Battery activation method and battery

Abstract

This application discloses a battery activation method and a battery. The battery activation method includes: obtaining battery parameters and determining the limiting diffusion current density of the battery based on the battery parameters; performing a first activation on the battery, controlling the charging current density of the first activation to be less than or equal to the limiting diffusion current density; calculating the diffusion length L of lithium ions in the battery electrode based on the formation time of the first activation; and comparing the diffusion length L with the median particle size D of the active material particles in the electrode. 50 Compare; in L>0.4D 50 In this case, the activation of the battery is terminated. This method can avoid uneven deposition of active lithium and ensure full utilization of active lithium; subsequently, targeted homogenization treatment of the electrodes is achieved to reduce the uneven lithium intercalation stress caused by interface inhomogeneity, reduce the crack growth rate of material particles, and improve the cycle performance and rate performance of lithium-ion batteries.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to a battery activation method and a battery. Background Technology

[0002] In the design and manufacturing process of lithium-ion batteries, the ratio of the specific capacity of the negative electrode active material to the specific capacity of the positive electrode active material is called the N / P ratio. During battery charging, lithium ions extracted from the positive electrode must be fully inserted into the negative electrode lattice. Therefore, designing an N / P ratio greater than 1 for a lithium-ion battery ensures that lithium ions are fully inserted through negative electrode capacity redundancy, thus preventing the risk of lithium deposition from the source.

[0003] However, in the overhang region of a lithium-ion battery, the N / P ratio is greater than 1, resulting in a low lithium-ion intercalation rate. This affects the driving force for local potential reduction and film formation. Uneven film formation leads to uneven stress on the electrode and active materials during subsequent lithium-ion intercalation, which in turn accelerates material crack growth and is detrimental to the long lifespan and rate performance of lithium-ion batteries.

[0004] Therefore, how to solve the problem of interface inhomogeneity caused by the N / P ratio being greater than 1 in lithium-ion batteries, and improve the cycle performance and rate performance of lithium-ion batteries, is a problem that needs to be solved. Summary of the Invention

[0005] The purpose of this application is to provide a battery activation method and a battery, which aims to solve the problem of uneven lithium intercalation stress in materials caused by uneven interface of lithium-ion batteries, and improve the cycle performance and rate performance of lithium-ion batteries.

[0006] The first embodiment of this application provides a battery activation method, including the following steps: Obtain the battery parameters, and calculate the limiting diffusion current density of the battery based on the battery parameters; The battery is activated for the first time, and the charging current density of the first activation is controlled to be less than or equal to the limiting diffusion current density. Based on the formation time of the first activation, the diffusion length L of lithium ions in the electrode of the battery is calculated; The diffusion length L is compared with the median particle size D of the active material particles in the electrode. 50 Compare; When L>0.4D 50 In this case, the activation of the battery is terminated.

[0007] In some embodiments, the battery parameters include the structural parameters of the electrode and the reaction parameters of the active material.

[0008] In some embodiments, the limiting diffusion current density is calculated using equation (I): ; In the formula, I d ε is the limiting diffusion current, in A; n is the number of moles of reactant particles in the electrode, dimensionless; F is the Faraday constant, in C / mol; ε is the porosity of the electrode, dimensionless; τ is the tortuosity of the active material in the electrode, dimensionless; D i Lithium-ion solid-state diffusion coefficient, in cm⁻¹ 2 / s;c 0 The lithium-ion reaction concentration on the electrode surface, in mol / cm³. 3 H represents the thickness of the electrode, in cm.

[0009] In some embodiments, the formation time for the first activation is 2 to 5 hours.

[0010] In some embodiments, the charging current for the first activation is 0.01~0.2C.

[0011] In some embodiments, the diffusion length L is calculated using equation (II): ; In the formula, L is the diffusion length in μm; t is the formation time of the first activation in seconds.

[0012] In some embodiments, the diffusion length L is compared with the median particle size D of the active material particles in the electrode. 50 Following the comparison step, the following also includes: When L≤0.4D 50 In this case, the battery is activated a second time.

