Internal Short Circuit Analysis of Lithium-Ion Batteries

In the complex charging and discharging conditions of practical operation, lithium-ion batteries, despite being managed by battery energy management systems to operate as normally as possible, can still experience mechanical abuse, electrical misuse, and thermal misuse during special situations such as overcharging, overdischarging, and overheating. These issues can lead to rapid performance degradation, internal short circuits, and ultimately result in thermal runaway safety problems.

This article conducts a systematic study on internal short circuit principles, induced experimental methods, internal short circuit identification methods, and preventive measures. It provides insights into the identification and prevention of internal short circuits in lithium-ion batteries, offering guidance for safety protection and applications.

Internal Short Circuit of Lithium-Ion Batteries

1. Study on Internal Short Circuit Mechanism

The triggering conditions for internal short circuits can be divided into three types: mechanical abuse, electrical misuse, and thermal misuse, as shown in Figure 1.

Mechanical abuseinvolves actions such as needle puncture and compression, causing mechanical deformation and partial membrane rupture, triggering an internal short circuit in the battery.
Electrical misuseresults in lithium deposition and dendrite growth, penetrating the membrane pores and connecting the positive and negative electrode portions of the battery, causing an internal short circuit.
Thermal misuseinvolves high temperatures causing extensive shrinkage and collapse of the membrane, leading to an internal short circuit in the battery.

When a lithium-ion battery experiences an internal short circuit, it generates high currents and a significant amount of local heat, ultimately resulting in thermal runaway.

Figure 1: Conditions Triggering Internal Short Circuits

Internal Short Circuits exist throughout the battery’s entire lifecycle. And its developmental evolution can be categorized into early, middle, and late stages, as shown in Table 1.

Table 1 Characteristic changes in the evolution process of internal short circuit

Internal short circuit development stage	Voltage	Temperature	Controllability	Duration
Initial stage	Decline slowly	No significant changes	Controllable	Longer
Mid stage	Significant decline	Significantly increased	Controllable	Shorter
Terminal stage	Down to 0V	Increase rapidly	Can not control	Extremely short

In the initial stage of internal short circuits, the voltage drop caused by the short circuit is relatively slow. At the same time,the generated heat, which is minimal, can be dissipated in a timely manner by the cooling system. There is no significant change in battery temperature during this stage, and it lasts for a long time, making it less likely to be detected.
In the middle stage of internal short circuits, there is a noticeable voltage drop, and the increased heat, which is more substantial, cannot be dissipated promptly, leading to heat accumulation. The battery temperature significantly rises during this stage, and it lasts for a shorter duration, exhibiting clear characteristics and being more easily identified.
In the late stage of internal short circuits, widespread short circuits in the battery result in the voltage dropping to 0V. A significant amount of heat is instantly generated, leading to thermal runaway of the battery. This stage has an extremely short duration and is unstoppable. The characteristic changes in the evolution process of internal short circuits are summarized in Table 1.

2. Methods for Inducing Internal Short Circuits

Currently, there are mainly three types of methods for inducing internal short circuits in lithium-ion batteries: the abuse condition method, the artificial design of internal defects method, and the equivalent resistance method.

The triggering mechanisms and the analysis of advantages and disadvantages of these methods are summarized in Table 2.

Table 2: Methods for Inducing Internal Short Circuits in Lithium-ion Batteries

Experimental Methods for Triggering Internal Short Circuits		Mechanisms for Triggering	Advantages	Disadvantages
Abusive conditions	Mechanical abuse	Applying mechanical pressure or puncturing the battery to induce internal short circuits.	Real internal short circuit	Not repeatable
	Electrical abuse	Cycling the battery through overcharging and discharging to promote the growth of metal dendrites, piercing the separator and inducing internal short circuits.	Real internal short circuit	Long test time, not repeatable
	Thermal abuse	Conducting high-temperature tests on the battery to induce internal short circuits.	Real internal short circuit	Not repeatable
Artificially designed internal defect	Phase-change material / low-melting-point alloy substitution experiment	Introducing phase-change materials/melting alloys into the battery, triggering internal short circuits by melting the material through heating.	Repeatable for multiple types of internal short circuit experiments	Complex experimental battery fabrication process
	Shape memory alloy substitution experiment	Incorporating shape memory alloys inside the battery, triggering internal short circuits by heating the battery, causing the tip of the component to lift.	Repeatable for multiple types of internal short circuit experiments	Complex experimental battery fabrication process
	Impurity particle substitution experiment	Adding impurity particles to the battery, triggering internal short circuits through the charge and discharge process.	Close to real experiments	Poor repeatability
	Hole squeezing experiment	Creating defects or holes inside the battery, triggering internal short circuits by mechanically squeezing the battery.	Repeatable for multiple types of internal short circuit experiments	Damages battery integrity
Equivalent resistance		Inserting an equivalent internal short circuit in batteries with special structures, controlling the occurrence of internal short circuits through switch control.	Repeatable experiments, batteries can be reused	Unable to observe real damage of internal materials caused by internal short circuit

3. Analysis of Internal Short Circuit Identification Methods

In order to prevent the development of internal short circuits to the point of uncontrollable thermal runaway, researchers have long been devoted to the study of accurate identification methods for early-stage internal short circuits in lithium batteries.

