1. Introduction
A full cell is a complete battery system that includes a positive electrode, a negative electrode, a separator, an electrolyte, and a shell. Unlike a half-cell, a full cell can provide an accurate assessment of the electrochemical and mechanical properties of an actual battery when it is in operation. A half-cell usually uses a metal sheet or foil (such as a lithium sheet or foil) as a counter electrode, while a full cell consists of two active electrodes, one as the positive electrode and the other as the negative electrode. The design and assembly of a full cell need to consider a variety of factors, including the choice of electrode materials, the type of electrolyte, the properties of the separator, and the structure of the battery shell to ensure the performance, safety, and reliability of the battery . Full cell testing is usually used to evaluate the degree of match between the positive and negative electrode materials and the rest of the battery, as well as the performance of the battery under actual use conditions. This article mainly introduces the design, assembly, and testing of full cells based on laboratory button-type full cells, and analyzes the factors affecting the design of full cells with examples.
2. Overview of full battery
2.1 Definition
A full battery is a complete battery system with all components, including positive electrode, negative electrode, electrolyte, separator and casing. It can actually store and release electrical energy and is an important tool for evaluating the performance of battery materials under actual working conditions. In a full battery, the positive and negative electrode materials convert electrical energy and chemical energy through redox reactions, while the electrolyte and separator are responsible for the migration of ions and preventing short circuits caused by direct contact between the two electrodes.
2.2 Working Principle
The working principle of the full battery is based on the redox reaction between the positive and negative electrode materials. Taking lithium iron phosphate( LiFePO 4 ) positive electrode and graphite ( C ) negative electrode as an example, the voltage applied by the outside during charging causes the Li + in LiFePO 4 to escape and migrate to the surface of lithium iron phosphate , and then enter the electrolyte under the continuous action of the external voltage and pass through the diaphragm and then enter the electrolyte, continue to migrate to the graphite surface, and finally embed into the graphite. At the same time, the electrons act on the external device and quickly reach the graphite side, so that the entire battery system maintains charge balance, thereby achieving a smooth charging process. During discharge, the direction of Li + movement and the electrochemical reaction of the battery are opposite to the charging process. The schematic diagram of the button battery composition is shown in Figure 1.
Figure 1. Schematic diagram of the components of a button cell
( a ) Two Celgard separators and ( b ) one polypropylene blown microfiber ( BMF ) separator. ( doi : 10.1149/2.1171902jes )
3. Full battery design and assembly
3.1 Design basis
The design basis of button-type full-cell mainly considers N/P ratio, Overhang and first coulombic efficiency ( ICE ). The above basis will be introduced below.
3.1.1 N/P ratio
1. Definition: It refers to the ratio of the excess capacity of the negative electrode material over the positive electrode material at the same stage and under the same conditions , also known as the CB value ( Cell balance ).
[ Note ] The N/P ratio is generally between 1.05 and 1.20 . The excess of negative electrode is to avoid the formation of dendrites caused by metal deposition , but considering the energy density and production cost, the excess of negative electrode should be as low as possible. In the laboratory, the setting of N/P ratio is usually more flexible, ensuring that the assembled full battery can achieve stable cycling.
2. N/P ratio calculation formula: N/P = (specific capacity of negative electrode active material × surface density of negative electrode active material × content of negative electrode active material) / (specific capacity of positive electrode active material × surface density of positive electrode active material × content of negative electrode active material)
[ Note ] When calculating N/P, ( 1 ) the same stage means that since the battery has a charging process and a discharging process, corresponding to different specific capacities, it will cause differences in the calculation results, so it is necessary to select the appropriate process for calculation; ( 2 ) the same conditions means that the test current density, electrolyte type, test temperature and other external factors are kept consistent, and the actual specific capacity of the positive and negative half-cells is selected for calculation to avoid the above factors causing a large difference in capacity, which affects the N/P ratio calculation. ( 3 ) Opposite refers to the area where the positive and negative electrode sheets are aligned.