[0013] In some embodiments, the second activation includes the following steps: The battery is subjected to low-rate charge-discharge cycles; The low-rate cyclic charge / discharge rate is 0.1~0.5C, and the number of cycles is 20~200.

[0014] In some embodiments, after the activation step of the battery is completed, the method further includes: The battery is stored at a state of SOC of 80% to 100%.

[0015] In some embodiments, the storage temperature is 45~55°C.

[0016] The second embodiment of this application provides a battery, which is activated by the battery activation method in any of the above embodiments, wherein the edge potential of the negative electrode suspension region of the battery is 0.001~0.6V.

[0017] This application provides a battery activation method, comprising the following steps: obtaining battery parameters and determining the limiting diffusion current density of the battery based on the battery parameters; performing a first activation on the battery, controlling the charging current density of the first activation to be less than or equal to the limiting diffusion current density; calculating the diffusion length L of lithium ions in the battery electrode based on the formation time of the first activation; and comparing the diffusion length L with the median particle size D of the active material particles in the electrode. 50 Compare; in L>0.4D 50 In this case, the battery activation is terminated. The battery activation method provided in this application, by controlling the current density of the first activation to be less than or equal to the limiting diffusion current density, can match the diffusion rate of lithium ions in the electrode thickness direction and inside the particles with the reaction rate, avoiding uneven deposition of active lithium and ensuring full utilization of active lithium; after the first activation is completed, the diffusion length of lithium ions in the battery electrode is compared with the median particle size of the active material particles in the electrode, which can effectively determine whether lithium ions have penetrated into the particle core after the first activation, thereby achieving targeted homogenization treatment of the electrode, thus obtaining a uniform electrode interface, reducing the uneven lithium intercalation stress caused by interface inhomogeneity, reducing the crack growth rate of material particles, and improving the cycle performance and rate performance of lithium-ion batteries. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 These are graphs showing the battery capacity loss test results provided in the embodiments and comparative examples of this application; Figure 2 The graph shows the battery coulombic efficiency test results provided in the embodiments and comparative examples of this application. Detailed Implementation

[0020] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0021] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection, an electrical connection, or a connection that allows for mutual communication; they can refer to a direct connection, an indirect connection through an intermediate medium, or an indirect connection through a pipe or conduit; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances. In the description of this application, "multiple" means two or more, unless otherwise expressly and specifically limited. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more features.

[0022] In lithium-ion battery design, the N / P ratio is greater than 1. After assembly and electrolyte injection, lithium ions migrate from the positive electrode to the negative electrode. The negative electrode potential decreases to the range where additives can reduce, forming an SEI film. The formation of the SEI film affects the subsequent rate performance and cycle performance of the lithium-ion battery. Under the drive of electric field and concentration, lithium ions are deposited and embedded in the electrode. However, in the overhang region of the electrode, the lithium ion embedding is low, which affects the driving force for local potential reduction and film formation. Uneven film formation leads to uneven stress on the electrode and active materials during the subsequent lithium ion embedding process, accelerating material crack growth, which is detrimental to the long life and rate performance of the lithium-ion battery.

[0023] Activation of lithium-ion batteries can be achieved through charge-discharge cycles. During these cycles, lithium ions repeatedly intercalate and deintercalate between the positive and negative electrodes, resulting in a more uniform distribution of lithium ions within the battery and a denser SEI film growth, thus improving battery performance and lifespan. However, these methods are not ideal for activating the overhang region, leading to rapid capacity decay in the early stages of battery cycling after activation.

[0024] The applicant discovered through research that by controlling the charging current density during the initial activation and then selectively performing a secondary activation based on the diffusion of lithium ions along the electrode thickness, a uniform electrode reaction interface can be effectively obtained, thereby improving the battery's cycle performance and rate performance.

