Application of low-power batteries

Application of low-power batteries

Rechargeable batteries can be used in commercial, medical, space and military applications. However, different applications have different performance requirements. The main design requirements for medical and portable diagnostic equipment are minimal packaging, ultra-low power consumption, low offset drift and excellent noise performance. The basic requirements for electric vehicles and hybrid electric vehicles are high power density, high energy density, a lifetime of more than 10 years, and cost. For space applications, rechargeable batteries must meet strict performance requirements, such as light weight, radiation resistance, compact packaging, high efficiency, long life cycles, and ultra-high reliability. In battlefield applications, batteries must meet strict performance requirements, that is, light weight, high efficiency, ultra-high reliability, and continuous operation until the mission is completed. The following paragraphs describe in more detail the performance requirements of batteries currently used in various commercial applications.

Low-power batteries are widely used in electronic circuits, digital sensors and electrical equipment, such as toys, electronic clocks, watches, radios, computers, medical equipment, electric toothbrushes, smoke detectors, parametric safety equipment and many other commercial applications. Old electrochemical systems such as carbon zinc, zinc air (Zn-air), nickel cadmium, and lead acid continue to improve in terms of performance, cost and size. They are ideal for medium power applications. Most low-power commercial batteries are called primary batteries. Primary batteries called D batteries (or dry batteries) are widely used in flashlights, smoke detectors, toys, electronic clocks, radios and other entertainment equipment. In the past 10 years or so, lithium manganese oxide (L-Mn-O) batteries have dominated the commercial market. The size of primary batteries is increasing because new electrical and electronic devices are designed around their higher voltage, higher energy capacity, and excellent service life.

Environmental regulations continue to affect the use and disposal of batteries. Therefore, secondary batteries or rechargeable batteries that can be reused many times before being discarded have attracted more interest. The use of rechargeable secondary batteries provides the most cost-effective battery usage. Existing and emerging battery systems will be discussed in terms of energy capacity, shelf life, lifespan, one-time purchase cost, discharge rate, duty cycle, operating voltage and other related characteristics. The advancement of battery technology has been mainly linked to the application of electronic devices and sensors. Advances in material science, design configuration, and packaging technology have played a key role in improving energy density, lifetime, reliability, size, and weight.

1. Batteries using thin film and nanotechnology
Materials scientists and battery designers have confirmed interesting developments including thin film (TF) and nanotechnology. The energy and power density, compared to its size, make it the most attractive in terms of energy harvesting. Fast charging capability and low ESR (equivalent series resistance) are the most notable features of this battery. In addition, these batteries do not self-discharge, allowing them to maintain normal use for 10 years or longer. In other words, these batteries do not need to be charged even after a long period of storage. When supercapacitors are added, these batteries fill up the energy storage picture for many mesh network applications. Materials scientists claim that TF batteries can be used to integrate photovoltaic (PV) batteries to maintain self-charging capabilities, providing another option when charging facilities cannot be used immediately.

The TF battery was originally designed and developed by the Oak Ridge National Laboratory (ORNL). TF batteries can be deposited directly on thin plastic sheets or chips. Unlike traditional batteries, TF batteries can provide maximum bending ability when manufactured on thin plastic sheets, and can be formed into any shape required for a specific application. The size and geometry of these TF batteries are also well expanded. Operational tests conducted on various users have shown that these batteries do not exhibit performance degradation when operating in a wide temperature range of -30 to 140°C. In addition, the performance of the battery is not affected under the automatic reflow soldering at a heating temperature of 280°C.

During the manufacturing process, different layers can be deposited by sputtering or evaporation techniques. The stack from the current collector to the anode is usually less than 5 μm. TF battery designers estimate that the total battery thickness will reach 0.35 ~ 0.62 mm everywhere. The charge and discharge characteristics of thin-film lithium-ion batteries are shown in Figure 1. In this particular battery design, the voltage starts at 4.0v, because lithium batteries have a lower operating voltage than lithium anode batteries.

Application of low-power batteries
Figure 1 Mass energy density and volume energy density of different rechargeable batteries

The most serious shortcoming of flexible TF battery is current limitation. Therefore, it is necessary to heat the cathode at a temperature higher than 700°C to achieve a high current density. This shortcoming makes it impossible to use flexible polymer substrates as cathode films in certain specific applications. The internal resistance of this type of battery depends on the thickness of the polyimide sheet and the annealing temperature, and it must not exceed 400°C. If the battery is made on a rigid ceramic substrate and the thickness of the cathode is comparable, the annealing temperature can reach 750°C.

2. TF micro battery
The most sophisticated, flexible, nanotechnology-based TF battery is most suitable for radio frequency identification (RFID) tags, smart cards, portable sensors and medical embedded devices. According to the battery designer, these TF batteries can be as thin as 0.002in, including packaging.

