Cycle life of lithium ion batteries
The battery is used, it feels not durable, and the capacity is not as much as before. These are the manifestations of the continuous decay of cycle life. Next, we will talk about two indicators related to the long-term stable and reliable use of lithium-ion batteries: cycle life and safety.
The decay of the cycle life is actually a change trend of the battery's current actual usable capacity, which is declining relative to its rated capacity at the factory.
For an ideal lithium-ion battery, the content balance does not change during its cycle, and the initial capacity in each cycle should be a certain value, but in reality the situation is much more complicated. Any side reaction that can generate or consume lithium ions can cause a change in battery capacity balance. Once the battery's capacity balance changes, the change is irreversible and can be accumulated through multiple cycles, causing serious battery cycle performance. influences.
There are many factors that affect the cycle life of lithium-ion batteries, but the underlying cause is that the amount of lithium ions involved in energy transfer is decreasing. It should be noted that the total amount of lithium in the battery is not reduced, but the amount of lithium ions that are "activated" is less. They are blocked in other places or the channels of activity are blocked, and they are not free to participate in the cycle of charge and discharge. process.
Then, as long as we figure out which lithium ions that should have been involved in the redox reaction, we will be able to figure out the mechanism of capacity reduction, and we can take measures to delay the decline of lithium battery capacity and improve The cycle life of lithium batteries.
1. Deposition of metallic lithium
Through the previous analysis, we know that there is no metal form of lithium in lithium-ion batteries. Lithium is either in the form of metal oxides or carbon-lithium compounds or in the form of ions.
The deposition of metallic lithium generally occurs on the surface of the negative electrode. For a certain reason, when lithium ions migrate to the surface of the negative electrode, part of the lithium ions do not enter the negative electrode active material to form a stable compound, but the electrons are deposited on the surface of the negative electrode to become metallic lithium, and no longer participate in the subsequent cycle process, resulting in The capacity is reduced.
In this case, there are generally several reasons: charging exceeds the cut-off voltage; large rate charging; insufficient anode material. When the overcharge or the negative electrode material is insufficient, the negative electrode cannot accommodate lithium ions that have migrated from the positive electrode, resulting in deposition of metallic lithium. When charging at a large rate, the amount of lithium ions reaching the negative electrode in a short time is excessive, causing clogging and deposition.
The deposition of metallic lithium not only causes a decrease in cycle life, but also causes a short circuit between the positive and negative electrodes, causing serious safety problems.
To solve this problem, a reasonable ratio of positive and negative materials is needed, and the conditions of use of the lithium battery are strictly limited to avoid exceeding the limit of use. Of course, starting from the rate performance, it is also possible to locally improve the cycle life.
2. Decomposition of the cathode material
Lithium-containing metal oxides as cathode materials, although having sufficient stability, will continue to decompose during long-term use, producing some electrochemically inert substances (such as Co3O4, Mn2O3, etc.) and some flammable gases. The capacity balance between the electrodes is destroyed, resulting in irreversible loss of capacity.
This situation is particularly noticeable in the case of overcharging, and sometimes even severe decomposition and gas release occur, which not only affects battery capacity, but also poses a serious safety risk.
In addition to strictly limiting the charge cut-off voltage of the battery, improving the chemical stability and thermal stability of the positive electrode material is also a feasible method for reducing the cycle life.
3. SEI film on the electrode surface
As mentioned above, in the lithium ion battery with carbon material as the negative electrode, the electrolyte will form a solid electrolyte (SEI) film on the surface of the electrode during the initial cycle. Different anode materials will have certain differences, but the SEI film The composition is mainly composed of lithium carbonate, lithium alkylate, lithium hydroxide, etc., of course, there are also decomposition products of salt, and some polymers and the like.
The formation process of the SEI film consumes lithium ions in the battery, and the SEI film is not stable, and will rupture continuously during the cycle, exposing a new carbon surface and reacting with the electrolyte to form a new SEI film. Constantly causing continuous loss of lithium ions and electrolytes, resulting in a decrease in battery capacity. The SEI film has a certain thickness. Although lithium ions can penetrate, the SEI film may cause clogging of a part of the diffusion channel on the surface of the negative electrode, which is disadvantageous for the diffusion of lithium ions in the negative electrode material, which also causes a decrease in battery capacity.
4. The influence of electrolytes
In the process of continuous circulation, due to the limitations of chemical stability and thermal stability, electrolytes will continue to decompose and volatilize, accumulating for a long time, resulting in a decrease in the total amount of electrolytes, insufficient infiltration of positive and negative materials, and incomplete charging and discharging reactions. , causing a decline in the actual use capacity.
The electrolyte contains active hydrogen and metal ion impurities such as iron, sodium, aluminum, and nickel. Because the oxidation potential of the impurity is generally lower than the positive electrode potential of the lithium ion battery, it is easy to oxidize on the surface of the positive electrode, and the oxide is reduced at the negative electrode, continuously consuming the positive and negative active materials, causing self-discharge, that is, changing the battery in the case of abnormal use. Discharge. Battery life is determined by the number of charge and discharge cycles, and the electrolyte containing impurities directly affects the number of battery cycles.
