The electrochemical properties

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History

Lithium-ion batteries were first proposed by M.S. Whittingham (Binghamton University), then at Exxon, in the 1970s. Whittingham used titanium(II) sulfide as the cathode and lithium metal as the anode.

The electrochemical properties of the lithium intercalation in graphite were first discovered in 1980 by Rachid Yazami et al. at the Grenoble Institute of Technology (INPG) and French National Centre for Scientific Research (CNRS) in France

Lithium batteries in which the anode is made from metallic lithium pose severe safety issues. As a result, lithium-ion batteries were developed in which the anode, like the cathode, is made of a material containing lithium ions. In 1981

In 1983, Michael Thackeray, John Goodenough, and coworkers identified manganese spinel as a cathode material.] Spinel showed great promise, since it is a low-cost material, has good electronic and lithium ion conductivity, and possesses a three-dimensional structure which gives it good structural stability

In 2002, Yet-Ming Chiang and his group at MIT published a paper in which they showed a dramatic improvement in the performance of lithium batteries by boosting the material's conductivity by doping it with aluminium, niobium and zirconium, though at the time, the exact mechanism causing the increase became the subject of a heated debate.

In 2004, Chiang again increased performance by utilizing iron-phosphate particles of less than 100nm in diameter. This miniaturized the particle density by almost a hundredfold, increased the surface area of the electrode and improved the battery's capacity and performance.

Electrochemistry

The three participants in the electrochemical reactions in a lithium-ion battery are the anode, cathode, and electrolyte.

Both the anode and cathode are materials into which and from which lithium can migrate. The process of lithium moving into the anode or cathode is referred to as insertion (or intercalation ), and the reverse process, in which lithium moves out of the anode or cathode is referred to as extraction (or deintercalation). When a lithium-based cell is discharging, the lithium is extracted from the anode and inserted into the cathode. When the cell is charging, the reverse process occurs: lithium is extracted from the cathode and inserted into the anode.

During discharge, the anode of a conventional Li-ion cell is made from carbon, the cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent.

The cathode half-reaction (with charging being forwards) is:

Advantages and disadvantages

Advantages

  • Lithium-ion batteries can be formed into a wide variety of shapes and sizes so as to efficiently fill available space in the devices they power.
  • Lithium-ion batteries are lighter than other energy-equivalent secondary batteries
  • Lithium-ion batteries do not suffer from the memory effect. They also have a self-discharge rate of approximately 5-10% per month.

Disadvantages of traditional Li-ion technology

Shelf life

  • A disadvantage of lithium-ion cells lies in their relatively poor cycle life: upon every (re)charge, deposits form inside the electrolyte that inhibits lithium ion transport, resulting in the capacity of the cell to diminish.
  • Also, high charge levels and elevated temperatures (whether resulting from charging or being ambient) hasten permanent capacity loss for lithium-ion batteries
  • At a 100% charge level, a typical Li-ion laptop battery that is full most of the time at 25 °C or 77 °F will irreversibly lose approximately 20% capacity per year.

Internal resistance

The internal resistance of lithium-ion batteries is high compared to other rechargeable chemistries such as nickel-metal hydride and nickel-cadmium. It increases with both cycling and chronological age. Rising internal resistance causes the voltage at the terminals to drop under load, reducing the maximum current that can be drawn from them. Eventually they reach a point at which the battery can no longer operate the equipment it is installed in for an adequate period.

Safety requirements

Li-ion batteries are not as durable as nickel metal hydride or nickel-cadmium designs and can be extremely dangerous if mistreated. They may explode if overheated or if charged to an excessively high voltage. Furthermore, they may be irreversibly damaged if discharged below a certain voltage. To reduce these risks, lithium-ion batteries generally contain a small circuit that shuts down the battery when it is discharged below about 3V or charged above about 4.2. In normal use, the battery is therefore prevented from being deeply discharged

Other safety features are also required for commercial lithium-ion batteries:

  • shut-down separator (for over temperature),
  • tear-away tab (for internal pressure),
  • vent (pressure relief), and
  • Thermal interrupt (over current/overcharging).

Specifications and design

A lithium-ion battery from a mobile phone.

  • Specific energy density: 150 to 200 Wh/kg (540 to 720 kJ/kg)
  • Volumetric energy density: 250 to 530 Wh/l (900 to 1900 J/cm³)
  • Specific power density: 300 to 1500 W/kg (@ 20 seconds and 285 Wh/l)

Because lithium-ion batteries can have a variety of cathode and anode materials, the energy density and voltage vary accordingly.

Charging procedure

Stage 1:

Apply charging current limit until the voltage limit per cell is reached.

Stage 2:

Apply maximum voltage per cell limit until the current declines below 3% of rated charge current.

Stage 3:

Periodically apply a top-off charge about once per 500 hours.

The charge time is about three to five hours, depending upon the charger used. Generally, cell phone batteries can be charged at 1C and laptop-types at 0.8C, where C is the current that would discharge the battery in one hour. Charging is usually stopped when the current goes below 0.03C but it can be left indefinitely depending on desired charging time.

Top-off charging is recommended to be initiated when voltage goes below 4.05 V/cell.

For safety reasons it is recommended to stay within the manufacturers stated voltage and current ratings during both charge and discharge cycles.

Guidelines for prolonging lithium-ion battery life

  • Lithium-ion batteries should never be depleted below their minimum voltage (2.4 to 2.8 V/cell, depending on chemistry). If a lithium-ion battery is stored with too low a charge, there is a risk that the charge will drop below the low-voltage threshold, resulting in an unrecoverable dead battery.
  • Lithium-ion batteries should be kept cool. Ideally they are stored in a refrigerator.
  • Aging will take its toll much faster at high temperatures.

Safety

Lithium-ion batteries can rupture, ignite, or explode when exposed to high-temperature environments, e.g. in an area that is prone to prolonged direct sunlight. Short-circuiting a lithium-ion battery can cause it to ignite or explode and any attempt to open or modify the casing or circuitry is dangerous. For this reason they normally contain safety devices that protect the cells from abuse.

Replacing the lithium cobalt oxide cathode material in lithium-ion batteries with lithiated metal phosphate leads to longer cycle and shelf life, improves safety, but lowers capacity.

Another option is to use a manganese oxide or iron phosphate cathode.

A new class of high power cathode materials, lithium nickel manganese cobalt (NMC) oxide has recently been introduced that have a significantly higher temperature tolerance compared to lithium cobalt oxide

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