What are the requirements for high-quality lithium-ion batteries? Generally speaking, long life, high energy density, and reliable safety performance are the prerequisites for measuring a high-quality lithium-ion battery. Lithium-ion batteries are currently used in all aspects of daily life, but the manufacturer or brand is different. There are some differences in the service life and safety performance of lithium-ion batteries, which are closely related to the production process standards and production materials; the following conditions must be the conditions for high-quality lithium-ion;

1. Dugi vijek trajanja

The life of the secondary battery includes two indicators: cycle life and calendar life. Cycle life means that after the battery has experienced the number of cycles promised by the manufacturer, the remaining capacity is still greater than or equal to 80%. The calendar life means that the remaining capacity shall not be less than 80% within the time period promised by the manufacturer, no matter whether it is used or not.

Life is one of the key indicators of power lithium batteries. On the one hand, the big action of replacing the battery is really troublesome and the user experience is not good; on the other hand, fundamentally, life is a cost issue.

Vijek trajanja litij-ionske baterije znači da se kapacitet baterije smanjuje nakon nominalnog kapaciteta (pri sobnoj temperaturi od 25 ° C, standardnom atmosferskom tlaku i 70% kapaciteta baterije ispražnjenom pri 0.2 ° C) , a život se može smatrati krajem života. U industriji se vijek trajanja općenito izračunava prema broju ciklusa potpuno napunjenih i ispražnjenih litij-ionskih baterija. U procesu uporabe unutar litij-ionske baterije dolazi do nepovratne elektrokemijske reakcije, što dovodi do smanjenja kapaciteta, poput raspadanja elektrolita, deaktiviranja aktivnih materijala i kolapsa pozitivnih i negativnih elektrodnih struktura dovesti do smanjenja broja interkalacija i deinterkalacije litijevih iona. Čekati. Eksperimenti pokazuju da će veća stopa pražnjenja dovesti do bržeg slabljenja kapaciteta. Ako je struja pražnjenja niska, napon baterije bit će blizu ravnotežnog napona, što može osloboditi više energije.

The theoretical life of a ternary lithium-ion battery is about 800 cycles, which is medium among commercial rechargeable lithium-ion batteries. Lithium iron phosphate is about 2,000 cycles, while lithium titanate is said to be able to reach 10,000 cycles. At present, mainstream battery manufacturers promise more than 500 times (charge and discharge under standard conditions) in the specifications of their ternary battery cells. However, after the batteries are assembled into a battery pack, due to consistency issues, the most important factors are voltage and internal The resistance can not be exactly the same, and its cycle life is about 400 times. The recommended SOC usage window is 10%~90%. Deep charging and discharging is not recommended, otherwise it will cause irreversible damage to the positive and negative structure of the battery. If it is calculated by shallow charge and shallow discharge, the cycle life will be at least 1000 times. In addition, if lithium-ion batteries are frequently discharged in high-rate and high-temperature environments, the battery life will be drastically reduced to less than 200 times.

2. Less maintenance, lower use cost

Sama baterija ima nisku cijenu po kilovat-satu, što je najintuitivniji trošak. Osim gore navedenog, za korisnike je li trošak stvarno nizak ovisi o „cijeni punog životnog ciklusa električne energije“.

“Full life cycle cost of electricity”, the total power of the power lithium battery is multiplied by the number of cycles to get the total amount of power that can be used in the full life cycle of the battery, and the total price of the battery pack is divided by this sum to get the price per kilowatt of electricity in the full life cycle.

The battery price we usually talk about, such as 1,500 yuan/kWh, is only based on the total energy of the new battery cell. In fact, the cost of electricity per unit of life is the direct benefit of the end customer. The most intuitive result is that if you buy two battery packs with the same power at the same price, one will reach the end of life after 50 times of charging and discharging, and the other can be reused after 100 times of charging and discharging. These two battery packs can be seen at a glance which is cheaper.

To put it bluntly, it is long life, durable and reduces costs.

In addition to the above two costs, the maintenance cost of the battery should also be considered. Simply consider the initial cost, select the problem cell, the later maintenance cost and labor cost are too high. Regarding the maintenance of the battery cell itself, it is important to refer to manual balancing. The BMS’s built-in equalization function is limited by the size of its own design equalization current, and may not be able to achieve the ideal balance between the cells. As time accumulates, the problem of excessive pressure difference in the battery pack will occur. In such situations, manual equalization has to be carried out, and the battery cells with too low voltage are charged separately. The lower the frequency of this situation, the lower the maintenance cost.

