For reference

Index of terms

Each term the course introduces, with the sentence that introduces it and a link to that place. The sentences are quoted from the chapters.

A

activation

The polarization itself has three parts: activation, from the charge-transfer reactions at the interfaces; ohmic, from the resistance of the cell’s components and contacts; and concentration, from the limits of mass transport. Chapter 5 · Polarization

additives

Real electrolytes also carry minute amounts of additives, each with a job: some decompose on purpose to build a thin, controlled SEI on the graphite (chapter 10), others protect the positive electrode or stop overcharge (chapter 17); no single additive can do everything, and finding the right combin… Chapter 8 · Liquid electrolytes

adhesion

A strip of the coating is pulled off its current collector at a right angle, slowly, and the force per width is the adhesion; it costs nothing and it decides which samples can be handled. Lab 6 · Post-mortem

aggregates

When an anion wins a place, the two ions form a contact ion pair; when solvent stays between them, they are a solvent-separated ion pair; larger clusters are aggregates. Chapter 8 · Liquid electrolytes

alloying

In alloying, lithium forms an alloy with an element such as silicon, germanium or tin, xLi⁺ + xe⁻ + M ⇌ Li_xM. Chapter 7 · Insertion hosts

alloys

Graphite’s 372 mAh/g now limits how much energy a cell can hold, and the search for something better has produced four families of material, named for how they store lithium: intercalation hosts, alloys, conversion materials and lithium metal. Chapter 12 · Negative electrodes

anode-free

An anode-free cell carries none: it is assembled with no lithium on the negative side at all, only a copper current collector. Chapter 12 · Negative electrodes

arcs

In the middle range come arcs from the interfaces, each a resistance in parallel with a capacitance; at low frequency a tail from lithium diffusing inside the particles. Lab 4 · EIS: measurement

areal loading

The areal loading, in mg/cm² or mAh/cm², is that mass over the area of the disc, and it should be reported with every result: a thick electrode and a thin one of the same material behave differently at the same C-rate (chapter 11). Lab 1 · Cell testing

artificial SEI

An artificial SEI, made beforehand on the copper or formed in the cell, guides the lithium to deposit evenly. Chapter 12 · Negative electrodes

B

battery

The International Electrotechnical Commission defines a battery as one or more electrochemical cells fitted with the devices needed for use, such as a case, terminals, marking and protective devices. Chapter 1 · Galvanic cells

battery management system

The battery management system measures voltage, current and temperature with its sensors, keeps the cells within their safe limits, regulates the charge, balances the cells, manages the heat and reports the state of the battery. Chapter 17 · Safety

bipolar

An n-type molecule is reduced, and a cation such as Li⁺ balances it; a p-type molecule is oxidized, and an anion from the electrolyte balances it; a bipolar one does both, in different windows of potential. Chapter 15 · Sodium and organic

Bode plot

The Bode plot shows |Z| and the phase against the logarithm of frequency, so every point’s frequency can be read and each decade gets equal room. Lab 4 · EIS: measurement

bulk

Far from either electrode, in the bulk, every small volume holds as much positive as negative charge: a separation of charge there would set up a field large enough to erase it at once. Chapter 9 · Ion transport

C

Capacity

Capacity is the charge a battery stores, in ampere-hours: a 10 Ah cell can supply 10 A for one hour, and since an ampere is a coulomb per second, 1 Ah is 3600 C. Chapter 4 · Capacity and energy

carbonates

The two solvents of a lithium-ion cell are both carbonates, built round a carbon atom bonded to three oxygen atoms. Chapter 8 · Liquid electrolytes

carbonyl oxygen

In both, one oxygen is double-bonded to the carbon and sticks out of the molecule: the carbonyl oxygen. Chapter 8 · Liquid electrolytes

causal

A spectrum is worth modelling only if it could have come from a system that was linear, causal (the response comes only from the excitation), stable (unchanged during the sweep) and finite. Lab 4 · EIS: measurement

charge

On charge the charger drives the current the other way: electrons are pushed into the negative electrode, which is now reduced, and pulled out of the positive electrode, which is now oxidized, so by the reaction definition the names swap, and the graphite that was the anode while you used the phone … Chapter 2 · Cell anatomy, Chapter 5, Lab 2

