Research progress and future prospects of 1.5v Alkaline battery

　　I. Overview

　　Compared with lithium-ion batteries, metal-air batteries have extremely high theoretical capacity and energy density and have attracted widespread research attention, as shown in Figure 1. Among them, rechargeable zinc-air batteries in alkaline systems and lithium-air batteries in organic systems, as typical representatives of aqueous and non-aqueous systems, have become hot spots in recent research. The working principle of the battery is shown in Figure 2. When making batteries flexible, it is necessary to design new flexible structures, prepare flexible electrode materials and solid electrolyte membranes, and thus face more challenges.

　　Figure 1 Comparison of capacity, energy density and voltage of different metal-air batteries

　　Figure 2 Schematic diagram of the working principles of alkaline system zinc-air batteries and non-aqueous system lithium-air batteries

　　2. Flexible battery structure and testing

　　Currently, the battery structure widely used in flexible zinc-air and lithium-air batteries is a sandwich structure composed of a flexible positive electrode, an electrolyte membrane and a negative electrode. The other uses linear metal electrodes, which are sequentially wrapped with an electrolyte layer and an air electrode layer on the surface to form a tubular structure. In addition, there are some new structures, such as foldable battery structures and flexible, ultra-lightweight lithium-air batteries inspired by bamboo slips.

　　In addition to the charge-discharge and cycle life tests in conventional batteries, the stability of flexible batteries under external force is crucial. Mainly including electrochemical stability under bending, twisting at different angles and stretching at different lengths, as well as performance retention under long-term fatigue.

　　3. Metal electrodes

　　Metal sheets are usually used directly as electrodes in flexible air batteries. However, metal sheets may suffer from fatigue failure during long-term bending. In flexible zinc-air batteries, metal powder is combined with binder and conductive carbon powder to form a composite electrode, which improves the flexibility and stability of the electrode. In flexible lithium-air batteries, metallic lithium and stainless steel mesh are rolled together to improve the fatigue resistance of the metal electrode. In addition, in order to achieve a certain stretchability of the flexible battery, the metal electrode can be made into a spring shape, or multiple small pieces of metal can be combined into a complete electrode to meet the stretching needs by "breaking it into parts".

　　4. Electrolyte membrane

　　In flexible zinc-air batteries, anion exchange membranes and alkaline gel electrolytes are mainly used as the electrolyte membranes of the battery. In flexible lithium-air batteries, electrolyte membranes mainly include gel, solid and composite polymer electrolyte membranes. In order to achieve good electrochemical performance of flexible batteries, in addition to requiring the electrolyte membrane to have good conductivity, chemical and electrochemical stability and other properties of traditional liquid electrolytes, the interface problem with metal and air electrodes is a difficult problem that needs to be solved.

　　For the electrolyte membrane-metal electrode interface, it is necessary to overcome the problems of dendrites and surface passivation. For the electrolyte membrane-air electrode interface, the solid electrolyte greatly reduces the effective reaction interface. For lithium-air batteries, since the product is solid lithium peroxide, the attenuation of the reaction area is further aggravated. Therefore, effective methods are needed to increase the reaction interface.

　　In addition, during the bending or twisting process of the battery, the separation of the electrode and electrolyte membrane may occur due to the different mechanical properties of the electrode and electrolyte membrane. How to maintain the stability of the interface is the key to ensuring long-term stable operation of the battery.

　　5. Air electrode

　　As an important component of metal-air batteries, air electrodes have always been a hot research topic. On the one hand, an effective catalyst is needed to achieve rapid charge and discharge of the battery; on the other hand, a suitable structure is needed to ensure the transmission of oxygen. In flexible batteries, it is more necessary for electrodes to have good flexibility to meet the needs of deformation. At present, the main flexible electrodes include: 1. Electrodes made of carbon cloth or carbon fiber mesh; 2. Electrodes made of nanocarbon materials (such as carbon nanotubes, graphene) such as carbon nanotube paper and graphite paper; 3. , Electrodes formed by metal substrates such as stainless steel mesh and nickel mesh; 4. Some other new flexible electrodes.

　　6. Operation management

　　Typically, zinc-air batteries operate directly in air, while lithium-air batteries operate in oxygen. And operating conditions can seriously affect battery performance. First of all, moisture in the air will affect the stability of the electrolyte membrane, and carbon dioxide in the air has a greater impact: in zinc-air batteries, carbonate will be formed, affecting the conductivity of the electrolyte; in lithium-air batteries, solid secondary The product lithium carbonate affects the charging performance of the battery. Secondly, battery performance is usually tested at room temperature, but the temperature changes significantly during actual use. For example, in wearable devices, the operating temperature of the battery may rise to 30 degrees or higher due to contact with the human body. In different seasons and regions, temperature changes will be greater. Therefore, future battery testing needs to examine the stability under different gas atmospheres and temperatures in more detail, and adopt appropriate management measures.

　　7. Summary and Outlook

　　In recent years, a series of progress have been made in 1.5v Alkaline battery, and the energy density, efficiency and cycle life of the batteries have been significantly improved. Future research needs to further address the following issues: 1. New battery structure design to maintain stable electrochemical performance under various deformation conditions; 2. Establishment of evaluation standards to standardize battery performance assessment indicators, such as based on unified quality or volume, stipulating recognized flexibility test standards (such as bending and twisting angles, tensile lengths, fatigue tests, etc.); 3. Development of flexible components, including metal and air electrodes, electrolyte membranes, current collectors, and packaging materials; 4. Management of operating conditions ensures stable electrochemical performance under different conditions.

　　All in all, future research needs to use a combination of experimental online monitoring and numerical simulation and other technical means to clearly clarify the relationship between material transport, structural changes and electrochemical reactions during battery operation, and provide information for the rational design of batteries. Important guidance.

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