Causes and control methods of lithium battery self-discharge

Lithium batteries frequently encounter voltage decreases during their usage or storage phases, attributed to factors such as electrolyte compatibility, the characteristics of graphite negative electrodes, and inconsistencies during assembly. A significant proportion of these voltage reductions stems from the inherent self-discharge of the battery core.

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The extent of self-discharge in batteries can be quantified in two ways: one involves measuring the daily voltage drop in millivolts (mV/day). A quality battery should not exhibit a drop exceeding 2mV in a day; the second method uses the K value, indicating voltage drop per time unit, specifically mV/h. A reliable battery typically maintains a K value of 0.08mV/h or less.

K = OCV2 - OCV1 / T

1. Factors Contributing to Self-Discharge

Excessive self-discharge in lithium batteries can be attributed to two main causes: physical micro-short circuits and chemical reactions. An in-depth analysis of these factors follows:

1. Physical Micro-Short Circuits

Physical micro-short circuits significantly contribute to lithium battery voltage reduction. This phenomenon is usually observed when a battery is stored at room or elevated temperatures for an extended duration, resulting in a voltage drop below the standard cutoff levels. Unlike self-discharge stemming from chemical reactions, self-discharge due to physical micro-short circuits does not result in irreversible capacity diminution. The causes of these micro-short circuits can be categorized as follows:

1. Dust and Burrs

Disassembly of a slightly short-circuited battery often reveals dark spots on its separator. If these spots are central on the diaphragm, it's likely due to dust breakdown; conversely, if located at the edges, burrs from the pole piece cutting process are typically responsible. Differentiating between these two causes is often straightforward.

1. Metal Impurities in the Electrodes

Within the battery, metal impurities undergo corrosive chemical and electrochemical reactions, dissolving into the electrolyte:

M → Mn + + ne−;

Following this, the Mn+ migrates to the negative electrode, resulting in metal deposition:

Mn + + ne− → M;

Over time, metal dendrites grow and penetrate the separator, leading to micro-short circuits between the positive and negative electrodes, thereby draining power and causing voltage reductions.

Cathode Metal Impurities

Metal impurities from the positive electrode can damage the separator after charging reactions, leading to small black spots that induce physical micro-short circuits. Generally, metal impurities significantly affect self-discharge rates, with copper (Cu) exhibiting the highest level of impact, followed by zinc (Zn), iron (Fe), and iron oxides (Fe2O3). For instance, excessive iron impurities are a common issue in many cathode lithium iron materials, leading to increased self-discharge rates.

Negative Metal Impurities

Metal impurities present in the negative electrode, as a result of primary battery formations, may also detach and settle on the separator, further contributing to micro-short circuits. While metal impurities in the negative electrode slurry pose a lesser threat to self-discharge compared to those in the positive electrode, copper and zinc are particularly impactful.

1. Metal Impurities in Auxiliary Materials

For instance, one can encounter metal impurities in materials such as CMC and tape.

1. Chemical Reactions
2. Moisture Presence

Moisture retention facilitates electrolyte decomposition, releasing a substantial quantity of electrons that embed within the oxidation structure of the positive electrode, thus lowering its potential and causing voltage reductions.

Additionally, H2O presence in batteries leads to reactions with LiPF6, producing corrosive gases such as HF alongside other gases, such as CO2, which can result in battery swelling. HF interactions with various battery components can deteriorate the SEI film, culminating in additional issues.

SEI Film: 1) The solvent permeates the graphite layer, reacting with LixC6 and leading to irreversible capacity loss; 2) Repair processes for the damaged SEI consume Li+ and solvent, further contributing to irreversible losses.

1. Electrolyte Solvent

The addition of certain electrolyte solvents can cause rapid voltage drops in batteries. For instance, one specific solvent increased ion conductivity yet resulted in a self-discharge rate three times faster than standard solvents after application.

The hypothesized mechanism is that these solvents are oxidatively unstable and undergo gradual chemical reactions during storage, yielding voltage reductions.

SEI Film Instability

Variations in warehouse temperatures can lead to SEI film instability, resulting in deterioration and subsequent reactions that cause battery swelling and lowered pressure.

Inadequate Packaging

Excessive sealing of the pole position may lead to corrosion whilst consuming the battery's lithium voltage. If other sections are over-sealed, electrolytes may infiltrate the CPP layer, corroding the aluminum foil and leading to perforations in the aluminum-plastic film, allowing moisture to enter and causing low-pressure swelling.

Instances of swelling and low pressure frequently occur simultaneously, typically indicating a severe issue that may ultimately result in battery disposal.

1. Strategies for Self-Discharge Management

Lithium battery self-discharge diminishes product quality and customer satisfaction. So, how can we effectively manage this phenomenon? Here are several approaches to consider:

1. Thorough Dust Control

Dust and burrs are prevalent causes of voltage declines in batteries; hence, controlling dust is a critical but challenging task. Many manufacturers recognize the importance of workshop dust control but often neglect it during practical implementation.

Firstly, factory architectural designs should be optimized. Areas involved in the pole piece manufacturing process need stringent dust controls, particularly in slurry preparation domains. However, during lithium battery assembly phases, the introduction of dust impurities must be rigorously restricted, requiring distinct separation, isolation, and protection of various zones.

Secondly, implementing robust 5S practices within operational areas is crucial. Developing stringent cleanliness habits and fostering high 5S competency can substantially raise product yield rates. For example, performing cleanups before work can assure dust-free environments, while cleaning equipment post-operation prevents impurity retention.

1. Enhancing Pole Piece Production Processes

Burr formations significantly contribute to lithium battery self-discharge. Burrs predominantly develop during pole piece slitting processes and can arise from various factors:

(1) The use of positive and negative electrode slurries derived from materials with higher BETs and excessive conductive agent additions can lead to weak bonding between active materials and conductive agents. Hence, selecting suitable materials and improving slurry preparation, coating, and related processes to prevent powder loss and burr generation is essential.

(2) Timely replacement of slicing tools is critical; familiarity with cutter lifespan and their timely exchange according to usage ensures minimal burr generation.

1. Raw Material Quality Management

As previously mentioned, metal impurities in electrodes contribute significantly to lithium battery self-discharge. Thus, companies must enhance incoming material inspection to ensure compliance with standards, as failure to do so can lead to substantial losses. Moreover, stringent quality control of raw materials is necessary during storage to prevent impurity and moisture ingress.

1. Environmental Factors Control

Effective environmental control not only includes monitoring dust particle counts but also managing moisture levels at crucial stages to avert adverse effects of excess moisture on lithium battery quality.