3. High energy density/high power density

Energy density refers to the energy contained in a unit weight or unit volume; the electric energy released by the average unit volume or mass of a battery. Generally, in the same volume, the energy density of lithium-ion batteries is 2.5 times that of nickel-cadmium batteries and 1.8 times that of nickel-hydrogen batteries. Therefore, when the battery capacity is equal, lithium-ion batteries will be better than nickel-cadmium and nickel-hydrogen batteries. Smaller size and lighter weight.

Battery energy density=battery capacity× discharge platform/battery thickness/battery width/battery length.

Gustoća snage odnosi se na vrijednost maksimalne snage pražnjenja po jedinici težine ili volumena. U ograničenom prostoru cestovnih vozila, samo povećanjem gustoće može se učinkovito poboljšati ukupna energija i ukupna snaga. Osim toga, sadašnje državne subvencije koriste gustoću energije i gustoću energije kao prag za mjerenje razine subvencija, što dodatno pojačava važnost gustoće.

Međutim, postoji određena kontradikcija između gustoće energije i sigurnosti. S povećanjem gustoće energije, sigurnost će se uvijek suočiti s novijim i težim izazovima.

4. Visoki napon

Budući da se grafitne elektrode u osnovi koriste kao anodni materijali, napon litij-ionskih baterija uglavnom je određen karakteristikama materijala katodnih materijala. Gornja granica napona litij-željezovog fosfata je 3.6 V, a maksimalni napon tropolnih litij-litijevih i litij-manganatnih baterija je oko 4.2 V (sljedeći dio će objasniti zašto maksimalni napon Li-ionske baterije ne može premašiti 4.2 V ). Razvoj visokonaponskih baterija tehnički je put za litij-ionske baterije za povećanje gustoće energije. Za povećanje izlaznog napona ćelije potreban je materijal pozitivne elektrode s visokim potencijalom, materijal negativne elektrode s niskim potencijalom i elektrolit sa stabilnim visokim naponom.

5. High energy efficiency

Coulombova učinkovitost, koja se naziva i učinkovitost punjenja, odnosi se na omjer kapaciteta pražnjenja baterije i kapaciteta punjenja tijekom istog ciklusa. Odnosno, postotak pražnjenja specifičnog kapaciteta za punjenje određenog kapaciteta.

For the positive electrode material, it is the lithium insertion capacity/delithium capacity, that is, the discharge capacity/charge capacity; for the negative electrode material, it is the lithium removal capacity/lithium insertion capacity, that is, the discharge capacity/charge capacity.

During the charging process, electrical energy is converted into chemical energy, and during the discharging process, chemical energy is converted into electrical energy. There is a certain efficiency in the input and output of electrical energy during the two conversion processes, and this efficiency directly reflects the performance of the battery.

From the perspective of professional physics, Coulomb efficiency and energy efficiency are different. One is the ratio of electricity and the other is the ratio of work.

The energy efficiency of the storage battery and the Coulomb efficiency, but from the mathematical expression, there is a voltage relationship between the two. The average voltage of charge and discharge is not equal, the average voltage of discharge is generally less than the average voltage of charge

Učinkovitost baterije može se procijeniti prema energetskoj učinkovitosti baterije. Iz očuvanja energije, izgubljena električna energija se uglavnom pretvara u toplinsku energiju. Stoga se energetskom učinkovitošću može analizirati toplina koju stvara baterija tijekom radnog procesa, a zatim se može analizirati odnos između unutarnjeg otpora i topline. Poznato je da energetska učinkovitost može predvidjeti preostalu energiju baterije i upravljati racionalnom uporabom baterije.

Because the input power is often not used to convert the active material into a charged state, but part of it is consumed (for example, irreversible side reactions occur), so the Coulomb efficiency is often less than 100%. But as far as current lithium-ion batteries are concerned, the Coulomb efficiency can basically reach 99.9% and above.

Influencing factors: electrolyte decomposition, interface passivation, changes in the structure, morphology, and conductivity of electrode active materials will reduce the Coulomb efficiency.

Osim toga, vrijedno je spomenuti da raspadanje baterija ima mali utjecaj na Coulombovu učinkovitost i ima malo veze s temperaturom.

Gustoća struje odražava veličinu struje koja prolazi po jedinici površine. S povećanjem gustoće struje povećava se i struja koja prolazi pored snopa, učinkovitost napona opada zbog unutarnjeg otpora, a Coulombova učinkovitost se smanjuje zbog polarizacije koncentracije i drugih razloga. Na kraju dovesti do smanjenja energetske učinkovitosti.

