Commercial viability of Sodium-ion battery for electric aviation

Dr Vilas Shelke, founder and Chief Executive Officer of Rechargion Energy, discusses the viability of sodium-ion batteries for drones.

The conventional modes of transport include surface, sea, and air vehicles. The surface mode is transforming rapidly from fossil fuel to electric or battery-powered vehicles. Small-scale aviation machines known as ‘Unmanned Aerial Vehicles (UAVs), or more commonly, drones, conveniently use rechargeable batteries. These vehicles are categorised as nano, micro, and mini drones Depending on the wing size, flight range, payload capacity, etc. The Li-ion batteries with LiFePO4 (LFP) or Li-Ni-Mn-Co-O (NMC) cathodes with liquid or polymer electrolyte (LiPo) are the dominant products in the energy storage market.  However, they have some inherent drawbacks:

• Minerals for key elements like Li, Co, and Ni are scarce and disproportionately distributed. The relevant resources are available in very few countries, particularly not in India.

• The excessive mining and hydro-extraction process creates a huge environmental impact.

• China dominates the end-to-end manufacturing of Li-ion batteries, and the dependence on imports for materials, components, and cells can seriously jeopardise national energy security for many countries.

• Li-ion batteries are prone to catch fire at slightly elevated temperatures due to thermal runaway. Several vehicle, cargo-ship, and factory-related fire incidents have created vital safety concerns.

• Li-ion batteries typically need 3-4 hours for full charging, which increases the wait time for mobility applications.

Therefore, searching for an alternative ‘market-fit’ energy storage technology is the need of the hour. In this context, sodium-ion technology is a potential competitor to Li-ion batteries. The U.S. Department of Energy recently sanctioned US$50 million to establish the Low-cost Earth-abundant Na-ion Storage (LENS) consortium. It aims to discover, develop, and demonstrate a new class of sodium-ion batteries.

Several reputed companies like Faradion, UK (now acquired by Reliance Energy, India); CATL, China; BYD, China; Hina, China; Natron Energy, USA; and Altris, Sweden are pursuing this emerging technology with some demoable products and full-scale commercialization plans by 2025-26. Nevertheless, there is some scepticism about the practical viability of this technology for electric mobility.

At Rechargion Energy, a deep-tech startup and spinoff from CSIR-National Chemical Laboratory, Pune, India, we addressed such concerns by demonstrating the deployment of a self-manufactured sodium-ion battery in a small drone. The successful takeoff and vertical flight illustrate the capability of sodium-ion batteries even for aerial mobility. Rechargion has developed an indigenous technology with a ‘powder-to-pouch cell’ approach that includes the synthesis of anode/cathode material and fabrication of pouch cells using in-house pilot plant facilities. The drone was powered by a small 3.7 V, 0.5 Ah pouch cell.

A rechargeable battery is an assortment of outer casing, current collectors, anode, cathode, separator, and electrolyte. The structure of a battery remains mostly the same across different chemistries, except for the change in anode, cathode, and electrolyte materials. A typical sodium ion battery includes a hard carbon anode, sodium fluorophosphate (NVPF) or transition-metal oxide (NTMO) cathode, NaPF6 electrolyte, aluminium current collector, and polypropylene separator stacked inside a flexible pouch or hard casing. There are few practical considerations or performance metrics to judge the commercial viability of any battery technology. The comparison of key parameters for commercial LFP and sodium-ion chemistries is given in the table.

• Battery Weight aka Energy Density

The Energy Density (Gravimetric) expressed as Watt-hour/kilogram (Wh/kg) directly translates to the weight of the battery for a specific capacity. Typically, commercial LFP batteries have an energy density of around 150 Wh/kg; sodium-ion batteries can catch up with this number. Thus, a representative 3 kWh battery for an electric 2-wheeler will weigh 20 kg, which is bearable for a significant kilometre range on a single charge. A small drone is powered by 4-8 Wh battery which will weigh 25-50 gm. Similarly, bigger-size drones can take off and fly conveniently with both types of batteries.

