electrode/electrolyte interface (SEI film, CEI film), ensuring efficient and stable lithium ion transport between the positive and negative electrodes.
I. Significant Capacity Decline (Low Capacity)
The most direct and intuitive impact of insufficient electrolyte is that the actual discharge capacity of the battery is significantly lower than the design capacity, and this capacity decline is irreversible, continuously worsening with increasing cycle count. The core mechanism lies in the fact that the positive and negative electrode active materials cannot be fully wetted by the electrolyte; only a small amount of surface active material can participate in the lithium ion insertion/extraction reaction, while a large amount of internal active material remains "idle" and cannot exert electrochemical activity.
From a structural perspective, both the positive and negative electrodes are porous (with a porosity typically between 30% and 50%). The electrolyte must fully fill these pores to allow lithium ions to contact every active particle. If the electrolyte is insufficient, a continuous electrolyte phase cannot form within the pores, and lithium ions can only move within a limited area on the electrode surface. This significantly reduces the number of lithium ions participating in the electrochemical reaction, preventing the release of the full designed capacity during discharge.
Furthermore, in a low-electrolyte state, during the first charge, some unwetted active materials cannot form a stable interfacial film. Even with subsequent electrolyte replenishment, these active materials are unlikely to regain activity, leading to irreversible capacity decay and preventing recovery to the design value through charging cycles.
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II. Sharp Increase in Battery Internal Resistance
The internal resistance of a lithium battery mainly consists of three parts: ohmic resistance, charge transfer resistance, and diffusion resistance. Insufficient electrolyte leads to a significant increase in the latter two, ultimately causing a sharp rise in the total battery internal resistance, which in turn affects charge/discharge efficiency and output performance.
On the one hand, increased charge transfer resistance: Charge transfer resistance mainly occurs at the electrode/electrolyte interface, relying on a stable interfacial film and sufficient electrolyte for charge transfer. When electrolyte is insufficient, the electrode surface is not adequately wetted, and the interfacial films (SEI film, CEI film) cannot be uniformly covered. The resistance to lithium ion insertion/extraction on the electrode surface increases, slowing down charge transfer and causing the charge transfer resistance to increase exponentially. On the other hand, increased diffusion resistance: The diffusion rate of lithium ions in the electrolyte is directly related to the continuity and concentration of the electrolyte. Insufficient electrolyte leads to uneven electrolyte concentration, with some areas even becoming "electrolyte-free" blank areas. The diffusion path of lithium ions is blocked, the diffusion distance becomes longer, and the diffusion resistance increases significantly.
III. Significantly Deteriorating Cycle Performance
Cycle performance is a core indicator of lithium battery lifespan, referring to the battery's ability to maintain stable capacity during repeated charge-discharge cycles. Insufficient electrolyte leads to a sharp deterioration in cycle performance and is prone to abnormal phenomena such as a sudden and significant drop in capacity after a single cycle. This is essentially a vicious cycle caused by increased internal resistance.
As mentioned earlier, insufficient electrolyte leads to increased internal resistance. The core consequence of increased internal resistance is intensified localized heating during battery charging and discharging (according to Joule's law Q=I²Rt, with constant current, higher internal resistance results in more heat generation). Localized overheating accelerates electrolyte decomposition-at high temperatures, the electrolyte undergoes redox reactions, generating gases such as CO₂ and HF, as well as inert substances, further consuming the remaining electrolyte and leading to even more insufficient electrolyte. Simultaneously, high temperatures also damage the stable film at the electrode/electrolyte interface (the SEI film will rupture and reconstruct). The ruptured SEI film will consume lithium ions and electrolyte again to repair itself, further increasing charge transfer resistance.
This vicious cycle of "low electrolyte level → increased internal resistance → localized heating → electrolyte decomposition → exacerbated low electrolyte level" causes the battery capacity to continuously decline during cycling, and the rate of decline accelerates. When the number of cycles reaches a certain level, the interfacial film completely fails, or the electrolyte is exhausted, a significant drop in capacity occurs. Furthermore, low electrolyte level also leads to poor capacity consistency during battery cycling. In a multi-cell series battery pack, the cells with low electrolyte level will decline first, thus dragging down the performance and lifespan of the entire battery pack.