[ Explanation ] ( 1 ) In the calculation of the N/P ratio of laboratory button-type full cells , the discharge capacity of the half-cell assembled with positive electrode materials is generally taken, and the charge capacity of the half-cell assembled with negative electrode materials is taken for calculation purposes; ( 2 ) If the electrode is wrinkled or bent, you can reduce the drying rate, lower the drying temperature, or fix the electrode with tape. The solution depends on the specific situation.
3. Impact of N/P ratio on battery: ( 1 ) If the N/P ratio is too large, the negative electrode will be too large. Although it can effectively avoid lithium plating, the effective loading amount of the positive electrode material will decrease, the gram capacity will decrease, and the battery energy density will decrease; ( 2 ) If the N/P ratio is too small, the negative electrode will be too small. The risk of lithium plating will increase during normal cycling and battery abuse such as overcharging and over-discharging, affecting battery performance.
3.1.2 Overhang
1. Definition: It refers to the extra part of the negative electrode in the length and width direction relative to the positive electrode . It is mainly based on the following considerations: ( 1 ) The overhang design of the negative electrode helps to avoid the precipitation of lithium ions on the surface of the negative electrode to form lithium dendrites during battery charging, which can reduce the risk of lithium dendrites penetrating the diaphragm and causing internal short circuits, thereby improving the safety of the battery. ( 2 ) When the negative electrode area is larger than the positive electrode, it can ensure that there is enough space to receive or release lithium ions during the battery charging and discharging process, thereby maintaining the battery capacity. ( 3 ) The increase in the negative electrode area in the overhang design may lead to a decrease in the first efficiency of the battery and a gradual decrease in capacity. This is because some lithium ions will diffuse into the excess area of the negative electrode , thereby affecting the electrochemical performance of the battery. ( 4 ) Although the overhang design can improve safety and capacity retention, excessive overhangs will lead to a decrease in battery energy density because they do not contribute to energy storage.
2. Design: In the laboratory, for circular pole pieces, Overhang can be considered as the area size. The size of the battery is designed according to the principle of diaphragm > negative pole piece > positive pole piece . For example, for a positive pole piece with a diameter of 10 mm (or 12 mm ), a negative pole piece with a diameter of 12 mm (or 14 mm ) can be selected, and a diaphragm with a diameter of 14 mm (or 16 mm , 19 mm ) can be selected to avoid pole piece misalignment leading to rapid battery capacity decay and battery matching failure. The corresponding size selection depends on the specific situation.
3.1.3 First Coulombic Efficiency ( ICE )
1. Definition: The ratio of the amount of charge released during discharge to the amount of charge input during charging in the first charge-discharge cycle of a battery. The first coulombic efficiency ( ICE ) reflects the energy conversion efficiency of a battery during its first use .
2. Design: During the first charge and discharge process, SEI film will form on the surface of the material, resulting in a decrease in active ions, which will lead to the first charge capacity > first discharge capacity > discharge capacity during subsequent cycles. Therefore, it is necessary to pay attention to the difference when designing. For laboratory button-type full cells, in order to avoid the influence of ICE , the positive and negative electrodes can generally be assembled into half cells separately, and then the full cells can be disassembled and assembled after several cycles. It is also possible to add sufficient electrolyte to the assembled full cell to compensate for the loss of active ions to a certain extent .