charge factor

The coulombic efficiency is the ratio of the charge taken out on discharge to the charge put in on charge, over one cycle; for a good cell it is close to 100 %, and its reciprocal is called the charge factor. Chapter 4 · Capacity and energy

charge under a peak

The charge under a peak, the area between the curve and its baseline divided by the scan rate, is the capacity of that reaction, in coulombs or, per gram of active material, in mAh/g at that rate. Lab 2 · Cyclic voltammetry

check-ups

Ageing studies run check-ups: the same measurement at fixed conditions every so many cycles, often enough to follow the ageing, rarely enough not to add to it. Lab 5 · EIS: analysis

closed-circuit voltage

The voltage at the end of each pulse is the closed-circuit voltage (CCV). Lab 3 · GITT

coin cell

A coin cell is a flat stainless-steel case, wider than it is high, with a gasket, a cap, a spacer plate and a spring holding the layers together; in the laboratory its negative electrode is often lithium foil. Chapter 2 · Cell anatomy

columnar silicon

Two other designs avoid the problem: columnar silicon, whose columns change volume anisotropically and so build up less stress, kept 92 % of its capacity after 50 cycles with a sulfide electrolyte against 50.6 % in a liquid one, at a Coulombic efficiency of about 99.95 %; and Samsung’s silver–carbon… Chapter 14 · Solid-state cells

compact layer

The excess of ions thins out over the diffuse layer, less than about 10 nm thick above 0.01 M; closest to the surface is the compact layer, one solvent shell thick. Chapter 9 · Ion transport

composite

It is a composite: particles of active material, often wired together by a conductive additive such as carbon, held by a binder, with about 30 % of its volume left as pores that the electrolyte fills. Chapter 11 · Composite electrodes

concentration

The polarization itself has three parts: activation, from the charge-transfer reactions at the interfaces; ohmic, from the resistance of the cell’s components and contacts; and concentration, from the limits of mass transport. Chapter 5 · Polarization

Constant current

Constant current (CC) until an upper cut-off voltage; a constant-voltage (CV) hold at that cut-off until the current has decayed to a set fraction of the C-rate; a rest; a constant-current discharge to a lower cut-off; a rest. Lab 1 · Cell testing

constant phase element

The usual fix replaces the capacitor with a constant phase element, Z=1/(Y0(jω)n) : n=1 is a capacitor, n=0 a resistor and n=0.5 a Warburg diffusion line. Lab 4 · EIS: measurement

constant-voltage

Charging often ends with a constant-voltage step: the cycler holds the upper cut-off, the current falls away by itself, and the step ends when it reaches a small limit. Chapter 5 · Polarization, Lab 1

contact ion pair

When an anion wins a place, the two ions form a contact ion pair; when solvent stays between them, they are a solvent-separated ion pair; larger clusters are aggregates. Chapter 8 · Liquid electrolytes

conversion

In conversion, a compound is taken apart into metal and a lithium compound and rebuilt on the way back. Chapter 7 · Insertion hosts, Chapter 12

coulombic efficiency

The coulombic efficiency is the ratio of the charge taken out on discharge to the charge put in on charge, over one cycle; for a good cell it is close to 100 %, and its reciprocal is called the charge factor. Chapter 4 · Capacity and energy, Chapter 16, Lab 1

counter electrode

The reference electrode fixes the potential scale; the counter electrode, an inert platinum wire larger than the working electrode, carries the current so that the reference does not have to. Lab 2 · Cyclic voltammetry

CR2032

We follow that guide here for the commonest format, the CR2032 half cell: 20 mm across and 3.2 mm tall, with the electrode under study facing a disc of lithium metal. Lab 1 · Cell testing

C-rate

The C-rate scales the current to the capacity, relative to a one-hour discharge: at 1C a cell empties in an hour, at 2C in 30 minutes, at C/5 in five hours and at C/20 in twenty. Chapter 4 · Capacity and energy, Lab 1