• Battery Life aka Charge-Discharge Cycles

Commercial Li-ion batteries are rated for 2000-3000 charge-discharge cycles. Everyday charging and discharging may result in a life span of 3000 days or 8 years maximum. Comparatively, sodium-ion batteries with sodium-transition metal oxide cathode and sodium-fluorophosphate cathode have life spans of 8 and 13 years, respectively.

• Charging time aka C rate

The C rate is the measure of battery current discharge relative to the capacity.  Thus, 1 C rate means discharging a battery of 1 A capacity in 1 hour.  The higher C rate means rapid movement of ions between electrodes. This movement depends more on the intercalation force than the mass of ions. Therefore, low intercalation force offers the advantage of faster charging up to 10 C rate for sodium-ion batteries. It can be charged in less than 30 minutes. Moreover, sodium-ion batteries have symmetric structures, with aluminium as a current collector for both electrodes. It allows the battery to discharge fully to zero volts and recharge to one hundred per cent.  The storage and transport can be done in a fully discharged state.

• Battery Safety aka Thermal Runaway

The thermal energy (heat) generated during charging/discharging is related to the intercalation/deintercalation force of ions at the electrodes. Sometimes, the process triggers excess heat generation, causing thermal runaway and, thereby, the battery catching fire. The intercalation force is inherently low for sodium ions as compared to Li ions. The small amount of heat can be dissipated easily, and the chance of thermal runaway is minimized. Therefore, a sodium-ion battery is much safer with a wide operating temperature range of -40 to 60 ⁰C.

• Range aka Battery Capacity

The range of flight or mileage depends on the capacity of the battery deployed for a specific vehicle load. A combination of voltage and current is required for the traction motor. The LFP and Na-ion batteries have similar voltage and energy density values, so the battery weight is similar for a particular capacity. As a result, the range will be the same, and replacing the LFP battery with a Na-ion battery will not affect the mileage.

• Cost aka Bill of Materials

The cost of the battery depends on the materials, fabrication process, utilities, manpower, and overhead/logistic expenses. Except for materials, other expenses are the same for multiple battery chemistries. Within materials major cost difference is on the account of cathode and anode materials. The abundance of sodium (2.4 wt %) is much higher than lithium (0.001 wt %), and it is extracted directly from seawater without extensive mining efforts. The costs of hard carbon and synthetic graphite anode materials are comparable. The use of aluminium (and exclusion of copper) current collector for both the electrodes of the Na-ion battery further reduces the cost of the materials. Effectively, the bill of materials is reduced by 30-40 %. Additional factors like no import dependence and extended battery life imply significantly low cost for the customer. 

• Supply Chain and Scale-up

The right types of ‘Materials-Machines-Manpower’ are essential for large-scale manufacturing of battery cells. In spite of a few ‘Giga-factory’ announcements, the ‘end-to-end’ manufacturing is still a distant goal in India. Importing processing machines, battery components, and materials is indispensable for Li-ion batteries. Comparatively, Na-ion is a nascent technology and building a supply chain is a herculean task. A translational approach from laboratory to pilot scale and Mega to Giga factory is necessary to scale up the technology.

In a nutshell, Sodium-ion technology offers ‘six S’ metrics viz., Sustainability, Safety, Scalability, Suitability, Speed and Self-reliance. Therefore, it can be viable alternative to the Li-ion battery for surface mobility as well as aviation sector.

About the author

Dr Vilas Shelke is a Physicist and an academic researcher turned entrepreneur. He has been the recipient of an Indo-US Science & Technology Forum Fellowship, a French Embassy Fellowship, the DST Young Scientist award and CSIR fellowships. He is the founder and Chief Executive Officer of Rechargion Energy Pvt Ltd., Pune. The company is developing fully indigenous, import-free, safer and cheaper rechargeable batteries based on sodium-ion chemistry. He can be reached at vilas.shelke@rechargion.com

Also read: Sodium-ion batteries: A real challenger or another passerby for Indian storage tech?