3. Prelithiation method: Prelithiation is a technology used to improve the initial coulombic efficiency and overall performance of lithium-ion batteries. It compensates for the irreversible loss of lithium ions caused by the formation of the solid electrolyte interface ( SEI ) layer and other side reactions by adding additional lithium sources to the battery during the battery manufacturing process. The following are some common prelithiation methods: ( 1 ) Chemical prelithiation . The lithium source is introduced on the surface of the negative electrode material through a chemical reaction, such as using a lithium salt solution to react with the negative electrode material to embed lithium ions into the negative electrode material. ( 2 ) Electrochemical prelithiation . The lithium foil, electrolyte and negative electrode are combined into a system, and the lithium ions are diffused from the electrolyte to the negative electrode through an external voltage to complete the prelithiation process. ( 3 ) Contact prelithiation . Metal lithium source is added to the negative electrode material, and prelithiation is performed by using the internal short-circuit micro-corrosion reaction that occurs when the metal lithium contacts the negative electrode material. ( 4 ) Lithium salt-assisted prelithiation . Add specific lithium salts to the electrolyte. These lithium salts can decompose when the battery is charged for the first time and release additional lithium ions to supplement the lithium loss during the formation of the SEI layer. ( 5 ) Surface coating pre-lithiation . Coat a layer of lithium-containing compounds, such as lithium oxides or lithium metal organic compounds, on the surface of the negative electrode material. These compounds decompose and release lithium ions when the battery is charged for the first time. ( 6 ) Lithium alloy pre-lithiation . Use lithium alloys (such as lithium silicon alloys and lithium germanium alloys) as part of the negative electrode material, and use the lithium in the alloy to provide additional lithium sources during the first charge. ( 7 ) Spray pre-lithiation . Spray a mixture of stable lithium metal particles and surfactants on the negative electrode to form a uniform lithium-containing coating to increase the battery life cycle. ( 8 ) Mechanical rolling . Introduce a sacrificial lithium film on the surface of the negative electrode by mechanical rolling to construct a pre-lithiation interface with different contact states to improve the utilization rate of lithium. ( 9 ) Vacuum thermal evaporation . A lithium thin film is formed on the surface of the negative electrode using vacuum thermal evaporation technology, achieving a more conformal contact through the high mobility of lithium vapor and having abundant electron channels.
[ Explanation ] The choice of pre-lithiation technology depends on the type of battery, the materials used and the manufacturing process. Pre-lithiation can improve the total capacity, energy density and cycle stability of the battery, thereby extending the service life of the battery.
3.1.4 Determination of voltage range
The voltage range of the full battery is the lowest voltage of the positive electrode minus the highest voltage of the negative electrode, or the highest voltage of the positive electrode minus the lowest voltage of the negative electrode . The larger difference between the two is taken to determine the maximum voltage range of the full battery.
The maximum voltage range can be roughly calculated using the following method: Maximum voltage range = positive electrode maximum voltage − negative electrode minimum voltage . However, in actual applications, the battery charge and discharge cut-off voltage usually varies according to battery design and safety considerations, and the entire voltage range may not be used , so it is necessary to optimize the design according to actual conditions .
3.2 Button full battery assembly
The assembly sequence of button-type full cells is similar to that of half cells, except that the negative electrode needs to be pre-lithiated to avoid low coulombic efficiency due to the formation of SEI film. In the laboratory, the negative electrode materials can be assembled into half cells (the positive electrode materials can also be assembled into half cells), pre-cycled for several cycles, and then disassembled and matched with the positive electrode (or the positive electrode after cycling) to assemble into a full cell. Please contact Xinwei staff for the following positive and negative electrode materials, separators, binders and battery shells.
Preparation of positive electrode sheet: After the positive electrode material (such as lithium iron phosphate, lithium cobalt oxide), conductive agent (such as Super P , SWCNTs ), and binder ( PVDF ) are uniformly mixed in a mass ratio of 8 : 1 : 1 (or 7 : 2 : 1 ), an appropriate amount of NMP is added and uniformly mixed again, and then coated on aluminum foil. Dry in a blast drying oven until there is no obvious solvent residue on the surface, and then transfer to a vacuum drying oven for vacuum drying. Cut to the required size to obtain the positive electrode sheet.
Preparation of negative electrode sheet: Mix the negative electrode material (such as graphite, activated carbon), conductive agent (such as Super P , SWCNTs ), and binder ( CMC & SBR , 1 : 1 ) in a mass ratio of 7 : 2 : 1 or 8 : 1 : 1 , add appropriate amount of water and mix evenly, then apply it on the copper foil. Dry at room temperature until there is no obvious solvent residue on the surface, and then transfer to a vacuum drying oven for vacuum drying. Cut to the required size to get the negative electrode sheet.
