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Unit 1 Battery Technology INTRODUCTION Can you imagine a world without batteries? The clock on your wall requires a battery for it to work. The watch on your wrist goes on a battery. The motor vehicle needs a battery f...

Unit 1 Battery Technology INTRODUCTION Can you imagine a world without batteries? The clock on your wall requires a battery for it to work. The watch on your wrist goes on a battery. The motor vehicle needs a battery for it to start and take you to your destination. Battery is the backbone of UPS to run your computer without interruption. After a day’s work, if you pick-up the remote control of the television, it is again the battery that helps you to change channels of the T.V. Cameras, laptop computers, cellular phones, key chain laser, heart pacemaker and children’s toys – they all require batteries. Batteries are used for so many different purposes and different applications require batteries with different properties. The battery required to start a car must be capable of delivering a large electrical current for a short period of time. The battery that powers a cardiac pacemaker must be small, rugged, leak proof, compact and capable of delivering a steady current for an extended period of time. In UPS systems, longer and consistent backup is needed. Batteries for hearing aids must be above all tiny. Batteries for torpedoes and submarines must be stable during storage and give high power for short times & certainly rechargeable. For a lap-top computer, a battery in the form of a flexible sheet distributed around the case is much preferred. Batteries remain an enormous industry with a turnover of billions of pounds worldwide. Primary batteries: They are galvanic cells which produce electricity from chemicals that are sealed into it when it is made. This type of cells cannot be recharged as the cell reaction cannot be reversed efficiently by recharging. Once the cell reaction has reached equilibrium the cell must be discarded. No more electricity is generated, and we say the battery is ‘dead’. These are also known as ‘throw – away’ batteries or irreversible batteries. Eg: Dry cell, Lithium copper sulfide cell. Secondary batteries: A secondary cell is rechargeable by passing current through it. In the charging process, an external source of electricity reverses the spontaneous cell reaction and restores a non- equilibrium mixture of reactants. After charging, the cell can be used for supplying current when required, as the reaction sinks toward equilibrium again. Thus, as the secondary cell can be used through a large number of cycles of discharging and charging. Such cells are also known as rechargeable cells, storage cells, or accumulators. Eg. Lead-acid batteries, Nickel-cadmium cell, Lithium- ion battery. Requirements of Primary Battery: Compact, lightweight and must be fabricated from easily available raw materials. Economic and have benign environmental properties Should have a high energy density, longer shelf life Provide constant voltage and should have a long discharge period Requirements of Secondary Battery Long shelf-life in both charged & discharged conditions Longer cycle life and design life High power to weight ratio Short time for a recharge. High voltage & high energy density BASIC PRINCIPLE BEHIND BATTERY OPERATION The basis for a battery operation is the exchange of electrons between two chemical reactions, an oxidation reaction and a reduction reaction. The key aspect of a battery which differentiates it from other oxidation/reduction reactions (such as rusting processes, etc) is that the oxidation and reduction reaction are physically separated. When the reactions are physically separated, a load can be inserted between the two reactions. The electrochemical potential difference between the two batteries corresponds to the voltage of the battery which drives the load, and the exchange of electrons between the two reactions corresponds to the current that passes through the load. The components of a battery, which are shown in the figure below, and consist of an electrode and electrolyte for both the reduction and oxidation reaction, a means to transfer electrons between the reduction and oxidation reaction (usually this is accomplished by a wire connected to each electrode) and a means to exchange charged ions between the two reactions. The key components which determine many of the basic properties of the battery are the materials used for the electrode and electrolyte for both the oxidation and reduction reactions. The electrode is the physical location where the core of the redox reaction – the transfer of electrons – takes place. In many battery systems, including lead acid and alkaline batteries, the electrode is not only where the electron transfer takes places but is also a component in the chemical reaction that either uses or produces the electron. However, in other battery systems (such as fuel cells) the electrode material is