These TF batteries use a lithium phosphorous oxynitride (LiPON) ceramic electrolyte developed by Oak Ridge National Laboratory. The battery cathode is made of lithium cobalt oxide (LiCoO2), and the anode is lithium. Neither the cathode nor the anode contains liquids or materials harmful to the environment. Although the lithium material is slightly toxic, if the vacuum seal is broken, the small amount of lithium in the micro battery will not cause a fire. Therefore, the battery provides the best reliability and safest operability in its use.

3. The number of charge-discharge cycles and charging time of low-power batteries
The charging time strictly depends on the capacity of the battery. The battery designer claims that a 0.25 mA·h TF battery can be charged to 70% of the rated capacity in less than 2 minutes and fully charged in 4 minutes. The discharge rate of each battery can exceed 10C, and a good battery can perform more than 1000 charge-discharge cycles under 100% deep discharge. The self-discharge capacity of this battery is less than 5% per year. During the discharge process, the battery experiences a loss of capacity. The annual capacity loss of various rechargeable batteries is shown in Figure 2. These batteries can be customized to meet specific size requirements. A battery with a capacity of 0.1mA·h can be designed by a battery design engineer to have a physical size not exceeding 20mm x 25 mm x 0.3 mm. The battery can be stored in an ambient temperature of -40~+85℃ without any structural damage or performance degradation. In addition, the design of the battery is such that the operating temperature does not significantly affect the performance. At high temperatures, these batteries can be charged and discharged at a higher rate and higher capacity. However, when a high temperature of +170°C is reached, the capacity of the battery will decrease at a faster rate during the cycle. In a cold environment where the temperature drops to -40°C, a decrease in the charging and discharging rate can be expected. The decreasing trend of the volumetric energy density of various rechargeable batteries with the working temperature is shown in Figure 3. It can be clearly seen from this figure that the volumetric energy density of the battery decreases at a faster rate at a lower operating temperature. Especially sealed lead-acid and nickel-cadmium batteries have the lowest energy drop in a wide temperature range.

Application of low-power batteries
Figure 2 The shelf life of various rechargeable batteries at different ambient temperatures
Application of low-power batteries
Figure 3 The volumetric energy density of various batteries at different temperatures

The charging of these batteries requires a constant voltage of 4.2V. In addition, according to the requirements of battery designers, these nano-energy batteries cannot be overcharged. When the battery is charged at 4.2V and discharged to 3.0V at 1mA, the battery will lose approximately 10% of its capacity after 1,000 charge-discharge cycles. The time required for charging to reach 95% of the rated capacity was less than 4 minutes in the first cycle and increased to 6 minutes after 1,000 cycles. As for the electrical performance of these batteries, a miniature version of a commercial battery will provide about 80mA·h per discharge cycle, and a volumetric energy density of more than 400w·h/L.

Low-power batteries are identified by their rated power, and cover capacities ranging from a few milliwatts of watches to 10 to 20 watts of laptop computers. The energy level and output power per unit volume are the most critical requirements for many portable devices. The energy provided by a particular battery depends on the rate at which electrical energy is consumed or retrieved. Energy density and power capacity are strictly limited by the structural size of the battery, the size of the battery and the duty cycle used. The discharge rate, frequency, and cut-off voltage are usually chosen to meet the power requirements of specific applications, such as smoke detectors or cameras. The rated current of the primary battery is 1/1000 of the battery capacity, and the rated current of the secondary battery or rechargeable battery is C/20, where C represents the full capacity rating of the battery.

4. Structure configuration of low-power battery
Most primary or dry cells that use aqueous electrolytes use a single, thick electrode arranged in parallel or concentrically. Typical battery structures are classified as “barrel”, “simple tube”, “button”, or “coin battery”. Some galvanic cells are made into prismatic, thin, flat structures to minimize their volume. These form factors produce poor energy density and power capacity levels.

5. The most popular material for low-power batteries
Carbon-zinc batteries continue to dominate low-power household batteries worldwide. These batteries were widely used from 1920 to 1990, and showed significant improvements in electrical performance, life cycle or life span and leakage. There are two different models of this battery. A “Premium” version, it uses manganese dioxide (MnO2) electrolyte and zinc chloride (Zn-CI) electrolyte, which provides better electrical performance and higher reliability in a longer duration. Most standard size batteries are D, C, AA, or other configurations and Zn-C1 electrolyte. According to market surveys, the unit sales ratio of alkaline Zn-CI batteries to carbon-zinc batteries is 1, while the ratio of alkaline batteries using manganese dioxide electrolyte in the United States is 3.5:1. According to international market surveys, China produces Approximately 6 million carbon-zinc batteries. The quality of Chinese batteries is slightly worse, but the cost of batteries is lower. Simply put, the use of mercury-free manganese dioxide alkaline dry batteries is the most popular option around the world because the recycling process is easy and environmentally friendly.

