Batteries PDF

BATTERIES  2 Redox reactions Many chemical reactions can be understood in terms of the transfer of electrons between the species involved. These reactions are called oxidation–reduction, or redox reac...

BATTERIES  2 Redox reactions Many chemical reactions can be understood in terms of the transfer of electrons between the species involved. These reactions are called oxidation–reduction, or redox reactions. Because electrons also move in generating and delivering electricity, oxidation–reduction reactions provide a connection to electrical systems. If the oxidation process (or half-reaction) is physically separated from the reduction half-reaction, electrons can be made to traverse a circuit. A chemical reaction used to generate an electric current is called a galvanic cell, and a commercially important example of such a cell is the battery. A number of factors influence the nature and function of a galvanic cell. DEFINITION AND CLASSIFICATION Battery: A device that can store chemical energy and later release it as electrical energy at a constant voltage. Types of batteries 1. Primary Batteries 2. Secondary Batteries 3. Flow Batteries and Fuel Cells  4 Primary cell A primary cell such as the typical alkaline battery becomes useless once the underlying chemical reaction has run its course. The lifetime of the battery is determined by the amounts of reactants present, so a relatively large D-cell would last longer than a smaller AA-cell in the same application. The battery “dies” when the reactants have been converted into products, bringing the reaction to a halt. In practice, the voltage output of a battery usually begins to decrease near the end of its lifetime, and the cell will generally fail before the reactants are completely consumed.  5 Secondary cell A secondary cell is one that can be recharged, allowing for a much longer life cycle. To make a rechargeable battery, we must be able to reverse the redox reaction, converting the products back into reactants. Because we know that the cell reaction must be exothermic to supply energy, we can also see that the reverse reaction must be endothermic. So some external energy source will be needed to “push” the reaction back toward the reactants. This is the role of a battery charger, which typically uses electrical energy from some other source to drive the redox reaction in the energetically “uphill” direction. The question of whether or not a given reaction can be made reversible in this way determines whether a particular type of battery can be recharged easily. 1. Primary batteries: In this batteries, the electrochemical reactions are not reversible. So, upon conversion of the major portion of the reactant to products, no further current can be produced and the battery will become exhausted. 2. Secondary batteries: In this case, the cell reactions can be reversed by passing the current in the inverse direction. Thus such a device can be used in successive number of cycles through charging and discharging. 3. Flow batteries and fuel cells: In this device, the materials (reactant, product, electrolyte) flow through the battery. Unlike batteries, they do not store electrical energy but rather directly convert chemical energy into electrical energy. The reagents (fuel and oxidant) are stored outside and supplied when it is required.  Principal Components Anode: Electrodes where materials spontaneously undergo oxidation Cathode: Electrodes where materials spontaneously undergo reduction Electrolyte: Helps in the migration of ions, leading to production of electrical current Separator: Thin porous membranes that prevent the mixing of the products formed at the electrodes. (e.g., polypropylene, polystyrenes). Separators ensure that only the ions moves through the electrolyte and electrons only move through the circuit Characteristics of a battery  Characteristics of a battery  Characteristics of a battery Cycle Life: It is also applicable for secondary cells only. It is the number of times the charging and discharging process can be repeated. Shelf-Life: It is the amount of time the battery can store the charge Self-Discharge: Several times, the battery undergoes discharging reactions even though it is not in operation. It is due to the loss of active materials due to localized actions in the electrode. The smaller the self-discharge, the longer will be the shelf life. Difference between primary and secondary battery Primary Battery Secondary Battery 1. Chemical Reactions are 1. Chemical Reactions are irreversible reversible 2. They can be used only one 2. The can by recharged and and can not be recharged used in repeated number of times 3. This cell are dry and 3. This thing contains liquid contains no fluids. (solution or molten salts). 4. The design is simpler and 4 The design is more heavy lighter and complex. 