Minimizing Self-Discharge with Optimized Electrolytes
Plugging holes in a leaky bucket: Self-Discharge and why electrolyte must be tailor-made for each application
In our previous post, we considered energy arbitrage, the idea that electrical power can be stored cheaply and sold off when demand is high. The concept is empowered by long term storage where, for example, cells are charged in the summer and discharged in the winter. We asked if Li-ion batteries will meet this need in the future. The answer: probably not. Why?
Money Talks and It’s Telling us Li-ion Is a Tough Sell for Long Term Storage
The first answer is the reality that every scientist wishes they could ignore: cost. Li-ion batteries have made great strides in price – dropping more than 5x in the last decade (Volta Fondation). This trend is expected to continue, but the competition for stationary storage at this timescale is fierce. With decreased charge/discharge cycles, the $/kWh for Li-ion increases steeply. That’s not to say that it won’t find a place in the medium timeframe energy storage (>10 hrs, <1 month), but likely not longer. Flow batteries, emerging metal anodes, chemical storage and others are likely better suited for this application (LDES Council).
The Technology Barrier: Holding Charge for as Long as Possible
The second answer is driving electrolyte research forward: self-discharge. As the name suggests, batteries lose their charge over time. For EV drivers, this means missing a few percent when you return to your car after 24 hours (more if you made the mistake of leaving it charged during vacation). For energy traders, charge lost is dollars lost. It is also an important metric for folks with battery back-ups in case of emergency. Even if Li-ion batteries aren’t going to be sitting charged for months, consumers don’t like leaking energy - and therefore $$.
The figure below shows what self-discharge looks like from Dahn’s research group. A cell (here a pouch cell, similar to a cell phone battery just smaller) starts fully charged at 4.2 V (100% state of charge). After charge, the voltage is allowed to move as it wishes (simulating a battery at rest, open-circuit) and we watch what happens. As the voltage drops, so does the extractable energy of the cell. Over the 500 hours shown (20 days), the voltage drops to 4.17 V in the best case and almost 4.13 V in the worst. This corresponds to ~25 mAh, or about 10% of the cell’s nominal capacity.
David Wang’s (Dahn group) work demonstrating self-discharge and the effect of electrolyte additives. Here 225 mAh LCO/graphite pouch cells are shown (J. Electrochem. Soc.).
Solid Electrolyte Interphase (SEI): Hitting the Goldilocks Zone
FEC and VC are two common electrolyte additives and shown in the figure above. They preferentially react with electrodes during cycling and form a layer on the surface (the SEI, Solid Electrolyte Interphase) that has a number of beneficial properties when appropriately formed. One major benefit is the layer’s ability to block unwanted side reactions that compete with Li+ storage. In the above figure, 2% VC (purple diamonds) prevents unwanted reactions during storage - as demonstrated by the lowest amount of voltage drop. FEC does as well, but clearly higher concentrations are required for the same effect. This process is impacted by temperature and voltage, where reactions are accelerated when the storage temperature is warmer and voltage is higher. The figure below demonstrates this mechanism, where electrolyte components react and form additional SEI during storage. The process locks away Li+ from future use. Too much additive and the SEI layer grows too thick - either during formation or during storage. Too little and there isn’t enough additive to form the SEI in the first place. This amount is sensitive to the form factor (pouch vs. cylindrical, etc.), loading, and surface area of the electrodes. No one electrolyte meets all needs - even if you have the magic additive.
Two degradation mechanisms and an electrolyte approach to solving them. At the cathode, during storage electrolyte decomposition negatively affects the cathode. At the anode, renegade side reactions drive too thick layers.
The above discussion is anode-focused, but the cathode plays a role in self-discharge as well. Historically, we consider the electrolyte oxidizing at the cathode surface during charged storage: producing degradation products and intercalating Li+ ions. The process mimics normal discharge where Li+ moves into the cathode driven by potential. Recently, researchers Wan et. al. used the Advanced Light Source at Stanford to publish a paper that made a splash and challenged this idea. They validate a theory originally advocated by Chen et. al. in 2013, who suggested it is H+ liberated during solvent degradation that is intercalated - not solely Li+. Further problems occur as H+ wreaks havoc on the morphology of the cathode particles. A protective cathode layer that prevents direct cathode/electrolyte reactivity can suppress this behavior. Cathodes are often purchased with a thin coating and coating development is an area of active technological advancement (see ForgeNano). The electrolyte can also form this layer in-situ working in parallel.
Both the mechanisms above have similar effects:
locking away Li+ so it can’t be used for charge/discharge (depleting Li+ inventory), lowering capacity
slowing Li+ movement with thicker layers, lowering available power
degrading the active material, lowering available capacity
Note: There is another family of degradation mechanisms, cross-talk, that encompasses coupled chemical reactions at the cathode and anode. These are important. For brevity, we leave it for a future article.
The LiNK-BT Approach
We recognize that one electrolyte doesn’t meet every cell. Even within one company with unique technology, different products need different electrolytes. We provide specific electrolytes co-developed with cell manufacturers to match active material loading, density, form factor, performance priorities, etc. Let’s find the appropriate formulation for you!
LiNK-BT is investing is production technology to rapidly and cost-effectively produce custom batches at any scale. We know this flexibility will help us meet the demand for the mosaic of emerging battery technologies and companies in the active battery community.