C-rates

Currents are quoted as C-rates: 1C is the current that passes the cell’s full nominal charge in one hour, so C/20 takes twenty hours. Chapter 5 · Polarization

cut-off voltage

Each step stops at a cut-off voltage. Chapter 5 · Polarization

cut-off voltages

The cut-off voltages bound the useful range: charging past the upper one is overcharge, which decomposes the electrolyte and releases gas, and discharging below the lower one is over-discharge. Chapter 4 · Capacity and energy

cycle life

The cycle life is the number of cycles a battery lasts at a given C-rate; by industry convention it is nearly over when the capacity has fallen to 80 % of its initial value (16.2). Chapter 4 · Capacity and energy, Chapter 16

cycler

Cells are studied, and their capacities measured, on a cycler: an instrument that sets the current and records the voltage at regular intervals (once a second in one set of tests). Chapter 5 · Polarization

cylindrical cell

In a cylindrical cell the electrodes and separator are wound into a spiral inside a cylindrical case, the roll of figure 2.2: good cycling stability, but round cells pack less densely. Chapter 2 · Cell anatomy

D

dead lithium

On stripping, pieces break off the dendrites and lose contact: dead lithium, which in one study thickened from about 2 µm to about 100 µm within a few dozen cycles, lowering the efficiency and raising the impedance. Chapter 12 · Negative electrodes

de-background

The third is de-background: the double-layer current, read from the flat region before the reaction and checked between rates by i₁/v₁ = i₂/v₂, is subtracted, because otherwise it is counted as pseudocapacitance and inflates it. Lab 2 · Cyclic voltammetry

dendrites

It was studied long before graphite, and kept out of commercial cells by dendrites. Chapter 12 · Negative electrodes

depressed

Arcs are depressed: their centres sit below the axis. Lab 4 · EIS: measurement

depth of discharge

The state of charge (SOC) says how much of the capacity is left; the depth of discharge (DOD) says how much has been used, so the two add up to 100 %. Chapter 4 · Capacity and energy

de-residual

The second is de-residual. Lab 2 · Cyclic voltammetry

diameter is the resistance

A resistor in parallel with a capacitor draws a semicircle whose diameter is the resistance: the curve leaves the real axis at R₀ and returns to it at R₀ + R₁. Lab 5 · EIS: analysis

differential capacity

Its charge per volt, the differential capacity dq/dU , is a single spike, and the energy it stores is simply proportional to the charge. Chapter 6 · Voltage profiles, Lab 1

differential voltage

Two plots are used: the differential voltage dV/dQ against charge, and the differential capacity dQ/dV against voltage. Lab 1 · Cell testing

diffuse layer

The excess of ions thins out over the diffuse layer, less than about 10 nm thick above 0.01 M; closest to the surface is the compact layer, one solvent shell thick. Chapter 9 · Ion transport

diffusion

This is diffusion, met inside the electrode particles in chapter 7. Chapter 9 · Ion transport, Lab 2, Lab 4

Direct recycling

Direct recycling, which keeps the electrode materials whole, is still immature. Chapter 18 · Recycling

discharge

On discharge the negative electrode is oxidized and the positive electrode is reduced, so the negative electrode is the anode and the positive electrode the cathode; that is the battery convention. Chapter 2 · Cell anatomy, Chapter 5

distribution of relaxation times

That is what the distribution of relaxation times (DRT) does. Lab 5 · EIS: analysis

double-layer capacitance

And the double-layer capacitance, read where nothing reacts: the flat current at the foot of the forward sweep divided by the scan rate and the area, or half the gap between the forward and reverse currents in a narrow window with no reaction in it (a separate scan in practice, because inside a full… Lab 2 · Cyclic voltammetry

E

electrochemical cell

IUPAC gives no formal definition of a battery: it defines an electrochemical cell as a system of at least two electron conductors, the electrodes, in contact with ionic conductors, the electrolytes, and mentions batteries only as examples of such cells. Chapter 1 · Galvanic cells

electrochemically reversible

Such a couple is electrochemically reversible: the peak potentials do not depend on the scan rate, the peak separation stays at about 57 to 59 mV for one electron, and the two peak currents are equal. Lab 2 · Cyclic voltammetry

