The constant quest for improvement in EV battery technology continues

BY RON FREUND: ELECTRIC AUTO ASSOCIATION BOARD MEMBER 

In the spring of 1992, the guest speaker at the chapter meeting of the Electric Auto Association of Silicon Valley was Dr. Steven Visco, founder, CEO, and CTO of PolyPlus in Berkeley. It’s been remembered to this day that he gave attendees a fascinating look into the future.

In the intervening 18 years, Visco has been on a  quest for a Lithium Sulphur (Li-S) battery, ostensibly for use in electric vehicles (EVs). Recently, another PolyPlus veteran, scientist  Bruce Katz, returned to the EAA podium to give an update on the direction their research had taken. It showed that Visco’s tenacity is certainly laudable and underscores that the trip from ‘research lab to production’ can sometimes be a long and winding road. 

Nearly three decades later, with much continued research, all rechargeable batteries are  improving. Many type have been developed, each with strengths and weaknesses. 

New records established 

The promise of ultra-high energy density chemistries has always enticed EV drivers. A scant 14 years ago, the promise of a 200+ mile electric sports car was ridiculed by many. With a limited production release, the 244-mile EPA rating was the best a growing community of believers had ever seen. A few years later, Simon Hackett broke the distance record for a production electric vehicle in his Tesla Roadster, driving 501 km (311 miles) from Alice Springs to Marla, South Australia, all on a single charge. The car had approximately 4.8 km (3 miles) of range remaining when the drive terminated. 

Fast forward to today. The quest for this new battery continues, and the bar has been raised with ever higher stakes as the world population of plug-in cars increases. Tesla has produced many capable sedans, and other Original Equipment Manufacturers (OEMs) are following. Lucid is now promising 517 miles of range on their high-end Air model with delivery from their new Casa Grande, Arizona plant starting in early 2021. Details on this ‘Dream Edition’ are here. 

With a new generation of rechargeable lithium batteries beckoning EV designers, systems have higher than ever voltages and faster recharge time as the evolution continues. Given the typical exponentially tapering charge currents used today, charge rates in excess of 1,000 miles gained per connected hour are possible. In practice, not all charging facilities offer that rate. No longer are the factors limiting recharge times simple, as new approaches are steadily devised.

Research continues on battery chemistry, too 

Just this year, PolyPlus devised a glass coating that prevents the dendritic growth between electrodes that becomes a major limiting factor of the cycle life of lithium rechargeable batteries. Assuming that the cells are operated within tight constraints, projected traction pack cycle lifetimes count in the middle thousands, meaning a possible battery longevity well past 10 years. With careful treatment, an EV pack could last even longer. For that, the term “million mile battery” was coined. This valuable asset would survive multiple vehicle owners and possibly have a chance at a healthy secondary life beyond automotive service. 

But increasing the energy density is still the quest. Gasoline has an incredible energy density of 12,000 watt-hours per kilogram (wh/kg), but batteries fall far short of that level, currently at approximately 300 wh/kg. Not only could higher densities spell longer ranges, but they could also result in lighter cars and potential applications in other transportation, such as short haul and Vertical Take-off and Landing (VTOL) airplanes. But along with higher density, we cannot sacrifice cycle life, nor have any temperature restriction, which just makes the fit more difficult.

Classified as lithium metal chemistry, specifically with sulphur as one of the agents, PolyPlus continues to look at solid-state lithium glass laminate solutions.

And speaking of flight...

This past August, UK-based-firm Oxis Energy announced electric airplane flight tests using Li-S cells packaged in foil as a pouch cell. They claim to have achieved twice the energy density of lithium cells currently in use in EVs. Although those specs are not readily available (of particular interest for comparison is the cycle-life projection), the content released by the company includes a simplified description of the chemical reactions that take place during charging and discharging (which together, constitute a ‘cycle’). 

The content also alludes to the parasitic side-reactions which inevitably take place and currently thwart achieving high cycle counts. The multi-stage reaction taking place in the cell involves lithium ions in the electrolyte flow to the cathode, where they form ‘polysulfides’ with ever higher sulfur-to-lithium ratios. Li-S batteries are unusual because they go through multiple stages as they discharge, each time forming a different, distinct molecular species of lithium and sulfur (the polysulfides).  

The actual charging and discharging process can become involved with various polysulfides, each releasing energy to power the load. In the Li-S battery, different polysulfides figure in the electrochemical process at different times during charge and discharge. So the key issue of an Li–S battery is that this polysulfide "shuttle" effect is responsible for a progressive leakage of active material from the cathode, resulting in low cycle life for the battery. 

This is so much more complicated than current LiNiMnCoAl formulations mass produced for EVs today. In theory, such Li-S cells would have a very high specific energy density [1].  But the “proof is in the pudding”: the actual $/kWh cost in production and the lifetime experience in the field in products. 

It’s all changing quickly

Perhaps other chemistries will emerge with additional capability, but we know the winner will not stand on the podium for long. Our quest continues, since gasoline is so conveniently portable and many times more dense. The path forward may mean shaving with Occam’s razor. These Li-S cell parasitic electrochemical reactions may be hard to contain and/or eliminate. 

Current Lithium ion cell formulations with their demonstrably powerful formulation may in the near future lose their cobalt component. Cell volume may increase nearly ten-fold, lessening the weight, allowing the total vehicle cell count to decrease, and enabling simpler manufacturing processes to be used. Clever packaging will benefit the next generation of products and further improve pack level energy density, while maintaining or reducing cost. 

1 “Shuttle phenomenon—The irreversible oxidation mechanism of sulfur active material in Li–S battery". Journal of Power Sources. 235: 181–186.