Battery

B. Conversion and storage of electrochemical energy Battery Technology Introduction: A battery is an arrangement consists of two or more galvanic cells connected in series. The battery is used as a source of direct electric current at constant voltage. Importance of Batteries: Various types of batteries are used in computers, pace makers, automobiles, calculators, watches, uninterrupted power supplies, industries, hospitals and space applications. In automobiles lead storage battery is used. The advantage of storage battery is that they can be recharged by reverse electrolysis. Even over charging does not cause any damage to lead storage battery. Basic concepts of batteries: The basic electrochemical unit in a battery is a galvanic cell, the components of such a cell are shown schematically as shown below

The basic components of a battery are 1. Anode; It is a negative electrode where oxidation takes place. It gives out electrons to the external circuit by a electro chemical reaction. 2. Cathode; It is a positive electrode where reduction takes place. It accepts the electrons from the external circuit.

3. The electrolyte (Active mass in the anode and cathode): Is commonly a solution or slurry of acids, alkalis or salts having high ionic conductivity. Solid electrolyte with appreciable ionic conductivity is also used at the operating temperature of the cell. 4. The separator: The material which electronically isolate anodes and cathodes in a battery to prevent internal short circuiting are referred to as separator. They are permeable to the electrolyte so as to maintain ionic conductivity. Their function is to transport ions from one comportment to other. Characteristics of Batteries: 1. It should be economically priced. 2. It should be light in weight and compact in size. 3.The voltage of the battery should not change much during its use. 4. It should provide power for longer time. 5. It should be rechargeable. Characteristic terminologies of a battery; I. current; Current is the rate at which the battery is discharging. For an efficient working of the cell there must be a large quantity of electro active species which help rapid electron transfer reaction. II. Voltage: The voltage available from a battery depends on the e.m.f of the cell which constitute the battery system. It is given by the Nerns’t equation E cell = E° cell – 2.303 RT log Q nF Q is the ratio of the products of the molar concentrations of product molecules to that of reactant molecules. III. Capacity:

The capacity is the amount of electricity that may be obtained from the battery and is given in ampere hours. (Ah). This depends on the size of the battery which is determined by the Faraday relation C = wnF M Where W is the mass, M is the molar mass of the active material and C is the capacity in ampere hours. IV. Energy Density: The ratio of the energy available from a cell or battery to its weight (volume) is referred to as energy density. It is expressed as avg

Energy density = i t E cell Where t is the time taken at the fixed current i to reach an average voltage E. It is measured by determining the capacity and recording the average voltage and the total weight of the battery.

V. Electric Storage Density: In the measure of charge per unit weight stored in the battery i.e., it is the capacity per unit weight. A high storage density depends on a good battery design and also the appropriate selection of electrode reaction. VI. Power Density: The ratio of power available from a battery to its weight (w /kg) or volume (w/ l) is called the power density. VII. Energy efficiency: The energy efficiency for storage batteries is given by % energy efficiency = energy released on discharge x 100 Energy required for charge VIII. Cycle life: Primary batteries are not rechargeable but secondary batteries are rechargeable and their cycle life must be high

Cycle life = Number of charge Number of discharge IX. Shelf life: The duration of storage under specified condition at the end of which a cell or battery retains the ability to give specified performance is called the shelf life. Classification of Batteries : Batteries are classified into three types a) Primary battery: These are the batteries in which the cell reaction is irreversible. After some times cell reaction stops and cell becomes dead. For this reason primary cells can not be recharged and reused. Ex; Zn-Hg cell, Zn- MnO2 cell ,etc b) Secondary batteries: These are the batteries in which the cell reaction is completely reversible. The cell can be recharged by passing electric current through them after every use. As a result a secondary cell can be used again and again for a long time . Ex: a) Lead – Acid battery b) Ni – Cd battery c) Reserve batteries: The batteries which may be stored in an inactive state and made ready for use by activating them prior to the application are referred to as the reserve batteries. Ex: Mg-water activated batteries Zn-Ag2O batteries, etc. Classical Batteries:

i) Zn-MnO2 Cell (Dry cell): Construction:

The dry cell is a primary cell, since various reactions involved can not be reversed by passing electricity. Which consists of Zn-container acts as anode. A graphite rod acts is placed in the center but does touch the base acts as cathode. The space between the anode and cathode is so packed that Zncontainer is filled with the paste of NH4Cl and ZnCl2 and graphite rod is surrounded by MnO2 powder and carbon. The graphite rod is fitted with a metal cap and the cylinder is sealed at the top with pitch. Zn container is covered with cardboard so that the container may be protected from the atmosphere. The dry cell is represented as Zn/Zn2+,NH4+/ MnO 2,C Cell reactions: The following reactions are takes place in the cell At anode; Zn Zn2+ + 2e – At cathode; 2MnO2(S)+H2O(l)+2e – 2NH4+(aq)+2OH - (aq) ZnCl2(S)+2NH3(g) The net reduction reaction at cathode is

Mn2O3(s)+2OH -(aq) 2NH3(g)+2H2O(l) [Zn(NH3)2Cl2]

2MnO2(S)+2NH4+(aq)+ZnCl2(s)+2eMn2O3 (s)+H2O(l)+[Zn(NH3)2Cl2]

Net cell reaction; Zn (s)+2MnO2(s)+2NH4+(aq)+ZnCl2(s) Mn 2O3 2+

(s)+H2O(l)+Zn (aq)+[Zn(NH3)2Cl2] The cell potential is in the range of 1.25to1.5V Application : dry cell finds application in flashlights, transistor radios, calculators, electronic watches, etc. ii) Lead – Acid battery (Lead storage battery) Construction:

It is the most frequently used battery in automobiles. It consists of six voltaic cells connected in series. In each cell anode is made of spongy lead and cathode is a grid of lead packed with lead dioxide. The electrolyte is the aqueous solution of H2SO4 which is 38% by mass. Anode and cathode are arranged alternatively. The cell representation is

Pb(s),PbSO4(s)/H2SO4(38%)/PbO2(s),Pb(s) Cell reactions: a. The following reactions take place in lead storage battery during discharging. At anode; Pb (s) Pb2 + (aq)+2e Pb2+ (aq) +SO42- (aq) PbSO4(s) Pb(s) +SO42- (aq) PbSO4(s) +2e At cathode; Pbo2(s) +2e-+4H+ (aq) Pb2+ (aq)+2H2O(aq) Pb2+ (aq) +SO42-(aq) PbSO4(s) Pbo2(s) + 4H+(aq) +SO42-(aq) +2e PbSO4(s)+2H2O(aq) Net cell reaction; Pb (s) + PbO2(s) + 4H+(aq) + 2SO42-(aq) 2PbSO4(s) + 2H2O(l) During the working of the cell, the concentration of H2SO4 decreases as SO42- ions precipitated as PbSO4. With the decrease in the concentration of H2SO4, the density of the solution also decreases. The condition of the battery can easily checked by measuring the density of the solution when the density falls below 1.20 gm/cm3, the battery require recharging. b. The following reactions take place in lead storage battery during Recharging: The battery can be recharged by connecting it to an external source of direct current. It would reverse the flow of electrons resulting in the deposition of Pb on the anode and PbO2 on the cathode. The charging reaction is as follows. PbSO4(s) + 2 H2O (l) Pb(s) + PbO 2 (s) + 4H+(aq)+ 2SO42-(aq)

Applications: Lead acid storage batteries are used for many purposes such as automobiles, gas engine ignition, in telephone exchanges, railway trains, mines, laboratories, hospitals broadcasting stations etc., iii) Nickel – Cadmium Battery: This is also a secondary battery. It consists of Cd anode and metal grid containing NiO2 as cathode. These electrodes immersed in KOH solution which acts as electrolyte. Cell reactions: At anode: Cd(s) → Cd+2(aq) + 2eCd+2 (aq)+ 2OH-(aq) → Cd(OH)2 (S) Cd (S)

+ 2OH-(aq) → Cd (OH)2(s)

+

2e -

At cathode: NiO2 (s) + 2e- + 2 H2O (l) Ni (OH)2 (s) +2OH-(aq) The net cell reaction. Cd(s) +NiO2(s) +2 H2O (l) Cd (OH)2(s)+Ni(OH)2(s) The reaction can be readily reversed because the reaction products Ni (OH) 2 and Cd(OH)2 deposit to the electrode surfaces. Ni-Cd battery is a portable re-chargeable cell and its cell voltage is fairly constant of about 1.4 V Like dry cell it can be packed in a sealed container. No gases are produced during charging or discharging. Applications: It is used in electronic calculators, electronic flash unit, cordless electronic shavers, transistors and other battery powered small tools.

Modern batteries:

i)

Zn-air battery Zn battery belongs to the class of air- metal battery, which uses oxygen from the atmosphere to produce electrochemical energy. It offers very high energy density because the oxygen does not contribute to the mass of the battery. This consists of porous carbon plate as cathode rectangular flat plates of zinc as anode are placed on either side of the cathode. The electrodes are immersed in 20% NaOH which acts as electrolyte. The outer container is made up of glass or ebonite. The cell is represented as Zn / NaOH (5M) / air, C Cell reactions: At anode; Zn →Zn2+ +2eZn2+ + 2OH →ZnO+H2O Zn + 2OH→ZnO+H2O+2eAt cathode; ½ O2 +H2O+2e-→2OH The net reaction; Zn + ½ O2→ ZnO The cell produces an open circuit potential of 1.4V. During charging the above reaction is reversed. Application ; 1. It is used as a power source for hearing aids. 2. used in electronic pagers 3.used in military radio receivers and Railway Station.

ii) Nickel-Metal hydride battery The active material for anode is a metal hydride such as VH2,ZrH2 and TiH2 with a hydrogen storage metal alloy such as LaNi5 or TiNi. The active

material for the cathode is NiO(OH). An aqueous solution of KOH acts as the electrolyte and polypropylene is used as the separator. Anode and cathode are stacked alternatively in a stainless steel container and connected in series to get the desired voltage. The battery is represented as MH / KOH (5.35M) / Ni(OH)2. Nio(OH) The reaction during the discharge are given by At anode: MH + OH → M +H2O + eAt cathode: NiO (OH) + H2O + e- → Ni (OH)2 + OH Net reaction MH + NiO (OH) → Ni (OH)2 + M During the charging the above reaction is reversed. The open circuit potential of the cell ranges from 1.25 – 1.35 V. Application: 1) It is used is consumer electronic devices such as cellular phones, computers other portable devices. iii) Lithium – MnO2 Battery: It consists of Lithium anode and manganese dioxide cathode. A solution of lithium halide in organic solvent served as the electrolyte. The manganese dioxide palate and lithium anode disc are separated by a no woven poly propylene separator. The electrode reaction At anode: LI → Li+ + eAt cathode MnO2 + Li + + e - → LiMnO2 Net cell reaction: Li + MnO2 → LiMnO2 Application: It is used in variety of application such as memory backup, safety and senility devices, watches, calculators, automotive cameras and many consumer electronic devices.