24 July 2018

Beyond Li-ion – batteries for transportation

Li-ion batteries have made amazing advances in the last few years, being used in a wide range of applications. But there is still the issue of range anxiety with many of these applications and if we want to remove that issue the batteries have to be made lighter and must carry more energy. Achieving a doubling of the energy density in a Li-ion battery will give an average eCar a range of 600 miles plus, comparable with their fossil fuel cousins. And so there are many companies ranging from classic OEMs like Toyota to start-ups such as OXIS Energy which are focused on developing the next generation Li-ion battery.

The technical bit…..

In order to understand the challenges of such an endeavour it is is first necessary to take a quick look under the hood of a “traditional” Li-ion battery. Such a battery has 3 main components (1) An anode, (2) a cathode and (3) an electrolyte. In a conventional Li-ion battery when we charge the battery we are taking lithium ions from the cathode, pushing them through the electrolyte and “inserting” them into the anode. To discharge the battery we just reverse the process. Typically the anode is made from carbon – graphite, whilst the cathode can be from a number of materials e.g.Lithium Manganese Oxide.

The amount of energy we can store in the battery is dependent at upon the amount of lithium ions we can store in the anode (more is better!). And this is where the new R&D is focused. Broadly speaking, two routes are being followed. In the first route silicon is being combined with the graphite, this increases the amount of lithium ions that the anode structure can hold. Unfortunately, this has some severe technical challenges related to volume changes when the lithium is inserted into the anode during charging. Various novel carbons I.e. graphemes or nano engineered carbon supports are used to solve the problem. This approach is largely incremental, every year there are reports of more and more silicon being integrated with corresponding incremental increases in the energy stored.

However, this approach will only ever likely get us an extra 50% increase on where we are today, and if we want to get the needed 100% increase then a different approach is needed. A more disruptive approach is to replace the anode with a pure lithium anode. This opens up a route to eventually tripling the energy stored with Lithium Air batteries. However, a pure lithium anode comes with a different set of challenges. Thin metallic fingers called dendrites can form during charging/discharging, and if these are large enough to reach across to the cathode then a catastrophic short circuit can occur. Lithium is also extremely reactive, and it is very difficult to build a battery that lasts a long time. See a representative roadmap for Li-ion based systems below

A roadmap for batteries for mobility applications

OXIS Energy and Lithium Sulphur

Despite these challenges there are a number of companies working on these solid anodes and one of them is OXIS Energy, headquartered in the UK. OXIS has developed a type of battery with a solid lithium anode called a lithium-sulphur battery. This uses lithium for the anode and sulphur for the cathode. It has successfully produced batteries for the aviation industry – their batteries are part of the Zephyr Innovation Program with Airbus. Zephyr is a high altitude, long range UAV designed to provide telecoms services similar to a satellite but at a fraction of the cost.

OXIS is also ramping up its production capability with a new manufacturing site in Brazil with investment from Aerotec, a Brazilian Private Equity fund. And Oxis is not just focusing on the aviation sector but also has plans to extend its offering to the eBus segment with a new technology program (vehicles Lithium Sulfur Future Automotive Battery (LiSFAB) project), focused around batteries for bus and utility vehicles. Such buses will be able to carry more people in more comfort, the extra energy being carried allowing better (and more energy hungry) air-conditioning and heating.

Other technologies

In coming blogs I will take a look at more of the solid lithium anode technology being developed as well as considering what technologies may be a better fit for the rapidly expanding stationary applications segment.

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