Semco university – All about the Lithium-Ion Batteries

semco-university-logo

Basic knowledge of lithium-ion battery commercialization – Semco University

Introduction

Li-ion batteries are the powerhouse for the digital electronic revolution in this modern mobile society, exclusively used in mobile phones and laptop computers. The success of Lithium-ion Battery Commercialization was not an overnight achievement, but a result of intensive research and contribution by many great scientists and engineers. Then much efforts have been put to further improve the performance of Li-ion batteries, achieved certain significant progress. To meet the increasing demand for energy storage, particularly from increasingly popular electric vehicles, intensified research is required to develop next-generation Li-ion batteries with dramatically improved performances, including improved specific energy and volumetric energy density, cyclability, charging rate, stability, and safety. studied “beyond Li-ion” batteries is also provided.

1. Why does the battery have voltage?

The voltage U (battery) of the battery is determined by the difference between the electrode potential E (positive electrode) of the positive e electrode and the electrode potential E (negative electrode) of the negative electrode, which is represented by the formula:                          

U (Battery) = E(Positive) – E(Negative)

In the battery system, the standard lithium electrode is generally used as the reference electrode, and the electrode potentials of the positive and negative electrode materials are generally the potentials generated by the reaction between the reactants and products and the reference lithium electrode. In the process of charging and discharging, the positive and negative materials are delithiated or intercalated, the electrode potential changes, and the battery voltage is the difference between the two.

2. Why do some materials have plateau voltage and some do not?

In thermodynamics, the degree of freedom F is a factor (such as temperature and pressure) that can be changed independently without changing the number of phases when the system is in equilibrium. The number of these variables is called the number of degrees of freedom. The relationship between the degrees of freedom of the system and other variables.

F = C – P + n

Among them, F: the degree of freedom of the system; C: the number of independent components of the system; P: the number of phases; n: external factors, most of which take n=2 , representing pressure and temperature. For the lithium-ion electrochemical system, the external factors n=2, and the voltage and temperature are taken respectively. It is assumed that the temperature and pressure of the lithium-ion electrode material are constant during the charging and discharging process. Here we discuss the binary system (C=2), if there is a phase in a particle, i.e., P=1, then F=1, the chemical potential is a degree of freedom that varies with the lithium concentration (e.g., solid solution Lithium cobaltate, one phase, with changing lithium concentration). If the particle contains two phases, i.e., P=2, then F=0. When two phases coexist, a flat voltage plateau exists in a binary system electro de material (eg, lithium iron phosphate, two phases coexist, and the lithium concentration in each phase is constant).

3. Polarization

When current passes through the electrodes, the phenomenon that the electrodes deviate from the equilibrium electrode potential is called the polarization of the battery, and the polarization produces an overpotential. According to the causes of polarization, polarization can be divided into ohmic polarization, concentration polarization and electrochemical polarization.

(1) Ohmic polarization: caused by the resistance of each part of the battery connection, the voltage drop value follows Ohm’s law, the current decreases, the polarization decreases immediately, and disappears immediately after the current stops.

(2) Electrochemical polarization: polarization is caused by the retardation of electrochemical reaction on the electrode surface. As the current gets smaller, it decreases significantly in the microsecond range.

(3) Concentration polarization: Due to the retardation of the ion diffusion process in the solution, the concentration difference between the electrode surface and the solution body is caused under a certain current, resulting in polarization. This polarization decreases or disappears on a macroscopic second scale (seconds to tens of seconds) as the current decreases.

4. Current Collector Selection

1. Why copper and aluminium can be used as current collectors?

Copper aluminium foil is also relatively stable in the air. Aluminium easily reacts chemically with oxygen in the air, forming a denseoxide film on the surface of aluminium to prevent further reaction of aluminium and this very thin oxide film also has a certain protective effect on aluminium in the electrolyte. Copper itself is relatively stable in air and basically does not react in dry air. In addition, copper and aluminium foil has good conductivity, soft texture and cheap price.

2. Why use aluminium for the positive electrode and copper for the negative electrode?

a. Why can’t aluminum be used for the negative electrode: The lattice octahedral void size of metal aluminum is similar to that of Li, and it is easy to form metal interstitial compounds with Li. Li and Al not only form an alloy with the chemical formula of LiAl, but also may form L i 3 Al 2 or Li 4 Al 3. Due to the high activity of the reaction between metal Al and Li, metal Al consumes a large amount of Li, and its structure and morphology are also damaged, so it cannot be used as a current collector for the negative electrode of lithium-ion batteries,
b. Why can’t copper be used for the positive electrode: When the Cu foil is at 3.75V, the polarization current begins to increase significantly, and it increases linearly, and the oxidation intensifies, indicating that Cu is unstable at this potential; while the aluminum foil is in the entire polarization potential range, The polarization current is small and constant, no obvious corrosion phenomenon is observed, and the electrochemical performance is stable. Due to the small lithium intercalation capacity of Al in the positive electrode potential range of lithium-ion batteries, and can maintain electrochemical stability, it is suitable as a positive electrode current collector for lithium-ion batteries,
c. Why aluminum can be used for the positive electrode: The positive electrode potential is high, and the aluminum thin oxide layer is v very dense, which can prevent the current collector from oxidizing,
d. Why copper can be used for the negative electrode: During the charging and discharging process of the battery, it is difficult for Li to f form a lithium intercalation alloy with Cu/Ni at a low potential, and the structure and electrochemical properties are kept stable. fluid;

5. Formation

Why make it into?