[0025] The first embodiment of this application provides a battery activation method, including the following steps: Obtain the battery parameters and then calculate the battery's limiting diffusion current density based on those parameters. The battery is activated for the first time, and the charging current density of the first activation is controlled to be less than or equal to the limiting diffusion current density. Based on the formation time of the first activation, the diffusion length L of lithium ions in the battery electrode is calculated; The diffusion length L is related to the median particle size D of the active material particles in the electrode. 50 Compare; When L>0.4D 50 In this case, the activation of the battery is terminated.

[0026] Understandably, by controlling the current density of the first activation to be less than or equal to the limiting diffusion current density, the diffusion rate of lithium ions in the electrode thickness direction and inside the particles can be matched with the reaction rate, avoiding uneven deposition of active lithium and ensuring full utilization of active lithium. In conjunction with the first activation, after the first activation is completed, comparing the diffusion length of lithium ions in the battery electrode with the median particle size of the active material particles in the electrode can effectively determine whether lithium ions have penetrated into the particle core after the first activation. This allows for targeted homogenization of the electrode, resulting in a uniform electrode interface, reducing the uneven lithium intercalation stress caused by interface inhomogeneity, reducing the crack growth rate of material particles, and improving the cycle performance and rate performance of lithium-ion batteries.

[0027] In some embodiments, battery parameters include the structural parameters of the electrodes and the reaction parameters of the active materials.

[0028] It is understandable that electrode structure parameters, such as porosity and thickness, can determine the physical channel characteristics of lithium-ion diffusion, while the reaction parameters of active materials, such as diffusion coefficient and molar number of reactant particles, can affect the chemical kinetics of lithium-ion reaction and diffusion. Combining the two can ensure that the calculated results of the limiting diffusion current density are consistent with the actual situation of the battery, providing a reliable reference for the current density control of the first activation, and avoiding calculation deviations and poor activation effects caused by missing parameters.

[0029] Specifically, the motor structural parameters include the porosity and thickness of the electrodes, while the reaction parameters of the active material include the number of moles of reactive particles in the electrode, the tortuosity of the active material in the electrode, the lithium-ion solid-phase diffusion coefficient, and the lithium-ion reaction concentration on the electrode surface.

[0030] In some embodiments, the limiting diffusion current density is calculated using equation (I): ; In the formula, I dε is the limiting diffusion current, in A; n is the number of moles of reactant particles in the electrode, dimensionless; F is the Faraday constant, in C / mol; ε is the porosity of the electrode, dimensionless; τ is the tortuosity of the active material in the electrode, dimensionless; D i Lithium-ion solid-state diffusion coefficient, in cm⁻¹ 2 / s;c 0 The lithium-ion reaction concentration at the electrode surface, in mol / cm³. 3 H represents the thickness of the electrode, in cm.

[0031] It is understandable that by quantifying the influence of electrode structure and material reaction characteristics on lithium-ion diffusion through Equation (I), the limit diffusion current density threshold of the adapted battery can be accurately derived, ensuring that the charging current density of the first activation is controlled within a reasonable range, effectively avoiding problems such as uneven lithium-ion deposition and SEI film quality defects, and laying a quantitative foundation for uniform electrode activation.

[0032] Specifically, the molar number n of reactant particles in the electrode can be determined according to the positive and negative electrode reaction equations of a lithium-ion battery. + In the insertion / extraction reaction, n is generally set to 1; F = 96485 C / mol (standard value); the porosity ε of the electrode can be calculated by the mass-volume method or directly measured by a mercury porosimeter; the tortuosity τ of the active material in the electrode can be obtained by measuring the effective diffusion coefficient D of the electrode. eff Then, according to the formula τ=ε×D i / D eff The lithium-ion solid-phase diffusion coefficient D can be calculated, or it can be obtained by observing the electrode pore structure through scanning electron microscopy and then calculating it using simulation software. i The lithium-ion reaction concentration c on the electrode surface can be calculated through symmetrical battery experiments. 0 It can be done through formula c 0 = (electrode loading × theoretical capacity) / (material molar mass × electrode volume) is calculated; the electrode thickness H can be obtained by direct measurement. The above parameters can also be obtained by other measurement or calculation methods commonly used in this field, which will not be elaborated here.