Electrolyte engineering

Electrolyte engineering: additives that build a better SEI, two-salt electrolytes, localized high-concentration electrolytes that mix a dissolving solvent with a diluent that does not dissolve the salt, and fluorinated solvents that are stable against both lithium and nickel-rich positive electrodes. Chapter 12 · Negative electrodes

electron microscope

The electron microscope showed a mossy layer on the aged graphite, absent from the fresh and the car-aged samples, with oxygen and fluorine hot spots where the metal had met the air during preparation; it cannot map lithium, and it sees a square centimetre at most. Lab 6 · Post-mortem

energy efficiency

The energy efficiency is the ratio of the energy delivered on discharge to the energy taken on charge, and it depends strongly on the charge and discharge rates. Chapter 4 · Capacity and energy

equivalent circuit

The usual analysis fits an equivalent circuit to the spectrum by complex nonlinear least squares and judges the fit by its χ². Lab 5 · EIS: analysis

exchange current

A reaction that runs fast in both directions at equilibrium, one with a large exchange current, needs only a small overpotential to carry a given current, and battery reactions with exchange current densities around 10⁻² A/cm² are favoured. Chapter 5 · Polarization

F

faradaic current

C2: energy is stored and released through reversible oxidation and reduction, that is, through faradaic reactions; the current they carry is what IUPAC calls a faradaic current, the current that results from the electrooxidation or electroreduction of an electroactive substance. Chapter 1 · Galvanic cells

first solvation shell

This group is the ion’s first solvation shell. Chapter 8 · Liquid electrolytes

G

galvanostatic

This is galvanostatic cycling, and its record, the voltage against the charge passed, is the raw material of everything else in this module. Chapter 5 · Polarization

graphene

Graphite is a stack of graphene sheets: carbon atoms joined in hexagonal rings, the sheets held to one another by weak van der Waals forces and π–π interactions, 0.335 nm apart; in natural graphite they are mostly stacked in the order A, B, A, B. Chapter 7 · Insertion hosts

H

high dielectric constant

A good solvent needs a high dielectric constant (relative permittivity), to pull the salt apart into separate ions, and a high fluidity, so that the ions can move. Chapter 8 · Liquid electrolytes

high fluidity

A good solvent needs a high dielectric constant (relative permittivity), to pull the salt apart into separate ions, and a high fluidity, so that the ions can move. Chapter 8 · Liquid electrolytes

hydrogen

…n the positive electrode, raising the cell’s resistance; and compounds such as cyclohexylbenzene decompose and give off hydrogen, so that a pressure-operated current interrupter cuts the charging current. Chapter 17 · Safety

hydrometallurgy

…ich smelts the cells at about 1000 °C into a copper-nickel-cobalt alloy while lithium and manganese go to the slag, and hydrometallurgy, which dissolves the electrode materials in acid and separates the metals, with higher recovery and lower energy use but more complex processing. Chapter 18 · Recycling

I

ICP-OES

ICP-OES of the lithium that dissolves when a weighed piece of electrode is stirred in water is quantitative, with errors of 1 to 3 %, because plated metal reacts with the water and is counted; but it is indirect, and lithium that has already become insoluble LiF is missed. Lab 6 · Post-mortem

ideal battery material

An ideal battery material is one that supplies all its capacity through one uniform chemical process at one fixed potential. Chapter 6 · Voltage profiles

ideal capacitor material

The opposite extreme is an ideal capacitor material: its differential capacity is constant, its specific capacitance Csp , so the potential rises in a straight line as charge goes in. Chapter 6 · Voltage profiles

inductance

At the top of the range the cables and the wound electrodes act as an inductance, so the spectrum starts below the axis. Lab 4 · EIS: measurement

inductive effect

In a polyanion oxide, the strongly covalent P–O or S–O bonds weaken the covalency of the metal–oxygen bonds, lower the energy of the metal’s redox couple and so raise the voltage, the inductive effect: Fe²⁺/Fe³⁺ works below 2.5 V in Fe₂O₃ but at 3.6 V in Fe₂(SO₄)₃. Chapter 13 · Positive electrodes

insertion

Both are insertion hosts: solids that conduct ions and electrons and let guest ions in and out by a solid-state redox reaction. Chapter 2 · Cell anatomy

intercalation

Because their structure and shape survive, insertion electrodes are highly reversible; for layered hosts the process is called intercalation. Chapter 2 · Cell anatomy, Chapter 7, Chapter 12

ionic potential

Mg²⁺ is small and doubly charged: it has a high ionic potential, its charge divided by its radius. Chapter 15 · Sodium and organic

irreversible

An irreversible part, often loosely called Joule heat, comes from the whole polarization: it is released whichever way the current flows. Chapter 5 · Polarization, Lab 2