5. More cost-effective 5. More costly 6. Can produce current 6. Needs to be charged first immediately before using. Discharging During discharging the chemical reactions occurs in the electrodes and generates flow of current through the circuit. The chemical on anode undergoes oxidation and releases the electrons on that electrode. These electrons passes through the external circuit to the cathode and causes reduction thereby completing the flow of electrons Charging Charging of a battery is exactly reverse of the discharging process. It involves reversal of chemical reaction by applying an external DC voltage. The negative end of the external source is connected with the anode. It injects electrons to the anode and causes reduction and the anode returns to its original state before discharging. The positive end of the external source is connected with the cathode. It removes electrons to the anode and causes oxidation and the cathode returns to its original state before discharging.  14 Lead-Acid Battery One of the first commercially available secondary batteries is the lead-acid battery, which is still in use today. This battery, invented by Gaston Plante in 1859, is the oldest type of wet cell battery. A single cell, shown in Fig., consists of a lead anode, a lead dioxide cathode, and a sulfuric acid electrolyte.  15 Lead-Acid Battery An insulating separator between the anode and cathode prevents short circuiting through physical contact of the electrodes, which would be most commonly caused by the accumulation of solids in the electrolyte. Although a single cell produces only about 2.1V, common lead- acid batteries used to start engines are constructed of six single cells in series (Fig.), producing a fully charged output voltage of 12.6V.  Lead-Acid Battery/Acid Storage Cell ✓Rechargeable battery first ever discovered. ✓Invented by Gaston Planté in 1859 ✓The anode consists of a lead grid filled with spongy lead ✓The cathode consists of a lead grid filled by PbO2. ✓Number of electrode pairs (generally six) are dipped in 20% sulphuric acid solution as electrolyte ✓The electrodes are separated by inert and porous partitions known as separators or cell dividers Lead-Acid Battery/Acid Storage Cell 18  Discharging Process At anode (oxidation): Pb(s) + HSO4−(aq) → PbSO4(s) + H+(aq) + 2e− H+(aq) and e− move to the cathode At cathode (reduction): PbO2(s) + HSO4− (aq) + 3H+(aq) + 2e− → PbSO4(s) + 2H2O(l) Overall Chemical Reaction: Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l) E0cell = 2.05 V Discharging Process ✓Each electrode pair gives rise to an EMF of 2.05 V ✓Whole cell (with 6 pairs) results in total EMF of ~12 V. ✓The concentration of H2SO4 falls with discharging. ✓When it falls below a certain limit, the battery can not produce enough current ✓Then it needs to be charged again.  Charging Process At anode: PbSO4(s) + H+(aq) + 2e−→ Pb(s) + HSO4−(aq) At cathode: PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4 − (aq) + 3H+(aq) + 2e− Overall Chemical Reaction: 2PbSO4(s) + 2H2O(l) →Pb(s) + PbO2(s) + 2H2SO4(aq)  22 Why lead-acid battery is rechargeable and at what point it cannot be recharged In an automobile, the battery produces a large initial current to start the car engine, discharging the battery. The battery is recharged during normal driving by an alternator that drives an electric current through the battery terminals. The recharging process is possible as long as the PbSO4(s) remains coated on the electrode surfaces. After time, the coatings will flake off the electrodes and fall to the bottom of the battery case. At this point the current forced through the battery by the alternator cannot reach the solid PbSO4 to return it to its original forms and the battery cannot be recharged.  23 Why lead-acid battery is rechargeable and at what point it cannot be recharged  24 Issues with lead-acid battery One issue with the lead-acid battery is that, as the battery is discharged, H2SO4 is consumed at both electrodes. This decreases the sulfuric acid concentration of the electrolyte and reduces the cell potential. At anode (oxidation): Pb(s) + HSO4 (aq) → PbSO4(s) + H (aq) + 2e − + − H+(aq) and e− move to the cathode At cathode (reduction): PbO2(s) + HSO4− (aq) + 3H+(aq) + 2e− → PbSO4(s) + 2H2O(l) Also, overcharging the battery will cause the water to undergo electrolysis producing hydrogen and oxygen gases, which leads to a decrease in water content and an increase in acid concentration causing an increase in electrode corrosion. Older lead acid batteries provided ports for the addition of water to compensate for any loss through electrolysis.  25 Effect of water evaporation Upon evaporation of water leads to the enhancement of H2SO4 concentration. The viscosity of the electrolyte increases resulting decrease in ionic mobility. The water needs to be added to the battery to maintain the H2SO4 concentration  26 Issues with lead-acid battery Newer sealed batteries may suffer from bulging of the case due to the gas buildup. Other drawbacks to this type of battery are that they are large and heavy and the toxic lead is an environmental hazard. Because of the high toxicity of the lead components, lead- acid batteries are required to be recycled at a certified recycling facility.  