Apr 19 2011
Today, the major battery systems are the rechargeable lead acid and the primary manganese dioxide-zinc: both have a long history and are at an advanced state of technical maturity.
The Lithium-Ion Battery (LIB), conceived and developed in Japan by Asahi Kasei Co. and first commercialized by Sony Co. in 1991, is poised to challenge these established systems as the demand for higher-performance battery systems continues.
Lithium has a low atomic number and a high electrode potential that results in significantly high energy density for the LIB compared to lead and zinc in the traditional batteries. However, the development of new high energy lithium systems has been neither simple nor easy. It has required a total system approach and the development of breakthrough technologies based on new anodes, cathodes and non-aqueous electrolytes to continue the steady improvement of high energy lithium battery systems.
The early rechargeable lithium cells were plagued with safety problems caused by the tendency of lithium metal anodes to form dendrites and powder deposits on recharging. The lithium metal rechargeable cells now are restricted mainly to small capacity coin cells.
Since lithium metal constituted a safety problem, attention shifted to the use of a lithium-intercalation material as an anode. H. Ikeda of Sanyo was the first to patent an intercalation material in an organic solvent such as graphite in his June 1981 Japanese Patent No. 1769661. Later, I. Kuribayashi and A. Yoshino developed a new cell design using an intercalation carbon anode and a LiCoO2 cathode and filed patents worldwide. Using a pilot plant developed for rechargeable Li-MnO2 cells, Sony Energytec Inc. began to produce commercial cells (called the Li-Ion Battery) based on the Asahi patents in 1991. They also introduced electronic circuitry to control the charge-discharge, the use of a current interrupt device to interrupt current flow on build-up of excessive internal cell pressure and the use of a “shut-down” polymer separator.
The name “Lithium-Ion” now is accepted by the battery community worldwide, although there is no lithium metal in the cell. However, very often lithium metal deposition occurs during charging with the graphite anode and it may cause the many troubles on the LIB. Both electrodes operated by intercalation of lithium ions into the structure of the active materials. AT Battery Co., a joint venture of Toshiba Battery Co. and Asahi Chemical Co., was the second to commercialize the technology using Asahi patent portfolio.
The early cylindrical 18650 cell LIBs (18 mm in diameter and 65.0 mm long) had a capacity of 800 mAh and an end-of-charge voltage of 4.1 V. The initial cells used hard-carbon anode materials, which had a capacity of about 200 mAh/g, and the LiCoO2 had a capacity of nearly 130 mAh/g due to 4.1 V charging voltage. The early lithium-ion cells used a propylene carbonate-based electrolyte. However, energy density of LIB improved rapidly and increased on average by 10% per year and has approached 3.1 Ah in 2010. Responding to pressure from device manufactures, the cell capacity improved through engineering and the introduction of graphite anodes, improved LiCoO2-based cathode materials and the introduction of electrolyte additives.
Other recent developments include the incorporation of a fire retardant, which retards the combustion of the solvent, and a new additive to improve the wetting of the separator. New electrolytes and/or additives also are under development.
Today, LIBs are used in very high volumes in low power applications, such as mobile phones, laptops, cameras and other consumer electronic products. They have many attractive performance advantages, which make them also ideal for higher power applications, such as automotive and standby power:
- High energy/power density (about 4 times better than lead acid) and potential for yet higher capacities (high cell voltage of 3.6 Volts means fewer cells and associated connections and electronics are needed for high voltage batteries, so that, for instance, one lithium cell can replace three NiCad or NiMH cells which have a cell voltage of only 1.2 Volts).
- Does not need prolonged priming when new; one regular charge is all that’s needed.
- Relatively low self-discharge (self-discharge is less than half that of nickel-based batteries and they can retain charge for up to ten years).
- Low Maintenance (no periodic discharge is needed and there is no memory).
- Specialty cells can provide very high current to applications such as power tools.
- No liquid electrolyte means they are immune from leaking.
- Very small batteries also available. Solid state chemistry can be printed on to ceramic or flexible substrates to form thin film batteries with unique properties.
- Low weight.
- Can be optimised for capacity or rate.
- Can be discharged at the 40C rate or more: the high discharge rate means that for automotive use the required cold cranking power or boost power for hybrid vehicles can be provided by a lower capacity battery.
- Fast charge possible.
- Can be deep cycled: the cell maintains a constant voltage for over 80% of its discharge curve. It thus delivers full power down to 80% DOD versus 50% for Lead acid. This means that in practice, for a given capacity, more of the stored energy is usable or that the battery will accept more starting attempts or boost power requests before becoming effectively discharged.
- Very high Coulombic efficiency (Capacity discharged over Capacity charged) of almost 100%: thus very little power is lost during the charge/discharge cycles.
- No memory effect and no reconditioning needed (as for nickel based batteries).
- Tolerates microcycles.
- Long cycle life: 1000 to 3000 deep cycles. Cycle life can be extended significantly by using protective circuits to limit the permissible DOD of the battery. This mitigates against the high initial costs of the battery.
- Variants of the basic cell chemistry allow the performance to be tuned for specific applications.