IV. Severe Heat Generation During Charge and Discharge
The heat generation caused by insufficient electrolyte is a critical link between performance degradation and safety failure. The heat generation mainly originates from two sources, which have a cumulative effect, leading to an abnormally high battery temperature and posing a potential early risk of thermal runaway.
- Catalytic Heat Generation Due to Internal Resistance
As mentioned earlier, insufficient electrolyte leads to a sharp increase in internal resistance, significantly increasing Joule heat generated during charge and discharge. Furthermore, due to insufficient electrolyte, its own heat dissipation capacity also decreases (electrolyte also has a certain heat dissipation function, conducting heat generated by the electrodes to the battery casing).
- Abnormal Reaction Heat Generation
Insufficient electrolyte causes instability in the electrode interface film, making it prone to side reactions. This manifests as a significant increase in the battery casing temperature during charge and discharge (normal charge and discharge temperatures are typically 20-45℃, but insufficient electrolyte batteries may rise above 50℃), especially noticeable during charging, sometimes even becoming "hot to the touch." If the battery is under high-rate charge and discharge, the heat generation will further intensify, potentially exceeding the electrolyte decomposition temperature (usually above 60℃), accelerating electrolyte decomposition and electrode aging, creating a potential safety hazard for subsequent failures. Furthermore, prolonged heat generation can lead to battery casing deformation and sealant aging, potentially causing electrolyte leakage and further deteriorating the battery's condition.
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V. Lithium Plating at the Negative Electrode, or Ignition
This is the most serious negative impact of insufficient electrolyte, directly affecting battery safety and representing one of the primary failure modes for under-electrolyte batteries. The core cause of lithium plating is: insufficient electrolyte leads to poor local wetting of the negative electrode, resulting in uneven formation of the SEI film. Lithium ions cannot properly embed between the graphite layers and can only deposit metallic lithium on the negative electrode surface (i.e., "lithium plating"), causing direct contact between the positive and negative electrodes inside the battery, triggering an internal short circuit.
An internal short circuit generates a large amount of heat instantaneously, causing the battery temperature to rise sharply (instantaneously exceeding 100°C), leading to thermal runaway. High temperatures cause the electrolyte to decompose violently, producing large amounts of flammable and explosive gases (such as CO and CH4). The internal pressure of the battery rises sharply, ultimately causing the battery casing to rupture and leak. If the gas comes into contact with air, or if the internal temperature reaches the ignition point of the electrolyte or electrode materials, it can ignite or even explode. Furthermore, even without an internal short circuit, the deposited metallic lithium will react with the electrolyte, consuming both electrolyte and lithium ions, further accelerating battery performance degradation and safety hazards. When disassembling a battery with severely insufficient electrolyte, metallic lithium deposits can even be directly observed on the negative electrode surface, and it is prone to ignition.
Summary
Insufficient electrolyte in lithium batteries is not a "minor defect," but rather causes irreversible damage to the battery from the inside out, affecting performance, lifespan, and safety. Its impact exhibits a "chain reaction" characteristic: insufficient electrolyte → inadequate wetting → obstructed lithium ion transport → increased internal resistance → increased heat generation → electrolyte decomposition → worsening insufficient electrolyte → lithium deposition → internal short circuit → fire and explosion. In actual production and use, the amount of electrolyte used must be strictly controlled, typically calculated precisely based on parameters such as electrode porosity and battery volume to avoid insufficient electrolyte problems.
If a battery exhibits significant reduction in range (capacity decay exceeding 20%), abnormal heating during charging and discharging, excessively high charging voltage, excessively low discharging voltage, or a sharp drop in capacity during cycling, it may indicate insufficient electrolyte. Timely inspection and maintenance are crucial to prevent further battery damage or safety incidents. For large battery packs such as power batteries and energy storage batteries, insufficient electrolyte can also affect the stability of the entire system, requiring focused attention and prevention.