The electrolyte also contains a certain amount of water, which chemically reacts with LiFP6 in the electrolyte to produce LiF and HF, which in turn destroys the SEI film, generates more LiF, causes LiF deposition, and continuously consumes active lithium ions. Causes a decrease in battery cycle life.
It can be seen from the above analysis that the electrolyte has a very important influence on the cycle life of the lithium ion battery, and selecting a suitable electrolyte will significantly improve the cycle life of the battery.
5. The diaphragm is blocked or damaged
The function of the separator is to separate the positive and negative electrodes of the battery to prevent short circuit. During the lithium-ion battery cycle, the failure of the separator to dry out is an important cause of the early performance degradation of the battery. This is mainly due to the insufficient electrochemical stability and mechanical properties of the separator itself, as well as the deterioration of the electrolyte's wettability to the separator during repeated charging. Due to the dryness of the isolation membrane, the ohmic internal resistance of the battery increases, resulting in blockage of the charge and discharge channels, incomplete charging and discharging, and the battery capacity cannot be restored to the initial state, which greatly reduces the capacity and service life of the battery.
6. The positive and negative materials fall off
The active material of the positive and negative electrodes is fixed on the substrate by the binder. During the long-term use, the active material of the positive and negative electrodes continuously falls off due to the failure of the binder and the mechanical vibration of the battery, and enters the electrolyte solution. This leads to a continuous decrease in the amount of active substances that can participate in the electrochemical reaction, and the cycle life of the battery is continuously declining.
The long-term stability of the binder and the good mechanical properties of the battery will delay the rate of decline in battery cycle life.
7. External use factors
Lithium-ion batteries have reasonable conditions and ranges, such as charge and discharge cut-off voltage, charge and discharge rate, operating temperature range, and storage temperature range. However, in actual use, abuse beyond the allowable range is very common. Long-term unreasonable use will lead to irreversible chemical reactions inside the battery, causing damage to the battery mechanism, accelerating the aging of the battery, resulting in a rapid decline in cycle life. It also creates a safety incident.
Lithium-ion battery safety
The internal safety of lithium-ion batteries is caused by thermal runaway inside the battery and the accumulation of heat, which causes the internal temperature of the battery to continuously rise. The external performance is the intense energy release phenomenon such as combustion and explosion.
The battery is a high-density carrier of energy. Intrinsically, there are insecure factors. The higher the energy density, the greater the impact of the intense release of energy and the more prominent the safety problem. High-energy carriers such as gasoline, natural gas, and acetylene all have the same problems, and there are countless safety incidents every year.
Different electrochemical systems, different capacities, process parameters, use environment, and degree of use have a great impact on the safety of lithium-ion batteries.
Since the battery stores energy, during the energy release process, when the heat generation and accumulation speed of the battery is greater than the heat dissipation speed, the internal temperature of the battery continues to rise. Lithium-ion battery consists of a highly active positive electrode material and an organic electrolyte. It is very prone to severe chemical side reactions under heated conditions. This reaction will generate a lot of heat, and even cause "thermal runaway", which is a dangerous battery. The main cause of the accident.
The thermal runaway inside the lithium-ion battery indicates that some chemical reactions inside the battery are not the "controllable" and "ordered" that we have expected before, but show an uncontrollable and disordered state, resulting in rapid and intense release of energy. .
So, let's see what chemical reactions are there, accompanied by a lot of heat generation, which leads to thermal runaway.
1. SEI membrane decomposition, electrolyte exothermic side reaction
The solid electrolyte membrane is formed during the initial cycle of the lithium ion battery, and we neither want the SEI membrane to be too thick nor hope that it is completely absent. The presence of a reasonable SEI film protects the negative active material from reacting with the electrolyte.
About the cycle life and safety of lithium-ion batteries
However, when the internal temperature of the battery reaches about 130 ° C, the SEI film will be decomposed, causing the negative electrode to be completely exposed, and the electrolyte is largely liberated on the surface of the electrode, causing the internal temperature of the battery to rise rapidly.
This is the first exothermic side reaction inside the lithium battery and the starting point for a series of thermal runaway problems.
2. Thermal decomposition of electrolyte
Due to the exothermic side reaction of the electrolyte in the negative electrode, the internal temperature of the battery is continuously increased, which further causes thermal decomposition of LiPF6 and the solvent in the electrolyte.
About the cycle life and safety of lithium-ion batteries
This side reaction occurs at a temperature range of approximately 130 ° C to 250 ° C, which is accompanied by a large amount of heat generation, which further pushes up the temperature inside the battery.
3. Thermal decomposition of the cathode material
As the internal temperature of the battery further rises, the active material of the positive electrode is decomposed, and this reaction generally occurs between 180 ° C and 500 ° C with a large amount of heat and oxygen.