itself inert and is only the site for the electron transfer from one reactant to another. For a discharging battery, the electrode at which the oxidation reaction occurs is called the anode and has a positive voltage, and the electrode at which the reduction reaction occurs is the cathode and is at a negative voltage. Battery Batteries are devices that store energy in the form of chemical energy and convert it when required to electrical energy. Through the electrochemical reactions that occur in a battery, electrons are released and flow from one conductor (electrode) to another through external electric circuit providing an electric current that is used to do tasks. At the same time, charged ions are transported through an electrically conducting solution (electrolyte), which is in contact with the electrodes to bring the reactants to the electrode/electrolyte interface. The electrodes and electrolyte can be made from various materials. Variations in the composition of the electrolyte and electrodes result in different electrochemical reactions and charged ions. These influence the extent to which energy can be stored, as well as the operating voltage and performance of the battery. A battery can be comprised of one or more electrochemical cells. Each electrochemical cell includes two electrodes which are separated by an electrolyte. During discharge, an electrode from which electrons flow is known as anode or negative electrode and an electrode which receives the electrons is known as cathode or positive electrode. Typically, the anode and cathode are made of different types of chemical compounds or metals. The electrolyte is a medium that allows the charged ions to be transported toward the electrodes. In fact, the flow of positive charged ions through the electrolyte balances the movement of negative electrons which makes the battery cell electrically neutral during its operation. Also, a porous and electronically insulating separator is often used between the anode and cathode to improve the mechanical strength of the electrolyte and reduce the risk of an internal short circuit. Typically, separators have a high ionic conductivity and provide electronic insulation. The current collectors can also provide efficient transfer of electrons and remove heat from the electrodes. Thin foils of aluminium or copper are typically used as current collectors. ELECTRODES (ANODE AND CATHODE) Any two electrodes that have different standard reaction potentials can create an electrochemical cell. The electrode with higher ability to absorb electrons (as happens at the cathode) has a higher standard potential (i.e., more positive potential) while the electrode with a higher tendency to lose electrons (as happens at the anode) has a lower standard potential (i.e., more negative potential). Therefore, an appropriate selection of materials for the anode and the cathode is based on more negative and more positive standard reaction potentials. The desired characteristics for the cathode are high redox potential (efficient oxidizing agent), high specific capacity (total amount of electricity generated in the electrochemical reactions per gram of cathode material), reversibility and stability while in contact with the electrolyte. For the anode, the desired characteristics are low redox potential (efficient reducing agent), high specific capacity, reversibility and good conductivity. Table No 1 lists several important parameters and their values for several anode and cathode materials used in different types of battery cells, including molecular weight, valence, standard reduction potential, gravimetric capacity, and volumetric capacity. The difference between the electrical potentials of the cathode and the anode gives the cell potential, i.e., Ecell = Ecathode (Reduction potential) – Eanode (Reduction potential) The greater the difference, the greater is the cell potential, and the higher is the voltage. A practical way to measure the electrical potential of an electrode (cathode or anode) is to assign zero for the electrical potential of the reaction that occurs at the electrode and then use it as a reference electrode. ELECTROLYTES The electrolyte can be a liquid, solid, polymer, or composite (hybrid) depending on the type of battery. The electrolyte should have a high ionic conductivity, no electric conductivity, nonreactivity with the electrode materials and a wide operating temperature range. The traditional liquid electrolyte generally has a low viscosity, high energy density, high charge/discharge rate capability, a relatively low operational temperature (between – 40 °C and 60 °C) and low flammability. The polymeric electrolyte can be either a gel or a solid. The solid polymeric electrolyte has the advantageous of high flexibility, high energy density, multifunctional applications, good safety and mechanical properties and thermal/chemical stability. But solid polymeric electrolytes typically have low ionic conductivities at room temperature (10−5 – 10−1 mS cm−1). Gel polymeric electrolytes, on the other hand, have relatively high ionic conductivities (1 mS cm−1), high flexibility, multifunctional applications, and chemically stability, but they have poor mechanical strength and poor interfacial properties. The main advantages of a solid polymeric electrolyte are no electrolyte leakage, high safety (nonflammability), nonvolatility, thermal and mechanical stability, ease of fabrication, and high achievable power density and cyclability. REDOX REACTIONS AND ELECTRONS TRANSFER A reaction in which electrons are exchanged is referred to a reduction oxidation reaction or redox reaction. The overall reaction is divided into two half reactions; in an electrochemical cell, one half reaction takes places at the cathode and the other at the anode. A general redox reaction can be written as follows: where O denotes the oxidized species and R the reduced species. Reduction refers to the gain of electrons (which happens at the cathode). The cathode is said to be reduced during the reaction. Oxidation refers to the loss of electrons (which happens at the anode), and the anode is said to be oxidized during the reaction. So, the reaction at the anode is oxidation and at the cathode is reduction. These reactions have their own standard potentials, which indicate the capability of the reactions for accepting or producing electrons. Fig. 1 shows a basic operational mechanism for an electrochemical battery cell. Each cell contains two electrodes and at least one electrolyte. During discharge of the battery, at the anode/electrolyte interface, an electrochemical reaction of R → O + nee− occurs between the anode and the electrolyte that produces electrons and ions. The electrons are transferred to the cathode through an external electrical circuit which connect the electrodes, and the ions are transferred to the cathode through an electrolyte. The electrons and ions that were produced in the anodic half reaction react at the cathode/electrolyte interface (i.e., O + ne e− → R). Fig 1: Generalized redox battery operation during discharge and charge KEY BATTERY PERFORMANCE METRICS The characteristic performance of a battery determines its suitability for the desired application. Some of them are – a. Cell potential: The voltage of a battery is given by the equation Cell potential = (EC - EA) -|ηA - ηC |- i Rcell Where, Ec and Ex are the electrode (reduction) potentials of the cathode and the anode respectively, ηA and ηC are the over-potentials at the anode and the cathode respectively and iRcell is the internal resistance. To derive maximum voltage from a battery (a) The difference in the electrode potentials must be high (b) The electrode reactions must be fast to minimize the overpotential. (c) The internal resistance of the cell must be low. The electrode systems should be such that the active mass at the positive electrode depletes readily and that at the negative electrode increases easily. This minimizes the over-potential at the anode and cathode. The cell should be appropriately designed to minimize internal resistance. This can be achieved by keeping the electrodes close to each other and by using an electrolyte of high conductivity. b. Current: For efficient discharge, electrons should flow at a uniform rate in the electrolyte. Current is a measure of the rate of flow of electrons during discharge. It is the amount of charge flowing per unit time and is expressed in ampere per second. For uniform current, electrolyte of high conductance is desirable. Batteries provide direct current. c. Capacity: Capacity is the charge in ampere hours (A h) that could be obtained from the battery. This depends on the size of the battery and is determined by the Faraday relation, 𝑚 ×𝑛 ×𝐹 𝐶= 𝑀 Where C is the capacity in A h, m is the mass of active material and M is the molar mass. Fig 2: Battery voltage during charging Fig 3: Battery voltage during discharging The amount of active materials consumed during discharge determines the capacity of the battery. The capacity also depends on the discharge conditions. It is measured by finding the time ‘t’ taken for the battery to reach a minimum voltage, E cellmin for a fixed current discharge (i amperes). (The cell is said to be dead at minimum voltage). A plot of time against voltage at a fixed current discharge is shown in Fig. 5 and 6 The length of the flat portion of the curve is a measure of the capacity of the battery, the longer the flat portion better the capacity. d. Electricity Storage Density: Electricity storage density is a measure of the charge per unit mass stored in the battery, The mass of the battery includes masses of the electrolyte, current collectors, terminals, the case, and other subsidiary elements. To get high storage density the mass of subsidiary elements should be minimum. For instance, 7g of lithium is required at anode to give 96500 C whereas, for the same charge, 65g of zinc would be required. e. Energy Efficiency: Energy efficiency