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Lithium ion batteries have raised increasing interest due to their high potential to provide efficient energy storage and environmental sustainability. At present, not only the battery market for portable electronics is dominated by them, but they are also widely used in the field of electric vehicles and energy storage. While these batteries are very impressive, as the saying goes, every coin has two sides. Increasing attention is being paid to the battery safety issue. The following analyzes various aspects of the safety issue, and then proposes solutions to improve the safety of batteries under normal and abusive conditions.
The movement of lithium ions through the electrolyte between the anode and the cathode generates a significant amount of heat, which is released as a result of the Joule heat and the chemical energy of the charge/discharge process. This heat release is a normal part of battery operations, but if the generated heat does not have an efficient dissipation path in certain charge/discharge states, the battery would become overheated. Even under normal operation conditions, the heat generated by the battery cannot be completely removed, especially in high temperature enviroments. Rising in battery temperature would trigger other undesirable parasitic reactions, causing thermal runaway, which seriously compromise the battery safety.
Undesirable Chemical Reactions
In the normal voltage and temperature range, only Li+ shuttle occurs in the electrolyte during the insertion/extraction cycles at the cathode and anode. In high temperature and high voltage conditions the electrochemical reactions become more complex, including decomposition of the solid electrolyte interface film, oxygen release at the cathode side, and additional electrolyte/electrode parasitic side reactions. Decomposition of the solid electrolyte interface film and interfacial reactions initially accelerate the temperature rising, thereby increasing risks of oxygen release from the active cathode materials. These reactions eventually lead to the thermal runaway, which causes battery rupture and explosion due to the reaction of hot flammable gases from the battery with the ambient oxygen.
Thermal Runaway
Thermal runaway is the most detrimental battery safety issue. The origins of battery thermal runaway include side reactions of electrolyte, cathode, anode, and interface reactions at the surface of electrodes and Li plating. These side reactions are triggered by mechanical, thermal, or electrical abuse. Rupture of the separator and evolution of oxygen from the cathode side are the root causes of thermal runaway.
There are five types of causes for this phenomenon. The first type is uncontrollable internal heat generation, which causes oxygen release from the cathode material, leading to numerous side reactions. In the second type, separator defects due to thermally-induced shrinkage or mechanical damage create short circuits in the battery and rapid discharge of the energy stored in it, accompanied by undesirable chemical chain reactions and release of massive amounts of heat. The third type is electrical abuse. Electrolyte decomposition, especially in a high state of charge, occurs at the cathode interface. This leads to heat accumulation and consequently release of oxygen from the cathode and damage to the separator. The fourth type consists of electrochemical side reactions caused by local thermal abuse. If the heat generated during normal battery operations cannot be dissipated quickly enough, the separator in that specific place will shrink or rupture. The fifth type occurs during mechanical battery damage, which causes short circuits and/or air to penetrate the battery. The main causes of battery safety accidents among these five categories are short-circuiting due to separator damage, electrical abuse, and mechanical abuse.
Mechanical Abuse
Due to the high energy density of lithium ion batteries, local damage caused by external influences, for example in case of collisions, will release a significant amount of heat, which can easily cause thermal runaway. The outer casing provides a cell’s first level of thermal and mechanical protection. The shell casing needs to withstand mechanical force and not break, and ensure that the internal structure is not damaged under certain deformation conditions. The mechanical behavior of the shell casing is an important aspect of the overall battery mechanical behavior, as it is the most vulnerable point during safety accidents, so it should be understood and taken into account during battery materials’ design and consideration. Even when the shell casing is only deformed, battery internal components might still be severely damaged. metallic current collectors and separators with insufficient flexibility would break, causing direct contact between the electrodes. When the heat generated by the battery after a localized short circuit is sufficient to trigger other internal short circuits in the area, thermal runaway occurs in overall battery.
Electrical Abuse
When a battery is in an overcharge or over-discharge state, or is undergoing an external short circuit, it experiences electrical abuse, and a series of undesirable electrochemical reactions occurs in it. Overcharge first causes electrolyte decomposition at the cathode interface. This reaction slowly increases the battery temperature. Subsequently, excessive Li+ deintercalation from the cathode occurs. The cathode material becomes unstable and start to release oxygen, while excess Li+ deposits on the anode to form Li dendrite. Heat and gas generation during the side reactions would lead to safety accidents, such as cell overheating and rupture. Charging rate is often the most significant factor affecting overcharge, as the overcharging current density determines the rate of heat generation by the battery reactions: the higher the current, the more heat is generated per unit time, thereby increasing the risks of uncontrollable battery behavior.
The principle of over-discharge is similar that of over-charge. Forced over-discharge continuously releases Li+ from the anode, which change the graphite structure and destroy the solid electrolyte interface film. At very deep set state of discharge, a copper current collector is oxidized, with the released copper ions potentially being deposited on the cathode surface. Too much copper deposition results in the short-circuit of cell. An external short circuit occurs when a cathode and an anode of the same cell are in direct contact through a conductor. In such cases, instead of decoupled electron and ion transportation, both electron and ion transfer occur at the same place and Li+ migrates quickly inside the cell, rapidly discharging the battery.
Overcharge is the most dangerous types of electrical abuse and one of the most frequently observed reasons for battery safety accidents. The other two types of electrical abuse, over-discharge and external short circuiting, are relatively benign and do not cause instant and fast-developing accidents. They can, however, still impair a battery state of health.
Thermal Abuse
In thermal abuse situations, a battery experiences thermal shock, or its local temperature is too high. In theory, battery cycling will not cause safety accidents because the heat generated during normal anodic and cathodic reactions is insufficient to cause a sharp temperature increase. In reality, however, the electrode heat release rate is often higher than its cooling rate. Heat dissipation of a battery depends on its external surface area and geometry. Heat dissipation by radiation helps to alleviate some of the generated heat. As a result, some of the heat remains stored inside the battery. At some point, if this heat continues to accumulate instead of being dissipated, exothermic side reactions start to occur, further concentrating thermal stress.
Improvement Solutions
Since undesirable and uncontrollable heat and gas generation from various parasitic reactions are the leading causes of battery safety accidents, efforts to improve battery safety should focus on ways to prevent battery from generating excessive heat, keeping them working at a suitable voltage range, and improving their cooling rates.
The battery safety is determined by the active material and electrolyte chemistry, the speed of heat generation and dissipation, and the tolerance of external forces. On one hand, safety analysis should start with evaluating the electrode active materials, electrolytes, and separators, as these are the most controllable factors. By carefully choosing electrode materials, separators, and electrolytes, and by optimizing battery design the chemical, thermal and structural stablility can be significantly improved. On the other hand, strategies for alleviating the consequences of mechanical, thermal and electrical abuse such as shell casing, cell cooling, cell balancing, charging monitoring and other cell protection and management technologies also need to be engineered into batteries.
Moreover, freshly-made batteries should be tested for safety before they are incorporated into devices. The safety standards and test methods are intended to be developed to ensure that lithium ion batteries and their components meet specified safety criterias. They are essential for ensuring that batteries available on the market are of sufficient quality for intended purposes. Most countries and international organizations have developed lithium ion battery safety oriented standards including International Electrotechnical Commission (IEC) standard IEC62133, United Nations (UN) standard UN38.3, Underwriters Laboratories (UL) standard UL1642/2054, Chinese standard GB/T18287.