① Low-power standard battery
The standard battery uses manganese dioxide as the cathode and ammonium chloride as the electrolyte. The battery is called Zn-CI or alkaline battery. Carbon-zinc batteries cannot meet the requirements of electrical equipment, such as video tape recorders and CD players, high-resolution automatic cameras, flashlights, and certain toys, because they cannot provide the power required for satisfactory and reliable operation. Due to the power limitation of carbon-zinc batteries, alkaline batteries are widely used in low-power devices.

The shelf life of medium and high power alkaline batteries is approximately 3 to 6 years based on the rated power. Alkaline batteries provide a very long life and maintain battery power for a longer period of time. In addition, alkaline batteries provide higher reliability and lower production costs. This is why alkaline rechargeable batteries are widely used in the United States.

②Miniature primary battery
Micro batteries are used in applications where the power consumption is tens to hundreds of microwatts. These batteries are best suited for watches, smoke alarms, temperature monitoring sensors and other low-power electronic components.

In the commercial market, “button” structure batteries are sold in large numbers. Market research shows that zinc-air, silver oxide, mercury oxide, and manganese dioxide button batteries are the most popular. Due to concerns about mercury content, mercury batteries are banned from being sold in industrial countries. Zinc-air batteries are suitable for hearing aid equipment, while zinc manganese dioxide batteries are widely used in watches. In addition to zinc-air batteries, micro batteries provide excellent shelf life and reliable service life. Most watch batteries provide high reliability and long life at an ambient temperature of 37°C. The service life of the watch battery is 5~7 years. According to the supplier of the micro battery, the unit volume of the zinc anode micro battery will be reduced due to the new device designed around the lithium button battery. Regardless of these shortcomings, zinc-air batteries have the highest level of energy density.

Miniature lithium battery. In the past 20 years, due to its best working life and ultra-high reliability, miniature lithium-ion batteries have been in great demand. Lithium batteries exhibit a very high level of energy density, but low power. Lithium iodine batteries are widely used in pacemakers. Lithium iodine is a low-conductivity solid electrolyte, which limits the output current to a few microamperes. The latest developments in battery materials and speedometer technology allow the most reliable battery to work for 10-12 years. High-power implanted batteries using lithium silver vanadium oxide (Li-AgV) are being used as power sources for cardiac pacemakers and portable automatic defibrillation devices. 1g of lithium is equal to 3.86A·h of stored energy. The disposal of a large number of lithium batteries can cause serious environmental problems and generate hazardous waste. These substances must be handled or stored under strict environmental guidelines. Despite these problems, lithium-based rechargeable batteries are experiencing a growth rate of up to 25%, driven by the surge in users of mobile phones, ipads, laptops, video cameras and entertainment devices.

6. Low-power batteries using nanotechnology
Smart batteries are developed using nanotechnology. The smart battery contains a “super-hydrophobic nanostructured surface” of carbon nanotubes, as shown in Figure 4. This battery technology separates the electrolyte from the anode and cathode electrodes. Under the action of an electric field, the electrolyte undergoes a process of “electrowetting”. This introduces a change in surface tension, allowing it to pass through the barrier flow, thereby creating a voltage between the electrodes of the battery. This type of battery has a low power rating, but it shows a significant improvement in size, weight, and performance. A typical application of smart or nano-battery may include a mission-critical mobile phone. Just as the battery of a traditional mobile phone is about to run out of power, for example, a spare battery can drive to provide 10 minutes of talk time, which may be crucial.

Application of low-power batteries
Figure 4 The surface structure and charge/discharge characteristics of a TF superhydrophobic nanostructure made of nanotubes

7.Paper batteries using nanotechnology
Scientists and postdoctoral researchers at Stanford University in California are working to develop paper batteries that use plain paper and inks that can deposit carbon nanotubes (CNT) and silver nanowires. Scientists believe that when paper is covered by ink injected with nanomaterials, the paper becomes highly conductive and can be used to produce ultra-low-cost, flexible, and light-weight miniature batteries and supercapacitors.

These nanomaterials are a one-dimensional structure with a very small diameter, which helps the injected ink adhere strongly to the fiber paper, making batteries and supercapacitors more durable and cost-effective. Scientists at Stanford University claim that nanomaterials are the most ideal conductors because they can conduct electricity more efficiently than ordinary conductors. The ink-covered paper is baked, and then folded into a power generation source to build a battery source. The potential applications of such paper batteries, including small electric and hybrid vehicles, depend strictly on the rapid transmission of electrical energy. Plastic sheets can be used to make such batteries. However, due to the porous texture of the paper, the ink will adhere to the surface of the paper more strongly.

This kind of paper can be crumpled, folded, or even soaked in acid, and the performance of the battery is not significantly reduced. The research work of scientists is also suitable for energy storage devices with lower cost and complexity. Scientists are conducting more tests on these paper batteries.