After the battery is manufactured, the internal positive and negative materials are activated by a certain charging and discharging method, and the process of improving the charging and discharging performance of the battery and the comprehensive performance such as self-discharge and storage is called formation.

What is Formation?

The formation of lithium cells is the initial activation of the battery, which activates the active substances of the cells, which is a process of energy conversion. The formation of lithium batteries is a very complex process, and it is also an important process that affects battery performance, because when Li+ is charged for the first time, Li+ is inserted into graphite for the first time, and an electrochemical reaction will occur in the batter y. During the first charging of the battery, it is inevitable to form a passivation thin layer covering the surface of the carbon electrode on the phase interface between the carbon negative electrode and the electrolyte, which is called the solid electrolyte phase interface or SEI fil m (SOLID ELECTROLYTE INTERFACE). On the one hand, the formation of the SEI film consumes the limited lithium ions in the battery, which requires the use of more lithium-containing cathode materials to compensate for the lithium consumption during the initial charging process on the other hand, it also increases the resistance of the electrode/electrolyte interface. cause a certain voltage hysteresis.

6. N/P ratio

N/P, Negative/Positive, Negative/Positive. Calculation formula: The gram capacity of negative electrode active material × negative electrode surface density × negative electrode active material content ratio ÷ (positive electrode active material gram capacity × positive electrode surface density × positive electrode active material content ratio).

It is generally believed that if the N/P ratio is too large, that is, the negative electrode is too large, which will cause shallow charge and dis charge of the negative electrode and deep charge and discharge of the positive electrode (and vice versa). The fully charged negative electrode is not easy to precipitate lithium (some materials, such as soft and hard carbon, LTO materials will not precipitate lithium), which is safer, but the increase of the oxidation state of the positive electrode increases the safety hazard. Since

When Li-ion batteries are charged, Li + is deintercalated from the positive electrode, and these Li + diffuses in the electrolyte to the surface of the negative electrode and intercalates into the negative electrode material. Taking the graphite anode as an example, when the anode potential drops to 200-65 mV vs. Li + /Li, the lithium intercalation process occurs; as the charging continues, the anode potential drops below 0 V vs. Li + /Li, which occurs in this case, the side reaction of lithium deposition and the lithium intercalation reaction of the negative electrode are carried out at the same time. Considering the effect of polarization, the lithium deposition side reaction occurs when the sum of the equilibrium potential and the overpotential (from ohmic resistance, charge transfer, and diffusion processes) is negative with respect to the Li + /Li pair performance of the gram capacity.

7. Lithium Precipitation

When Li-ion batteries are charged, Li + is deintercalated from the positive electrode, and these Li + diffuses in the electrolyte to the surface of the negative electrode and intercalates into the negative electrode material. Taking the graphite anode as an example, when the anode potential drops to 200-65 mV vs. Li + /Li, the lithium intercalation process occurs; as the charging continues, the anode potential drops below 0 V vs. Li + /Li, which occurs in this case, the side reaction of lithium deposition and the lithium intercalation reaction of the negative electrode are carried out at the same time. Considering the effect of polarization, the lithium deposition side reaction occurs when the sum of the equilibrium potential and the overpotential (from ohmic resistance, charge transfer, and diffusion processes) is negative with respect to the Li + /Li pair.

Conclusion

The demand for Li-ion batteries increases rapidly, especially with the demand from electric-powered vehicles. It is expected that nearly 100 GW hours of Li-ion batteries are required to meet the needs from consumer use and electric-powered vehicles with the later takes about 50% of Li-ion battery sale. There are still notable challenges in the development of next-generation Li-ion batteries. New battery concepts have to be further developed to go beyond Li-ion batteries in the future. The focus of this Article is to introduce the basic concepts, highlight the recent progress, and discuss the challenges regarding Li-ion Batteries.

More Articles:

Lithium-ion Battery Packaging Information for Beginners,
Lithium Battery Protection Board,
Application of Isolation testing Technology,
Action protector in electric vehicle charging system,
History of Lithium Battery Development,
Production problems of lithium batteries,

Formation Process of Lithium Battery,
Model S Plaid Battery System Design,
Battery Pole Piece Coating Process,
Point Defects in Pole Piece Coating,

Leave a Comment

Your email address will not be published.