L

layered

Almost every commercial positive electrode belongs to one of three families of oxide, identified in Goodenough’s group in the 1980s: layered oxides such as LiCoO₂, spinels such as LiMn₂O₄, and polyanion oxides such as LiFePO₄. Chapter 13 · Positive electrodes

linear

A spectrum is worth modelling only if it could have come from a system that was linear, causal (the response comes only from the excitation), stable (unchanged during the sweep) and finite. Lab 4 · EIS: measurement

lithium metal

Graphite’s 372 mAh/g now limits how much energy a cell can hold, and the search for something better has produced four families of material, named for how they store lithium: intercalation hosts, alloys, conversion materials and lithium metal. Chapter 12 · Negative electrodes

Lithium titanate

Lithium titanate, Li4Ti5O12, works at about 1.55 V, where little SEI forms, and changes volume by only about 0.2 %, a “zero-strain” material: it is safe, long-lived and fast, but stores only 175 mAh/g, conducts poorly and can give off gas (H2, CO2, CO) by decomposing the electrolyte on its surface, … Chapter 12 · Negative electrodes

Local impedance

Local impedance, measured on one spot of an electrode, is a further step, since a whole-cell spectrum averages over a structure that is not uniform. Lab 5 · EIS: analysis

Loss of active material

Loss of active material (LAM) is material that has changed irreversibly, dissolved or come loose from the current collector. Lab 1 · Cell testing

Loss of lithium inventory

Loss of lithium inventory (LLI) is cyclable lithium used up by side reactions. Lab 1 · Cell testing

M

Machine learning

Machine learning is the newest route: train a model on features from many cells, then predict the state of health or charge of a new one. Lab 5 · EIS: analysis

MacMullin number

The ratio NM=κ/κeff=τ/ε , the MacMullin number, says how many times worse the filled pores conduct than the free liquid. Chapter 11 · Composite electrodes

migration

That steady bias is migration, and the Nernst-Planck equation adds the two together (chapter 0). Chapter 9 · Ion transport

modified current collectors

And modified current collectors, such as three-dimensional porous copper that spreads the current and gives the lithium room. Chapter 12 · Negative electrodes

molecule

A molecule is a group of atoms held together by chemical bonds; the figure draws each atom as a ball and each bond as a stick. Chapter 8 · Liquid electrolytes

N

negative electrode

To avoid the confusion this page says negative electrode and positive electrode from here on. Chapter 2 · Cell anatomy

negolyte

Redox-flow cells pass even though their two liquids sit in tanks outside the cell, because the reactants stay inside the system; the liquids are called the negolyte and the posolyte, not anolyte and catholyte, because anode and cathode swap between charge and discharge. Chapter 1 · Galvanic cells

Nernst equation

The first is the Nernst equation: at every moment the potential sets the ratio of the two forms at the electrode surface, Lab 2 · Cyclic voltammetry

Niobium oxides

Niobium oxides such as Nb₂O₅, TiNb₂O₇ and Nb₁₄W₃O₄₄ also work at 1 to 2 V, store more (about 200 to 350 mAh/g), change little in volume and charge very fast; they are already being made at industrial scale for high-power cells. Chapter 12 · Negative electrodes

nominal capacity

Data sheets add two reference values: the nominal voltage, the average voltage over a whole discharge at 0.2 C, and the nominal capacity, the capacity of that discharge. Chapter 4 · Capacity and energy

nominal voltage

Data sheets add two reference values: the nominal voltage, the average voltage over a whole discharge at 0.2 C, and the nominal capacity, the capacity of that discharge. Chapter 4 · Capacity and energy

not

A faradaic reaction that is not limited by diffusion, because the migrating electrolyte keeps the surface supplied, scales with neither v^1/2 nor v; written against the time of the sweep, its current goes as v^3/2, and the full regression becomes |i| = a v^1/2 + b v + c v^3/2 at every potential. Lab 2 · Cyclic voltammetry

n-type

An n-type molecule is reduced, and a cation such as Li⁺ balances it; a p-type molecule is oxidized, and an anion from the electrolyte balances it; a bipolar one does both, in different windows of potential. Chapter 15 · Sodium and organic

