The article by Blomgren provides an overview of lithium ion batteries, giving some background on their early development, then describing the current state of the technology, and finally going into their current deficiencies and prospects for likely improvement in the future.
Early Lithium Ion Batteries
The Li-ion battery was first developed by Sony in 1991, although the idea of using Li ions going back and forth reversibly between the two electrodes in a battery goes back to the 1970s. At the time there was interest in developing Li battery technology, due to the high specific energy density, but other lithium battery technologies were plagued with safety problems.
It had been discovered that lithiated transition metal oxides of the NaFeO2 structure would reversibly deintercalate and reintercalate at relatively high potentials. Sony ended up using LiCoO2 for the active positive (cathode) material in their battery. Finding a suitable material for the negative electrode (anode) however, proved more difficult. With graphite and other carbonaceous materials, solvent molecules would co-intercalate along with the lithium ions, leading to disruption of the carbon structure and reduction of the solvent. Petroleum coke, with the addition of ethylene carbonate, proved to be more resistant to co-intercalation. Later generations of batteries used hard carbon prepared from polyfurfuryl alcohol, or mesophase carbon microbeads for the negative electrode, due to their higher specific energy.
The electrolyte that was used was ethylene carbonate with a linear dialkyl carbonate, with LiPF6 for the salt. This carbonate solvent is oxidation resistant up to 4.5 V.
The original Sony battery had an energy density of 200 Wh/l and a specific energy of 80 Wh/kg. Modern Li-ion batteries have an energy density of about 300-400 Wh/l and a specific energy of 120-150 Wh/kg, depending upon the materials used.
Present Day Lithium Ion Batteries
Since first introduced in 1991, Li-ion batteries have increased in both size and power, and are now used in a much wider range of applications.
An important issue with Li-ion batteries is short circuiting due to metal particles on the surface of the electrode. To prevent this, ceramic coatings, both for the separator and the electrode were developed. These ceramic coatings also have the added benefit of improving electrolyte wetting.
In modern Li-ion batteries, small quantities of Si are added to graphite-based anodes because it increases the specific capacity while adding little to the cost.
For the cathode, the common materials currently in use are LiCoO2 (LCO) and LiMn2O4 (LMO). Safety still remains an issue with these materials. Newer cathode materials are being developed, such as LiNixMnyCo1-x-yO2 (NMC), which is safer, more flexible, and has higher power, but whose wider implementation is hampered by ongoing patent issues.
Deficiencies of Present Lithium Ion Batteries and Likely Improvements
Lithium ion batteries are of great interest in the development of electric vehicles. According to current estimates, the current cost of Li-ion battery packs is $190 per kWh. A general goal for the auto industry and the Department of Energy is to get this cost down $125 per kWh. It is also considered essential for mass adoption of electric vehicles that they have a range of at least 200 miles. Currently there are few models of electric vehicles that have such a range, and they are expensive, high-end vehicles, such as those produced by Tesla.
The anodes of current Li-ion batteries suffer from low capacity density. Most alloys experience significant volume change during operation, which can result in a disruption of the protective layer on the anode particles. It is noted that progress in this area is likely to be made by using composites of alloying metals with various types of carbons.
Another issue currently facing Li-ion batteries is the instability of LCO at higher voltages due to the loss of oxygen, limiting charging potential of the battery to about 4.2 V. Some newer materials, such as NMC do not have this oxygen loss issue, but do experience slow oxidative degradation of the electrolyte. It has been shown that the life cycle of batteries made with NMC can be improved with the use of a protective coating on the cathode particles and/or additives to the electrolyte.
Lithium ion batteries are likely to continue to be an area of focus for the research efforts of industry, and therefore we will likely see continued improvement in the technology in coming years.
Source: G. E. Blomgren. J. Electrochem. Soc. 164, A5019 (2017).
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