6. Dobre performanse pri visokim temperaturama

Lithium-ion batteries have good high-temperature performance, which means that the battery core is in a higher temperature environment, and the battery’s positive and negative materials, separators and electrolyte can also maintain good stability, can work normally at high temperatures, and the life will not be accelerated. High temperature is not easy to cause thermal runaway accidents.

The temperature of the lithium-ion battery shows the thermal state of the battery, and the essence of it is the result of the heat generation and heat transfer of the lithium-ion battery. Studying the thermal characteristics of lithium-ion batteries, and their heat generation and heat transfer characteristics under different conditions, can make us realize the important way of exothermic chemical reactions inside lithium-ion batteries.

Unsafe behaviors of lithium-ion batteries, including battery overcharge and overdischarge, rapid charge and discharge, short circuit, mechanical abuse conditions, and high temperature thermal shock, can easily trigger dangerous side reactions inside the battery and generate heat, directly destroying the negative and positive electrodes Passivation film on the surface.

When the cell temperature rises to 130°C, the SEI film on the surface of the negative electrode decomposes, causing the high-activity lithium carbon negative electrode to be exposed to the electrolyte to undergo a violent oxidation-reduction reaction, and the heat that occurs makes the battery enter a high-risk state.

When the internal temperature of the battery rises above 200°C, the passivation film on the positive electrode surface decomposes the positive electrode to generate oxygen, and continues to react violently with the electrolyte to generate a large amount of heat and form a high internal pressure. When the battery temperature reaches above 240°C, it is accompanied by a violent exothermic reaction between the lithium carbon negative electrode and the binder.

The temperature problem of lithium-ion batteries has a great impact on the safety of lithium-ion batteries. The environment of use itself has a certain temperature, and the temperature of the lithium ion battery will also appear when it is used. The important thing is that temperature will have a greater impact on the chemical reaction inside the lithium-ion battery. Too high temperature can even damage the service life of the lithium-ion battery, and in severe cases, it will cause safety problems for the lithium-ion battery.

7. Good low temperature performance

Lithium-ion batteries have good low-temperature performance, which means that at low temperatures, the lithium ions and electrode materials inside the battery still maintain high activity, high residual capacity, reduced discharge capacity degradation, and large allowable charging rate.

As the temperature drops, the remaining capacity of the lithium-ion battery decays into an accelerated situation. The lower the temperature, the faster the capacity decay. Forcible charging at low temperatures is extremely harmful, and it is very easy to cause thermal runaway accidents. At low temperatures, the activity of lithium ions and electrode active materials decreases, and the rate at which lithium ions are inserted into the negative electrode material is severely reduced. When the external power supply is charged at a power exceeding the allowable power of the battery, a large amount of lithium ions accumulate around the negative electrode, and the lithium ions embedded in the electrode are too late to get electrons and then directly deposit on the surface of the electrode to form lithium elemental crystals. The dendrite grows, penetrates the diaphragm directly, and pierces the positive electrode. Causes a short circuit between the positive and negative electrodes, which in turn leads to thermal runaway.

In addition to the severe deterioration of the discharge capacity, lithium-ion batteries cannot be charged at low temperatures. During low-temperature charging, the intercalation of lithium ions on the graphite electrode of the battery and the lithium plating reaction coexist and compete with each other. Under low temperature conditions, the diffusion of lithium ions in graphite is inhibited, and the conductivity of the electrolyte decreases, which leads to a decrease in the intercalation rate and makes the lithium plating reaction more likely to occur on the graphite surface. The main reasons for the decrease in the life of lithium-ion batteries when used at low temperatures are the increase in internal impedance and the degradation of the capacity due to the precipitation of lithium ions.

8. Good security

The safety of lithium-ion batteries includes not only the stability of internal materials, but also the effectiveness of battery safety auxiliary measures. The safety of internal materials refers to the positive and negative materials, diaphragm and electrolyte, which have good thermal stability, good compatibility between the electrolyte and the electrode material, and good flame retardancy of the electrolyte itself. Safety auxiliary measures refer to the safety valve design of the cell, the fuse design, the temperature-sensitive resistance design, and the sensitivity is appropriate. After a single cell fails, it can prevent the fault from spreading and serve the purpose of isolation.

9. Good consistency

Through the “barrel effect” we understand the importance of battery consistency. Consistency refers to the battery cells used in the same battery pack, the capacity, open circuit voltage, internal resistance, self-discharge and other parameters are extremely small, and the performance is similar. If the consistency of the battery cell with its own excellent performance is not good, its superiority is often smoothed out after the group is formed. Studies have shown that the capacity of the battery pack after grouping is determined by the smallest capacity cell, and the battery pack life is less than the life of the shortest cell.