This article by C.S. Ramanathan suggests the importers of Lithium-ion cells and batteries to procure that the cathodes made of single crystals (not polycrystals as is being done at present).
Problems with polycrystal cathodes
Most cathodes used presently all over the world are Polycrystal cathodes.
Each of the oxides of Nickel, Manganese, Cobalt with Lithium exist as different crystals side by side. This gives rise to the lack of homogeneity. Upon cycling over a period of time, microcracks develop causing structural degradation. Gas release and poor thermal stability are also some of the resultant drawbacks.
Use of Single Crystal Cathodes gives a tremendous boost to the electrical performance and life of LIBs by overcoming the structural damage caused by cracking of polycrystals.
The table below shows the major differences between poly and single crystals and also the reasons why we need to switch over to single crystal cathodes for all LIBs. The superiority and benefits of Single crystal cathodes are highlighted.
Differences between polycrystalline and single crystal cathodes
|Properties||Polycrystal Cathode Conventional (PC)||Single Crystal Cathode (SC)||Remarks|
|Arrangement of atoms||Agglomerates of particles||Regular & periodical arrangement of atoms in 3 -dimensional space||Different crystal arrangements|
|Size||Large separation of nano size between agglomerates||Homogeneous; Ordered structure. large crystal of micron size|
|Manufacturing of cathode material||Spray Pyrolysis – Salts of Nickel, Manganese, Cobalt and Lithium are dissolved in water and passed through a nozzle. The emerging droplets strike a hot plate. Powder obtained is heat-treated to get Polycrystalline cathode||Additional process of ANNEALING in inert atmosphere and temperature of 650ᴼC to get Single Crystal Cathode material or SCC||Spray Pyrolysis is a procedure to make Poly crystals for all chemistries. Several other procedures exist.|
|Advantage||Good fluidity, high initial capacity, good rate performance||Good crystals, high mechanical strength, homogeneous reactions, small specific area, remarkable cyclability||Single Crysal Cathodes have superior performance, good thermal stability and longer life|
|Disadvantage in long term service||Microcracks, structural degradation, gas release, poor thermal stability||Cracks are suppressed. Controlled side-reactions and gas release|
|Diffusion of Lithium-ion||Slow||Faster by 10 times within crystal||Li-ion moves 6-14 times faster within single crystals|
Process to make polycrystals and convert them into single crystals.
Spray Pyrolysis method of making cathodes
There are several methods of making cathode materials. Out of these, Spray Pyrolysis is a simple process and is now being used by many manufacturers. In particular, the Berkeley National Laboratory in USA has recognized this as a suitable process for making almost all types of cathodes. An aqueous solution of the salts of Nickel, Manganese, Cobalt and Lithium are sucked through a nozzle using compressed air (Ventury Effect) as the carrier. The solution breaks into droplets and falls on a heated surface resulting in a dry powder. Subsequently, the powder is heated and cooled a few times. The polycrystals so obtained is found to have Manganese on the surface and the Nickel locked in inside. Nickel is very reactive; but packing it inside reduces the instability of Nickel. The higher energy density which is the main advantage of Nickel is realised. This is an unexpected benefit from spray pyrolysis procedure.
Making Single Crystal Cathode Powder
The polycrystalline crystals are subjected to another step called ANNEALING. This involves heating the material to a high temperature in an inert atmosphere for several hours. The crystals rearrange themselves in an orderly fashion.
- Single-crystal layered oxide cathodes have superior cycle life due to reduced cracking
- Single Crystals can be synthesized through a variety of methods
- Synthesis process benefits from post-synthesis treatments such as Annealing
- There is significant space to optimize the synthesis and interest is growing rapidly
TESLA’s focus on Single Crystal Cathodes for its batteries
Long range and longer life of batteries is the target of every EV car maker. A longer range can be achieved by using a larger battery of higher capacity. But this comes at a higher cost and more importantly an increase in weight ratio of battery to kerb weight of cars. At present for some cars, battery weight is about 25% of weight of the car. The Battery Group 500 of USA (supported by DOE) has a target specific energy density of 500 Wh/kg. Currently, it is still below 250 -280 Wh/kg.
Researchers at Tesla are working on LIB technology are continuously working to improve the performance and range of batteries. Chief of Technology – Jeff Dahn and co-researchers made a breakthrough which improves the performance and more importantly the life of batteries.
NCA 811 (Nickel 80%, Cobalt 10%, Aluminium 10% – combined with Lithium) is a commonly used cathode in Tesla LIBs. The breakthrough is the use of Single Crystal Cathodes in place of the Polycrystalline Cathode used at present. In polycrystals, the different crystalline species are oriented in different directions; while in Single crystals which are larger, the orientation is the same in all directions.
Suggestions for importers of Lithium–ion cells and batteries
For Lithium –ion battery, cathodes with single crystals have been of exceptional interest to both academics and industry in the last few years. The SCCs ( Single Crystal Cathodes) give better electrical performance and more importantly longer Life and higher safety. My purpose is to bring this development to the notice of many importers of Lithium–ion cells and batteries. Therefore, when ordering cells or batteries, SPECIFY THAT THE CATHODE SHOULD BE MADE OF SINGLE CRYSTALS. The small increase in price (about 1% increase in cell price) will be more than justified by the superior characteristics of SCCs. The higher cost is attributable to the addition step of heat treatment of polycrystals required to obtain single crystal cathodes. Single crystal cathodes of all chemistries will benefit- LiFePo4, MNC, NCA etc. Some manufacturers are already making SSCs for in-house consumption and will be more than willing to market the same. It may take a while for the widespread production of SCCs but I hope that by creating awareness among our entrepreneurs, we will be hastening its adoption.
Obviously, further improvement can be made by using better electrolytes, and Anodes to make the Lithium–ion battery as robust as possible.
Matching Electrolyte for single crystal
The Lithium salt commonly used is Lithium hexafluoro phosphate or LPF₆. This decomposes in presence of moisture of 300 ppm. Therefore, LPF is treated with Oxalic acid to get more stable compound – Lithium difluorobis (oxalato) phosphate.
About the Author
C.S. Ramanathan is a Battery Consultant, and holds M.Sc; D.I.I.Sc. degrees from the Indian Institute of Science. Earlier, he was Head of R&D at AMCO Batteries Ltd and has been a consultant to many battery manufacturers in India and abroad. Authored the book ‘Manufacture of Lithium-Ion Battery (LiFePO4 based) – An introduction for MSMEs‘.
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