for a storage battery is given by - % Energy efficiency = (Energy released on discharge / Energy required for charging) × 100 Energy efficiency depends on the current efficiency of the electrode processes, the overpotentials during discharge and charge, and internal resistance. A battery should have high energy efficiency. f. Cycle Life: The cycle life of a battery is the number of charge-discharge cycles that can be achieved before failure occurs. (Note that cycle life applies to secondary batteries). It is necessary that during charging the active material is regenerated in a suitable state for discharge. The discharge-charge cycle depends on chemical composition, morphological changes, and distribution of active materials in the cell. The cycle life of a battery is affected by corrosion at contact points, shedding of the active material from the plates, and shorting between the electrodes due to irregular crystal growth and changes in morphology. g. Shelf Life Shelf life is the period of storage under specified conditions during which a battery retains its performance level. Shelf life is affected by self-discharge. Self-discharge occurs when there is a reaction between the anode and the cathode active materials or corrosion of current collectors. In addition, a commercial battery should have tolerance to service conditions such as variation in temperature, vibration and shock, and should have reliable output. h. Energy Density: Energy density is the ratio of the energy available from a battery to its mass (or volume) (Wh/Kg) or Wh/L. Energy density is determined by measuring the capacity and recording the average voltage (voltage averaged during the discharge) and the total mass (or volume) of the battery. A battery should have continuous energy density above a certain value or a very high energy density for a short period. i. Power Density: Power density is the ratio of the power available from a battery to its mass (W/kg) or volume (W/L). A battery should have continuous power density above a certain value or a high value for a short period EMERGING BATTERY TECHNOLOGIES Emerging battery technologies are constantly developing to fulfil the increasing requirements of many industries, such as consumer electronics, electric vehicles (EVs), renewable energy storage, and grid stability. Presented below are many noteworthy developing battery technologies: a. Solid-State Batteries: These batteries substitute the liquid or gel electrolyte present in conventional lithium-ion batteries with a solid electrolyte, resulting in increased energy density, enhanced safety, and perhaps extended longevity. The improved energy density and lower fire danger of solid-state batteries are expected to bring about a revolution in electric vehicles (EVs) and portable gadgets. b. Lithium-Sulfur Batteries: Lithium-sulfur batteries possess the capacity to provide greater energy density in comparison to lithium-ion batteries. They employ sulfur as the cathode material, which is abundant and affordable, resulting in the possibility of reduced production expenses. Nevertheless, there are still unresolved issues that need to be tackled, such as the low electrical conductivity of sulfur and the dissolution of polysulfides. c. Lithium-air batteries: Lithium-air batteries have an exceedingly elevated theoretical energy density, with the potential to surpass even that of gasoline. The anode is composed of lithium metal, while the cathode is made up of oxygen obtained from the  air. Nevertheless, it is necessary to address practical obstacles such as the restricted number of cycles that lithium metal anodes may undergo and concerns over their stability. d. Flow Batteries: Flow batteries utilize liquid electrolytes stored in separate external tanks to store energy, providing the advantages of scalability and flexibility in terms of capacity. Vanadium redox flow batteries (VRFBs) are now the most prominent form. However, continuing research aims to investigate other chemistries that can provide better energy density and cheaper prices. e. Metal-Air Batteries: Metal-air batteries, such as zinc-air and aluminum-air batteries, employ a metal anode and oxygen derived from the atmosphere as the cathode. These materials have large theoretical energy densities and may be inexpensive since their component materials are abundant. Nevertheless, the issues concerning reversibility, efficiency, and cycle life must be resolved to achieve commercial feasibility. f. Sodium-Ion Batteries: Sodium-ion batteries are being investigated as a more economical substitute for lithium-ion batteries, capitalizing on the ample availability of sodium minerals. Although they generally have lower energy density than lithium- ion batteries, they show potential for use in stationary energy storage applications that emphasize cost-effectiveness above energy density. These nascent battery technologies are at different phases of advancement, spanning from initial investigation to commercial