Batteries Flashcards
Name some energy storage systems for transport use? (Inc +ve and -ve)
Petrol, diesel, biofuel - high energy density, established supply network, rapid start up, CO2 footprint, unsecure supply
Batteries and fuel cells - greener, wide range of supply sources, limited supply network, poor energy density, poor lifetime
Name some energy storage for the power grid (inc +ve and -ve)
Fission/fusion, oil, gas - high energy density, established network, rapid response, CO2 footprint, security supply, waste disposal, flexible use reduces lifetime
Hydro, geothermal, renewables - renewable, geographically limited, varying power, poor transport
Name a type of energy storage technique
Hydroelectric power - pump water into dam, release when needed
Name some requirements for batteries
Low density, high voltage, high charge density, fast responding, long lifetime, cheap, readily available materials (rarer = better performance)
Why are there limited energy storage techniques?
Means exiting a material to higher energy state and preventing it from decomposing
Too high = spontaneous decomposition, too low = never decompose
Describe SEP
Standard electrode potentials - only possible to measure Pd across electrodes so 0 is defined as hydrogen half cell reaction
Positive SEP means wont oxidise, negative will oxidise
More -ve or +ve will react stronger
Describe the structure of a voltaic pile and half cell reactions
Electrolyte (Eg brine) separates a Zn and Cu player
Half cell reactions:
Zn -> Zn(2+) + 2e-
2H+ + 2e- = H2
Reaction stops when all 2H+ are used up, Cu is inert as it has the most positive SEP
Describe the Daniell cell structure and half cell reactions
Zn anode in sulphuric acid, surrounded by porous container (allows e- through) with Cu sulphate in a Cu cathode around
Half cell: Zn = Zn(2+) + 2e- = -0.76V
Cu(2+) + 2e- = Cu = 0.34V
EMF = 1.1V
Describe a dry cell structure and half cell reactions
Metal cap, cathode in magnesium oxide, surrounded by NH4Cl paste with Zn casing
Half cell: Zn = Zn(2+) + 2e-
2MnO2 + 2e- + 2NH4Cl = Mn2O3 + 2NH3 + H2O + 2Cl -> EMF = 1.26V
Compare dry cell, voltaic pile and daniell cell
Dry cell is more portable, more lightweight and cheaper but Zn casing so full reaction not achieved
Describe an alkaline battery’s structure and half cell reactions
Metal connector in a Zn and KOH powder, with ion conductor separating a MnO2 layer, metal cap (cathode) surrounding cell, all encased in insulator
Half cell: Zn + 2OH- = ZnO + H2O +2e-, 2MnO2 + H2O + 2e- = Mn2O3 + 2OH-
What were the advantage of an alkaline battery?
Reactants separated = better shelf life
Recombining half reactions = linger life span
However, still non-rechargeable
What is the structure and half cell reactions of a Li battery?
Swiss roll structure - Li and FeS2 separated by electrolyte means more complete reactions possible
Half cell: 2Li + 1/2FeS2 = Li2S + Fe/2
Compare a lithium battery to an alkaline battery
Replace Zn with Li and MnO2 with FeS2 (longer lifetime)
Li is monovalent, Zn is divalent (Zn creates twice e-), although Zn and MnO2 are both smaller, half cell restrooms means smaller Li battery needed for same energy (= greater charge and longer lifetime)
How do rechargeable batteries store energy?
In a meta stable state, activation energy stops battery from discharging too quickly (too high and battery won’t discharge)
To recharge battery = Eact + energy between charge and discharge
Describe a lead-acid battery
Pb and PbO2 separated by dilute acid
Voltage applied in reverse direction (into anode) = recharges battery
Pb can branch over acid centre & short battery
Each cell produces 2V, very bulky and rechargeable limit of 350
Describe a NiCd battery
Still roll structure with layers of NiCd, safety spring on top to release H that’s released via charging
Produces 1.2V and a recharge limit of 2000, if not fully charged or discharged can create charge memory
Define charge memory
When a battery isn’t discharge or recharged fully so over time charge capacity is reduced
Describe a nickel metal hydride battery structure and the limitations and advantages
Ni and metal hydride in Swiss roll structure
Metal selection is limited by hydride need (can’t be too stable = difficult recharge, not stable = self-discharge)
Use RE alloy with polymer based spectators, as high capacity and self-discharge limited. Very high charge density
Describe lithium ion batteries
Much higher capacity and voltage than lithium battery
A Li salt separates a Li ion intercalated Graphite layer and a layered lithium compound (different compounds used = different half cells) - then Swiss roll
e- can’t travel through Li salt so travels round external circuit, Li+ goes through salt.
What determines if a battery can be rechargeable?
If it is a reversible reaction or not
How can lithium ions become unstable (combust)? And what is the most common Li ion battery (inc half cell)?
Salt is a source of fire risk if gets too hot - thermal run away
Most common is LiCO battery, half cell:
LiC6 = C6 + Li+ + e-
What are the benefits of a Li ion battery?
Thin film structure, very large charge density = 1200 recharges with no charge memory
