Chapter 3: Batteries
How is Li-ion storage capacity determined?
1. the capability of the host, or the electrode, to change valence states 2. the available space to accommodate Li ions 3. the reversibility of the intercalation reactions
What is the energy density of gasoline?
12,000 Wh/Kg (80x that of Li ion)
What is the difference between a battery and a cell?
A cell is a single electrochemical device, consisting of two plates and an electrolyte. The cell voltage is determined by the materials used, not the size. Batteries are made up of one or more cells.
What is a battery?
A device that transforms chemical energy into electricity
What are a few common modern primary batteries?
Alkaline, Zinc-Carbon, Lithium-metal
What are common Li-ion battery materials?
Anode: graphite, Cathode: a lithium metal oxide, Electrolyte: lithium salt (may be liquid, gel, or solid)
What is the difference between capacity and specific capacity?
Capacity is amp-hours while specific capacity is amp-hours per mass
Explain the charge storage mechanism in Li-ion batteries
Charge is stored both at the interface of the electrode/electrolyte and more importantly inside the electrode materials, resulting in higher capacity and energy density, but lower power density since ions and electrons need to travel farther distances.
How do we get a joule?
Coulomb * Volt
What is the unit for capacity?
Current * time (A.h, mA.h)
T or F: in a battery or other source of direct current, the anode is the positive terminal
False, the anode is the negative terminal
T or F: We use the equation C = Q/V to determine a battery's capacity
False. This equation is used to determine capacitance. Batteries have negligible capacitance
How do we assemble battery stacks?
In series or parallel; in series when we want higher voltage of the same current, in parallel when we want higher current of the same voltage
Who invented the modern battery?
Italian physicist Alessandro Volta in 1799. It was composed of alternating copper and zinc discs (with saltwater paper in between)
What is the difference between Li-ion and Li-metal batteries?
Li-ion cells do not contain metallic lithium, the ions are inserted into the structure of the electrode materials or electrolyte. Li-metal batteries are primary (not rechargeable) and are unsafe.
Which type of battery has the best energy density?
Lithium ion (150 Wh/kg)
How do ions flow during charging/discharging in Li-ion batteries?
Lithium ions leave the cathode, travel through the electrolyte and intercalate into the anode during charging; during discharging, lithium ions leave the anode and intercalate into the cathode
What are some common modern secondary batteries?
Nickel-cadmium (NiCd), Nickel-metal hydride (NiMH), Lead-Acid, Lithium-ion
What is the difference between practical specific energy and theoretical specific energy?
Practical specific energy is typically only 30% of theoretical specific energy. Losses of energy occur from voltage loss (~10%), electrolyte (poor ionic conductivity), current collector (poor electrical conductivity between electrode materials and current collectors), and utilization of active materials (intercalation electrodes, ~50%)
Is a primary battery voltaic/galvanic or electrolytic?
Primary batteries are only voltaic/galvanic because they cannot be charged
Is a secondary battery voltaic/galvanic or electrolytic?
Secondary batteries are both. They are voltaic/galvanic when discharging and electrolytic when being charged
What material is a desired replacement for graphite in Li-ion batteries?
Silicon, because it has higher specific capacity and energy density, although it is not as elastic and cannot cope with the strain of being repeatedly charged and discharged
What are some other rechargeable batteries?
Sodium ion, Aluminum ion, flow batteries
Explain the charge storage mechanism in supercapacitors
Supercapacitors store charge at the interface of the electrode/electrolyte, so both electrons and ions do not need to travel far to reach the current collector or electrode materials, resulting in low energy density and high power density.
What do letters mean for battery sizes?
The later the letter, the larger the battery; AA means 50.5mm x 14.5mm
Why does faster discharge times lead to less extracted energy and lower battery capacity?
The necessary components needed for the reaction to occur may not have the requisite time to move to their needed positions. Only a fraction of the total reactants are converted to other forms, and available energy is reduced. On the contrary, if the battery is discharged slowly, more energy can be extracted from the battery and the battery capacity is higher.
How do you calculate cell voltage in batteries?
The nominal cell voltage is the potential difference between the two electrodes
What is the difference in the chemistry between primary and secondary batteries?
The principle of their work is the same (chemical reaction involving anode, cathode and electrolyte), but the chemical composition between the two is different
what is system voltage?
The sum of battery voltages connected in series
What is cut-off voltage?
The voltage at which a battery is considered fully discharged, beyond which further discharge could cause harm. Below this voltage, a battery is considered depleted.
Why are secondary batteries rechargeable?
Their electrochemical reaction can be reversed by applying a certain voltage to the battery in the opposite direction of the discharge
Why are primary batteries disposable?
Their electrochemical reaction cannot be reversed
Why are lithium-air batteries so desirable?
They are extremely lightweight and thus have higher energy density
What is wrong with NiCd batteries?
They forget their initial capacity if not discharged fully
Why are Li-ion batteries so popular?
They have much higher energy density than competitor secondaries, no memory effect, they produce higher voltage than any other secondaries, they have a high cell voltage (3.6 V) so batteries can be designed with only one cell
T or F: Batteries are charged and discharged at constant current
True
T or F: Generally speaking, higher C-rates for batteries results in lower energy density for a given capacity
True
T or F: in a passive load, the anode of a battery is the positive terminal
True
True or false: A, AA, AAA, C and D batteries have the same voltage
True
True or false: NiCd were the first secondary batteries used everywhere in the world
True
What is the unit of joule?
