The Future of Battery Technologies – Part 2: Focus on Lithium-Ion Batteries

This article is the second in our ongoing series about batteries. This installment provides an overview of lithium-ion batteries – typical properties, principal applications, and trends.

The expression “lithium batteries” is a generic name for battery technologies in which lithium ions play a part in the primary electrochemical discharge and charge reactions. Lithium batteries are available both as primary batteries (disposable batteries) and secondary batteries (rechargeable batteries). The choice of the best battery for a specific purpose is determined by a number of parameters, such as the power and energy requirements of the application, the operating environment, the electrical preconditions and the cost aspects. The most important factors are:

1. type of battery – disposable or rechargeable
2. electrochemical system – match benefits and disadvantages of battery type to the requirements of the application
3. voltage requirements – rated and operating voltage respectively, minimum and maximum levels, voltage regulation, discharge profile, etc.
4. load profile – resistive, current or power discharge, single vs. variable load, etc.
5. work cycle – continual or intermittent
6. temperature specifications
7. lifetime
8. physical requirements – size, shape, connections, etc.
9. storage requirements – shelf life, charge state when stored, environment, etc.
10. charge and discharge states (rechargeable batteries) – cycling batteries vs. energy storage batteries (flow charging), charge characteristics, efficiency and accessibility, discharge requirements
11. environmental conditions – climate (temperature, humidity), pressure conditions, physical environment (impact, vibration, dust, acceleration, etc.)
12. safety and reliability – permitted variance, acceptable faults, sensitivity to gas leaks or leaks of electrolyte from the battery, etc.
13. unusual or demanding operating conditions – e.g. extreme temperature conditions, standby times, high reliability, rapid activation, etc.
14. maintenance and replacement of unserviceable batteries
15. costs – initial, and operating costs

Lithium-based battery systems are characterised by high energy density levels, relatively high voltages and low weight-to-volume ratio, but generally tend to be more expensive than equivalent battery technologies with aqueous electrolytes, such as alkali disposable batteries and zinc/air batteries in the primary battery sector, and nickel metal hydride, nickel cadmium and lead batteries in the secondary battery sector.

Primary Lithium Batteries

Primary batteries have existed since the 1970s, are easy to use and provide convenient sources of energy for portable applications. The batteries usually require no or very little maintenance and they have a long shelf life; modern lithium batteries can usually be stored for up to 10 years, and there are special batteries with solid state electrolytes that can be stored for more than 20 years. The storage tolerance at elevated temperatures is generally good, in some cases up to 70°C.

There are 3 classes of primary lithium batteries:

1. Solid state batteries – Characterized by low power but superior shelf lives and used, for example, in pacemakers and computer memory backup. The expression ‘solid state’ is used because the electrolyte is solid and the ion transmission between the electrodes takes place in a solid, non-electrically conductive material, usually a polymer.
2. Batteries with a solid cathode – Usually found in coin cells or small cylindrical batteries and used in a huge numbers of applications, such as clocks, memory circuits, photo and communication equipment, toys, flashlights, etc.
3. Batteries with soluble cathodes – Industrial and military batteries manufactured in large cell sizes, up to 35-40 Ah, but also available in smaller sizes that are used, for example, in medical and civilian applications.

All of these batteries have lithium anodes (the battery’s negative terminal). The cathode material determines the battery system.

The most common primary lithium batteries on the market are lithium disulphide (LiFeS₂) and lithium manganese dioxide (LiMnO₂) batteries. Both of these are of the solid cathode type and are sold as consumer batteries from electrical goods stores and supermarkets. Other primary lithium batteries are mainly intended for the professional market. Table 1 lists the most common primary lithium battery chemistries and their properties.

Over the last few years the market has seen the introduction of battery types in which the cathode consists of a mixture of different electrode materials. The purpose of these batteries is to provide a system that can accommodate both high and low loads. One example is the battery used for implanted defibrillators that consist of silver-vanadium oxide (SVO) cathodes to absorb the peak load of a defibrillation, and carbon monofluoride ((CF)x) for the baseline load between defibrillation pulses.

Secondary Lithium Batteries

There are two main groups of rechargeable lithium batteries, one of which uses lithium metal as the negative electrode. These are called lithium metal batteries. Lithium metal batteries were launched on a limited scale for consumer electronic products in the 1980s, but were withdrawn quickly because of safety problems with the lithium electrode. Lithium reacts with the electrolyte, forming dendrites on the surface of the electrode. Under repeated charging, the surface of the anode increases, with a corresponding increase in its reactivity and thermal sensitivity. More recently, however, the lithium metal anodes have once again found practical application in research, although they have not yet been marketed on a large scale. One way of reducing the fire risk in lithium metal batteries is to replace the electrolyte with a solid state polymer that does not react with lithium.

The second type of rechargeable lithium battery is called a lithium ion battery, which has a negative terminal that consists of a carbon-based material, usually graphite or another type of alloy or material that permits intercalation (i.e. storage) of lithium in the structure. This category includes lithium polymer batteries, which differ from traditional lithium ion batteries in that they have an electrolyte bound within a non-conducting polymer matrix. Lithium ion batteries were introduced by SONY in 1991, and were first used in advanced consumer electronics products, mobile phones, digital still and video cameras, as well as laptop computers. This type of battery is constantly gaining new ground, and the areas of use are constantly expanding. Lithium ion batteries are currently used in all sorts of portable applications within consumer electronics, medical technology and military systems. The general properties that contribute to this popular usage are the high energy density of the lithium ion battery and its specific energy, i.e. a lot of energy from a small volume and weight compared with other rechargeable types of battery. Other properties include low self-discharge and relatively long recharge lifetimes (300-500 complete charges, and even more if the battery is used within a more restricted charge state range). High costs and the safety aspects are the main barriers, as well as the fact that access to large cell types (battery cells with a capacity exceeding 5 Ah) is limited. Existing lithium ion batteries also suffer from functional limitations at low temperatures.