implementation. Ongoing research and innovation in battery technology are essential for tackling difficulties and fully harnessing their capacity to fuel the future. Lead Acid Battery/Storage Battery (Lead Accumulator or Car Battery or the acid battery) Construction: The electrodes are lead grids. Grids are used to maximize the surface area. The anode grid is filled with finely divided spongy lead (Pb) and the cathode grid is packed with lead dioxide (PbO2). Both electrodes are submerged in a sulfuric acid solution (H2SO4) having a specific gravity of about 1.25 that acts as the electrolyte. The anode and cathode grids are separated by insulators like strips of wood, rubber, or glass fiber. The battery is encased in a plastic container or hard vulcanized rubber vessel. The cell doesn’t need separate anode and cathode compartments because the oxidizing and reducing agents are both solids (PbO2 & Pb) that are kept from coming into contact by insulating spacers between the grids. Both anode and cathode are immersed in a common electrolyte (i.e. cell without liquid junction). This is an example of a wet cell because the electrolyte is an aqueous sulphuric acid solution. Discharging reactions At the anode Pb (s) → Pb2+ (aq) + 2e- Pb2+(aq) + SO42-(aq) → PbSO4(s) Pb(s)+ SO42-(aq) → PbSO4(aq) + 2e- At the cathode: PbO2(s) + 4H+(aq) + 2e− → Pb2+(aq) + 2H2O(l) Pb2+(aq) + SO42−(aq) → PbSO4(s) PbO2(s) + 4H+(aq) + SO42−(aq) + 2e− → 2 PbSO4(s) + 2H2O(l) Overall: Pb(s) + PbO2(s) +4H+(aq) + 2SO42−(aq) → 2PbSO4(s) + 2H2O(l) Explanation: At the anode, lead atoms lose two electrons (e-) and become positively charged lead ions (Pb2+). The lead ions combine with the sulfate ions (SO42-) in the sulfuric acid solution to produce lead sulfate (PbSO4). At the cathode, lead dioxide gains electrons, releasing oxygen, which attaches to hydrogen ions (H+) to produce water (H2O (l) and lead ions. The lead ions react with sulfate ions to produce lead sulfate. As this reaction progresses, the flow of electrons creates an electric current. Lead sulfate adheres to each electrode and water increases. The concentration of sulfuric acid decreases and the amount of lead and lead dioxide decreases. When the reactants are depleted, the battery stops producing electricity. The reaction can be reversed by recharging the battery & the PbSO4 formed during discharge remains adhered to each electrode and is available at the site during recharging. The nominal voltage of each cell is about 2.1 V. The lead storage battery is designed to operate reversibly so that it can be used for storing electrical energy. They are called storage batteries because their essential function is to store electrical energy. It is used to store energy in chemical form. Reactions during Charging: Anode: PbSO4(s) + 2e- → Pb(s)+ SO42-(aq) Cathode: PbSO4(s) + 2H2O(l) → PbO2(s) + SO42-(aq) + 4H +(aq) +2e- Net reaction: 2PbSO4(s) + 2H2O(l) → Pb(s) + PbO2(s) +2H2SO4 Recharging is possible because PbSO4 formed during discharge adheres to the electrodes. By attaching an external power source to a battery ( >2 volts), a current runs through the poles in the opposite direction from normal discharge. This changes the lead sulfate and water back into the original reactants, lead dioxide and sulfuric acid, i.e. the electrodes return to their former composition and the sulfuric acid is regenerated. Charging, therefore, produces a gradual increase in sulfuric acid concentration. Since the level of charge on a storage battery is related to sulfuric acid concentration, the specific gravity of the (H2SO4) solution is a measure of the operational condition of a battery. A charged battery at room temperature with its electrolyte at normal concentration supplies a potential difference of 2.1 to 2.2 V. The complete reaction cycle of a lead acid storage battery is as follows. Discharge Pb(s) + PbO2 + 2 H2SO4(aq) 2 PbSO4(s) + 2 H2O(l) Charge In an automobile, the energy necessary for recharging the battery is provided by the generator driven by the engine. As the external source forces electrons from one electrode to another, the PbSO4 is converted to Pb at one electrode and PbO2 at the other. Overcharging: Electrolysis of water: The net reaction can be summarized by the equation. 2H2O (l) + electrical energy → 2H2(g) + O2(g) No gases will be liberated during charging if lead ions are present in the solution. If the electrolysis is permitted to proceed further, hydrogen gas is formed at the cathode, and oxygen gas is evolved at the anode. The hydrogen ions are discharged at the cathode through the reaction. 