Why Mobile Phone Batteries Are Less Durable

Smartphones are basically owned by everyone now. Many people should have the same question as the author, why are our mobile phones becoming less and less durable? This has to start with our batteries.

01. Early product of mobile phone battery

In 1973, the world's first mobile phone was born in Motorola's laboratory. This mobile phone is very bulky, but thanks to the built-in nickel-cadmium battery of the mobile phone, this mobile phone can realize real-time mobile calls without complicated electronic circuits.

As the first battery built into a mobile phone, the nickel-cadmium battery itself is relatively bulky. Most of the "big phone" popular in the last century used nickel-cadmium batteries. The capacity of nickel-cadmium batteries is low, and contains highly toxic cadmium, which is not conducive to the protection of the ecological environment. And nickel-cadmium batteries also have a very obvious memory effect: if the power is not completely discharged before charging, the battery capacity will decrease over time.

Figure 1: Basic structure of nickel-cadmium battery

In 1990, Sony Corporation of Japan first developed the Ni-MH battery. Compared with its predecessors, Ni-MH batteries can not only be made thinner and thinner, but also have effectively improved capacity. The appearance of nickel metal hydride batteries makes mobile phones more portable, and mobile phones can also support longer calls. Therefore, with the appearance of NiMH batteries, bulky NiCd batteries were gradually replaced, and small and exquisite mobile phones became popular. But Ni-MH batteries still have a memory effect, which is why the previous generation of mobile phones needed to be fully discharged before being recharged. Moreover, due to the limited energy density of nickel-cadmium batteries, mobile phones at that time could only support relatively simple tasks such as making calls, which is still far from our current smartphone form.

02. The Rise of Lithium Batteries

Lithium metal was discovered in the nineteenth century. Lithium has unique advantages as a primary battery due to its relatively low density, high capacity and relatively low potential. However, lithium is a very active alkali metal element, which leads to very high environmental requirements for the storage, use or processing of lithium metal, and is much more complicated than other metals. Therefore, in the process of researching lithium batteries with lithium as the electrode material, scientists have overcome many research difficulties through continuous development and improvement of lithium batteries, and finally made it what it is today after going through many stages.

The lithium battery using metal lithium as the negative electrode first achieved commercialization. In 1970, Panasonic Corporation of Japan invented fluorocarbon lithium batteries. This type of battery has a large theoretical capacity, stable discharge power, and small self-discharge phenomenon. However, this type of battery cannot be recharged and is a primary lithium battery.

In the 1970s, M. Stanley Whittingham, a researcher from ExxonMobil, proposed the battery charging and discharging principle of ion intercalation, and published it in 1975 Patented titanium disulfide lithium battery. In 1977, the Whittingham team working for Exxon Corporation developed a secondary battery with the aluminum-lithium alloy Li-Al as the negative electrode and titanium disulfide TiS2 as the positive electrode, in which the aluminum-lithium alloy can improve The stability of lithium metal enhances the safety of batteries. During the discharge process, the electrochemical process that occurs in the battery is:

Negative electrode: Li - e- → Li+

Positive electrode: xLi+ + TiS₂+ xe- → LixTiS₂

Among them, TiS2 is a layered compound, and the interaction between layers is weak Van der Waals Force (Van der Waals Force). Smaller lithium ions can enter the TiS2 layer and undergo charge transfer, and store Lithium ions are similar to squeezing jam into a sandwich. This process is ion intercalation. During discharge, the TiS₂ layers of the positive electrode intercalate Li+ ions in the electrolyte, accept charges and form LixTiS₂.

Figure 2: The structure of TiS₂ and the principle of intercalation reaction during discharge

The secondary lithium batteries at this stage mainly use metallic lithium as the negative electrode material, and improve the life and safety of the battery by improving the positive electrode material. As the earliest commercialized secondary lithium battery, the use of metal lithium as the negative electrode material has the lowest potential of the negative electrode, the highest energy density of the battery is high, and it is relatively portable, but it is safe Sex has also been widely questioned. In the late spring of 1989, the first-generation metal lithium battery produced by the Canadian company Moli Energy experienced a explosion, which also brought the commercialization of metal lithium batteries to a standstill.

In order to improve the safety of lithium batteries, it is very important to develop new electrode materials for lithium batteries. However, using other lithium compounds as the negative electrode instead of lithium will greatly increase the potential of the negative electrode, reduce the energy density of the lithium battery, and reduce the battery capacity. Therefore, finding suitable new electrode materials has also become a difficult problem in the field of lithium battery research.

Around 1980, John Bannister Goodenough, who taught at the University of Oxford, UK, and others discovered the compound lithium cobaltate LiCoO2 (LCO), which can accommodate lithium ions. LiCoO₂ has a higher potential compared to other types of cathode materials at that time. This enables lithium batteries with LiCoO₂ as the cathode to deliver higher voltage and have higher battery capacity.