[0033] In some embodiments, the formation time for the first activation is 2 to 5 hours.

[0034] It is understandable that the formation time for the first activation can be any value from 2h, 3h, 3.5h, 4h, 4.5h, 5h, or any value within a range of two. When the formation time for the first activation meets the above range, it can ensure that lithium ions can fully diffuse in the electrode initially and adapt to the thickness of the electrode.

[0035] In some embodiments, the charging current for the first activation is 0.01~0.2C.

[0036] It is understandable that the initial activation charging current can be any value from 0.01C, 0.04C, 0.08C, 0.12C, 0.16C, or 0.2C, or any value within a range of any two. If the initial activation charging current (i.e., the formation current) is too high, it will lead to a lower overall quality of the SEI film. High currents result in uneven lithium intercalation at locations with significant polarization overhang and on the negative electrode surface, requiring long-term cycling for interface homogenization activation and SEI film repair. This process consumes a large number of active lithium ions, leading to faster early battery degradation. Therefore, a low current can mitigate these negative effects to some extent, but too low a current may result in insufficient activation. When the initial activation charging current meets the above-mentioned range, it can ensure interface homogenization while avoiding insufficient activation.

[0037] In some embodiments, the diffusion length L is calculated using equation (II): ; In the formula, L is the diffusion length in μm; t is the formation time of the first activation in seconds.

[0038] It is understandable that the actual diffusion length of lithium ions can be directly calculated based on the formation time of the first activation using formula (II). This allows for a rapid assessment of the activation status inside the particles in the electrode without the need for complex detection, providing an objective and accurate basis for determining whether a second activation is needed, and ensuring the relevance and necessity of the second activation.

[0039] In some embodiments, the diffusion length L is correlated with the median particle size D of the active material particles in the electrode. 50 Following the comparison step, the following also includes: When L≤0.4D 50 In this case, the battery is activated a second time.

[0040] It is understandable that when the activation of particles inside the electrode is insufficient after the first activation, the second activation can further promote interface uniformity.

[0041] In some embodiments, the second activation includes the following steps: Perform low-rate charge and discharge cycles on the battery.

[0042] The low-rate charge-discharge cycle rate is 0.1~0.5C, and the number of cycles is 20~200.

[0043] Understandably, low-rate charge / discharge can reduce the matching pressure between lithium-ion diffusion rate and reaction rate, prompting lithium ions to slowly penetrate into the core region of the particles. Simultaneously, it promotes the further dense and uniform growth of the SEI film, effectively eliminating unintercalated lithium regions within the particles. This reduces uneven lithium intercalation stress, achieves electrode homogenization, and improves battery cycle performance and rate performance. The low-rate charge / discharge rate can be any value from 0.1C, 0.2C, 0.3C, 0.4C, 0.5C, or any value within a range of any two values. The number of cycles can be any value from 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, or any value within a range of any two values. When the rate and number of low-rate charge-discharge cycles meet the above-mentioned range, it can further ensure that lithium ions slowly penetrate into the core area of ​​the particles, while promoting the further dense and uniform growth of the SEI film, effectively eliminating the unintercalated lithium areas inside the particles, thereby reducing uneven lithium intercalation stress, achieving homogenization of the electrodes, and improving the battery cycle performance and rate performance.

[0044] In some embodiments, after the step of activating the battery is completed, the method further includes: Store the battery at a state of 80%~100% SOC.

[0045] It is understandable that the State of Charge (SOC) of the stored battery can be any value from 80%, 84%, 86%, 92%, 96%, and 100%, or any value within a range of any two values. In a fully charged or near-fully charged state, the lithium intercalation interface and the overhang site have a significant potential and concentration difference, which can drive lithium-ion diffusion. Controlling the battery to be stored at 80%~100% SOC effectively drives lithium-ion diffusion and promotes interface homogenization.

[0046] In some embodiments, the storage temperature is 45~55°C.

