Lithium ion battery charging and discharging process, lithium battery discharge curve comprehensive analysis

The determination of battery discharge curve, is one of the basic method of the battery performance, according to the discharge curve, it can be judged whether the battery performance stability, and the battery in the allowed maximum current stable work. This article comprehensively introduced in detail lithium ion batteries basic knowledge of discharge curve. Due to the limited level of the author, the wrong place, welcome to comment. In this paper, longer, more than 10000 words, the main content includes:

1 the voltage of the battery

1.1 lithium ion battery materials of electrode potential

1.2 open circuit voltage of the battery

1.3 the battery polarization

1.4 working voltage of the battery

The basic principle of 2 discharge test

2.1 discharge test mode

The information contained in 2.2 discharge curve

2.3 the basic form of discharge curve

3 discharge curve of the differential treatment

The working voltage of lithium ion battery discharge, it is always constantly changing, with a continuation of the time made ordinate, the working voltage of the battery discharge time, or capacity, or charged state of (SOC), or the depth of discharge (DOD) do abscissa, drawn curve is called the discharge curve.To know the discharge characteristic curve of the battery, first need to understand from the principle of voltage of the battery.

1 the voltage of the battery

Electrode reaction to form the cell must meet the following conditions: losing electrons in chemical reaction process (oxidation) and get the electronic process (i.e. the reduction reaction process) must be separated in two different areas, this differs from the general REDOX reaction;Two electrode active material to meet the need of the REDOX reaction of electronic must by external transfer, this difference in micro cell reaction of metal corrosion process.Battery voltage is a potential difference between the positive and negative, specific key parameters including open circuit voltage and working voltage, charge and discharge cut-off voltage, etc.

1.1 lithium ion battery materials of electrode potential

Electrode potential refers to solid material immersed in electrolyte solution, shows the effect of electricity, i.e. the potential difference between the surface of the metal and solution, this potential difference is called the metal in the solution of the potential or electrode potential.Simple said electrode potential is said by reduction of certain ions or atoms for electronic trend.

Therefore, for some the positive or negative material, when in a lithium salt electrolyte, the electrode potential is expressed as:

Among them, the phi c is the material of electrode potential.The standard electrode potential listed in table 1 (25.0 ℃ and 101.325 kPa) is relative to the standard hydrogen electrode potential value.The standard hydrogen electrode potential is 0.0 V.

Table 1 standard electrode potential of common materials in aqueous solution

1.2 open circuit voltage of the battery

Battery electromotive force is based on cell reaction, thermodynamic method is applied to calculate the theoretical value, namely the battery in the circuit in reversible equilibrium state, the balance between is negative electrode potential difference, is produces the maximum voltage battery.And, in fact, is negative in the electrolyte is not necessarily in the thermodynamic equilibrium state, across the electrodes of the battery in the established electrode potential in electrolyte solution is usually not equilibrium electrode potential, therefore the open circuit voltage of battery is less than its electromotive force.The electrode reaction:

Consider and nonstandard status of reactant composition activity of active components (or concentration) changes over time, the nernst equation is modified actual battery open circuit voltage:

Among them, R is the gas constant, T is the reaction temperature, part a is activity or concentration.Open circuit voltage of battery depends on the battery is the nature of the anode materials, electrolytes, and temperature conditions, etc., and has nothing to do with the geometric structure and size of battery.

Lithium ion electrode materials preparation into pole piece, and lithium metal pieces assembled into button batteries, electrode materials can be measured in the condition of different SOC of open circuit voltage, open circuit voltage response curve is charged electrode material status, figure 1 is iron phosphate lithium electricity materials open circuit voltage curve, from the open circuit voltage curve can determine the corresponding electrode materials that the intercalated-li state.And the open circuit voltage of battery anode materials are information of superposition state.

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