Nuclear magnetic resonance

Nuclear magnetic resonance of 7Li, on active material scraped from the copper and sealed in a rotor inside the glovebox, is direct: metallic lithium has a Knight shift of 245 to 270 ppm, far from the ions near 0, and the peak appeared in both artificially aged cells and in neither the fresh cell nor… Lab 6 · Post-mortem

Nyquist plot

The Nyquist plot puts −Z″ upward against Z′ , one point per frequency, high frequencies on the left; a resistor in parallel with a capacitor draws a semicircle of diameter R . Lab 4 · EIS: measurement

O

ohmic

The polarization itself has three parts: activation, from the charge-transfer reactions at the interfaces; ohmic, from the resistance of the cell’s components and contacts; and concentration, from the limits of mass transport. Chapter 5 · Polarization, Lab 2

ohmic resistance

It jumps at once; the jump divided by the current, ΔV/ΔI , is the ohmic resistance of electrolyte, active material, current collectors and contacts. Lab 3 · GITT, Lab 4

open-circuit voltage as assembled

Then the first measurement, with a hand-held meter: the open-circuit voltage as assembled. Lab 1 · Cell testing

overpotential

And each boundary has to be pushed off its equilibrium before its reaction keeps pace with the current; that extra push is the electrode’s overpotential. Chapter 5 · Polarization

P

path dependence

This is path dependence, and it fades at low rates. Chapter 6 · Voltage profiles

peak currents

The peak currents i_pa and i_pc, each measured from a baseline, not from zero: the flat foot of the sweep before the reaction, fitted by a line and extrapolated under the peak, because the current there is the capacitive current C_dlA v and whatever reaction is still tailing off from the previous sweep. Lab 2 · Cyclic voltammetry

peak potentials

The peak potentials E_pa and E_pc, their separation ΔE_p, and their midpoint E½, the estimate of the formal potential. Lab 2 · Cyclic voltammetry

plate

Lithium can arrive faster than it can enter the graphite and plate as metal, a risk made worse by graphite’s low potential; the particles are stressed and crack; and the heat destabilizes the SEI. Chapter 12 · Negative electrodes

polyanion

Almost every commercial positive electrode belongs to one of three families of oxide, identified in Goodenough’s group in the 1980s: layered oxides such as LiCoO₂, spinels such as LiMn₂O₄, and polyanion oxides such as LiFePO₄. Chapter 13 · Positive electrodes

polymerize

…s current through a redox process and keep the voltage from rising; aromatic compounds such as biphenyl are believed to polymerize above the maximum operating voltage into an insulating layer on the positive electrode, raising the cell’s resistance; and compounds such as cyclohexylbenzene decompose … Chapter 17 · Safety

porosity

The porosity ε is the fraction of the electrode’s volume that is pore. Chapter 11 · Composite electrodes

porous electrode

A porous electrode is a ladder: the electrolyte resistance runs down each pore, and every grain wall has its own interface. Lab 4 · EIS: measurement

positive electrode

To avoid the confusion this page says negative electrode and positive electrode from here on. Chapter 2 · Cell anatomy

posolyte

Redox-flow cells pass even though their two liquids sit in tanks outside the cell, because the reactants stay inside the system; the liquids are called the negolyte and the posolyte, not anolyte and catholyte, because anode and cathode swap between charge and discharge. Chapter 1 · Galvanic cells

pouch cell

A pouch cell stacks its electrodes in a sealed, soft, flat case: light and cheap to make. Chapter 2 · Cell anatomy

Power density

Power density is the greatest power a battery can deliver per kilogram (W/kg) or per litre (W/L), and it is measured by cycling the battery at different rates. Chapter 4 · Capacity and energy