2H+(aq) + 2e-→ H2(g) Sulphate ions are resistant to oxidation and are not discharged at the cathode. Water is oxidized at the anode in preference to SO42- 2H2O(l) →O2(g) + 4H+(aq) + 4e- Consequences: (i) Excessive charging may reduce the acid level and may damage the exposed electrode grids (ii) In extreme cases, there will be dangerous high-pressure build-up that can lead to a serious risk of explosion. In more normal circumstances, the older version of the battery needs to be ‘topped up’ from time to time. Recent years have seen the introduction of “maintenance–free batteries” without a gas– release vent. Here the gassing is controlled by careful choice of the composition of the lead alloys used i.e. by using a Pb-Ca (0.1 %) as the anode which inhibits the electrolysis of water. Alternatively, some modern batteries contain a catalyst (e.g. a mixture of 98% ceria (cerium oxide) & 2% platinum) that combines the hydrogen and oxygen produced during discharge back into the water. Thus, the battery retains its potency and requires no maintenance. Such batteries are sealed as there is no need to add water and this sealing prevents leakage of cell materials. Applications: Many battery designs are available for a wide variety of uses that can be classified into three main categories. (a) Automotive (b) Industrial & (c) Consumer batteries. The automotive type is used in cars and trucks, to provide a short burst of power for starting the engine. It is incorporated as an essential accessory into the starting circuit of internal combustion engines for starting, lighting & ignition (SLI). The industrial batteries are used for heavy-duty applications such as motive and standby power. This class of batteries is used to operate electric trucks, submarines, and mine locomotives. They also provide power for the air conditioning and lighting systems. It is used in stationary backup power applications such as telecommunication systems, to ensure that, for example, the telephone network will continue to operate even in the event of a mains power failure. Such batteries are kept in hospital operating theaters, railway signal centers, and other places where a power failure might be disastrous and is used to supply electrical power during emergencies. The consumer batteries are used in emergency lighting systems, security and alarm systems, public address systems, power tools, UPS in computers, and vehicles. Advantages: A lead storage battery is highly efficient. The voltage efficiency of the cell is defined as follows. Voltage efficiency = average voltage during discharge average voltage during charge The voltage efficiency of the lead–acid cell is about 80 %. The near reversibility is a consequence of the faster rate of chemical reactions in the cell, i.e. anode oxidizes easily, and the cathode reduces easily leading to an overall reaction with a high negative free energy change. A lead – acid battery provides a good service for several years. The number of recharges possible ranges from 300 to 1500, depending on the battery’s design and conditions. The sealed lead-acid batteries can withstand up to 2000 – recharging. The time required for recharging is relatively low i.e about 2-8 hours. The battery has low internal self–discharge. A typical car battery provides a voltage of 12 V. This is not a large voltage, but the battery can provide a large current. E.g. over 10 A, without being destroyed. Disadvantages: If left unused in partially charged condition, it can be ruined in a short time by nucleation and the growth of relatively large PbSO4 crystals, which are not easily reduced or oxidized by the charging current. This disaster is known as ‘sulfation’. The lead acid battery has low energy storage to weight ratio i.e. its chief disadvantage is its great weight. (energy density ~ 35 wh / kg) The cell potential decreases with a decrease in concentration of sulfuric acid. During the discharge process sulfuric acid is consumed and water is produced.  So, the solution of sulfuric acid in the cell becomes less concentrated. The state of charge of the cell can be judged by the concentration of sulfuric acid in the liquid and this is usually done with a simple device to measure the density of the liquid. The density of a healthy, fully charged battery is ≥ 1.25 g/mL. The battery is not efficient at a lower temperature. Foe a battery to function properly, the electrolyte must be fully conducting. There will be an increase in the viscosity of the electrolyte due to a decrease in the temperature. This leads to a decrease in cell potential. Overcharging may damage the exposed electrodes and may also lead to an explosion in extreme cases. Sulfuric acid is a highly corrosive liquid. In the event of an explosion, the electrolyte is sprayed on to the individual who is working on the battery. Therefore, safety goggles must be worn when working with these batteries. Lead is toxic and hence environmental & health problems arise due to careless disposal by consumers. The corrosion of the lead grid at the lead dioxide electrode is one of the primary causes of lead acid battery failure. Nickel-Metal Hydride (NiMH) battery: As electronic products have come to feature more sophisticated functions, more compact sizes, and lighter weights, the sources of power that operate these products have been required to deliver increasingly higher levels of energy. To meet this requirement, nickel-metal hydride batteries