The current internal short circuit identification methods can be summarized into the following five categories:

(1) Empirical Data Deviation Identification Method

This method requires the establishment of a reliable battery state prediction model. Real-time measured values of parameters such as voltage and temperature during the battery charging and discharging process are then compared and analyzed against the model’s predicted values.

If the calculated deviation exceeds the allowable range of error, it is determined that the battery has experienced an internal short circuit.

Since the characteristic parameters of the battery, such as voltage and temperature, do not change significantly in the early stages of an internal short circuit, this method has limited effectiveness in identifying early-stage internal short circuits and cannot identify internal short circuits in parallel battery packs.

(2) Voltage Signal Anomaly Recognition Method

This method is based on the principle that there will be an abnormal voltage drop-recovery phenomenon when a ceramic diaphragm-type battery experiences an internal short circuit. By detecting whether there is a voltage drop-recovery anomaly in the battery voltage signal during the battery charging and discharging process, once an anomaly is detected, it is determined that the battery has experienced an internal short circuit.

Since only ceramic diaphragm-type batteries coated with porous protective materials exhibit the voltage drop-recovery anomaly when experiencing an internal short circuit, this method can only identify internal short circuits in series-connected battery packs of specific battery types, with significant limitations.

(3) Battery Self-Discharge Identification Method

Internal short circuits in batteries inevitably cause self-discharge processes that exceed the normal range. By comparing voltage levels before and after static storage and using constant voltage source benchmarking, the method detects whether the battery undergoes an abnormal self-discharge process. If it does, it is determined that the battery has experienced an internal short circuit.

Since this method requires the battery to be in a static state and not in operation, it cannot be used for real-time identification of internal short circuits during battery operation or for identifying internal short circuits in parallel battery packs.

(4) Battery Consistency Identification Method

Based on the assumption of consistency among individual batteries, this method monitors parameters such as voltage, capacity, and remaining charge of each battery cell within the same battery pack.

If a particular battery cell deviates significantly from the normal parameters of other cells, disrupting the overall consistency of the battery, it is determined that an internal short circuit has occurred in that specific battery cell.

Since the initial changes in voltage and capacity characteristics are not noticeable in the early stages of an internal short circuit, this method has limited effectiveness in identifying early-stage internal short circuits and is unable to identify internal short circuits in parallel battery packs.

(5) Special Circuit Identification Method

By examining parameters such as voltage and current in the symmetrical ring circuit topology, this method accurately identifies the position of a battery cell where an internal short circuit has occurred when changes in circuit parameter symmetry are detected.

This method addresses the high-precision identification and resistance estimation of internal short circuits in battery packs under parallel conditions. However, it faces challenges such as high costs for detection equipment, impacting the dynamic consistency of the batteries.

4. Internal Short Circuit Suppression Measures

The factors causing internal short circuits in batteries can generally be categorized into two types:

① those related to battery materials and processes

② those related to battery design and usage

The methods for suppressing and preventing internal short circuits from occurring are summarized from these two aspects as follows:

4.1 Battery Materials and Processes

This is mainly achieved through improvements in separator materials, electrolyte materials, electrode coatings, and optimizing production processes to reduce production defects.

The use of high-temperature-resistant, low self-discharge-rate ceramic separators and flame-retardant electrolytes or ionic liquid electrolytes can effectively suppress dendrite growth, reducing the risk of internal short circuits.
Coating low-conductivity layers or positive temperature coefficient materials on the current collector or positive/negative electrodes of battery cells can effectively reduce the internal short-circuit current and heat generation capacity during internal short circuits, thereby lowering the probability of triggering thermal runaway in the battery.
Optimizing the production processes of battery cores, separators, and other materials, along with impurity removal processes, can effectively filter out metal impurities, prevent irreversible side reactions between metal impurities and electrolytes, and reduce the risk of metal particles puncturing separators, causing internal short circuits.
Additionally, adopting advanced detection technologies to assess the internal structural integrity, processing accuracy, and alignment of electrode sheets can also help avoid potential risks of internal short circuits.

4.2 Battery Design and Usage Aspects

(1) Battery Software Design:

Setting up reasonable battery warnings and safety control strategies through Battery Management System (BMS) to achieve real-time monitoring of individual battery cell states. This facilitates the timely detection of the position of battery cells experiencing internal short circuits and the prompt elimination of safety hazards.
Implementing redundancy and balance design in cell charging and discharging to reduce the risk of internal short circuits caused by high battery loads.

(2) Battery Hardware Design:

Subdividing battery fuses into individual cell fuses, module fuses, pack fuses, and vehicle load fuses, among others, through layered management. This allows for the timely disconnection of the circuit of battery cells experiencing internal short circuits, preventing the continuous development of internal short circuits.
Designing a rational internal cooling system for the battery to enhance thermal conductivity and prevent thermal runaway caused by overheating, which can lead to decomposition reactions of electrodes, electrolytes, and separators.
Implementing a rational internal heating system for the battery, warming the battery to an appropriate operating temperature during low-temperature charging to prevent thermal runaway caused by dendrite penetration of the separator during cold charging.