[0047] It is understandable that the storage temperature can be any value from 45℃, 47℃, 49℃, 51℃, 53℃, and 55℃, or any value within a range of two. By controlling the storage temperature to meet the above range, the diffusion rate of lithium ions in the electrolyte and electrodes can be accelerated, the homogenization time required can be shortened, and lithium ions can be more evenly distributed in all areas of the electrodes, especially the overhang region. At the same time, it avoids the aggravation of side reactions caused by high temperatures or the stagnation of diffusion caused by low temperatures, ensuring the homogenization effect during the storage stage, and further improving the battery cycle life and rate performance.

[0048] In some embodiments, after battery activation is completed, the battery is further disassembled to confirm the lithium intercalation status of the fully charged negative electrode and to confirm the activation effect. The negative electrode exhibits different colors at different lithium intercalation concentrations: a fully lithium-intercalated negative electrode appears golden yellow, a low-concentration lithium-intercalated negative electrode appears purple, and an unintercalated negative electrode appears black. Judging the lithium intercalation concentration of the negative electrode based on its different colors allows for a direct confirmation of the activation effect.

[0049] In some embodiments, after battery activation is completed, the activation effect can be evaluated by testing the battery's coulombic efficiency (CE). The battery coulombic efficiency can be tested using commercially available discharge cabinets such as those from Blue Electric or Newway, or through other testing procedures commonly used in the art. Current methods for confirming activation effects require at least 100 cycles of data or interface verification by disassembling a fully charged battery, which is costly. This application confirms the activation effect by observing the CE changes during the standard capacity process after activation, which not only saves costs but also significantly improves testing efficiency.

[0050] The second embodiment of this application provides a battery, which is activated by the battery activation method in any of the above embodiments, and the edge potential of the negative electrode suspension region of the battery is 0.001~0.6V.

[0051] The battery activation method and battery provided in this application are described below with reference to specific embodiments: Example 1 Example 1 provides a battery activation method, comprising the following steps: For the NCM-Gr system battery, the N / P ratio is 1.3 and the diffusion coefficient D is... i =10E-8, electrode thickness is 80μm on both sides, D 50 =17μm, and the calculated limiting diffusion current density of the material is 1.4mA / cm. 2 ; The battery underwent its first activation, achieving a maximum current density of 0.67 mA / cm². 2 The conversion time is 2 hours; Based on the formation time of the first activation, the diffusion length of lithium ions in the battery electrode was calculated to be L = 8.4 μm; 2L<0.8D 50 The two are not much different, but there is a certain deviation in theoretical calculation. Therefore, the battery is activated a second time with a cycle rate of 0.1C and a cycle count of 1. After the second activation, the lithium intercalation on the front side of the electrode can be basically uniform, but the activation at the suspended position is poor. After formation, the battery was fully charged and stored at 55°C for 5 days for activation in a suspended position.

[0052] Example 2 The battery activation method provided in Example 2 is the same as that in Example 1, except that after the formation is completed, the battery is stored at 80% SOC at 55°C for 5 days.

[0053] Comparative Example 1 The battery activation method provided in Comparative Example 1 is the same as that in Example 1, except that after the first activation, a 1C cycle is used, and the number of cycles is 100.

[0054] After activation, the batteries in Examples 1-2 and Comparative Example 1 were disassembled. In Examples 2 and Comparative Example 1, the negative electrode overhang region was purple, indicating that all overhang regions in Examples 2 and Comparative Example 1 were utilized and fully activated. In Example 1, the overhang region showed two colors: purple near the negative electrode active area and black at the electrode edge, indicating that the overhang region of Example 1 was not fully utilized and lithium intercalation was incomplete. The electrode showed that most of the overhang region was black. The rate of cycle capacity decay corresponds to the activation effect. This indicates that the activation method of full-charge storage is more beneficial to battery performance than the commonly used cycle activation method.

[0055] The batteries of Examples 1-2 and Comparative Example 1 were subjected to capacity loss tests and coulombic efficiency tests. The results are shown in Table 1 and 2. Figures 1-2 As shown.

