Life and Safety of Polymer Lithium-ion Battery

Lithium-ion battery cycle life

Batteries are used, feel not durable, capacity is not more than before, these are the continuous decay of the cycle life.

The decay of the cycle life is actually a changing trend of the actual available capacity of the battery, relative to its rated capacity at the time of delivery.

For an ideal lithium-ion battery, the capacity balance within its cycle does not change, and the initial capacity in each cycle should be a certain value, but in fact the situation is much more complicated. Any side reaction that can produce or consume lithium ions may lead to a change in the battery capacity balance. Once the battery's capacity balance state changes, this change is irreversible and can be accumulated through multiple cycles, which can have serious effects on the battery cycle performance..

There are many factors that affect the cycle life of lithium ion batteries, but the underlying reason is that the number of lithium ions involved in energy transfer is constantly decreasing. It should be noted that the total number of lithium elements in the battery has not decreased, but there are fewer "activated" lithium ions. They are trapped in other places or the channels of activity are blocked and can not freely participate in the process of circular charging and discharging.

So we just need to figure out where the lithium ions that should have been involved in the Redox reaction have gone, and we can figure out the mechanism of the reduction in capacity, and we can also take targeted measures to slow down the decline in the capacity of lithium batteries and increase the recycling life of lithium batteries.

1. Sedimentation of metallic lithium

From the previous analysis, we know that lithium ion batteries should not exist in the metal form of lithium, lithium elements are either in the form of metal oxides, carbon lithium compounds, or in the form of ions.

The deposition of metallic lithium generally occurs on the negative surface. For some reason, when lithium ions migrate to the negative electrode surface, some lithium ions do not enter the negatively active material to form a stable compound. Instead, electrons are obtained and deposited on the negative electrode surface to become metallic lithium, and no longer participate in subsequent cycles. The process leads to a decrease in capacity.

In this case, there are generally several reasons: charging exceeds the cut-off voltage; High rate charging; Negative materials are insufficient. When overcharging or negative material is insufficient, the negative pole can not accommodate lithium ions that have migrated from the positive pole, resulting in the deposition of metallic lithium. When the large ratio is charged, due to the excessive number of lithium ions reaching the negative pole in a short period of time, congestion and deposition are caused.

The deposition of metallic lithium will not only cause a decrease in the cycle life, but also lead to short circuits of positive and negative poles in serious cases, causing serious safety problems.

To solve this problem, it is necessary to have a reasonable ratio of positive and negative electrode materials, and at the same time strictly limit the conditions for the use of lithium batteries to avoid exceeding the use limit. Of course, starting from the ratio performance, it can also partially improve the cycle life.

2. Decomposition of Positive Material

Lithium-containing metal oxides as positive materials, although they have sufficient stability, continue to decompose during long-term use, producing some electrochemical inert substances(such as Co 3O4, Mn2O3, etc.) and some flammable gases., It destroys the capacity balance between electrodes and causes irreversible loss of capacity.

This situation is particularly evident in over-charging, and sometimes even violent decomposition and gas release can affect not only battery capacity but also serious safety risks.

In addition to strictly limiting the charging cut-off voltage of the battery, improving the chemical stability and thermal stability of the positive electrode material is also a feasible method to reduce the rate of decline in the cycle life.

3. SEI membrane on electrode surface

As mentioned earlier, lithium ion batteries with carbon as the negative electrode will form a layer of solid electrolyte(SEI) membrane on the surface of the electrode during the initial cycle, and different negative electrode materials will have certain differences. However, the composition of the SEI membrane is mainly composed of lithium carbonate, alkyl Ester lithium, lithium hydroxide, etc.. Of course, there are also decomposition products of salt, and there are also some polymers.

The formation process of the SEI membrane consumes lithium ions in the battery, and the SEI membrane is not stable and will continue to break during the cycle process, exposing new carbon surfaces and reacting with electrolytes to form a new SEI membrane. This will continue to cause continuous losses of lithium ions and electrolytes, resulting in a reduction in battery capacity. The SEI membrane has a certain thickness. Although lithium ions can penetrate, the SEI membrane will cause blockages in the diffusion of the negative surface part, which is not conducive to the diffusion of lithium ions in the negative electrode material. This will also cause the reduction of battery capacity.

4. Effects of electrolytes

In the continuous cycle process, due to the limitations of chemical stability and thermal stability, electrolytes will continue to decompose and evaporate, which will accumulate over the long term, 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 decrease in the actual capacity of use.

The electrolyte contains active hydrogen substances and metal ions such as iron, sodium, aluminum, and nickel. Because the oxidation potential of impurities is generally lower than the positive potential of lithium-ion batteries, it is easy to oxidize on the positive surface, and the oxide is reduced at the negative electrode, constantly consuming positive and negative active substances, causing self-discharge, that is, changing the battery discharge in the case of abnormal use.. The battery life is determined by the number of cycles, and the electrolyte containing impurities directly affects the number of cycles.

The electrolyte also contains a certain amount of water. Water reacts with LiFP6 in the electrolyte to produce LiF and HF. HF, in turn, destroys the SEI membrane and generates more LiF, causing LiF deposition and continuous consumption of active lithium ions. Causes the battery cycle life to decline.

From the above analysis, it can be seen that electrolytes have a very important influence on the cycle life of lithium-ion batteries. Choosing the appropriate electrolytes will significantly increase the cycle life of batteries.

5. Isolation membrane blockage or damage

The role of the isolation membrane is to separate the positive and negative poles of the battery to prevent short circuits. In the process of lithium ion battery cycle, the separation membrane drying up and failure is an important reason for the early deterioration of battery performance. This is mainly due to the lack of electrochemical stability and mechanical properties of the isolation film itself, and the infiltration of the electrolyte to the isolation film during repeated charging process. Due to the drying up of the isolation membrane, the ohmic resistance of the battery increases, resulting in congestion of the charging and discharging channel, incomplete charging and discharging, and the battery capacity can not return to the initial state, greatly reducing the battery capacity and service life.

6. Positive and negative materials fall off

The positive and negative active substances are fixed to the matrix by binders. During the long-term use process, due to the failure of binders and the mechanical vibration of the battery, the positive and negative active substances continue to fall off and enter the electrolyte solution. This leads to the continuous reduction of active substances that can participate in electrochemical reactions and the continuous reduction of battery cycle life.

The long-term stability of the binder and the good mechanical properties of the battery will slow down the decline of the battery cycle life.

7. External use factors

Lithium-ion batteries have reasonable conditions and range of use, such as charging and discharging off voltage, charging and discharging ratio, operating temperature range, storage temperature range, etc.. However, in actual use, abuse beyond the allowed range is very common. Long-term unreasonable use will lead to irreversible chemical reactions within the battery, causing damage to the battery mechanism, accelerating the aging of the battery, and causing a rapid decline in the cycle life., severe, There are also safety incidents.

The page contains the contents of the machine translation.