Different positive electrode materials have different heat generated by decomposition of active substances, and the amount of oxygen released is also different. The lithium iron phosphate cathode material has the most heat generation in all the cathode materials because of the low heat generated during the decomposition. When the nickel-cobalt-manganese ternary material decomposes, it generates more heat, accompanied by a large amount of oxygen released, which is prone to combustion or explosion, so the safety is relatively low.
4. Reaction of binder with negative active substance
The reaction temperature of the negative active material LixC6 and the PVDF binder starts from about 240 ° C, the peak appears at 290 ° C, and the reaction exotherm reaches 1500 J / g.
It can be seen from the above analysis that the thermal runaway of the lithium ion battery is not instantaneous, but a gradual process. This process, generally caused by overcharge, large rate charge and discharge, internal short circuit, external short circuit, vibration, collision, drop, impact, etc., causes a large amount of heat inside the battery in a short time, and continuously accumulates, pushing the temperature of the battery continuously rise.
Once the temperature rises to the threshold temperature of the internal chain reaction (about 130 ° C), the interior of the lithium-ion battery will spontaneously produce a series of exothermic side reactions, and further increase the heat accumulation and temperature rise inside the battery. A large amount of flammable gas will be precipitated. When the temperature rises to the flash point and ignition point of internal solvents and flammable gases, it will cause safety accidents such as burning and explosion.
The lithium-ion battery that has just been manufactured has passed the safety test certification and does not represent the safety of the lithium-ion battery during its life cycle. According to our previous analysis, in the long-term use process, lithium metal deposition on the surface of the negative electrode occurs, the electrolyte is decomposed and volatilized, the positive and negative active materials are detached, the internal structure of the battery is deformed, metal impurities are mixed in the material, and others. Many unintended changes, which can cause internal short circuits in the battery, which in turn generates a lot of heat. In addition, external abuse, such as overcharge, extrusion, metal puncture, collision, drop, impact, etc., will also cause the battery to generate a large amount of heat in a short period of time, which becomes the cause of thermal runaway.
In the use of lithium-ion batteries, there is no absolute safety, only relative security. We should try our best to avoid abuse and reduce the probability of occurrence of hazardous events. At the same time, we must start with the main components such as positive and negative materials, electrolytes and separators, and choose materials with excellent chemical stability and thermal stability. Flame-retardant properties, in the presence of internal and external thermal runaway incentives, reduce the heat of internal side reactions, or have a high ignition temperature to avoid thermal runaway. In the design of the battery structure and the housing, the structural stability should be fully considered to achieve sufficient mechanical strength to withstand external stresses and ensure that no significant deformation occurs inside. In addition, the heat dissipation performance needs to be considered. If the heat can be dissipated in time, the internal temperature will not continue to rise, and thermal runaway will not occur.
The safety design of lithium-ion batteries is systematic. It is not comprehensive to measure the safety of lithium-ion batteries by simply decomposing the heat of the positive electrode materials. From a system perspective, lithium iron phosphate batteries are not necessarily safer than ternary materials bec
ause there are many factors that ultimately affect thermal runaway, and the heat generated by the decomposition of the positive electrode material is only one factor.
Summary and outlook
About 13.5 billion years ago, after the so-called "big bang", the matter, energy, time and space in the universe formed what it is today. These basic characteristics of the universe become "physics."
After about 300,000 years later, matter and energy began to form complex structures called "atoms", which further formed "molecules". As for the stories of these atoms and molecules and how they interact, it becomes "chemistry."
All the principles of the battery must be explained by the theory of physics and chemistry, and subject to the objective laws. From this category, we can neither invent the battery nor use the battery properly.
Humans have been researching and using batteries for nearly 200 years. In large-scale commercial applications, lead-acid batteries, alkaline batteries, zinc-manganese batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and lithium-ion batteries have already penetrated. All aspects of human society play an irreplaceable role in supporting the normal operatio of industrialized society.
The pursuit of human energy for mobile storage, with the expansion of economic scale, shows a rapid growth trend, which also objectively promotes the development and transformation of battery technology, to be faster, stronger, longer, and safer. More environmentally friendly, and the unit price is even cheaper.
Since the commercialization of lithium-ion batteries by SONY in the 1990s, after more than 20 years of development, the existing electrochemical system has gradually approached the bottleneck, and will gradually enter the era of “post-lithium battery” in the future. The strong demand of the market will certainly promote and promote the application of new materials, new chemical systems, and new processes in the battery field, thus achieving a major breakthrough.
In the battery industry, new research directions are emerging, and there are more promising commercialization directions, such as all-solid lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, and lithium-air batteries. In the era of “post-lithium battery”, it will be a situation where hundreds of flowers bloom and hundreds of schools contend, the diversity of market demand, the diversity of technical routes, and the geographical factors of raw material supply will bring us more choices and better experiences. .