[ Note ] ( 1 ) PVDF can be first dissolved in NMP to prepare a solution with a certain mass fraction. Similarly, CMC can be first dissolved in water to prepare a solution with a certain mass fraction.
( 2 ) The selection and proportion of conductive agent and binder shall be adjusted according to the specific materials .
( 3 ) After coating, the entire electrode should be dried immediately. The actual humidity and temperature should be adjusted. The above steps are for reference only .
The assembly steps can refer to the following order: taking CR2032 as an example, positive electrode shell - gasket ( 15.6 mm × 0.5 mm ) - positive electrode sheet - electrolyte - diaphragm - electrolyte - negative electrode sheet - gasket ( 15.6 mm × 0.5 mm ) - spring - negative electrode shell. The assembly order and gasket thickness selection depend on personal habits.
4. Button-type full battery example analysis
4.1 Introduction to test equipment
The Xinwei multi-channel battery testing system (as shown in Figure 2 ) was selected to perform constant current charge and discharge tests to obtain the actual specific capacity of the button half-cell assembled with positive and negative electrode materials, and then the full battery was designed and assembled and tested according to the above calculation method.
Figure 2. Xinwei battery testing system
The Xinwei multi-channel battery test system also integrates many working modes: ( 1 ) Charging mode: constant current charging, constant voltage charging, constant current constant voltage charging, constant power charging; ( 2 ) Discharging mode: constant current discharge, constant voltage discharge, constant current constant voltage discharge, constant power discharge, constant resistance discharge; ( 3 ) Direct current internal resistance ( DCIR ) test; ( 4 ) Cycle test; ( 5 ) Loop nesting: It has nested loop function and supports up to 3 layers of nesting. For more detailed test functions, please consult Xinwei staff.
4.2 Test parameter settings
The test parameter setting selects the constant current cycle mode of the Xinwei tester, and the rate mode can be selected. The parameter setting can refer to the following settings: assemble the positive and negative electrodes with calculated N/P ratio into half-cells respectively, and cycle the half-cells at a current density of 0.1 A g − 1 (or less current) for 3 to 5 cycles within the corresponding voltage range. Then, disassemble the half-cells in the glove box, take out the positive and negative electrodes after cycle activation, and assemble the full cell and conduct subsequent tests. Taking lithium iron phosphate positive electrode and graphite negative electrode as an example, the half-cell test voltage range in the screenshot below is for reference only, and the specific voltage range depends on the specific material.
Figure 4. Graphite // Li Button Cell Cycling Procedure
4.3 Example Analysis
Taking the sodium -ion button -type full cell assembled with crumpled Cp-MoS2 /CNTs negative electrode material and Na3V2(PO4)2F3 positive electrode material as an example , the N / P ratio of the full cell was calculated and the battery design was carried out. The relevant pictures and data are from SCI papers ( 1 ) " Crumpling Carbon-Pillared Atomic-Thin Dichalcogenides and CNTs into Elastic Balls as Superior Anodes for Sodium/Potassium-Ion Batteries " and ( 2 ) " On-site conversion reaction enables ion-conducting surface on red phosphorus/carbon anode for durable and fast sodium-ion batteries ".
In this paper, the preparation of Cp-MoS 2 /CNTs negative electrode sheet is to mix the negative electrode material, Super P , CMC and SBR according to the mass ratio of 70:15:15 ( where CMC/SBR=1 : 1 ) , and then coat them on copper foil. After vacuum drying, the electrode sheet is obtained. The electrode sheet has a diameter of 10 mm and a loading capacity of 1.2-1.5 mg cm − 2. The voltage range of the assembled sodium ion half-cell is 0.01-2.5 V ( vs. Na + /Na ), and the electrolyte is 1 M NaClO 4 +EC/PC ( 1 : 1, v/v ) +5% FEC . The relevant test data are as follows.