Figure 3: Schematic diagram of lithium cobalt oxide crystal structure

Lithium cobaltate crystals have layered structure and belong to hexagonal system. Among them, the octahedral lattice composed of O and Co atoms is arranged on a plane to form CoO2 layers, and the CoO2 layers are separated by lithium ions to form a planar lithium ion transport channel. This enables lithium cobalt oxide to transport lithium ions faster through planar lithium ion channels. The detachment and intercalation process of lithium ions in lithium cobalt oxide is similar to an intercalation process. In the process of light charge and discharge, lithium cobalt oxide can maintain the stability of the crystal structure. However, with the gradual extraction of lithium ions, lithium cobalt oxide has a tendency to transform to a monoclinic crystal system. In a lithium battery with lithium cobalt oxide as the positive electrode, during the discharge process, the reaction that occurs at the positive electrode is:

Positive electrode: Li1-xCoO₂ + xLi+ + xe- → LiCoO₂

Figure 4: Schematic diagram of the extraction of lithium ions from lithium cobalt oxide during discharge

Compared with titanium disulfide, the lithium cobalt oxide cathode material has a higher positive electrode potential, and at the same time, the layered lithium cobalt oxide can transmit lithium ions faster, so it is an excellent lithium-ion battery Cathode material.

In the same year, Rachid Yazami discovered the recyclable ion intercalation phenomenon of lithium ions in graphite, and verified the feasibility of graphite as the positive electrode of lithium batteries. Graphite has a lamellar structure, and similar to TiS2, the layers in graphite are connected by weak van der Waals force, which allows smaller lithium ions to enter between graphite layers and undergo charge transfer.

Figure 5: Graphite has a layered structure, and the layers are connected to each other by van der Waals force

In the 1983 paper, Yazami used polyethylene oxide-lithium perchlorate solid-state electrolysis, and used lithium metal as the negative electrode and graphite as the positive electrode to form a primary battery. During the discharge process, graphite as the positive electrode undergoes the following reactions:

nC + e- + Li+ → (nC, Li)

Then it happens: (nC, Li) → LiCn

During the discharge process of the primary battery with graphite as the positive electrode, lithium ions undergo intercalation reaction in the graphite layer, charge transfer occurs and the compound LiCn is formed.

03. The arrival of lithium-ion batteries

In 1982, Yoshino Akira, who worked for Asahi Kasei Corporation in Japan, used lithium cobaltate as the positive electrode and polyacetylene (C2H2)n as the negative electrode to construct A sample of a Li-ion battery. During the discharge process of the lithium cobalt oxide battery, lithium ions migrate from the positive electrode of the battery to the lithium cobalt oxide through the electrolyte to realize battery discharge.

However, lithium cobalt oxide batteries still have many problems. The negative electrode polyacetylene of the battery has low energy density and low stability. Therefore, Akira Yoshino adopted a new type of graphite-like material "soft carbon" instead of polyacetylene as the negative electrode material of the battery, and prepared the first lithium-ion battery prototype in 1985, and A patent has been applied for . The prototype lithium-ion battery designed by Akira Yoshino became the prototype of many modern batteries.

Figure 6: Lithium-ion battery discharge, schematic diagram of lithium ion migration process

Compared with lithium batteries, the primary battery designed by Yoshino Akira with carbonaceous materials as the negative electrode and lithium cobalt oxide as the positive electrode has completely got rid of the lithium metal, so this type of battery is also called "lithium-ion battery". Because in the lithium cobalt oxide lithium-ion battery, lithium ions undergo intercalation reactions at both the positive and negative electrodes, and the rapid charge transfer is achieved through the rapid intercalation of lithium ions, so this battery structure is also vividly called a rocking chair battery.

In 2019, the Nobel Prize in Chemistry was awarded to American scientist John B. Goodenough , British scientist M. Stanley Whittingham and Japanese scientist Akira Yoshino for their research contributions to lithium-ion batteries.

Figure 7: Nobel laureates: from left to right are American scientist John B. Goodenough, British scientist M. Stanley Whittingham and Japanese scientist Akira Yoshino

The emergence of lithium-ion batteries with carbon materials as the negative electrode and lithium cobalt oxide as the positive electrode has promoted the development of lithium-ion batteries. With the deepening of researchers' research on lithium-ion batteries, three systems have been developed for the positive electrode materials of lithium-ion batteries: lithium cobalt oxide (LCO), lithium iron phosphate (LFP) and ternary nickel-cobalt-manganese (NMC / NCM) system. Among them, the lithium cobalt oxide system has a relatively higher battery capacity, and plays a pivotal role in the field of 3C electronic products such as mobile phones and computers that we usually use. The lithium iron phosphate system and the ternary lithium system have higher stability, so they are widely used in new energy vehicles.