27 Effect of low temperature Upon lowering the temperature, the viscosity of the electrolyte increases resulting decrease in ionic mobility. So, under extremely cold conditions, the battery can not produce current, and it becomes dead So, the battery needs to be warmed up to become operational again. Charging process The concentration of H2SO4 increases during charging process. The potential developed by each electrode pair becomes 2.05 V Chagring process becomes complete. At anode: PbSO4(s) + H+(aq) + 2e−→ Pb(s) + HSO4−(aq) At cathode: PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4 − (aq) + 3H+(aq) + 2e− Overall Chemical Reaction: 2PbSO4(s) + 2H2O(l) →Pb(s) + PbO2(s) + 2H2SO4(aq) Applications 1. Extensively used in automobiles for starting and lighting 2. Widely used in UPS 3. Used in trains for lighting 4. In emergency tube lights 5. In Telephone exchages  30 Lithium-ion battery (LIBs) The lithium-ion battery was first marketed in 1991 and is now the most commonly used rechargeable battery for many applications from laptop computers to electric cars. The lithium-ion battery is constructed with an anode made of a mixture of graphitic carbon and lithium and a cathode of a transition metal oxide, usually cobalt oxide. Since lithium is very easily oxidized with an oxidation potential of 3.04, lithium reacts readily with water. Because of this, the lithium-ion battery cannot use an aqueous electrolyte. Instead, the electrolyte is a lithium salt in an organic solvent. A porous membrane separator prevents short circuiting while allowing the flow of ions. 31 32  33 Chemistry Nobel for Lithium-Ion Batteries That Power Smartphones to Spacecraft John B. Goodenough [Chemistry Nobel @97 years]  34 Lithium-ion battery The structure of both electrodes is in the form of layered plates and the mechanism for the oxidation-reduction reaction involves the intercalation of lithium between the plates of the electrodes as shown in Fig. The construction of a lithium-ion battery where the lithium ions (blue circles) reside between the plates of a carbon graphite electrode. Intercalation is the reversible insertion of an ion, atom, or molecule into a chemical species with a layered structure.  35 Lithium-ion battery During discharge, the lithium ions move from the open space between the plates of the graphitic carbon electrode through the electrolyte into the open spaces between the plates of the cobalt oxide electrodes. During the recharging cycle, the lithium ions move in the reverse direction back into the carbon anode. The construction of a lithium-ion battery where the lithium ions (blue circles) reside between the plates of a carbon graphite electrode.  36 Graphite LiCoO2 Graphite LiCoO2  37 Lithium-ion battery The half reactions of the lithium-ion battery are; where C6 indicates the structure of graphite. The lithium is oxidized at the anode to lithium ion, which then travels through the electrolyte to the cathode. The cobalt is reduced at the cathode and combines with the lithium ion to form the mixed metal oxide. During the recharging cycle, cobalt is oxidized releasing the lithium ions.  38 Lithium-ion battery Lithium-ion batteries provide lightweight, high energy density power sources for a variety of devices. To power larger devices, such as electric cars, many cells must be connected in a parallel circuit. While a mobile phone uses a single cell battery, an 85kW battery in an electric car uses 7104 lithium-ion cells. Battery lifetimes typically consist of 1000 charge-recharge cycles. Some batteries, constructed with a lithium titanium oxide anode instead of lithiumcarbon, have resulted in more than 4000 recharge cycles.  39 Lithium-ion battery Lithium-ion batteries are generally categorized as nontoxic waste since the components are less toxic than other types of batteries. These components, which include iron, copper, nickel, cobalt, and carbon, are considered safe for disposal in both incinerators and landfills and so the batteries can be discarded in household waste.  40 Effect of overcharging Overcharging causes the formation of Lithium oxide (Li2O) Li+ + LiCoO2 + e− → Li2O + CoO This reaction is irreversible Results loss in efficiency of the battery It is important to discharge the battery regularly to avoid overcharging.  41 Rechargeable nature of lithium-ion batteries The rechargeable nature of lithium-ion batteries means that, under some circumstances, they may be overcharged. When this happens, too much lithium moves to the anode side of the battery, and small whisker-like lithium structures called dendrites can form.  42 Rechargeable nature of lithium-ion batteries By bridging the gap between the anode and the cathode, these structures may also cause short circuits that can contribute to the types of failures that potentially lead to fires.  43 Why Li ion batteries Except H+ , Li+ ions are the smallest possible cation Low ionic radius and high charge density Under non-aqueous medium Li+ ions will have very high ionic mobility due to the lack of solvation It will generate quite high current and EMF.