Primary

Primary batteries are single-use: they store and give energy but cannot be recharged, so they are discarded after use. Chapter 1 · Galvanic cells

prismatic cell

A prismatic cell holds large stacked sheets in a rectangular case of aluminium, steel or semi-hard plastic, usually with a vent, and suits uses where swelling matters; its six digits give the thickness in tenths of a millimetre, then the width and the length in millimetres, so 504050 is 5.0 by 40 by 50 mm. Chapter 2 · Cell anatomy

pseudocapacitor

Capacitors and supercapacitors store charge without faradaic reactions and are not batteries; a pseudocapacitor, which stores charge through fast faradaic reactions confined to surfaces while behaving like a capacitor, meets two of the tests only in part and is not a battery either. Chapter 1 · Galvanic cells

pseudo-open-circuit curve

A slow cycle at C/20 also gives the pseudo-open-circuit curve of the cell: “pseudo” because the cell is never at open circuit and a small polarization remains in it. Lab 1 · Cell testing

p-type

An n-type molecule is reduced, and a cation such as Li⁺ balances it; a p-type molecule is oxidized, and an anion from the electrolyte balances it; a bipolar one does both, in different windows of potential. Chapter 15 · Sodium and organic

pyrometallurgy

Two chemical routes dominate: pyrometallurgy, which smelts the cells at about 1000 °C into a copper-nickel-cobalt alloy while lithium and manganese go to the slag, and hydrometallurgy, which dissolves the electrode materials in acid and separates the metals, with higher recovery and lower energy use… Chapter 18 · Recycling

Q

quasi-open-circuit voltage

The voltage at the end of each rest is a quasi-open-circuit voltage (QOCV); joined up, these trace the equilibrium curve of the electrode. Lab 3 · GITT

quasi-reversible

When electron transfer is slower, extra overpotential is needed to drive it, the peaks move apart and the separation grows with scan rate: a quasi-reversible process. Lab 2 · Cyclic voltammetry

R

Randles-Sevcik equation

For a reversible couple whose species diffuse freely in solution, the peak current follows the Randles-Sevcik equation, Lab 2 · Cyclic voltammetry

redox shuttles

Additives in small quantities answer this in three main ways: redox shuttles consume the excess current through a redox process and keep the voltage from rising; aromatic compounds such as biphenyl are believed to polymerize above the maximum operating voltage into an insulating layer on the positiv… Chapter 17 · Safety

reference electrode

The reference electrode fixes the potential scale; the counter electrode, an inert platinum wire larger than the working electrode, carries the current so that the reference does not have to. Lab 2 · Cyclic voltammetry

residual capacity

The capacity it delivers, compared with a disc from a fresh cell, is the residual capacity of that piece of electrode; the rate test that follows says whether the material is intact. Lab 6 · Post-mortem

resistive reactant

A simple model builds that in, a resistive reactant: four groups of identical particles, differing only in their contact resistance. Chapter 6 · Voltage profiles

rest

The pulse itself is the weak point, so a more robust route reads diffusion from the rest after the pulse. Lab 3 · GITT

reversible

A reversible part comes from the entropy change of the reaction, and its sign follows the temperature coefficient of the voltage, dE/dT . Chapter 5 · Polarization

rocking-chair

Replacing their lithium-metal negative electrode with a second insertion host gave the lithium-ion or rocking-chair cell: demonstrated by Murphy and by Scrosati, put on the market by Sony in June 1991 as carbon against LiCoO₂. Chapter 2 · Cell anatomy

S

Secondary

Secondary batteries are rechargeable: an external current reverses the reaction that makes the electricity, so they store and supply energy over many cycles. Chapter 1 · Galvanic cells

SEI

On the first charge the electrolyte is reduced on its surface and leaves the SEI (chapter 10), consuming lithium; once the layer is complete it protects the graphite and lets it cycle. Chapter 10 · The SEI

shelf life

The shelf life is how long a battery can be stored without a load, and self-discharge, the slow loss of charge at open circuit through internal reactions, is what shortens it (chapter 16). Chapter 4 · Capacity and energy

silver–carbon

…es with a sulfide electrolyte against 50.6 % in a liquid one, at a Coulombic efficiency of about 99.95 %; and Samsung’s silver–carbon layer, which needs no excess lithium and gave a prototype pouch cell of more than 900 Wh/L that lasted 1000 cycles. Chapter 14 · Solid-state cells