The advent of lithium-ion batteries has revolutionized our way of life. Compared with nickel-cadmium batteries and nickel-metal hydride batteries, lithium-ion batteries have higher energy density, and lithium-ion batteries with the same battery capacity are more portable and can support smart phones with integrated rich functions High power consumption. At the same time, most lithium-ion batteries have no memory effect and do not need to be fully discharged before recharging, so lithium-ion batteries can achieveon-demand charging. Lithium-ion batteries offer significantly faster charge rates compared to lithium batteries. And the charging rate of lithium-ion batteries is fast, which greatly facilitates our lives. Therefore, in application scenarios such as mobile phones, mobile computers, and new energy vehicles, lithium-ion batteries have gradually replaced nickel-cadmium batteries and nickel-metal hydride batteries in some scenarios due to their excellent performance.

04. Why is the battery life of the mobile phone getting shorter and shorter?

The Pain of NiCd Batteries——Memory Effect

For nickel-cadmium batteries, the cadmium grains of the negative electrode of the nickel-cadmium battery prepared by sintering are relatively thick. When the nickel-cadmium battery is not fully charged and discharged for a long time, the cadmium grains are easy to aggregate and eventually aggregate into blocks. At this time, a secondary discharge platform is formed when the battery is discharged. The battery will use this secondary discharge platform as the end point of battery discharge, the capacity of the battery will become lower, and the battery will only remember this low capacity in the subsequent discharge process. This is why older cell phones with nickel-cadmium batteries are often recommended to be fully discharged before recharging. However, with the continuous improvement of the processing technology of nickel-cadmium batteries and nickel-metal hydride batteries, the impact of memory effect on battery capacity has been continuously reduced, and the harm of full charge and discharge to battery life has gradually emerged.

Nickel-cadmium batteries have a significant memory effect, while lithium-ion batteries have almost no memory effect. And because the energy density of lithium-ion batteries is higher than that of nickel-cadmium batteries, lithium-ion batteries are mainly used in our mobile phones, computers and other products. Therefore, when we use smartphones or computers loaded with lithium-ion batteries every day, we don’t need to worry about the memory effect of the battery.

Excessive charge and discharge of lithium-ion battery leads to life attenuation

Lithium cobalt oxide has a high theoretical capacity, but the actual capacity of lithium cobalt oxide is far below the theoretical capacity during use. Because after we charge and discharge the lithium-ion battery beyond this capacity, lithium cobalt oxide will undergo an irreversible charge and discharge process, which is what we often call battery overcharge or overdischarge. This process is accompanied by the structural phase change of lithium cobalt oxide, which reduces the capacity of the battery.

Figure 9: Schematic diagram of six-direction monoclinic phase transition of lithium cobalt oxide

When the battery is overcharged, a large amount of lithium ions are released from the lithium cobalt oxide at the negative electrode of the lithium ion battery, and the remaining lithium ions are not enough to support the original structure of lithium cobalt oxide, resulting in the transformation of the Li1-xCoO₂ crystal from the hexagonal system to the monoclinic system. , the original hexagonal structure collapsed due to the lack of ionic support. In this process, the phase transition of lithium cobalt oxide is not completely reversible. The unit cell parameters of lithium cobalt oxide change completely, the stress changes, and the lithium ion vacancies are compressed, which leads to the capacity decay of lithium ion batteries.

Instability of High Voltage Li-Ion Batteries

In addition to the irreversible change in battery capacity caused by the structural phase change of lithium cobaltate, the increase in the output voltage of lithium-ion batteries also leads to other side reactions in lithium-ion batteries, and the life of lithium-ion batteries is attenuated. Currently, smartphones on the market usually use a charging and discharging voltage of around 4.4V . High voltage can increase the capacity of the lithium-ion battery and speed up the charge and discharge rate of the lithium-ion battery. But what follows is a series of side effects such as the increase of the side reaction on the surface of the lithium-ion battery electrode, the instability of the electrolyte under high voltage, and so on.

Figure 10: Influence mechanism of life decay of high voltage lithium-ion battery

The electrolyte of the lithium-ion battery reacts with the solid-liquid phase interface of the positive and negative electrodes to form a passivation layer covering the surface of the electrodes. This passivation layer has the characteristics of a solid electrolyte through which Li ions can be inserted and extracted freely, so this passivation film is called "solid electrolyte interface", or SEI film for short. The process of forming the SEI film will consume part of the lithium ions, resulting in an irreversible loss of lithium-ion battery capacity. Under the action of high voltage, the side reactions on the surface of such electrodes are serious, which gradually reduces the battery capacity.

05.What should be paid attention to when using a mobile phone

High temperature without charging

Do not charge the phone when the phone is overheated or the temperature is extremely low. When the mobile phone is overheated, charging the lithium-ion battery under high temperature conditions will also change the structure of the positive and negative electrodes of the lithium-ion battery, resulting in irreversible attenuation of the battery capacity. Therefore, Try to avoid charging your mobile phone when it is too cold or too hot, which can also effectively extend its service life.

Change battery in time

In the process of using digital products such as mobile phones, laptops or tablets, if we find that the battery back cover is deformed, the battery has a bulge, etc., we must stop using it in time and replace the battery with the manufacturer, as much as possible Avoid potential safety hazards caused by improper use of batteries.