have been developed and manufactured with nickel hydroxide for the positive electrode and hydrogen absorbing alloys, capable of absorbing and releasing hydrogen at high-density levels, for the negative electrode. Because NiMH batteries have about twice the energy density of Ni-Cd batteries and a similar operating voltage as that of Ni-Cd batteries, they have become a mainstay in rechargeable batteries. Construction Nickel-metal hydride batteries consist of a positive plate containing nickel hydroxide as its principal active material, a negative plate mainly composed of hydrogen-absorbing alloys, a separator made of fine fibers, an alkaline electrolyte, a metal case, and a sealing plate provided with a self-resealing safety vent. Their basic structure is identical to that of Ni-Cd batteries. With cylindrical nickel-metal hydride batteries, the positive and negative plates are separated by the separator, wound into a coil, inserted into the case, and sealed by the sealing plate through an electrically insulated gasket. Structure of Nickel-Metal Hydride Batteries Hydrogen-absorbing alloys have a comparatively short history which dates back about 20 years to the discovery of NiFe, MgNi and LaNi5 alloys. They are capable of absorbing hydrogen equivalent to about a thousand times of their own volume, generating metal hydrides and also of releasing the hydrogen that they absorbed. These hydrogen- absorbing alloys combine metal (A) whose hydrides generate heat exothermically with metal (B) whose hydrides generate heat endothermically to produce the suitable binding energy so that hydrogen can be absorbed and released at or around normal temperature and pressure levels. Depending on how metals A and B are combined, the alloys are classified into the following types: AB (TiFe, etc.), AB2 (ZnMn2, etc.), AB5 (LaNi5, etc.) and A2B (Mg2Ni, etc.). From the perspective of charge and discharge efficiency and durability, the field of candidate metals suited for use as electrodes in storage batteries is now being narrowed down to AB5 type alloys in which rare-earth metals, especially metals in the lanthanum group, and nickel serve as the host metals; and to AB2 type alloys in which the titanium and nickel serve as the host metals. Nickel-metal hydride batteries employ nickel hydroxide for the positive electrode like Ni- Cd batteries. Hydrogen is stored in a hydrogen-absorbing alloy for the negative electrode, and an aqueous solution consisting mainly of potassium hydroxide for the electrolyte. Their charge and discharge reactions are shown below. As can be seen by the overall reaction given above, the chief characteristics of the principle behind a nickel-metal hydride battery is that hydrogen moves from the positive to negative electrode during charge and reverse during discharge, with the electrolyte taking no part in the reaction, which means that there is no accompanying increase or decrease in the electrolyte. A model of this battery’s charge and discharge mechanism is shown in the figure below. The hydrogen-absorbing alloy negative electrode successfully reduces the gaseous oxygen given off from the positive electrode during overcharge by sufficiently increasing the capacity of the negative electrode which is the same method employed by NiCd batteries. By keeping the battery’s internal pressure constant in this manner, it is possible to seal the battery. Advantages of nickel metal hydride batteries 1. Energy density and capacity NiMH batteries boast commendable energy density, surpassing traditional nickel- cadmium batteries. Their capacity ranges from approximately 1000mAh to 3000mAh or higher, providing reliable and sustained power for various devices. 2. Rechargeability and cycle life These batteries excel in longevity, enduring hundreds to thousands of charge-discharge cycles. Their rechargeability and ability to retain capacity over multiple cycles make them a cost-effective and sustainable option for numerous consumer electronics and portable gadgets. 3. Environmental friendliness Comprising fewer harmful materials compared to certain battery types, NiMH batteries present an environmentally friendly choice. The absence of toxic cadmium reduces environmental impact during disposal or recycling, aligning with eco-conscious practices. 4. Enhanced safety features Relative to some battery chemistries, NiMH batteries possess a safer profile, exhibiting stability and lower risk of thermal runaway or fire hazards. This safety factor contributes to their suitability in various applications where reliability is crucial. 5. Cost-effectiveness NiMH batteries are known for their cost efficiency. With the capability to be recharged hundreds to thousands of times before significant capacity loss, they offer a long-term economic power solution, reducing the need for frequent replacements. Disadvantages of nickel metal hydride batteries 1. High self-discharge rate A notable drawback of NiMH batteries is their relatively high self-discharge rate. They can lose around 1-5% of their charge per day when idle, affecting their shelf life and necessitating regular recharging. 