single-ion conductor

The logical end point is a single-ion conductor, in which the anions are tied to the polymer so that t₊ approaches 1 and no salt gradient builds up; one block copolymer of this kind reached t₊ above 0.85, though at only 1.3 × 10⁻⁵ S/cm at 60 °C, and conducting 10⁻⁴ S/cm at room temperature remains the challenge. Chapter 9 · Ion transport

solid-electrolyte interphase

The electrolyte is reduced at the graphite surface, and its products form a passivating layer, the solid-electrolyte interphase or SEI. Chapter 10 · The SEI

solution

It is a solution: a salt dissolved in a liquid, the solvent. Chapter 8 · Liquid electrolytes

solvent-separated ion pair

When an anion wins a place, the two ions form a contact ion pair; when solvent stays between them, they are a solvent-separated ion pair; larger clusters are aggregates. Chapter 8 · Liquid electrolytes

spectator cation

The magnesium stays in place while lithium comes and goes, so it is called a spectator cation. Chapter 15 · Sodium and organic

spinels

Almost every commercial positive electrode belongs to one of three families of oxide, identified in Goodenough’s group in the 1980s: layered oxides such as LiCoO₂, spinels such as LiMn₂O₄, and polyanion oxides such as LiFePO₄. Chapter 13 · Positive electrodes

square plot

To see both electrodes at once, we draw the state of a cell on a square plot: the negative electrode’s lithium content across, the positive electrode’s (inverted) up. Lab 1 · Cell testing

stable

A spectrum is worth modelling only if it could have come from a system that was linear, causal (the response comes only from the excitation), stable (unchanged during the sweep) and finite. Lab 4 · EIS: measurement

stages

It fills some gaps and leaves the others empty, forming stages: the stage number is the number of graphene sheets between two lithium layers. Chapter 7 · Insertion hosts

state of charge

The state of charge (SOC) says how much of the capacity is left; the depth of discharge (DOD) says how much has been used, so the two add up to 100 %. Chapter 4 · Capacity and energy

steady-state

After the rest the electrode has settled at a slightly different composition: the steady-state change ΔEs=E4−E1 , a step along the equilibrium curve. Lab 3 · GITT

symmetric cells

Then the cell is discharged slowly and opened in an argon glove box; its electrodes can be rebuilt into half cells or symmetric cells of two identical electrodes, whose spectra show which electrode carries which arc, and examined by microscopy and spectroscopy. Lab 6 · Post-mortem

T

tail

In the middle range come arcs from the interfaces, each a resistance in parallel with a capacitance; at low frequency a tail from lithium diffusing inside the particles. Lab 4 · EIS: measurement

thermal runaway

The worst case is thermal runaway: the reaction of an electrode with the electrolyte becomes self-sustaining and enters an autocatalytic mode, the heat it releases speeding it up. Chapter 17 · Safety

time constant

At its top, where the imaginary part is largest, ωRC = 1, so the frequency at the top gives the time constant τ = RC, and the capacitance follows from R and τ. Lab 5 · EIS: analysis

tortuosity

The tortuosity τ collects these effects. Chapter 11 · Composite electrodes

total harmonic distortion

The total harmonic distortion, the harmonics’ combined size over the fundamental, checks linearity at every frequency. Lab 4 · EIS: measurement

transference number

The transference number t₊ is the fraction of the ionic conductivity carried by Li⁺; in a dilute electrolyte it is the ratio of the lithium diffusion coefficient to the sum of both. Chapter 9 · Ion transport

transient

During the pulse, lithium is taken from the surface faster than diffusion can refill it from inside, so the surface composition runs ahead and the voltage climbs: the transient change ΔEt=E3−E2 , measured after the immediate IR jump. Lab 3 · GITT

V

voltage efficiency

The voltage efficiency is the ratio of the average discharge voltage to the average charge voltage. Chapter 4 · Capacity and energy

W

working electrode

The working electrode is where the reaction of interest happens: an inert disc of glassy carbon, platinum or gold, polished before each run, or, in battery work, the composite electrode itself. Lab 2 · Cyclic voltammetry