Voltage * Current * time (Watt.sec)
What happens to voltage and Li ions during discharge?
Voltage decreases steadily to zero and Li ions are extracted from anode and move to cathode
What happens to voltage and Li ions during battery charging?
Voltage increases steadily and lithium ions are extracted from the cathode and intercalate into the anode's graphite layers
What is the unit of specific energy?
Wh/kg; nFE/3600
What is C-Rate?
a measure of the rate at which a battery is discharged; higher C-Rate equals faster discharge rate; 1C means that the discharge current will discharge the entire battery in 1 hour (2C = 30 min, 3C = 20 min, 0.5C = 2h, etc); most portable batteries are rated at 1C
What is a dry cell?
a primary cell because there are no fluids in the battery
What is a wet cell?
a secondary cell because they are full of paste that allows the movement of ions
How many watt-hours can a 1 kg Li-ion battery store?
about 150
What is galvanostatic testing?
applying constant current while measuring the change in voltage
What is the unit for energy?
capacity * voltage (A.h.V)
What is the "intercalation process"?
charge storage process in Li-ion batteries where lithium ions insert or de-insert from the active electrode materials
What are the advantages of primary batteries?
convenient, higher energy density, and less cost per battery, although they cost more over the long term. Generally have higher capacity and initial voltage than rechargeable batteries
What is the effect of battery size?
different capacities, different discharge time, and therefore different energy density; bigger batteries last longer
What is potentiostatic testing?
imposes a constant potential (voltage) on the working electrode for a specific time period; the measured current is plotted with time
What is battery cell testing?
includes cyclic voltammetry tests, galvanostatic/potentiostatic tests, such as C/D tests, electrochemical impedence spectroscopy
What is battery module testing?
involves little testing of the internal chemical reactions; evaluates the overall battery performance, safety, battery management, cooling system, mechanical testing, etc.
What are the disadvantages of Li-ion batteries?
involves risk of bursting, costly, complete discharge can damage the battery, extremely sensitive to high temperatures, short lifespan, not available in standard cell sizes
Why is graphite used as the anode material in Li-ion batteries?
it is layered, so intercalation can easily take place because the bond between layers is weak
What are the advantages of Li-ion batteries?
lightweight, higher energy density than other secondaries, rate of charge loss is less, greater number of charge and discharge cycles, need not be discharged completely, operates at higher voltage than competitors
what is system capacity?
measured in mAh, does not increase in the series string compared to the individual battery, but will increase with the number of batteries in parallel
Talk about Lead-Acid batteries
most common battery type in cars and to store solar energy because they can provide high current and their cost is relatively low; maintain 40-45% of the battery market because of their role in automobiles; the electrodes are lead(-) and lead-oxide(+) with an acidic electrolyte
What are some characteristics of electric cars?
no tailpipe emissions, energy comes from a utility company, usually around a 100 mile range, takes hours to recharge, approx 2 cents/mile
What are the two electrochemical testing modes for batteries?
potentiostatic and galvanostatic
What are some characteristics of gasoline cars?
produce greenhouse gases, energy comes from fossil fuel companies, 300+ mile range, takes minutes to refuel, 12+ cents/mile
Talk about Nickel-Metal hydride (NiMH) batteries
rechargeable with no memory effect, the positive electrode contains nickel hydroxide and the negative electrode contains hydrogen-absorbing nickel alloys; the electrolyte is an alkaline electrolyte (usually KOH); they have very good life cycle and capacity, although they self-discharge relatively quickly without an applied load
What is a secondary battery?
rechargeable-multi use (i.e. car and cell phone batteries)
Shape of cyclic voltammetry in primary vs secondary batteries
see image on desktop
What is a primary battery?
single use, disposable, cannot be recharged
what is system current?
system current increases as more parallel strings are added
Why do Li-ion batteries produce higher specific energy and energy density than other batteries?
the electrodes are made from lightweight lithium and carbon (nickel and lead are much heavier), lithium is highly reactive so lots of energy can be stored in its atomic bonds
What is energy density?
the measure of how much energy a battery contains in comparison to its volume; generally measured in Watt-hours/liter
What is specific energy?
the measure of how much energy a battery contains in comparison to its weight; measured in Watt-hours/kilogram (W-hr/kg), generally
What is battery capacity?
the number of coulombs (charge) in amp-hours that a battery can deliver; usually defined by single electrode materials, not cell; capacity of electrode material depends on amount of Li ions can be intercalated into the host structure
How does a Volta (Zn/Cu) battery work?
there is a electrochemical reaction, oxidation occurs at the anode (which creates electrons), reduction occurs at the cathode (which consumes electrons)
Battery structure?
two terminals: cathode and anode
What is an alkaline battery?
used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline (high pH such as KOH) electrolyte
Talk about Li-ion batteries
used in computers and consumer electronics; the negative electrode is made of carbon, the positive electrode is a lithium metal oxide, the electrolyte is a lithium salt in an organic solvent
What is the unit for power (Watt)?
voltage * current (V.A, V.mA)
What is open circuit voltage (OCV)?
voltage at zero current, when no external load is connected