Carbon materials are used in lithium ion batteries because carbon has the ability to reversibly absorb and release large quantities of lithium (Li:C=1:6) without altering the mechanical and electrical properties of the material. Graphite and coke also have an electrode potential that is close to that of lithium; 0.0­-0.1 for graphite, and 0.0-0.3 for coke. On the first charge of lithium in the carbon material, an outer layer is formed (Solid Electrolyte Interface, SEI). The SEI layer is important because it prevents the carbon anode from reacting with the electrolyte. Dissolution of the SEI layer at elevated temperature poses an important safety risk to lithium ion batteries.

There are a number of different chemistries within the lithium ion family that are based on the choice of cathode material. Traditional lithium ion batteries use cathodes made from cobalt dioxide (LiCoO2). By adding nickel (LiNi₁-xCoxO₂), a layered material is obtained that is more stable and less expensive than pure CoO₂. ­Manganese oxide spinel (LiMn₂O₄) is another common cathode material for lithium ion batteries. The manganese oxide has a lower energy density but is cheaper and more stable than cobalt. Extensive research is being carried out into new cathode materials, and in the last few years lithium ion batteries with cathodes made from lithium iron phosphate (LiFePO₄) have been marketed on a large scale based on their high safety levels and low cost compared with conventional lithium ion systems. The benefits of LiFePO₄ have been achieved at the expense of their energy density and voltage. New electrode materials are being developed and tested constantly in order to produce lithium ion batteries with properties that are suitable for different applications and areas of use. Table 2 shows a comparison of the main types of lithium ion batteries that are currently on the market. All of them have graphite as the most commonly occurring anode, but there are also high power batteries using hard carbon. At present, research into rechargeable lithium ion batteries is being driven mainly by the requirement in the automotive industry for long calendar and charge lifetimes, high discharge currents, the need for rapid charging, and, additionally, cost targets.

Trends in Lithium Ion Batteries

Trends that are noticeable within research and development of lithium ion batteries are:

1. transition to cheaper and less toxic electrode materials (cathodes), e.g. phosphates and silicates.
2. transition to materials with higher reversible lithium reception (the more lithium atoms the material can absorb in each unit cell, the higher the potential battery capacity)
3. materials that can withstand rapid charges (from 0 to 90% SOC in ten minutes)
4. power and energy batteries for the automotive industry and stationary installations
5. increased cell size in the form of stored energy capacity
6. high voltage levels, including electrolytes that can withstand higher electrode potential without degrading or reacting with the environment
7. enhanced safety compared to current battery types

Titanate, silicon and silicides, as well as tin and tin alloys, are the new anode materials that have been discussed in the context of research and development. All of these materials are regarded as offering improved safety since they do not form an SEI layer. The main advantage of titanate is that its structure is highly stable and there is no increase in volume during battery charging. Lithium’s mobility within the material is very rapid, which permits the battery to be charged and discharged at high currents. Charging times of less than 10 minutes have been postulated. The limitations include low cell voltage due to the high electrode potential of titanate compared to that of lithium. The electrodes are also more expensive than carbon-based electrodes. Silicon and tin are both limited by very large electrode expansion during charging, around 300 and 250% respectively. These two material groups have relatively low electrode potential compared with lithium, which results in battery cells with acceptable cell voltages of between 3.5-4 V, depending on the choice of cathode material. Silicon is especially interesting as it permits storage of large quantities of lithium in the anode structure (e.g. the reversible Li ion insertion capacity is >1). Moreover, the material is neither expensive nor toxic.

Development on the cathode side is more multi-faceted. The list of substances and variants of substances that are being studied is extremely long, and many of them will never be marketed. Some of them will only be of interest for niche applications on cost grounds. Among the materials being discussed are metal phosphates (in particular vanadium, magnesium and manganese, as well as various compounds), layered metal oxides (often with manganese oxide bases) and silicates (e.g. iron silicate). The metal phosphates are being promoted thanks to their chemical stability, leading to increased safety compared to traditional cobalt-based cathode materials. Magnesium and manganese phosphates are also relatively cheap and non-toxic materials, which is also one of the strengths of iron silicate. The layered metal oxide structures are expected to offer rapid lithium mobility, thereby permitting high charge and discharge currents, but the material is still relatively expensive and associated with safety problems.

Rich Byczek
is the Technical Lead for Electric Vehicle & Energy Storage for Intertek and has been with the company since 2003.  Over the past year, he has been developing Intertek’s North American center of excellence for battery testing at the Detroit site.  Rich sits on multiple battery-related standards committees.  His work in the past has included EMC (Electro-Magnetic Compatibility) testing as well as audio and navigation equipment validation and product development. Rich is a member of SAE and IEEE.  He holds a B.S. in Electrical Engineering from Lawrence Technological University.