Transcript
Introduction A lithium-ion battery (sometimes Li-ion battery or LIB) is a member of a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging. Li-ion batteries use anintercalated lithium compound as one electrode material, compared to the metallic lithium used in a non-rechargeable lithium battery. The electrolyte which allows for ionic movement, and the two electrodes are the consistent components of a lithium-ion cell. Lithium-ion batteries are common in consumer electronics. They are one of the most popular types of rechargeable batteries forportable electronics, with a high energy density, no memory effect, and only a slow loss of charge when not in use. Beyond consumer electronics, LIBs are also growing in popularity for military, electric vehicle and aerospace applications.[6] For example, lithium-ion batteries are becoming a common replacement for the lead acid batteries that have been used historically for golf carts and utility vehicles. Instead of heavy lead plates and acid electrolyte, the trend is to use lightweight lithium-ion battery packs that can provide the same voltage as lead-acid batteries, so no modification to the vehicle's drive system is required. Chemistry, performance, cost and safety characteristics vary across LIB types. Handheld electronics mostly use LIBs based on lithium cobalt oxide (LiCoO 2), which offers high energy density, but presents safety risks, especially when damaged. Lithium
iron phosphate(LFP), lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide (NMC) offer lower energy density, but longer lives and inherent safety. Such batteries are widely used for electric tools, medical equipment and other roles. NMC in particular is a leading contender for automotive applications. Lithium nickel cobalt aluminum oxide (NCA) and lithium titanate (LTO) are specialty designs aimed at particular niche roles. Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard since they contain, unlike other rechargeable batteries, a flammable electrolyte and are also kept pressurised. Because of this the testing standards for these batteries are more stringent than those for acid-electrolyte batteries, requiring both a broader range of test conditions and additional battery-specific tests.[7][8] This is in response to reported accidents and failures, and there have been battery-related recalls by some companies.
Construction The three primary functional components of a lithium-ion battery are the positive and negative electrodes and electrolyte. Generally, the negative electrode of a conventional lithium-ion cell is made from carbon. The positive electrode is a metal oxide, and the electrolyte is alithium salt in an organic solvent.[36] The electrochemical roles of the electrodes reverse between anode and cathode, depending on the direction of current flow through the cell. The most commercially popular negative electrode is graphite. The positive electrode is generally one of three materials: a layered oxide(such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as lithium manganese oxide).[37] The electrolyte is typically a mixture of organic carbonates such as ethylene carbonate or diethyl carbonate containing complexes of lithium ions.[38] These non-aqueous electrolytes generally use non-coordinating anion salts such as lithium hexafluorophosphate (LiPF 6), lithium hexafluoroarsenate monohydrate (LiAsF 6), lithium perchlorate (LiClO 4), lithium tetrafluoroborate (LiBF 4) and lithium triflate (LiCF 3SO 3).
Depending on materials choices, the voltage, energy density, life and safety of a lithium-ion battery can change dramatically. Recently, novel architectures using nanotechnology have been employed to improve performance. Pure lithium is highly reactive. It reacts vigorously with water to form lithium hydroxide and hydrogen gas. Thus, a non-aqueous electrolyte is typically used, and a sealed container rigidly excludes moisture from the battery pack. Lithium ion batteries are more expensive than NiCd batteries but operate over a wider temperature range with higher energy densities. They require a protective circuit to limit peak voltage. For notebooks or laptops, lithium-ion cells are supplied as part of a battery pack with temperature sensors, voltage converter/regulator circuit, voltage tap, battery charge state monitor and the main connector. These components monitor the state of charge and current in and out of each cell, capacities of each individual cell (drastic change can lead to reverse polarities which is dangerous),[39] temperature of each cell and minimize the risk of short circuits.[40]
Electrochemistry The participants in the electrochemical reactions in a lithium-ion battery are the negative and positive electrodes with the electrolyte providing a conductive medium for Lithium-ions to move between the electrodes. Both electrodes allow lithium ions to move in and out of their interiors. During insertion (or intercalation) ions move into the electrode. During the reverse process, extraction (ordeintercalation), ions move back out. When a lithium-ion based cell is discharging, the positive Lithium ion moves from the negative electrode (usually graphite) and enters the positive electrode (lithium containing compound). When the cell is charging, the reverse occurs. Useful work is performed when electrons flow through a closed external circuit. The following equations show one example of the chemistry, in units of moles, making it possible to use coefficient
.
The positive electrode half-reaction is:[44]
The negative electrode half reaction is:
The overall reaction has its limits. Overdischarge supersaturates lithium cobalt oxide, leading to the production of lithium oxide,[45] possibly by the following irreversible reaction:
Overcharge up to 5.2 volts leads to the synthesis of cobalt(IV) oxide, as evidenced by x-ray diffraction:[46]
In a lithium-ion battery the lithium ions are transported to and from the positive or negative electrodes by oxidizing the transition metal, cobalt (Co).
The cell's energy is equal to the voltage times the charge. Each gram of lithium represents Faraday's constant/6.941 or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kg. This is a bit more than the heat of combustion of gasoline, but does
not consider the other materials that go into a lithium battery and that make lithium batteries many times heavier per unit of energy.