- Available in a wide range of cell constructions with capacities from less than 500 mAh to 1000 Ah from a large number (over 100) of suppliers world-wide.
- Flexible form factor (manufacturers are not bound by standard cell formats): with high volume, any reasonable size can be produced economically.
For high power applications, which require large high cost batteries, the price premium of lithium batteries over the older lead acid batteries becomes a significant factor, impeding widespread acceptance of the technology. This in turn has discouraged investment in high volume production facilities keeping prices high and has for some time discouraged take up of the new technology. This is gradually changing and lithium is also becoming cost competitive for high power applications.
Among the limitations of LIBs, we can cite:
- Requires protection circuit to maintain voltage and current within safe limits.
- Subject to aging, even if not in use (but storage in a cool place at 40% charge reduces the aging effect).
- Transportation restrictions (shipment of larger quantities may be subject to regulatory control); this restriction does not apply to personal carry-on batteries.
- Internal impedance higher than equivalent NiCads.
- Expensive to manufacture (about 40 percent higher in cost than nickel-cadmium).
- Not fully mature (metals and chemicals are changing on a continuing basis).
- Degrades at high temperatures and when discharged below 2 Volts.
- Capacity loss or thermal runaway when overcharged.
- Venting and possible thermal runaway when crushed.
Measurement of the state of charge of the cell is more complex than for most common cell chemistries: the state of charge is normally extrapolated from a simple measurement of the cell voltage, but the flat discharge characteristic of lithium cells, so desirable for applications, renders it unsuitable as a measure of the state of charge and other more costly techniques such as coulomb counting have to be employed.
Although lithium cell technology has been used in low power applications for some time now, there is still not a lot of field data available about long term performance in high power applications. Reliability predictions based on accelerated life testing however shows that the cycle life matches or exceeds that of the most common technologies currently in use.
These drawbacks are far out weighed by the advantages of lithium cells and are now being used in an ever widening range of applications.
In order to control battery performance and safety it is necessary to understand what needs to be controlled and why it needs controlling. This requires an in depth understanding of the fundamental cell chemistries, performance characteristics and battery failure modes, particularly lithium battery failures. The battery can not simply be treated as a black box.
The Battery Management Systems (BMSs) encompass not only the monitoring and protection of the battery, but also methods for keeping it ready to deliver full power when called upon and methods for prolonging its life. This includes everything from controlling the charging regime to planned maintenance. There are many varieties of BMS.
There are three main objectives common to all Battery Management Systems:
- protect the cells or the battery from damage;
- prolong the life of the battery;
- maintain the battery in a state in which it can fulfil the functional requirements of the application for which it was specified.
To achieve these objectives, the BMS may incorporate one or more of the following functions: cell protection, charge control, demand management, SOC (State of Charge) determination, SOH (State of Health) determination, cell balancing, battery’s history, authentication and identification, communications between the battery and the charger or test equipment and many others.
The cell voltage used for charging LIB is typically 4.2 Volts (charging to 4.1 Volts will increase the cycle life but reduces the effective cell capacity by about 10%). Battery lasts longer with partial charges rather than full charges. They can not tolerate overcharging and hence should not be trickle charged. Fast chargers typically operate during the constant current charging phase only when the charging current is at a maximum. They switch off at the point when the constant voltage, reducing current phase, starts. At this point the battery will only be charged to about 70% of its capacity.
Rechargeable lithium cells are used in a wide range of consumer products, including cameras, camcorders, electric razors, toothbrushes, calculators, medical equipment, communications equipment, instruments, portable radios and TVs, pagers and PDA’s.
They are fast replacing Nickel Metal Hydride cells as the preferred power in mobile phones. Laptop computers almost exclusively use lithium batteries.
Now high power versions of up to 1000Ah capacity and more are becoming available for use in traction applications in electric and hybrid vehicles as well as for standby power.
The price of Lithium cells continues to fall as the technology gains more acceptance: the target price for high power cells is around 300 $/kWh, but cell makers are still quite some way from achieving that.
Although lithium secondary batteries may cost two or three times more than the cost of equivalent lead acid batteries and even more when the necessary battery management electronics are taken into account, this is more than compensated for by their longer cycle life, which may be five to ten times the life of lead acid batteries. Valid cost comparisons should therefore take into account the lifetime costs as well as the initial capital costs.
As society advances, a variety of new technologies, machines and systems have been developed and more efficient industrial operations have been adopted. The concern for global warming and a clean environment along with the development of advanced electric and hybrid vehicles may be served well by advanced LIB. This situation has resulted in increased demand for high-performance batteries and power generation and storage. At the same time, demand has increased for a cleaner environment and a more efficient energy production, coupled with low power consumption systems. In this context, the LIB has made and will continue to make significant contributions and advances.
In the pictures below, you can see, respectively, the charging characteristics and the CC/CV 3 cycles charge/discharge and temperature monitoring (at 1C and 23°C) of 18650 LIB packs.
GENPORT‘s activity in designing Li-Ion battery pack BMSs involves, among others, the acquisition of cell’s electrochemical data using an Arbin battery tester, the configuration and parametrization of the fuel gauge and protection the integrated circuit and the BMS PCB (Printed Circuit Board) final design.