[0056] Table 1

[0057] As shown in Table 1, the CE change in Example 1 exhibits a process of first increasing and then stabilizing, with an initial CE value less than 1. The CE changes in Example 2 and Comparative Example 1 exhibit a process of first decreasing and then stabilizing, with initial CE values ​​greater than 1. This is mainly because the fully activated negative electrode overhang region is also sufficiently diffused and in a partially lithium-intercalated state. During subsequent discharge, lithium ions slowly diffuse from the overhang region to the negative electrode active region and then intercalate into the positive electrode, resulting in a discharge capacity greater than the charging capacity, and ultimately a CE value greater than 1. In Example 1, the negative electrode overhang region is not sufficiently diffused and remains in an unintercalated state. During subsequent cyclic discharge, a small amount of lithium ions slowly diffuse from the negative electrode active region to the overhang region, leading to a decrease in the amount intercalated into the positive electrode, resulting in a discharge capacity less than the charging capacity, and ultimately a CE value less than 1. Generally, the activation effect can be confirmed after about 20 cycles. After 20 cycles, the CE value gradually stabilizes. Comparing the CE values ​​of the stable phase of different activation methods reveals that the better the activation effect, the closer the CE value is to 100%, the less irreversible capacity loss, and the better the cycle performance. Based on this principle, it is possible to perform gradient verification of storage time, storage SOC, cycle time, and cycle rate parameters for storage and cycle activation methods of batteries with different designs. Activation process design can be carried out for batteries with different designs, providing the optimal activation method and parameters.

[0058] The battery activation method and battery provided in the embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A battery activation method, characterized in that, Includes the following steps: Obtain battery parameters, and then determine the limiting diffusion current density of the battery based on the battery parameters; The battery is activated for the first time, and the charging current density of the first activation is controlled to be less than or equal to the limiting diffusion current density. Based on the formation time of the first activation, the diffusion length L of lithium ions in the electrode of the battery is calculated; The diffusion length L is compared with the median particle size D of the active material particles in the electrode. 50 Compare; When L>0.4D 50 In this case, the activation of the battery is terminated.

2. The battery activation method according to claim 1, characterized in that, The battery parameters include the structural parameters of the electrodes and the reaction parameters of the active materials.

3. The battery activation method according to claim 2, characterized in that, The limiting diffusion current is calculated using equation (I): ； In the formula, I d ε is the limiting diffusion current, in A; n is the number of reactant particles in the electrode, dimensionless; F is the Faraday constant, in C / mol; ε is the porosity of the electrode, dimensionless; τ is the tortuosity of the active material in the electrode, dimensionless; D i Lithium-ion solid-state diffusion coefficient, in cm⁻¹ 2 / s;c 0 The lithium-ion reaction concentration on the electrode surface, in mol / cm³. 3 H represents the thickness of the electrode, in cm.

4. The battery activation method according to claim 1, characterized in that, The formation time for the first activation is 2-5 hours; and / or, The charging current for the first activation is 0.01~0.2C.

5. The battery activation method according to claim 1, characterized in that, The diffusion length L is calculated using equation (II): ； In the formula, L is the diffusion length in μm; t is the formation time of the first activation in seconds.

6. The battery activation method according to claim 1, characterized in that, The diffusion length L is compared with the median particle size D of the active material particles in the electrode. 50 Following the comparison step, the following also includes: When L≤0.4D 50 In this case, the battery is activated a second time.

7. The battery activation method according to claim 6, characterized in that, The second activation includes the following steps: The battery is subjected to low-rate charge-discharge cycles; The low-rate cyclic charge / discharge rate is 0.1~0.5C, and the number of cycles is 20~200.

8. The battery activation method according to claim 1, characterized in that, After the activation step of the battery is completed, the process further includes: The battery is stored at a state of SOC of 80% to 100%.

9. The battery activation method according to claim 8, characterized in that, The storage temperature is 45~55℃.

10. A battery activated by the battery activation method according to any one of claims 1 to 9, characterized in that, The edge potential of the negative electrode suspension region of the battery is 0.001~0.6V.

Images