In the preparation of inorganic electrode materials for lithium-ion batteries, the most commonly used high-temperature solid-phase reaction is the sol-gel method, co-precipitation method, hydrothermal method, solvothermal method, etc. Solid-phase reaction or solid-phase sintering at temperature. It is a subject of great scientific significance and practical value to discuss and study the thermodynamics and kinetics in the synthesis reaction of cathode materials for lithium-ion batteries. In this paper, some basic thermodynamic discussions of high-temperature solid-phase reactions will be performed.

Basic Concepts and Laws of Thermodynamics of Synthetic Reactions for Li-ion Batteries

Lithium-ion battery synthesis reaction thermodynamics studies the change of thermodynamic parameters of lithium-ion battery synthesis reaction in the process of a certain system interacting with the environment. It describes and determines the state of the system through observable macroscopic state quantities, such as temperature, pressure, volume, concentration, etc. There is a relationship between these macroscopic state quantities, and their changes restrict each other. In addition to the properties of substances, the constraints must also follow some basic thermal laws that are applicable to any substance, such as the zeroth, first, second and third laws of synthesis reaction thermodynamics for lithium-ion batteries. In practical applications, the first and second laws of synthesis reaction thermodynamics for lithium-ion batteries are the basic starting points for the thermodynamic calculations of various lithium-ion battery synthesis reactions.

The first law of thermodynamics in the synthesis reaction of lithium-ion batteries

The development of the thermodynamics of the synthesis reaction of lithium-ion batteries started from people's understanding of heat [1]. People's feelings about cold and heat and their changes made people start to think about what heat is. Before the 18th century, the "caloric theory" was popular, that is, people regarded heat as an invisible and weightless substance filled with tangible matter. But then more and more scientific facts denied this view, and new correct theories emerged. In 1842, Julius R.Mayer put forward the theory of energy conservation, thinking that heat is a form of energy that can be converted into mechanical energy, and the thermal work equivalent is calculated from the difference between the specific heat capacity at constant pressure and the specific heat capacity at constant volume. . British physicist James P. Joule has done a lot of experiments and used various methods to find the thermal work equivalent, and the results are consistent. That is to say, there is a certain conversion relationship between heat and work. Later, it was determined by a precise experiment that 1cal=4.184J. In 1850, Joule's experimental results have completely abandoned the "caloric and mass theory" in the scientific community, it is recognized that energy is conserved, and the form of energy can be interchanged, thus establishing the first law of thermodynamics for the synthesis reaction of lithium-ion batteries: energy can neither be out of thin air. Generation also does not disappear out of thin air, it only transfers from one object to another, or from one form to another. In the process of transformation or transfer, the total amount of energy remains the same.

It is the internal energy that characterizes the energy of the thermodynamic system of the synthesis reaction of lithium-ion batteries. The system exchanges energy with the outside world through work and heat transfer so that the internal energy changes. According to the general law of conservation of energy, after the system reaches the final state 2 from the initial state 1 through any process, the increment of internal energy ΔU should be equal to the difference between the heat Q transferred from the outside to the system and the work W done by the system to the outside world in this process, namely ΔU=U2-U1=Q-W or write dU=δQ+δW This is the expression of the first law of thermodynamics for the synthesis reaction of lithium-ion batteries. ΔU is the change in the internal energy of the research system, Q is the heat exchanged between the system and the environment (the heat absorption is positive, and the heat release is negative), and W is the work exchanged between the system and the environment (the external work on the system is positive, and the system external work is done externally). is negative).

The second law of thermodynamics in the synthesis reaction of lithium-ion batteries

The first law of thermodynamics in the synthesis reaction of lithium-ion batteries defines the conservation and transformation of energy but does not describe the direction of energy change. Because according to the law of conservation of energy, heat and work should be equivalent. But in practice, people find that heat and work are not exactly the same, because work can be completely turned into heat without any conditions, while heat generation work must be accompanied by dissipation of heat to cold. That is, heat can spontaneously transfer from a hotter object to a cooler object, but cannot spontaneously transfer from a colder object to a warmer object. Work is converted into heat as a result of friction between two objects, but it is impossible to convert this frictional heat back into work without other effects. For the synthesis reaction thermodynamic process of lithium-ion battery such as diffusion, permeation, mixing, combustion, electrothermal and magnetic hysteresis, although the reverse process still conforms to the first law of synthesis reaction thermodynamics of lithium-ion battery, it cannot occur spontaneously. Therefore, any process in nature cannot be automatically restored. To make the system return from the final state to the initial state, it must rely on the action of the outside world. It can be seen that there is a significant difference between the initial state and the final state of the irreversible process carried out by the synthesis reaction thermodynamic system of lithium-ion batteries. This difference determines the direction of the process. For this, people use the state function entropy (S) to determine and Describe this difference.

In an isolated system, for a reversible process, the entropy of the system always remains unchanged; for an irreversible process, the entropy of the system always increases. This law is called the principle of increasing entropy. This is also an expression of the second law of thermodynamics for the synthesis reaction of lithium-ion batteries. The increase of entropy means that the system evolves from a state with low probability to a state with high probability, that is, from a relatively regular and orderly state to irregular and disordered state. Entropy reflects the statistical properties of the system. The expression of the second law of thermodynamics for the synthesis reaction of lithium-ion batteries can be written as: In the formula, the inequality sign should be used for the irreversible process, and the equal sign should be used for the reversible process. δQ refers to the actual process heat of the system, and T is the system temperature.