2. Memory effect and voltage sag While less prone than nickel-cadmium batteries, NiMH cells can still suffer from memory effect issues if not fully discharged before recharging. Moreover, they might exhibit voltage sag under heavy loads, impacting performance in devices requiring consistent power output. 3. Sensitivity to temperature extremes Extreme temperatures adversely affect NiMH battery performance. High temperatures accelerate self-discharge and degrade the battery, while low temperatures reduce efficiency and capacity, limiting functionality in extreme environmental conditions. 4. Limited fast charging capability NiMH batteries have slower charging rates compared to newer technologies. Their limited ability for rapid charging requires longer charging times, affecting convenience in fast-paced scenarios. 5. Reduced voltage output Compared to some newer battery chemistries, NiMH batteries exhibit lower voltage outputs, affecting their compatibility with devices requiring higher voltage levels for optimal performance. Applications of NiMH batteries NiMH batteries have found applications across various industries and consumer devices, catering to diverse power needs. Some of the typical applications include Consumer electronics, Power tools, medical devices, Hybrid vehicles, Emergency lighting and backup power, Renewable energy storage, Flashlights and portable devices, and Electric bicycles and scooters. Li-Ion batteries Li-ion batteries have revolutionized the world of portable electronics and are making significant strides in electric vehicles and grid storage. Construction: The anode is a lithium-carbide intercalate (LixC6). The cathode is a transition metal oxide MO2 of variable oxidation state (MnO2, CoO2, NiO2) which can intercalate lithium usually cobalt dioxide, CoO2 is used. The electrolyte is usually inert polar dry ether or carbonate (diethyl carbonate or propylene carbonate), in which a conductivity salt such as LiPF6 or LiBF4 is dissolved. Schematic diagram of Rechargeable lithium ion battery Working: Graphite has a layered structure and its electrochemically reduced in an aprotic organic electrolyte containing lithium salts and lithium in interclated (or doped) between the layers of graphite to form Lithium-Graphite interclated compound (GIC). Li-GIC undergoes oxidation leaving an electron. At anode: LixC6 xLi+(solv) + 6C(s) + xe-………………………….(1) When a negative electrode is discharged, lithium is deinterclated (undoped) from lithiated graphite and lithium ions dissolve into the elctrolyte. The lithium content in the LiCoO2 electrode reversibly changes during charge and discharge as indicated in the following equation: At cathode: CoO2(s) + xLi+(solv) + xe- LixCoO2(s)……………….(2) Net cell reaction: LixC6(s) + CoO2 Discharge LixCoO2(s) + 6C………………….(3) Charge Explanations: It is called as a lithium ion battery to emphasise that it contains no lithium metal. Both the electrodes are intercalation compounds. The electrode on the left serves as the anode when the cell discharges. It is a special intercalation compound consisting of a graphite host into which lithium ions have been electrochemically inserted between the carbon atom layers. The lithiated graphite is written as LixC6. During the discharge, the lithium ions are extracted by the half-reaction (1). The electrode that serves as cathode during discharge is normally cobalt dioxide. The reduction half reaction is cathode during discharge is normally cobalt dioxide. The reduction half reaction shown in eq.(2), where cobalt undergoes reduction from IV to III oxidation state. The overall reaction is shown in eq.(3). During discharge, the Li+ ions spontaneously migrate from the lithium graphite anode to the CoO2 cathode, enabling the current to flow through the external circuit. When charged, cobalt ions are oxidized and lithium ions migrate into the graphite, when discharging the battery delivers energy to the external load and when charging it receives energy from a D.C. power source. The electrode that acts as an anode, during discharging becomes a cathode when its charging. Advantages  Designed to overcome the safety problems associated with the highly reactive properties of Lithium metal. Long cycle life (400-1200 cycles). Smaller, lighter and provide greater energy density than either nickel-cadmium or nickel-metal-hydride batteries Can be operated in a wide temperature range and can be recharged before they are fully charged. Typically designed to be recharged in the device rather than in an external charger. The average voltage of a Li-ion battery is equivalent to three Ni-Cd cells. A typical Li-ion battery can store 150 watt-hours of electricity in 1 kilogram of battery as compared to lead acid batteries can sore only 25 watt-hours of electricity in one kilogram. Limitations Poor charge retention Self discharge rate is about 10% per month High cost