Charge and discharge[edit] During discharge, lithium ions Li+ carry the current from the negative to the positive electrode, through the nonaqueous electrolyte and separator diaphragm.[53] During charging, an external electrical power source (the charging circuit) applies an overvoltage (a higher voltage but of the same polarity) than that produced by the battery, forcing the current to pass in the reverse direction. The lithium ions then migrate from the positive to the negative electrode, where they become embedded in the porous electrode material in a process known as intercalation.
Charging procedure The charging procedures for single Li-ion cells, and complete Li-ion batteries, are slightly different.
A single Li-ion cell is charged in two stages:[39] 1. Constant current (CC) 2. Voltage source (CV)
A Li-ion battery (a set of Li-ion cells in series) is charged in three stages: 1. Constant current 2. Balance (not required once a battery is balanced) 3. Voltage source
During the constant current phase, the charger applies a constant current to the battery at a steadily increasing voltage, until the voltage limit per cell is reached.
During the balance phase, the charger reduces the charging current (or cycle the charging on and off to reduce the average current) while the state of charge of individual cells is brought to the same level by a balancing circuit, until the battery is balanced. Some fast chargers skip this stage. Some chargers accomplish the balance by charging each cell independently. During the constant voltage phase, the charger applies a voltage equal to the maximum cell voltage times the number of cells in series to the battery, as the current gradually declines towards 0, until the current is below a set threshold of about 3% of initial constant charge current. Periodic topping charge about once per 500 hours. Top charging is recommended to be initiated when voltage goes below 4.05 V/cell. Failure to follow current and voltage limitations can result in an explosion. [54]
Specifications[edit]
Specific energy density: 100 to 250 W·h/kg (360 to 900 kJ/kg)[60]
Volumetric energy density: 250 to 620 W·h/L (900 to 1900 J/cm³)[2]
Specific power density: 300 to 1500 W/kg (@ 20 seconds and 285 W·h/l) [1][not in citation given]
Because lithium-ion batteries can have a variety of positive and negative electrode materials, the energy density and voltage vary accordingly. The open circuit voltage is higher than aqueous batteries (such as lead acid, nickel-metal hydride and nickel-cadmium).[61] Internal resistance increases with both cycling and age.[61] [62]
Rising internal resistance causes the voltage at the terminals to drop under load, which
reduces the maximum current draw. Eventually increasing resistance means that the battery can no longer operate for an adequate period. Batteries with a lithium iron phosphate positive and graphite negative electrodes have a nominal open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese cobalt (NMC) oxide positives with graphite negatives have a 3.7 V nominal voltage with a 4.2 V maximum while charging. The charging procedure is performed at constant voltage with currentlimiting circuitry (i.e., charging with constant current until a voltage of 4.2 V is reached in the cell and continuing with a constant voltage applied until the current drops close to zero). Typically, the charge is terminated at 3% of the initial charge current. In the past, lithium-ion batteries could not be fast-charged and needed at least two hours to fully charge. Current-generation cells can be fully charged in 45 minutes or less.
Comparison of battery types Four figures help choose which battery type is best for a given application: The energy/weight ratio, the energy/volume ratio, the power to weight ratio, and the cost in watt-hours per dollar. In comparison to the common lead-acid battery, which is probably the least desirable to run an electric vehicle, the numbers are
Battery Type
Energy/weight Energy/volume Power/weight Energy/US$ Watt-hours/Kg watt-hours/L watt/Kg watt-hr/$
Lead-acid
30-40
60-75
180
4-10
NIckel-Zinc
60-70
170
900
2-3
Lithium-Ion
160
270
1800
3-5
Lithium-Polymer
130-200
300
to 2800
3-5
Two other values should also be considered, the self-discharge rate, which causes the charge to diminish over time, and the cycle life of the batteries, the number of times the batteries can undergo a deep discharge and still accept charge. The batteries listed above perform well in those categories.
Lithium-ion battery
A Nokia Li-ion battery used on the Nokia 3310mobile phone.
Specific energy
100–265 W·h/kg[1][2] (0.36–0.95 MJ/kg)
Energy density
250–730 W·h/L[2] (0.90–2.23 MJ/L)
Specific power
~250-~340 W/kg[1]
Charge/discharge efficiency
80–90%[3]
Energy/consumer-price
2.5 W·h/US$
Self-discharge rate
8% at 21 °C 15% at 40 °C 31% at 60 °C (per month)[4]
Cycle durability
400–1200 cycles [5]
Nominal cell voltage
NMC 3.6 / 3.7 V,LiFePO4 3.2 V
Adaptable for both the ASC and WSC regulations. Extra batteries and fixing holes can be included so that the solar car can be raced at the World’s major events. The nominal battery voltage for the ASC is 100.8V (28 cells) and for the WSC 122.4V (34 cells).
2.3 BATTERY CAPACITY: Motor energy needed, EMO = (P * (d)) / V where Total power, P d – total dist. Topspeed, V The car however is expected to accelerate numerous times during the journey which will deplete more energy from the battery. Thus a battery capacity of roughly 3 times higher must be selected.
